Merck & Co. Inc. announced on Wednesday that it isdiscontinuing development of the investigational AIDS drug L-697,661 in combination with AZT because of HIV resistance.The company first noticed the resistance problem inmonotherapy trials in 1991 and had hoped to overcome theproblem by combining the drug with AZT.

"Unfortunately, Merck's own researchers and our outsideclinical investigators have concluded that the viral resistanceproblem is so clinically significant, particularly in the absenceof any sustained antiviral effect, that development should bediscontinued," the company said. A company spokespersonnoted that HIV was mutating within weeks of L-661 treatment,whereas with AZT treatment, the virus changed after a periodof months.

Merck, based in Whitehouse Station, N.J., conductedpreliminary clinical trials of four compounds from a class ofnon-nucleoside reverse transcriptase inhibitors and chose topursue development of L-661.

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