Effect of high-dose vitamin C on the steady-state pharmacokinetics of the protease inhibitor indinavir in healthy volunteers.

STUDY OBJECTIVE

To determine whether daily high-dose vitamin C alters the steady-state pharmacokinetics of indinavir, a protease inhibitor indicated for treatment of the human immunodeficiency virus type 1.

DESIGN

Prospective, open-label, longitudinal, two-period time series.

SETTING

University medical center.

SUBJECTS

Seven healthy volunteers.

INTERVENTION

Indinavir 800 mg every 8 hours was given to subjects for four doses on days 1 and 2. Plasma samples were then collected for indinavir pharmacokinetic determination. After a 7-day washout period, subjects were given vitamin C 1000 mg/day for 7 days. Beginning on day 6 of vitamin C administration, indinavir 800 mg every 8 hours was restarted for four doses. Plasma was then collected from subjects to determine indinavir pharmacokinetics. All subjects were given a vitamin C content-controlled diet for 1 week before the study began and throughout the study period.

MEASUREMENTS AND MAIN RESULTS

Steady-state plasma samples were collected before dosing (0 hr) and 0.5, 1, 2, 3, 4, and 5 hours after dosing to determine indinavir pharmacokinetics. Parameters of interest were maximum plasma concentration (C max ), time to C max , area under the plasma concentration-time curve from 0-5 hours after the dose (AUC 0-5 ), an extrapolated 8-hour AUC (AUC 0-8 ), trough (minimum) plasma concentration (C min ), and oral clearance. Mean steady-state indinavir C max was significantly reduced (20%) after 7 days of vitamin C administration (10.3 +/- 1.5 vs 8.2 +/- 2.9 microg/ml, p=0.04). The corresponding mean AUC 0-8 was also significantly decreased (14%; 26.4 +/- 7.2 vs 22.7 +/- 8.1 microg*hr/ml, p=0.05). Although not statistically significant, the mean indinavir C min was 32% lower in the presence of vitamin C (0.27 +/- 0.17 C vs 0.18 +/- 0.08 microg/ml, p=0.09). Indinavir oral clearance and half-life were not significantly different.

CONCLUSION

Concomitant administration of high doses of vitamin C can reduce steady-state indinavir plasma concentrations. Subtherapeutic concentrations of antiretroviral agents have been associated with viral resistance and regimen failure, but the clinical significance of our findings remains to be established.