Dec 22 (Reuters) - The U.S. Food and Drug Administration (FDA) on Thursday approved Gilead Sciences Inc's (GILD.O) Sunlenca therapy for HIV infections, paving the way for a drug that requires less frequent dosing than existing treatments. Sunlenca injection and tablets are expected to cost $42,250 in the first year of therapy and $39,000 annually after that, the company told Reuters. Treatment of HIV infections typically involves daily administration of a combination of drugs - such as Gilead's Biktarvy and ViiV Healthcare's Triumeq - that suppress replication of the virus. The initial doses of Sunlenca, whose chemical name is lenacapavir, are given as oral tablets and injections to get the drug up to therapeutic levels quickly, followed by injections every six months to sustain the treatment. ViiV Healthcare, majority owned by GSK Plc (GSK.L), makes rival drug Cabenuva, which is the longest-acting HIV drug, given at an interval of up to two months. Gilead was working to ha...