The pulsatile release of GnRH stimulates the synthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). The frequency of LH pulses in the mid and late follicular phase is approximately 1 pulse per hour. These pulses can be detected as transient rises in serum LH. At midcycle, a large increase in GnRH release results in an LH surge. The midcycle LH surge initiates several physiologic actions including: ovulation, resumption of meiosis in the oocyte, and luteinization. Luteinization results in a rise in serum progesterone with an accompanying decrease in estradiol levels.

Ganirelix Acetate acts by competitively blocking the GnRH receptors on the pituitary gonadotroph and subsequent transduction pathway. It induces a rapid, reversible suppression of gonadotropin secretion. The suppression of pituitary LH secretion by Ganirelix Acetate is more pronounced than that of FSH. An initial release of endogenous gonadotropins has not been detected with Ganirelix Acetate, which is consistent with an antagonist effect. Upon discontinuation of Ganirelix Acetate, pituitary LH and FSH levels are fully recovered within 48 hours.

Pharmacokinetics

The pharmacokinetic parameters of single and multiple injections of Ganirelix Acetate Injection in healthy adult females are summarized in Table I. Steady-state serum concentrations are reached after 3 days of treatment. The pharmacokinetics of Ganirelix Acetate are dose-proportional in the dose range of 125 to 500 µg.

TABLE I: Mean (SD) pharmacokinetic parameters of 250 µg of Ganirelix Acetate following a single subcutaneous (SC) injection (n=15) and daily SC injections (n=15) for seven days.

Absorption

Ganirelix Acetate is rapidly absorbed following subcutaneous injection with maximum serum concentrations reached approximately one hour after dosing. The mean absolute bioavailability of Ganirelix Acetate following a single 250 µg subcutaneous injection to healthy female volunteers is 91.1%

Distribution

The mean (SD) volume of distribution of Ganirelix Acetate in healthy females following intravenous administration of a single 250 µg dose is 43.7 (11.4) liters (L). In vitro protein binding to human plasma is 81.9%.

Metabolism

Following single dose intravenous administration of radiolabeled Ganirelix Acetate to healthy female volunteers, Ganirelix Acetate is the major compound present in the plasma (50-70% of total radioactivity in the plasma) up to 4 hours and urine (17.1-18.4% of administered dose) up to 24 hours. Ganirelix Acetate is not found in the feces. The 1-4 peptide and 1-6 peptide of Ganirelix Acetate are the primary metabolites observed in the feces.

Excretion

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