➦ To determine the inhibitory potential of a test item the intracellular accumulation of the labeled probe substrate is performed in the absence or presence of increasing concentrations of the test item.
Experience has shown that a two-step procedure is recommendable:
Two point Inhibition: Inhibition of transporter-mediated uptake of a probe substrate by two different concentrations of a test item. Example: Test item identified as an inhibitor showed a significant inhibitory effect.
Determination of IC50 value: Concentration dependent inhibition of a probe substrate by a test item and determination of the half-maximal inhibition value (IC50).
Alternative Second Step
Determination of the inhibition character (Ki value): Dixon Plot analysis to determine competitive or non-competitive inhibition.