Why use of dienogest for the first contraceptive pill with estradiol?

Dienogest (DNG) has the essential properties of an effective progestogen for use in a new contraceptive pill using estradiol valerate as estrogenic component -- it inhibits ovulation and protects against endometrial proliferation. DNG is a derivative of norethisterone (NET), but has a cyanomethyl- instead of an ethinyl-group in C17 position which may offer a variety of benefits regarding hepatic effects. The similarity to NET is reflected in the high endometriotropy and in similar pharmacokinetics like short plasma half-live and high bioavailability. However, DNG also elicits properties of progesterone derivatives like neutrality in metabolic and cardiovascular system and considerable antiandrogenic activity, the latter increased by lack of binding to SHBG as specific property of DNG. It has no glucocorticoid and antimineralocorticoid activity and has no antiestrogenic activity with the consequence that possible beneficial estradiol effects should not be antagonized. This may be of special importance for the tolerability and safety of the first pill with estradiol valerate instead of ethinylestradiol, although well-designed postmarketing studies are still ongoing to demonstrate what can be expected on the basis of pharmacology.