Ritonavir's role in reducing fentanyl clearance and prolonging its half-life.

BACKGROUND: The human immunodeficiency virus protease inhibitor ritonavir is a potent inhibitor of the cytochrome P450 3A4 enzyme, and ritonavir's concomitant administration with the substrates of this enzyme may lead to dangerous drug interactions. METHODS: The authors investigated possible interactions between ritonavir and intravenously administered fentanyl in a double-blind, placebo-controlled, cross-over study in two phases. Twelve healthy volunteers received orally ritonavir or placebo for 3 days; the dose of ritonavir was 200 mg three times on the first day and 300 mg three times on the second. The last dose of ritonavir 300 mg or placebo was given on the morning of the third day. On the second day, 2 h after the afternoon pretreatment dose, fentanyl 5 microg/kg was injected intravenously in 2 min with naloxone to moderate its effects, and 15 timed venous blood samples were collected for 18 h. RESULTS: Ritonavir reduced the clearance of fentanyl by 67% from 15.6+/-8.2 to 5.2+/-2.0 ml x min(-1) x kg(-1) (P<0.01). The area under the fentanyl plasma concentration-time curve from 0 to 18 h was increased from 4.8+/-2.7 to 8.8+/-2.3 ng x ml(-1) x h(-1) by ritonavir (P<0.01). Ritonavir did not affect the initial concentrations and the steady-state volume of distribution of fentanyl. One subject discontinued participation before fentanyl administration because of severe side effects, and during the study 8 of the remaining 11 subjects reported nausea. CONCLUSIONS: Ritonavir can inhibit the metabolism of fentanyl significantly, so caution should be exercised if fentanyl is given to patients receiving ritonavir medication.

Effect of saquinavir on the pharmacokinetics and pharmacodynamics of oral and intravenous midazolam.

OBJECTIVE: To assess the effect of human immunodeficiency virus protease inhibitor saquinavir on the pharmacokinetics and pharmacodynamics of oral and intravenous midazolam. METHODS: In a double-blind, randomized, two-phase crossover study, 12 healthy volunteers (six men and six women; age range, 21 to 32 years) received oral doses of either 1200 mg saquinavir (Fortovase soft-gel capsule formulation) or placebo three times a day for 5 days. On day 3, six subjects were given 7.5 mg oral midazolam and the other six subjects received 0.05 mg/kg intravenous midazolam. On day 5, the subjects who had received oral midazolam on day 3 received intravenously midazolam and vice versa. Plasma concentrations of midazolam, alpha-hydroxymidazolam, and saquinavir were determined for 18 hours after midazolam administration, and midazolam effects were measured up to 7 hours by six psychomotor tests. RESULTS: Saquinavir increased the bioavailability of oral midazolam from 41% to 90% (P < .005), the peak midazolam plasma concentration more than twofold, and the area under plasma concentration-time curve more than fivefold (P < .001). During saquinavir treatment, five of the six psychomotor tests revealed impaired skills and increased sedative effects after midazolam ingestion (P < .05). Saquinavir decreased the clearance of intravenous midazolam by 56% (P < .001) and increased its elimination half-life from 4.1 to 9.5 hours (P < .01). After intravenous midazolam, only the subjective feeling of drug effect was increased significantly (P < .05) by saquinavir. CONCLUSION: The dose of oral midazolam should be greatly reduced or avoided with saquinavir, but bolus doses of intravenous midazolam can probably be used quite safely. During a prolonged midazolam infusion, an initial dose reduction of 50% followed by careful titration is recommended to counteract the reduced clearance caused by saquinavir.

Effect of itraconazole on the pharmacokinetics of bupivacaine enantiomers in healthy volunteers.

We studied seven healthy volunteers given itraconazole 200 mg orally or placebo, once daily for 4 days, in a crossover study. On day 4, racemic bupivacaine 0.3 mg kg-1 was given i.v. over 60 min and venous plasma samples were collected for 23 h. Plasma concentrations of R- and S-bupivacaine, itraconazole and hydroxyitraconazole were measured. Itraconazole reduced the clearance of R-bupivacaine by 21% (P < 0.05) and that of S-bupivacaine by 25% (P < 0.05), while it had no significant effect on other pharmacokinetic variables of the enantiomers. Reduction of bupivacaine clearance by itraconazole probably increases the steady-state concentration of bupivacaine enantiomers by 20-25%. This should be taken into account in the concomitant use of bupivacaine and itraconazole, although the interaction seems to be of limited clinical significance.

Effect of erythromycin and itraconazole on the pharmacokinetics of intravenous lignocaine.

OBJECTIVE: We have studied the possible interaction of erythromycin and itraconazole, both inhibitors of cytochrome P450 3A4 isoenzyme (CYP3A4), with intravenous lignocaine in nine healthy volunteers using a randomized cross-over study design. METHODS: The subjects were given oral placebo, erythromycin (500 mg three times a day) or itraconazole (200 mg once a day) for 4 days. Intravenous lignocaine 1.5 mg x kg(-1) was given with an infusion for 60 min on the fourth day of pretreatment with placebo, erythromycin or itraconazole. Timed plasma samples were collected until 11 h. The concentrations of lignocaine and its metabolite monoethylglycinexylidide (MEGX) were measured by gas chromatography. RESULTS: The area under the lignocaine concentration-time curve was similar during all three phases but erythromycin significantly increased the elimination half-life of lignocaine from 2.5 to 2.9 (0.7) h compared with placebo. Following itraconazole administration, t1/2 was 2.6 h. The values for plasma clearance and volume of distribution at steady state were similar during all the phases. Compared with placebo and itraconazole, erythromycin significantly increased MEGX peak concentrations by approximately 40% and AUC(0-11 h) by 45-60%. CONCLUSION: The plasma decay of lignocaine administered intravenously is virtually unaffected by the concomitant administration of erythromycin and itraconazole. However, erythromycin increases the concentrations of MEGX, which indicates that erythromycin either increases the relative amount of lignocaine metabolized via N-de-ethylation or decreases the further metabolism of MEGX. Further studies are necessary to elucidate the clinical significance of the erythromycin-induced elevated concentrations of MEGX during prolonged intravenous infusions of lignocaine.

The effect of intravenous and oral fluconazole on the pharmacokinetics and pharmacodynamics of intravenous alfentanil.

UNLABELLED: The azole antimycotic fluconazole inhibits cytochrome P450 enzymes. We studied the possible interaction of alfentanil with oral and i.v. fluconazole in a three-phase, double-blind, placebo-controlled, randomized, cross-over study at intervals of 4 wk. Nine healthy volunteers were given a single oral dose of either fluconazole 400 mg or placebo, followed within 60 min by either i.v. fluconazole 400 mg in saline or saline only. The dose of 20 micrograms/kg of alfentanil was given i.v. 1 h after the fluconazole/placebo. Venous plasma samples were collected, and pharmacodynamic measurements were performed for 10 h. Alfentanil clearance was decreased from 3.1 +/- 1.1 mL.min-1.kg-1 to 1.3 +/- 0.3 mL.min-1. kg-1 after i.v. fluconazole and to 1.4 +/- 0.4 mL.min-1. kg-1 after oral fluconazole (P < 0.001). The mean elimination half-life of alfentanil was almost doubled after both oral and i.v. fluconazole (P < 0.001). Fluconazole increased the subjective effects of alfentanil as quantified on visual analog scales (P < 0.01). Correspondingly, both i.v. and oral fluconazole also increased the alfentanil-induced respiratory depression by decreasing the respiratory rate by 10%-15% (P < 0.05). We conclude that metabolism of alfentanil is inhibited by oral and i.v. fluconazole and that caution should be exercised when alfentanil is given to patients receiving fluconazole. IMPLICATIONS: We studied the interaction of cytochrome P450 3A4 enzyme inhibitor fluconazole with alfentanil. Nine volunteers were given placebo or fluconazole 400 mg orally or i.v. Alfentanil 20 micrograms/kg was given i.v. 1 h after pretreatment. Fluconazole decreased the clearance of alfentanil by 55% and increased the alfentanil-induced subjective effects.

The CYP 3A4 inhibitor itraconazole has no effect on the pharmacokinetics of i.v. fentanyl.

We studied 10 healthy volunteers given itraconazole 200 mg orally, once daily or placebo for 4 days in a crossover study. i.v. fentanyl 3 micrograms kg-1 was given on day 4. Plasma concentrations of fentanyl were measured by radioimmunoassay and ventilatory frequency and peripheral arteriolar oxygen saturation were also measured. Fentanyl-induced subjective effects (drowsiness, itching, nausea, performance, feeling of drug effect) were measured by visual analogue scales. The pharmacokinetics and pharmacodynamics of fentanyl were similar after both itraconazole and placebo. Thus although itraconazole is a strong inhibitor of the cytochrome 3A enzymes responsible for metabolism of fentanyl in vitro, it did not affect the i.v. pharmacokinetics of fentanyl in humans.