RT Journal Article SR Electronic T1 Bioactivation of the Anticancer Agent CPT-11 to SN-38 by Human Hepatic Microsomal Carboxylesterases and the in VitroAssessment of Potential Drug Interactions JF Drug Metabolism and Disposition JO Drug Metab Dispos FD American Society for Pharmacology and Experimental Therapeutics SP 1157 OP 1164 VO 25 IS 10 A1 J. Greg Slatter A1 Ping Su A1 James P. Sams A1 Larry J. Schaaf A1 Larry C. Wienkers YR 1997 UL http://dmd.aspetjournals.org/content/25/10/1157.abstract AB Human hepatic microsomes were used to investigate the carboxylesterase-mediated bioactivation of CPT-11 to the active metabolite, SN-38. SN-38 formation velocity was determined by HPLC over a concentration range of 0.25–200 μM CPT-11. Biphasic Eadie Hofstee plots were observed in seven donors, suggesting that two isoforms catalyzed the reaction. Analysis by nonlinear least squares regression gave KM estimates of 129–164 μM with a Vmax of 5.3–17 pmol/mg/min for the low affinity isoform. The high affinity isoform hadKM estimates of 1.4–3.9 μM withVmax of 1.2–2.6 pmol/mg/min. The lowKM carboxylesterase may be the main contributor to SN-38 formation at clinically relevant hepatic concentrations of CPT-11. Using standard incubation conditions, the effects of potential inhibitors of carboxylesterase-mediated CPT-11 hydrolysis were evaluated at concentrations ≥ 21 μM. Positive controls bis-nitrophenylphosphate (BNPP) and physostigmine decreased CPT-11 hydrolysis to 1.3–3.3% and 23% of control values, respectively. Caffeine, acetylsalicylic acid, coumarin, cisplatin, ethanol, dexamethasone, 5-fluorouracil, loperamide, and prochlorperazine had no statistically significant effect on CPT-11 hydrolysis. Small decreases were observed with metoclopramide (91% of control), acetaminophen (93% of control), probenecid (87% of control), and fluoride (91% of control). Of the compounds tested above, based on these in vitro data, only the potent inhibitors of carboxylesterase (BNPP, physostigmine) have the potential to inhibit CPT-11 bioactivation if administered concurrently. The carboxylesterase-mediated hydrolysis of α-naphthyl acetate (α-NA) was used to determine whether CPT-11 was an inhibitor of hydrolysis of high turnover substrates of carboxylesterases. Inhibition of α-NA hydrolysis by CPT-11 was determined relative to positive controls BNPP and NaF. Incubation with microsomes pretreated with CPT-11 (80–440 μM) decreased α-naphthol formation to approximately 80% of control at α-NA concentrations of 50–800 μM. The inhibitors BNPP (360 μM) and NaF (500 μM) inhibited α-naphthol formation to 9–10% of control and to 14–20% of control, respectively. Therefore, CPT-11-sensitive carboxylesterase isoforms may account for only 20% of total α-NA hydrolases. Thus, CPT-11 is unlikely to significantly inhibit high turnover, nonselective substrates of carboxylesterases. The American Society for Pharmacology and Experimental Therapeutics