Submission Control Number: 117774

is a registered trademark

Table of Contents

PART I: HEALTH PROFESSIONAL INFORMATION 3

SUMMARY PRODUCT INFORMATION 3 INDICATIONS AND CLINICAL USE 3 CONTRAINDICATIONS 4 WARNINGS AND PRECAUTIONS 4 ADVERSE REACTIONS 6 DRUG INTERACTIONS 10 DOSAGE AND ADMINISTRATION 11 OVERDOSAGE 13 ACTION AND CLINICAL PHARMACOLOGY 13 STORAGE AND STABILITY 16 DOSAGE FORMS, COMPOSITION AND PACKAGING 16

PART II: SCIENTIFIC INFORMATION 18

PHARMACEUTICAL INFORMATION 18 CLINICAL TRIALS 19 TOXICOLOGY 24 REFERENCES 27

PART III: CONSUMER INFORMATION. 29

PrFAMVIR * (famciclovir)

PART I: HEALTH PROFESSIONAL INFORMATION SUMMARY PRODUCT INFORMATION

CONTRAINDICATIONS

Patients who have known hypersensitivity to FAMVIR * (famciclovir) or to any ingredient in the formulation or component of the container. For a complete listing, see the Dosage Forms, Composition and Packaging section of this product monograph.

WARNINGS AND PRECAUTIONS

General: The efficacy of FAMVIR * (famciclovir) has not been established for first episode genital herpes infections, disseminated zoster, or in immunocompromised patients with herpes zoster (see Actions and Clinical Pharmacology). Dosage adjustment is required when administering famciclovir to patients with moderate or severe renal dysfunction (see Dosage and Administration). No special precautions are required for patients with well-compensated hepatic impairment. Famciclovir has not been studied in patients with severe uncompensated hepatic impairment (see Action and Clinical pharmacology. Genital herpes is a sexually transmitted disease with an increased risk of transmission during acute episodes. There are no data evaluating whether FAMVIR * will prevent transmission of infection to others. Patients should be advised to avoid intercourse when lesions and/or symptoms are present (even if treatment with an anti-viral has been initiated) in order to avoid infecting partners. Genital herpes can also be transmitted in the absence of symptoms through asymptomatic viral shedding. FAMVIR * 125mg, 250 mg and 500 mg tablets contain lactose (26.9 mg, 53.7 mg and 107.4 mg, respectively). Patients with rare heredity problems of galactose intolerance, a severe case of lactase deficiency or glucose-galactose malabsorption should not take FAMVIR * 125mg, 250 mg and 500 mg tablets.

Pregnancy: Although animal studies have not shown any embryotoxic or teratogenic effects with famciclovir or penciclovir, the safety of famciclovir in human pregnancy has not been established. Because animal reproductive studies are not always predictive of human response, famciclovir should, therefore, not be used in pregnancy unless the potential benefits are considered to outweigh the potential risks associated with treatment.

Lactation: Following oral administration of famciclovir to lactating rats, penciclovir is excreted in milk. It is not known whether it (penciclovir) is excreted in human milk, thus, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

Use in Children:

Safety and efficacy in children under the age of 18 years has not been established. Geriatric Use: Of 816 patients with herpes zoster in clinical studies who were treated with famciclovir, 248 (30.4%) were >65 years of age and 103 (13%) were >75 years of age. No overall differences were observed in safety between younger and older patients (see Adverse Events).

Impairment of Fertility

As with other drugs of this class, testicular toxicity has been observed in animals receiving both famciclovir and penciclovir. Two placebo-controlled studies in a total of 130 otherwise healthy men with a normal sperm profile over an 8 week baseline period and recurrent genital herpes receiving oral famciclovir (250 mg bid) (n=66) or placebo (n=64) therapy for 18 weeks showed no evidence of significant effects on sperm count, motility or morphology during treatment or during an 8 week follow-up. Preliminary results of another placebo-controlled trial in a total of 117 otherwise healthy men with recurrent genital herpes and a normal sperm profile over an 8 week baseline period receiving famciclovir (250 mg bid, n=59) and placebo (n=58) therapy for 52 weeks showed no evidence of significant effects in sperm concentration, total sperm count, percent motility, percent abnormal morphology and percent dead sperm during treatment or during a 12 week follow-up.

Effects on ability to drive and use machines

FAMVIR * can cause dizziness, drowsiness or confusion in very rare cases. Patients who experience any of these symptoms while taking FAMVIR * should take special care when driving or using machines (see Adverse events - Post-marketing Experience).

DRUG INTERACTIONS

Effects of other medicinal products on famciclovir

No clinically significant alterations in penciclovir (active metabolite of famciclovir) pharmacokinetics were observed following single dose administration of 500 mg famciclovir after pretreatment with multiple doses of cimetidine, allopurinol, theophylline, zidovudine, or promethazine or when given shortly after an antacid (magnesium and aluminium hydroxide), or concomitantly with emtricitabine. Furthermore, no clinically significant effect on penciclovir pharmacokinetics was observed following multiple-dose (t.i.d.) administration of famciclovir (500 mg) with multiple doses of digoxin. Probenecid and other drugs that affect renal physiology could affect plasma levels of penciclovir. The conversion of the inactive metabolite 6-deoxy penciclovir (formed by deacetylation of famciclovir) to penciclovir is catalysed by aldehyde oxidase. Interactions with other drugs metabolized by this enzyme and/or inhibiting this enzyme could potentially occur. Clinical interaction studies of famciclovir with cimetidine and promethazine, in vitro inhibitors of aldehyde oxidase, did not show relevant effects on the formation of penciclovir. However. raloxifene, the most potent aldehyde oxidase inhibitor observed in vitro, could affect the formation of penciclovir.

Effects of famciclovir on other medicinal products

The pharmacokinetics of digoxin were not altered by concomitant administration of single or multiple (t.i.d) doses of famciclovir (500 mg). No clinically significant effects on the pharmacokinetics of zidovudine, its metabolite zidovudine glucuronide or emtricitabine were observed following a single oral dose of 500 mg famciclovir co-administered with zidovudine or emtricitabine. Although famciclovir is only a weak inhibitor of aldehyde oxidase in vitro, interactions with drugs metabolized by aldehyde oxidase could potentially occur. Evidence from preclinical studies has shown no potential for induction of cytochrome P450 enzymes and inhibition of CYP3A4.

DOSAGE AND ADMINISTRATION

Recommended dose

Herpes zoster infections: The recommended dose is 500 mg 3 times per day for 7 days. Therapy should be initiated within 72 hours of the onset of the rash.

Herpes simplex infections:

Immunocompetent Patients

Recurrent genital herpes episodes

: The recommended dosage is 125 mg twice a day for 5 days. Initiation of treatment is recommended during the prodromal period or as soon as possible after onset of lesions.

Suppression of recurrent genital herpes episodes:

The recommended dosage is 250 mg twice daily for up to 1 year. The safety and efficacy of Famvir * therapy beyond one year of treatment has not been established.

HIV-Infected Patients

For recurrent episodes of mucocutaneous herpes simplex infection, the recommended dosage is 500 mg twice a day for 7 days. FAMVIR * (famciclovir) tablets should be swallowed whole and may be taken with or without food.

Dosage Adjustment

Dosage in renally impaired patients:

In patients with moderately or severely reduced renal function, dosage reduction is recommended: Indication Creatinine clearance (mL/min/1.73m2)

Dosage

500 mg every 8 hours 500 mg every 12 hours 500 mg every 24 hours 250 mg every 48 hours Recurrent Genital Herpes >40 20-39 <20 125 mg every 12 hours 125 mg every 24 hours 125 mg every 48 hours Suppression of Recurrent Genital Herpes >40 20-39 <20 250 mg every 12 hours 125 mg every 12 hours 125 mg every 24 hours Recurrent episodes of mucocutaneous herpes simplex infections in HIV- infected patients >40 20-39 <20 500 mg every 12 hours 500 mg every 24 hours 250 mg every 24 hours Hemodialysis patients: Following each dialysis treatment, the recommended dose of famciclovir is 250 mg (herpes zoster) or 125 mg (genital herpes) in immunocompetent patients and 250 mg (recurrent episodes of mucocutaneous herpes simplex) in HIV-infected patients.

Dosage in hepatically impaired patients:

No dosage adjustment is required in patients with well-compensated hepatic impairment. No data are available for patients with severe uncompensated hepatic impairment (see ACTION and CLINICAL PHARMACOLOGY).

Missed Dose

If a dose of FAMVIR * is missed, it should be taken as soon as the patient remembers. The next dose should be taken at the normal time. The patient should carry on as normal until they have finished all the tablets. Do not double-dose.

OVERDOSAGE

No acute overdosage has been reported. Appropriate symptomatic and supportive therapy should be given. Penciclovir is dialyzable; plasma concentrations are reduced by approximately 75% following 4 h hemodialysis. In patients with underlying renal disease who have received inappropriately high doses of famciclovir for their level of renal function, acute renal failure has been reported frequently.

ACTION AND CLINICAL PHARMACOLOGY

FAMVIR * (famciclovir) is the orally administered pro-drug of the antiviral agent penciclovir. Famciclovir itself has no antiviral activity until it is biotransformed to penciclovir. Studies in volunteers have shown that famciclovir is well absorbed and produces plasma penciclovir concentrations superior to those obtained following oral administration of penciclovir alone. The mean bioavailability of penciclovir after administration of oral famciclovir is 77%. The mean peak plasma concentration of penciclovir, following a 500 mg oral dose of famciclovir was 3.3 mcg/mL and occurred at a mean time of 0.89 hours post-dose. Plasma concentration time curves of penciclovir are similar following single and repeat dosing. The terminal plasma elimination half-life of penciclovir after both single and repeat oral dosing with famciclovir is 2.3 hours. The elimination of famciclovir is by metabolism, principally to penciclovir and its 6- deoxy precursor, which are subsequently excreted in urine (See Pharmacokinetics).

Mechanism of Action

Penciclovir is a substituted guanine analogue with potent and selective antiviral activity against varicella zoster virus and other human herpes viruses (see Virology). Penciclovir is in the same class of antiviral drugs as acyclovir, and both are phosphorylated by viral thymidine kinase and then by cellular enzymes to the active triphosphate form in virus-infected cells. Penciclovir triphosphate inhibits viral DNA polymerase competitively with deoxyguanosine triphosphate and is incorporated into the extending DNA chain, preventing significant chain elongation. Consequently, viral DNA synthesis and, therefore, viral replication are inhibited. Inhibition of the virus reduces the period of viral shedding, limits the degree of spread and level of pathology, and thereby facilitates healing. Penciclovir is not readily phosphorylated in uninfected cells and does not inhibit cellular DNA synthesis even at concentrations > 20 times those achieved in clinical usage.

Pharmacokinetics

Absorption

Following oral administration, famciclovir is rapidly, extensively and consistently absorbed and converted to the antivirally active compound, penciclovir. The mean (range) bioavailability of penciclovir after oral famciclovir is 77% (69.5 - 84.5%). The extent of systemic availability (AUC) of penciclovir from oral famciclovir is unaffected by food. In a healthy male volunteer study using a single oral dose of famciclovir, the pharmacokinetics of penciclovir were linear over the famciclovir dose range 125 to 750 mg. The mean (range) peak plasma concentration of penciclovir, calculated from dose normalised estimates across all single dose healthy male volunteer studies, following a single 500 mg dose of famciclovir was 3.3 ug/ml (range 1.3-6.3 ug/ml) and occurred at a mean time of 0.89 hours post-dose (range 0.5-5.0 hours). The mean terminal half-life of penciclovir was 2.3 hours (range 0.99-5.26 hours) Pharmacokinetic parameter estimates of penciclovir following oral administration of a single dose of famciclovir to patients with uncomplicated herpes zoster were essentially identical to values reported in healthy volunteers matched for age. Repeated oral dosing of famciclovir every 8 hours for up to 7 days in patients with herpes zoster infections had no significant effect on the pharmacokinetics of penciclovir compared to that described after single doses of famciclovir. The terminal plasma half-life of penciclovir in patients with herpes zoster was 2.8 h and 2.7 h, respectively, after single and repeated doses of famciclovir.

Distribution

Plasma protein binding of penciclovir and its 6-deoxy precursor is low (<20%) and penciclovir distributes freely between plasma and blood cells.

Metabolism

Following oral administration little or no famciclovir is detected in plasma or urine since famciclovir is rapidly converted via deacetylation and oxidation to penciclovir. An in vitro study using human liver microsomes demonstrated that cytochrome P450 does not play an important role in famcyclovir metabolism. The conversion of B-deoxy penciclovir is catalyzed by aldehyde oxidase.

Excretion

Little or no famciclovir is detected in plasma or urine since famciclovir undergoes extensive first-pass metabolism to penciclovir. The major metabolites identified in plasma and urine are penciclovir (67 +- 4% of radioactivity in plasma at 1.5 h following a 500 mg oral dose of [14C]famciclovir and 82 +- 2.2% of radioactivity in 0-24 h urine) and, to a lesser extent, its 6- deoxy precursor, which has no antiviral activity (11 +- 4% in plasma and 7 +- 0.5% in urine at the corresponding time points). Other minor, virally inactive metabolites identified in human urine are monoacetylated penciclovir and 6-deoxy monoacetylated penciclovir (each < 0.5% of the dose). Renal clearance values for penciclovir exceed creatinine clearance indicating that net active tubular secretion and glomerular filtration contribute to renal elimination.

Renal impaired patients:

The apparent plasma clearance, renal clearance, and plasma elimination rate constant of penciclovir decreased linearly with reductions in renal function, both after single and repeated dosing. Dose adjustment is necessary in patients with renal insufficiency (see Dosage and Administration).

Hepatically impaired patients:

Following single oral administration of famciclovir to patients with well-compensated hepatic impairment, there was no change in the extent of availability of penciclovir compared with healthy volunteers. There was, however, a decrease in the rate of availability of penciclovir in the hepatically impaired subjects. Mean maximum plasma concentrations of penciclovir were decreased by 43% and the time to maximum plasma concentrations increased by 0.75 hours. However, no dosage adjustment for patients with well-compensated hepatic impairment is recommended. The pharmacokinetics of penciclovir following oral famciclovir in patients with severe uncompensated hepatic impairment has not been studied.

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.

Elderly subjects:

Based on cross-study comparisons, the mean penciclovir AUC was about 40 % higher and penciclovir renal clearance about 20% lower after oral administration of famciclovir in elderly volunteers (65-79 years) compared to younger volunteers. Some of this difference may be due to differences in renal function between the two age groups. No dose adjustment based on age is recommended unless renal function is impaired (see Dosage and Administration).

Gender:

Small differences in renal clearance of penciclovir between females and males have been reported and were attributed to gender differences in renal function. No dose adjustment based on gender is recommended.

STORAGE AND STABILITY

Store at controlled room temperature (15-30degC).

DOSAGE FORMS, COMPOSITION AND PACKAGING

Each white, film-coated debossed tablet contains either 125 mg, 250 mg or 500 mg famciclovir. Inactive ingredients include hydroxypropyl cellulose, hydroxypropyl methylcellulose, lactose, magnesium stearate, polyethylene glycols, sodium starch glycolate and titanium dioxide. FAMVIR * (famciclovir) is supplied as 125 mg, white, round and biconvex film-coated tablets, with bevelled edges debossed with Famvir * on one side and 125 on the other, in blister packages of 10. FAMVIR * (famciclovir) is supplied as 250 mg, white, round and biconvex film-coated tablets, with bevelled edges debossed with Famvir * on one side and 250 on the other, in bottles of 30 or 60 and in blister packages of 30. FAMVIR * (famciclovir) is supplied as 500 mg, white, oval and biconvex film-coated tablets with bevelled edges, debossed with FAMVIR * on one side and 500 on the other, in bottles of 30 or 60 and in blister packages of 21.

PART II: SCIENTIFIC INFORMATION

PHARMACEUTICAL INFORMATION

Drug Substance:

Proper Name:

famciclovir

Chemical Name: 2-[2-(2 amino-9H-purine-9-yl)-ethyl]-1,3-propanediol diacetate Molecular structure: Molecular Formula: C14H19N5O4 Molecular Weight: 321.34

Description:

White to pale yellow crystalline solid with a melting range of 101-103.5degC

Solubility:

Soluble in water up to initial concentrations of >25% w/v, but subsequently precipitates as the crystalline monohydrate. Solubility of the monohydrate in water is approximately 2.2% w/v at 25degC and approximately 9.1% w/v at 37degC

pKa and pH Values:

The pKa of famciclovir is 3.84 and the pH of a 1 mM solution is 4.8.

CLINICAL TRIALS

Herpes Zoster and Post-Herpetic Neuralgia:

In patients with uncomplicated herpes zoster, famciclovir has been shown to significantly reduce the duration of virus shedding and to relieve the signs and symptoms of the disease. A 7-day double-blind placebo-controlled trial was conducted in 419 patients with uncomplicated herpes zoster treated within 72 hours of initial lesion appearance. According to the randomization scheme, 138 patients were given famciclovir 500 mg t.i.d., 135 patients famciclovir 750 mg t.i.d. and 146 patients given placebo. No additional efficacy was demonstrated with the higher dose of famciclovir (750 mg t.i.d. ), when compared to famciclovir 500 mg t.i.d. In the total population, 65.2% of patients had a positive viral culture at some time during their acute infection. Patients treated with famciclovir 500 mg had a shorter median duration of viral shedding (time to last positive viral culture) than did placebo-treated patients (1 day and 2 days, respectively; p=0.0001). The times to loss of vesicles (p=0.01), loss of ulcers (p=0.01), and loss of crusts (p=0.05), were shorter for famciclovir 500 mg-treated patients than for placebo-treated patients in the overall study population. The follow-up phase of this trial was designed to monitor the progression of post-herpetic neuralgia (PHN) after treatment with either famciclovir or placebo for seven days during acute infection. There was no difference in the incidence of postherpetic neuralgia between the treatment groups at the time of rash resolution. In the 186 patients (44.4% of total study population) who did develop postherpetic neuralgia, the median duration of postherpetic neuralgia was shorter in patients treated with famciclovir 500 mg than in those treated with placebo (63 days and 119 days, respectively; p=0.02). A second 7-day double-blind controlled trial involved 545 patients with uncomplicated herpes zoster treated within 72 hours of initial lesion appearance. According to the randomization scheme, 134 patients received 250 mg t.i.d. of famciclovir, 134 patients received 500 mg t.i.d. of famciclovir, 138 patients received 750 mg t.i.d. of famciclovir and 139 patients received 800 mg of acyclovir given 5 times a day. Famciclovir was found to be as effective as acyclovir at all dose levels for cutaneous lesion healing parameters, time to loss of pain and viral shedding. A double-blind controlled trial in 497 adult patients with ophthalmic zoster treated within 72 hours of initial lesion appearance compared FAMVIR * (famciclovir) 500 mg three times daily for 7 days (n=251) to acyclovir 5 times per day for 7 days (n=246). FAMVIR * was comparable to acyclovir in preventing ocular complications due to herpes zoster infection.

Herpes Simplex Infections:

Treatment of Recurrent Genital Herpes Episodes:

Famciclovir was studied in two placebo-controlled trials of 626 otherwise healthy patients with a recurrence of genital herpes who were treated with famciclovir 125 mg b.i.d. (n=160), famciclovir 250 mg b.i.d. (n=169), famciclovir 500 mg b.i.d. (n=154) or placebo (n=143) for 5 days. In the two studies combined, the median time to healing in famciclovir 125 mg-treated patients was 4 days compared to 5 days in placebo treated patients (p= 0.0001) and the median time to cessation of viral shedding was 1.8 vs. 3.4 days in famciclovir 125 mg and placebo recipients, respectively (p= 0.0001). The median time to loss of all symptoms was 3.2 days in famciclovir 125 mg-treated patients vs. 3.8 days in placebo treated patients (p= 0.0001). Pre- treatment, self-obtained viral cultures were positive in 31%, 25%, 30% and 24% for the famciclovir 125 mg, 250 mg, 500 mg and placebo recipients respectively in the patient-initiated study. Of those patients whose pre-treatment culture was negative, significantly fewer patients self-initiating famciclovir treatment went on to become viral culture positive compared to placebo. Patients initiating treatment early (during the prodrome) were half as likely to commence viral shedding compared to placebo patients. Additionally, in the clinic-initiated study, famciclovir reduced the number of patients who developed new lesions. Famciclovir was also studied in three acyclovir-controlled, double-blind trials in 951 otherwise healthy patients with first episode genital herpes. Famciclovir for 5 or 10 days provided comparable efficacy to acyclovir although the studies were not powered to show statistical equivalence.

Suppression of Recurrent Genital Herpes Episodes:

A total of 934 otherwise healthy adults with frequently recurring genital herpes, were enrolled in two 12-month, placebo-controlled studies. Patients either had at least six recurrences 12 months prior to study entry or a history of at least six recurrences per year while not receiving other suppressive therapies. Sixty-two percent of patients had experienced at least 12 genital herpes recurrences in the previous 24 months. Treatment arms consisted of Famvir * 125 mg t.i.d. (n=233), 250 mg b.i.d. (n=236), 250 mg t.i.d. (n=232) and placebo (n=233). Compared to placebo, Famvir * 250 mg b.i.d. significantly delayed the time to developing the first clinically confirmed recurrence by 10 months in one study (medians: >365 days for Famvir * vs. 67 days for placebo; p=0.0001) and 9.5 months in another study (medians: 336 days for Famvir * vs. 47 days for placebo; p=0.0001). Approximately 80% of Famvir *-treated patients in both studies remained free from HSV recurrences documented by viral culture for up to 6 months compared with approximately 25% of patients treated with placebo (p<0.001). Treatment effects were sustained for 12 months. Treatment of Recurrent Mucocutaneous Herpes Simplex Infection in HIV-Infected Patients A randomized, double-blind, multicenter study 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 mucocutaneous HSV infection. Approximately 40% of patients had a CD4 count below 200 cells/mm3; 54% of patients had anogenital lesions and 35% had orolabial lesions. Twice-daily oral famciclovir was comparable to five-times daily oral acyclovir in preventing new lesion formation, in time to complete healing (median of 7 days in both groups), time to loss of all lesion-associated symptoms (median of 4 days in both groups) and time to cessation of viral shedding (median of 2 days in both groups). Efficacy was maintained regardless of the degree of immunosuppression or location of lesions.

VIROLOGY

Penciclovir is a highly potent and selective antiviral agent. Inhibitory activity in animals has been shown against herpes simplex virus type 1 and 2 (HSV-1 and HSV-2) infections. It also has inhibitory activity in cell cultures against HSV-1, HSV-2 and varicella zoster virus (VZV). Penciclovir enters cells rapidly. In HSV-1, HSV-2 and VZV infected cells, viral thymidine kinase rapidly converts penciclovir to a monophosphate, which host cell enzymes convert to penciclovir triphosphate. The triphosphate inhibits viral DNA polymerase competitively with deoxyguanosine triphosphate and is incorporated into the extending DNA chain, preventing significant chain elongation. Consequently, viral DNA synthesis and, therefore, viral replication are inhibited. Following the removal of the acyclonucleoside from MRC-5 cell culture medium, the stability of intracellular penciclovir triphosphate was much greater than that of acyclovir triphosphate. The intracellular half-life of penciclovir triphosphate in virus-infected cells was: 10 hours for HSV-1; 20 hours for HSV-2; 7 hours for VZV. The corresponding values for acyclovir triphosphate were 0.7 hours for HSV-1 and 1.0 hours for HSV-2. No value was derived for VZV-infected cells since acyclovir triphosphate concentrations were undetectable. In other human cells, penciclovir triphosphate was invariably a more stable entity than acyclovir triphosphate. The long intracellular half-life of penciclovir triphosphate ensures prolonged antiviral activity as demonstrated in cell cultures with HSV-1, HSV-2 and VZV, and in animal studies with HSV-1 and HSV-2. In cell culture, penciclovir has the highest antiviral activity against the following herpes viruses (listed in decreasing order of potency and detailed below): HSV-1, HSV-2, VZV, Epstein-Barr virus (EBV) and cytomegalovirus (CMV). The degree of inhibition is dependent upon a number of variables, including the assay method, the host cell, virus type and multiplicity of infection. Comparative in vitro data for the antiviral potency of penciclovir and acyclovir are shown in the following table.

IC99 (mcg/mL)

* Data were obtained using clinical isolates, except for DNA synthesis assays in which the specified laboratory strains were used.

In an assay measuring inhibition of EBV DNA synthesis in P3HR1 cells, the IC50 for penciclovir was 1.5 mcg/mL. In plaque-reduction assays in MRC-5 cells against CMV, the IC50 for penciclovir was 52 mcg/mL. The most common form of resistance encountered with acyclovir among HSV strains is a deficiency in the production of the thymidine kinase (TK) enzyme. Such TK deficient strains would be expected to be cross-resistant to both penciclovir and acyclovir. However, penciclovir is effective against some acyclovir-resistant strains of HSV-1 with an altered DNA polymerase. In plaque-reduction assays against laboratory-produced mutants with altered DNA polymerase, Vero (human lung fibroblast) cells infected with HSV-1 were inhibited by penciclovir to the same extent as the wild type (IC50 was 1.5 mcg/mL for the mutant and 1.0 mcg/mL for the wild type). In contrast, resistance to acyclovir was demonstrated as the IC50 of acyclovir was 47 fold higher (20 mcg/mL) for the mutant compared to (0.43 mcg/mL) the wild type. In a clinically isolated strain of acyclovir-resistant HSV-1 tested in Hs68 (human foreskin fibroblast) cells, resistance to acyclovir was demonstrated, whereas the IC50 values for penciclovir in the wild type and mutant strains were almost identical (1.3 mcg/mL vs. 1.0 mcg/mL, respectively). Penciclovir sensitivity testing was performed on 84 HSV-2 isolates from 50 famciclovir-treated patients (59 isolates) and 21 placebo-treated patients (25 isolates) who participated in a 4-month genital herpes suppression trial. All HSV-2 isolates on-treatment and after-treatment were sensitive (mean IC50 1.7 mcg/mL for famciclovir recipients and 1.5 mcg/mL for placebo recipients). Short-term treatment of HSV-1-infected MRC-5 cells with penciclovir (3 mM) for 2 hours reduced viral DNA by 76% compared with only 17% following similar treatment with acyclovir (3 mM). Treatment of HSV-2-infected cells with penciclovir (3 mM) for 1.5 hours reduced viral DNA by 52% vs. 20% with acyclovir (3 mM). Studies of VZV-infected MRC-5 cells exposed to pulse treatment for 8 hours on days 0, 1, 2 and 3 produced an IC50 of 5.0 mcg/mL for penciclovir and 24 mcg/mL for acyclovir. The activity of penciclovir seen after short or pulse treatment is consistent with the stability of intracellular penciclovir triphosphate. The presence of penciclovir at 10 or 100 mcg/mL did not decrease the activity of zidovudine against human immunodeficiency virus in M8166 cells. As there is no appropriate animal model that mimics VZV infection in humans, the antiviral activity of penciclovir has not been evaluated in animals infected with VZV. However, penciclovir has been shown to have inhibitory activity against HSV-1 and HSV-2 infections in mice and guinea pigs. The level of antiviral activity depends on a number of factors including the route of infection, route of administration of penciclovir and time between virus infection and treatment with penciclovir. The clinical significance of inhibitory activity of penciclovir against HSV-1 and HSV-2 in in vitro animal models is unknown at this time.

TOXICOLOGY

Both famciclovir and its active metabolite penciclovir have undergone a comprehensive toxicological evaluation; principal findings from the key studies are summarised below. Exposure to drug-related material in animals compared with that in humans given 500 mg famciclovir t.i.d. is expressed, where appropriate, as multiples of the amount of penciclovir and 6-deoxy-penciclovir systemically available.

Acute Toxicity:

Famciclovir has a low order of acute toxicity, with median lethal doses in excess of 5000 mg/kg orally and around 500 mg/kg or higher intravenously; penciclovir intravenously was slightly better tolerated. With both compounds the majority of adverse effects were indicative of central nervous system disturbances, undoubtedly reflecting the high doses administered since no such effects were detected at the lower doses used in the studies to assess general pharmacological actions.

Subacute Toxicity Studies:

In studies of 1 month's duration, famciclovir was administered at oral doses of up to 4000 mg/kg/day in the rat and up to 500 mg/kg/day in the dog. In rats, there were early degenerative changes in the seminiferous epithelium at 4000 and 400 mg/kg/day (35- and 9- fold human exposure to penciclovir respectively), but the effects at the lower dose were minimal. Microscopically there was also a slight reduction in the number of lymphocytes in the thymic cortex confined to the high dose of 4000 mg/kg/day; there were no effects at 400 mg/kg/day. In dogs, doses of up to 500 mg/kg/day ( 3-fold human exposure to penciclovir), which was the maximum practicable dose, were tolerated with no indications of definitive target organ toxicity.

Chronic Toxicity Studies:

In 6 and 12 month studies with famciclovir, oral doses of up to 600 mg/kg/day in rats and up to 500 mg/kg/day in dogs were without significant effect on any tissue except the testes (See Reproduction). At these doses, a high systemic exposure to both penciclovir ( 4- and 3-fold human values in rat and dog respectively) and 6-deoxy penciclovir (at least 40- and 150-fold human values in rat and dog respectively) were achieved.

Carcinogenicity Studies:

In lifetime (2-year) studies there was an increase in the incidence of mammary adenocarcinoma in female rats receiving the maximally tolerated dose of 600 mg/kg/day ( 4- and 40-fold human exposure to penciclovir and 6-deoxy penciclovir respectively), but not in the number of high dose females with mammary tumours of any type (benign or malignant). No effects were seen in females receiving 200 mg/kg/day ( 2- and 8-fold human exposure to penciclovir and 6-deoxy penciclovir respectively) or in males receiving doses of up to 240 mg/kg/day, the maximum tolerated dose. There was also no effect on the incidence or type of tumours in mice given the maximum tolerated dose of 600 mg/kg/day ( 6- and 65-fold human exposure to penciclovir and 6-deoxy penciclovir respectively).

Mutagenicity Studies:

Famciclovir was not found to be genotoxic in a comprehensive battery of in vivo and in vitro tests designed to detect gene mutation, chromosomal damage and repairable damage to DNA. Penciclovir, in common with other drugs of this class, has been shown to cause chromosomal damage, but did not induce gene mutation in bacterial or mammalian cell system. There was also no evidence of increased DNA repair in vitro.

Reproduction: As with other drugs of this class, testicular toxicity has been observed in animals receiving both famciclovir and penciclovir, but there was evidence of a return towards normal on withdrawal of treatment. The testicular effects seen following famciclovir are very likely attributable to penciclovir; therefore, systemic exposure comparisons in animals and man are shown for penciclovir. Mating performance was unaffected in male rats treated with famciclovir for up to 6 months, but fertility decreased progressively with time following 500 mg/kg/day ( 5-fold human exposure) and there were abnormal sperm profiles, degenerative changes in the seminiferous epithelium and raised FSH levels. Fertility was unaffected at 150 mg/kg/day ( 2-fold human exposure). Only minimal testicular degeneration was apparent in rats after 1 month's dosing at 400 mg/kg/day ( 9-fold human exposure), and no effects were seen following 240 mg/kg/day for 2 years ( 3-fold human exposure). In dogs there were no effects after 1 month's dosing at 500 mg/kg/day ( 3-fold human exposure) and, except for minor histological changes in a single dog after 12 months' treatment, testicular toxicity was confined to chronic dosing with $150 mg/kg/day. There were no significant effects on fertility or on peri- and post-natal development in female rats given famciclovir at doses of up to 1000 mg/kg/day. There were also no embryotoxic or teratogenic effects in rats and rabbits at oral doses of up to 1000 mg/kg/day or intravenous doses of 360 mg/kg/day (rats) and 120 mg/kg/day (rabbits). Penciclovir administered intravenously at 80 mg/kg/day or 60 mg/kg/day was also without effect in these species. There were no clinically significant effects on sperm count, morphology or motility in male patients receiving famciclovir 250 mg b.i.d. for 18 weeks. Although the daily dosage in this study was lower than the recommended dose for zoster, the duration of treatment was longer at 18 weeks compared with 7 days, and patients therefore received a cumulative (total) dose which was 6-fold higher.

REFERENCES

Boyd M.R., Safrin S., Kern E.R.. Penciclovir: A Review of its Spectrum of Activity, Selectivity, and Cross-resistance pattern. Antiviral Chemistry and Chemotherapy 1993; 4 (Suppl. 1): 3-11 Daniels S., Schentag J.J.. Drug Interaction Studies and Safety of Famciclovir in Healthy Volunteers: A Review. Antiviral Chemistry and Chemotherapy 1993; 4 (Suppl. 1): 57- Degreef H. et al. (Famciclovir Herpes Zoster Clinical Study Group). Famciclovir, a New Oral Antiherpes Drug: Results of the First Controlled Clinical Study Demonstrating its Efficacy and Safety in the Treatment of Uncomplicated Herpes Zoster in Immunocompetent Patients. International Journal of Antimicrobial Agents 1994; 4 (4): 241-246 Portnoy J. Famciclovir in the Treatment of Herpes Zoster (HZ) Infection. Antiviral Research 1994; 23(Suppl. 1): 98 Pue M.A., Benet L.Z.. Pharmacokinetics of Famciclovir in Man. Antiviral Chemistry and Chemotherapy 1993; 4 (Suppl. 1): 47-55 Saltzman R., Jurewicz R., Boon R.. Safety of Famciclovir in Patients with Herpes Zoster and Genital Herpes. Antimicrobial Agents and Chemotherapy 1994; 38(10): 2454-2457 Tyring S., Barbarash R., Nahlik J., Cunningham A., Marley J., Heng M., Jones T. Efficacy of Famciclovir on Herpes Zoster Rash Resolution and Post-herpetic Neuralgia. Antiviral Research 1994; 23(Suppl. 1): 73 Tyring S., Barbarash R., Nahlik J. et al. Famciclovir for the Treatment of Acute Herpes Zoster: Effects on Acute Disease and Postherpetic Neuralgia. Ann Int Med 1995; 123:89- 96. Vere Hodge R.A., Cheng Y-C. The Mode of Action of Penciclovir. Antiviral Chemistry and Chemotherapy 1993; 4 (Suppl. 1): 13-24 Whitley R.J.. Penciclovir/Famciclovir: Parent and Child in the Treatment of Herpes Virus Infections. Expert Opinion on Investigational Drugs 1994; 3 (7): 759-761 Sacks S, Aoki F, Diaz-Mitoma F, Sellors J, Shafran S et al. Patient-Initiated, Twice-Daily Oral Famciclovir for Early Recurrent Genital Herpes A Randomized, Double-blind Multicentre Trial. JAMA 1996; 276: 44-49. Perry CM, Wagstaff AJ. Famciclovir - A Review of its Pharmacological Properties and Therapeutic Efficacy in HerpesVirus Infections. Drugs 1995; 50: 396-415. Thackray AM, Field H. Differential Effects of Famciclovir and Valacyclovir on the Pathogenesis of Herpes Simplex Virus in a Murine Infection Model Including Reactivation from Latency. Journal of Infectious Diseases 1996; 173: 291-299 Thackray AM, Field H. Comparison of Effects of Famciclovir and Valaciclovir on Pathogenesis of Herpes Simplex Virus Type 2 in a Murine Infection Model. Antimicrobial Agents and Chemotherapy 1996; 40: 846-851. Sacks S, Martel A, Aoki F, Shafran S, St-Pierre C, Lassonde M et al. Early Clinic- initiated Treatment of Recurrent Genital Herpes using Famciclovir, Results of a Canadian Multicentre Study. Clin Res 1994; 42: 300A. Sacks SL, Aoki FY, Diaz-Mitoma F, Sellors J, Shafran S, Romanowski B, Martel A, Williams K, Haase D et al. Patient and Clinic-initiated Treatment of Recurrent Genital Herpes with Twice-Daily Oral Famciclovir. 7th European Congress of Clinical Microbiology and Infectious Diseases 1995; 866A. Loveless M, Harris W, Sacks S. Treatment of First Episode Genital Herpes with Famciclovir. 35th ICAAC 1995; Abstract H12. Frechette G, Romanowski B. Efficacy and safety of famciclovir for the treatment of HSV infection in HIV+ patients. Can J Infect Dis 1997; 8(suppl A): 44A: 301.