European regulators last week gave Cambridge biotech Biogen Idec Inc. approval to sell a new kind of injectable drug to treat multiple sclerosis. The medicine, Plegridy, treats adults with relapsing-remitting MS, the most common form of the disease. An application to sell Plegridy in the United States is pending with the Food and Drug Administration and a decision is expected later this summer. Plegridy is an interferon drug, and can be taken by injection once every two weeks and administered under the skin.