Stavudine clinical pharmacology

# Clinical Pharmacology

## Mechanism of Action
Stavudine is an antiviral drug.

## Pharmacokinetics
The pharmacokinetics of stavudine have been evaluated in HIV-1-infected adult and pediatric patients (Tables 7, 8, and 9). Peak plasma concentrations (Cmax) and area under the plasma concentration-time curve (AUC) increased in proportion to dose after both single and multiple doses ranging from 0.03 to 4 mg/kg. There was no significant accumulation of stavudine with repeated administration every 6, 8, or 12 hours.

## Absorption
Following oral administration, stavudine is rapidly absorbed, with peak plasma concentrations occurring within 1 hour after dosing. The systemic exposure to stavudine is the same following administration as capsules or solution. Steady-state pharmacokinetic parameters of ZERIT (stavudine) in HIV-1-infected adults are shown in Table 7.

## Distribution
Binding of stavudine to serum proteins was negligible over the concentration range of 0.01 to 11.4 g/mL. Stavudine distributes equally between red blood cells and plasma. Volume of distribution is shown in Table 8.

## Metabolism
Metabolism plays a limited role in the clearance of stavudine. Unchanged stavudine was the major drug-related component circulating in plasma after an 80-mg dose of 14C-stavudine, while metabolites constituted minor components of the circulating radioactivity. Minor metabolites include oxidized stavudine, glucuronide conjugates of stavudine and its oxidized metabolite, and an Nacetylcysteine conjugate of the ribose after glycosidic cleavage, suggesting that thymine is also a metabolite of stavudine.

## Elimination
Following an 80-mg dose of 14C-stavudine to healthy subjects, approximately 95% and 3% of the total radioactivity was recovered in urine and feces, respectively. Radioactivity due to parent drug in urine and feces was 73.7% and 62.0%, respectively. The mean terminal elimination half-life is approximately 2.3 hours following single oral doses. Mean renal clearance of the parent compound is approximately 272 mL/min, accounting for approximately 67% of the apparent oral clearance.
In HIV-1-infected patients, renal elimination of unchanged drug accounts for about 40% of the overall clearance regardless of the route of administration (Table 8). The mean renal clearance was about twice the average endogenous creatinine clearance, indicating active tubular secretion in addition to glomerular filtration.

## Special Populations

### Pediatric
Pharmacokinetic parameters of stavudine in pediatric patients are presented in Table 9.

## Renal Impairment
Data from two studies in adults indicated that the apparent oral clearance of stavudine decreased and the terminal elimination half-life increased as creatinine clearance decreased (see Table 10). Cmax and Tmax were not significantly altered by renal impairment. The mean  SD hemodialysis clearance value of stavudine was 120  18 mL/min (n=12); the mean  SD percentage of the stavudine dose recovered in the dialysate, timed to occur between 26 hours post-dose, was 31  5%. Based on these observations, it is recommended that ZERIT (stavudine) dosage be modified in patients with reduced creatinine clearance and in patients receiving maintenance hemodialysis .

## Hepatic Impairment
Stavudine pharmacokinetics were not altered in five non-HIV-infected patients with hepatic impairment secondary to cirrhosis (Child-Pugh classification B or C) following the administration of a single 40-mg dose.

## Geriatric
Stavudine pharmacokinetics have not been studied in patients >65 years of age. 

## Gender
A population pharmacokinetic analysis of data collected during a controlled clinical study in HIV-1-infected patients showed no clinically important differences between males (n=291) and females (n=27).

## Race
A population pharmacokinetic analysis of data collected during a controlled clinical study in HIV-1-infected patients showed no clinically important differences between races (n=233 Caucasian, 39 African-American, 41 Hispanic, 1 Asian, and 4 other).

## Drug Interaction Studies
Stavudine does not inhibit the major cytochrome P450 isoforms CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4; therefore, it is unlikely that clinically significant drug interactions will occur with drugs metabolized through these pathways. Because stavudine is not protein-bound, it is not expected to affect the pharmacokinetics of protein-bound drugs.
Tables 11 and 12 summarize the effects on AUC and Cmax, with a 95% confidence interval (CI) when available, following coadministration of ZERIT with didanosine, lamivudine, and nelfinavir. No clinically significant pharmacokinetic interactions were observed.