The Orphan Drug Act provides market incentives to develop new therapeutics for diseases affecting relatively small numbers of persons. (Providing a seven year period of market exclusivity.) The act applies specifically to therapies for treating diseases or conditions which affect less than 200,000 or affects more than 200,000 persons but for which there is no reasonable expectation that the costs of developing the drug will be recovered from U.S. sales of the drug.
One problem for the FDA has been determining whether the market exclusivity should apply for new entrants who produce a slightly different drug or who treat a slightly different condition.
Robert Bohrer argues (txt)(pdf) that, in the case of monoclonal antibody drugs, the approach to orphan drug exclusivity should be for the FDA to presume antibodies to the same antigen, of the same immunoglobin class and with the same mechanism of action to be the same drug unless the second antibody is shown to be clinically superior to the first.
David Rohde agrees (pdf) that the Act has had a great impact on the development of drugs for treating rare diseases. According to David, “ten times the number of orphan products were approved in the decade following the Act than in the decade preceding Act.”