Adefovir dipivoxil sustained viral load reductions in HIV-1-infected patients receiving extended therapy for ≥6 months with or without concomitant treatment with antiretroviral agents (Fig. ). This pattern of viral load suppression is in contrast to that achieved by many other nucleoside inhibitors such as AZT, 3TC, or ddI (11
), with which a gradual return toward baseline is usually seen, often coincident with the development of drug-resistant viruses. Since adefovir dipivoxil is known to have activity in a variety of cell types (2
), it is possible that the prolonged decrease in viral load observed during adefovir dipivoxil therapy is due to its activity in the longer-lived cells of the monocyte/macrophage lineage as well as the lack of significant resistance development.
The K70E and K65R mutations were selected for in vitro and exhibited approximately 9- to 16-fold decreases in sensitivity to adefovir (9
). Notably, viruses from two patients developed the K70E mutation during maintenance therapy, and both patients responded with better than median decreases in viral load. One patient (Table , patient A; Fig. A) was receiving monotherapy, and the viruses from that patient developed a mixture of wild-type and mutant codons at amino acid 70 (K70E/K) after 3 months of maintenance dosing. Even in the presence of this mutation the patient had a decrease in viral RNA load of 0.9 log10
copies/ml after 6 months of maintenance-phase dosing. Interestingly, the HIV in this patient’s plasma remained a mixture of mutant K70E and wild-type virus between months 3 and 6, despite continued treatment with adefovir dipivoxil monotherapy. It has previously been shown that the K70E recombinant virus demonstrated modestly reduced growth kinetics in vitro (9
). The K70E virus may also have a reduced replication capacity in vivo, perhaps contributing to the sustained activity of adefovir dipivoxil in this patient. The moderate decrease in susceptibility to adefovir in recombinants generated from patient A is also consistent with the lack of a viral load rebound in this patient. Viruses from the second patient (Table , patient G) developed the K70E mutation in RT after 12 months of adefovir dipivoxil therapy while the patient was concomitantly receiving AZT and 3TC. Since this patient was receiving concomitant therapy and also developed the M184V mutation during the maintenance phase, it is not clear what role the K70E mutation played in the virological response (decrease in viral load of 0.9 log10
copies/ml from maintenance-phase baseline at month 12) of this patient. No patient entered the maintenance phase with a virus with a K65R mutation in RT or was infected with a virus that developed that mutation during therapy, so the effect of this mutation on the virological response to adefovir dipivoxil is as yet unknown.
Viruses from two patients (Table , patients E and F) developed the T69D mutation while the patients were receiving adefovir dipivoxil therapy. While this mutation has previously been associated with ddC therapy (18
), it has not been associated directly with treatment with other nucleosides and was not selected for in vitro by adefovir. Neither of the patients whose viruses developed the T69D mutation reported prior ddC use. Both patients had sustained decreases in viral load (−1.4 and −1.2 log10
copies/ml) subsequent to the development of this mutation. However, since both patients were concomitantly receiving other medications during the maintenance phase, the effects of the T69D mutation on the virus responses to adefovir dipivoxil treatment are not clear. The minor decrease in adefovir susceptibility measured for the posttherapy recombinant virus from patient E may suggest that adefovir dipivoxil is still able to effectively suppress replication of the T69D virus.
Interestingly, during the maintenance phase viruses from four patients (Table , patients B, C, D, and H) developed mutations that are characteristically associated with AZT resistance, although the patients did not report receiving concomitant AZT therapy. It is curious that among the viruses from these patients, the majority of the viruses with AZT-associated resistance mutations, once identified, maintained mixtures of mutations but did not develop into full mutants. This finding suggests that the selective pressure exerted by adefovir dipivoxil that allows the outgrowth of viruses harboring the AZT-associated resistance mutations may be quite moderate. Previous phenotypic assays performed with clinical isolates containing different combinations of AZT-resistant mutations exhibited little to no decrease in susceptibility to adefovir in vitro (8
). Clearly, recombinant viruses from patients B and C showed no measurable decrease in susceptibility to adefovir in vitro (Table ), and the viral loads in these patients did not rebound in the presence of viruses with these mutations (Fig. ). Patient D is the only patient in this study whose recombinant viruses showed a greater than threefold decrease in adefovir susceptibility, yet the patient still had a notable viral RNA decrease over the course of therapy.
Three of these four patients (Table , patients B, C, and H) had received AZT therapy prior to the maintenance phase, and although AZT-associated mutations were not detectable at baseline, viruses with these mutations may have existed as a minority viral population in these patients. These viruses expressing AZT-associated mutations may have a slight selective advantage over wild-type viruses in the presence of adefovir dipivoxil and may therefore be enriched for during adefovir dipivoxil therapy, thus becoming a larger percentage of the viral population after the significant decrease in HIV load experienced by all three patients (−0.7, −0.8, and −1.8 log10 copies/ml, respectively; the last two patients were on monotherapy). The fourth patient (Table , patient D) was reportedly AZT naive, yet viruses from this patient still developed a complex mixture of AZT-associated resistance mutations after 12 months of monotherapy. Even so, this patient demonstrated a decrease in HIV RNA load of 1.0 log10 copies/ml after 12 months of adefovir dipivoxil monotherapy, making the clinical significance of these genotypic findings unclear. Alternatively, it is possible that patient D was originally infected with an AZT-resistant virus.
The development of AZT-associated resistance mutations (M41L, D67N, K70R, L210W, T215Y, and K219Q) in viruses from patients receiving RT inhibitors other than AZT has been documented previously. Winters et al. (51
) reported that viruses from 6 of 23 patients receiving ddI monotherapy for 56 to 104 weeks developed the M41L, K70R, L210W, T215Y, and/or K219Q mutations. Two of these patients did not report receiving prior AZT therapy. Viruses from the patients who developed AZT-associated resistance mutations maintained wild-type ddI susceptibilities when phenotypic assays were performed. Demeter et al. (16
) also reported that viruses from two patients receiving ddI monotherapy for >2 years developed M41L, D67N, K70R, and/or T215Y mutations. One of these patients had not received prior AZT therapy. Lin et al. (33
) reported that viruses from 8 of 13 patients analyzed after ≥18 months of d4T treatment developed the AZT-associated resistance mutations M41L, D67N, L210W, T215Y, and/or K219Q or K219E. Two of these eight patients did not report receiving prior AZT therapy. Thus, as suggested for adefovir dipivoxil, both ddI and d4T may exert enough selective pressure to allow the outgrowth of viruses carrying AZT-associated resistance mutations. Infrequently, however, it appears that AZT-associated resistance mutations may be selected for in vivo in AZT-naive patients during therapy with an RT inhibitor other than AZT.
It has been shown in numerous studies that antiretroviral agent-naive patients have a greater decrease in viral load than antiretroviral agent-experienced patients. The reasons for this finding are likely numerous (stage of disease, compliance, pharmacokinetics, etc.), but at least for some patients, baseline preexisting virus mutations influence the in vivo response to a new agent. For example, patients whose viruses develop a T215Y or T215F mutation in RT during AZT therapy have been shown to have a diminished virological response to subsequent treatment with AZT-ddI, ddI alone, or AZT-ddI-delavirdine (12
) compared to the response of patients whose viruses have the wild-type amino acid at this RT codon. The role played by other baseline nucleoside resistance mutations on the virological response to a subsequent agent is less well described to date. Interestingly, it was reported recently that patients whose viruses possess an M184V mutation at baseline and who later added AZT to their 3TC therapy and whose viruses developed multiple AZT-resistant mutations had poor virological responses to AZT-3TC combination therapy (35
As shown in Fig. , the viral load responses of patients whose viruses had AZT-associated resistance mutations at baseline were similar to the overall median viral load changes among all patients in this study. However, the concomitant use of other medications by 50% of the patients, coupled with the limited number of patients in each group and the complex pattern of representation of the six individual AZT mutations present at the baseline, precludes a complete evaluation of the effects of preexisting AZT resistance mutations on the response to adefovir dipivoxil therapy. Additional studies are needed to directly address the effects of not only AZT-associated resistance mutations but also those associated with other HIV therapies on the response to adefovir dipivoxil therapy.
Few changes in the sequence of RT from baseline that could be potentially attributed to adefovir dipivoxil therapy were detected in viruses from patients who received up to 12 months of maintenance-phase dosing with or without the concomitant use of other antiretroviral agents. The lack of significant genotypic or phenotypic resistance development as well as adefovir dipivoxil’s documented activity in resting and activated lymphocytes and macrophages/monocytes (2
) are consistent with the durable anti-HIV activity observed in this study. Ongoing blinded, controlled clinical trials will further investigate the resistance profile and antiretroviral activity of adefovir dipivoxil.