# Challenges in the Investigation of Therapeutic Equivalence of Locally Applied/Locally Acting Drugs in the Gastrointestinal Tract: The Rifaximin Case

## TL;DR

This paper discusses the difficulties in proving that generic versions of gut-targeted drugs like rifaximin are as effective as the original, due to limitations in current testing methods.

## Contribution

The paper highlights the need for new, more sensitive methods and harmonized regulations to assess bioequivalence for locally acting gastrointestinal drugs.

## Key findings

- Standard bioequivalence methods are inadequate for drugs like rifaximin due to minimal absorption and low solubility.
- Clinical trials for such drugs are costly and may not detect formulation differences effectively.
- Regulatory differences between FDA and EMA complicate global generic drug development.

## Abstract

Background: Locally acting gastrointestinal (GI) drugs present challenges for generic drug development because traditional bioequivalence measures, which rely on systemic drug levels, do not reflect local efficacy. This review examines regulatory guidelines for establishing therapeutic equivalence for such drugs, using rifaximin—a minimally absorbed, gut-localized antibiotic—as a case study. Methods: We reviewed bioequivalence guidelines from the United States Food and Drug Administration (FDA) and European Medicines Agency (EMA), along with the literature on rifaximin’s biopharmaceutical and clinical properties, to identify strategies and challenges for establishing equivalence for locally acting GI drugs. Results: Rifaximin exemplifies the limitations of standard bioequivalence methods: as a Biopharmaceutics Classification System (BCS) class IV drug with minimal absorption and low solubility, in vitro dissolution may not predict local drug availability. Clinical endpoint trials (e.g., traveler’s diarrhea, hepatic encephalopathy, IBS-D) are resource-intensive and insensitive to formulation differences. Pharmacokinetic (PK) studies in healthy volunteers show low, variable plasma levels, which may inaccurately discriminate between formulations. The EMA requires evidence of non-saturable absorption to accept PK data, a difficult-to-establish but potentially irrelevant criterion. Differences between FDA and EMA approaches highlight a lack of harmonization, complicating global generic development. Conclusions: A tailored, multifaceted approach is needed to demonstrate bioequivalence for GI-localized drugs like rifaximin. This case underscores the need for more sensitive surrogate methods (e.g. advanced in vitro or pharmacodynamic models) and flexible regulatory criteria. Harmonization across international guidelines and innovative bioequivalence study designs are key to facilitating the approval of safe and effective generic alternatives in this drug class.