The HIV integrase inhibitor Isentress (raltegravir) was approved in the European Union today for use in combination with other antiretrovirals for treatment-experienced patients who have detectable viral loads despite ongoing antiretroviral therapy.

The approval of Isentress, developed and manufactured by Merck & Co., allows for access to the drug in the 27 European Union countries, as well as Norway and Iceland. It was approved earlier this year in the United States, Canada and Mexico.

Isentress is a promising HIV drug for treatment-experienced patients with drug-resistant virus because it works differently than other available antiretrovirals. After HIV’s genetic material is deposited inside a cell, its RNA must be converted (reverse transcribed) into DNA. A viral enzyme called integrase then helps to hide HIV’s DNA inside the cell’s DNA. Once this happens, the cell can begin producing genetic material for new viruses. Integrase inhibitors, such as Isentress, are designed to block the activity of the integrase enzyme and to prevent HIV DNA from meshing with healthy cell DNA.