Chemistry

Chloramphenicol is a stable, lipid‐soluble, neutral compound. It is a derivative of dichloracetic acid and contains a nitrobenzene moiety. This para‐nitro group is associated with idiosyncratic (nondose‐dependent) aplastic anemia in humans (Figure 15.1). Thiamphenicol has a similar antibacterial spectrum to chloramphenicol but differs from the parent compound in that the p‐nitro group attached to the benzene ring is replaced by a sulfomethyl group. Florfenicol is a structural analogue of thiamphenicol that also lacks the p‐nitro group, and it is more active than thiamphenicol against bacteria. Neither thiamphenicol nor florfenicol is associated with dose‐independent aplastic anemia in humans or any other species, but both are associated with dose‐dependent bone marrow suppression.

Mechanism of Action

Chloramphenicol is a potent inhibitor of microbial protein synthesis. It binds irreversibly to a receptor site on the 50S subunit of the bacterial ribosome, inhibiting peptidyl transferase and preventing the amino acid transfer to growing peptide chains and subsequently inhibiting protein formation. Chloramphenicol also inhibits mitochondrial protein synthesis in mammalian bone marrow cells in a dose‐dependent manner.

Antimicrobial Activity

Chloramphenicol is active against a wide range of Gram‐positive and many Gram‐negative and anaerobic bacteria. It is bacteriostatic at usual therapeutic concentrations. Chloramphenicol is active against Hemobartonella, Rickettsia, and Chlamydia. While mycoplasma often show susceptibility in vitro, chloramphenicol therapy of mycoplasma pulmonary infections is often ineffective.

• Susceptible organisms (MIC ≤8 μg/ml) are Gram‐positive aerobic bacteria, including Actinomyces spp., Trueperella pyogenes, Bacillus anthracis, Corynebacterium spp., Erysipelothrix rhusiopathiae, Listeria monocytogenes, many Enterococcus spp., Staphylococcus spp., and Streptococcus spp. Methicillin‐resistant Staphylococcus aureus (MRSA) and Staphylococcus pseudintermedius (MRSP) have emerged as significant pathogens in companion animals. Two major clonal MRSP lineages have disseminated in Europe and North America. Isolates originating from North America are often susceptible to chloramphenicol, whereas isolates from Europe are often resistant to chloramphenicol (Perreten et al., 2010). Staphylococcus schleiferi, a coagulase‐negative staphylococcus and emerging zoonotic pathogen isolated from dogs, is typically susceptible (Lee et al., 2019). Typically susceptible Gram‐negative aerobic bacteria include Actinobacillus spp., Bordetella bronchiseptica, Brucella canis, Enterobacterales (including many E. coli), Klebsiella spp., Proteus spp., and Salmonella spp., Haemophilus spp., Histophilus somni, Leptospira spp., Moraxella bovis, Mannheimia haemolytica, and Pasteurella spp. Anaerobes (Bacteroides spp., Clostridium spp., Prevotella spp., Porphyromonas spp.) are commonly susceptible, including penicillin‐resistant Bacteroides fragilis.
  
• Intermediately susceptible organisms (MIC = 16 μg/ml) include Rhodococcus equi.
  
• Resistant organisms (MIC ≥32 μg/ml) include Pseudomonas spp., Mycobacterium spp., and Nocardia spp. Resistance often emerges in Gram‐negative enteric bacteria such as E. coli.

Resistance to Chloramphenicol

The most common mechanism of bacterial resistance to chloramphenicol is enzymatic inactivation by acetylation by chloramphenicol acetyltransferases (CATs). Acetylation of the hydroxyl groups on chloramphenicol prevents drug binding to the 50S ribosomal subunit. There are also reports of other mechanisms of resistance, such as efflux systems, inactivation by phosphotransferases, and mutations of the target site or permeability barriers (Schwarz et al., 2004). Many of the genes coding for the CAT genes or specific transporters are located on mobile genetic elements, such as plasmids, transposons or gene cassettes. The CAT genes are commonly found on plasmids in Enterobacterales and Pasteurellaceae, and most of these plasmids carry one or more additional resistance genes.

The other mechanisms of chloramphenicol resistance include an efflux mechanism due to chloramphenicol/florfenicol exporter ( fexA) and the 23S rRNA methyl transferase (cfr) that also mediates resistance to linezolid. Increasing rates of chloramphenicol resistance in MRSA appear to be due to acquisition of novel MRSA clones carrying a fexA variant associated with florfenicol use in animals (Udo et al., 2021).

Pharmacokinetic Properties

In monogastric animals and preruminant calves, chloramphenicol is well absorbed from the gastrointestinal tract. The oral bioavailability of chloramphenicol in foals is 83%, but only 40% after a single administration in mares, declining to 20% after five doses (Brumbaugh et al., 1983; Gronwall et al., 1986). Chloramphenicol palmitate is poorly absorbed in cats. In ruminants, orally administered chloramphenicol is inactivated in the rumen. The apparent volume of distribution of chloramphenicol is large (>1 l/kg) in all species. This can be attributed to widespread distribution, as partitioning of the drug is independent of pH and there is no evidence of selective tissue binding.

Because of its lipid solubility and moderately low protein binding (30–46%), chloramphenicol attains effective concentrations in most tissues and body fluids, including cerebrospinal fluid (CSF) and the central nervous system. Chloramphenicol may achieve CSF concentrations up to 50% of plasma concentrations when the meninges are normal and more when inflammation is present. Topical ophthalmic formulations achieve therapeutic concentrations in the aqueous humor. Chloramphenicol readily diffuses into milk, and pleural and ascitic fluids. It readily crosses the placenta, achieving concentrations 75% of those in maternal plasma. This may be of clinical significance, as the fetal liver is deficient in glucuronyl transferase activity. Penetration of the blood–prostate barrier is relatively poor unless inflammation is present.

The elimination half‐life of chloramphenicol varies widely between species. Elimination is primarily by hepatic metabolism by conjugation with glucuronic acid. Its elimination is short in horses (one hour) and long in cats (5–6 hours) because of feline deficiencies in glucuronide conjugation. A fraction of the dose is excreted unchanged by glomerular filtration in the urine of dogs (10%) and cats (25%), while a negligible amount is eliminated by renal excretion in herbivores. The hepatic metabolites, which are inactive, are excreted in the urine and to a much lesser extent in the bile. The glucuronide conjugate excreted in bile can be hydrolyzed by intestinal flora to liberate the parent drug.

In newborn animals, the elimination half‐life of chloramphenicol is considerably longer than in adult animals of the same species. This is due mainly to immature glucuronide conjugation mechanisms. Glucuronide conjugation develops most rapidly in foals, so that the half‐life in the one‐week‐old foal approaches that of the adult horse.

Drug Interactions

Chloramphenicol should not be used concurrently with bactericidal antimicrobials in treating infections where host defenses are poor. Concurrent chloramphenicol and penicillin G are antagonistic in treating bacterial meningitis and endocarditis in humans. Chloramphenicol acts on the same ribosomal site as the macrolides. Chloramphenicol is antagonistic to the fluoroquinolones, as inhibition of protein synthesis by chloramphenicol interferes with the production of autolysins necessary for cell lysis after the fluoroquinolone interferes with bacterial DNA supercoiling.

Chloramphenicol inhibits the oxidase activity of cytochrome P450, and decreases the clearance of drugs metabolized through the same pathway, resulting in prolonged pharmacological effect. For example, chloramphenicol markedly prolongs the effect of barbiturates, opioids, xylazine, and propofol.

Toxicity and Adverse Effects

The main toxic effects of chloramphenicol in humans are bone marrow depression, which can be either an idiosyncratic, nondose‐dependent aplastic anemia or a dose‐dependent anemia from suppression of protein synthesis associated with the p‐nitro group (Yunis et al., 1980). Aplastic anemia appears to be a genetically determined idiosyncrasy of individual humans. The incidence of fatal aplastic anemia has been estimated as 1 in every 25 000–60 000 humans who use the drug. A few cases of aplastic anemia in humans have occurred following contact exposure (ophthalmic use, medicated sprays, handling), so veterinarians and owners should wear protective gloves and face masks when handling chloramphenicol products.

A “gray baby” syndrome occurs in newborn infants because their deficiency in glucuronic acid conjugation causes a dose‐dependent anemia. In animals, chloramphenicol toxicity is related to both the dose and duration of treatment, and cats are more likely than dogs to develop toxicity. In cats, clinical signs of toxicity may be seen when the usual maintenance dosage of 25 mg/kg of base or palmitate ester is given twice daily for 21 days (Watson, 1991). Chloramphenicol causes changes in the peripheral blood and bone marrow due to reversible, dose‐related disturbances in red cell maturation. Administration for less than 10 days using the maintenance dose is not likely to cause toxicity in either dogs or cats, unless the animals have depressed hepatic microsomal enzyme activity or severely impaired renal function. Use in dogs is associated with frequent adverse gastrointestinal effects (e.g., vomiting, diarrhea, weight loss, nausea, anorexia, decreased appetite), as well as lethargy, shaking, and hindlimb weakness (especially in large dogs) (Short et al., 2014).

Dosage Considerations

Therapeutic efficacy of chloramphenicol is maximized by maintaining an average steady‐state plasma concentration of 5–10 μg/ml.

Chloramphenicol is available for either oral (free base or palmitate ester) or parenteral (sodium succinate) administration. For local treatment of eye or ear infections caused by susceptible organisms, topical preparations are available.

Because chloramphenicol is well absorbed from the gastrointestinal tract in small animals, it can be given orally as either the base or palmitate ester. The ester is hydrolyzed in the small intestine prior to absorption of the active free base. Subcutaneous injection of chloramphenicol sodium succinate is an alternative to oral administration. While both routes may provide equivalent concentrations, the oral route is preferable as injection of the parenteral preparation is painful. Intramuscular injection provides significantly lower plasma levels than subcutaneous and oral routes, and is not recommended. The total length of treatment should not exceed 10 days, especially in cats. Do not administer chloramphenicol to patients with evidence of bone marrow suppression or renal/hepatic insufficiency.

The short half‐life of chloramphenicol in horses (one hour), together with its generally bacteriostatic action, makes IV administration impractical. Tablets of the free base drug can be administered orally or the sodium succinate formulation can be given by IM injection. After absorption from injection sites, the inactive succinate ester is rapidly hydrolyzed to the active drug.

Because of the risks of idiosyncratic aplastic anemia in humans, chloramphenicol is banned for use in food animals in most countries. The drug should not be used in the early neonatal period unless plasma concentrations are monitored, and should be used with caution in pregnant animals because of the potential adverse effects on the fetus.