WASHINGTON (AP) — The FDA has approved expanded use of a Merck & Co. HIV drug, Isentress. Known chemically as raltegravir, it is now approved for treating new and existing patients with the virus that causes AIDS, in combination with other types of HIV drugs. The drug got an accelerated approval from FDA in October 2007, limiting it to use in patients who had drug-resistant strains or were failing on other therapies, also in combination with other HIV drugs. Now it can be used in all adult patients. The recommended dose for new adult patients is 400 milligrams twice daily, with or without food, FDA said. The ruling was made Wednesday but announced late Thursday. The approval was based on a 48-week study comparing patients who took Isentress with patients who took another HIV drug, Sustiva. Both groups also took two other HIV drugs. Isentress is an integrase inhibitor, meaning it works by blocking the enzyme integrase, one of three types of enzymes the AIDS virus uses to reproduce and infect cells.