Received for publication August 31, 2007.
Revised December 12, 2007.
Accepted for publication January 2, 2008.

Identification of the In Vitro Metabolites of ARQ 501 (-Lapachone) in Whole Blood

Abstract

ARQ 501 (3,4-dihydro-2,2-dimethyl-2H-naphthol[1,2-b] pyran-5,6-dione, -lapachone) showed promising anticancer activity in Phase I clinical trials as monotherapy and in combination with cytotoxic drugs. ARQ 501 is currently in multiple Phase II clinical trials. In vitro incubation in fresh whole blood at 37ºC revealed that ARQ 501 is stable in plasma, but disappears rapidly in whole blood. Our data showed that extensive metabolism in red blood cells (RBCs) was mainly responsible for the rapid disappearance of ARQ 501 in whole blood. By comparison, covalent binding of ARQ 501 and/or its metabolites to whole blood components was a minor contributor to the disappearance of this compound. Sequestration of intact ARQ 501 in RBCs was not observed. Cross-species metabolite profiles from incubating [¹⁴C]-ARQ 501 in freshly drawn blood were characterized using liquid chromatography-mass spectrometry-accurate radioactivity counter. The results show that ARQ 501 was metabolized more rapidly in mouse and rat blood than in dog, monkey and human blood, with qualitatively similar metabolite profiles. Six metabolites were identified in human blood using ultra-high performance liquid chromatography/time-of-flight mass spectrometry, and the postulated structure of five metabolites was confirmed using synthesized authentic standards. We conclude that the primary metabolic pathway of ARQ 501 in human blood involved oxidation of the two adjacent carbonyl groups to produce dicarboxylic and monocarboxylic metabolites, elimination of a carbonyl group to form a ring-contracted metabolite, and lactonization to produce two metabolites with a pyrone ring to form a ring-contracted metabolite. Metabolism by RBCs may play a role in clearance of ARQ 501 from the blood compartment in cancer patients.

Key words: anticancer agents, covalent drug binding, drug development, drug discovery, HPLC, mass spectrometry, metabolite identification, structure elucidation