(FORTUNE Magazine) – An unexpected front has opened in the national war on AIDS: a battle for market share among the drugs used to combat the virus. Until recently the best-known "nucleoside reverse transcriptase inhibitor" was Glaxo Wellcome's AZT, which had established itself over the past decade as the dominant drug in the $3 billion market. But in late 1994, Bristol-Myers Squibb introduced Zerit, a drug patients seem to tolerate better than AZT and to which they build resistance more slowly. In December, Zerit shot past AZT in monthly prescription counts--and finally buried the idea that AIDS drug development is a selfless, cooperative effort.

"It's market war, I guess," says Amy Keller, Glaxo's international product development leader for AZT. Bracing for the fight, Glaxo introduced Combivir in October--a new pill that contains both AZT and 3Tc, another Glaxo drug used in the triple-combination therapy (a.k.a. "the cocktail") now used against the AIDS virus. But despite the fact that Glaxo rolled AZT and Combivir annual sales together, that figure rose only 7% in 1997--to $472 million--while year-over-year market share as of February actually fell from 29.3% to 16.5%. Zerit, on the other hand, saw its 1997 worldwide sales jump 185%, to $389 million, and that number is expected to surpass $600 million in 1998. Zerit's year-over-year market share spiked from 19.6% to 27.1%, according to auditor IMS America.

"Because of its toxicity profile, AZT is destined to fade away," said Rich van den Broek, biotech analyst at Hambrecht & Quist. It certainly looks that way.

Ironically, Bristol may be succeeding despite itself. In mid-1995, when triple-combination therapy was emerging, Bristol was the only company manufacturing FDA-approved drugs in two of the three categories used in triple-combo therapy. They failed to capitalize, losing a chance to get in on the ground floor of a growing market. (It is growing not only because more people are being infected with HIV, but because patients are living longer.)

What Bristol has done right is to market Zerit to African Americans, the fastest-growing group of newly infected HIV patients, by using an African-American model in their ads. "Choosing a black male as their primary image and using the word 'freedom' [in the ads] resonates not only with young black men, but also with their physicians," says A. Cornelius Baker, executive director of the National Association of People with AIDS in Washington. And it certainly resonates with Bristol.

--Liz Smith