About The Study

This study described the rate of in vitro (in the test tube) resistance of HIV to AZT and how resistance could point to subsequent disease progression.

Study Approach

This phase III study measured how well a drug works in large groups of people. It studied different doses of the drug. Participants were evaluated clinically and in the lab at regular intervals. Viral cultures and sensitivity testing were performed every 12 weeks in a subset of 50 volunteers.

Study population

Seventy-four volunteers enrolled in three Canadian, university hospitals. All were HIV-positive with CD4 counts over 270.


In vitro resistance to AZT developed in 64% of the volunteers after 180 weeks of therapy. Lower CD4:CD8 ratio (the balance between the two types of white cells) at the beginning of the study was associated with faster development of resistance.


The development of in vitro AZT-resistance was found to be a marker of subsequent disease progression. In other words, the more resistance demonstrated in the lab, the faster HIV disease was likely to progress. Whether in vitro resistance to AZT is merely an indicator of disease progression, rather than the cause, remains to be established.