Gonadotropin-releasing hormone antagonists:

Pharmacokinetics and Pharmacodynamics of a Novel Depot Formulation of Abarelix, a Gonadotropin-Releasing Hormone (GnRH) Antagonist, in Healthy Men Ages 50 to 75

This study evaluated the safety, pharmacokinetics (PK), and pharmacodynamics (PD) of a novel depot formulation of abarelix, a new gonadotropin-releasing hormone (GnRH) antagonist. This was an open-label, sequential two-phase study in healthy male subjects ages 50 to 75. Subjects received a single intramuscular (IM) dose of 15 microg/kg abarelix injectable solution, followed by a 21-day washout period and a subsequent intramuscular dose of 100 mg abarelix depot. The PK and the hormonal suppression effects of abarelix were evaluated based on testosterone (T), dihydrotestosterone (DHT), follicle-stimulating hormone (FSH), and luteinizing hormone (LH) levels. Abarelix provides immediate competitive blocking of the GnRH receptors on pituitary gonadotropes without causing release of gonadotropins, and these effects are reversible. The mean IC(50)s of abarelix for T, DHT, FSH, and LH were 2.08, 3.42, 6.43, 4.25 ng/mL, respectively. The mean relative bioavailability of the depot formulation in reference to the injectable solution was 0.52. The mean t(max) and terminal t(1/2) for abarelix after administration of abarelix injectable solution and abarelix depot injection were 1 hour and 3 days and 0.22 days (5.3 h) and 13.2 days, respectively. The novel abarelix depot formulation used in this study significantly improved the duration of abarelix delivery and pharmacological activities compared to the injectable formulation, without causing any safety issues.

Drug delivery for in vitro fertilization: rationale, current strategies and challenges

In vitro fertilization has experienced phenomenal progress in the last thirty years and awaits the additional refinement and enhancement of medication delivery systems. Opportunity exists for the novel delivery of gonadotropins, progesterone and other adjuvants. This review highlights the rationale for various medications, present delivery methods and introduces the status of novel ideas and possibilities.

Hormone therapy in prostate cancer: LHRH antagonists versus LHRH analogues

GnRH agonists have a proven and well-established role in the management of prostate cancer. Further adaptations of the amino-acid sequence led to the development of antagonists with potential therapeutic uses, including a possible role in prostate cancer patients. Treatment of prostate cancer with GnRH agonists results in an initial flare of symptoms that may be prevented by co-administration of a steroidal or non-steroidal antiandrogen. However, this can be associated with additional adverse effects. Clinical studies have shown that GnRH antagonists produce a rapid decline in testosterone but without the disease flare. However these short-term effects have yet to be proven to lead to long-term survival benefits. There have been some reports that antagonists may be associated with adverse effects due to histamine release leading to severe allergic reactions. GnRH agonists are currently available in a range of depot formulations, allowing treatment to be tailored to the patient's needs. At present, the antagonists are only available as on-month depot formulations, which may limit their clinical use. Abarelix should be given intramuscularly. It is the first GnRH antagonist which is approved by the FDA for patients with advanced prostate cancer who should be treated under a risk management program. In Europe, abarelix has not been registered yet.

Gonadotropin-releasing hormone and reproductive medicine

The hypothalamic gonadotropin-releasing hormone (GnRH) is a key regulator of the reproductive system, mainly through its effects on pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release. Gonadotropin-releasing hormone analogues are modifications of the natural decapeptide, being either agonists (GnRHa) or antagonists. GnRHa may imitate the native hormone and induce an endogenous LH surge; however, sustained treatment with GnRHa results in complete refractoriness of LH and FSH production. This aspect of GnRHa action is the main application of the GnRHa in reproduction today. The hypogonadotropic hypogonadal state that results from the sustained treatment is the basis for additional applications of the GnRHa. The agonists appear to be effective, to some extent, for the treatment of endometriosis and were proved to be beneficial prior to surgery for fibroma uteri. GnRHa is used in assisted reproduction for both induction of an endogenous LH surge and for induction of ovulation, and its capacity to cause refractoriness of the pituitary may be used to avoid a premature LH surge. It is suggested that the GnRHa have a direct effect on ovarian steroidogenesis, which is independent of its action on the pituitary. This unwanted effect and other possible drawbacks of the agonists are thought to be eliminated with the use of the antagonists. The mechanism of action of these antagonists is through competitive blocking of the GnRH receptor, which results in a rapid but reversible suppression of gonadotropin secretion. As for GnRHa, GnRH antagonists have been proven effective to prevent the LH surge and to reduce the incidence of severe ovarian hyperstimulation syndrome in controlled ovarian stimulation cycles.