IDENTIFICATION OF A NEW METABOLITE OF CPT-11 (IRINOTECAN) - PHARMACOLOGICAL PROPERTIES AND ACTIVATION TO SN-38

Irinotecan, or CPT-II iperidino)-1-piperidino]carbonyloxycamptothecine ), is a water-soluble derivative of camptothecine with promising activ ity against several types of malignancies. In addition to 7-ethyl-10-h ydroxycamptothecine (SN-38), its active metabolite, we were able to id entify several metabolites in the plasma of patients treated with this drug, especially an oxidative metabolite, 7-ethyl-10[4-N-(5-aminopent anoic acid)-1-piperidino] carbonyloxy-camptothecine. During our study of the biosynthesis of 7-ethyl-10[4-N-(5-aminopentanoic acid)-1-piperi dino] carbonyloxy-camptothecine from CPT-11 by human liver microsomes, we were able to detect another quantitatively important polar metabol ite, which was also present in the plasma and urine of patients treate d with CPT-II. On the basis of preliminary experiments, the structure of this compound was postulated to be -10(4-amino-1-piperidino)carbony loxycamptothecine, and this structure was synthesized by Rhone-Poulenc Rorer. Urine samples and human liver microsomal extracts were studied by high-performance liquid chromatography/atmospheric pressure chemic al ionization/tandem mass spectrometry to identify its structure forma lly. The identification of the metabolite was supported by identical r etention time, mass-to-charge ratio and tandem mass spectrometry fragm entation as a synthetic standard. Like irinotecan, 7-ethyl-10-(4-amino -1-piperidino) carbonyloxycamptothecine was a weak inhibitor of cell g rowth of P388 cells in culture (IC50 = 3.4 mu g/ml vs. 2.8 mu g/ml for irinotecan and 0.001 mu g/ml for SN-38). It was also a poor inducer o f topoisomerase I-DNA cleavable complexes (100-fold less potent than S N-38). However, unlike 7-ethyl-10[4-N-(5-aminopentanoic acid)-1-piperi dino] carbonyloxy-camptothecine, this new metabolite could be hydrolyz ed to SN-38 by human liver microsomes and purified human liver carboxy lesterase, though to a lesser extent than irinotecan. This compound ca n therefore contribute to the activity and toxicity profile of irinote can in vivo.