No interaction studies have been performed with SITAVIG. Acyclovir is primarily eliminated unchanged in the urine via active tubular secretion. Drugs administered concomitantly that compete with tubular secretion may increase acyclovir plasma concentrations. However, due to the low dose and minimal systemic absorption of SITAVIG, systemic drug interactions are unlikely.
Pregnancy Category B
No studies with SITAVIG have been performed in pregnant women. Systemic exposure of acyclovir following buccal administration of SITAVIG is minimal. SITAVIG should be used during pregnancy only if the potential benefit outweighs the potential risk to the fetus. There are no adequate and well-controlled studies of systemic acyclovir in pregnant women. A prospective epidemiologic registry of acyclovir use during pregnancy between 1984 and 1999 followed 749 pregnancies in women exposed to systemic acyclovir during the first trimester of pregnancy resulting in 756 outcomes. The occurrence rate of birth defects approximated that found in the general population. However, the size of the registry was insufficient to evaluate the risk for less common defects or to permit reliable or definitive conclusions regarding the safety of acyclovir in pregnant women and their developing fetuses.
Animal reproduction studies have not been conducted with SITAVIG. Acyclovir was not teratogenic in the mouse, rabbit or rat at exposures greatly in excess of human exposure.
SITAVIG should not be administered during labor and delivery as there is no experience with SITAVIG.
It is not known whether topically applied acyclovir is excreted in breast milk. Systemic exposure following buccal administration is minimal. After oral administration of acyclovir, concentrations have been documented in breast milk in 2 women and ranged from 0.6 to 4.1 times the corresponding plasma levels. These concentrations would potentially expose the nursing infant to a dose of acyclovir up to 0.3 mg/kg/day. There is no experience with SITAVIG in nursing mothers. SITAVIG should be administered to a nursing mother with caution.
Safety and effectiveness of SITAVIG in pediatric patients have not been established. The ability of pediatric patients to comply with the application instructions has not been evaluated. Use in younger children is not recommended due to potential risk of choking.
Clinical studies of SITAVIG did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects.
The safety of SITAVIG has not been studied in immunocompromised subjects.
Acyclovir absorption and systemic exposure following application of SITAVIG are minimal. Overdose is therefore unlikely [see Clinical Pharmacology (12.3)].
Symptomatic and supportive care is the basis for management.
SITAVIG (acyclovir) buccal tablet is applied topically to the gum and releases acyclovir as the buccal tablet gradually dissolves [see Clinical
Pharmacology (12.3)]. Acyclovir is a synthetic purine nucleoside analogue active against herpes viruses. The chemical name of acyclovir is 2-amino-1,9-dihydro-9-[(2-hydroxyethoxy)methyl]-6H-purin-6-one; it has a molecular formula of C8 H11 N5 O3 and a molecular weight of 225. The structural formula is shown in Figure 1.
Acyclovir drug substance is a white or almost white crystalline powder. SITAVIG contains 50 mg of acyclovir, USP and the following inactive
ingredients: hypromellose, USP; milk protein concentrate; sodium lauryl sulfate, NF; magnesium stearate, NF; microcrystalline cellulose,
NF; povidone, USP; colloidal silicon dioxide, NF.
Acyclovir is an antiviral drug [see Microbiology (12.4)].
Absorption and Distribution
Single dose application of SITAVIG containing 50 mg of acyclovir to the buccal mucosa in 12 healthy volunteers provided mean maximum
salivary concentrations of 440 μg/mL 8 hours after application of the tablet. The pharmacokinetic parameters of acyclovir in the saliva of
healthy volunteers are provided in Table 2.
Table 2: Pharmacokinetic (PK) Parameters of Acyclovir in Saliva Following Application of a Single SITAVIG 50 mg Tablet in Healthy Volunteers (N = 12)
Salivary PK Parameters(N = 12) Mean ±SD (Min — Max)
AUC0-24h (mcg.h/mL) 2900 ± 2400 (849 — 9450)
Cmax (mcg/mL) 440 ± 241 (149 – 959)
Tmax (hour) 7.95 ± 4.08 (3.07 – 18.05)
In the Phase 3 study, the levels of acyclovir in saliva were measured within 24 hours of SITAVIG application in 56 patients with recurrent
herpes labialis (mean value 88.1 micrograms per mL) and were within the range of those observed in the PK study in healthy volunteers.
In healthy volunteers, the median duration of buccal adhesion was 14 hours following application of a single SITAVIG 50 mg tablet.
Plasma concentrations of acyclovir were measured in 12 healthy volunteers after a single-dose application of SITAVIG 50 mg buccal tablet. Acyclovir concentrations had a delayed appearance (undetectable at 5 hours) and were below the concentrations required for antiviral activity (range: 17.5 to 55.3 nanogram per mL).
Metabolism and Excretion
Acyclovir is metabolized to 9-[(carboxymethoxy)methyl]guanine (CMMG) and 8-hydroxy-acyclovir (8-OH-ACV) by oxidation and hydroxylation, and is primarily excreted unchanged by the kidneys.
There was no formal food effect study conducted with SITAVIG; however, in clinical studies patients were allowed to eat and drink while taking SITAVIG.
Mechanism of Action
Acyclovir is a synthetic purine nucleoside that is phosphorylated intracellularly by the viral encoded thymidine kinase (TK) of HSV into acyclovir monophosphate, a nucleotide analogue. The monophosphate is further converted into diphosphate by cellular guanylate kinase and into triphosphate by a number of cellular enzymes. In a biochemical reaction, acyclovir triphosphate inhibits replication of herpes viral DNA by competing with nucleotides for binding to the viral DNA polymerase and by incorporation into and termination of the growing viral DNA chain. The cellular thymidine kinase of normal, uninfected cells does not use acyclovir effectively as a substrate, hence toxicity to mammalian host cells is low.
The quantitative relationship between the cell culture susceptibility of herpes viruses to antivirals 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 median EC50 value of acyclovir against clinical herpes virus isolates (subjects
receiving placebo) was 1.3 μM (range: < 0.56 to 3.3 μM).
Resistance of HSV to acyclovir can result from qualitative and quantitative changes in the viral TK and/or DNA polymerase. Clinical
isolates of HSV with reduced susceptibility to acyclovir have been recovered from immunocompromised subjects, especially with
advanced HIV infection. While most of the acyclovir-resistant mutant isolates from immunocompromised subjects thus far have been
found to be TK-deficient, other mutant isolates involving the viral TK gene (TK partial and TK altered) or DNA polymerase have been
identified. TK-negative mutants may cause severe disease in infants and immunocompromised adults.
The possibility of viral resistance to acyclovir should be considered in immunocompromised subjects who show poor clinical response during therapy.
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