Adefovir dipivoxil (bis-POM PMEA) is a member of a new class of antiretroviral agents. This orally bioavailable prodrug of PMEA (adefovir [9-(2-phosphonyl- methoxyethyl)adenine]) belongs to the nucleotide analogue class of RT inhibitors. Similar to the nucleoside RT inhibitors (AZT, ddI, d4T, etc), the drug needs to be phosphorylated by cellular enzymes into its active molecule (in this case, adefovir diphosphate), which then functions as a competitive inhibitor of one of the natural nucleotides. In the case of adefovir dipivoxil, this is dATP. In this way, it serves as a competitive substrate for incorporation into viral DNA, functioning as a chain terminator of viral DNA synthesis.
Both in vitro and in vivo, adefovir dipivoxil has been shown to be active against retroviruses, hepatitis viruses (HBV) and herpesviruses (HSV-1 and HSV-2, CMV, EBV). Activity against HIV has been described in a variety of host cells: resting and activated T-cells and monocytes/macrophages.
The resistance profile of adefovir dipivoxil has not been fully described. Two in vitro studies have been reported. These selection experiments revealed that amino acid substitutions at two positions in the RT enzyme (65 and 70) lead to a reduction in sensitivity of approximately 10-fold. Interestingly, a change at codon 70 is the primary substitution associated with AZT resistance in vivo. However, for AZT a change from lysine to arginine is observed whereas for adefovir dipivoxil a change to glutamic acid was seen. Various other positions (67, 69, 70, 74, 75) in this region have been shown to change as a consequence of nucleoside RT inhibition reflecting that this domain of RT is involved in recognition of the natural nucleotides and that amino acid substitutions in this region lead to an altered specificity of the enzyme.
Mulato et al were interested in determining the resistance profile of adefovir dipivoxil in vivo. The patient population was far from perfect from a scientific point of view, since most patients were pretreated with other nucleoside analogues, primarily with AZT or AZT plus ddI. However, this is a natural consequence of the therapeutic developments in the field. A subgroup was studied from a larger randomized, double blind, placebo controlled study which enrolled in 1994/5. Patients who completed the initial phase of 12 weeks of adefovir dipivoxil monotherapy or placebo were carried over into a maintenance phase in which the concomitant use of other antiretroviral agents (mostly 3TC and d4T) was permitted.
A subgroup of 29 patients was selected for resistance studies on the basis of the availability of a plasma sample obtained after at least six months in the maintenance phase. However it is not quite clear how representative this subgroup is of the whole group. Both phenotypic and genotypic analyses were performed using state-of-the-art assays. After 6 months of therapy the decrease in HIV-RNA from baseline was 0.6 log. Since the patients were allowed concomitant therapy it is difficult to determine to what extent adefovir dipivoxil was causing this decline. Five patients developed mutations during the monotherapy phase: 1 patient showed the change at codon 70 (K70E) which was also reported in vitro, 3 patients developed AZT-associated mutations (K70R and also changes at positions 41, 67, 215), and 1 patient had a change at codon 69 (T69D). This latter change was also observed in an additional patient during his maintenance phase. Phenotypic changes were analysed in these 5 patients using a recombinant virus assay. In viruses from 3 out of 5 patients a significant increase in the 50% inhibitory concentration to adefovir dipivoxil was observed.
During the maintenance phase, viruses from 4 patients developed characteristic mutations for AZT resistance, although the patients were not reported to have received concomitant AZT therapy. The explanation for this remains unclear, especially since a change in sensitivity to adefovir dipivoxil could not be measured. Three of these 4 patients had been pretreated with AZT prior to the maintenance period, and it may be that the AZT-associated mutations give a slight selective advantage over wild type virus which is not detected by our phenotypic assays.
In conclusion, this study of the resistance profiles of viruses from patients taking adefovir dipivoxil monotherapy followed by combination therapy shows that one of the mutations recognized in vitro may appear in vivo. These changes may lead to an approximately 10-fold increase in IC50. In addition, amino acid substitutions previously observed with other nucleoside RT inhibitors were observed. Given the relatively low (0.5 log) decreases in HIV-RNA observed during monotherapy, combined with the design of the study (pretreated patients, combination therapy during the maintenance), it remains difficult to determine the exact relationship between the development of drug resistance and therapeutic failure. Adefovir dipivoxil will be used in combination studies in the future, and it will be interesting to see the results of additional studies in order to make final conclusions on the resistance profile associated with adefovir dipivoxil therapy.
