Pharmacokinetics of three bioequivalent norethindrone/mestranol-50 micrograms and three norethindrone/ethinyl estradiol-35 micrograms OC formulations: are "low-dose" pills really lower?

We have examined the pharmacokinetic parameters derived from the analysis of plasma ethinyl estradiol (EE) and norethindrone levels after administration of a single dose of three bioequivalent norethindrone-1mg/mestranol (ME)-50 micrograms formulations (Ortho-NovumR 1/50, NorinylR 1/50 and Norcept-MR 1/50) and three norethindrone-1mg/ethinyl estradiol-35 micrograms formulations (Ortho-Novum 1/35R, NorinylR 1/35, Norcept-ER 1/35) in a randomized crossover design involving 24 women for the 35 micrograms and 27 women for the 50 micrograms agents. Differences between the AUC-EE of pairs from the same manufacturer (1 + 35 and 1 + 50) were not significantly different, indicating that 50 micrograms of mestranol was equivalent to 35 micrograms ethinyl estradiol with respect to this pharmacokinetic parameter. The Cmax values were also similar. Inter-individual coefficients of variation (C.V.) for the AUC-EE were 47% and 57% for the 1 + 35 and 1 + 50 agents, respectively. Intra-individual C.V.s were 41% and 42%, respectively. For norethindrone, the AUC was larger with the 1 + 50 formulations than with the 1 + 35 group (87.9 vs. 72.8 pg hr/ml). Additionally, the Cmax values were larger for the 1/50 group (17.7 vs. 14.0). Since the amount of norethindrone in the two dosage groups was the same, this difference in the pharmacokinetics between the 35 micrograms EE and the 50 micrograms ME formulations remains unexplained. The inter-individual C.V. averaged 56% for both dosage groups. The intra-individual C.V.s were 17% and 46% for the 1 + 35 and 1 + 50 groups, respectively. The large variation in blood levels of ethinyl estradiol and norethindrone between and within individuals may overshadow clinical differences attributable to differences in dosage.