Agonistssupraphysiological doses of hormone

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This approach is epitomized by the use of gonadotrophin-relating hormone agonists (GnRHa or LHRHa). The gonadotrophins LH and FSH provide the stimulus for gonads to produce steroid hormones; in turn their synthesis and release from the pituitary is regulated by the hypothalamic factor GnRH (or LHRH). Highly potent agonist analogues of GnRH have been synthesized by introducing unusual amino acids into the native peptide. When administered for short periods they cause a rapid release of gonadotrophins, but in the long term these agonists down regulate gonadotrophic receptors and desensitize the pituitary. As a result circulating gonadotrophins fall, the trophic drive to the gonads is abolished, and circulating sex hormones are reduced to castration levels. Depot formulations of LHRH agonists are available so that a single injection can maintain effective medical castration over prolonged periods. The use of GnRH analogues in pre-menopausal women with breast cancer and men with prostate cancer has produced anti-tumour effects equivalent to surgical castration.

A similar mechanism of action underpins the response seen in hormone-dependent cancers following use of pharmacological doses of steroid hormones such as:

♦ Oestrogen (diethyl stilboestrol).

♦ Progestogens (medroxyprogesterone and megestrol).

♦ Androgens (testolactone and fluoxymesterone).

Lower physiological doses of the same hormones may accelerate tumour growth.

While down regulation of steroid hormone receptors occurs in target organs, other non-specific effects can occur, and these agents may be associated with poor toxicity profiles. Also, tumour flare may occur at the start of treatment. Despite this they are of clinical benefit e.g. high-dose progestogens for endometrial and breast cancer.

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