Is there a direct relationship between narrow therapeutic index and substitutability?
FDA recognizes the scientific concept that drugs differ in their therapeutic range. However, because of FDA’s strict bioequivalence criteria, we believe that drugs do not fall into discrete groups that would allow one to consider NTI drugs as being clearly different from other drugs for purposes of therapeutic substitution. No data has been submitted to FDA to cause any revision in the bioequivalence criteria for these products. Therefore, there has been no scientific or regulatory purpose at this time for the agency to create and implement a mechanism to designate some products as being narrow therapeutic index products, or to define any other specific group of products. The FDA is now considering a different approach to documenting bioequivalence. This approach is termed ‘individual bioequivalence.’ This approach allows the possibility of scaling the bioequivalence ‘goalposts’ (e.g., the boundary of 80 – 125%) based on variability of the reference listed (innovator) drug. One possibl
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