Pharmacodynamics MGA has progestogenic activity, antigonadotropic effects, weak partial androgenic activity, and weak glucocorticoid activity. Relative affinities (%) of<unk>  Compound<unk>  65 5 0 30 0 0 0 Notes: Values are percentages (%). Reference ligands (100%) were promegestone for the , metribolone for the , estradiol for the , dexamethasone for the , aldosterone for the , dihydrotestosterone for , and cortisol for . Sources: Progestogenic activity MGA is a progestogen, or an agonist of the progesterone receptor (PR). It has about 65% of the affinity of promegestone and 130% of the affinity of progesterone for the PR. Like other progestogens, MGA has functional antiestrogenic effects in certain tissues such as the endometrium and has antigonadotropic effects. The total endometrial transformation dose of MGA is 50 mg per cycle. Antigonadotropic and anticorticotropic effects MGA has antigonadotropic effects in humans at sufficient doses, capable of profoundly suppressing circulating androgen and estrogen concentrations. The antigonadotropic effects of MGA are the result of activation of the PR, which suppresses the secretion of the gonadotropins—peptide hormones responsible for signaling the body to produce not only progesterone but also the androgens and the estrogens—from the pituitary gland as a form of negative feedback inhibition, and hence downregulates the hypothalamic–pituitary–gonadal axis (HPG axis), resulting in decreased levels of the sex hormones and interference with fertility. As such, MGA has functional antiandrogenic and antiestrogenic effects as well as contraceptive effects via its antigonadotropic effects. The precise ovulation-inhibiting dosage of MGA is unknown. However, doses of 1 to 5 mg MGA were previously used in combined birth control pills in combination with the estrogen ethinylestradiol or mestranol. MGA is an effective contraceptive by itself at dosages of 0.35 to 0.5 mg/day, but is not effective at a dosage of 0.25 mg/day. MGA alone does not inhibit ovulation at a dosage of 0.5 mg/day, nor does it fully inhibit ovulation at a dosage of 0.7 mg/day or even at a dosage of 5 mg/day. The combination of 2 to 5 mg/day MGA and 100 μg/day mestranol has been found to consistently inhibit ovulation, whereas either medication alone did not completely inhibit ovulation in all women. Suppression of testosterone levels by MGA is responsible for its effectiveness in the treatment of conditions like prostate cancer and benign prostatic hyperplasia. In one study, 120 to 160 mg/day MGA suppressed testosterone levels in men by 72%. However, a recovery or "escape" of testosterone levels, gradually returning to near-normal values, has been observed in most men after 2 to 6 months of MGA therapy, and this has limited the usefulness of the medication. The combination of a lower dosage of MGA (40–80 mg/day) and a low oral dosage of an estrogen such as estradiol (0.5–1.5 mg/day), diethylstilbestrol (0.1–0.2 mg/day) or ethinylestradiol (50 µg/day) is able to suppress testosterone levels into the castrate range in men, maintain this suppression long-term, and achieve equivalent effectiveness to high-dosage estrogen monotherapy in the treatment of prostate cancer with comparatively greatly reduced toxicity and side effects. In spite of these results, however, this combination has been very rarely used to treat prostate cancer in the United States. The antigonadotropic as well as anticorticotropic effects of MGA may be involved in its effectiveness in the treatment of postmenopausal breast cancer via substantially decreasing gonadal and adrenal production of sex steroids and by extension circulating levels of estrogens, by about 80%. Androgenic and antiandrogenic activity MGA is a weak partial agonist of the androgen receptor (AR). It has been reported to bind to this receptor with 5% of the affinity of the anabolic steroid metribolone. Despite its weak intrinsic activity at the AR, at clinical doses in humans, MGA appears to behave, for all intents and purposes, purely as an antiandrogen. This is based on the fact that no virilizing side effects have been observed with the use of MGA in patients of either sex at dosages up to as high as 1,600 mg per day, the highest that has been assessed. Furthermore, MGA produces detectable androgenic effects in animals only at a dose that is the equivalent of approximately 200 times that typically used for the treatment of prostate cancer in men. However, the medication does have moderate androgenic effects on serum lipids in humans, causing a significant reduction of and cholesterol levels and no change in triglyceride levels at a dosage of only 5 mg/day. Conversely, MGA does not decrease SHBG levels. The weak but significant androgenic activity of MGA may serve to limit its clinical effectiveness in the treatment of prostate cancer. Glucocorticoid activity MGA is an agonist of the glucocorticoid receptor (GR), the biological target of glucocorticoids like cortisol. It has been found to possess 30% of the affinity of the corticosteroid dexamethasone for this receptor. MGA shows the lowest ratio of PR affinity to GR affinity of a broad selection of marketed progestins, suggesting that it may have among the highest relative glucocorticoid effect of the progestins used in medicine. MGA produces observable glucocorticoid effects, with one study finding that, in the dose range tested, it possessed about 50% of the eosinopenic and hyperglycemic activity (markers of glucocorticoid activity) of an equal amount of medroxyprogesterone acetate and about 25% that of hydrocortisone. Accordingly, manifestations of its glucocorticoid activity, including symptoms of Cushing's syndrome, steroid diabetes, and adrenal insufficiency, have been reported with the use of MGA in the literature, albeit sporadically. Appetite stimulation MGA is frequently used as an appetite stimulant to promote weight gain. The direct mechanism of appetite enhancement is unclear, but it is known that MGA induces a variety of downstream changes to cause the effect, including stimulation of the release of neuropeptide Y in the hypothalamus, neurosteroid-like modulation of calcium channels in the ventromedial hypothalamus, and inhibition of the secretion of proinflammatory cytokines including interleukin 1α, interleukin 1β, interleukin 6, and tumor necrosis factor α, all of which have been implicated in facilitation of appetite. Increased levels of insulin-like growth factor 1 (IGF-1) may also be involved, specifically in its anabolic effects. Studies of MGA in elderly patients who experience weight loss are limited and of poor quality with most showing minimal or no weight gain, with no nutritional or clinically significant beneficial outcomes observed. In patients who take MGA, one in 12 will have an increase in weight. Miscellaneous Unlike the case of the AR, MGA has no significant affinity for the ER. As such, it does not possess the capacity to directly activate the ER. Furthermore, unlike antiandrogens such as spironolactone and bicalutamide but similarly to cyproterone acetate, there is relatively little risk of indirectly mediated estrogenic side effects (e.g., gynecomastia) with MGA. This is because MGA strongly suppresses both androgen and estrogen levels at the same time. Similarly to the case of the ER, MGA has negligible affinity for the mineralocorticoid receptor (MR), and hence does not possess mineralocorticoid or antimineralocorticoid activity. MGA has been found to dose-dependently increase total and free IGF-1 levels up to a dosage of 120 mg/day. Total IGF-1 levels were described as "profoundly" increased, gradually increasing, significantly by 3 days of treatment, up to a maximum of 2.66-fold by 5 to 6 months of treatment. Free (readily dissociable) concentrations of IGF-1 were increased to a smaller extent, by 1.23–2.15-fold, and were described as increasing "moderately". It was suggested that the increase in IGF-1 levels with high-dosage MGA therapy may explain the anabolic effects of MGA in patients with cachexia. Pharmacokinetics The oral bioavailability of MGA is approximately 100%. After a single low oral dose of 4 mg MGA, peak serum concentrations of MGA were about 7 ng/dL (18 nmol/L) and occurred after 3 hours. Following a single high oral dose of 160 mg micronized MGA in men, peak circulating levels of MGA were 125 ng/mL (325 nmol/L) and occurred after 6.3 hours. This study found that micronized MGA at this dose showed considerably improved absorption relative to its conventional tablet form. In terms of plasma protein binding, MGA is bound mostly to albumin (82.4%) and is not bound to sex hormone-binding globulin or to corticosteroid-binding globulin. MGA metabolized in the liver mainly by hydroxylation of the C21, C2α, and C6 positions, as well as by reduction and conjugation. Its elimination half-life is 34 hours on average, with a range of 13 to 105 hours. MGA is excreted 57 to 78% in urine and 8 to 30% in feces. At high doses, MGA appears to have far greater bioavailability and potency than medroxyprogesterone acetate, regardless of whether the route of administration of the latter is oral or parenteral. Following oral administration of 80 to 160 mg MGA or 500 to 1,000 mg medroxyprogesterone acetate, circulating levels of MGA were 2- to 10-fold higher than those of medroxyprogesterone acetate. Similar findings have been found for oral MGA relative to medroxyprogesterone acetate administered via intramuscular injection. MGA also reaches steady-state levels more quickly than medroxyprogesterone acetate. The improved potency of MGA compared to medroxyprogesterone acetate may be due to increased resistance to metabolism
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