Abstract
Identification of the human cytochrome P450 (P450) enzymes involved in the metabolism of cisapride and racemic norcisapride [(±)-norcisapride] was investigated at 0.1 and 1 μM, concentrations that span the mean plasmaCmax for cisapride. Formation of norcisapride (Nor), 3-fluoro-4-hydroxycisapride (3F), and 4-fluoro-2-hydroxycisapride (4F) from cisapride and an uncharacterized metabolite (UNK) from (±)-norcisapride in human liver microsomes (HLMs) were consistent with Michaelis-Menten kinetics for a single enzyme (Km, 6.0, 14.3, 13.9, and 107 μM;Vmax, 1350, 696, 568, and 25 pmol/mg of protein, respectively). HLMs converted cisapride to Nor at rates that were at least 3 orders of magnitude greater than those observed for (±)-norcisapride conversion to UNK. The sample-to-sample variation in the rates of Nor, 3F, 4F, and UNK formation correlated strongly (r2 > 0.796) with CYP3A4/5 activity in a panel of HLMs (n = 7) and was markedly reduced by ketoconazole, a potent CYP3A inhibitor. Ketoconazole virtually eliminated (±)-norcisapride conversion to UNK (94 ± 0.5%). Studies with 10 cDNA-expressed enzymes revealed that CYP3A4 catalyzed the formation of Nor and 4F at rates >100 times those of non-CYP3A enzymes and >100- and 50-fold higher than CYP3A5 and CYP3A7, respectively. CYP3A4 was the only P450 capable of UNK formation. Therefore, CYP3A4 is the principal P450 enzyme responsible for the conversion of cisapride to Nor, 3F, and 4F and of (±)-norcisapride to UNK. Compared with cisapride, factors related to CYP3A4-mediated (±)-norcisapride metabolism (e.g., ontogeny of drug-metabolizing enzymes, inhibition, and induction) should be clinically unimportant due to the apparent lack of dependence on cytochromes P450 for elimination.
Footnotes
-
Supported in part by Grant 1U01HD31313-08 (G.L.K.), from the Network of Pediatric Pharmacology Research Units, National Institute of Child Health and Human Development, Bethesda, MD.
- Abbreviations used are::
- Nor
- norcisapride
- 3F
- 3-fluoro-4-hydroxycisapride
- 4F
- 4-fluoro-2-hydroxycisapride
- P450
- cytochrome P450
- HPLC
- high-pressure liquid chromatography
- MS
- mass spectrometry
- UNK
- unidentified metabolite of (±)-norcisapride
- CLint
- intrinsic clearance
- Received May 8, 2001.
- Accepted July 19, 2001.
- The American Society for Pharmacology and Experimental Therapeutics
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|