Combination oral contraceptives act by suppression of gonadotropins. Although the primary mechanism of this action is inhibition of ovulation, other alterations include changes in the cervical mucus (which increase the difficulty of sperm entry into the uterus) and the endometrium (which reduce the likelihood of implantation).

Pharmacokinetics

Absorption

Ethinyl estradiol and norethindrone are rapidly absorbed with maximum plasma concentrations occurring within 2 hours after FEMCON Fe administration (see Table 1). Norethindrone appears to be completely absorbed following oral administration; however, it is subject to first-pass metabolism resulting in an absolute bioavailability of approximately 65%. Large intersubject variability is reflected in a 3- to 5-fold variation in norethindrone bioavailability. Ethinyl estradiol bioavailability is approximately 43% due to small-intestinal and hepatic first-pass metabolism.

Table 1. Mean ±SD Pharmacokinetic Parameters Following Single Dose Administration of FEMCON Fe in Healthy Female Subjects Under Fasting Conditions.

Effect of Food. Single-dose administration of FEMCON Fe tablets with food decreased the maximum norethindrone and ethinyl estradiol concentration by 53% and 47%, respectively; the extent of norethindrone and ethinyl estradiol absorption (AUC values) was not affected by food administration.

Distribution

Norethindrone is 36% bound to sex hormone-binding globulin (SHBG) and 61% bound to albumin. Ethinyl estradiol is not bound to SHBG but is highly (98.5%) bound to albumin. Volume of distribution of norethindrone and ethinyl estradiol ranges from 2 to 4 L/kg.

Metabolism

Norethindrone undergoes extensive biotransformation, primarily via reduction, followed by sulfate and glucuronide conjugation; less than 5% of a norethindrone dose is excreted unchanged; greater than 50% and 20-40% of a dose is excreted in urine and feces, respectively. The majority of metabolites in the circulation are sulfates, with glucuronides accounting for most of the urinary metabolites.

Ethinyl estradiol is also extensively metabolized, both by oxidation and by conjugation with sulfate and glucuronide. Sulfates are the major circulating conjugates of ethinyl estradiol, and glucuronides predominate in urine. The primary oxidative metabolite is 2-hydroxy-ethinyl estradiol which is formed by the CYP3A4 isoform of cytochrome P450.

Excretion

Plasma clearance values for norethindrone and ethinyl estradiol are similar (approximately 0.4 L/hr/kg). Ethinyl estradiol and norethindrone are excreted in both urine and feces, primarily as metabolites. Ethinyl estradiol is excreted in urine and feces as glucuronides and sulfates, and about 28-43% undergoes enterohepatic circulation. The mean terminal elimination half-lives of norethindrone and ethinyl estradiol following single dose administration of FEMCON Fe are approximately 9 hours and 17 hours, respectively.

Special Populations

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