Adefovir dipivoxil

Adefovir dipivoxil is an oral prodrug of adefovir, an acyclic nucleotide phosphonate analogue of adenosine monophosphate, which is actively transported into mammalian cells where it is converted by host enzymes to adefovir diphosphate. Adefovir diphosphate inhibits viral polymerases by competing for direct binding with the natural substrate (deoxyadenosine triphosphate) and, after incorporation into viral DNA, causes DNA chain termination. Adefovir diphosphate selectively inhibits HBV DNA polymerases at concentrations 12-, 700-, and 10-fold lower than those needed to inhibit human DNA polymerases α, β, and γ, respectively. Adefovir diphosphate has an intracellular half-life of 12 to 36 hours in activated and resting lymphocytes.

Pharmacodynamic effects

Adefovir is active against hepadnaviruses in vitro, including all common forms of lamivudineresistant HBV (rtL180M, rtM204I, rtM204V, rtL180M/rtM204V), famciclovir-associated mutations (rtV173L, rtP177L, rtL180M, rtT184S or rtV207I) and hepatitis B immunoglobulin escape mutations (rtT128N and rtW153Q), and in in vivo animal models of hepadnavirus replication.

Adefovir dipivoxil is a dipivaloyloxymethyl ester prodrug of the active substance adefovir, an acyclic nucleotide analogue which is actively transported into cells where it is converted by host enzymes to adefovir diphosphate.

Absorption

The oral bioavailability of adefovir from 10 mg adefovir dipivoxil is 59%. Following oral administration of a single dose of 10 mg adefovir dipivoxil to chronic hepatitis B patients, the median (range) peak serum concentration (C_max) was achieved after 1.75 h (0.58-4.0 h). Median C_max and AUC_0-∞ values were 16.70 (9.66-30.56) ng/ml and 204.40 (109.75-356.05) ng·h/ml, respectively. Systemic exposure to adefovir was not affected when 10 mg adefovir dipivoxil was taken with a high fat meal. The t_max was delayed by two hours.

Distribution

Preclinical studies show that after oral administration of adefovir dipivoxil, adefovir is distributed to most tissues with the highest concentrations occurring in kidney, liver and intestinal tissues. 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.

Biotransformation

Following oral administration, adefovir dipivoxil is rapidly converted to adefovir. At concentrations substantially higher (>4,000-fold) than those observed in vivo, adefovir did not inhibit any of the following human CYP450 isoforms, CYP1A2, CYP2D6, CYP2C9, CYP2C19, CYP3A4. Based on the results of these in vitro experiments and the known elimination pathway of adefovir, the potential for CYP450 mediated interactions involving adefovir with other medicinal products is low.

Elimination

Adefovir is excreted renally by a combination of glomerular filtration and active tubular secretion. The median (min-max) renal clearance of adefovir in subjects with normal renal function (Cl_cr >80 ml/min) is 211 ml/min (172-316 ml/min), approximately twice calculated creatinine clearance (Cockroft-Gault method). After repeated administration of 10 mg adefovir dipivoxil, 45 % of the dose is recovered as adefovir in the urine over 24 hours. Plasma adefovir concentrations declined in a biexponential manner with a median terminal elimination half-life of 7.22 h (4.72-10.70 h).

Linearity/non-linearity

The pharmacokinetics of adefovir are proportional to dose when given as adefovir dipivoxil over the dose range of 10 to 60 mg. Repeated dosing of adefovir dipivoxil 10 mg daily did not influence the pharmacokinetics of adefovir.

Gender, age and ethnicity

The pharmacokinetics of adefovir were similar in male and female patients. Pharmacokinetic studies have not been conducted in the elderly. Pharmacokinetic studies were principally conducted in Caucasian patients. The available data do not appear to indicate any difference in pharmacokinetics with regard to race.

Renal impairment

The mean (± SD) pharmacokinetic parameters of adefovir following administration of a single dose of 10 mg adefovir dipivoxil to patients with varying degrees of renal impairment are described in the table below:

A four-hour period of haemodialysis removed approximately 35% of the adefovir dose. The effect of peritoneal dialysis on adefovir removal has not been evaluated.

It is recommended that the dosing interval of 10 mg adefovir dipivoxil is modified in patients with creatinine clearance between 30 and 49 ml/min. Adefovir dipivoxil is not recommended in patients with creatinine clearance of <30 ml/min or in patients on dialysis.

Hepatic impairment

Pharmacokinetic properties were similar in patients with moderate and severe hepatic impairment compared to healthy volunteers.

Paediatric population

The pharmacokinetics of adefovir dipivoxil were studied in an efficacy and safety study of a daily dose of 0.25 mg/kg to 10 mg adefovir dipivoxil in children (aged 2 to <18 years). Pharmacokinetic analysis revealed that adefovir exposure was comparable among 3 age groups, 2 to 6 years (0.3 mg/kg), 7 to 11 years (0.25 mg/kg) and 12 to 17 years (10 mg) and all age groups achieved adefovir exposure in the target range, which was based on adefovir plasma concentrations in adult patients with chronic hepatitis B with established safety and efficacy profiles.

The primary dose-limiting toxic effect associated with administration of adefovir dipivoxil in animals (mice, rats and monkeys) was renal tubular nephropathy characterised by histological alterations and/or increases in blood urea nitrogen and serum creatinine. Nephrotoxicity was observed in animals at systemic exposures at least 3-10 times higher than those achieved in humans at the recommended therapeutic dose of 10 mg/day.

No effects on male or female fertility, or reproductive performance, occurred in rats and there was no embryotoxicity or teratogenicity in rats or rabbits administered adefovir dipivoxil orally.

When adefovir was administered intravenously to pregnant rats at doses associated with notable maternal toxicity (systemic exposure 38 times that achieved in humans at the therapeutic dose) embryotoxicity and an increased incidence of foetal malformations (anasarca, depressed eye bulge, umbilical hernia and kinked tail) were observed. No adverse effects on development were seen at systemic exposures approximately 12 times that achieved in humans at the therapeutic dose.

Adefovir dipivoxil was mutagenic in the in vitro mouse lymphoma cell assay (with or without metabolic activation), but was not clastogenic in the in vivo mouse micronucleus assay.

Adefovir was not mutagenic in microbial mutagenicity assays involving Salmonella typhimurium (Ames) and Escherichia coli in the presence and absence of metabolic activation. Adefovir induced chromosomal aberrations in the in vitro human peripheral blood lymphocyte assay without metabolic activation.

In long-term carcinogenicity studies in rats and mice with adefovir dipivoxil, no treatment-related increase in tumour incidence was found in mice or rats (systemic exposures approximately 10 and 4 times those achieved in humans at the therapeutic dose of 10 mg/day, respectively).