Epistaxis associated with elevation of INR in a patient switched to generic warfarin--another view

Narrow Therapeutic Index drugs: clinical pharmacology perspective

OBJECTIVES

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.

KEY FINDINGS

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.

SUMMARY

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.

Substituting Warfarin Products: What's the Source of the Problem?

Many clinicians have expressed concerns about the bioequivalence of warfarin products, and data suggest that substituting warfarin products may increase the risk of major bleeding and thromboembolic complications. Anecdotal reports and some retrospective studies have reported differences in anticoagulation control after a warfarin product substitution. But the best available evidence—prospective, randomized, blinded clinical trials—has failed to validate these observations. Indeed, interpatient and intrapatient variability in anticoagulation control observed before and after warfarin product substitution is very similar. So, while differences in product standardization, bioavailability, and bioequivalence make a convenient explanation, the problem lies elsewhere. Perhaps poor communication, fractionated systems of care, and errors are the culprits.

Subtherapeutic INR values associated with a switch to generic warfarin

OBJECTIVE

To discuss the effect on the international normalized ratio (INR) when patients are converted from therapy with Coumadin to generic warfarin.

CASE SUMMARY

Two cases at a family medicine center anticoagulation clinic in Oklahoma City, OK, in which subtherapeutic INR values occurred after a switch from Coumadin to generic warfarin, while all other variables were kept consistent demonstrate the need for close monitoring of the INR when patients are switched between brands of warfarin.

DISCUSSION

Warfarin is a narrow therapeutic index drug since small changes in systemic concentration can lead to significant variation in phamacodynamic response. In March 1997, the Food and Drug Administration (FDA) approved generic warfare based on the FDA and United States Pharmacopoeia (USP) standards. Due to different specifications between Dupont Pharma and the USP, content uniformity may differ between brand and generic forms of warfarin. United studies have demonstrated bioequivalence between brand and generic. The two cases presented demonstrate the potential for fluctuations in the INR when switching between generic and brand. Thus, these eases illustrate the need for close monitoring if a change is made.

CONCLUSIONS

While considered bioequivalent by the FDA, switching between Coumadin and generic warfarin may lead to significant changes in the INR, warranting dose alterations due to different content uniformity specifications between Dupont Pharma and the USP. Patients switching between Coumadin and generic warfarin should have their INR monitored more carefully.