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Mechanism of Action
Famciclovir is the diacetyl 6-deoxy analog of the active antiviral compound penciclovir. Following oral administration famciclovir undergoes rapid and extensive metabolism to penciclovir and little or no famciclovir is detected in plasma or urine. Penciclovir is predominantly eliminated unchanged by the kidney. Therefore, the dose of FAMVIR needs to be adjusted in patients with different degrees of renal impairment [see DOSAGE AND ADMINISTRATION].
Pharmacokinetics in adults
Absorption and Bioavailability
The absolute bioavailability of penciclovir is 77 ± 8% as determined following the administration of a 500 mg famciclovir oral dose and a 400 mg penciclovir intravenous dose to 12 healthy male subjects.
Penciclovir concentrations increased in proportion to dose over a famciclovir dose range of 125 mg to 1000 mg administered as a single dose. Table 5 shows the mean pharmacokinetic parameters of penciclovir after single administration of FAMVIR to healthy male volunteers.
Table 5 : Mean
Pharmacokinetic Parameters of Penciclovir in Healthy Adult Subjects*
|Dose||AUC (0-inf) † (mcg hr/mL)||Cmax‡ (mcg/mL)||t max§ (h)|
|* Based on pharmacokinetic data
from 17 studies
† AUC (0-inf) (mcg hr/mL)=area under the plasma concentration-time profile extrapolated to infinity.
‡ Cmax (mcg/mL)=maximum observed plasma concentration.
§ tmax (h)= time to Cmax.
Following oral single-dose administration of 500 mg famciclovir to 7 patients with herpes zoster, the AUC (mean ± SD), Cmax, and tmax were 12.1±1.7 mcg hr/mL, 4.0±0.7 mcg/mL, and 0.7±0.2 hours, respectively. The AUC of penciclovir was approximately 35% greater in patients with herpes zoster as compared to healthy volunteers. Some of this difference may be due to differences in renal function between the 2 groups.
There is no accumulation of penciclovir after the administration of 500 mg famciclovir three times daily for 7 days.
Penciclovir Cmax decreased approximately 50% and tmax was delayed by 1.5 hours when a capsule formulation of famciclovir was administered with food (nutritional content was approximately 910 Kcal and 26% fat). There was no effect on the extent of availability (AUC) of penciclovir. There was an 18% decrease in Cmax and a delay in tmax of about 1 hour when famciclovir was given 2 hours after a meal as compared to its administration 2 hours before a meal. Because there was no effect on the extent of systemic availability of penciclovir, FAMVIR can be taken without regard to meals.
The volume of distribution (Vdβ) was 1.08±0.17 L/kg in 12 healthy male subjects following a single intravenous dose of penciclovir at 400 mg administered as a 1-hour intravenous infusion. Penciclovir is < 20% bound to plasma proteins over the concentration range of 0.1 to 20 mcg/mL. The blood/plasma ratio of penciclovir is approximately 1.
Following oral administration, famciclovir is deacetylated and oxidized to form penciclovir. Metabolites that are inactive include 6-deoxy penciclovir, monoacetylated penciclovir, and 6-deoxy monoacetylated penciclovir (5%, < 0.5% and < 0.5% of the dose in the urine, respectively). Little or no famciclovir is detected in plasma or urine. An in vitro study using human liver microsomes demonstrated that cytochrome P450 does not play an important role in famciclovir metabolism. The conversion of 6-deoxy penciclovir to penciclovir is catalyzed by aldehyde oxidase. Cimetidine and promethazine, in vitro inhibitors of aldehyde oxidase, did not show relevant effects on the formation of penciclovir in vivo [see DRUG INTERACTIONS].
Approximately 94% of administered radioactivity was recovered in urine over 24 hours (83% of the dose was excreted in the first 6 hours) after the administration of 5 mg/kg radiolabeled penciclovir as a 1-hour infusion to 3 healthy male volunteers. Penciclovir accounted for 91% of the radioactivity excreted in the urine.
Following the oral administration of a single 500 mg dose of radiolabeled famciclovir to 3 healthy male volunteers, 73% and 27% of administered radioactivity were recovered in urine and feces over 72 hours, respectively. Penciclovir accounted for 82% and 6-deoxy penciclovir accounted for 7% of the radioactivity excreted in the urine. Approximately 60% of the administered radiolabeled dose was collected in urine in the first 6 hours.
After intravenous administration of penciclovir in 48 healthy male volunteers, mean ± SD total plasma clearance of penciclovir was 36.6±6.3 L/hr (0.48±0.09 L/hr/kg). Penciclovir renal clearance accounted for 74.5±8.8% of total plasma clearance.
Renal clearance of penciclovir following the oral administration of a single 500 mg dose of famciclovir to 109 healthy male volunteers was 27.7±7.6 L/hr. Active tubular secretion contributes to the renal elimination of penciclovir.
The plasma elimination half-life of penciclovir was 2.0±0.3 hours after intravenous administration of penciclovir to 48 healthy male volunteers and 2.3±0.4 hours after oral administration of 500 mg famciclovir to 124 healthy male volunteers. The half-life in 17 patients with herpes zoster was 2.8±1.0 hours and 2.7±1.0 hours after single and repeated doses, respectively.
Geriatric patients: Based on cross study comparison, penciclovir AUC was 40% higher and penciclovir renal clearance was 22% lower in elderly subjects (n=18, age 65-79 years) as compared with younger subjects Some of this difference may be due to differences in renal function between the 2 groups. No famciclovir dosage adjustment based on age is recommended unless renal function is impaired [see DOSAGE AND ADMINISTRATION, Use In Specific Populations.]
Patients with renal impairment: In subjects with varying degrees of renal impairment, apparent plasma clearance, renal clearance, and the plasma-elimination rate constant of penciclovir decreased linearly with reductions in renal function, after both single and repeated dosing [see Use In Specific Populations]. A dosage adjustment is recommended for patients with renal impairment [see DOSAGE AND ADMINISTRATION].
Patients with hepatic impairment: Mild or moderate hepatic impairment had no effect on the extent of availability (AUC) of penciclovir [see Use In Specific Populations]. No dosage adjustment is recommended for patients with mild or moderate hepatic impairment. The effect of severe hepatic impairment on the pharmacokinetics of penciclovir has not been evaluated.
HIV-infected patients: Following oral administration of a single dose of 500 mg famciclovir to HIV-positive patients, the pharmacokinetic parameters of penciclovir were comparable to those observed in healthy subjects.
Gender: The pharmacokinetics of penciclovir were evaluated in 18 healthy male and 18 healthy female volunteers after single-dose oral administration of 500 mg famciclovir. AUC of penciclovir was 9.3±1.9 mcg hr/mL and 11.1±2.1 mcg hr/mL in males and females, respectively. Penciclovir renal clearance was 28.5±8.9 L/hr and 21.8±4.3 L/hr, respectively.
These differences were attributed to differences in renal function between the 2 groups. No famciclovir dosage adjustment based on gender is recommended.
Race: A retrospective evaluation was performed to compare the pharmacokinetic parameters obtained in black and Caucasian subjects after single and repeat once-daily, twice-daily, or three times-daily administration of famciclovir 500 mg. Data from a study in healthy volunteers (single dose), a study in subjects with varying degrees of renal impairment (single and repeat dose) and a study in subjects with hepatic impairment (single dose) did not indicate any significant differences in the pharmacokinetics of penciclovir between black and Caucasian subjects.
Mechanism of action
Famciclovir is a prodrug of penciclovir, which has demonstrated inhibitory activity against herpes simplex virus types 1 (HSV-1) and 2 (HSV-2) and varicella zoster virus (VZV). In cells infected with HSV-1, HSV-2 or VZV, the viral thymidine kinase phosphorylates penciclovir to a monophosphate form that, in turn, is converted by cellular kinases to the active form penciclovir triphosphate. Biochemical studies demonstrate that penciclovir triphosphate inhibits HSV-2 DNA polymerase competitively with deoxyguanosine triphosphate. Consequently, herpes viral DNA synthesis and, therefore, replication are selectively inhibited. Penciclovir triphosphate has an intracellular half-life of 10 hours in HSV-1-, 20 hours in HSV-2-and 7 hours in VZV-infected cells grown in culture. However, the clinical significance of the intracellular half-life is unknown.
In cell culture studies, penciclovir is inhibitory to the following herpes viruses: HSV-1, HSV-2 and VZV. The antiviral activity of penciclovir against wild type strains grown on human foreskin fibroblasts was assessed with a plaque reduction assay and staining with crystal violet 3 days postinfection for HSV and 10 days postinfection for VZV. The median EC50 values of penciclovir against laboratory and clinical isolates of HSV-1, HSV-2, and VZV were 2 μM (range 1.2 to 2.4 μM, n = 7), 2.6 μM (range 1.6 to 11 μM, n = 6), and 34 μM (range 6.7 to 71 μM, n = 6), respectively.
Penciclovir-resistant mutants of HSV and VZV can result from mutations in the viral thymidine kinase (TK) and DNA polymerase genes. Mutations in the viral TK gene may lead to complete loss of TK activity (TK negative), reduced levels of TK activity (TK partial), or alteration in the ability of viral TK to phosphorylate the drug without an equivalent loss in the ability to phosphorylate thymidine (TK altered). The median EC50 values observed in a plaque reduction assay with penciclovir resistant HSV-1, HSV-2, and VZV were 69 μM (range 14 to 115 μM, n = 6), 46 μM (range 4 to > 395 μM, n = 9), and 92 μM (range 51 to 148 μM, n = 4), respectively. The possibility of viral resistance to penciclovir should be considered in patients who fail to respond or experience recurrent viral shedding during therapy.
Cross-resistance has been observed among HSV DNA polymerase inhibitors. The most commonly encountered acyclovir resistant mutants that are TK negative are also resistant to penciclovir.
Herpes Labialis (Cold Sores)
A randomized, double-blind, placebo-controlled trial was conducted in 701 immunocompetent adults with recurrent herpes labialis. Patients self-initiated therapy within 1 hour of first onset of signs or symptoms of a recurrent herpes labialis episode with FAMVIR 1500 mg as a single dose (n=227), FAMVIR 750 mg twice daily (n=220) or placebo (n=254) for 1 day. The median time to healing among patients with non-aborted lesions (progressing beyond the papule stage) was 4.4 days in the FAMVIR 1500 mg single-dose group (n=152) as compared to 6.2 days in the placebo group (n=168). The median difference in time to healing between the placebo and FAMVIR 1500 mg treated groups was 1.3 days (95% CI: 0.6 – 2.0). No differences in proportion of patients with aborted lesions (not progressing beyond the papule stage) were observed between patients receiving FAMVIR or placebo: 33% for FAMVIR 1500 mg single dose and 34% for placebo. The median time to loss of pain and tenderness was 1.7 days in FAMVIR 1500 mg single dose-treated patients vs. 2.9 days in placebo-treated patients.
A randomized, double-blind, placebo-controlled trial was conducted in 329 immunocompetent adults with recurrent genital herpes. Patients self-initiated therapy within 6 hours of the first sign or symptom of a recurrent genital herpes episode with either FAMVIR 1000 mg twice daily (n=163) or placebo (n=166) for 1 day. The median time to healing among patients with non-aborted lesions (progressing beyond the papule stage) was 4.3 days in FAMVIR-treated patients (n=125) as compared to 6.1 days in placebo-treated patients (n=145). The median difference in time to healing between the placebo and FAMVIR-treated groups was 1.2 days (95% CI: 0.5 to 2.0). Twenty-three percent of FAMVIR-treated patients had aborted lesions (no lesion development beyond erythema) vs. 13% in placebo-treated patients. The median time to loss of all symptoms (e.g., tingling, itching, burning, pain, or tenderness) was 3.3 days in FAMVIR-treated patients vs. 5.4 days in placebo-treated patients.
A randomized (2:1), double-blind, placebo-controlled trial was conducted in 304 immunocompetent black and African American adults with recurrent genital herpes. Patients self-initiated therapy within 6 hours of the first sign or symptom of a recurrent genital herpes episode with either FAMVIR 1000 mg twice daily (n=206) or placebo (n=98) for 1 day. The median time to healing among patients with non-aborted lesions was 5.4 days in FAMVIR-treated patients (n=152) as compared to 4.8 days in placebo-treated patients (n=78). The median difference in time to healing between the placebo and FAMVIR-treated groups was -0.26 days (95% CI: -0.98 to 0.40).
Two randomized, double-blind, placebo-controlled, 12-month trials were conducted in 934 immunocompetent adults with a history of 6 or more recurrences of genital herpes episodes per year. Comparisons included FAMVIR 125 mg three times daily, 250 mg twice daily, 250 mg three times daily, and placebo. At 12 months, 60% to 65% of patients were still receiving FAMVIR and 25% were receiving placebo treatment. Recurrence rates at 6 and 12 months in patients treated with the 250 mg twice daily dose are shown in Table 6.
Table 6 : Recurrence Rates at 6 and 12 Months in
Adults with Recurrent Genital Herpes on Suppressive Therapy
|Recurrence Rates at 6 Months||Recurrence Rates at 12 Months|
|Famvir 250 mg twice daily
|Famvir 250 mg twice daily
|Lost to follow-up*||14%||16%||11%||16%|
|†Based on patient reported data; not necessarily
confirmed by a physician.
‡Patients recurrence-free at time of last contact prior to withdrawal.
FAMVIR-treated patients had approximately 1/5 the median number of recurrences as compared to placebo-treated patients. Higher doses of FAMVIR were not associated with an increase in efficacy.
Recurrent Orolabial or Genital Herpes in HIV-Infected Patients
A randomized, double-blind trial compared famciclovir 500 mg twice daily for 7 days (n=150) with oral acyclovir 400 mg 5 times daily for 7 days (n=143) in HIV-infected patients with recurrent orolabial or genital herpes treated within 48 hours of lesion onset. Approximately 40% of patients had a CD4+ count below 200 cells/mm³, 54% of patients had anogenital lesions and 35% had orolabial lesions. Famciclovir therapy was comparable to oral acyclovir in reducing new lesion formation and in time to complete healing.
Herpes Zoster (Shingles)
Two randomized, double-blind trials, 1 placebo-controlled and 1 active-controlled, were conducted in 964 immunocompetent adults with uncomplicated herpes zoster. Treatment was initiated within 72 hours of first lesion appearance and was continued for 7 days.
In the placebo-controlled trial, 419 patients were treated with either FAMVIR 500 mg three times daily (n=138), FAMVIR 750 mg three times daily (n=135) or placebo (n=146). The median time to full crusting was 5 days among FAMVIR 500 mg-treated patients as compared to 7 days in placebo-treated patients. The times to full crusting, loss of vesicles, loss of ulcers, and loss of crusts were shorter for FAMVIR 500 mg-treated patients than for placebo-treated patients in the overall study population. The effects of FAMVIR were greater when therapy was initiated within 48 hours of rash onset; it was also more profound in patients 50 years of age or older. Among the 65.2% of patients with at least 1 positive viral culture, FAMVIR treated patients had a shorter median duration of viral shedding than placebo-treated patients (1 day and 2 days, respectively).
There were no overall differences in the duration of pain before rash healing between FAMVIR-and placebo-treated groups. In addition, there was no difference in the incidence of pain after rash healing (postherpetic neuralgia) between the treatment groups. In the 186 patients (44.4% of total study population) who developed postherpetic neuralgia, the median duration of postherpetic neuralgia was shorter in patients treated with FAMVIR 500 mg than in those treated with placebo (63 days and 119 days, respectively). No additional efficacy was demonstrated with higher dose of FAMVIR.
In the active-controlled trial, 545 patients were treated with 1 of 3 doses of FAMVIR three times daily or with acyclovir 800 mg five times daily. Times to full lesion crusting and times to loss of acute pain were comparable for all groups and there were no statistically significant differences in the time to loss of postherpetic neuralgia between FAMVIR and acyclovir-treated groups.
Last reviewed on RxList: 5/14/2013
This monograph has been modified to include the generic and brand name in many instances.
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