United Therapeutics, of Silver Spring, has received Food and Drug Administration approval for its cancer drug Unituxin, the company announced Tuesday. Unituxin treats the rare pediatric cancer neuroblastoma. Roger Jeffs, Ph.D., United Therapeutics’ president and co-chief executive officer, said in a statement: “We are grateful for the FDA’s thorough review and collaboration on this program, and we look forward to expanding our research efforts in the area of pediatric oncology.” Unituxin works with the body’s immune system to attack cancer cells. Neuroblastoma generally affects children 5 and under.