Narrow Therapeutic Index drugs: clinical pharmacology perspective

Historically, the identification of drugs with the Narrow Therapeutic Index (NTI) has been empirically based on the clinical practice. In general terms, NTI drugs can be defined based on the steepness of the dose-response relationship and the degree of overlap between the effective and the toxic concentrations.

The current definition in the Code of Federal Regulations is based on animal data and as such lacks direct clinical relevance. By basing these criteria on those factors that affect the degree of separation of the concentrations that elicit the therapeutic and the adverse effects, it was attempted to avoid the vagaries inherent in preparing a list of individual drugs themselves.

The overall conclusions are: (1) The issue of NTI drugs is still highly controversial, (2) The lists of NTI drugs are arbitrary and (3) Variability may contribute to the safety and efficacy. The objectives are to (1) To facilitate consensus on the definition of a NTI, (2) To identify characteristics of NTI and (3) To develop criteria for identifying NTI. The FDA should consider not only the pharmacokinetics (PK) studies, but also comparative efficacy and safety studies, for the NTI drugs to further to try 'convince or educate' the prescribers.