Evaluating the Utility and Constraints of the Competitive Counter Flow (CCF) Assay in OATP1B1 Substrate Profiling

Area of Science:

• Pharmacology
• Drug Metabolism
• Biochemistry

Background:

• Organic anion transporting polypeptide 1B1 (OATP1B1) is crucial for hepatic uptake of anionic molecules.
• Conventional OATP1B1 assays can produce false negatives for permeable compounds due to passive diffusion.
• Extended Clearance Classification System (ECCS) categorizes compounds based on clearance mechanisms.

Purpose of the Study:

• To evaluate the competitive counterflow (CCF) assay for assessing OATP1B1 substrates.
• To identify novel OATP1B1 substrates using the CCF assay.
• To compare CCF assay performance with traditional OATP1B1 uptake assays.

Main Methods:

• Applied the competitive counterflow (CCF) assay to 58 compounds across ECCS classes.
• Measured intracellular [3H]-estradiol-17β-glucuronide retention after test compound exposure.
• Utilized a three-tier classification system based on tracer retention levels.

Main Results:

• The CCF assay demonstrated high sensitivity in detecting OATP1B1 substrates.
• Identified four new OATP1B1 substrates: bumetanide, candesartan, naringin, and sulfasalazine.
• Several known OATP1B1 substrates showed significant tracer efflux in the CCF assay.

Conclusions:

• The CCF assay is a valuable tool for early-stage screening and triage of OATP1B1 substrates.
• An integrative strategy combining CCF data with other evidence is essential to avoid false conclusions.
• The CCF assay offers improved sensitivity, especially for challenging ECCS Class 1B compounds.