The Renaissance of Chloramphenicol

Symposium on Progress in Drug Therapy for Children

The Renaissance of Chloramphenicol Adnan S. Dajani, M.D., * and Ralph E. Kauffman, M.D. t

Chloramphenicol was first isolated in 1947 from a soil bacterium, Streptomyces venezuelae. Subsequently, the antibiotic was prepared synthetically and marketed in 1949. Chloramphenicol was the first "broad spectrum" antibiotic, with activity against most gram-positive and many gram-negative bacteria, anaerobic bacteria, and rickettsiae. To date, this broad activity is still retained. Shortly after its introduction, chloramphenicol was incriminated as a cause of serious blood dyscrasias, and probably became better known for its undesirable side effects than for its broad spectrum of activity. In 1958 the unique toxic effect of chloramphenicol on the newborn infant, which came to be known as the "gray baby syndrome," was first recognized, and the use of the antibiotic was curtailed further.3 The introduction of other broad spectrum antibiotics (primarily ampicillin and the aminoglycosides) in the late 1950s and early 1960s provided attractive alternatives to chloramphenicol and reduced its use to a minimum. A resurgence in use of chloramphenicol occurred in the 1970s and can be attributed to several developments. The emergence of ampicillin-resistant Haemophilus injluenzae is unquestionably a major factor in reintroduction of chloramphenicol to treat childhood meningitis and other serious infections caused by this bacterium. Recent interest in and increased awareness of anaerobic infections have also contributed to an increased use of chloramphenicol. Equally important has been the development of better analytical methods to measure the drug, leading to an improved understanding of the metabolism and excretion of chloramphenicol in infants and children. This has allowed the drug to be used effectively and safely for treatment of selected serious infections in all age groups.

ANTIMICROBIAL ACTIVITY Chloramphenicol inhibits bacterial ribosomal protein synthesis, acting primarily on the 50S ribosomes. The drug inhibits the enzyme peptidyltransferase resulting in inhibition of peptide-bond formation. 14 Mammalian • Professor of Pediatrics, Wayne State University School of Medicine, and Director, Division of Infectious Diseases, Children's Hospital of Michigan, Detroit, Michigan t Associate Professor of Pediatrics and Phannacology, Wayne State University School of Medicine, and Associate Director, Division of Clinical Phannacoiogy, Children's Hospital of Michigan, Detroit, Michigan

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mitochondria contain 70S ribosomes which also bind chloramphenicol leading to inhibition of mitochondrial protein synthesis in mammalian cells. Chloramphenicol is a wide-spectrum antimicrobial agent. Although it is classified as a bacteriostatic agent, it may be bactericidal to certain species such as H. injluenzae and Neisseria meningitidis. Table 1 shows the susceptibility of various bacteria to chloramphenicol. This classification is arbitrary, and different strains within a species may have varying susceptibilities to chloramphenicol. It is imperative, therefore, to check in vitro susceptibility of a particular isolate by standardized methods. Although many species are susceptible to chloramphenicol, the use of this antibiotic should be restricted to certain serious infections or conditions (Table 2). Factors that favor the use of chloramphenicol are determined by the following characteristics of the drug: 1. Chloramphenicol diffuses well into all body fluids (cerebrospinal fluid, vitreous humor, joint fluid, and so forth), thus its usefulness in meningitis, ventriculitis, and bacterial ophthalInitis. Concentrations in cerebrospinal fluid average 0.5 to 0.66 of the serum concentration. s 2. Chloramphenicol penetrates leukocytes and tissues, thus its usefulness in typhoid fever and chronic granulomatous disease of childhood. 3. The diffusion of chloramphenicol into the central nervous system tissue is superior to any other antibiotic due to its high lipid solubility. Chloramphenicol levels in brain tissue are approximately nine times the simultaneous serum level. 8 This fact, and the knowledge that anaerobic bacteria are almost universally present in brain abscesses, make chloramphenicol an ideal antibiotic to treat intracranial infections. 4. Resistance to chloramphenicol among gram-positive and gram-negative bacteria is slow to develop.

Table 1. Susceptibility of Various Bacteria to Chloramphenicol VERY SUSCEPTIBLE (MIC

< 4.0 /Lglml)

Groups A and B streptococci C. diphtheriae B. anthracis N. meningitidis N. gonoTThoeae H. inJluenzae B. pertussiS Brucella sp. P. multocida Shigella sp. Y. enterocolitica Peptococci and peptostreptococci C. perfringens Veillonella sp. Fusobacterium fusiforme

SUSCEPTIBLE (MIC

4.0-12.5 /Lglml)

S. aureus S. pneumoniae Alpha hemolytic streptococci L. monocytogenes E. coli K. pneumoniae P. mirabilis Salmonella sp. V. cholerae Ps. pseudomallei Clostridium sp. B·fragilis

MIC = minimum inhibitory concentration.

RELATIVELY RESISTANT (MIC

12.5-25.0 /Lglml)

Group D streptococci Enterobacter sp. S. marcescens

RESISTANT (MIC

> 25.0 /Lglml)

Ps. aeruginosa Proteus (Indole +)

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Table 2. Diseases in which Chloramphenicol Is a First Choice Antibiotic Typhoid fever and other invasive salmonella infections H aemophilus inf/uenzae systemic infections Anaerobic infections: Brain abscess Intra-abdominal abscess and bowel perforation Ventriculitis and meningitis Bacterial ophthalmitis Chronic granulomatous disease Rickettsial infections

Resistance to chloramphenicol is primarily due to the presence of an inactivating enzyme, chloramphenicol acetyltransferase. The production of this enzyme is plasmid (R factor) mediated.1 6 Whereas other bacterial enzymes may modify chloramphenicol chemically, such enzymes do not seem to be responsible for resistance. Nonenzymatic resistance (permeability barrier to the antibiotic or step-wise mutation in the bacterial 50S ribosome) is rare. Several new {:l-lactam antibiotics now in clinical trials are effective in vitro against {:l-lactamase-positive Haemophilus injluenzae and also appear to diffuse well into cerebrospinal fluid. These drugs may offer attractive alternatives to chloramphenicol in the future for treatment of H. injluenzae infections because of their reduced risk of serious toxicity.

TOXICITY In spite of the efficacy of chloramphenicol against a wide variety of microorganisms, its use is limited due to potential toxicity. The serious potential toxic effects of chloramphenicol include the "gray syndrome," reversible bone marrow suppression, and "idiosyncratic" aplastic anemia. With the exception of aplastic anemia, the risk of adverse effects may be minimized by careful control of serum concentrations since such effects are dose related and reversible. Most reported cases of the gray baby syndrome involved infants who were receiving chloramphenicol in doses of 100 to 200 mg per kg per day and had serum concentrations between 70 j.tg per ml and 250 j.tg per m!, 10 times the concentrations necessary to treat susceptible infections.10 Symptoms in affected babies usually developed within 60 to 72 hours after initiation of chloramphenicol therapy and consisted of abdominal distention, pallid cyanosis, and vasomotor collapse progressing to death within a few hours. The mechanism of this toxic syndrome may be related to the ability of chloramphenicol to disrupt energy metabolism at the cellular level. Chloramphenicol has been shown to inhibit mitochondrial electron transport and reduce oxygen consumption at concentrations in excess of 63 j.tg per m!, 4.6 consistent with concentrations at which the gray baby syndrome has been reported. The gray syndrome has also been reported in older children and adults who were receiving excessive doses. This toxic effect does not occur when such excessively high concentrations are avoided. Suppression of erythropoiesis predictably occurs if excessive concentrations of chloramphenicol are maintained for a sufficient length of time. 12 This toxic

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effect is dose related and usually reversible. The ability of chloramphenicol to reversibly bind 70S ribosomes in mammalian cells at concentrations as low as 10 p.g per ml, therefore inhibiting mitochondrial protein synthesis, may be related to this dose-dependent bone marrow tOxicity.18 Bone marrow suppression tends to occur with increasing frequency when peak chloramphenicol concentrations consistently exceed 25 p.g per ml or concentrations at 6 hours after a dose are above 15 p.g per ml.1 1 Overt signs of bone marrow toxicity usually are not evident until four to five days after initiating therapy. Early signs reflect arrest of erythropoiesis and include reticulocytopenia, increased serum iron, increased free erythrocyte protoporphyrin, and eventually a decrease in erythrocyte count. If chloramphenicol administration is continued at sufficient doses, thrombocytopenia and neutropenia may occur within two to three weeks. The risk of dose-related bone marrow toxicity is minimized by maintaining the serum chloramphenicol concentration below 25 p.g per ml and limiting the duration of therapy to the minimum required for adequate treatment. Although numerous theories have been advanced, the mechanism of idiosyncratic aplastic anemia associated with chloramphenicol administration is unknown. This toxic reaction is not dose related and typically occurs after therapy is discontinued rather than while receiving chloramphenicol. Fortunately, this potentially lethal reaction is rare. The estimated incidence varies between .002 and .004 per cent,17 which is considerably less than the incidence of anaphylactic reactions to penicillin, .02 to .11 per cent. 15 All blood cell lines are affected. Pancytopenia with hypocellular bone marrow may occur and is frequently irreversible.

METABOLISM AND ELIMINATION The metabolism and elimination of chloramphenicol vary widely among patients (Fig. 1). This variability has profound implications for the safe and effective use of the drug. Chloramphenicol is very lipid soluble, but is only sparingly soluble in water. Therefore, aqueous solubility for preparation of liquid dosage forms must be achieved by attaching a polar group to the chloramphenicol molecule by an ester linkage. The succinate ester is provided for intravenous use and the palmitate ester for liquid oral preparations. Both esters are biologically inactive and must be hydrolyzed following administration to release the active compound. Chloramphenicol palmitate is readily hydrolyzed in the proximal small bowel by pancreatic lipases, thereby releasing free chloramphenicol which is then absorbed as the active compound. The solubility characteristics of the drug result in its rapid and efficient absorption from the gastrointestinal tract when it is administered orally as the crystalline form (in capsules). Chloramphenicol succinate is hydrolyzed in vivo following intravenous administration. The mechanism by which hydrolYSiS occurs is not clear. In a recent study the disappearance of chloramphenicol succinate from serum of infants and children was found to be highly variable and unpredictable with the inactive ester perSisting in the serum of some patients for up to six hours following a dose. 7 In 14 infants under one month of age, a mean of 37 per cent of the total circulating chloramphenicol was in the esterified form at peak chloram-

199

THE RENAISSANCE OF CHLORAMPHENICOL

"Ec c:

~ 0

:H ~

Chloramphenicol succinate

Crystalline chloramphenicol

Chloramphenicol palmitate

( Parenteral)

(Capsules)

(Suspension)

c: 0

~

;;::: ~

.Q :::3 ~

c

II)

F

01

"3 E .c .Q

~ Esteroses (?)

I I.~J

Chloramphenicol (serum)

Liposes

Glomerular filtration

Urine .......___li_ub_u_la~r_ secretion

Chloramphenicol monoglucuronide

o

o Figure 1.

Active Inactive

Metabolism of chlorampheniCOl.

phenicol concentrations, and a mean of 11 per cent remained unhydrolyzed at six hours after administration. In contrast, chloramphenicol succinate comprised a mean of 25 per cent of total chloramphenicol at peak chloramphenicol concentrations and 1.4 per cent at six hours in 11 children one to 16 years of age. Therefore, infants during the first month of life appear to hydrolyze the succinate ester less readily than older infants and children. The succinate ester is also subject to renal elimination because of its high water solubility and dissociation as a weak acid. In 15 children receiving intravenous chloramphenicol, an average of 33 per cent, with a range of 6 to 80 per cent, of the administered dose was recovered in the urine as unhydrolyzed ester. 7 The renal clearance of chloramphenicol succinate in these patients was four times their concomitant creatinine clearance, indicating the ester is actively secreted by the renal tubules. The variation in rate of in vivo hydrolysis and variable renal elimination of chloramphenicol succinate markedly influence the serum concentrations of active chloramphenicol which are achieved following intravenous administration. While the succinate ester persists in the body, it serves as a "pro drug" reservoir, continually releasing active chloramphenicol. This results in lower and later peak levels of the drug and accounts, in part, for the wide variation in calculated apparent half-lives. Furthermore, in contrast to free chloramphenicol, little, if any, of the succinate ester diffuses into cerebrospinal fluid. We observed apparent half-lives ranging from 1. 7 to 12.0 hours with a mean of 5.1 hours in 26 of 41 patients in whom the pharmacokinetics of chloramphenicol were studied. This mean and range of apparent half-lives are consistent

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with those reported by others.5 There is a tendency for apparent half-lives to be more variable and more prolonged in newborn infants compared with older infants and children. In 15 of the 41 patients we studied, the serum concentration of chloramphenicol remained essentially unchanged between doses resulting in indeterminant apparent half-lives. This phenomenon occurred with significantly greater frequency in young infants. Indeterminant apparent half-lives were observed in 73 per cent of infants under one month of age compared with 26 per cent of infants one to 12 months and 18 per cent of children older than one year. This reflects a decreased ability in the younger infants to hydrolyze the ester. The renal elimination of unhydrolyzed chloramphenicol succinate results in an unusual bioavailability problem since that portion of the dose excreted as the ester is never available in active form. The fraction of the dose lost by this route is so variable that it is impossible to compensate by increasing the dose by some predetermined percentage. Nonesterified, active chloramphenicol is predominantly eliminated as the inactive, water soluble glucuronide which is formed in the liver and then excreted in the urine. Eighty-five to 90 per cent of a dose is excreted as the glucuronide, with 10 to 15 per cent being eliminated as chloramphenicol base and small amounts of minor metabolites. 7 Interpatient variability in metabolism and excretion contributes to the wide variation in apparent half-life of chloramphenicol. Impaired liver function may potentially diminish the ability to conjugate chloramphenicol resulting in increased risk of toxicity if the dose is not reduced. Renal failure may result in accumulation of the glucuronide conjugate, but neither does this increase the risk of toxicity nor does it usually require dosage adjustment.

DOSAGE The range of "therapeutic" concentrations of chloramphenicol defined by the sensitivity of susceptible organisms and the dose-dependent toxiCity of the drug is comparatively narrow. The minimal inhibitory concentrations for most organisms susceptible to chloramphenicol are below 12.5 fJ-g per ml (see Table 1), whereas the risk of dose-dependent toxicity is minimal at serum concentrations below 25 fJ-g per ml. It follows, therefore, that a dose providing chloramphenicol concentrations between 10 and 25 fJ-g per ml would be generally effective and acceptably safe for treatment of serious infections. Theoretically a dose could be calculated which would provide concentrations within this "therapeutic" range for most patients. However, this is not the case owing to the wide variability in metabolism and excretion of the drug. Current official dosing recommendations call for 25 mg per kg per day for premature infants or term infants under two weeks of age and 50 mg per kg per day for older term infants and children. Doses up to 100 mg per kg per day are allowed for infants greater than two weeks of age and for children when required for the treatment of severe infections, with the recommendation to decrease the dose as soon as possible. The total daily dose is divided into four doses administered at six-hour intervals. Unfortunately, these doses have been shown to result in serum concentrations outside the therapeutic range in a

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majority of patients. Lietman 9 found that only 33 per cent of 107 neonatal patients had serum concentrations of chloramphenicol between 10 and 20 f..tg per ml while receiving recommended doses. Fifty per cent had concentrations above 20 f..tg per ml and 11 per cent had concentrations below 10 f..tg per mI. Our experience has been consistent with Lietman's. Only 47 per cent of our patients had peak serum concentrations between 10 and 25 f..tg per ml. Seven of 42 patients had concentrations in excess of 25 f..tg per mI. Furthermore, we found no statistical correlation between chloramphenicol dose and peak serum concentration. The narrow therapeutic range of chloramphenicol concentrations and the lack of correlation between dose and serum concentration have led to the recommendation that serum concentrations be monitored during therapy, if at all possible.2.5·7.9 This is the only way the physician can be reasonably assured that the patient is receiving an adequate but safe dose. It is our practice to initiate chloramphenicol therapy at the appropriate recommended dose and check the gO-minute and six-hour post-dose serum concentrations at some point during the first 36 hours of therapy. Dosage adjustments and further monitoring are contingent on the results of the initial serum concentrations. When monitoring chloramphenicol concentrations, the physician must be aware of the analytical method utilized since different methods measure different compounds. Older colorimetric procedures are unsatisfactory because they measure chloramphenicol plus chloramphenicol succinate and the inactive metabolites, leading to erroneously high estimates of available active chloramphenicol. Radioenzymatic lO and high pressure liqUid chromatographic 13 assays are now available which are rapid, require small serum volumes (10 to 50 f..t 1), and are specific for active chloramphenicol. These are the methods of choice. Recently a high pressure liquid chromatographic method has been developed with which chloramphenicol succinate is quantitated Simultaneously with active chloramphenicol. 1 This provides information regarding hydrolysis of the succinate ester which is helpful when the serum concentration of active chloramphenicol is inappropriately low for the administered dose. The patient should be carefully monitored for signs of hematopoietic toxicity during therapy, including a complete blood count with reticulocyte count before initiating treatment and at least twice weekly. Decrease in erythrocytes, moderate anemia, or moderate thrombocytopenia are not, in themselves, indications for discontinuing chloramphenicol. However, the drug should be discontinued if neutropenia develops.

CONCLUSIONS Chloramphenicol is a broad-spectrum antibiotic, but its use has been limited because of potential toxicity. Emergence of ampicillin-resistant Haemophilus injluenzae isolates, increased interest in anaerobic infections, and the recent development of improved analytical methods to measure the drug have all contributed to the resurgence in the use of chloramphenicol. The metabolism and elimination of chloramphenicol vary widely among patients, and the drug has a relatively narrow therapeutic range. Therefore, it is desirable to monitor peak and trough chloramphenicol levels early in the course

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of therapy. Peak concentrations between 10 and 25 JLg per ml are generally effective and acceptably safe for the treatment of most infections. Patients receiving chloramphenicol should be carefully monitored for signs of hematopoietic toxicity during therapy. ACKNOWLEDGMENT

We appreciate the secretarial assistance of Deborah Riley.

REFERENCES 1. Aravind, M. K., Miceli, J N., Kauffman, R. E., et al.: Simultaneous measurements of chloramphenicol and chloramphenicol succinate in body fluids utilizing HPLC. J Chrom., 221:176-181, 1980. 2. Black, S. B., Levine, P., and Shinefield, H. R.: The necessity for monitoring chloramphenicol levels when treating neonatal meningitis. J Pediatr., 92:235-236, 1978. 3. Burns, L. E., Hodgman, J E., and Cass, A. B: Fatal circulatory collapse in premature infants receiving chloramphenicol. N. Engl. J Med., 261: 1318-1321, 1959. 4. Freeman, K. B., and Haldar, D: The inhibition of mammalian NADH oxidation by chloramphenicol and its isomers and analogues. Canad. J Biochem., 46:1003-1008,1968. 5. Friedman, C. A., Lovejoy, F. L., and Smith, A. L: Chloramphenicol disposition in infants and children. J Pediatr., 95: 1071-1078, 1979. 6. Hallman, M: Oxygen uptake in neonatal rats: A developmental study with particular reference to the effects of chloramphenicol. Pediatr. Res., 7:923-930, 1973. 7. Kauffman, R. E., Miceli, J N., Strebel, L., et al.: Pharmocokinetics of chloramphenicol and chloramphenicol succinate in infants and children. J Pediatr., in press. 8. Kramer, P. W., Griffith, R. S., and Campbell, R. L: Antibiotic penetration of the brain: A comparative study. J Neurosurg., 31:295-302,1969. 9. Lietrnan, P. S: Chloramphenicol and the neonate-1979 view. Clin. Perinatol., 6:151-162, 1979. 10. Lietman, P. S., White, T. 1.. and Shaw, W. V: Chloramphenicol: An enzymological microassay. Antimicrob. Agents Chemother., 10:347-353, 1976. 11. O'Gorman Hughes, D. W: Studies on chloramphenicol. II. Possible determinants and progress of haemopoietic toxicity during chloramphenicol therapy. Med. J Aust., 2: 1142-1146, 1973. 12. Oski, F. A: HematologiC consequences of chloramphenicol therapy. J Pediatr., 94:515-516, 1979. 13. Petersdorf, S. H., Raisys, V. A., and Opheim, K E: Micro-scale method for liqUid-chromatographic determination of chloramphenicol in serum. Clin. Chern., 25: 1300-1302, 1979. 14. Pongs, 0., Bald, R., and Erdmann, V. A.: Identification of chloramphenicol-binding protein in Escherichia coli ribosomes by affinity labeling. Proc. Nat. Acad. Sci. USA, 70:2229-2233, 1973. 15. Rudolph, A. E., and Price, E. V: Penicillin reactions among patients in venereal disease clinics. A national survey. JA.M.A., 223:499-501, 1973. 16. Suzuki, Y., and Okamoto, S: The enzymatic acetylation of chloramphenicol by the multiple drug resistant Escherichia coli carrying R factor. J BioI. Chern., 242:4722-4730, 1967. 17. Wallerstein, R. 0., Condit, P. K, Kasper, C. K, et al.: Statewide study of chloramphenicol treatment and fatal aplastic anemia. JA.M.A. 208:2045-2051, 1969. 18. Yunis, A. A: Chloramphenicol-induced bone marrow suppression. Semin. Hematol., 10:225-234, 1973. Children's Hospital of Michigan 3901 Beaubien Street Detroit, Michigan 48201