Chloramphenicol palmitate

The absorption and disposition of orally administered chloramphenicol palmitate (chloramphenicol-P) was studied in seven neonates (four preterm, three term). The highest measured chloramphenicol serum concentrations occurred greater than or equal to 4 hours after the dose, and ranged from 5.5 to 23 micrograms/ml after doses of chloramphenicol-P 50 mg/kg/day orally. The dosage had to be increased in all preterm neonates from 25 mg/kg/day to 50 mg/kg/day to obtain adequate serum levels during therapy. In four neonates the apparent half-life could not be estimated, because there was no decline in serum concentrations. The apparent half-life was 3 and 6 hours, respectively, in two neonates in whom the serum concentration declined during the dosing interval. Urinary excretion of chloramphenicol and the glucoronide ester in three neonates varied from 24% to 55% of the total dose administered. These preliminary data suggest considerable variability in serum chloramphenicol levels when chloramphenicol-P is administered orally in neonates. The delay in achieving the maximum serum concentration, nondeclining serum curve, and low renal recovery is indicative of incomplete, prolonged, and erratic absorption, possibly related to delayed gastric emptying or decreased intraluminal hydrolysis of the palmitate ester.

The relative bioavailability of intravenously administered chloramphenicol succinate and orally administered chloramphenicol palmitate was compared in 18 children, age 2 months to 14 years. The area under the serum concentration vs time curve of chloramphenicol and urinary excretion of chloramphenicol succinate were determined in each child under steady-state conditions while receiving chloramphenicol succinate and again while receiving chloramphenicol palmitate. The mean AUC was significantly greater during oral therapy compared to intravenous therapy (110 vs 78 mg hr/L, P less than 0.001). The relative bioavailability of chloramphenicol succinate was 70% compared to chloramphenicol palmitate. This could be explained by the mean loss of 36% of the intravenous dose in the urine as unhydrolyzed chloramphenicol succinate. The intravenous dose of chloramphenicol succinate did not correlate with AUC (r = 0.193). However, there was a significant correlation between the oral dose of chloramphenicol palmitate and AUC (r = 0.429, P = 0.025). The bioavailability of orally administered chloramphenicol palmitate is superior to that of chloramphenicol succinate given intravenously. Furthermore, there is a greater correlation between dose and amount of active drug in the body when the oral preparation is used. Oral administration of chloramphenicol palmitate appears to offer significant therapeutic advantages in patients who can tolerate medication given orally.

In recent years there has been a renewal of interest in chloramphenicol, predominantly because of the emergence of ampicillin-resistant Haemophilus influenzae, the leading cause of bacterial meningitis in infants and children. Three preparations of chloramphenicol are most commonly used in clinical practice: a crystalline powder for oral administration, a palmitate ester for oral administration as a suspension, and a succinate ester for parenteral administration. Both esters are inactive, requiring hydrolysis to chloramphenicol for anti-bacterial activity. The palmitate ester is hydrolysed in the small intestine to active chloramphenicol prior to absorption. Chloramphenicol succinate acts as a prodrug, being converted to active chloramphenicol while it is circulating in the body. Various assays have been developed to determine the concentration of chloramphenicol in biological fluids. Of these, high-performance liquid chromatographic and radioenzymatic assays are accurate, precise, specific, and have excellent sensitivities for chloramphenicol. They are rapid and have made therapeutic drug monitoring practical for chloramphenicol. The bioavailability of oral crystalline chloramphenicol and chloramphenicol palmitate is approximately 80%. The time for peak plasma concentrations is dependent on particle size and correlates with in vitro dissolution and deaggregation rates. The bioavailability of chloramphenicol after intravenous administration of the succinate ester averages approximately 70%, but the range is quite variable. Incomplete bioavailability is the result of renal excretion of unchanged chloramphenicol succinate prior to it being hydrolysed to active chloramphenicol. Plasma protein binding of chloramphenicol is approximately 60% in healthy adults. The drug is extensively distributed to many tissues and body fluids, including cerebrospinal fluid and breast milk, and it crosses the placenta. Reported mean values for the apparent volume of distribution range from 0.6 to 1.0 L/kg. Most of a chloramphenicol dose is metabolised by the liver to inactive products, the chief metabolite being a glucuronide conjugate; only 5 to 15% of chloramphenicol is excreted unchanged in the urine. The elimination half-life is approximately 4 hours. Inaccurate determinations of the pharmacokinetic parameters may result by incorrectly assuming rapid and complete hydrolysis of chloramphenicol succinate. The pharmacokinetics of chloramphenicol succinate have been described by a 2-compartment model. The reported values for the apparent volume of distribution range from 0.2 to 3.1 L/kg.(ABSTRACT TRUNCATED AT 400 WORDS)