By Natalie Grover

Aug 6 (Reuters) - The U.S. Food and Drug Administration approved The Medicines Co's single-dose intravenous drug to treat acute bacterial skin infections, the agency's third approval for the same condition this year.

Drugmakers need to constantly devise new therapies to fight bacterial infections as patients build up resistance to older antibiotics.

All the three drugs target acute bacterial skin and skin structure infections (ABSSSI), caused by the Gram positive strain of bacteria. These infections involve deep tissue or are associated with an underlying disease such as diabetes.

Medicines Co's treatment, Orbactiv, is in the same class of drugs as generic vancomycin, the standard-of-care for methicillin-resistant Staphylococcus aureus (MRSA), a serious Gram-positive infection.

ABSSSI, which leads to hospitalization of about 5.2 million patients in the United States and Western Europe each year, often involves intravenous therapies that require hospitalization over multiple days, the company said.

A single 1200 mg dose of Orbactiv comprises a full course of therapy - unlike vancomycin which requires twice-daily intravenous infusions for seven to 10 days - giving it an edge over competition.

The regulator in recent months approved Cubist Pharmaceuticals Inc's Sivextro and Durata Therapeutics Inc's Dalvance for the same condition.

Sivextro, available for intravenous and oral use, is administered once daily for six days. Dalvance, an intravenous drug, is given in two doses, eight days apart.

Jefferies & Co analyst Biren Amin told Reuters he expects Orbactiv to be launched in the fourth quarter and estimated peak U.S. sales of $150 million by 2021.

Orbactiv, known generically as oritavancin, was found to be non-inferior to vancomycin in 1,959 patients in late-stage trials. Patients on the drug most frequently reported adverse events that included nausea, headache, vomiting and diarrhea.

The antibiotic received an expedited review by the FDA and was designated a qualified infectious disease product, qualifying it for an extra five years of marketing exclusivity.

(Additional reporting by Anjali Rao Koppala in Bangalore)