Brand name: Famvir
Drug class: Nucleosides and Nucleotides
VA class: AM800
Chemical name: 2-[2-(2-Amino-9H-purin-9-yl)ethyl]-1,3-propanediol diacetate (ester)
Molecular formula: C₁₄H₁₉N₅O₄
CAS number: 104227-87-4

Medically reviewed by Drugs.com on Aug 22, 2023. Written by ASHP.

Introduction

Antiviral; prodrug of penciclovir; nucleoside derived from guanine.^{1 3 4}

Uses for Famciclovir

Genital Herpes

Treatment of initial episodes of genital herpes^{† [off-label]} in immunocompetent or HIV-infected adults and adolescents.^{26 36 37}

Episodic treatment of recurrent episodes of genital herpes in immunocompetent^{1 26 27 36 37 43} or HIV-infected adults and adolescents.^{1 26 37}

Chronic suppressive therapy of recurrent episodes of genital herpes in immunocompetent^{1 26 36 37} or HIV-infected adults and adolescents.²⁸

CDC and others recommend oral acyclovir, oral famciclovir, or oral valacyclovir as drugs of choice for treatment of initial episodes of genital herpes and for episodic treatment or chronic suppressive therapy of recurrent genital herpes.^{26 36 37}

Herpes Labialis

Episodic treatment of herpes labialis (perioral herpes, cold sores, fever blisters) in immunocompetent adults^{1 43} or HIV-infected adults and adolescents.

Mucocutaneous Herpes Simplex Virus (HSV) Infections

Treatment of recurrent mucocutaneous HSV infections in HIV-infected adults and adolescents.^{1 46}

Chronic suppressive or maintenance therapy (secondary prophylaxis) against recurrence of HSV infections^{† [off-label]} in HIV-infected adults or adolescents who have frequent or severe recurrences.^{27 28 35 46}

Herpes Zoster

Treatment of acute, localized herpes zoster (shingles, zoster) in immunocompetent adults and adolescents.^{1 11 27 37}

Treatment of localized dermatomal herpes zoster in HIV-infected adults or adolescents^{† [off-label]}.⁴⁶ If cutaneous lesions are extensive or there is clinical evidence of visceral involvement, IV acyclovir should be used for initial treatment.⁴⁶

Hepatitis B Virus Infection

Has been used for management of chronic hepatitis B virus (HBV) infection^{† [off-label]}, including control of HBV recurrence in organ or bone marrow transplant recipients^{† [off-label]}.^{30 32 33 34} Safety and efficacy for HBV infection not established.^{29 30 31 32 33 34}

CDC, National Institutes of Health (NIH), and IDSA state that famciclovir is not recommended for treatment of HBV infection in HIV-infected individuals since the drug is less active than lamivudine against HBV and is not active against lamivudine-resistant HBV.⁴⁶

Related/similar drugs

prednisone, acyclovir, valacyclovir, Valtrex, Zovirax, Deltasone, lysine

Famciclovir Dosage and Administration

Administration

Oral Administration

Administer orally without regard to meals.¹

Dosage

Pediatric Patients

Genital Herpes

Oral

Adolescents should receive dosage recommended for adults with genital herpes.²⁷ (See Adults under Dosage and Administration.)

Mucocutaneous Herpes Simplex Virus (HSV) Infections

Chronic Suppression of Recurrent Episodes

Oral

HIV-infected adolescents: 250 mg twice daily for chronic suppressive or maintenance therapy (secondary prophylaxis) of HSV^† infections in those with frequent or severe recurrences.²⁸

Herpes Zoster

Oral

Local dermatomal herpes zoster in HIV-infected adolescents^†: 500 mg three times daily for 7–10 days recommended by CDC and other experts.⁴⁶

Adults

Genital Herpes

Treatment of First Episodes†

Oral

Immunocompetent adults: 250 mg 3 times daily for 7–10 days recommended by CDC and others; duration of treatment may be extended if healing is incomplete after 10 days.^{26 36 37}

HIV-infected adults: 500 mg twice daily for 7–14 days recommended by CDC and others.⁴⁶

Episodic Treatment of Recurrent Episodes

Oral

Immunocompetent adults: 1 g twice daily for 1 day^{1 26 27} or 125 mg twice daily for 5 days.^{26 27 36 37}

HIV-infected adults: 500 mg twice daily for 5–10 days recommended by CDC and others;^{26 37 46} alternatively, continue for 7–14 days.⁴⁶

Initiate therapy at first sign or symptom of an episode;¹ efficacy not established if initiated >6 hours after onset of signs or symptoms.¹

Suppressive Therapy of Recurrent Episodes

Oral

Immunocompetent adults: 250 mg twice daily.^{1 26 36 37}

HIV-infected adults: 500 mg twice daily recommended by CDC.²⁶

Manufacturer states chronic suppressive therapy may be given for up to 1 year.¹

Because frequency of recurrent episodes diminishes over time in many patients, CDC and others recommend that suppressive antiviral therapy be discontinued periodically (e.g., once yearly) to assess the need for continued therapy.^{26 36}

Herpes Labialis

Oral

Immunocompetent adults: 1.5 g as a single dose.¹

Initiate therapy at first prodromal symptom (e.g., tingling, itching, burning).¹

Mucocutaneous Herpes Simplex Virus (HSV) Infections

Episodic Treatment of Recurrent Episodes

Oral

HIV-infected adults: 500 mg every 12 hours for 7 days for treatment of recurrent infections (orolabial or genital herpes).¹ Some experts recommend 7–14 days.⁴⁶

Suppressive Therapy of Recurrent Episodes

Oral

HIV-infected adults: 250 mg twice daily for chronic suppressive or maintenance therapy (secondary prophylaxis) of HSV^† in those with frequent or severe recurrences.²⁸

Herpes Zoster

Oral

Immunocompetent adults: 500 mg every 8 hours for 7 days.^{1 27 37 47}

Local dermatomal herpes zoster in HIV-infected adults^†: 500 mg three times daily for 7–10 days recommended by CDC and other experts.⁴⁶

Initiate therapy promptly as soon as diagnosed;¹ efficacy not established if initiated >72 hours after rash onset.¹

Special Populations

Renal Impairment

Genital Herpes

Herpes Labialis

Mucocutaneous Herpes Simplex Virus (HSV) Infections

Herpes Zoster

Cautions for Famciclovir

Contraindications

• Known hypersensitivity to famciclovir or penciclovir or any ingredient in the formulation.¹

Warnings/Precautions

General Precautions

Renal Effects

Use of inappropriately high dosage for the level of renal function has resulted in acute renal failure in patients with underlying renal disease.¹ Adjust dosage based on Cl_cr.^{1 2 14} (See Renal Impairment under Dosage and Administration.)

Genital Herpes

Not a cure for genital herpes.¹

Avoid sexual contact while lesions and/or symptoms are present due to risk of infecting sexual partners.¹ Infection can be transmitted in the absence of symptoms through asymptomatic viral shedding.¹

Although recommended by CDC and others for treatment of initial episodes of genital herpes,^{26 36 37} manufacturer says efficacy not established.¹

Herpes Zoster

Efficacy not established for treatment of disseminated or ophthalmic herpes zoster or for treatment of herpes zoster in immunocompromised individuals.¹

Lactose Intolerance

Patients with galactose intolerance, severe lactase deficiency, or glucose-galactose malabsorption should not receive famciclovir tablets.¹ Each 125-, 250-, or 500-mg tablet contains 26.9, 53.7, or 107.4 mg of lactose, respectively.¹

Specific Populations

Pregnancy

Category B.¹

Lactation

Distributed into milk in rats; not known whether distributed into human milk.^{1 18}

Pediatric Use

Safety and efficacy not established in infants or children <18 years of age.^{1 4}

Geriatric Use

Experience in geriatric adults (≥65 years of age) with herpes zoster indicate adverse effects are similar to those in younger adults.^{1 14}

Insufficient experience in geriatric adults with recurrent herpes simplex to determine whether they respond differently than younger adults.¹

Use with caution because of age-related decreases in hepatic, renal, and/or cardiac function and concomitant disease and drug therapy.¹

Hepatic Impairment

Pharmacokinetics not evaluated in patients with severe uncompensated hepatic impairment.^{1 4}

Renal Impairment

Dosage adjustment necessary based on degree of renal impairment.¹ (See Renal Impairment under Dosage and Administration.)

Common Adverse Effects

Headache,^{1 4 6 11 14 15 20 21} nausea,^{1 4 6 11 14 18 21} diarrhea,^{1 4 6 14 15 20 21} vomiting.^{1 4 6 14 15 21}

Drug Interactions

Not metabolized by CYP isoenzymes.¹

Drugs Eliminated by Renal Excretion

Potential increased plasma penciclovir concentrations when used concomitantly with other drugs eliminated by active renal tubular secretion (e.g., probenecid).¹

Drugs Metabolized by Aldehyde Oxidase

Potential pharmacokinetic interaction with other drugs metabolized by aldehyde oxidase.¹

Specific Drugs

Famciclovir Pharmacokinetics

Absorption

Bioavailability

Famciclovir, a prodrug of penciclovir, is rapidly and well absorbed following oral administration and metabolized to penciclovir.^{1 40 41} Little or no prodrug is present in plasma or urine.¹

Absolute bioavailability of penciclovir is 77% following oral administration of famciclovir;^{1 41} peak penciclovir plasma concentrations attained within 0.5–0.9 hours.^{1 2 40 42}

Pharmacokinetics in HIV-infected patients similar to healthy individuals.¹

Food

Administration of famciclovir with food decreases peak penciclovir plasma concentrations and delays time to peak concentrations but does not affect penciclovir AUC.¹

Distribution

Extent

Not known whether penciclovir crosses the placenta.

Not known whether penciclovir is distributed into human milk.¹

Plasma Protein Binding

Penciclovir <20% bound to plasma proteins.¹

Elimination

Metabolism

Famciclovir is deacetylated and oxidized to penciclovir.¹ Penciclovir is phosphorylated to penciclovir triphosphate (the active metabolite) in cells infected with HSV-1, HSV-2, or VZV.^{1 41} The inactive metabolite 6-deoxy penciclovir is converted to penciclovir by aldehyde oxidase.¹

Famciclovir not metabolized by CYP enzymes.¹

Elimination Route

Famciclovir eliminated principally by the kidneys as penciclovir and other metabolites.^{1 40 42} 73% of an oral famciclovir dose eliminated in urine and 27% eliminated in feces within 72 hours.^{1 40}

Half-life

Elimination half-life of penciclovir after oral administration of famciclovir 1.6–3 hours.^{1 2 3 40}

Intracellular half-life of penciclovir triphosphate in cells infected with HSV-1 or HSV-2 is 10 and 20 hours, respectively;^{1 41} intracellular half-life in VZV-infected cells is 7–14 hours.^{1 41}

Special Populations

AUC of penciclovir not affected when oral famciclovir used in patients with well-compensated chronic liver disease (chronic hepatitis, chronic ethanol abuse, primary biliary cirrhosis).¹ Pharmacokinetics not evaluated in severe uncompensated hepatic impairment.¹

Renal clearance decreased and terminal elimination half-life increased in patients with renal impairment;^{2 40} half-life 6.2 hours if Cl_cr 20–39 mL/minute and 13.4 hours if Cl_cr <20 mL/minute.¹

AUC may be greater and renal clearance decreased in geriatric patients ≥65 years of age, presumably because of decreased renal function.¹

Stability

Storage

Oral

Tablets

15–30°C.¹

Actions and Spectrum

• Famciclovir is the diacetyl 6-deoxy analog of penciclovir.¹ Prodrug with no antiviral activity until converted in vivo to penciclovir and subsequently to the active penciclovir triphosphate.^{1 5 6 12 13 17}

• Active against various Herpesviridae including herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), and Epstein-Barr virus (EBV).^{1 5 6 7 8 9 12 13 17} Also active against hepatitis B virus (HBV).^{29 30 31 32 33 34} Only limited activity in vitro against cytomegalovirus (CMV).^{6 9 17}

• Penciclovir triphosphate exerts its antiviral activity by interfering with DNA synthesis; the drug competes with deoxyguanosine triphosphate for viral DNA polymerase, inhibits DNA chain elongation, and inhibits viral replication.¹

• Resistance to penciclovir reported in HSV and VZV.¹

Advice to Patients

• Advise patients that famciclovir is not a cure for genital herpes, and there are no data evaluating whether famciclovir prevents transmission of genital herpes to others.¹

• Importance of avoiding sexual contact with uninfected partners while lesions and/or symptoms of genital herpes are present since there is a risk of transmission.¹ Genital herpes can be transmitted in the absence of symptoms through asymptomatic viral shedding.¹

• Advise patients to avoid driving or operating machinery if they experience dizziness, drowsiness, confusion, or other CNS disturbances; there is no evidence that famciclovir affects ability to drive or operate machinery.¹

• Importance of informing clinician of existing or contemplated concomitant therapy, including prescription or nonprescription drugs.¹

• Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.¹

• Importance of advising patients of other important precautionary information.¹ (See Cautions.)

Preparations

Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

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