The Food and Drug Administration on Tuesday approved a supplemental new drug application by Roche for the combined use of its protease inhibitor Invirase with a booster dose of Abbott Laboratories' protease inhibitor Norvir. Both Norvir and Invirase have previously been approved separately to treat HIV disease. However, Invirase, the first protease inhibitor ever approved by the FDA, is rarely used in HIV therapy because in its unboosted form it fails to reach adequate levels in the bloodstream. Roche began marketing an improved version of Invirase, called Fortovase, in 1997. New studies by the company showed that a 1,000-milligram Invirase/100-milligram Norvir combination is as effective in reducing HIV viral loads as treatment with Fortovase. The medications will be dosed in five capsules taken twice per day within two hours of eating. Company officials say Invirase should not be taken without a booster dose of Norvir because the drug when used alone is ineffective in slowing or preventing the spread of HIV in the body.