Hormonal anti-implantation agents: antiprogestins

Oestrogens and progestogens separately or in combination are able to prevent implantation with high efficiency, thus acting as interceptive agents. Current interceptive medical regimens include high-dose oestrogens, the association of oestrogens with progestogens or progestogens alone. Compounds with antiprogesterone properties, such as RU 486 (mifepristone) or ZK 98734 (lilopristone), also exhibit a strong interceptive action which, as shown in animal models, is proportional to the dose and the day(s) of administration. Recent clinical studies show that RU 486 can be used successfully for postcoital interception. The regimen applied for this purpose consists of the intake of a single dose of 600 mg RU 486 within 72 h of a single act of unprotected intercourse. This treatment was found to be highly effective and to have a more favourably side-effect profile in comparison with the oestrogen-progestogen interceptive regimen. However, because of the induced irregularities of the cycle, the mifepristone regimen, as with the other hormonal methods, should not be used on a regular basis. Currently, all interceptive hormonal regimens are emergency methods. Their occasional use to prevent unwanted pregnancies may reduce the number of therapeutic abortions. However, the frequency and extent of their side-effects do not allow for a repeated postcoital use after every act of unprotected intercourse. Obviously, the development of an effective and safe 'morning after pill' requires further basic and clinical investigations.