Does the Orphan Drug Act unambiguously foreclose FDA's interpretation that the scope of orphan-drug exclusivity is tied to a drug's approved use?

QUESTION PRESENTED Congress intended the Orphan Drug Act (ODA) to incentivize the development of drugs for the treatment of rare diseases. To be eligible for the incentives, a sponsor must obtain an orphan-drug designation from the Food and Drug Administration (FDA) for a drug that “is being or will be investigated for a rare disease or condition.” 21 USC § 360bb(a)(1). The primary incentive to do so is a period of exclusivity, which prevents FDA from approving another sponsor’s application for the “same drug for the same rare disease or condition” for seven years. 21 USC § 360cc(a). For the past 30 years, FDA regulations have interpreted ODA exclusivity to prohibit the agency only from approving a second “same drug” for the same “use” as the prior-approved drug. Here, the Eleventh Circuit created a split with the Fourth and D.C. Circuits, concluding that the ODA unambiguously foreclosed FDA’s regulation. FDA designated Respondent’s drug, Firdapse® (amifampridine), as an orphan drug to treat LambertEaton Myasthenic Syndrome (LEMS) = and subsequently approved it for adults with LEMS (the only population Respondent sought approval to treat). FDA approved’ Petitioners drug, Ruzurgi® (amifampridine), for a pediatric population, which Firdapse® was not approved to treat. The Eleventh Circuit held that the ODA foreclosed this result. The question presented is: Does the ODA unambiguously foreclose FDA’s decades-long, consistent interpretation that the scope of orphan-drug exclusivity is tied to a drug’s approved use?