Acyclovir Cream, 5%

SECTION 2: Hazards identification

Dose and Administration Acyclovir cream should be applied 5 times per day for 4 days. Therapy should be initiated as early as possible following the onset of signs or symptoms of herpes labialis i.e., during the prodrome or when lesions appear. For adolescents 12 years of age and older, the dosage is the same as in adults. Adverse effects Clinical trial experience: Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug, and may not reflect the rates observed in clinical practice. In five double-blind, placebo-controlled trials, 1,124 patients were treated with acyclovir cream and 1,161 with placebo (vehicle) cream. Local application site reactions were reported by 5% of patients receiving acyclovir cream and 4% of patients receiving placebo. The most common adverse reactions at the site of topical application were dry lips, desquamation, dryness of skin, cracked lips, burning skin, pruritus, flakiness of skin, and stinging on skin; each adverse reaction occurred in less than 1% of patients receiving acyclovir cream and placebo. Three patients on acyclovir cream and one patient on placebo discontinued treatment due to an adverse event. An additional study, enrolling 22 healthy adults, was conducted to evaluate the dermal tolerance of acyclovir cream compared with vehicle using single occluded and semi-occluded patch testing methodology. Both acyclovir cream and placebo showed a high and cumulative irritation potential. Another study, enrolling 251 healthy adults, was conducted to evaluate the contact sensitization potential of acyclovir cream using repeat insult patch testing methodology. Of 202 evaluable subjects, possible cutaneous sensitization reactions were observed in the same 4 (2%) subjects with both acyclovir cream and placebo, and these reactions to both acyclovir cream and placebo were confirmed in 3 subjects upon re-challenge. The sensitizing ingredient(s) has not been identified. The safety profile in patients 12 to 17 years of age was similar to that observed in adults. Over Dose Effect Overdosage by topical application of acyclovir cream is unlikely because of minimal systemic exposure. There is no information available for overdose. Pregnancy Comments Risk summary Acyclovir is minimally absorbed systemically following topical route of administration, and maternal use is not expected to result in fetal exposure to the acyclovir cream. Experience with topical acyclovir use in pregnant women over several decades, based on published literature including observational studies, has not identified a drug-associated risk of major birth defects, miscarriage or adverse maternal or fetal outcomes. Animal reproduction studies with systemic exposure of acyclovir have been conducted. The estimated background risk of major birth defects and miscarriage for the indicated population is unknown. All pregnancies have a background risk of birth defect, loss, or other adverse outcomes. In the U.S. general population, the estimated background risk of major birth defects and miscarriage in clinically recognized pregnancies is 2% to 4% and 15% to 20%, respectively. Pharmacokinetics A clinical pharmacology study was performed with acyclovir cream in adult volunteers to evaluate the percutaneous absorption of acyclovir. In this study, which included 6 male volunteers, the cream was applied to an area of 710 cm2 on the backs of the volunteers 5 times daily at intervals of 2 hours for a total of 4 days. The weight of cream applied and urinary excretion of acyclovir were measured daily. Plasma concentration of acyclovir was assayed 1 hour after the final application. The average daily urinary excretion of acyclovir was approximately 0.04% of the daily applied dose. Plasma acyclovir concentrations were below the limit of detection (0.01 µM) in 5 subjects and barely detectable (0.014 µM) in 1 subject. Systemic absorption of acyclovir from acyclovir cream is minimal in adults. The systemic absorption of acyclovir following topical application of cream has not been evaluated in patients <18 years of age. Microbiology Mechanism of Action: Acyclovir is a synthetic purine deoxynucleoside analogue with cell culture and in vivo inhibitory activity against HSV types 1 (HSV-1) and 2 (HSV-2) DNA polymerases. It inhibits HSV-1 and HSV-2 replication in cell culture and in vivo. The inhibitory activity of acyclovir is selective due to its affinity for the enzyme thymidine kinase (TK) encoded by HSV. This viral enzyme converts acyclovir into acyclovir monophosphate, a deoxynucleotide analogue. The monophosphate is further converted into diphosphate by cellular guanylate kinase and into triphosphate by a number of cellular enzymes. In biochemical assays, acyclovir triphosphate inhibits replication of α-herpes viral DNA. This inhibition is accomplished in 3 ways: 1) competitive inhibition of viral DNA polymerase, 2) incorporation into and termination of the growing viral DNA chain, and 3) inactivation of the viral DNA polymerase. Antiviral Activity The quantitative relationship between the susceptibility of herpes viruses to antivirals in cell culture and the clinical response to therapy has not been established in humans, and virus sensitivity testing has not been standardized. Sensitivity testing results, expressed as the concentration of drug required to inhibit by 50% the growth of virus in cell culture (EC50), vary greatly depending upon a number of factors. Using plaque-reduction assays on Vero cells, the EC50 values of acyclovir against herpes simplex virus isolates range from 0.09 to 59.9 µM (0.02 to 13.5 mcg/mL) for HSV‑1 and from 0.04 to 44 µM (0.01 to 9.9 mcg/mL) for HSV‑2.