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By Toni Clarke WASHINGTON, April 29 (Reuters) - Gilead Sciences Inc said on Monday that U.S. health officials rejected two of its HIV drugs as standalone therapies, citing deficiencies in documentation and validation of certain quality testing procedures. Gilead said it is working with the U.S. Food and Drug Administration to address the questions raised in the rejection letter in order to prove the application forward. The company is seeking approval for its drug elvitegravir for people with HIV, the virus that causes AIDS, who have already been treated with other products. The drug blocks the enzyme integrase which is needed for the HIV virus to replicate. Gilead is also seeking approval of cobicistat, a drug that does not itself fight the virus but boosts the function of other HIV medicines. Both drugs are already contained in Gilead's once-daily single-tablet HIV treatment Stribild, which combines four different medications and was approved in the United States last August. "The FDA, in its review, did not question the safety profile of either drug, which leads us to believe this is more of a delay than an overall drug profile problem," said Matthew Roden, an analyst at UBS Securities LLC, in a research note, adding that the findings "do not impact the marketing and commercialization of Stribild." Analysts on average expect elvitegravir, if eventually approved, to generate annual sales of about $300 million by 2016 according to Thomson Reuters data. They expect sales of cobicistat of roughly $242 million over the same period. Overall, Gilead's portfolio of antiviral drugs generated sales in 2012 of $8.14 billion. In themselves, elvitegravir and cobicistat comprise only a small portion of future Gilead revenue and "as such, we view a delay as immaterial," Brian Abrahams, an analyst at Wells Fargo Securities, said in a research note. "There are multiple facilities which produce Stribild and Gilead's other products, suggesting sufficient supply even if there are any overlapping issues that escalate," he said. HIV treatment typically involves the use of multiple drugs designed to attack the virus from different angles. Gilead licensed elvitegravir from Japan Tobacco Inc in 2005 and has the right to market it anywhere except Japan. Elvitegravir works in a similar manner to Merck & Co's drug Isentress, also known as raltegravir. Isentress is typically given twice a day. Elvitegravir only needs to be taken once a day, but it's action needs to be boosted by cobicistat or an alternative drug that delays the metabolism of elvitegravir. Gilead has given a number of Indian generic drugmakers, including Ranbaxy Laboratories Ltd rights to make generic versions of elvitegravir and other HIV drugs to be sold in developing countries. Gilead's shares were more or less unchanged at $51.27 in early trading on Nasdaq.