Mechanism of Action
Adefovir is an antiviral drug. [see Clinical Pharmacology].
The pharmacokinetics of adefovir have been evaluated in healthy volunteers and patients with chronic hepatitis B. Adefovir pharmacokinetics are similar between these populations.
Adefovir dipivoxil is a diester prodrug of the active moiety adefovir. Based on a cross study comparison, the approximate oral bioavailability of adefovir from HEPSERA is 59%.
Following oral administration of a 10 mg single dose of HEPSERA to chronic hepatitis B patients (N=14), the peak adefovir plasma concentration (Cmax) was 18.4 ± 6.26 ng/mL (mean ± SD) and occurred between 0.58 and 4.00 hours (median=1.75 hours) post dose. The adefovir area under the plasma concentration-time curve (AUC0–∞) was 220 ± 70.0 ng∙h/mL. Plasma adefovir concentrations declined in a biexponential manner with a terminal elimination half-life of 7.48 ± 1.65 hours.
The pharmacokinetics of adefovir in subjects with adequate renal function were not affected by once daily dosing of 10 mg HEPSERA over seven days. The impact of long-term once daily administration of 10 mg HEPSERA on adefovir pharmacokinetics has not been evaluated.
Effects of Food on Oral Absorption
Adefovir exposure was unaffected when a 10 mg single dose of HEPSERA was administered with food (an approximately 1000 kcal high-fat meal). HEPSERA may be taken without regard to food.
In vitro binding of adefovir to human plasma or human serum proteins is ≤4% over the adefovir concentration range of 0.1 to 25 µg/mL. The volume of distribution at steady-state following intravenous administration of 1.0 or 3.0 mg/kg/day is 392 ± 75 and 352 ± 9 mL/kg, respectively.
Metabolism and Elimination
Following oral administration, adefovir dipivoxil is rapidly converted to adefovir. Forty-five percent of the dose is recovered as adefovir in the urine over 24 hours at steady state following 10 mg oral doses of HEPSERA. Adefovir is renally excreted by a combination of glomerular filtration and active tubular secretion [see Drug Interactions and Clinical Pharmacology].
Assessment of Drug Interactions
Adefovir dipivoxil is rapidly converted to adefovir in vivo. At concentrations substantially higher (>4000-fold) than those observed in vivo, adefovir did not inhibit any of the common human CYP450 enzymes, CYP1A2, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. Adefovir is not a substrate for these enzymes. However, the potential for adefovir to induce CYP450 enzymes is unknown. Based on the results of these in vitro experiments and the renal elimination pathway of adefovir, the potential for CYP450 mediated interactions involving adefovir as an inhibitor or substrate with other medicinal products is low.
The pharmacokinetics of adefovir have been evaluated in healthy adult volunteers following multiple dose administration of HEPSERA (10 mg once daily) in combination with lamivudine (100 mg once daily) (N=18), trimethoprim/sulfamethoxazole (160/800 mg twice daily) (N=18), acetaminophen (1000 mg four times daily) (N=20), ibuprofen (800 mg three times daily) (N=18), and enteric coated didanosine (400 mg) (N=21). The pharmacokinetics of adefovir have also been evaluated in post-liver transplantation patients following multiple dose administration of HEPSERA (10 mg once daily) in combination with tacrolimus (N=16). The pharmacokinetics of adefovir have been evaluated in healthy volunteers following single dose pegylated interferon α-2a (PEG-IFN) (180 µg) (N=15).
Adefovir did not alter the pharmacokinetics of lamivudine, trimethoprim/sulfamethoxazole, acetaminophen, ibuprofen, enteric coated didanosine (didanosine EC), or tacrolimus. The evaluation of the effect of adefovir on the pharmacokinetics of pegylated interferon α-2a was inconclusive due to the high variability of pegylated interferon alpha-2a.
The pharmacokinetics of adefovir were unchanged when HEPSERA was coadministered with lamivudine, trimethoprim/sulfamethoxazole, acetaminophen, didanosine EC, tacrolimus (based on cross study comparison), and pegylated interferon α-2a. When HEPSERA was coadministered with ibuprofen (800 mg three times daily) increases in adefovir Cmax (33%), AUC (23%) and urinary recovery were observed. This increase appears to be due to higher oral bioavailability, not a reduction in renal clearance of adefovir.
Apart from lamivudine, trimethoprim/sulfamethoxazole, and acetaminophen, the effects of co-administration of HEPSERA with drugs that are excreted renally, or other drugs known to affect renal function have not been evaluated.
The effect of adefovir on cyclosporine concentrations is not known.
No drug interaction studies have been performed in adolescent patients aged ≥12 years to <18 years.
The pharmacokinetics of adefovir were similar in male and female patients.
The pharmacokinetics of adefovir have been shown to be comparable in Caucasians and Asians. Pharmacokinetic data are not available for other racial groups.
Pharmacokinetic studies have not been conducted in the elderly.
The pharmacokinetics of adefovir were assessed from drug plasma concentrations in 53 HBeAg positive hepatitis B pediatric patients with compensated liver disease. The exposure of adefovir following a 48 week daily treatment with adefovir dipivoxil 10 mg tablet in pediatric patients aged ≥ 12 to <18 years (Cmax = 23.3 ng/ml and AUC0–24 = 248.8 ng∙h/ml) was comparable to that observed in adult patients.
In adults with moderately or severely impaired renal function or with end-stage renal disease (ESRD) requiring hemodialysis, Cmax, AUC, and half-life (T1/2) were increased compared to adults with normal renal function. It is recommended that the dosing interval of HEPSERA be modified in these patients [see Dosage and Administration].
The pharmacokinetics of adefovir in non-chronic hepatitis B patients with varying degrees of renal impairment are described in Table 3. In this study, subjects received a 10 mg single dose of HEPSERA.
Table 3. Pharmacokinetic Parameters (Mean ± SD) of Adefovir in Patients with Varying Degrees of Renal Function
|Renal Function Group
|Baseline creatinine clearance (mL/min)
||17.8 ± 3.22
||22.4 ± 4.04
||28.5 ± 8.57
||51.6 ± 10.3
|AUC 0–∞ (ng∙h/mL)
||201 ± 40.8
||266 ± 55.7
||455 ± 176
||1240 ± 629
||469 ± 99.0
||356 ± 85.6
||237 ± 118
||91.7 ± 51.3
||231 ± 48.9
||148 ± 39.3
||83.9 ± 27.5
||37.0 ± 18.4
A four-hour period of hemodialysis removed approximately 35% of the adefovir dose. The effect of peritoneal dialysis on adefovir removal has not been evaluated.
The pharmacokinetics of adefovir have not been studied in adolescent patients with renal dysfunction [see Use in Specific Populations].
The pharmacokinetics of adefovir following a 10 mg single dose of HEPSERA have been studied in non-chronic hepatitis B patients with hepatic impairment. There were no substantial alterations in adefovir pharmacokinetics in patients with moderate and severe hepatic impairment compared to unimpaired patients. No change in HEPSERA dosing is required in patients with hepatic impairment.
Mechanism of Action
Adefovir is an acyclic nucleotide analog of adenosine monophosphate which is phosphorylated to the active metabolite adefovir diphosphate by cellular kinases. Adefovir diphosphate inhibits HBV DNA polymerase (reverse transcriptase) by competing with the natural substrate deoxyadenosine triphosphate and by causing DNA chain termination after its incorporation into viral DNA. The inhibition constant (Ki) for adefovir diphosphate for HBV DNA polymerase was 0.1 µM. Adefovir diphosphate is a weak inhibitor of human DNA polymerases α and γ with Ki values of 1.18 µM and 0.97 µM, respectively.
The concentration of adefovir that inhibited 50% of viral DNA synthesis (EC50) in HBV transfected human hepatoma cell lines ranged from 0.2 to 2.5 µM. The combination of adefovir with lamivudine showed additive anti-HBV activity.
Clinical isolates with genotypic changes conferring reduced susceptibility in cell culture to nucleoside analog reverse transcriptase inhibitors for the treatment of HBV infection have been observed. Long-term resistance analyses performed by genotyping samples from all adefovir dipivoxil-treated patients with detectable serum HBV DNA demonstrated that amino acid substitutions rtN236T and rtA181T/V have been observed in association with adefovir resistance. In cell culture, the rtN236T substitution demonstrated 4- to 14-fold, the rtA181V substitution 2.5- to 4.2-fold, and the rtA181T substitution 1.3- to 1.9-fold reduced susceptibility to adefovir.
In HBeAg-positive nucleoside-naïve patient isolates (Study GS-98-437, N=171), no adefovir resistance-associated substitutions were observed at Week 48. Sixty-five patients continued on long term treatment after a median duration on adefovir dipivoxil of 235 weeks (range 110–279 weeks). Isolates from 16 of 38 (42%) patients developed adefovir resistance-associated substitutions in the setting of virologic failure (confirmed increase of ≥1 log10 HBV DNA copies/mL above nadir or never suppressed below 103 copies/mL). The substitutions included rtN236T (n=2), rtA181V (n=4), rtA181T (n=3), rtA181T+rtN236T (n=5), and rtA181V+rtN236T (n=2). In HBeAg-negative nucleoside-naïve patients (Study GS-98-438), isolates from 30 patients were identified with adefovir resistance-associated substitutions with a cumulative probability of 0%, 3%, 11%, 19%, and 30% at 48, 96, 144, 192, and 240 weeks, respectively. Of those 30 patients, 22 had a confirmed increase of ≥1 log10 HBV DNA copies/mL above nadir or never achieved HBV DNA levels below 103 copies/mL; an additional 8 patients had adefovir resistance-associated substitutions without virologic failure. In addition, the long term (4 to 5 years) development of resistance to adefovir dipivoxil was significantly lower in patients who had serum HBV DNA below the limit of quantification (less than 1,000 copies/mL) at Week 48 as compared to patients who had serum HBV DNA above 1,000 copies/mL at Week 48.
In an open-label study of pre- and post-liver transplantation patients (Study GS-98-435), isolates from 129 patients with clinical evidence of lamivudine-resistant hepatitis B virus at baseline were evaluated for adefovir resistance-associated substitutions. The incidence of adefovir resistance-associated (rtN236T or rtA181T/V) substitutions was 0% at 48 weeks. Isolates from four patients developed the rtN236T substitution after 72 weeks of adefovir dipivoxil therapy. Development of the rtN236T substitution was associated with serum HBV DNA rebound. All 4 patients who developed the rtN236T substitution in their HBV had discontinued lamivudine therapy before the development of genotypic resistance and all 4 lost the lamivudine resistance-associated substitutions present at baseline. In a study of 35 HIV/HBV co-infected patients with lamivudine-resistant HBV (Study 460i) who added adefovir dipivoxil to lamivudine, no adefovir resistance-associated substitutions were observed in HBV isolates from 15/35 patients tested up to 144 weeks of therapy.
Clinical resistance in pediatric patients
In a Phase 3 pediatric Study GS-US-103-518, HBV isolates from 49 of 56 pediatric subjects (aged 12 to 17 years) had serum HBV DNA >169 copies/mL and were evaluated for adefovir resistance-associated substitutions. rtN236T and/or rtA181V adefovir resistance-associated substitutions were not observed at Week 48. However, the rtA181T substitution was present in baseline and Week 48 isolates from 2 pediatric patients.
Recombinant HBV variants containing lamivudine-resistance-associated substitutions (rtL180M, rtM204I, rtM204V, rtL180M + rtM204V, rtV173L + rtL180M + rtM204V) were susceptible to adefovir in cell culture. Adefovir dipivoxil has also demonstrated anti-HBV activity (median reduction in serum HBV DNA of 4.1 log10 copies/mL) in patients with HBV containing lamivudine-resistance-associated substitutions (Study 435). Adefovir also demonstrated in cell culture activity against HBV variants with entecavir resistance-associated substitutions (rtT184G, rtS202I, rtM250V). HBV variants with DNA polymerase substitutions rtT128N and rtR153Q or rtW153Q associated with resistance to hepatitis B virus immunoglobulin were susceptible to adefovir in cell culture.
HBV variants expressing the adefovir resistance-associated substitution rtN236T showed no change in susceptibility to entecavir in cell culture, and a 2- to 3-fold decrease in lamivudine susceptibility. HBV mutants with the adefovir resistance-associated substitution rtA181V showed a range of decreased susceptibilities to lamivudine of 1- to 14-fold and a 12-fold decrease in susceptibility to entecavir. In patients whose HBV expressed the rtA181V substitution (n=2) or the rtN236T substitution (n=3), a reduction in serum HBV DNA of 2.4 to 3.1 and 2.0 to 5.1 log10 copies/mL, respectively, was observed when treatment with lamivudine was added to treatment with adefovir dipivoxil.
Carcinogenesis, Mutagenesis, Impairment of Fertility
Long-term oral carcinogenicity studies of adefovir dipivoxil in mice and rats were carried out at exposures up to approximately 10 times (mice) and 4 times (rats) those observed in humans at the therapeutic dose for HBV infection. In both mouse and rat studies, adefovir dipivoxil was negative for carcinogenic findings. Adefovir dipivoxil was mutagenic in the in vitro mouse lymphoma cell assay (with or without metabolic activation). Adefovir induced chromosomal aberrations in the in vitro human peripheral blood lymphocyte assay without metabolic activation. Adefovir dipivoxil was not clastogenic in the in vivo mouse micronucleus assay and adefovir was not mutagenic in the Ames bacterial reverse mutation assay using S. typhimurium and E. coli strains in the presence or absence of metabolic activation. In reproductive toxicology studies, no evidence of impaired fertility was seen in male or female rats at systemic exposure approximately 19 times that achieved in humans at the therapeutic dose.
Animal Toxicology and/or Pharmacology
Animal reproduction studies were conducted in rats and rabbits with orally administered adefovir dipivoxil and intravenously administered adefovir.
In rats and rabbits, no embryotoxicity or teratogenicity was shown from oral administration of adefovir dipivoxil at maternal doses producing systemic exposures approximately 23 times (rats) and 40 times (rabbits) that achieved in humans at the therapeutic dose of 10 mg/day.
When pregnant rats were administered intravenous adefovir at maternally toxic doses associated with systemic exposure 38 times that in humans, embryotoxicity and an increased incidence of fetal malformations (anasarca, depressed eye bulge, umbilical hernia, and kinked tail) were observed. No adverse effects on development were seen with intravenous adefovir administered to pregnant rats at a systemic exposure 12 times that in humans.
Animal Toxicology Studies
Renal tubular nephropathy characterized by histological alterations and/or increases in BUN and serum creatinine was the primary dose-limiting toxicity associated with administration of adefovir dipivoxil in animals. Nephrotoxicity was observed in animals at systemic exposures approximately 3–10 times higher than those in humans at the recommended therapeutic dose of 10 mg/day.