Atazanavir/ritonavir and combined oral contraceptives

This was an open-label, three-period, single-sequence student assessing the effect of atazanavir/ritonavir on the pharmacokinetics of oral contraceptive containing either 35 microgram ethinyl estradiol plus norgestimate (OC given alone for days 1-28) or 25 microgram ethinyl estradiol plus norgestimate (OC given with atazanavir/r on days 29-42).

Reductions of approximately 20% in ethinyl estradiol levels were seen, whereas the concentrations of 17-deacetyl norgestimate (active metabolite) were increased (AUC by 85%, Cmin by 102%).

The authors concluded that increasing the ethinyl estradiol component to 30-35 microgram when co-administered with atazanavir/ritonavir might be desirable to minimise breakthrough bleeding.

Zhang J et al. The effect of atazanavir/ritonavir on the pharmacokinetics of an oral contraceptive containing ethinyl estradiol and norgestimate in healthy women. Antivir Ther, 2011, 16(2): 157-164.
http://www.ncbi.nlm.nih.gov/pubmed/21447864