What are progestins?

Progestins are a synthetic version of natural progesterone, with similar bioactivity.  They were originally developed to for their contraceptive potential and to counteract the problems of administering natural progesterone.  When taken orally natural progesterone has low bioavailability because it is rapidly metabolised in the gastrointestinal tract by up to 90% on first pass through the liver and therefore can only be used in high doses or when administered by injection.  Other administrative forms of natural progesterone have been developed to protect from metabolism, including micronized progesterone, in which a plant source of progesterone is used and reduced to tiny particles and delivered in oil filled capsules so that it is more readily absorbed.^{1, 2}

Several generations of progestins have been developed for use in contraceptives, to suppress ovulation usually in combination with an oestrogen; also for hormone replacement therapy to counteract the effects of oestrogen to reduce risk of endometrial cancer.  Most progestins, like norethisterone a first generation progestin and desogestrel a third generation progestin, have weak androgenic activity, although some progestins like medroxyprogesterone do have more noticeable androgenic side effects.  Other porgestins like dydrogesterone do not exhibit androgenic side effects such as hirsutism, acne.

What progestins are available?

Several progestins are available and they can be delivered in various forms.  These include:

• Desogestrel tablets
• Dydrogesterone tablets
• Norethisterone tablets
• Medroxyprogesterone liquid for injection and tablets
• Micronized progesterone in a topical gel or capsules
• Natural Progesterone injection

Clinical uses for progestins

Progestins are used to treat several conditions, including the following:

• Endometriosis, which is growth of endometrial tissue outside the uterus causing pain and bleeding.  Progestins used include medroxyprogesterone injection and tablets, dydrogesterone tablets and norethisterone tablets.
• Menstrual disorders and uterine bleeding disorders.  Progestins used include dydrogesterone, norethisterone, and the natural progesterone injection.
• To assist with infertility treatments by providing luteal support for infertility treatment.  Progestins used include micronized progesterone gel.
• To prevent spontaneous abortion (miscarriage).  Natural progesterone injection is used for this purpose.
• To counteract the stimulating effect of oestrogen on the endometrium, in hormone replacement therapy (HRT) for menopausal symptoms, thereby reducing the risk of endometrial cancer.  Progestins used include Dydrogesterone tablets, micronized progesterone, and medroxyprogesterone tablets.
• Contraception.  Progestins used include desogestrel tablets
• To treat (but not cure) certain oestrogen-related cancers like breast, endometrial and renal carcinomas.   When used at high doses, progestins can inhibit the growth of these tumours.  Progestins used include medroxyprogesterone injection. 

References

1. Warren MP and Shantha S.  Uses of progesterone in clinical practice.  Int J Fertil Womens Med.  1999: 44; 96-103
2. Hargrove JT, Maxson WS, Wentz AC.  Absorption of oral progesterone is influenced by vehicle and particle size. Am J Obstet and Gynecol. 1989: 161; 948–951.