Many scientists believe that the HIV virus mutates so rapidly within the body that it soon develops strains resistant to AZT. Using two or more drugs, they say, makes this more difficult.
Britain will take a leading role in the drugs trial, called Delta, which it is hoped will discover the benefits - if any - of using AZT in conjunction with ddI, made by Bristol-Myers Squibb of the United States, and ddC, made by Roche of Switzerland. Both ddI and ddC work in a similar way to AZT by blocking a vital enzyme of HIV that it needs for replication.
Like the Anglo-French Concorde trial of AZT, the Delta investigation will be 'double- blind, placebo-controlled and randomised'. Neither doctor nor patient with know whether they are taking a placebo (dummy) pill or the real thing.
About 2,500 people will take part and the Medical Research Council hopes to have recruited the 700 British volunteers by the end of this month. It will be several years before the results emerge.
David Warrell, professor of tropical medicine and infectious diseases at Oxford University and head of an MRC committee on Aids, said two drugs working together may have a 'synergistic' effect, producing a greater benefit than each separately. He said there is much research on new anti-HIV drugs but the prospects are not promising.
Two alternatives to drugs are aimed at boosting the body's own defences against HIV before the virus can destroy the vital cells of the immune system.
Scientists from St Mary's Hospital and the Middlesex Hospital in London, working with the company British Biotechnology, have just begun the second phase of preliminary trials of a 'therapeutic vaccine' based on one of the proteins in the virus.
The aim is to encourage the immune system to recognise this viral protein and attack it, keeping virus replication in check. Researchers are testing the idea on 72 HIV-positive, healthy volunteers at clinics in London, Amsterdam and Antwerp.
Another approach, 'passive immunisation', is to take the natural antibodies produced by healthy HIV-positive people and infuse them into patients with Aids. Preliminary results have been promising, but Professor Warrell said: 'I have only seen preliminary data and although it's an interesting approach, I'm not convinced.'
Despite the 'enormous disappointment' of the AZT drugs trial, Professor Warrell said there is every hope that Aids may one day be treatable. 'I am reasonably confident because of the breadth and depth of the work. I don't believe it is an unconquerable virus.'