This study has demonstrated the potent dose-related antihepadnaviral efficacy of oral adefovir dipivoxil in chronically WHV-infected woodchucks. Serum WHV DNA levels were reduced by more than 2.5 logs in woodchucks that received 15 mg of adefovir dipivoxil/kg daily and more than 1.6 logs in those treated with 5 mg of adefovir dipivoxil/kg daily in the course of 12 weeks of treatment. Diminution of serum WHV levels was evident within 14 days, at the first sample period. Viral levels remained suppressed throughout the course of treatment and rebounded to pretreatment levels once drug exposure was discontinued. These findings are in accord with studies in WHV-infected primary hepatocytes, duck HBV-infected ducks, HBV-expressing cell lines, and HBV-infected humans (5
). Although direct comparisons are not possible because of different dosage regimens and methods of analysis, the antiviral activity of adefovir in chronically WHV-infected woodchucks appears to be similar to activities of other antihepadnaviral nucleoside analogs, such as emtricitabine, adenine-5′-arabinoside monophosphate (Ara-AMP), famciclovir, and zidovudine (6
Although adefovir has a potent and selective activity against hepadnavirus and retrovirus replication (3
) it is not absorbed well from the digestive tract because of limited intestinal permeativity of the phosphonate group, which is charged at physiologic pH (25
). The prodrug, adefovir dipivoxil, is better absorbed than adefovir in several animal species, including humans, nonhuman primates, dogs, and rats (3
). The oral bioavailability of adefovir dipivoxil in woodchucks was quite similar to that in humans and the other species despite the differences in the anatomy of the digestive tracts. As a result, a single daily oral dose was sufficient to reduce serum WHV DNA levels and to maintain the reduction during the course of therapy.
The antiviral efficacy of nucleoside analogues is usually evident within 1 or 2 weeks of treatment in vivo; however, toxic effects can develop when treatment is extended beyond several weeks. Long-term treatment with Ara-AMP can lead to neuropathy, and long-term ribavirin can lead to disturbed erythropoietic activity (20
). WHV-infected woodchucks treated with fialuridine had a marked reduction in serum WHV DNA within 1 to 2 weeks of treatment. Toxicity was not evident in treated woodchucks until they had been exposed for approximately 8 weeks, after which time lethal liver damage ensued (31
). Fialuridine-treated patients have also experienced severe toxic effects (23
). In order to assess the risk of delayed toxicity in this study, adefovir dipivoxil was administered to woodchucks for 12 weeks and animals were monitored for an additional 6 weeks after treatment. Analysis of a broad series of serum biochemical analytes, targeting liver and renal function in particular, showed no evidence of clinically significant toxicity in treated woodchucks. In addition to standard serum biochemistry, serum bicarbonate and lactate levels were measured in all the woodchucks because the levels of these analytes have been abnormal in patients with fialuridine-induced hepatic mitochondrial damage (4
). In clinical studies with high doses of adefovir dipivoxil (60 and 120 mg daily) for the treatment of HIV infection, the most frequent adverse events were a mild nephrotoxicity characterized by elevated serum creatinine and/or hypophosphatemia that was reversible upon dose discontinuation. In ongoing clinical studies for the treatment of HBV infection, a much lower daily dose of 10 mg is being tested and serum creatinine and phosphate levels are being monitored closely. In the woodchuck study, there were no clinically relevant or statistically significant changes in serum creatinine or serum phosphate at week 12 compared to those in control animals, suggesting that there is no evidence of nephrotoxicity in woodchucks treated with these relatively high doses. There was no evidence of clinically significant abnormalities in the hematological data from the adefovir dipivoxil-treated woodchucks. Importantly, serum bicarbonate remained within normal limits, thus showing no evidence of metabolic acidosis. Also, there was no evidence of the marked treatment-related microvesicular and macrovesicular vacuolization seen in fialuridine-treated woodchucks or humans in the livers of high-dose (15 mg of adefovir dipivoxil/kg daily) woodchucks after the recovery period (16
The pathogenesis of the cheilitis is unclear. Irritation of the lips appeared to be related to exposure to the adefovir, since it occurred only in the animals that were treated and resolved quickly when exposure was discontinued. Physical trauma seems unlikely since the animals accepted the drug in grape juice without reluctance. Although it is possible that there was a direct irritant effect of the drug to the mucous membranes, this seems unlikely in view of the fact that there were no lesions in the oral cavity of any of the animals. Possibly there was more salivation in response to the drug that led to persistent moisture and a secondary bacterial dermatitis at the commissural region of the lips that led to the cheilitis.
In summary, adefovir dipivoxil was shown to be an effective antihepadnaviral agent in chronically WHV-infected feral woodchucks. Serum WHV DNA levels were markedly suppressed in a dose-related fashion during 12 weeks of treatment. The oral bioavailability of the prodrug, adefovir dipivoxil, was sufficient to permit an efficacious single daily oral administration regime. There was no evidence of toxicity during the 12-week treatment period or during the 6-week follow-up period. Continued development of adefovir dipivoxil for the treatment of chronic hepatitis B in patients is warranted.