CHLOROAMPHENICOL OXAMYLETHANOLAMINE AS AN END-PRODUCT OF CHLORAMPHENICOL METABOLISM IN RAT AND HUMANS - EVIDENCE FOR THE FORMATION OF A PHOSPHOLIPID ADDUCT

Chloramphenicol (CP) has been implicated as, although not proven to be , a causative agent of aplastic anemia in humans. Recent studies from our laboratory have presented evidence that CP-oxamylethanolamine was an end product of CP biotransformation in birds. Because this novel me tabolic pathway has never been reported in other species, we have now expanded these investigations to rat and humans. [H-3]CP was administe red po (10 mg/kg) to adult male Wistar rats and to a human volunteer. Urine was collected and analyzed by HPLC and GC-MS for CP metabolite d etermination. In rat, the two most important metabolites in 0-24 h uri ne were CP-base and CP-acetylarylamine which together accounted for ab out 50% of the ingested radioactivity. The remainder was due to unchan ged CP, CP-oxamic acid, CP-alcohol, CP-glucuronide, and CP-oxamylethan olamine. The presence of these end products was also demonstrated in m an. CP-oxamylethanolamine represented 0.74% and 1.37% of the ingested radioactivity in rat and human urine samples, respectively. CP-oxamyle thanolamine formation was confirmed in vitro with isolated rat hepatoc ytes, suggesting the involvement of liver in the production of this me tabolite. The origin of CP-oxamylethanolamine has been investigated wi th the use of hepatic liver microsomes from phenobarbital-treated rats . The incubation of [H-3]CP with this subcellular fraction led to the binding of a radiolabeled compound to the microsomal lipids, whereas n o binding occurred when CP-oxamic acid was incubated with the microsom es. Enzymatic hydrolysis of the microsome lipid fraction with phosphol ipase D from Streptomyces chromofuscus released CP-oxamylethanolamine. These data are consistent with a mechanism that involves an initial a ctivation of CP to CP-acyl chloride by cytochrome P450 dependent monoo xygenases, a subsequent covalent binding of this reactive intermediate metabolite with phosphatidylethanolamine present in the microsomal me mbrane, followed by the breakdown of the phospholipid adduct and the e limination of CP-oxamylethanolamine in urine.