Induction of zidovudine glucuronidation and amination pathways by rifampicin in HIV-infected patients

AIMS

The objective of the study was to determine the effect of multiple doses of rifampicin on the steady-state pharmacokinetics of zidovudine and its 5'-glucuronosyl (GZDV) and 3'-amino (AMT) metabolites.

METHODS

Eight asymptomatic HIV-infected patients (seven male, one female) participated in this three-period longitudinal study. Each patient received zidovudine (200 mg every 8 h) for 14 days (period 1), followed by rifampicin (600 mg every 24 h) with zidovudine for 14 days (period 2), and then zidovudine alone for a further 14 days (period 3). Blood and urine samples were collected over 6 h on the last day of each period for measurements of zidovudine and GZDV by h.p.l.c.-u.v. and AMT by h.p.l.c.-m.s-m.s.

RESULTS

Compared with zidovudine-alone values in period 1, 14 days of coadministration with rifampicin significantly increased zidovudine oral clearance (89%) and formation clearances to GZDV (100%) and AMT (82%). Correspondingly, there were decreases in maximum plasma concentration (43%), AUC (47%) and urine recovery (37%) of zidovudine. GZDV/zidovudine and AMT/zidovudine AUC ratios increased by 99% and 36%, respectively, despite a significant 29% decrease in AMT AUC. After stopping rifampicin for 14 days, values of these pharmacokinetic parameters returned to within 26% of baseline. Over the three periods AMT plasma levels were <18 ng ml-1 (n=6) and <40 ng ml-1 (n=2), and molar AMT/zidovudine AUC ratios ranged from 1.7% to 4.5%.

CONCLUSIONS

Rifampicin induced zidovudine glucuronidation and amination pathways resulting in decreased plasma and urine exposures to zidovudine. AMT plasma exposure decreased because induction was more pronounced for the major GZDV metabolite. The magnitude of the residual inductive effect was minimal at 14 days after stopping rifampicin.

Effect of rifabutin on the pharmacokinetics of zidovudine in patients infected with human immunodeficiency virus

We investigated the effects of rifabutin (300 mg daily administered for 7 or 14 days) on the pharmacokinetics of zidovudine in nine patients who were infected with human immunodeficiency virus (HIV). Serial blood and urine samples were collected over a 6-hour period on each day that the pharmacokinetics of zidovudine were studied. Pharmacokinetic parameters were determined for zidovudine and its glucuronide metabolite and compared with use of analysis of variance (ANOVA) appropriate for a repeated-measures design. Except for a statistically significant decrease (28%) in the terminal half-life of zidovudine from 1.5 to 1.1 hours (P = .005) after coadministration of both agents for 14 days, concurrent administration of rifabutin for 7 or 14 days had no statistically significant effects on zidovudine plasma and urine pharmacokinetic parameters (the difference among treatment means was < 25%). Treatment with rifabutin is unlikely to influence the effectiveness of treating HIV-infected patients with zidovudine because of any pharmacokinetic interaction between these drugs.

Pharmacokinetics of simultaneously administered zidovudine and didanosine in HIV-seropositive male patients

Our objective was to determine whether a pharmacokinetic interaction exists between zidovudine and didanosine when they are coadministered. This was designed as a randomized, three-period, three-treatment, six-sequence, crossover study with a 1-week washout period between treatments. The patients were six men infected with human immunodeficiency virus who were asymptomatic. On three separate occasions, patients received zidovudine alone (200 mg every 8 h) for 3 days, didanosine alone (200 mg every 12 h) for 3 days, or zidovudine and didanosine for 3 days. On the fourth day, each patient received the final dose of each regimen, and blood and urine were serially collected for 8 h. Pharmacokinetic parameters were assessed for zidovudine, its glucuronide metabolite (GZDV), and didanosine. Coadministration of zidovudine had no significant effect on didanosine pharmacokinetic parameters (< 12% difference between treatment means, p > 0.1). Coadministration of didanosine did not significantly alter zidovudine pharmacokinetic parameters but did cause statistically significant increases in the renal and apparent formation clearances of GZDV (18.5% and 30.5% difference between the treatment means, respectively, p < 0.025). Therapeutic doses of zidovudine did not alter didanosine pharmacokinetic parameters. Coadministration of didanosine did not affect zidovudine parameters but did cause small alterations in GZDV pharmacokinetic values. These changes are unlikely to be clinically significant.

Effect of fluconazole on zidovudine pharmacokinetics in patients infected with human immunodeficiency virus

The effect of a therapeutic dose of fluconazole on the disposition of zidovudine was evaluated in 12 men infected with human immunodeficiency virus. The study was designed as a randomized, two-period, two-treatment, crossover trial. On two occasions, 21 days apart, patients received either zidovudine alone or zidovudine (each, 200 mg every 8 h) and fluconazole (400 mg daily) for 7 days. Fluconazole coadministration decreased (P < .001) the apparent oral serum clearance of zidovudine by 43% and the apparent oral formation clearance to zidovudine glucuronide (GZDV) by 48%, resulting in increases (P < .002) in the area under the serum concentration time curve (74%), the maximum serum concentration (84%), and the terminal half-life (128%) of zidovudine. The molar ratio of GZDV to zidovudine recovered in urine was reduced by 34% with fluconazole (P < .001). These pharmacokinetic changes suggest that 400 mg of fluconazole inhibited the conversion of zidovudine to GZDV. Patients receiving this combination should be monitored for the development of zidovudine-related adverse reactions.

Effect of antacids in didanosine tablet on bioavailability of isoniazid

The antacids in two didanosine placebo tablets had no significant effect on the plasma pharmacokinetics of a single oral dose of 300 mg of isoniazid administered to 12 healthy volunteers. These results suggest that isoniazid bioavailability will be unaffected by the antacids in didanosine tablets when the two medications are administered simultaneously to human immunodeficiency virus-seropositive patients.

Pharmacokinetics of zidovudine after the initial single dose and during chronic-dose therapy in HIV-infected patients

The pharmacokinetics of zidovudine in 10 symptomatic HIV-infected men were assessed after administration of the initial 200 mg oral dose at the start of therapy (day 1) and on one occasion during non steady-state conditions of chronic-dose therapy (200 mg every 4 h while awake on day 21-66; 5 doses per day). The following mean (+/- s.d.) pharmacokinetic parameters were determined on day 1 and during chronic therapy, respectively: Cmax (4.09 +/- 2.54 vs 3.82 +/- 1.03 mumol l-1), oral clearance (40.7 +/- 12.9 vs 41.1 +/- 8.4 ml min-1 kg-1) and terminal half-life (68.4 +/- 29.4 vs 65.1 +/- 13.1 min). Mean pharmacokinetic parameters on day 1 were < 10% different (P > 0.34) from those determined during chronic therapy. Differences in means of 21% for clearance and 36% for Cmax could be detected with a power of 80% at the 5% significance level. Intra-subject coefficients of variation were 17% for clearance and 32% for Cmax, respectively. This study suggests that within a group of patients whose stage of HIV infection and general health are relatively stable, first-dose pharmacokinetic parameters provide a good estimate of multiple-dose pharmacokinetic parameters.

Evaluation of the in vivo effect of naproxen on zidovudine pharmacokinetics in patients infected with human immunodeficiency virus

OBJECTIVE

To determine if therapeutic doses of naproxen affect the in vivo disposition of zidovudine.

METHODS

This was designed as a randomized, two-period, two-treatment, crossover study. The patients were 12 men infected with human immunodeficiency virus who had acquired immunodeficiency syndrome (AIDS) or AIDS-related complex. On two separate occasions 14 days apart, patients received either zidovudine alone (200 mg every 4 hours while awake) or zidovudine (200 mg every 4 hours while awake) and naproxen (500 mg every 12 hours for 4 days). On the morning of the fifth day, each patient received the final dose of each regimen and blood and urine were serially collected for 8 hours. Pharmacokinetic parameters (area under the serum concentration-time curve [AUC], maximum plasma concentration, terminal half-life, renal clearance, and urinary recovery) were assessed for zidovudine and its glucuronide metabolite.

MAIN RESULTS

Naproxen had no significant effect (< 10% difference between treatment means, p > 0.15, ANOVA) on the above pharmacokinetic parameters for both zidovudine and its metabolite. Although the power of the study to detect these small differences was < 80% at the 5% significance level, differences ranging from 12.6% for AUC to 38.8% for urinary recovery could be detected with 80% power.

CONCLUSION

Therapeutic doses of naproxen do not significantly affect the pharmacokinetic disposition of zidovudine.

Postabsorption antidotal effects of N-acetylcysteine on acetaminophen-induced hepatotoxicity in the mouse

Male Swiss Webster mice, treated with N-acetylcysteine (NAC, 500 mg/kg po) 1 h following acetaminophen (NAPA, 350 mg/kg po) administration, had control levels of transaminases indicating that NAC protects against NAPA-induced hepatotoxicity by postabsorption antidotal mechanism(s). Hepatic congestion induced by NAPA was reduced by NAC. Significantly higher elimination rate constants (K) for indocyanine green (500 micrograms/kg, iv) in mice treated with NAPA and NAC (K = 0.676 +/- 0.062) than in animals receiving NAPA alone (0.341 +/- 0.105) suggested NAC improved or preserved the hepatic circulation of the compromised liver. This NAC-induced improvement and (or) preservation of hepatic circulation was reflected in biliary and urinary excretion of acetaminophen and its metabolites by a general increase in elimination during the first 6 h (70.2 +/- 2.6 vs. 32.6 +/- 7.1%), and in the repletion of glutathione (GSH) in the liver by a return to control levels more quickly (3 vs. greater than 5 h) following depletion by NAPA. The metabolic consequences of the postabsorption antidotal effect of NAC in the compromised liver was a preferential excretion of sulphydryl-derived metabolites in the 1-4 h bile (GSH conjugate 11.30 +/- 1.25 vs. 7.25 +/- 0.39%) which was subsequently observed in the urine by preferential excretion of glutathione degradation products.