Valacyclovir Hydrochloride (Page 7 of 13)

12 CLINICAL PHARMACOLOGY

12.1 Mechanism of Action

Valacyclovir is an antiviral drug active against α-herpes viruses [see Microbiology (12.4)].

12.3 Pharmacokinetics

The pharmacokinetics of valacyclovir and acyclovir after oral administration of valacyclovir tablets have been investigated in 14 volunteer trials involving 283 adults and in 3 trials involving 112 pediatric subjects aged 1 month to less than 12 years.

Pharmacokinetics in Adults

Absorption and Bioavailability

After oral administration, valacyclovir hydrochloride is rapidly absorbed from the gastrointestinal tract and nearly completely converted to acyclovir and L- valine by first-pass intestinal and/or hepatic metabolism.

The absolute bioavailability of acyclovir after administration of valacyclovir tablets is 54.5% ± 9.1% as determined following a 1-gram oral dose of valacyclovir tablets and a 350-mg intravenous acyclovir dose to 12 healthy volunteers. Acyclovir bioavailability from the administration of valacyclovir tablets is not altered by administration with food (30 minutes after an 873 Kcal breakfast, which included 51 grams of fat).

Acyclovir pharmacokinetic parameter estimates following administration of valacyclovir tablets to healthy adult volunteers are presented in Table 3. There was a less than dose-proportional increase in acyclovir maximum concentration (Cmax ) and area under the acyclovir concentration-time curve (AUC) after single-dose and multiple-dose administration (4 times daily) of valacyclovir tablets from doses between 250 mg to 1 gram.

There is no accumulation of acyclovir after the administration of valacyclovir at the recommended dosage regimens in adults with normal renal function.

Table 3. Mean (± SD) Plasma Acyclovir Pharmacokinetic Parameters Following Administration of Valacyclovir Tablets to Healthy Adult Volunteers
ND = not done.
*
Administered 4 times daily for 11 days.

Dose

Single-Dose Administration

(N = 8)

Multiple-Dose Administration *

(N = 24, 8 per treatment arm)

Cmax (± SD)

(mcg/mL)

AUC (± SD) (hr • mcg/mL)

Cmax (± SD) (mcg/mL)

AUC (± SD) (hr • mcg/mL)

100 mg

0.83 (± 0.14)

2.28 (± 0.40)

ND

ND

250 mg

2.15 (± 0.50)

5.76 (± 0.60)

2.11 (± 0.33)

5.66 (± 1.09)

500 mg

3.28 (± 0.83)

11.59 (± 1.79)

3.69 (± 0.87)

9.88 (± 2.01)

750 mg

4.17 (± 1.14)

14.11 (± 3.54)

ND

ND

1,000 mg

5.65 (± 2.37)

19.52 (± 6.04)

4.96 (± 0.64)

15.70 (± 2.27)

Distribution

The binding of valacyclovir to human plasma proteins ranges from 13.5% to 17.9%. The binding of acyclovir to human plasma proteins ranges from 9% to 33%.

Metabolism

Valacyclovir is converted to acyclovir and L-valine by first-pass intestinal and/or hepatic metabolism. Acyclovir is converted to a small extent to inactive metabolites by aldehyde oxidase and by alcohol and aldehyde dehydrogenase. Neither valacyclovir nor acyclovir is metabolized by cytochrome P450 enzymes. Plasma concentrations of unconverted valacyclovir are low and transient, generally becoming non-quantifiable by 3 hours after administration. Peak plasma valacyclovir concentrations are generally less than 0.5 mcg/mL at all doses. After single-dose administration of 1 gram of valacyclovir tablets, average plasma valacyclovir concentrations observed were 0.5, 0.4, and 0.8 mcg/mL in subjects with hepatic dysfunction, renal insufficiency, and in healthy subjects who received concomitant cimetidine and probenecid, respectively.

Elimination

The pharmacokinetic disposition of acyclovir delivered by valacyclovir is consistent with previous experience from intravenous and oral acyclovir. Following the oral administration of a single 1-gram dose of radiolabeled valacyclovir to 4 healthy subjects, 46% and 47% of administered radioactivity was recovered in urine and feces, respectively, over 96 hours. Acyclovir accounted for 89% of the radioactivity excreted in the urine. Renal clearance of acyclovir following the administration of a single 1-gram dose of valacyclovir tablets to 12 healthy subjects was approximately 255 ± 86 mL/min which represents 42% of total acyclovir apparent plasma clearance.

The plasma elimination half-life of acyclovir typically averaged 2.5 to 3.3 hours in all trials of valacyclovir tablets in subjects with normal renal function.

Specific Populations

Patients with Renal Impairment

Reduction in dosage is recommended in patients with renal impairment [see Dosage and Administration (2.4), Use in Specific Populations (8.5, 8.6)].

Following administration of valacyclovir tablets to subjects with ESRD, the average acyclovir half-life is approximately 14 hours. During hemodialysis, the acyclovir half-life is approximately 4 hours. Approximately one-third of acyclovir in the body is removed by dialysis during a 4-hour hemodialysis session. Apparent plasma clearance of acyclovir in subjects on dialysis was 86.3 ± 21.3 mL/min/1.73 m2 compared with 679.16 ± 162.76 mL/min/1.73 m2 in healthy subjects.

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