Structure Chemistry Relationship

As derivatives of sulfanilamide, all sulfonamides have the same nucleus. Functional groups have been added to produce compounds with varying physical, chemical, pharmacologic, and antibacterial properties, but the active amine is in position 4 and any substitutions at this position must be freed in vivo (Figure 7-13). Although amphoteric, sulfonamides generally behave as weak organic acids and are much more soluble in an alkaline than in an acidic environment. Those of therapeutic interest have pK_a values between 4.8 and 8.6. Water-soluble sodium or disodium salts are used for parenteral administration. Such solutions are highly alkaline, somewhat unstable, and readily precipitate out with the addition of polyionic electrolytes. In a mixture of sulfonamides (e.g., the sulfapyrimidine group), each component drug exhibits its own solubility; therefore, a combination of sulfonamides is more water soluble than a single drug at the same total concentration. This is the basis of triple sulfonamide mixtures used clinically (primarily in large animals). The N-4 acetylated sulfonamides, except for the sulfapyrimidine group (sulfadiazine), are less water soluble than their nonacetylated forms. Highly insoluble sulfonamides are retained in the lumen of the gastrointestinal tract for prolonged periods and are known as “gut-active” sulfonamides. Most sulfonamides used clinically for treatment of bacterial infections are “potentiated.” The “potentiator” of sulfonamides is a diaminopyrimadine; examples include trimethoprim, ormetoprim, and pyrimethamine (the latter being the preferred drug for toxoplasmosis) (see Figure 7-13).

Figure 7-13 `The mechanism of action of the sulfonamides and the diaminopyrimidines. By itself, either type of drug is bacteriostatic, but the two-point sequential inhibition of folic acid synthesis results in bactericidal effects. The progenitor of the sulfonamides is sulfanilamide (inset). As such, all are arylamines. Metabolism of the arylamine to a hydroxyalamine and nitroso compound contributes to the toxicities, including drug allergies, associated with sulfonamides.

Mechanism of Action

Folic acid is an essential bacterial substrate necessary for protein and nucleic acid metabolism. Bacterial synthesis of folic acid is accomplished in several sequential steps (see Figure 7-13). The sulfonamides are structurally similar to PABA and act as competitive substrates (antimetabolites) for the synthetase enzyme. Among the many sulfonamides used clinically are sulfadiazine, sulfamethoxazole, sulfachlorpyridazine, sulfadimethoxine, and sulfasalazine; sulfisoxazole is the model drug upon which C&S testing is based.

Because folate metabolism is required for many cellular functions, bacterial growth is inhibited; consequently, the antibacterial effects of sulfonamides as sole agents are bacteriostatic. The diaminopyrimidines trimethoprim and ormetoprim also impair folic acid synthesis but at a different point in the metabolic pathway. They prevent the conversion of dihydrofolate to tetrahydrofolate by inhibiting the reductase enzyme. By themselves, these drugs also are bacteriostatic. It is the combination of a sulfonamide antimicrobial with a diaminopyrimidine antimicrobial (“potentiated”) that results in subsequent two-point inhibition of bacterial folic acid synthesis and thus bactericidal rather than bacteriostatic activity (see Figure 7-13). ^5,246 Mammalian cells are not affected by these drugs because they are dependent on dietary sources of folic acid; in contrast, microbes cannot use external sources of the substrates. Further, the affinity of bacterial enzymes for the drugs is much higher than the mammalian enzymes. The competitive nature of the mechanism of killing activity of potentiated sulfonamides leads to a time-dependent effect. High inoculums may require higher doses for efficacy.

Spectrum of Activity

The spectrum of activity of sulfonamides is considered broad, but efficacy is variable because of acquired resistance. However, a decline in their use during the last decades (due to concerns regarding allergies) appears to be associated with an increased in susceptibility for a number of organisms. The spectrum of combined products includes gram-positive, gram-negative, and anaerobic organisms. The sulfonamides exhibit good to moderate activity against E. coli; Enterobacter spp.; Klebsiella spp.; Proteus spp.; Pasteurella spp.; and anaerobic organisms such as Actinomyces, Bacteroides, Fusobacterium spp., and selected clostridia.^247-249 The spectrum of these drugs does not include Serratia spp., P. aeruginosa, Rickettsia, or Mycoplasma spp. The sulfonamides exhibit good efficacy against Brucella spp., Actinomyces spp., and selected protozoal organisms such as Pneumocystis carinii and Cryptosporidium spp. Some Chlamydia spp. are susceptible to sulfonamides, whereas others are not. The difference appears to be based on whether the organism can obtain folic acid from the host.^249a Mycoplasma organisms are not susceptible to sulfonamides. By itself, trimethoprim has a potency that is twentyfold to 200-fold less than that of sulfonamides.²⁴⁶ Potentiated sulfonamides are generally useful for uncomplicated infections of many body systems.

Resistance

Inherent resistance to sulfonamides reflects, in part, the ability of the microbe to make use of host folic acid. Resistance to the sulfonamides and to trimethoprim or ormetoprim occurs relatively rapidly. Chromosomal resistance results in impaired drug penetration, reduced affinity of the enzyme for the substrate, or increased bacterial production of PABA. Plasmid-mediated resistance occurs rapidly because of altered drug penetration and decreased affinity of the enzyme for the substrate. Resistance to one sulfonamide generally results in resistance to all sulfonamides.²⁴⁶ The increasing emergence of resistance has sharply curtailed the use of these drugs. The role of trimethoprim/sulfonamide combinations for the critically ill patient or for chronic infections should be based on C&S information because of the incidence of resistance.

Pharmacokinetics

The sulfonamides are generally rapidly and completely absorbed after oral administration, although there are exceptions (see the discussion of structure and chemistry). Trimethoprim and ormetoprim are well absorbed after oral administration. Sulfasalazine is poorly absorbed as an intact molecule and is used primarily for gastrointestinal diseases. After oral administration sulfasalazine is partially absorbed in the small intestine. It undergoes enterohepatic circulation and ultimately is eliminated in the urine. Most of the drug (70%) is metabolized by colonic bacteria to its component parts: sulfapyridine and 5-aminosalicylic acid. Sulfapyridine is rapidly absorbed and subsequently eliminated in urine. The 5-aminosalicylic acid may provide the major therapeutic benefit for chronic inflammatory bowel disease.²⁴⁶

Solutions intended for parenteral administration must be buffered to prevent pain and irritation caused by the alkalinity of the compounds. Topical administration is not recommended because of the effects of these drugs on wound healing. An exception is made for silver sulfadiazine and mafenide, which are used primarily for burn patients in human.²⁴⁶ Sulfadiazine is combined with silver in a topical otic preparation approved for use in dogs. Protein binding of the sulfonamides varies from 15% to 99%. Examples include sulfadiazine at 30% to 50% bound, sulfadiamethoxine, at greater than 75%, and sulfasalazine up to 99% bound. Protein binding contributes to a relatively long half-life, allowing for convenient dosing intervals.

The tissue penetrability of the sulfonamides varies. All are distributed at least to extracellular fluid. Sulfamethoxazole (the model drug for susceptibility testing) is limited to interstitial fluid, whereas sulfadiazine is distributed to total body water.²⁴⁶ Sulfadiazine penetrates most body tissues extremely well, including the prostate.²⁵⁰ The penetration of these drugs varies with the sulfonamide component. Prostatic penetration is facilitated by a high pK_a. Sulfadiazine (pK_a 6.4) is among the best distributed sulfonamides but only achieved 11% of serum concentration in the prostate of dogs in one study (the original reference for this study could not be found). Drugs with a more basic pK_a may appear to better penetrate the prostate, although this may reflect ion trapping in prostatic fluids. Sulfadiazine can attain therapeutic concentrations in CSF, particularly if given intravenously, and is the preferred sulfonamide for CNS infections.²⁴⁶ Trimethoprim achieves tissue concentrations four times higher than that in plasma. The combination of a sulfonamide with a diaminopyramidine at a ratio of 1:5 trimethoprim/sulfonamide results in a bactericidal effect and a tissue distribution ratio of 1:20 in most tissues.⁵ This ratio, however, is described in humans, and information in dogs or for sulfadimethoxine and ormetoprim does not appear to be available.

Sulfonamides that undergo hepatic metabolism are generally acetylated. All sulfonamide antimicrobials are arylamines. The dog lacks some genes that encode for N-acetyltransferases responsible for metabolism of arylamines.²⁵¹ Thus metabolism in the dog may involve other pathways, facilitating the formation of potentially nitroso metabolites that are responsible for allergic or other idiosyncratic reactions (see the section on adverse reactions) (see Figure 7-13).²⁵² Drugs are renally excreted as either the parent compound or the conjugated metabolite by either glomerular filtration or active tubular secretion. Both passive reabsorption and enterohepatic circulation can prolong the elimination half-life of selected sulfonamides.²⁴⁶ Acetylated metabolites of sulfonamides are often less soluble than the parent compounds, which increases the risk of renal damage should drug precipitate and form crystals. However, this is unlikely in dogs because of deficient acetylation. The risk is reduced in other species because of the use of combination products, which reduces the total amount of dose needed for efficacy. The elimination half-lives of the drugs vary with the sulfonamide component and among the species. The duration at which sulfonamides remain in the body leads to classification as short-acting (12 hours or less: sulfacetamide, sulfathiazole, and sulfisoxazole), intermediate-acting (12 to 24 hours: sulfadimethoxine, sulfisoxazole, sulfamethoxazole, sulfapyridine, sulfamethazine, and sulfadiazine), and long-acting (longer than 24 hours).²⁴⁶ In the dog, according to the package insert, sulfadimethoxine concentrations are 39 μg/mL 24 hours after dosing. Peak ormetoprim at 2 hours was 1.09 μg/mL in dogs but was 0.09 μg/mL at 24 hours, indicating a half-life of about 6 hours. It is not clear whether the differences in half-life between sulfadimethoxine and ormetoprim “match” in terms of ideal proportion throughout the labeled 24-hour dosing interval.

Adverse Effects

Reactions to sulfonamide antimicrobials reflect the greatest proportion of antimicrobial adversities in the dog.²⁵² The adversities to sulfonamide antimicrobials but not other sulfonamides (e.g., nonsteroidal antiinflammatories, zonisamide, furosemide) probably reflect the basic structure of the sulfanilamide molecule, which is an arylamine, in which the amine group is directly attached to the benzene ring (see Figure 7-13). The susceptibility of dogs to sulfonamide toxicity may reflect the species’ deficiency in acetylation and specifically N-acetylation. The proposed mechanism of toxicity reflects shunting of the sulfanilamide arylamine to an oxidative phase I pathway (see Figure 7-13). Oxidation of the arylamines yields hydroxylamine, a metabolite that can be cytotoxic at high concentrations; the metabolite also is somewhat allergenic. Hydroxylamine can be further metabolized (often spontaneously) to a nitroso compound, which is somewhat cytotoxic but is more immunogenic than the hydroxyarylamine. The potential role of the arylamine as a cause of sulfonamide toxicity is supported by the lack of apparent toxicity by other sulfonamide drugs used in dogs, which, lacking a primary arylamine, are not converted to hydroxylamine. The likelihood of adversity may be related to the type of metabolites formed and the rate of acetylation. As such, the likelihood of toxicity occurring may vary among the sulfonamide antimicrobials. The mechanism of hypersensitivity may reflect haptenization of the metabolite and a subsequent T-cell response, although other mechanisms (e.g., humoral response or cytotoxicity) may contribute.²⁵² Deficiencies in glutathione, ascorbic acid, or other radical scavengers may increase the risk of either type A or B reactions; the role of supplementation in preventing or treating adversities apparently has not been addressed scientifically but may be prudent. Controversy exists as to whether the parent sulfonamide might be immunogenic.²⁵² The “potentiator” may also be responsible for some reactions; for example, trimethoprim has been associated with skin eruptions or hepatopathy in humans; further, use of sulfadiazine as the sole coccidiostat in dogs has not been associated with drug allergies.

Drug Interactions

The sulfonamides have been associated with a number of drug interactions in humans.² Inhibition of elimination with subsequent prolonged or increased effects have been reported for oral hypoglycemic agents, dapsone when combined with trimethoprim, folate antagonists (increased risk of megaloblastic anemia), methanamine (increased risk of crystalluria), procainamide (decreased metabolism when combined with trimethoprim), and warfarin (increased anticoagulant activity with trimethoprim). In contrast, increased elimination has been reported for cyclosporine when combined with either sulfonamides or trimethoprim.²

Therapeutic Use

Because of the advent of resistance, the use of sulfonamides is limited to uncomplicated infections of most body systems. The concentration in urine supports the use of potentiated sulfonamides for urinary tract infections. Trimethoprim/sulfonamide combinations are indicated for treatment of infections caused by susceptible bacteria in difficult-to-penetrate tissues such as the prostate and CNS.²⁴⁶ These drugs are among the drugs of choice for treating Nocardia and Actinomyces spp. Synergistic effects have been cited toward these organisms when used in combination with beta-lactam antibiotics.

Structure–Activity Relationship

Three naturally occurring tetracyclines are obtained from Streptomyces: chlortetracycline (the prototypic drug but no longer available in human-medicine preparations), oxytetracycline, and demethylchlortetracycline (see Figure 7-5). Several tetracyclines have been derived semisynthetically (tetracycline from chlortetracycline, rolitetracycline, methacycline, minocycline, doxycycline, lymecycline, and others). Elimination half-lives permit a further classification into short-acting (tetracycline, oxytetracycline, chlortetracycline), intermediate-acting (demethylchlortetracycline and methacycline), and long-acting (doxycycline and minocycline) formulations. All of the tetracycline derivatives are crystalline, yellowish, amphoteric substances that, in aqueous solution, form salts with both acids and bases. They characteristically fluoresce when exposed to ultraviolet light. The most common salt form is the hydrochloride, except for doxycycline, which also is available as doxycycline hyclate. The tetracyclines are stable as dry powders but not in aqueous solution, particularly at higher pH ranges (7-8.5). Preparations for parenteral administration must be carefully formulated, often in propylene glycol or polyvinyl pyrrolidone with additional dispersing agents, to provide stable solutions. Tetracyclines form poorly soluble chelates with bivalent and trivalent cations, particularly calcium, magnesium, aluminum, and iron. Doxycycline and minocycline exhibit the greatest liposolubility and better penetration of bacteria.

Mechanism of Action

Tetracyclines bind bacterial ribosomes and impair protein synthesis (see Figure 7-6). Bacterial ribosomal activity was described in the section on aminoglycosides. The tetracyclines bind to the 16S portion of the 30S ribosomal subunits, preventing access of the amino-acyl tRNA to the acceptor site on the mRNA ribosome complex⁸⁰ (see Figure 7-6). Because tRNA binding is prevented, amino acids cannot be added to the peptide chain, and protein synthesis is impaired. Tetracyclines are bacteriostatic in action and should not be used in the immunocompromised patient, whether disease or drug induced (i.e., glucocorticoids or anticancer drugs). Their effects are described with other bacteriostatic ribosomal inhibitors as time dependent but are probably related to AUC. The tetracyclines also inhibit matrix metalloproteinases, an action separate from their antibacterial properties.

Spectrum of Activity

Tetracyclines enter cells either through porins or active transport pumps.⁸⁰ They are considered broad spectrum (see Table 7-2), being effective against gram-positive, gram–negative, anaerobic organisms, as well as cell wall–deficient and rickettsial organisms and others. Their spectrum includes gram-negative organisms, particularly Pasteurella spp., and often E. coli, Klebsiella, and Salmonella spp. P. aeruginosa is generally not included; although susceptibility may be indicated on C&S data, caution should be exercised when selecting tetracyclines. They generally are intrinsically more effective against gram-positive organisms (see Tables 7-3 and 7-4). As such, Staphylococcus and Streptococcus spp. generally are included in the spectrum. However, the broad general use of these drugs has led to resistance by many organisms and use against gram-positive organisms should be based on C&S testing. The spectrum of action also includes Chlamydia, Mycoplasma, Rickettsia, and Hemobartonella organisms. Spirochetes (Borrelia, Leptospirosis spp. also are generally susceptible, and several mycobacterial organisms are susceptible. Tetracyclines target Brucella spp.) although in human medicine generally they are combined with rifampin or gentamicin. Tetracyclines generally are effective toward actinomycosis and are generally considered more effective than chloramphenicol.⁸⁰

Tigecycline is a glycylcycline, a class of drugs that are synthetic analogs of the tetracyclines. Specifically, it is a glycolamide derivative of minocycline. The spectrum of this class is similar to that of the tetracyclines; however, they often remain effective against strains that have developed resistance to tetracyclines through increased efflux transport mechanisms.⁸⁰

Resistance

Resistance to tetracyclines is plasmid mediated and inducible.⁸⁰ Most resistance to tetracyclines results from either decreased influx or increased transport of the drug out of the microbial cell. Other mechanisms include altered binding site (which may reflect a mutation) and enzymatic destruction. Cross-resistance does not necessarily occur and depends on the mechanism. Drugs that minimize the impact of efflux pumps have been developed, including the glycylcyclines.²⁵⁵ These drugs also have a higher binding affinity than tetracyclines.

Pharmacokinetics

The oral absorption of tetracyclines is variable, with chlortetracycline being the least bioavailable, oxytetracycline more so, and doxycycline the most lipid soluble of the tetracyclines, being 100% bioavailable. Absorption is decreased in the presence of divalent and trivalent cations such as those present in milk products or antacids; exceptions occur for doxycycline and minocycline. Tetracyclines, particularly doxycycline, are widely distributed to most body tissues, and theoretically, inflammation need not be present for distribution into the brain⁸⁰ (see Table 7-5). Drugs will distribute through the placenta into the fetus and into milk. Doxycycline is able to penetrate cell membranes and thus gain access to intracellular organisms. Doxycycline is 99% protein bound, which prolongs its elimination half-life; note that concentrations in body fluids (see Table 7-5) are likely to reflect unbound drug, whereas that in plasma may reflect bound drug, decreasing ratios. Tetracyclines, with the exception of lipophilic tetracyclines such as minocycline and doxycycline, do not penetrate the CSF. The latter drugs are thus preferred because of better tissue penetrability for treatment of infections caused by susceptible bacteria in difficult-to-penetrate tissues, reaching 30% to 40% of plasma concentrations. Minocycline is characterized by a larger Vd in people than is doxycycline, suggesting the potential of better tissue penetrability, but may also be more bound to bone or other tissues containing cations. Tetracyclines accumulate in reticuloendothelial cells.⁸⁰ Tetracyclines are incorporated into forming bone and the enamel and dentin of teeth and cause discoloration of teeth upon eruption. The age at which this occurs in dogs and cats is not clear.

Doxycycline (PC 0.68 and pKa 3.09)⁵⁷ was studied in the dog in both plasma and interstitial fluid (using ultrafiltration) after intravenous and constant-rate influsion (to allow establishment of steady-state concentrations). The drug is 91% bound to plasma proteins in dogs, resulting in a total AUC difference sixfold higher in plasma compared with interstitial fluid. Further, the interstitial fluid C_max (of unbound drug) was only 0.14 μg/mL at steady-state conditions; in contrast, PDCs extrapolated from the terminal component of the elimination curve was 1.6 μg/mL. The concentration of interstitial fluid drug was equivalent to the concentration of unbound drug in plasma.⁵⁷ Vd of unbound drug was 0.65 ± 0.08 L/kg; clearance was 1.66 ± 2.21 mL∗kg/min.

With the exception of doxycycline and minocycline, the tetracyclines are eliminated by both renal (approximately 60%) and biliary (40%) excretion. Presumably, minocycline is eliminated essentially in the bile, whereas the route of elimination of doxycycline is less obvious. In humans it is eliminated by both renal (41%) and biliary (59%) mechanisms. In dogs intestinal elimination of the unchanged drug appears to be the predominant route, with only about 16% of a given dose being excreted unchanged in the urine. Tetracyclines undergo enterohepatic circulation. Toxic concentrations may accumulate in patients with renal disease. Differences that justify use of minocycline instead of doxycycline are difficult to ascertain. Adverse reactions to minocycline may, however, be more likely.

The tetracyclines are available as intravenous, parenteral, and ocular preparations. Tetracyclines should not be given intramuscularly because of local tissue damage and irritation. For the same reason, tetracyclines are not indicated for topical treatment other than the eye.

Adverse Effects

Tetracyclines cause several adverse effects in small animals. Toxicity may be worsened in patients with renal disease because of decreased elimination. Gastrointestinal upset follows direct irritation of the gastrointestinal mucosa after oral administration; administration of doxycycline with food will reduce gastrointestinal side effects. Rarely, hepatotoxicity may occur. Rapid intravenous administration may result in collapse. Although the likelihood of this occurring in small animals is not clear, prudence dictates slow administration of a diluted solution (i.e., 1:10) when tetracyclines are administered intravenously. Although the mechanism is not certain, calcium binding may be important. Intravenous administration of tetracycline has caused anaphylactic shock in dogs. Diluting fluids should not contain calcium or other cations to which tetracylines might chelate. Hypersensitivity has also been reported in a dog after intramuscular administration of tetracycline. Minocycline may be more likely to cause allergic drug reactions in drugs. Lesions characterized by erythema of the skin and mucous membranes occurred in dogs after administration of most doses of minocycline. Anemia may also occur (10 mg/kg, administered intravenously). Brown to gray discoloration of teeth may occur because of chelation of tetracyclines in calcium deposits of dentin and, to a lesser degree, enamel. Tetracycline and oxytetracycline cause a yellow discoloration, whereas chlortetracycline produces a gray-brown discoloration; of all the tetracyclines, oxytetracycline causes the least tooth discoloration. Because chelation might occur in forming dentin as well as enamel, tetracyclines should be avoided from 3 weeks’ gestation to at least 1 month after birth. Among the lipid-soluble tetracyclines, doxycycline may be less likely to cause discoloration. In humans minocycline may stain teeth regardless of tooth development because of chelation with iron; the drug probably has not been used sufficiently in animals to determine whether a similar effect will occur. Other side effects caused by tetracyclines include drug fever (in cats), an antianabolic effect, and a Fanconi-like syndrome in the kidneys, with the latter more likely with expired or degraded tetracyclines.²⁵⁵

Doxycycline has been associated with esophageal erosions in cats (and humans).⁸⁰ In a study of 30 cats, no orally administered tablets had passed in 30 seconds; only 40% had entered in 5 minutes. In contrast, 90% of tablets passed within 30 seconds when followed with 6 mL of water, with 100% passage at 90 seconds. For capsules only 17% had passed by 30 seconds, but 93% had passed by 60 seconds.²⁵⁶ The impact of esophageal damage is not unique to doxycline; other drugs are ulcerogenic because of local effects. Indeed, the cat has been used as a model to assess the ulcerogenic potential of orally administered drugs.²⁵⁷ For doxycycline, the risk may be decreased with use of the monohydrate salt. In the event that erosions do occur, among the treatments to consider would be pentoxifylline.²⁵⁸

Drug Interactions

Because of chelation with cations (magnesium, calcium, aluminum, and so on), tetracyclines should not be simultaneously administered with cation-containing drugs (e.g., antacids, sucralfate, buffered aspirin, calcium-containing supplements, fluids). Cholestyramine may also bind to tetracyclines. Tetracyclines, with the exception of doxycycline, should not be administered with food.

Because tetracyclines bind to the 30s ribosomal subunit, combination with antimicrobials that target the 50s subunit might be considered (e.g., the phenicols, macrolides, and lincosamides) with scientific support. One study indicates an in vitro synergistic effect of the combined use of doxycycline and azithtromycin against P. aeruginosa.²⁵⁹

Therapeutic Use

The therapeutic indications for tetracyclines are many but have decreased in recent years because of the advent of resistance. Treatment of microbial infections is best based on C&S data. Doxycycline is the preferred tetracycline because of its ability to move intracellularly compared to other tetracyclines. Doxycycline generally is indicated among first-choice therapies for obligate intracellular organisms, including ehrlichiosis, Rocky Mountain spotted fever, chlamydiosis, mycoplasmosis, and hemobartenellosis. Doxycycline also has been used to treat canine brucellosis. Other potential indications include leptospirosis and Lyme disease.

Mechanism of Action

Like tetracyclines, chloramphenicol and florfenicol bind bacterial ribosomes and impair protein synthesis (see Figure 7-6). However, binding occurs at the 50s subunit with inhibition of peptidyl transferase. Actions are bacteriostatic in action, and these drugs should not be used in immunocompromised patients. As with other bacteriostatic ribosomal inhibitors, the effects of chloramphenicol and florfenicol should be considered time dependent. As with tetracyclines, although host ribosomes do not bind as effectively as do bacterial ribosomes, some host ribosomal activity will be impaired. Binding sites for chloramphenicol are close to those for clindamycin, which it competitively inhibits.⁸⁰ Chloramphenicol also inhibits mitochondrial protein synthesis in mammalian cells, with erythropoietic cells particularly sensitive.

Spectrum of Activity

Chloramphenicol is considered broad spectrum (see Table 7-1), being effective against gram-positive, gram-negative, and anaerobic organisms. P. aeruginosa is generally not included. The spectrum of action also includes Chlamydia, Mycoplasma, Rickettsia, and Hemobartonella organisms. As previously noted, tetracyclines are considered more effective than chloramphenicol for the latter organisms, but chloramphenicol tends to be more clinically effective for other organisms. The spectrum of activity of florfenicol is similar to chloramphenicol; although the anaerobic spectrum has not been described, it is assumed to be similar. The MIC for florfenicol of small animals generally reflects 1.0 to 8.0 μg/mL.

Resistance

Resistance to chloramphenicol is caused by destruction (acetylation) of the drug by microbial enzymes. The fluorine ring of florfenicol may impair bacterial acetylation, and thus florfenicol is more resistant to bacterial deactivation;²⁶⁰ selected organisms resistant to chloramphenicol may be susceptible to florfenicol.²

Pharmacokinetics

Chloramphenicol is very well absorbed after oral administration in its crystalline form. Many of the originally-approved preparations are no longer available in the United States. The liquid form is less well absorbed, so much so that the palmitate form should not be used for cats because of variability in oral absorption. The chloramphenicol succinate ester is the water-soluble form intended for injection (see Table 7-1). The succinate must be hydrolyzed by plasma, hepatic, pulmonary, or renal esterases before activity. Chloramphenicol palmitate is a suspension for oral administration. Its ester is hydrolyzed by small intestinal lipases; the freed chloramphenicol is then orally absorbed. The freed chloramphenicol is among the most lipid soluble of the clinically used drugs and achieves moderate to high concentrations in most body tissues, including the CSF. It is, however, unlikely to achieve bactericidal concentrations in most tissues, including the CNS. Most of the drug is eliminated by hepatic metabolism. Glucuronidation is a major route of elimination of chloramphenicol. Cats eliminate chloramphenicol more slowly because of deficiencies in both phase I and phase II metabolism. Greater concentrations may occur in cat urine than in dog urine as a result.²⁶¹ Pediatric patients also may not eliminate chloramphenicol as efficiently as young adult dogs.

Chloramphenicol was studied after single oral dose as the commercially available Chloromycetin (50 mg/kg) in dogs.²⁶² Although pharmacokinetics were not reported, the C_max (μg/mL) at T_max were, respectively, for Greyhounds with feeding, 21.6 ± 4.8 at 1.5 hours, or without feeding, 18.6 ± 6.7 at 3 hours; large dogs (22-26 kg), 20.0 ± 4.8 at 1.5 hours; and small dogs (11.4 to 15.5 kg), 27.5 ± 7.0 at 3 hours. Peak concentrations were notably higher in small dogs than large dogs. Half-life in Greyhounds was 3.2 hours in fasted dogs versus 1.9 hours in fed dogs; the elimination half-life (based on noncompartmental anaylsis of published data) in large dogs was 2.3 hours compared with 3.4 hours in small dogs. Average half-life among all groups was 2.7 ± 0.7 hours; mean residence time was 4.6 ± 0.67 hours. Neither oral bioavailability nor clearance was determined.

During approval for use in humans, chloramphenicol was studied in dogs.²⁶³ Chloramphenicol was measured on the basis of an analytic procedure that detected chloramphenicol and its metabolites; therefore the relevance of the data must be considered. Homogenate tissue concentrations were described after subcutaneous administration of 35 mg/kg for 2 dogs: at 1.5 and 3 hours, plasma concentrations were 21 and 13, respectively, yielding an elimination half-life of 2.9 hours. Concentrations in the brain and CSF at the same time were 15 and 8 μg/mL (brain) and 7 and 9 μg/mL (CSF), yielding a 3-hour plasma:tissue ratio of 0.7. A second study measured chloramphenicol using a bioassay. However, only a single dog was studied after oral administration of 150 mg/kg of the crystalline powder form. The C_max was 45 μg/mL at 4 hours and approximately 15 μg/mL at 8 hours, yielding a disappearance half-life of 2.5 hours. This should extrapolate to a C_max of approximately 14 μg/mL when 50 mg/kg is administered. Although 54% of the drug was eliminated in the urine, only 6.3% of the drug in the urine was pharmacologically active. Intravenous administration of 50 mg/kg yielded an initial plasma concentration of approximately 39 μg/mL and a concentration of approximately 5 μg/mL at 8 hours, yielding a half-life of about 1.5 hours.²⁶³

Chloramphenicol has been studied in cats (n = 5). Oral administration of the crystalline powder in capsules yielded C_max (μg/mL) of 43 to 62 at 40 mg/kg, 25 to 42 at 20 mg/kg tid, and 8 to 25 at 50 mg bid.²⁶⁴ Cats were also dosed with succinate intravenously, intramuscularly, subcutaneously, or orally (crystalline powder in capsules).²⁶⁵ Concentrations at 30 minutes (T_max for each route except oral) were, respectively (μg/mL) 19.5 ± 1.5 (intravenous), 18.6 ± 2.6 (intramuscular), 14.8 ± 2.9 (subcutaneous) and 9.8 ± 2.61 (oral) after administration of 20 mg/kg (n = 5). The mean half-life of all three routes was 4.4 ± 1.38; range was 3.3 hours for subcutaneous and 6.9 hours for intravenous. AUC for each route was similar (lowest at 55 ± 7 μg∗hr/mL for intravenous, highest at 67 ± 9 for subcutaneous). Finally, the bioavailability of the palmitate salt suspension is poor in cats, particularly in the fasted state.²⁶⁶ Peak concentrations of the crystalline form following 100 mg/cat was 25 ± 5 (fasted) or 31 ± 3 (fed) versus 6.5 ± 1.3 (fasted) and 16 ± 3 (fed) for the succinate form.

Florfenicol has been studied in dogs and cats.^260,267 In dogs, although predictable PDCs (1.64 μg/mL) are achieved at 20 mg/kg after intramuscular administration, concentrations are unpredictable after subcutaneous administration. The drug appears to follow a “flip-flop” model, with the elimination half-life in dogs following intramuscular administration much longer (9 hours) compared with intravenous administration (<1 hour). A second study determined the oral bioavailability and described in more detail the PK of florfenicol (based on HPLC) in dogs (n = 6) after intravenous and oral administration (20 mg/kg).²⁶⁷ Florenicol clearance was 1.03 ± 0.49 l∗kg/hr, and the Vdss 1.45 ± 0.82 L/kg.The elimination half life is 1.11 + 0.94 hour after intravenous and 1.24 ± 0.64 after oral administration. Oral bioavailability was 95 ± 11%, with C_max reaching 6.18 μg/mL at a T_max of 0.94 hour. Florfenicol amine is a major metabolite of florfenicol, with a longer half-life in dogs (2.26 hours), but it has only 1/90 the activity of the parent compound, and its contribution to microbiological activity is considered negligible. Dogs showed no evidence of side effects after either intravenous or oral administration. The disposition of florfenicol by the intramuscular route appears to be more predicatable in cats than dogs, with C_max (22 mg/kg) reaching 20 μg/mL after IM administration and 27 μg/mL after oral administration (see Table 7-1).²⁶⁷ Oral administration was based on a solution of 100 mg/mL. The elimination half-life was 8 hours in cats after oral administration, supporting a 12-hour dosing interval. The distribution volume in cats is supportive of a lipid-soluble drug. Adverse reactions were not noted in the six cats studied.

Adverse Effects

A major toxic concern with chloramphenicol for humans is both reversible dose-dependent and irreversible dose-independent (rare) bone marrow suppression. Reversible bone marrow suppression can also occur in animals. Dose-dependent bone marrow effects may reflect suppression of bone marrow precursor cells after mitochondrial damage. Irreversible bone marrow suppression may reflect reduction of the NO₂ group to a toxic metabolite that causes stem cell damage. Irreversible suppression should be avoided with florfenicol, which lacks the NO₂ group. Although cats appear more sensitive to chloramphenicol-induced reversible bone marrow suppression than do dogs, toxicosis appears rapidly reversible once the drug is discontinued. Toxicity to chloramphenicol occurs in cats after 7 days of therapy at 50 mg/kg, administered intramuscularly. The drug can, however, be used for 7 to 10 days safely in cats after oral administration of the crystalline form (capsules) at the rate of 50 mg/cat.^265,266 The antianabolic effects of chloramphenicol may result in impaired protein synthesis in the patient; however, despite earlier concerns, impaired immune response to vaccines does not appear to occur.

Drug Interactions

Because they compete for the same ribosomal binding site, chloramphenicol should not be used in combination with macrolides. Because they target two different ribosomal sites, the combination of chloramphenicol with tetracyclines is appealing. Interestingly, the combined use of chloramphenicol with penicillins has been demonstrated to enhance penicillin (bacteriostatic) activity in Enterobacteraceae that are otherwise resistant to penicillins because of beta-lactamase production. Chloramphenicol inhibits product of beta-lactamases in these organisms.^267a Chloramphenicol is a potent inhibitor of drug-metabolizing enzymes and inhibits the hepatic metabolism of other drugs, potentially causing toxicity should drug concentrations increase. Prolonged sleeping times have been documented after administration of pentobarbital to dogs and cats also receiving chloramphenicol;²⁶⁸ chloramphenicol has markedly prolonged phenytoin half-life²⁶⁹ and phenobarbital half-life (see Chapter 2) in dogs. Phenobarbital-induced sedation and ataxia have occurred in as few as 3 days of chloramphenicol therapy. Chloramphenicol decreases the rate of elimination of digoxin.²⁷⁰

Therapeutic Use

Chloramphenicol has been commercially available as a palmitate (oral) salt and a sodium succinate injectable preparation. Florfenicol is commercially available only as solution intended for (intramuscular) injection, which has been studied in dogs and cats.

Pharmacokinetics

Only oral preparations of clindamycin are approved in the dog and cat; an injectable preparation is approved for use in humans. Both clindamycin and lincomycin are bioavailable after oral administration, although clindamycin is more so. Food does not impair the absorption of clindamycin but does appear to impair absorption of lincomycin. Clindamycin is available as the hydrochloride, palmitate, or phosphate salts. The palmitate form is an oral prodrug, with the ester being rapidly hydrolyzed to yield free drug. The phosphate form is intended for parenteral administration, including subcutaneous, intramuscular (although it is painful), and intravenous routes. In the cat administration of 5.5 and 11 mg/kg orally generates serum concentrations above the MIC of most S. pseudintermedius organisms and previously S. aureus, but it is likely that resistance has resulted in less favorable PDI. Higher doses (11 to 20 mg/kg) will generate concentrations above the MIC of most susceptible anaerobes (see Table 7-1). In dogs oral administration of 11 mg/kg every 12 hours has been sufficient for treatment of most Staphylococcus spp. infections, but current MIC₉₀ for clindamycin and susceptible Staphyloccocus spp. have not been reported. As a time-dependent drug, decreasing the interval to 8 to 6 hours may increase efficacy. Clindamycin is highly (>90%) protein bound. Distribution of the lincosamides includes most body tissues, with excellent concentrations being achieved in the skin and bones. However, it does not substantially penetrate the brain or CSF, although it can achieve concentrations effective for toxoplasmosis.⁸⁰ Its Vd in both dogs and cats approximates 1.5 L/kg. Clindamycin has been cited for its efficacy in the treatment of chronic gingivitis or periodontal disease. Unlike many other drugs with a favorable spectrum, it is able to penetrate the biofilm that protects the causative organisms. Accumulation of clindamycin in white blood cells up to fortyfold or more may increase the probability of reaching bactericidal concentrations at some sites of infection. The lincosamides are eliminated primarily by biliary excretion.

After administration of 10 mg/kg intravenously, intramuscularly, and subcutaneously in dogs, in addition to C_max and elimination half-life (see Table 7-1), the following were achieved: T_max occurred at 73 ± 16 min (intramuscular) or 47 ± 20 min (subcutaneous) and CL (mL/min/kg) 6.1 ± 1.1. The elimination half-life may vary with the route (see Table 7-2), as does mean residence time at 143 ± 34, (intravenous), 700 ± 246 (intramuscular), or 364 ± 147 (subcutaneous) minutes. Bioavailability was 115% after intramuscular and 310% after subcutaneous administration. The long half-life coupled with the highest C_max suggests that the subcutaneous route of administration is the preferred parenteral route for clindamycin.²⁷² The reason for the very high bioavailability after subcutaneous administration is not clear, although enterohepatic circulation is anticipated to increase bioavailability regardless of route of administration.

Clindamycin disposition has been reported in cats after oral administration of either a capsule or aqueous solution (see Table 7-1). ²⁷¹ Peak PDCs are equivalent for both preparations, but a longer half-life for the capsule may contribute to a (not statistically significant) greater AUC for the capsule (42.6 ± 12.2) compared with the solution (35 ± 9.2). The lack of statistical difference may reflect the marked variability in half-life mean residence time for both preparations, which was approximately 6.5 hours.

Adverse Reactions, Drug Interactions, and Indications

Pseudomembranous colitis is a reported side effect in humans caused by overgrowth of C. difficile. The negative impact on the intestinal microbiota may persist for more than 2 weeks.⁸⁰ Because of similar mechanisms of action, this drug should not be combined with chloramphenicol or erythromycin. It has been combined with aminoglycoside treatment of polymicrobial infections involving gram-negative and anaerobic organisms. The use of clindamycin as combination antimicrobial therapy was addressed in the preceding section. Because of its ability to impair pili formation and thus adherance to tracheal epithelium, clindamycin has been associated with treatment of cystic fibrsosis associated with P. aeruginosa in humans, generally in association with antipseudomonadal antimicrobials.²⁷³ However, the macrolides are more generally accepted for this use. The use of clindamycin as part of combination chemotherapy targeting protozoal disease (toxoplasmosis) is addressed in Chapter 12).

Structure–Activity Relationship

The macrolides are named for their chemical structure, composed of a very large lactone (MW >750 to >1000) ring attached to a number of sugars.²⁷⁴ They include the azalides, which contain a nitrogen in the ring structure (see Figure 7-13). No macrolide derivative is approved for use in dogs or cats at the time of this publication. Human-medicine drugs include the14-member rings erythromycin and clarithromycin and the 15-member ring azithromycin (an azalide semisynthetic derivative of erythromycin), spiramycin, and dirithromycin (a prodrug converted to the active erythromycylamine). The methyl group that distinguishes clarithromycin from erythromycin and the additional methyl group on azithromycin increases acid stability and enhances tissue distribution. Telithromycin is a ketolide macrolide (discussed later). Tylosin, a drug approved for use in food animals, is used to treat intestinal disorders, largely in dogs. Of the human drugs, erythromycin (first-generation), azithromycin, and to a lesser extent, clarithromycin (second-generation) are used in dogs and cats.²⁷⁴ Tilmicosin is approved for use in selected food animals, but toxicity precludes its use in the injectable form in dogs and cats; information is not available regarding safety of other preparations. The second-generation macrolides differ from erythromycin only by the addition of a methyl group substitution. However, this simple change improves acid stability and tissue penetration. Further, because the methyl group enhances interaction with bacterial ribosomes, the spectrum also is improved.²⁷⁴

Mechanism of Action

Macrolides inhibit bacterial ribosomal action by binding to the 50s subunit of susceptible organisms (see Figure 7-6), and impairing the translocation step of protein synthesis. The azalides macrolides bind the ribosome at two sites.²⁷⁵ Although macrolides are classified as bacteriostatic in vitro, they are bactericidal against very susceptible organisms. Further, selected drugs (e.g., azithromycin) accumulate in selected tissues at bactericidal concentrations. All macrolides generally accumulate in phagocytic white blood cells, which may facilitate distribution to the site of infection. Efficacy is enhanced in an alkaline pH, probably because of increased diffusion of the nonionized drug into organisms; as such, intracellular activity may be decreased in phagocytic cells. The antibacterial effects of the macrolides vary with the drug and are time dependent for erythromycin; antibacterial effects for azithromycin and clarithromycin are time dependent for some organisms and concentration dependent for others.

Spectrum of Activity

Like the lincosamides, the macrolides are often used in humans as penicillin substitutes to minimize the risk of penicillin hypersensitivity. Organisms are considered susceptible to the macrolides at an MIC below 2 μg/mL. For the first-generation drugs, gram-positive organisms accumulate erythromycin at concentrations that exceed that of gram-negative organisms by a hundredfold. As such, erythromycin is most effective against gram-positive organisms. Streptococcus spp. are susceptible at a range of 0.015 to 1 μg/mL, although resistance is increasing. Many Staphyloccocus organisms have remained susceptible to erythromycin, but MIC ranges of 0.12 to > 128 μg/mL for S. aureus indicate an increasing trend of resistance. Among the staphyloccoci, S. pseudintermedius remains the most susceptible. P. multocida, Bordetella pertussis, and Mycoplasma spp. are among the organisms susceptible to erythromycin. However, use should be based on C&S testing. Erythromycin generally is effective against anaerobic organisms, with the exception of Bacterioides spp. Macrolides are generally effective against Campylobacter spp.

The azolides were designed to overcome barriers presented to penetration of gram-negative organisms. Thus the spectrum of azithromycin and clarithromycin increases, particularly in terms of gram-negative bacteria, although efficacy toward selected gram-positive microbes may decrease, requiring higher MIC.²⁷⁵ The actions of the azolides are bactericidal for Streptococcus pyogenes and S. pneumoniae but bacteriostatic toward staphylococci and most aerobic gram-negative organisms. Clarithromycin is effective at lower concentrations than erythromycin against Streptococcus and Staphyloccus spp. but is similar to erythromycin in efficacy against other organisms. Azithromycin has less activity against gram-positive organisms compared with erythromycin and greater activity against selected gram-negative organisms and Mycoplasma spp.⁸⁰ Although the spectrum of the macrolides generally includes Actinomyces spp., efficacy is generally less for Nocardia spp. Clarithromycin and azithromycin are effective against the Mycobacterium avium complex, Mycobacterium leprae, and Toxoplasma gondii. Compared to erythromycin, azithromycin and clarithromycin have enhanced activity against selected protozoa (e.g., T. gondii, Cryptosporidium spp.).

Controversy surrounds the classification of macrolides as either concentration or time dependent. The macrolides do exhibit a postantibiotic effect, with that of clarithromycin and azithromycin being longer than that of erythromycin. Azithromycin appears to be bacteriostatic against Staphylococcus or Streptococcus spp.; in vitro killing did not increase in a dose-dependent manner, suggesting that the drug is a time-dependent antimicrobial.²⁷⁶

Resistance

Acquired mechanisms of resistance to macrolides include pump-driven drug efflux from the cell (particularly in staphylococci, group A streptococci, and S. pneumoniae) and altered ribosomal targets (methylase enyzme; MLSB phenotype) that also confer resistance to lincosamides, which bind at the same ribosomal site. Efflux pumps contribute to resistance in E. coli as well.¹⁷⁹ Chromosomal mutations in Bacillus subtilis, Campylobacter spp., and gram-positive cocci alter the ribosomal binding site. Resistance of S. aureus to erythromycin generally is indicative of resistance to azithromycin and clarithromycin as well.The Enterobacteriaceae produce an esterase that hydrolyses the drug.

Pharmacokinetics

The macrolides and azolides are largely water insoluble and are unstable in the acidic gastric environment.²⁷⁴ However, each of the macrolides is available as an oral preparation. Erythromycin also is available as a topical and ophthalmic preparation. Erythromycin base preparations generally are coated to prevent gastric degradation. Oral absorption of enteric-coated or delayed-release products designed for humans may be unpredicatable in animals.² Oral salts include the estolate and ethylsuccinate salts, which must be de-esterified after oral absorption, and the stearate (octadecanoate) and phosphate salts. The former (and possibly the latter) dissociate in the duodenum to be absorbed as the free base. The disposition of selected erythromycin salts has been described in dogs.²⁷⁷

After oral administration, the erythromycin base is incompletely but adequately absorbed. Food may increase acidity and thus delay absorption. Esters (stearate, estolate, ethylsuccinate) improve stability and absorption but do not appear to increase PDCs. Among the salts, estolate appears to be best absorbed orally and minimally affected by food. For the azolides, clarithromycin is characterized by greater acid stability compared with erythromycin. Clarithromycin is more rapidly absorbed (in humans), but food delays absorption and first-pass metabolism (to an active metabolite) further reduces oral bioavailability of the parent compound to 50%. Azithromycin also is absorbed rapidly, but, again, food decreases bioavailability to 43% (in humans). Erythromycin is approximately 75% protein bound; binding is as high as 96% (in humans) for the estolate salt. Protein binding for clarithromycin is concentration dependent and ranges from 40% to 70%. Despite their large moleculer size, macrolides are sufficiently lipid soluble that they diffuse through membranes, albeit slowly. With a Vd of 2 L/kg in dogs, erythromycin will reach effective concentrations in all tissues except the brain and CSF. In general, the macrolides act as weak bases and, as such, trapped in an acidic environment, including acidic intracellular organelles. Consequently, tissue concentrations will exceed plasma in many tissues. Although accumulation occurs in selected tissues (e.g., bile, bronchial secretions, phagocytic white blood cells), concentrations reach only 50% of plasma in the prostate and aqueous humor and less than 15% in the CSF. Concentrations in the middle ear will approximate 50% of those in plasma. Clarithromycin and its active metabolite are well distributed, achieving higher concentrations than erythromycin in both the middle ear and CNS. Among the macrolides, azithromycin distributes the most extensively, with a Vd that exceeds (in humans) 30 L/kg. Fibroblasts act as a reservoir, with transfer to phagocytic cells. Whereas erythromycin and azithyromycin are eliminated principally in the bile, clarithromycin is extensively metabolized to an active (14 hydroxy derivative) metabolite. Excretion is primarily by biliary secretion into the feces; enterohepatic circulation of active drug might be anticipated. Urine excretion is not significant (3% to 5%), with concentrations in urine being low (approximately 50% of plasma); an exception is clarithromycin, for which the active metabolite might achieve high concentrations in urine. The elimination half-life for azithromycin has been reported at 1 to 1.5 hours in dogs^279,280 and cats.

The disposition of erythromycin as the estolate tablet and ethylsuccinate suspension and tablet has recently been described in dogs.²⁷⁷ Intravenous administration revealed a Vd of 4.8 L/kg (see Table 7-1) and a clearance of 2.64 ± 0.84 L/hr/kg. Oral absorption of all three products was poor: the ethylsuccinate tablets did not yield predictably detectable concentrations, whereas, based on mean AUC adjusted for differences in dose, the bioavailability of the estolate tablet was only 11% (T_max 1.7 hr) and the ethylsuccinate suspension only 3% (T_max, 0.7 hr). Absorption of the suspension, in particular, was described by the authors as erratic. Peak concentrations did not reach MIC₉₀ for susceptible Staphylococcus spp. of 0.5 μg/mL (reported by the authors) for any of the oral preparations. The apparent efficacy of erythromycin, despite poor absorption, may reflect accumulation of drug in tissues such that higher concentrations are achieved at the site of infection.²⁷⁷ All dogs vomited after dosing, regardless of route of administration, with vomiting apparent 5 to 10 minutes after intravenous administration, approximately 45 minutes after oral succinate preparations, and 1 to 2 hours after the estolate tablet administration.

Limited information is available for the second-generation macrolides in animals. Azithromycin and clarithromycin absorption is influenced by uptake transporters in the intestinal epithelium. Whereas efflux transporters, such as P-glycoprotein, decrease absorption, others (organic anion-transporting proteins) facilitate uptake.²⁷⁸ Azithromycin has been studied in cats and dogs (see Table 7-1).^279,280 Bioavailability in the dog is greater than 97%. Serum protein binding is less than 25%.

Clearance is 6.0 mL∗min/kg. In dogs 67% of the drug is eliminated in the bile and 33% in the urine.²⁷⁹ The majority of the drug (75%) is eliminated unchanged. The remaining portion is metabolized by cytochrome P450s into a number of metabolites, which, with one exception, are inactive. Tissue concentrations (based on homogenate) at 24 hours after 20 mg/kg orally were over 101, 20, and 39 μg/mL, respectively, for liver, kidney, and lungs. After 5 days of dosing, 23 μg/mL was achieved 24 hours after the last dose in the eye but only 1.2 μg/mL in the brain (at 30 mg/kg for 5 days). In cats the maximum drug concentration (C_max) of 0.97 ± 0.65 μg/mL occurs at T_max of 0.85 ± 0.72 hr. Plasma concentrations (μg/mL) range from approximately 8 at 1 hour to 0.1 at 12 hours after intravenous administration of 5 mg/kg and approximately 1 μg/mL to 0.1 μg/mL during the same times after oral administration of 5 mg/kg. Although the elimination half-life is long, concentrations in plasma are below 0.1 μg/mL after 12 hours. However, concentrations of azithromycin approximate 0.75 to 1 μg/mL in the femur, skin, and muscle versus 10 μg/mL in tissues characterized by reticuloendothelial cells (liver, spleen, and to a lesser degree lung) and the kidney with concentrations persisting for 72 hours or more. Because tissue concentrations were based on homogenate, it is not clear how much drug is available to interstitial fluid. Clearance is 0.64 ± 0.24 L∗hr/kg. Oral bioavailability is 52 ± 22%. The elimination half-life is 35 (range 29 to 51 hours).²⁸⁰ The Clinical Laboratory and Standards Institute susceptible breakpoint for azithromycin (human pathogens) is 4 μg/mL. Because concentrations decline to less than 0.1 μg/mL by 12 hours, daily dosing should be considered in both cats and dogs; because time to steady state will approximate 3 to 5 days, a 15 mg/kg loading dose should be considered followed by once-daily dosing at a minimum of 5 mg/kg. Although cats do metabolize azithromycin, the unchanged drug is the predominant form in tissues. Biliary excretion is a major route of clearance in the cat.²⁸⁰ Kinetics of clarithromycin become zero order (saturated) at higher doses. The large Vd of the macrolides contributes to their long elimination half-life. The half-life in cats exceeds 72 hours in some tissues.²⁸⁰ In contrast to azithromycin, urinary concentrations of clarithromycin can be signifcant: up to 40% of the parent drug or its metabolite are eliminated in the urine. The mean half-life in plasma is 35 hours but varies in tissues from a low of 13 hours (fat) to a high of 72 hours (cardiac muscle).

Adverse Effects

Side effects of the macrolides are limited. With injectable products, pain may occur with intramuscular injection and thrombophlebitis with intravenous injection. Reversible cholestatic hepatitis accompanied by jaundice has been reported in humans 10 to 20 days into erythromycin therapy, especially with the estolate preparation.

Gastrointestinal upset is the most common adverse effect of the macrolides. Up to 50% of animals treated with erythromycin may exhibit vomiting. Erythromicin is motilin-like in action and characterized by marked prokinetic effects on gastrointestinal motility. This effect is dose dependent in humans and may occur more commonly in younger animals. Abdominal cramping, epigastric pain, and increased gastric emptying resulting from increased gastric motility also may occur. However, because contraction is not coordinated, efficacy as a prokinetic is limited. Gastric emptying may decrease gastric maceration of ingested food, although the impact on digestion is not likely to be significant. Azithromycin and clarithromycin do not appear to have the same gastrointestinal side effects of erythromycin. In humans allergic reactions occur rarely and are manifested as fever or skin eruptions, which resolve once therapy is discontinued. Cholestatic hepatitis is an infrequent side effect in humans.

Drug Interactions

Antacids decrease the rate (and thus peak) but not extent of absorption of azithromycin, whereas food decreases the extent by close to 50%. The macrolides may inhibit cytochrome P450 enzymes, and CYP 3A4 in particular, impairing the metabolism of other drugs.^280a Among the macrolides, erythromycin followed by clarithromycin is most likely to be involved in significant drug interactions, although all three drugs inhibit drug-metabolizing enzymes. The effects of drugs metabolized by the liver, including selected anticonvulsants, cardiac drugs, and theophylline, are likely to increase. Drugs affected in humans include glucocorticoids, digoxin, theophylline, and warfarin. The macrolide antimicrobials (clarithromycin, roxithromycin) also increase the risk of digoxin toxicity, although this effect may be more reflective of competitive interactions with P-glycoprotein transport proteins.²⁸¹ Azithromycin is a substrate; others may be as well.²⁸² Among the P-glycoprotein interactions with azithromycin in cats is cyclosporine; peak cyclosporine concentrations exceeded 4500 ng/mL in a cat receiving 5 mg/kg while being treated with azithromycin.

Because they are ribosomal inhibitors, care must be taken not to combine the macrolides with drugs whose efficacy requires rapid bacterial growth, unless scientific support exists, or “-cidal” concentrations of the macrolide are achieved at the target site for both drugs. For example, synergistic effects have been documented against B. fragilis when erythromycin is combined with cefamandole and against Nocardia asteroides when combined with ampicillin. The use of erythromycin in combination with other antimicrobials is limited in small animals. Erythromycin has been used in combination with rifampin to treat Rhodococcus equi in horses; a similar application has not been identified in dogs or cats. Synergistic antimicrobial actions also have been reported against P. aeruginosa for either azithromycin or clarithromycin when combined with sulfadiazine/trimethoprim or doxycycline. In humans azithromycin has been combined with antipseudomonadal drugs, particularly for treatment of cystic fibrosis–associated P. aeruginosa infections. This may reflect an apparent immunomodulatory effect of azithromycin or its ability to inhibit adherence of pseudomonad organisms to respiratory epithelium. Less commonly, synergism has been demonstrated for azithromycin when combined with ticarcillin/clavulanic acid, piperacillin/tazobactam, ceftazidime, meropenem, imipenem, ciprofloxacin, travofloxacin, chloramphenicol, or tobramycin.²⁵⁹

Although not included in their spectrum, the macrolides, like clindamycin, impair the ability of P. aeruginosa to adhere to tracheal epithelium, the first step in respiratory tract infection. The effect occurs at least at subinhibitory concentrations and reflects decreased ability to form pili.²⁷³ Decreased adherence to human mucins also has been demonstrated for azithromycin.²⁸³ Other proposed effects of azithromycin include decreased alginate formation and decreased biofilm. Azithromycin has been demonstrated to impede, but not prevent, biofilm formation by Pseudomonas spp.²⁸⁴ These attributes have led to its long-term use for treatment of cystic fibrosis in humans, generally in association with some level of antipseudomonadal antimicrobials. Antiinflammatory effects have also been attributed to azithromycin’s apparent long-term efficacy for treatment of cystic fibrosis.^285,286

Oxazolidinones

Oxazolidinones are a new group of synthetic antimicrobials effective against gram-positive bacteria, including methicillin- and vancomycin-resistant staphylococci, vancomycin-resistant enterococci, penicillin-resistant pneumococci, and anaerobes.²⁸⁸ Linezolid is the first of this class of drugs to be approved for use in the United States (see Figure 7-4). Oxazolidinones inhibit the initiation of protein synthesis by binding at the P site of 50S ribosomal subunit; it also binds to the 70S subunit. Oxazolidinones compete with chloramphenicol and lincomycin for binding of the 50S subunit, which indicates that they have close binding sites, even though oxazolidinones do not inhibit peptidyl transferase as do chloramphenicol and lincomycin. Oxazolidinones may inhibit formation of the ribosomal initiation complex, similar to aminoglycosides. The mechanism is sufficiently different from other 50S binders that resistance to other protein synthesis inhibitors does not cross over to the oxazolidinones. Efficacy against Staphylococcus spp. is characterized by an MIC₉₀ between 1 and 4 μg/mL in humans; methicillin resistance does not appear to affect susceptibility. Linezolid also is effective against enterococci; even intermediate isolates appear to be susceptible at 1 μg/mL.²⁸⁸ Streptococci also are susceptible. Anaerobic activity is comparable to clindamycin.²⁸⁹ Linezolid is effective toward atypical mycobacterium²⁹⁰ and both Actinomyces and Nocardia sp. Activity toward S. pneumoniae is generally bactericidal but bacteriostatic against staphylococci and enterococci. ²⁹¹, ²⁹² Antibacterial effects appear to be time dependent, with efficacy related to AUC/MIC. Resistance thus far is rare.

Disposition includes good oral absorption and good tissue penetration. Linezolid accumulates in bone, lung, vegetations, hematoma, and CSF. Concentrations in sanctuaries are lower than those in plasma.²⁹³ Linezolid has been approved by the FDA for treatment in humans of complicated skin infections or nosocomial pneumonia caused by MRSA, concurrent bacteremia associated with either vancomycin-resistant E. faecium or CA pneumonia caused by penicillin-resistant S. pneumoniae.²⁸⁸ It has become the drug of choice for treatment of resistant gram-positive infections. The oxazolidinones have been minimally used in dogs and cats, and their use is discouraged unless warranted on the basis of C&S testing and until kinetic studies are available in the target species (e.g., cats).

Linezolid PK has been described in the dog after oral and intravenous administration (see Table 7-1).²⁹³ Oral absorption is rapid and complete, allowing intravenous and oral dosing to be the same. The drug is minimally protein bound. Clearance is 2.0 ± 0.3 mL∗min/kg. The drug appears to undergo limited metabolism to inactive metabolites that are extensively enterohepatic recycled. Renal excretion occurs for parent compounds and metabolites. In humans renal disease causes accumulation of metabolites that may contribute to adverse effects.²⁹⁴

Linezolide appears to be well tolerated in humans. Myelosuppression has occurred in humans. Additionally, it is an inhibitor of monoamine oxidases, and care should be taken in patients also receiving serotonergic or adrenergic drugs or dietary supplements. Peripheral neuropathies have been associated with long-term use. Drug interactions involving cytochrome P450 do not appear to occur. Linezolide inhibits mitochondrial protein synthesis, causing hyperlactatemia in humans.²⁹⁴ Linezolid may decrease intracellular movement of aminoglycosides, affecting rapid killing. ⁹⁴ Based on in vitro killing curve studies, linezolid efficacy against MRSA was enhanced most by rifampin, compared with vancomycin or gentamicin; indeed, efficacy of the latter was reduced by linezolid, with activity antagonistic toward gentamicin.

MISCELLANEOUS ANTIBIOTICS

Bacitracin

Bacitracin is a complex polypeptide isolated from B. subtilis. It inhibits peptidoglycan synthesis in bacteria by interfering with the enzyme responsible for movement of cell components through the membrane. Its spectrum of activity includes gram-positive and very few gram-negative organisms. Systemic use causes nephrotoxicity, and use is limited to topical administration. The drug is not absorbed after oral administration and can be used to treat gastrointestinal infections caused by susceptible organisms.^5,297

Polymyxins

Polymyxins are a group of large acetylated decapeptides produced by Bacillus spp. At least six compounds have been identified, of which only two, polymyxin (polymyxin B) and colistin (polymyxin E), are used clinically. Polymyxins are cationic detergents that interact and interfere with the phospholipid of the bacterial cell membrane, resulting in increased permeability. The polymyxins are thus bactericidal. However, a number of compounds can interfere with their activity, including divalent cations, purulent exudate, fatty acids, and quaternary ammonium compounds. The spectrum of activity of the polymyxins includes most gram-negative organisms, including P. aeruginosa. Two exceptions include Proteus spp. and most Serratia spp. The drugs are weak bases (pK_a 8 to 9) and are not orally bioavailable. As such, they have been used to “sterilize” the gastrointestinal tract.

Elimination is principally by way of the kidneys, which are also the primary sites of toxicity. Glomerular and tubular epithelial damage has limited their usefulness. Other side effects include respiratory paralysis (after rapid intravenous administration), CNS dysfunction, fever, and anorexia. Use of the polymyxins is primarily limited to topical administration. However, pemphigus vulgaris has been reported in association with topical use for otitis externa in the dog.²⁹⁶ Polymyxin protects against gram-negative endotoxemia by binding to the anionic lipid component of the lipopolysaccharide at concentrations much lower than those associated with toxicity. Relevance to treatment in dogs or cats is not established.

Novobiocin

Novobiocin is derived from coumarin and is effective against both gram-positive and gram-negative organisms. The drug is particularly efficacious against Staphylococcus spp. Its mechanism of action is not certain but involves both cell membrane and cell wall synthesis. Novobiocin causes a number of toxic effects when used systemically, including bone marrow suppression, nausea, vomiting, and diarrhea. Its use is limited to topical application.^5,297

Mupirocin

Mupirocin (pseudomonic acid) is a naturally occurring fermentation product of Pseudomonas fluorescens. It is available as a cream or ointment, and its use has been largely limited to topical application. Although it acts to inhibit protein synthesis, its mechanism is novel in that it prevents incorporation of isoleucine into proteins by binding to isoleucyl transfer-RNA synthetase.²⁹⁷ Its unique mechanism precludes cross-resistance with other antibacterials. Resistance is unusual, low level, and generally overcome by higher concentrations. The spectrum of mupirocin includes aerobic gram-positive cocci (high efficacy toward S. aureus, S. epidermidis, and beta-hemolytic streptococci) and selected gram-negative cocci. An advantage to mucopirin is that it minimally affects normal flora. Its indications in human medicine include prophylaxis in ulcers, operative wounds, and burns and treatment of skin infections. In humans mupirocin has proved efficacious as an oral antibiotic. In addition, mupirocin has proven useful in the management of secondary pyodermas or superinfection of chronic dermatoses. Mupirocin is generally not associated with side effects; local burning, stinging, itching, or pain has been reported in about 1% of human patients.²⁹⁷

Silver Sulfadiazine

Silver sulfadiazine (see the discussion of sulfonamides) is approved for use in humans in a polypropylene glycol vehicle and in a water-soluble gel. It is approved for use in dogs combined with enrofloxacin as an otic preparation. The synergistic coupling of the silver with sulfadiazine results in efficacy against P. aeruginosa as well as a broad range of gram-positive and other gram-negative organisms. The silver component interferes with the cell wall. Silver sulfadiazine has been approved for use in the treatment of human burn patients, but other antimicrobials have proved more efficacious (e.g., iodophors; combinations of povidone iodine with neomycin, polymyxin, and bacitracin [Neosporin]; and silver sulfadiazine–cerium nitrate cream). However, the low toxicity, low hypersensitivity, and low level of resistance warrants its continued use in veterinary patients.²⁹⁷

Urinary Antiomicrobials