P48 Rat Phenotypic Evaluation of Several Human Cytochrome P450 3A4 Substrates

Jenny Wen , Dart Neuroscience LLC, San Diego, CA
Jane Angeles , Dart Neuroscience LLC, San Diego, CA
Anne Danks , Dart Neuroscience LLC, San Diego, CA
Kathe Stauber , Dart Neuroscience LLC, San Diego, CA
Human cytochrome P-450 (CYP) 3A4 substrates could be expected to be primarily metabolized by gender-specific rat CYP3A family and thus could show a gender difference in pharmacokinetics (PK) where plasma concentrations in male rats are lower than those of females (Wahajuddin, Sheelendra Pratap Singh, and Girish Kumar Jain, Gender differences in pharmacokinetics of lumefantrine and its metabolite desbutyl-lumefantrine in rats, Biopharmaceutics & Drug Disposition, 2012, 33(4):229-234). Here we have identified four compounds (compounds A, B, C, and D) that are exclusively (> 90%) metabolized by CYP3A4 but they vary in the degree of gender differences in rat PK. Studies using individual rat recombinant CYP isoforms showed that although these compounds were substrates for rat CYP3A, other rat CYP isoforms also demonstrated affinity for each of these compounds.

The rat CYP families, CYP2A, CYP3A and CYP2C, are expressed in a gender-specific manner. Since CYP2A2, CYP2C11, CYP2C13 and CYP3A2 are male-specific enzymes, the relative contribution of these isoforms towards the metabolism of a particular compound could modulate the pharmacokinetic differences between male and female rats. Compound A demonstrated a 4-fold lower exposure in males compared to females. Phenotypic analysis showed that the rat recombinant isoforms that highly turned over Compound A in vitro were male-specific (CYP3A2 and CYP2C11). Compound B and Compound C also demonstrated lower plasma exposures in males, however, the difference was only 2-fold. Similar phenotypic screening revealed that while CYP3A2 was the major isoform involved in the metabolism of these two compounds, there was a significant contribution of isoforms that were gender-independent, thus attenuating the gender difference in PK. Finally, Compound D while showing affinity for CYP3A2, its metabolic clearance was mediated mainly by CYP1A1. Since CYP1A1 is not a gender specific enzyme in rats, no noticeable gender difference in PK was observed for compound D.

In conclusion, we observed that four exclusive CYP3A4 substrates were also metabolized by the gender-dependent rat CYP3A2 family. However, while each CYP3A4 substrate showed turnover by rat CYP3A2, it was also metabolized by other CYP isoforms in the rat. The relative contribution of CYP3A2 and other male specific CYP isoforms resulted in varying gender difference in pharmacokinetics of each compound. The use of in vitro rat CYP phenotypic screening studies could assist in predicting the potential for a PK gender difference in the rat.