D. Ouellet et al., EFFECT OF RITONAVIR ON THE PHARMACOKINETICS OF ETHINYL ESTRADIOL IN HEALTHY FEMALE VOLUNTEERS, British journal of clinical pharmacology, 46(2), 1998, pp. 111-116
Aims To assess the effects of the protease inhibitor ritonavir on the
pharmacokinetics of ethinyl oestradiol in healthy female volunteers. M
ethods This was an open-label, single centre study in 23 subjects who
received two single doses of oral contraceptive containing 50 mu g eth
inyl oestradiol on Day 1 (alone) and on Day 29 during concomitant rito
navir. Each subject received 16 days of every 12 h doses of ritonavir
from Day 15 through Day 30. Blood samples were collected for serum eth
inyl oestradiol concentrations for 48 h after each dose and for plasma
ritonavir on Day 29 at 0 and 4 h postdose. Results Statistically sign
ificant decreases in ethinyl oestradiol mean C-max (-32%) and mean AUC
(-41%), and a statistically significant increase in the mean terminal
elimination rats constant (+31%) were observed during concomitant rit
onavir. The harmonic mean terminal half-life decreased from 17 h to 13
h during concomitant ritonavir. No statistically significant change w
as noted in t(max). The ratios of means (95% confidence intervals) for
C-max and AUC were 0.682 (0.612-0.758) and 0.595 (0.506-0.694), respe
ctively. The changes in ethinyl oestradiol pharmacokinetics were consi
stent with an increase in clearance from enzymatic induction of glucur
onidation and/or cytochrome P450 hydroxylation. Mean steady-state rito
navir concentrations of 6.5 and 13.4 mu g ml(-1) were observed at 0 an
d 4 h postdose, respectively. Conclusions Considering the extent of th
e decrease in ethinyl oestradiol concentrations, the use of alternate
contraceptive measures should be considered when ritonavir is being ad
ministered.