Characterizing Riboflavin Antagonists for Targeted Drug Delivery Applications

Abstract

Riboflavin ligands present an alternative pathway for targeted drug delivery as riboflavin receptors are over-expressed in breast and prostate cancer cells. We have examined riboflavin and several riboflavin mimicking molecules (antagonists) for targeting the riboflavin binding protein, which acts as a model protein for the riboflavin receptor. Isothermal titration calorimetry (ITC) was used for determining the binding constant of riboflavin (RF) and RF antagonists to chicken riboflavin binding protein (RfBP). The equilibrium dissociation constants determined for riboflavin (Kd = 1.4 nM) and lumiflavin (Kd = 64.2 nM) in 0.1 M phosphate buffer (pH 7.4) by the microcalorimetric method are in close agreement with the binding data previously determined by fluorescence quenching spectrometry. Several of the RF antagonists screened showed dissociation constants in the micromolar range while some exhibited no binding. The RF antagonists will be used in future studies to target riboflavin receptors for cellular uptake as a potential route for the selective delivery of drug molecules to cancer cells that over-express riboflavin receptors.