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Avodart (Dutasteride)

August 25, 2019
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Dutasteride, sold under the brand name Avodart among others, is a medication primarily used to treat the symptoms of an enlarged prostate.[4][5] A few months may be required before benefits occur.[5] It is also used for scalp hair loss in men and as a part of hormone therapy in transgender women.[6][7] It is taken by mouth.[4][8]

Common side effects include sexual problems, breast tenderness, and breast enlargement.[4] Other side effects include an increased risked of certain forms of prostate cancer, depression, and angioedema.[4][5] Exposure during pregnancy, including use by the partner of a pregnant women may result in harm to the baby.[4][5] Dutasteride is a 5α-reductase inhibitor, and hence is a type of antiandrogen.[3] It works by decreasing the production of dihydrotestosterone (DHT), an androgen sex hormone.[9][4]

Dutasteride was patented in 1993 by GlaxoSmithKline and was approved for medical use in 2001.[10][4] It is available as a generic medication.[5] A month supply in the United Kingdom costs the NHS about £12 as of 2019.[5] In the United States, the wholesale cost of this amount is about $6.66.[11] In 2016, it was the 274th most prescribed medication in the United States with more than a million prescriptions.[12]Dutasteride has overall been found to be well-tolerated in studies of both men and women, producing minimal side effects.[16] Adverse effects include headache and gastrointestinal discomfort.[16] Isolated reports of menstrual changes, acne, and dizziness also exist.[16] There is a small risk of gynecomastia (breast development or enlargement) in men.[16] The risk of gynecomastia with 5α-reductase inhibitors is about 2.8%.[23]

The FDA has added a warning to dutasteride about an increased risk of high-grade prostate cancer.[24] While the potential for positive, negative or neutral changes to the potential risk of developing prostate cancer with dutasteride has not been established, evidence has suggested it may temporarily reduce the growth and prevalence of benign prostate tumors, but could also mask the early detection of prostate cancer. The primary area for concern is for patients who may develop prostate cancer whilst taking dutasteride for benign prostatic hyperplasia, which in turn could delay diagnosis and early treatment of the prostate cancer, thereby potentially increasing the risk of these patients developing high-grade prostate cancer.[25]

Sexual dysfunction, such as erectile dysfunction, loss of libido, or reduced semen volume, may occur in 3.4 to 15.8% of men treated with finasteride or dutasteride.[16][26] This is linked to lower quality of life and can cause stress in relationships.[27] It has been reported that in a subset of men, these adverse sexual side effects may persist even after discontinuation of finasteride or dutasteride.[28] Some have decreased sperm numbers as low as 10% of pretreatment values.[29]

Several small studies have reported an association between 5α-reductase inhibitors and depression.[16] However, most studies have not observed this side effect.[16] There have also been reports in a subset of men of long-lasting depression persisting even after discontinuation of dutasteride.[16][28]forms of 5α-reductase, and can decrease DHT levels in the blood by up to 98%.[1][37][38] Specifically it is a competitive, mechanism-based (irreversible) inhibitor of all three isoforms of 5α-reductase, types I, II, and III (IC50 values are 3.9 nM for type I and 1.8 nM for type II).[1][37][39][40] This is in contrast to finasteride, which is similarly an irreversible inhibitor of 5α-reductase[40][41] but only inhibits the type II and III isoenzymes.[37] As a result of this difference, dutasteride is able to achieve a reduction in circulating DHT levels of up to 98%, whereas finasteride is able to achieve a reduction of only 65 to 70%.[38][2][36][42] In spite of the differential reduction in circulating DHT levels, the two drugs decrease levels of DHT to a similar extent of approximately 85 to 90% in the prostate gland,[42] where the type II isoform of 5α-reductase predominates.[39]

Since 5α-reductases degrade testosterone to DHT, the inhibition of them could cause an increase in testosterone. However, a 2018 review found that initiation of 5α-reductase inhibitors did not result in a consistent increase in testosterone levels, with some studies showing increases and others showing no change.[43] There was no statistically significant change in testosterone levels from 5α-reductase inhibitors in the overall meta-analysis, though men with lower baseline testosterone levels may a rise in testosterone levels.

In addition to inhibition of DHT production, 5α-reductase inhibitors like dutasteride are also neurosteroidogenesis inhibitors, preventing the 5α-reductase-mediated biosynthesis of various neurosteroids including allopregnanolone (from progesterone), THDOC (from deoxycorticosterone), and 3α-androstanediol (from testosterone).[44] These neurosteroids are potent positive allosteric modulators of the GABAA receptor and have been found to possess antidepressant, anxiolytic, and pro-sexual effects in animal research.[44][45][46] For this reason, prevention of neurosteroid formation may be involved in the sexual dysfunction and depression that has been associated with 5α-reductase inhibitors like dutasteride.[44The oral bioavailability of dutasteride is approximately 60%.[1] Food does not adversely affect the absorption of dutasteride.[1] Peak plasma levels occur 2 to 3 hours after administration.[1] Levels of dutasteride in semen have been found to be 3 ng/mL, with no significant effects on DHT levels in sexual partners.[1] The drug is extensively metabolized in the liver by CYP3A4.[1] It has three major metabolites, including 6'-hydroxydutasteride, 4'-hydroxydutasteride, and 1,2-dihydrodutasteride; the former two are formed by CYP3A4, while the latter is not.[1] All three metabolites are active; 6'-hydroxydutasteride has similar potency as a 5α-reductase inhibitor to dutasteride, while the other two are less potent.[1] Dutasteride has an extremely long terminal or elimination half-life of about 4 or 5 weeks.[2][3] The elimination half-life of dutasteride is increased in the elderly (170 hours for men age 20–49 years, 300 hours for men age >70 years).[1] No dosage adjustment is necessary in the elderly nor in renal impairment.[1] Because of its long elimination half-life, dutasteride remains in the body for a long time after discontinuation and can be detected for up to 4 to 6 months.[1][2] In contrast to dutasteride, finasteride has a short terminal half-life of only 5 to 8 hours.[3][1] Dutasteride is eliminated mainly in the feces (40%) as metabolites.[1] A small portion (5%) is eliminated unchanged in the urine.[1]

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