Investigation of the in vitro metabolism profile of a phosphodiesterase-IV inhibitor, CDP-840: leading to structural optimization.

CDP-840 is a selective and potent phosphodiesterase type IV inhibitor, whose in vitro metabolism profile was first investigated using liver microsomes from different species. At least 10 phase I oxidative metabolites (M1-M10) were detected in the microsomal incubations and characterized by capillary high-performance liquid chromatography continuous-flow liquid secondary ion mass spectrometry (CF-LSIMS). Significant differences in the microsomal metabolism of CDP-840 were found between rat and other species. The major route of metabolism in rat involved para-hydroxylation on the R4 phenyl. This pathway was not observed in human and several other species. The in vitro metabolism profile of CDP-840 was further examined using freshly isolated hepatocytes from rat, rabbit, and human. The hepatocyte incubations indicated more extensive metabolism relative to that in microsomes. In addition to the phase I oxidative metabolites observed in microsomal incubations, several phase II conjugates were identified and characterized by CF-LSIMS. Interspecies differences in phase II metabolism were also found in these hepatocyte incubations. The major metabolite in human hepatocytes was identified as the pyridinium glucuronide, which was not detected in rat hepatocytes. Simple structural modification on R4, such as p-Cl substitution, greatly reduced the species differences in microsomal metabolism. Furthermore, modifications on R3, such as the N-oxide, eliminated the N-glucuronide formation in human. These results not only helped in determining the suitability of animal species used in the preclinical safety studies but also provided valuable directions for the synthetic efforts in finding backup compounds that are more metabolically stable.