Disease-Modifying Agents: Chondroprotectants

A number of compounds are able to modify the progression of osteoarthritis, most commonly by supporting the cartilage. These products help achieve the goals of preventing further degradation and providing support for the reparative cartilage.Although variable, evidence exists that supports the use of these products as sole or adjuvant therapy in the treatment of the damaged joint (see the discussion of recommendations regarding the use of disease-modifying agents in the treatment of joint disease). Care should be taken to not overinterpret studies that fail to demonstrate a statistical difference between treatment groups (including placebo); such studies do not demonstrate a lack of treatment effect, but rather, are unable to demonstrate a treatment effect. Commonly, treatment design fails to effectively address the complex nature of the conditions being treated, particularly in terms of sample size.

Injectable Products

Polysulfated Glycosaminoglycans

Efforts to treat osteoarthritis have focused on drugs that favorably shift the balance from degradation to synthesis of cartilage matrix. Two compounds composed of PGAGs are available in the United States and Canada: Adequan and pentosan polysulfate. Hyaluronic acid (e.g., Legend) is also a PGAG but differs in structure sufficiently to be addressed separately. Because of similarities in structure, however, the disposition of these drugs (at least as much as is understood) and their assumed mechanism of action are similar. On the one hand, subtle differences in chemical structure may impact physiologic function (e.g., water retention in cartilage), and thus change efficacy.On the other hand, for products with demonstrated efficacy but presumed different mechanisms of action, combination therapy in the damaged joint. Their use should not be limited to osteoarthritis, but should include any conditions associated with of joint damage—trauma, immune-mediated diseases, septic or drug-induced damage, surgery (including prophylaxis) and others. Further, prophylactic use should be considered in animals predisposed to joint damage (e.g., conformation predisposing to joint damage or intensive sports training). Because PGAGs are responsible for normal functions in a variety of body tissues, their potential applications include disorders other than osteoarthritis (e.g., interstitial cystitis and glomerulonephritis).

Chemistry

A PGAG s a polymeric chain of repeating units of hexosamine and hexuronic acid. Considered a hypersulfated compound, approximately 14% of the drug is sulfated. It is extracted and purified from bovine tracheal tissues.²⁹⁹ Normal cartilage matrix is composed of proteoglycan complexes, collagen, and water. Side chains of glycosaminoglycans (keratin and chondroitin) are attached to the core protein of the proteoglycan molecule by a strand of hyaluronate (see Figures 29-12 and 29-13). Water trapped between these complexes accounts for the resiliency of cartilage. PGAGs closely mimic the proteoglycan complexes found in normal articular cartilage.

Pharmacologic effects

PGAGs appear to be chondroprotective in both in vitro and in vivo models. In vivo models have included chemically and traumatically induced cartilage damage.^300,³⁰¹ Cartilage degradation is retarded in the presence of PGAG. Although the mechanisms of these protective actions are not known, chondrocyte proliferation and matrix biosynthesis appear to be important.³⁰¹ Collagen, proteoglycan, and hyaluronic acid syntheses increase.³⁰² In addition, proteolytic enzymes such as collagenase,^302,³⁰³ leukocyte elastase,³⁰⁴ proteases,^299,³⁰⁵ and lysosomes are inhibited,³⁰⁵ although these actions are likely to be complex.³⁰² Complement activity is also inhibited; the degree of inhibition appears to be related to the sulfate load of the chondroitin sulfate matrix.³⁰⁶ PGAGs appear to have no effect on the ability of IL-1 to stimulate metalloproteinase activity in cartilage.³⁰⁷

Disposition and safety

Deposition of PGAG in normal and damaged cartilage has been demonstrated after parenteral administration. Drug that is not retained in cartilage is excreted primarily by the kidneys with minimal degradation of the parent compound. Toxicity is limited in all species studied. In dogs the LD₅₀ is 1000 mg/kg.²⁹⁹ Heparin and PGAG are chemically similar. Adverse effects related to the anticoagulant activity of PGAG have been suggested. One study found coagulation times to be prolonged after administration (see below). This suggests that PGAGs should not be administered at the time surrounding a surgical procedure. Heparin-associated thrombocytopenia, a presumed immunologically mediated decrease in circulating platelet numbers, has been reported in human patients receiving PGAGs.³⁰⁸

At the time of this publication, several injectable PGAG products are being marketed as “generic” Adequan. These products (e.g., Chondroprotec for horses) are not “generic” in that they have undergone no FDA approval process. That such products contain the same ingredients and will provide the same level of response as approved products cannot be assumed without inbiased scientific evaluation. Although a peer review paper disputing the efficacy of such products could not be identified at the time of this publication, a review of the internet reveals testimonial based evidence of the lack of efficacy of these products and the lack of understanding regarding their approval status. Although veterinarians might be legally empowered to prescribe or recommend such products, liability and standard of care concerns should lead to caution regarding their use. Differences in chemistry may be very subtle but may impact efficacy both safety and efficacy. Among the biggest concerns might be the impact of adulteration, as has occurred for heparin, also a PGAG (see Chapter 17). That these products can become unsafe if composed of improper PGAGs has been recently demonstrated.

Clinical use

Adequan has been approved for use in dogs for the treatment of osteoarthritis. The drug might, however, be considered in any situation in which the joint has been or will be injured. This includes trauma, elective surgical procedures, and arthritis associated with immune-mediated or infectious conditions. Additionally, PGAGs should be considered in conjunction with NSAIDs for their chondroprotective effects, as well as with the intent to potentially discontinue the NSAID. Disease-modifying agents including PGAG or its precursors (see later discussion of nutraceuticals) might also be considered as preventive therapy in animals that are likely to develop osteoarthritis for whatever reason, including conformation problems. Use of these drugs before clinical signs of osteoarthritis develop may prolong the time until NSAIDs are necessary. In patients with osteoarthritis, the time to clinical response is likely to be directly related to the severity of disease. Treatment that is begun before the joint is markedly damaged is more likely to be successful. Adequan may negatively impact hemostasis. Dogs treated at 10 times the recommended dose develop hemotomas and prolonged protime (package insert). Surgical candidates probably should not be treated with Adequan on the day of or prior to surgery unless coagulation studies indicated no effect. Care should be taken to not use Adequan in patients with bleeding disorders (see package insert), which might include patients receiving aspirin. The impact on patients receiving COX-2 preferential drugs is not clear.

KEY POINT 29-23

Injectable polysulfated glycosaminoglycans should be anticipated to act more rapidly than orally administered products.

Pentosan Polysulfate

Pentosan polysulfate (PPS) is isolated from beechwood hemicellulose and synthetically modified by adding sulfates to its repeating units of xylan pyranoses. Thus, unlike Adequan, it is not derived from animal sources. It is available as an injectable product, and an oral product has been approved in the United States for people with interstitial cystitis, a syndrome that may reflect a quantitative and qualitative defect in bladder mucosal glycosaminoglycans.^309,³¹⁰ It is approved for use under the market name Cartrophen for treatment of osteoarthritis in dogs in Canada and Europe, where it has been marked for at least 15 years.

PPS has a number of pharmacologic effects. Response of interstitial cystitis (based on resolution of pain in humans) ranges from 6% to 20%. In Europe PPS is used to treat thrombosis and hyperlipidemia, and an application for its use in treatment of osteoarthritis is pending. For cartilage PPS may improve subchondral and synovial membrane blood flow. In addition, it modulates cytokine actions, stimulates hyaluronic acid synthesis, and maintains PGAG content in joints.^302,³¹¹ When PPS was administered intramuscularly (2 mg/kg once weekly) in a model of osteoarthritis in dogs, cartilage damage was significantly decreased. In a double-blind clinical trial in 40 dogs, after 3 mg/kg intramuscularly per week, lameness, body condition, pain on joint manipulation, and willingness to exercise were improved at 4 weeks.³¹² The work of Budsberg and coworkers³¹³ supports the efficacy of PPS in selected surgeries. The investigators prospectively studied the efficacy of PPS in dogs (n = 40; 10 per group) with spontaneous cranial cruciate rupture using a parallel, randomized, blinded, placebo-controlled design. Dogs were divided into four groups on the basis of radiographic score (low versus high) and a partial meniscectomy was performed. Within each group animals were randomly assigned to receive either placebo or 3 mg/kg PPS weekly for 4 weeks after surgery. Animals were evaluated for 48 weeks, with biomarkers of collagen cleavage, aggrecan activity, and osteocalcin activity measured in serum. Radiographs and gait analysis were included in the postoperative evaluation. A total of 10 response variables were compared among the groups. Although lameness scores did not differ, PPS-treated dogs generally improved faster than placebo-treated dogs, and in those dogs with partial meniscectomies, biochemical markers were reduced compared with placebo-treated dogs. An oral dose has not been established for dogs.

PPS appears to be safe, but like other PGAG-like compounds, it appears to prolong clotting times and may cause thrombocytopenia. Safety information from the manufacturer available online³¹⁴ indicated an adverse event reporting incident of 0.01% with no signalment predisoposition. Clinical signs considered to be “probably related” to the drug, based on their appearance within 10 to 15 minutes of administration, included vomiting (78% of those reports, or 0.0047% of animals dosed) and a change in demeanor (i.e., quiet, etc; 0.0062% of animals). Experimentally, at 3 mg/kg, PPS increased partial thromboplastin time (PTT) and thrombin time (TT) but not prothrombin time (PT) above baseline in dogs, with the peak effect at 2 hours and resolution by 8 hours. Although hemorrhage has been reported in clinical cases using PPS, few of those reports have been designated as probable cause. Local reactions at the injection site did not occur in experimental studies, even at 30 mg/kg. The posibility that PPS and similar products worsens gastrointestinal hemorrhage induced by NSAIDs has not been addressed; indeed, evidence suggests a protective nature is imparted by related products in the gastrointestinal tract. It should be noted that information provided by the manufacturer of Cartrophen indicates that generic products are not therapeutically equivalent because they differ both chemically and thus in their interation with target proteins. As such, differences in therapeutic response among these products in general might be anticipated.

The use of this product for syndromes other than osteoarthritis in humans (e.g., interstitial cystitis, thrombosis) potentially might lead to similar uses in animals. For example, a study of mouse mesangial cells found that PPS decreased proliferation and net extracellular matrix production, mechanisms that may explain its apparent ability to slow the progression of glomerular sclerosis.^315,³¹⁶ PPS inhibits calcium oxalate crystallization in vitro³¹⁷ and is being studied for possible use in vivo. The compound is being studied for its apparently clinically beneficial effects for the treatment of acquired immune deficiency syndrome–related Kaposi’s sarcoma in human patients.³¹⁸

Hyaluronic Acid

Hyaluronic acid is a linear polydisaccharide (glucuronic acid combined with glucosamine) that is an essential component of synovial fluid, where it is chemically linked to proteoglycans in articular cartilage. As such, it helps to form large, aggregating proteoglycans in articular cartilage (Figure 16-12).²⁹⁶ Its mode of action is not certain, but it is assumed to function as a lubricant by increasing viscosity of synovial fluid. It may also act as an antiinflammatory. Studies in horses support its efficacy in the treatment of osteoarthritis. After intraarticular injection, the drug persists in joints for several days. The drug also has been given intravenously; the half-life in horses is 96 hours, but no studies could be found regarding dogs. Hyaluronic acid exists in variable molecular weights. High-molecular-weight hyaluronic acid inhibits phagocytosis, lymphocyte migration, and synovial permeability and stimulates hyaluronic acid synthesis.³¹⁹ Prior treatment with glucocorticosteroids or bony changes limits response. Hyaluronic acid appears to be very safe; side effects tend to be associated with administration of the drug. The drug has been used with variable success after intraarticular injection in horses³²⁰ and dogs.³²¹

The role of viscosupplementation in the treatment of human osteoarthritis has been reviewed.³²² A meta-analysis of 22 studies of hyaluron use in humans found it to have a small effect compared with placebo, but the study also found publication bias.³²² Accordingly, as with glucocorticoids, intraarticular administration of hyaluron tends to be limited in humans to patients who have not responded to other therapies.

Oral Disease-Modifying Agents

Veterinary Nutraceuticals

The use of oral disease-modifying agents (slow-acting disease-modifying agents)^323,³²⁴ for the treatment of damaged joints remains controversial despite increasing evidence regarding their potential contributions. Currently, with the exception of oral PPS (for which an approved drug version exists), these products can be classified as veterinary nutraceuticals.³²³^-³²⁷ The North American Veterinary Nutraceutical Council (no longer active) defined a veterinary nutraceutical as “a [non-drug] substance that is produced in a purified or extracted form and administered orally to a patient to provide agents required for normal body structure and function and administered with the intent of improving the health and well-being of animals.”^325,³²⁶

Regulatory considerations

Neither human nor veterinary nutraceuticals, botanicals, herbs or other novel ingredients (including botanicals or biological) undergo any mandated federal approval process, and therefore neither quality, safety nor efficacy necessarily have been documented before they are marketed. Chapter 4 addressed some of the safety issues associated with these products, and particularly those of plant origin. Unfortunately, clients may not realize the lack of regulation of these products, and counseling by veterinarians regarding the implications may be important.

The issues relating to quality, safety, and efficacy of disease-modifying agents have been reviewed,³²⁷ but the complexity of their nature and the impact the lack of regulation has on products warrant a short review. The FDA historically has held dietary supplements to the same standards applied to foods, requiring evidence of safety as well as evidence that labeling was truthful and not misleading. In 1994 the dietary supplement industry was successful in lobbying Congress to pass the Dietary Supplement Health and Education Act of 1994 (DSHEA), which amends the Food, Drug and Cosmetic Act such that it addresses these products. The amendment, which legally defines a dietary supplement, effectively restricts the FDA’s ability to regulate dietary supplements. Various label claims that refer to effects on structure or function of the body are now allowed on these products. Premarket safety evaluation is no longer required; rather, the manufacturer is responsible for ensuring safety, However, the FDA is responsible for monitoring the safety, despite the lack of a mandated mechanism for adverse event reporting. Rather, all adverse event reporting is voluntary, which may preclude effective safety assessment of products by the FDA. Consequently, the burden of proof that a product is unsafe rests with the FDA. Since DSHEA passage, the FDA has been able to require that manufacturers of hebal products include proper herbal names, and when relevant, the generic drug name for the product (e.g., ma guang for caffeine), and to indicate the part of the plant (e.g, stem, leaf, flower), from which the ingredient is derived.

The Center for Veterinary Medicine (CVM) of the FDA has determined that DSHEA does not apply to animals or animal feeds, including veterinary nutraceuticals. Therefore the term dietary supplements does not apply to veterinary products and theoretically should not be used when referring to products marketed for veterinary use. The term animal dietary supplements will nonetheless be used in this document. The CVM believes that public health is better served if the special concessions given to human dietary health supplements by the DSHEA do not apply to those given to food-producing animals. This reflects their goal of ensuring that harmful residues from either the compound or its metabolite do not reach food intended for human consumption and their concern that extrapolation of information among species receiving novel ingredients is complex and difficult. Most notably, the CVM is concerned that current ‘‘production drugs’’ (i.e., those that increase the production of food) might be considered by manufacturers or users of these products to fall under the lax guidelines of DSHEA, thus increasing the risk of human exposure to unapproved products. Thus, unlike human products, the FDA-CVM has retained its ability to regulate animal dietary supplements, although the lack of resources necessary to adequately regulate their sale will limit regulation. However, in contrast to dietary supplements, states can and do restrict the sale of products for animals through an alternative mechanism. Federal and state feed officials have organized the American Association of Feed Control Officials (AAFCO),³²⁸ whose stated goal is to ‘‘provide a mechanism for developing and implementing uniform and equitable laws, regulation, standards, and enforcement policies for regulating the manufacture, distribution, and sale of animal feeds’’ such that the use of these products is ‘‘safe, effective and useful.’’ Although nonregulatory, the AAFCO nonetheless influences state regulation and thus the marketing of oral products administered to animals in many (although not all) states. For those states that follow the AAFCO’s guidelines, a manufacturer seeking the sale of an unapproved oral product in a state must provide information necessary for the product to be recognized as an ‘‘ingredient’’ that has been ‘‘defined’’ by the AAFCO. Sale of a product that is not a drug, food, or feed additive is more likely to be denied by a state feed official if the product is not listed as a defined ingredient in the AAFCO publication.

The dramatic increase in veterinary nutraceutical use in the last 15 years led the AAFCO to focus more aggressively on the regulation of these products. The National Animal Supplement Council (NASC),³²⁹ based in the United States, has taken an active approach in working with the AAFCO to implement voluntary actions among nutraceutical manufacturers that will cause the AAFCO to respond to their products positively, thus allowing their sale. Veterinarians should be reminded that because DSHEA specifically applies only to human beings and neither dietary supplements (humans) nor novel ingredients (other animals) are not approved drugs, the use or prescription of these products is not protected by the Animal Medical Drug Use Clarification Act of 1994, which otherwise legalizes veterinary extralabel drug use.

Quality assurance

Several sources of scientific information indicate the need to focus on those products whose quality assurance can be verified. Chondroitin sulfate products (CDS) offer an example of consistent mislabeling. A University of Maryland study, funded in part by Nutramax Laboratories, found deviations from label claims for CDS in 84% (9 of 11) of the products studied; the amount by which products were mislabeled ranged from 0% to 115%.^329a Further, the study found that products costing less than or equal to $1 per 1200 mg of CDS were seriously deficient (less than 10% of the label claim), which suggests that cheaper products should be avoided. Costliness did not guarantee accuracy, however. Several of the most expensive products also were found in this study to be mislabeled. In contrast to CDS, the glucosamine of only 1 of 14 products was mislabeled in this study. However, another study found glucosamine sulfate to vary 60% to 140% of the label claim.^329b ConsumerLab³³⁰ (www.Consumerlabs.com) is a for-profit laboratory (with income based largely on subscription to their site) that offers a seal of ‘‘validation’’ for dietary supplements that are appropriately labeled. The efforts of the laboratory are two fold in origin: dependent, based on requests from a sponsoring manufacturer or independent, that is, an unsponsored investigation. In either case, the products to be tested are obtained from commercial sites (grocery or health food stores) rather than from the manufacturer, thus avoiding manufacturer-induced bias through product selection. The samples are subjected to ingredient analysis by independent laboratories. The ‘‘pass’’ criteria vary for each ingredient but are based on comparing content as determined from ingredient analysis to the labeled ingredients. Criteria include labeling of ingredients by proper name (for herbs, this includes the part of the plant and herbal names); a misatch between the listed and measured ingredients; and lack of contaminants, including toxicants (heavy metals, cleaning agents, and [particularly for herbs] pesticides or insecticides), metabolites, and other degradative products of the active ingredient. Products that pass analysis are allowed to place the ConsumerLab seal on the product label. Members of the public can read selected information on the ConsumerLab website for a fee ($23 annual fee at the time of this printing) for ingredients that have been tested. Criteria for passing and failing and reasons for failure for specific ingredients can be viewed, as can a list of proprietary products that have passed. Animal products have just recently been included in the review. A review of the website reveals that an important proportion of both human and animal products fail quality assessment; in general, animal products have fared more poorly than products marketed to humans. Among the passing products are those manufactured by Nutramax Laboratories, although a number of other human products also have passed. The USP¹²⁷ has also recently implemented the Dietary Supplement Verification Program (DSVP), a voluntary standards assessment program for human dietary supplements; veterinary products have not yet been included in their review. Although its activities are nonregulatory, the USP criteria are recognized in the Food, Drug and Cosmetic Act and its amendments, including the DSHEA. The NASC Compliance Plus program offers a potential mechanism for ensuring the accuracy in labeling of veterinary products; however, it is not clear if their program is is based on independent product analysis or if their criteria include the the use of ingredients that meet USP standards.

KEY POINT 29-24

Because they undergo no premarket approval process, dietary supplements for animals or humans often lack in quality.

Assessing Quality

Veterinarians and consumers should fully evaluate a product under consideration.³²⁷ First, the user should establish whether the product has been manufactured according to good manufacturing practices. The label should include the exact amount of each active ingredient; labels that combine metric and apothecary systems may be intentionally misleading because conclusions regarding product content are more difficult to determine. Each product listed on the label should contain a specific dosage. The source of each compound should be noted on the label. Note that products based on whole body tissues (e.g., mussel-containing glycosaminoglycans) will lack milligram contents of specific compounds (e.g., chondroitin sulfate). When individual products are listed, the purity of the compounds is likely to vary among and within products. The stated purity may not be the actual purity, particularly if good manufacturing procedures are not followed. Anderson³²⁴ noted that 70% of products analyzed for glucosamine and chondroitin sulfate did not meet the labeled claims.

Safety and Efficacy Considerations

The lack of efficacy data should not lead to the assumption that the benefits of these products are negligible. Establishing efficacy for many novel ingredients, including disease-modifying agents targeting cartilage, may be very difficult because of the complex nature of their actions and the dependence of their actions on other endogenous molecules.³²⁶ For example, the disease-modifying agents accumulate in tissues and have a carryover effect that requires a minimum of 6 to 8 weeks of therapy, necessitating long studies.³³¹ A placebo effect of more than 40% mandates the need for controls. Studies that do not demonstrate a significant difference between treatment groups should not be considered evidence of “no effect” unless the power of the study to demonstrate an effect is sufficient. Establishing safety may be easier simply because many adverse events are dose and duration dependent. It should be noted, however, that with poorly manufactured products, harm may occur not only because of the compounds themselves but also because of possible contaminants. Additional harm may occur if the client neglects traditional therapies in the belief that the nutraceutical agent will be sufficiently effective.

Nutraceutical Disease-Modifying Agents

Nutraceutical products that contain various forms of glycosaminoglycans or their component parts (aggregates form proteoglycans, the major constituent of cartilage matrix), such as glucosamines or chondroitin sulfates, appear most promising for treatment of osteoarthritis according to studies supporting their efficacy³³²^-³⁴⁰ and safety.³⁴¹ Presumably, as precursor nutrients, chondroitin sulfates, glucosamines, and other ingredients that comprise these will be extracted from the serum by chondrocytes and used to synthesize proteoglycans. During periods in which cartilage degradation exceeds cartilage formation, the need for precursor molecules may exceed availability, inhibiting the repair process. The availability of orally administered compounds not only increases the efficiency of the ability of the chondrocytes to repair damaged cartilage, as is demonstrated by increased synthesis, but also leaves less opportunity for formation of inappropriate molecules. The role of glucosamine and chondroitin sulfate, the major component of oral disease-modifying agents, in veterinary medicine has recently been reviewed.^342,^342a

Glucosamine

Glucosamine is an amino sugar that is among the aminosugars necessary for synthesis of mucopolysaccharides such as chondroitin, heparin and hyaluronic acid. As such, it is important in chondrocyte synthesis of PGAG. A deficiency of the compound has been implicated as a cause of decreased PGAG synthesis in early osteoarthritis. Alternatively, glucosamine also may stimulate synovial production of hyaluronic acid.³³⁹

Glucosamine is not available in foods. It is derived from either bovine cartilage or chitin, the hard outer shells of shrimp, lobsters, and crabs. Several glucosamine salts are available, including sulfate, hydrochloride, N-acetyl, and hydroiodide; although it is glucosamine that is active, the oral absorption of the salts varies.

After intravenous injection of radiolabeled (at a carbon atom) glucosamine sulfate, about 10% appears in plasma, with the rest rapidly disappearing from plasma as it is incorporated into plasma globulins. After oral administration, at least 88% of the compound is absorbed, with oral bioavailability being only 44%, due to first-pass extraction. Radioactivity remains in the body, with a half-time of approximately 95 hours. Radioactivity rapidly appears in multiple tissues, including articular cartilage. After intravenous administration, 49% of the dose is excreted in the lungs as radiolabled CO₂, presumably after hepatic or other peripheral tissue metabolism. However, another 29% is excreted in the urine.³⁴⁴ A meta-analysis of studies on glucosamine in humans concluded that the compound is well absorbed after oral administration; first-pass metabolism due to incorporation into proteins reduced oral bioavailability to 26%.^340,³⁴³ Glucosamine sulfate kinetics have been studied in dogs as well and appear to resemble that in humans.^344,³⁴⁵ After oral administration, absorption was rapid and nearly complete in dogs (87%). Glucosamine hydrochloride also is well absorbed in dogs, with peak concentrations occurring in 1.5 to 2 hours; bioavailability after a single dose is 12% although bioavailability is likely to increase, as it does with chondroitin sulfates, after multiple dosing.³³¹ Glucosamine hydrochloride exhibits a dose-dependent effect on plasma glucosamine concentrations, although, unlike chondroitin sulfates, glucosamine does not appear to accumulate in dogs with multiple dosing.³³¹

KEY POINT 29-25

If only a single disease-modifying agent is chosen, glucosamine is a reasonable first choice because it is the rate-limiting step in polysulfated glycosaminoglycans synthesis; its oral absorption and safety have been demonstrated; and among the supplements, it tends to be one that is labeled appropriately.

The safety and efficacy of glucosamine have been well reviewed in human medicine.^287,^342a, ³⁴³ Glucosamine appears to be safe; an LD₅₀ value cannot be established in mice or rats, even at doses of 5000 mg/kg orally or 3000 mg/kg intramuscularly, or 1500 mg/kg IV. Safety has been established both in the dog and cat after medium to high doses for 30 days.^341,³⁴⁶ Oral doses ranging from 160 to 2000 mg/kg for up to 180 days were associated with no adverse effects in dogs.³⁴³ Safety in animals with comorbidity may be a concern. The impact of glucosamine on glucose metabolism, particularly through the hexosamine pathway,³⁴³ has led to concern regarding diabetic control. However, glucosamine metabolism follows a different path than radiolabeled glucose,³⁴⁴ and glucosamine does not appear to interact with glucose disposition.³⁴⁷ Diabetes could not be induced in rats genetically predisposed to sugar-induced diabetes mellitus after treatment with glucosamine, chondroitin sulfate, or the combination of the two at 3 to 7 times the recommended dose.³⁴⁸ In a separate series of studies in humans, glucose concentrations did not change and histologic lesions could not be identified as a result of glucosamine administration.³⁴³ Using a placebo-controlled, double-blinded, randomized clinical trial, human patients requiring medical management of type 2 diabetes mellitus and receiving Cosequin (1200 mg CDS, 1500 mg glucosamine) for 90 days were monitored in an outpatient clinic. Diabetic control did not change in either group during the study period, and concentrations of glycosylated hemoglobin did not differ between treatment or placebo groups.³⁴⁹ With rare to no exceptions, reviews of clinical trials in humans reveal that the use of glucosamine is not associated with adverse effects in humans.³⁴³ Indeed, some trials find fewer adverse effects associated with glucosamine than with placebo.³⁴³

The use of glucosamine for treatment of osteoarthritis remains controversial despite in vitro and in vivo studies supporting its efficacy in improving lameness scores and mobility in human and animal models of degenerative joint disease. Some of the controversy probably reflects poor design of selected clinical trials.³⁴⁰ However, several meta-anlayses and reviews of clinical trials generally find glucosamine (hydrochloride or sulfate) moderately effective for treatment of osteoarthritis in humans.^342a,³⁴³ A prospective study found glucosamine to be highly cost effective in the treatment of human osteoarthritis.^343a

All glucosamine salts appear to be equally effective,³³⁶ although one in vitro study suggests that the N-acetyl salt may be less efficacious, perhaps owing to less absorption. This is in contrast to galactosamine salts (also found in PGAGs) and glucuronide salts, which do not appear to be effective in damaged joints.³³⁶ Dosing differences should be expected among the glucosamine products. The differences reflect, in part, the different salts, with the molecular weight of the salts being hydrochloride (36.5), N-acetyl (58), and sulfate (96). This compares to a molecular weight of 192 for glucosamine. Thus approximately 16% of the molecular weight of glucosamine hydrochloride is represented by the (hydrochloride) salt, compared with 24% of the N-acetyl salt and 33% of the sulfate salt. Consequently, on the basis of weight alone (of the total salt), the dose of glucosamine sulfate should be 15% higher. An additional adjustment must be made for differences in bioavailability. Some glucosamine salts, including sulfate, are accompanied by sodium or potassium, whose molecular weight also may be included with the active moiety. Sulfate has been proposed as the active moiety responsible for efficacy of glucosamine sulfate insofar as it is required for glycosaminoglycan synthesis. Because sulfate, but not glucosamine, increases in plasma and sulfate concentrations appear in the joint, some investigators believe that it is the sulfate moiety of glucosamine sulfate that is responsible for its effects.³⁵⁰ Therefore the sulfate salt might be the preferred form, but clinical trials comparing the salts are warranted. An injectable glucosamine preparation is available, and the acetyl salt is available as an enema preparation.

A number of studies support the use of glucosamine for treatment of osteoarthritis. In 2001 Reginster and coworkers³⁵¹ reported on the success of glucosamine sulfate in the treatment of knee osteoarthritis in humans in a non–manufacturer-sponsored study. The placebo-controlled, blinded study encompassed 3 years, a sample size that exceeded 200, and included outcome measures that were less subjective to bias (radiographic measurements) compared with clinical assessment. The investigators have since published additional studies³⁵² in different sample populations, with similar success. Glucosamine has been compared to NSAIDs as well. A meta-analysis³⁵³ comparing ibuprofen and glucosamine for treatment of osteoarthritis pain found glucosamine to be a reasonable alternative to or adjuvant with ibuprofen or other NSAIDs.

Chondroitin Sulfate

Chondroitin sulfates are glycosaminoglycans (repeating units of galactosamine sulfate and glucuronic acid) that can be found in many tissues (see Figure 29-13). In cartilage matrix they bind to and support collagen. Differences in molecular weight result in variable oral bioavailability with lower weight molecules being more bioavailable. Chondroitin 4-sulfate is mammalian in origin, and it is the most abundant chondroitin in growing mammalian cartilage. Chondroitin sulfate generally is derived from bovine trachea. Processing is costly and variable, with differences in fractionation, particle size, molecular weight, location, purity (presence of other PGAGs, such as keratan or dermatan sulfate), and degree of sulfation evident among products. As such, products containing CDS are more likely to be mislabeled with regard to CDS content compared with glucosamine, which is less expensive to manufacture.

With age, chondrocyte secretion of chondroitin 4-sulfate may decline, contributing to the initiation of degenerative joint disease. Chondroitin 6-sulfate is derived from shark cartilage and conceptually may be less ideal than chondroitin 4-sulfate. Chondroitin sulfates appear not only to increase synthesis of PGAGs but also to competitively inhibit the actions of metalloproteases in cartilage matrix. They have a variety of other in vitro and in vivo effects on cartilage. In humans (dosed at 1 to 1.5 g/day) they decrease the need for NSAIDs.

Despite its large molecular size, 70% of CDS is absorbed in various sizes ranging from intact chains to monomer subunits after oral administration. In humans more than 70% of radioactivity was absorbed and distributed to urine and tissues after oral administration of radiolabled low molecular weight (14,000) CDS.^347,³⁵⁴ The presence of intestinal chondroitinases in carnivorous and omnivorous animals has been postulated as the reason for absorption.³³⁶ In contrast to glucosamine, chondrotin sulfate accumulates after multiple dosing (for 7 days); bioavailability of 200% was reported, indicating that a carryover or residual effect might be expected after dosing is discontinued.³³¹

In dogs oral chondroitin increases serum glycosaminoglycans. Because of its ubiquitous location in the body, indications other than joint disease should be considered for chondroitin sulfates, including cardiovascular diseases associated with thrombogenesis and indications previously noted for PPS. Attention should be paid to the source of chondroitin sulfates in nutraceuticals. Syndromes such as interveterbral disc disease (discs are comparised of CDS), tracheal collapse (a hylaline cartilage related problem), and chronic urinary (cystitis) infection may be rationale indications. Purified preparations are expensive, but the amount of chondroitin in whole animal tissues (mussel, shark cartilage, sea cucumber, or sea algae) cannot be determined from the label. Bioavailability of the chondroitin sulfates in such products is not known.

Efficacy of Glucosamine–Chondrotin Sulfate Combination

Glucosamine by itself might be expected to be the most effective of the oral nutraceuticals for the following reasons: (1) It is the rate-limiting step in PGAG synthesis; (2) it is generally labeled more accurately than chondroitin sulfate; and (3) clinical trials have demonstrated a clinical effect using glucosamine alone. However, the combination product might offer some advantages. A number of studies support the efficacy of the combination of chondroitin sulfate and glucosamine.^355,³⁵⁶ Many are sponsored by the manufactuer, most commonly Nutramax Laboratories. This should not be surprising insofar as this company holds the patent for their glucosamine–chondrotin sulfate product (Cosamin for humans and Cosequin for animals) and strives to ensure product quality and safety. In a single-center study sponsored by the manufacturer, humans with mild to moderate radiographic damage in the knee improved more when receiving Cosamine compared with placebo after 6 months of therapy.³⁵⁷ Some evidence supports the possible efficacy of combination products in lower-back degenerative diseases. Leffler and coworkers³⁵⁸ reported improvement in Navy personnel (n = 34) afflicted with knee or lower-back cartilage degeneration osteoarthritis after 16 weeks of Cosamin therapy compared with control. A European meta-analysis of studies found a large treatment effect in persons receiving chondroitin sulfate (of similar molecular weight to the Nutramax products Cosamin or Cosequin) and a moderate treatment effect for glucoasmine.³⁵⁹ Fajardo and Di Cesare³⁶⁰ reviewed meta-analyses examining the effect of glucosamine and chondroitin on treatment of human osteoarthritis. In general, the studies revealed that glucosamine provides significant effects on most outcome measures, including structural efficacy (based on radiography of joint spaces), whereas effects of chondroitin tend to be significant for symptomatic outcome measures.

Clegg and coworkers³⁶¹ reported the results of a large multicenter double-blinded clinical trial that compared placebo (negative control), celecoxib (positive control), glucosamine (1500 mg), chondroitin sulfate (1200 mg), and the combination of glucosamine and chondroitin sulfate in humans (n = 1583), with osteoarthritis graded as mild (n = 1229) and moderate and severe (n = 354). The primary outcome measure was a 20% decrease in knee pain by week 24 of treatment. The only treatment that significantly reduced pain compared with placebo was celecoxib; however, the placebo effect was 60%, underscoring the importance of negative control groups. More important, the group receiving the combination therapy tended to respond better than the placebo group (p = 0.09). Further, within the subgroup of patients with moderate to severe pain, the combination product, but no other treatment, was significantly more effective than placebo.

Efficacy also has been demonstrated in dogs using a variety of models, including in vitro studies and experimental and spontaneous disease. Most of these studies also have been manufacturer sponsored. With use of in vitro methods, biosynthetic activity was greater in canine cartilage cores incubated in serum collected from dogs receiving Cosequin for 1 month compared to incubation in serum collected from the same dogs before Cosequin administration.³⁶² Under experimental conditions, in surgically induced instability in rabbits, the combination product was found to be superior to placebo, glucosamine hydrochloride, chondroitin sulfate, or manganese alone (Figure 29-14). The results of this study led investigators to suggest that the combination product Cosequin was more effective (and perhaps acted synergistically) than individual ingredients. However, the dose that animals received was higher than that recommended. Using an experimental model in dogs, Canapp and coworkers³⁶³ demonstrated a protective effect of Cosequin in dogs when administered 21 days before induction of chemical synovitis. These studies do not reflect a comprehensive review of the literature but nonetheless provide evidence of efficacy.

Figure 29-14 Histologic evidence of the clinical efficacy of disease-modifying agents. A surgically induced instability model of osteoarthritis was induced in rabbits. Treated rabbits received 0.38 g glucosamine hydrogen chloride and 0.304 g sodium chondroitin sulfate (Cosequin, Nutramax Laboratories, Inc., Edgewood, Md.) per kilogram body weight per day. Histologic samples were collected at 16 weeks from the center of the medial condyle. Differences in lesions between the groups were significant (p <0.02). (Photographs courtesy Nutramax Laboratories.)

Other Oral Supplements

Nutritional products for treatment of osteoarthritis were scientifically reviewed in human medicine.³⁶⁴ These studies were randomized (human or animal) clinical trials that focused on osteoarthritis, published in peer-reviewed journals, and based on nonsynthesized (i.e., natural) orally administered products. Because it is considered synthetic, SAMe was excluded (it is not stable in its endogenous form), whereas chondroitin sulfates and glucosamine were excluded because they were already well reviewed. Surprisingly, only of 52 of 2026 potential studies met all criteria. Each was then scored on a scale of −2 (evidence of no efficacy) to 2 (very good evidence of efficacy). Funding sponsorship for the trial as a possible source of bias was not addressed. No positive effects were found in 11 of 52 trials. An example of some of the major nutrients that emerged from the review and their respective scores are as follows: avocado soybean unsaponifiables ([ASU] score 1.58); methylsulfonylmethane (MSM) (1.21), vitamins B₃ or C (0.75); lipids from green-lipped mussels (0.58); boron (a nonmetallic trivalent chemical whose concentration is less in femoral osteoarthritis than in normal osteoarthritis; score of 0.50); cetyl myristoleate (a lipid of sperm whale and beaver gland origin; score of 0.58); ginger [0.42], vitamin E (0.17); hyperimmune milk (−0.9); and collagen hydrosylate (−0.17).

Avocado Soybean Unsaponifiable Lipids

The term ASU lipids refers to the residue left behind after the lipids associated with avocados and soybeans are saponified (ie “soapafied”). Saponification occurs through the addition of an alkali. For ASU the primary ingrediants associated with the beneficial effects are phytosterols. They appear to inhibit cholesterol absorption and endogenous cholesterol biosynthesis.³⁶⁵ Among those present in ASU are beta-sitoseterol, which is particularly potent as an antiinflammatory. Among the anabolic, anticatabolic, and antiinflammatory effects cited for ASU are increased collagen and aggrecan synthesis, decreased collagenase, aggrecanase and matrix metalloproteinase activity, and decreased production of IL-6 and IL-8 and PGE₂ activity. ASU increases transforming growth factor in normal canine synovial fluid. The most common avocado to soybean ratio studied was 1:2; it was the most effective, followed by products with a ratio of 1:1; products with a ratio of 2:1 were less effective. In general, ASU appears to improve symptoms of osteoarthritis after several months and may slow down joint space loss. However, further studies are needed to confirm structure-modifying effects and long-term effects.

KEY POINT 29-26

A variety of nutraceutical ingredients target inflammation. The ideal supplement or supplement combination, however, should always include orally bioavailable glucosamine and chondroitin sulfates.

S-Adenosylmethionine

S-Adenosylmethionine (also addressed in Chapter 19) also is a nutraceutical product, but its mechanism for treatment of osteoarthritis is less clear and probably reflects antiinflammatory effects. It is synthesized in the body from methionine and is responsible for a number of biological reactions, serving as a methyl donor. In the joint it may act to transulfate glycosaminoglycans. Its precursor (methionine) cannot be administered during states of deficiency without toxicity. The product must be prepared as a salt because it is unstable; it is extremely hygroscopic, and the tablet cannot be broken without loss of efficacy. In human clinical trials (controlled and uncontrolled), SAMe has improved lameness scores and mobility. In vitro studies suggest that SAMe increases proteoglycan synthesis and protects the cartilage.³²³ This is being studied in dogs. The human dose is 600 mg daily for the first 2 weeks, followed by 400 mg daily. Clinical response may not be evident for 1 or 2 months.

The efficacy of SAMe for treatment of osteoarthritis in humans compared with placebo or an NSAID was studied through meta-analysis.³⁶⁶ Eleven studies (controlled clinical trials) met the critiera of the study. The authors concluded that SAMe was as effective as NSAIDS in reducing pain and improving function, without the adverse effects traditionally associated with NSAIDs. Limitations of the studies included higher than recommended doses for some studies; a short intervention for most studies (28 to 30 days), which may have underestimated NSAID efficacy; and, in general, study of only osteoarthritis of the knee or hip, which limited extrapolation of data to osteoarthritis of other joints. The mechanisms and other aspects of SAMe are addressed in greater depth in Chapter 19.

Phycocyanin

Phycocyanin (PC) is a phycobiliprotein chromoprotein that is a major pigment of the microalgae Spirulina. Among its function in algae is to harvest light, which also serves to protect the organism from undue damage associated with oxygen radical formation produced by exposure to ultraviolet light. Like many other light-harvesting pigments, it is an effective antioxidant compound. Structurally, it is similar to bilirubin, which also has antioxidant activities.³⁶⁷ Romay and coworkers³⁶⁷ reviewed the evidence supporting PC as a selective COX-2 inhibitor. First, antioxidant properties in general have been demonstrated in vitro, including scavenging of hydroxyl, alkoyl, peroxyl, and peroxynitrite radicals. Lipid peroxidation has been demonstrated, also in vitro. Neutrophil activation is decreased. Antiinflammatory effects have been demonstrated using inflammatory rodent models in a dose-dependent model. Because the models are also used to demonstrate inhibition of COX or lipoxygenase activities, the antiinflammatory effects of PC were considered to potentially reflect inhibition of AA metabolism, as well as prevention of lipid peroxidation. Separating out the direct inhibitionof COX or lipoxygenase from inhibition of fatty acid (i.e., AA) release from cell membranes is difficult. Studies in human whole blood have supported COX-2 inhibition, although selectivity was not demonstrated. However, a subsequent in vitro study using cloned human COX-1 and COX-2 does demonstrate selectivity.³⁶⁷ The appropriateness of using COX-1 to COX-2 ratios as indicators of efficacy or safety has already been discussed. By virtue of its antioxidant effects, largely through animal models, PC has demonstrated potential efficacy as a hepatoprotectant and a treatment for arthritis and inflammatory bowel disease. Because efficacy has been demonstrated after oral absorption, it is possible that it is not active as the intact compound. Although its combined use as an antiinflammatory with disease-modifying agents containing glucosamine and chondroitin sulfates is reasonable, issues associated with dietary supplments in general should be addressed, including species differences, safety, and quality assurance of the product. This is particularly true if PC is an inhibitor of COX; its safety should be confirmed in target species, particularly when combined with NSAIDs.

Use of Modulators of Inflammation in the Treatment of Shock and Central Nervous System Trauma

A number of drugs that modulate the inflammatory response have been studied for their effect in the patient suffering from shock, particularly that associated with the release of bacterial toxins such as endotoxin (septic shock). Because of the oxygen radical scavenging ability of some of these drugs, studies have also focused on their use in treatment of damage to the CNS.

Pathophysiology of Septic Shock

The pathophysiology of septic shock is addressed in Chapter 8. This discussion focuses on the inflammatory mediators associated with the syndrome. Increasingly, sepsis is recognized to reflect an exaggerated systemic inflammatory response to infectious organisms. Mediators of the response include lipopolysaccharide of gram-negative bacteria, lipoteichoic acid of gram-positive bacteria, and peptidoglycan from both. Cytokines released by host cells (macrophages and circulating monocytes) play a pivotal role in the pathyophysiology.

Gram-positive bacteria appear to secrete superantigens, which bind to both major histocompatibility complex molecules and T-cell receptors, initiating massive cytokine production. Lipoteichoic acid and endotoxin both stimulate cytokine production. The lipid A component of lipopolysaccharide, endotoxin, found on the surface of gram-negative organisms, is a highly conserved molecule responsible for the sequelae of endotoxic shock. The manifestation begins as endotoxin released from dying gram-negative organisms interacts with receptors on cells of the host defense system: macrophages, neutrophils, platelets, and lymphocytes. Endotoxin also directly interacts with vascular endothelial cells. In response, cells either release the mediators of endotoxic shock or render other cells more reactive to cellular signals and subsequent mediator release. Mediators of endotoxic shock are grouped as cytokines, lipid mediators, or secondary mediators.^377,³⁷⁸ Cytokines are small polypeptides released from inflammatory cells, especially macrophages. TNF and IL-1 are the two cytokines that appear to be primarily responsible for the cascade of endotoxemia. Their effects in turn are often mediated by nitric oxide. Lipid mediators are derived from AA, located in the phospholipids of cell membranes, particularly those of neutrophils, platelets, vascular endothelium, and vascular smooth muscle. Examples include PGs (including thromboxane), LTs, and platelet-activating factor.

Both cytokines and the lipid mediators act as signaling mechanisms among inflammatory cells, platelets, and the vascular endothelium through negative and positive feedback mechanisms. When the positive feedback loops overwhelm the negative feedback loops, the pathophysiology of endotoxic shock becomes a clinical reality. The pathophysiology reflects, in part, the direct effects of endotoxin (e.g., it directs activation of Hageman factor and complement components) and the combined or individual effects of the mediators. Secondary mediators (e.g., histamine, serotonin, vasopressin, angiotensin II, catecholamines, and opioids) are released in response to cytokines and lipid mediators, resulting in the general signs of endotoxic shock. Adhesion molecules, selectins, and leukocyte integrins are among the humoral mediators associated with the pathophysiology. Disruption in hemostasis balance also is affected by inflammatory cytokines. Interleukins 1α, 1β, and TNF-α activate tissue factor and subsequent coagulation; endotoxin also increased the activity of fibrinolytic inhibitors, contributing to an imbalance in the coagulation cascade. Changes in peripheral vasculature (i.e., constriction and dilation) coupled with activation of clotting factors and inhibition of fibrinolysis, results in widespread coagulopathy, microbembolization, and vascular endothelial damage.The clinical signs associated with each stage of shock depend on the mediators released during that stage and vary among species. The complex interactions of these mediators, however, if allowed to progress, can result in multiple organ failure in any species simply because of the cumulative effects of hypoxia: oxygen radical, lysosomal enzymes, thrombosis, and metabolic derangements.

Drug therapy is most likely to be successful when initiated early during the course of endotoxic shock. The role of steroidal compounds in the treatment of shock in animals was reviewed by Howe.³⁷⁹ A number of investigators have recently reviewed the role of mediator-specific antiinflammatory agents in the prevention and treatment of septic shock.^380,³⁸¹ Examples of these drugs include antibiodies to TNF, soluble TNF receptors, agonists of IL-1or platelet activating receptors, and drugs that target bradykinin or PGs. Thus far, these drugs appear to have demonstrated efficacy in clinical trials. Meta-analyses indicate that their use should be associated with clinical benefits in the individual patient.³⁸¹ However, because the targets of these drugs play complex roles in the pathophysiology of septic shock, their general use should be discouraged; rather their use should be matched to patient conditions that are more likely to benefit from treatment.

Role of Lipid Peroxidation in Tissue Injury

Lipid peroxide formation is the result of free radical–mediated cell and tissue injury caused by lipid peroxides within cell membranes and organelles. Both structure and function of the membranes and organelles are disrupted by lipid peroxide formation. Lipid peroxidation is a potentially geometrically progressing reaction that spreads over the surface of cell membranes, impairing phospholipid-dependent enzymes; ionic gradients across the cell membrane,;and, if sufficiently severe, membrane lysis (Figure 29-15). Its importance, along with the generation of oxygen radicals, is evident early in the pathophysiology of CNS trauma. Lipid peroxidation is only one of several sources of oxygen radial formation. Other sources include the AA cascade; catecholamine oxidation; mitochondrial “leak”; oxidation of extravasated hemoglobin; and, as the inflammatoryprocess proceeds, infiltrating neutrophils.

Figure 29-15 Lipid peroxidation of cell membranes reflects a cascade of events that begins with neurologic damage. Oxygen radical formation is potentiated in the presence of iron and is inhibited by endogenous protectants such as superoxide dismutase (SOD) (which converts the superoxide radical to hydrogen peroxide) and catalase. Catecholamine release can contribute to damage. The ability of the radicals to move down the cell membrane is facilitated by the fluidity of the membrane. Calcium influx accompanying cell membrane damage contributes to the formation of arachidonic acid metabolites and subsequent inflammation. AA, Arachidonic acid; CNS, central nervous system; PAF, platelet-activating factor.

Lipid peroxidation is initiated by a reactive oxygen molecule. After CNS trauma, the generation of oxygen radicals during the normal reduction of oxygen overwhelms normal control mechanisms. Xanthine/xanthine oxidase, PG synthetase, and other mechanisms result in superoxide anion formation. Although superoxide anion is not in itself very reactive, it becomes more so by accepting a proton, thereby becoming more able to penetrate cell membranes. Other sources of superoxide anion include catecholamines; ascorbic acid and glutathione act to inhibit superoxide anion. The superoxide anion can also become more dangerous by conversion by way of superoxide dismutase to hydrogen peroxide. Inflammatory cells are an important source of hydrogen peroxide, as is degradation of monoamines mediated by monoamine oxidase. Mitochondria contain high concentrations of superoxide dismutase. Although hydrogen peroxide is not very damaging to intact tissues, it can easily penetrate cell membranes unless destroyed first by catalase. In the cell membrane, hydrogen peroxide interacts with iron to yield the highly reactive hydroxyl radical.

Normally, the conversion of oxygen to water is well controlled by the presence of superoxide dismutase, catalase, and endogenous antioxidants, with vitamin E being one of the most important membrane-bound antioxidants. After CNS trauma, normal control mechanisms are lost, and lipid peroxidase formation begins. Iron plays a crucial role in this process. Free iron released from hemoglobin, transferrin, or ferritin in the presence of a lowered tissue pH or oxygen radicals catalyzes radical-initiated peroxidation (see Figure 29-15). Iron–oxygen complexes probably initiate lipid peroxidase formation; damage during ischemic injury from radicals will be affected by the amount and location of iron (and copper) ions. Unfortunately, these ions become more available during injury. Acidosis, which often accompanies ischemia (anaerobic environment and lactic acidosis), increases the solubility of iron. Free calcium released during the injury stimulates phosopholipase A₂ and the AA cascade. Metabolites of AA are important sources of reactive oxygen species. Inflammatory cells become an important source of continued AA metabolism. Decreased concentrations of vitamin E, ascorbic acid, and glutathione (induced by scavenging of oxygen radicals) predicate the occurrence of lipid peroxidation.

Drugs That Impair Mediators of Sepsis

Nonsteroidal Antiinflammatory Drugs

A number of NSAIDs have been studied for their ability to block response to mediators of endotoxic shock. Indomethacin and ibuprofen have shown efficacy in human patients.^380,³⁸² Flunixin meglumine has been studied in horses and dogs.^141,³⁸³ As with glucocorticoids, however, the effects of NSAIDs must be realized within the first 2 hours of the onset of endotoxic shock—that is, before mediators have been able to stimulate response. The use of NSAIDs may shunt AA substrate to the lipoxygenase pathway, which may cause detrimental effects. Thus drugs that impair both arms of the AA cascade may prove more useful. The efficacy of ketoprofen, an NSAID that appears to inhibit both PGs and LTs, has been shown to ameliorate many of the effects of endotoxin infusion.³⁸⁴ The combined use of NSAIDs with LT antagonists apparently has not been reported in endotoxic shock, but the advent of dual inhibitors warrants further studies. Prolonged therapy with NSAIDs is not advisable because of toxic effects. Although gastrointestinal toxicity is the major concern in most animals, the patient suffering from endotoxic shock may be more predisposed.

Glucocorticoids

Glucocorticoids are discussed in Chapter 17. Glucocorticoids inhibit the enzyme phospholipase A₂ and the release of TNF and IL-2 from activated macrophages.³⁸⁵ Glucocorticoids also alter synthesis of and biologic response to collagenase, lipase, and plasminogen activator. The immunosuppressive actions of glucocorticoids are more pronounced on the cellular arm than the humoral arm of the immune system. Glucocorticoids have minimal effects on plasma immunoglobulin concentrations but can modulate immunoglobulin function. Immunosuppressive actions of glucocorticoids, like their antiinflammatory actions, involve disruption of intercellular communication of leukocytes through interference with lymphokine production and biological action; however, these effects are largely transrepressive (see Chapter 17). Glucocorticoids block the effects of macrophage-inhibiting factor and interferon-γ (IFN-γ) on macrophages. IFN-γ, which is released from activated T cells, plays an important role in facilitating antigen processing by macrophages. Glucocorticoids inhibit the synthesis and release of IL-1 by macrophages, thereby suppressing the activation of T cells. Glucocorticoids also inhibit IL-2 synthesis by activated T cells. Interleukin-2 plays a critical role in amplification of cell-mediated immunity. Additionally, glucocorticoids suppress the bactericidal and fungicidal actions of macrophages.

Septic shock

Earlier experimental models of septic shock in animals indicated that glucocorticoids can be of benefit but only if administered before or concurrently with endotoxin administration—that is, within the first 2 hours. In canine models severe mesenteric vasoconstriction within the first 15 minutes can lead to irreversible shock. Thus glucocorticoids provided no beneficial effects when administered 30 to 60 minutes after administration of the endotoxin. Rapid-acting, water-soluble agents such as dexamethasone sodium phosphate (4 to 8 mg/kg intravenously), prednisolone sodium succinate or sodium phosphate (30 mg/kg intravenously), or methylprednisolone sodium succinate (30 mg/kg intravenously) have been recommended, at shock doses, which are 5 to 10 times the immunosuppressive dose. However, more recent data suggest that the use of glucocorticoids in patients with sepsis is controversial. Multiple meta-analyses of randomized human studies have demonstrated that high doses of glucocorticoids (e.g., methylprednisolone at 30 mg/kg) invariably do not prevent septic shock, reverse the shock state, or improve the 14-day mortality rate, despite theoretical and experimental animal evidence to the contrary. Indeed, mortality rates were greater in one study, presumably because of immunosuppression and secondary infection.³⁸¹ In contrast, stress-dose (or physiologic dose) glucocorticoids (e.g., hydrocortisone at 200 mg/day; approximately 3 mg/kg) intended to replace deficient corticosteroids (due to adrenal suppression) in patients with severe and refractory shock may be of benefit (see Chapter 17). Patients with persistent vasopressor-dependent shock should be targeted,³⁸¹ although controversies continue regarding this indication as well.³⁸⁰ In human patients use of glucocorticoids is controversial, with no improvement in survival in some studies.³⁸⁶

Hemorrhagic shock

The use of glucocorticoids for treatment of hemorrhagic shock is controversial. Some studies in dogs suggest that dexamethasone sodium phosphate (5 mg/kg intravenously) may improve blood flow to the kidneys, lungs, and gastrointestinal tract. Other supportive measures, particularly aggressive fluid therapy, must also be instituted. Appropriate fluid replacement therapy will ensure adequate drug distribution to target tissues.

Oxygen radical scavengers

A neuroprotective role has been recognized for certain glucocorticoids and, most notably, methylprednisolone. Interest stemmed from the observation that the ability to inhibit CNS lipid peroxidation and influence other pathophysiologic processes strongly correlated with neurologic recovery. The neuroprotective effects have been separated from the glucocorticoid activity by the discovery of nonglucocorticoid steroids that are able to equal or surpass the antioxidant effects of methylprednisolone (see later discussion of lazaroids).

In a feline model of spinal injury, methylprednisolone (30 mg/kg) attenuates posttraumatic lipid peroxidation. In addition, perhaps because of the inhibitory effect on lipid peroxidation, methylprednisolone supports energy metabolism, reduces or prevents posttraumatic ischemia and neurofilament degradation, reduces intracellular calcium accumulation (resulting in the AA cascade), and inhibits vasoactive PGs (PGF_2α and thromboxane). In addition, like other steroids, methylprednisolone may increase spinal neuronal excitability, which may also be important to neurophysiologic recovery. Several pertinent points must be appreciated regarding these effects of methylprednisolone on spinal cord injury. First, these effects occur only at a high concentration (i.e., that achieved with an intravenous dose of 30 mg/kg). Second, the effects are biphasic, with loss at 60 mg/kg. As with many protective mechanisms, the drug must be administered early in the pathophysiologic process because spinal uptake of methylprednisolone decreases rapidly with time after injury. Loss of effect may reflect a decrease in blood flow to damaged tissues or the irreversible nature of lipid peroxidase. Finally, the time course of neuroprotection of methylprednisolone follows the disappearance of the drug from plasma or tissue—that is, it lasts only 2 to 6 hours (the half-life of the drug in feline spinal tissue). Thus the drug must be administered frequently to preserve tissues and maximize the potential for recovery. The role of glucocorticoids in treatment of intervertebral disc disease is addressed in Chapter 27).

Lazaroids

After the unique protective effect of methylprednisolone among the glucocorticoids in damaged nerve tissue was recognized, attempts were made to refine the structure of the steroidal molecule such that the neuroprotective (anti lipid peroxidase) effects would be maintained but the glucocorticoid effects minimized.³⁹³ The result of these efforts was the synthesis of the 21-aminosteroids or lazaroids. Tirilazad mesylate is the prototypic drug (Figure 29-16). Lazaroids have been specifically designed to localize in cell membranes and inhibit (iron-mediated) lipid peroxidase. Although initial investigations were oriented toward acute trauma, because inhibition of lipid peroxidase may decrease neuronal degeneration, these drugs may also be useful for chronic neurodegenerative processes.

Figure 29-16 Tirilazad mesylate is an example of a lazaroid (a 21-aminosteroid). Compared with methylprednisolone (left), a glucocortiocoid capable of scavenging oxygen radicals, the 21-aminosteroids are devoid of steroidal activity. Their ability to inhibit lipid peroxidation and provide a neuroprotective effect occur in part by insertion into the lipid bilayer of the cell membrane. Cell membrane fluidity is decreased, thus impairing the ability of oxygen radical formation to cascade across the lipid layer. Lipid peroxidation is thus minimized.

Tirilazad mesylate is a very lipophilic drug that preferentially distributes to the lipid components of cell membranes. Because neuronal tissue is composed of a greater proportion of lipid components, these drugs preferentially accumulate in neuronal tissue. Its pharmacologic actions are complex and include a radical scavenging–antioxidant effect and a physiochemical interaction with the cell membrane such that the fluidity of the membrane is decreased (see Figure 29-16). Although action against iron-mediated lipid peroxidase was sought for these drugs, they will in fact inhibit lipid peroxidase in iron-free systems as well. Tirilazad has proved to be an effective inhibitor of lipid peroxidase in all in vitro models studied. It also acts to reduce hydroxyl radicals by either direct scavenging abilities or decreased lipid peroxidase. Stabilization of cell membranes is considered an important part of its protective action. The nitrogen component of the steroid is thought to interact with the phosphate of the “head” groups (hydrophilic portion) of the bilipid layer by way of ionic interactions. The steroidal component localizes in the lipophilic portion of the membrane, compressing the phospholipid groups. Restriction of the movement of the cell membrane reduces the potential for lipid peroxidase by restricting the movement of lipid peroxyl and alkoxyl radicals in the membrane.

Tirilazad also has a high affinity for vascular endotheliulm. It appears to be able to protect vascular endothelium from damage by reactive oxygen species, possibly by preserving endothelium-derived relaxing function. It also appears to protect the blood–brain barrier against traumatically or chemically induced permeability. Tirilazad may protect other endothelial cells during trauma or hypoxic damage, such as the hepatic endothelium during hemorrhagic shock.

Animal models used to study the effects (not safety) of tirilazad have been primarily rats and monkeys. A few studies have focused on dogs or cats. Models of CNS damage have included that induced by cardiac arrest, altered cerebral blood flow, and subarachnoid hemorrhage. The clinical pharmacology of tirilazad has been studied in humans. It appears to be a flow-limited hepatically cleared drug. In humans there is a discrepancy in the elimination half-life of approximately 4 hours in one report and, after steady state, 35 hours. The difference appears to be a longer terminal phase after multiple dosing compared with single dosing; it is likely that volume of distribution cannot be accurately assessed after single dosing. The disposition of tirilazad is linear and does not appear to change with plasma drug concentration. Safety studies in humans have revealed the drug to be safe. Pain occurs at the site of injection, but this has been overcome by dilution of the drug, a change in the site of injection, and frequent catheter changes. Tirilazad does not appear to adversely affect the heart, blood pressure, or hepatic or renal function. Tirilazad has no glucocorticoid activity and will not alter parameters indicative of glucocorticoids (i.e., glucose, hematologic indices, adrenocorticotropic hormone, or cortisol). Apparent indications for tirilazad in humans include acute head or spinal injury, subarachnoid hemorrhage, and ischemic stroke.

The lazaroids have also demonstrated efficacy in traumatic shock,³⁸⁷ hemorrhagic shock,³⁸⁸ and endotoxemia.^389,³⁹⁰ Lazaroids decrease neutrophil accumulation, maintain arterial pressure, decrease myocardial injury, and increase survival. Lazaroids decrease formation of the eicosanoid mediators and production of TNF. When administered to dogs within 30 minutes of endotoxin infusion, lazaroids attenuated the effects of endotoxin.³⁹¹ Further studies are indicated before the use of lazaroids is confirmed. Currently, no lazaroid is approved for use in the United States.

Recombinant Human Activated Protein C

Activated protein C (APC) is an endogenous protein that simultaneously promotes fibrinolysis and inhibits thrombosis and inflammation, thus potentially modulating some of the negative sequelae of sepsis. In septic patients concentrations of protein C are decreased and conversion of protein C to activated protein C is inhibited. The role of protein C in septic shock led to the development and subsequent approval of a recombinant product, drotrecogin-activated, used in the treatment of human septic shock. In humans clinical trials largely support the safety and efficacy of the drotrecogin-alfa, activated, as indicated by decreased mortality rates.³⁸⁰ Its efficacy has been attributed to effects beyond those on the coagulation cascade, with antiapoptotic effects on endothelial cells suggested. The use of APC may be of most benefit in patients with a high risk of death but may be harmful in patients with a low risk of death.^381,³⁹² Consequently, the use of this product in small animals should be based on scientific studies that establish the optimal pharmacokinetic–pharmacodynamic relationship.

Inhibitors of Soluble Epoxide Hydrolase

The role of cytochrome P450 in the production of biologically active compounds, including the patient with septic shock, is largely overlooked. However, EETs or HETEs and EpOMEs are among the chemicals derived from the vascular endothelium that mediate vascular relaxation responses, as well as antiinflammatory effects in septic shock. Proposed mechanisms by which EETs modify the response of septic shock include inhibition of transcription factor nuclear factor-κB and IB kinase, thus preventing amplification resulting from macrophage production of proinflammatory proteins, such as TNF-α, IL-6, iNOS, and COX-2. Once formed, EETs and EpOMEs are further metabolized by soluble (in the cytoplasm) epoxide hydrolase (SEH). Persistence of these mediators by inhibition of SHE is being investigated as a therapeutic target in the patient in septic shock.²⁶¹ Drugs that target SHE not only facilitate EETs but also may facilitate the antiendotoxic effects of lipoxin.

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Treatment of Osteoarthritis

Physiology and Pathophysiology

Osteoarthritis Defined

Cartilage Physiology and Pathophysiology

Therapy with Drugs and Other Agents