AZT has been shown to be effective in the treatment of AIDS and advanced ARC but not without toxicity. The most clinically significant toxicity is dose related inhibition of bone marrow function. Furthermore, HIV-1 isolates from patients treated for more than 6 months with AZT have now been found which appear to have reduced sensitivity to AZT. The incidence of toxicity and occurrence of virus with reduced sensitivity to AZT may result in the inability to administer AZT long-term to patients with AIDS and advanced ARC. Recombinant soluble CD4 (rCD4) has shown antiretroviral effects and has been shown to be safe when given to AIDS and ARC patients either as a single agent or in combination with AZT.

Part 1A: Twenty p24+ patients with AIDS or advanced ARC are randomized to 4 dosing groups of 5 patients each. Patients are treated with AZT for 6 weeks at ranging doses to determine a minimally effective dose (MED). At the end of week 6, each patient is sequentially assigned to 1 of 5 groups of rCD4 / AZT combination treatment. The first 3 patients to complete treatment through week 6 are treated in group A, the next 3 patients in group B, and so on.

The treatment period is 4 weeks. Each patient continues on his or her AZT dose as initially administered. The highest dose of AZT that produces less than a 50 percent drop in HIV-1 p24 antigen levels in at least 3 of 5 patients over 6 weeks will be the MED of AZT and will be known as the AZT MED. After the MED is determined, Part 1B begins. Part 1B: 20 patients are randomized to 2 different dosing groups: Group 5: AZT MED (weeks 1 to 8), then AZT MED plus rCD4 (weeks 9 to 16); Group 6: AZT MED plus rCD4 (weeks 1 to 8), then AZT MED (weeks 9 to 16). Part 2: Part 2 begins once accrual to Part 1B is completed. If fewer than 10 of 20 patients exhibit a drop of at least 50 percent in p24 antigen level after receiving AZT and rsCD4 combination treatment, entry to Part 2 will be interrupted and the study design will be reevaluated. In Part 2, 10 patients are randomly assigned to one of four groups. Patients within each group receive two 8 week treatment courses. These two treatment courses are 8 weeks with the AZT MED alone, and 8 weeks of treatment with the combination of the AZT MED plus rCD4.