Comparative Contraceptive Efficacy and Mechanism of Action of the Norgestimate-Containing Triphasic Oral Contraceptive

Norgestimate (NGM), a derivative of 19-nortestosterone with very specific affinity for the progesterone receptor, has been used in combination with ethinyl estradiol (EE) at low doses in both monophasic and triphasic oral contraceptives (OCs). An open-label comparative clinical trial was conducted with 4,234 healthy women using triphasic levonorgestrel (LNG)/EE and NGM/EE through a total of 22,312 menstrual cycles. Contraceptive efficacy was excellent with both preparations, with no statistically significant between-regimen differences in pregnancy rates. The theoretical Pearl index was 0.12 for the NGM/EE triphasic, and 0.34 for the LNG/EE triphasic. Adverse experiences in both groups were typical of those that may occur among women taking low-dose OC agents. The percentage of subjects discontinuing treatment because of use-related adverse experiences was similar with the two preparations: 8.6% for the NGM/EE triphasic and 6.8% for the LNG/EE triphasic. In a separate mechanism of action study, specific endocrine parameters were investigated in 20 subjects using the NGM/EE triphasic for 4 cycles. Ovulation suppression was demonstrated in statistically significant decreases from pre-treatment values in serum levels of luteinizing hormone, follicle-stimulating hormone, progesterone, and estradiol. Significant on-treatment increases in serum levels of sex hormone binding globulin evidenced minimal androgenicity. All hormonal values returned to or toward normal in the post-treatment cycle. The study results support those obtained in large noncomparative studies of the NGM/EE triphasic. This phased-dose combination suppresses ovulation and is a very effective, minimally androgenic contraceptive agent with a good safety profile.