Despite the success of potent combination therapies in patients with limited or no prior antiretroviral therapy, clinical experience demonstrates that these regimens are less likely to achieve durable suppression of HIV-1 replication in patients with extensive prior treatment with nucleoside analogues. The response to treatment of patients who have failed multiple previous regimens has been disappointing. Thus, there is an urgent need for new approaches to the treatment of such patients. Recent studies have shown that it is effective to add investigational drugs to optimized background drug regimens that have been selected based on resistance testing. This study will assess the virologic and immunologic activity of amdoxovir (DAPD) versus placebo in combination with enfuvirtide (T-20, or ENF) plus optimized background (OB) antiretroviral therapy for highly treatment-experienced patients.

Patients in this study will continue to take their current (failing) antiretroviral regimen until they are registered to the study. Patients will be randomized to receive DAPD or placebo. Patients will receive DAPD or placebo together with ENF plus an OB regimen containing at least three but not more than five antiretroviral agents. The OB regimen will be selected based on the results of a screening HIV-1 drug resistance test and is expected to remain stable for at least the first 24 weeks of the study. Only ENF and DAPD will be supplied by this study, but they will not be provided to participants beyond the end of the study.

ENF is injected into the abdomen, deltoid, or the anterior aspect of the thigh. Patients will be taught how to self-administer ENF. Medical staff will observe self-injection of the first dose of ENF and at clinic visits scheduled for Weeks 1, 2, and 4. During Week 4, patients will undergo pharmacokinetic testing. This requires that patients come to the clinic for approximately 12 hours so that blood can be tested at different times after taking the study drugs.

After Week 4, there are follow-up visits every 4 weeks until Week 48. Blood work, ophthalmologic exams, and urinalysis are done at all clinic visits, except for Week 1. Participants may continue to receive study treatment beyond Week 24 for up to 48 weeks total, unless they experience a confirmed loss of virologic and immunologic response. Regardless of treatment, all patients are followed for 48 weeks.

In March 2004, participants in this study were unblinded. Participants on DAPD placebo were given the option of discontinuing the placebo and replacing it with an active antiretroviral, if one is available. Participants on active DAPD were given the option of continuing DAPD through Week 48, or discontinuing it and replacing it with another antiretroviral agent. All participants may continue to receive ENF through Week 48.