Ortho-Tri-Cyclen® (OTC), a two drug cocktail comprising of ethinylestradiol (EE) and norgestimate (13-ethyl-17-acetoxy-18, 19-dinor-17α-pregn-4-en-20yn-3 oxime), is commonly prescribed to avert unwanted pregnancies in women of reproductive age. In vivo, norgestimate undergoes extensive and rapid deacetylation to produce 17-deacetylnorgestimate (NGMN), an active circulating metabolite that likely contributes significantly to norgestimate efficacy. Despite being of primary significance, the metabolism and reaction phenotyping of NGMN have not been previously reported. Hence, detailed biotransformation and reaction phenotyping studies of NGMN with recombinant cytochrome P450s (rCYPs), recombinant uridine 5'-diphospho-glucuronosyltransferases (rUGTs) and human liver microsomes (HLM) in the presence and absence of selective cytochrome P450 (CYP) inhibitors were conducted. It was found that cytochrome P450 3A4 (CYP3A4) plays a key role in NGNM metabolism with a fraction metabolized (fm) of 0.57. CYP2B6 and to an even lesser extent CYP2C9 were also observed to catalyze NGMN metabolism. Using this CYP3A4 fm, the predicted area under the plasma concentration vs. time curve (AUC) change in NGMN using a basic/mechanistic static model was found to be within 1.3-fold of reported NGMN AUC changes for four modulators of CYP3A4. In addition to NGMN, we have also elucidated the biotransformation of norgestrel (NG), a downstream norgestimate and NGMN metabolite, and found that CYP3A4 and UGT1A1 have a major contribution to the elimination of NG with a combined fm of 1. The data presented in this manuscript will lead to a better understanding and management of NGMN based drug-drug interactions (DDIs) when norgestimate is co-administered with CYP3A4 modulators.
- The American Society for Pharmacology and Experimental Therapeutics