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Pharmacology: Pharmacodynamics: RITA produce a contraceptive effect mainly by suppressing the hypothalamic-pituitary system resulting in prevention of ovulation. The estrogen acts mainly by suppressing secretion of follicle-stimulating hormone (FSH), resulting in prevention of follicular development and the rise of plasma estradiol concentration which is thought to be the stimulus for release of luteinizing hormone (LH). The progestin appears to act mainly by inhibiting the preovulatory rise of LH. Long term administration of these combination products results in inhibition of both FSH and LH secretion. It has been suggested that oral contraceptives may also produce a direct effect on ovarian steroidogenesis or the response of the ovary to gonadotropins. In addition, changes in the cervical mucus may prevent sperm penetration.
Pharmacokinetics: Ethinylestradiol is rapidly absorbed with peak concentrations attained within 2 hours. It undergoes considerable first-pass elimination. Ethinylestradiol is 97% to 98% bound to plasma albumin. The mean elimination half-life varies from 6 to 20 hours. It is excreted in bile and urine as conjugates and undergoes some enterohepatic recirculation.
Desogestrel is rapidly and completely absorbed and converted into 3-keto-desogestrel, the biologically active metabolite. Relative bioavailability is about 84%. Maximum concentrations of the metabolite are reached at about 1.4 hours. Desogestrel is bound to albumin and to sex hormone binding globulins. The mean elimination half-life of desogestrel (metabolite) is 38 ± 20 hours.
