In vitro biodistribution studies on clinically approved FGFR inhibitors ponatinib, nintedanib, erlotinib and the investigational inhibitor KP2692.

Binding towards human serum albumin (HSA) and α1-acid glycoprotein (AGP) of three approved fibroblast growth factor receptor (FGFR) inhibitors ponatinib (PON), nintedanib (NIN) and erdafitinib (ERD), as well as the experimental drug KP2692 was studied by means of spectrofluorometric and UV-visible spectrophotometric methods. Additionally, proton dissociation processes, lipophilicity, and fluorescence properties of these four molecules were investigated in detail. The FGFR inhibitors were predominantly presented in their single protonated form (HL) at pH 7.4 (at blood pH). At gastric pH (pH 1-2) the protonated forms (+1 - +3) are present, which provide relatively good aqueous solubility of the drugs. All of the four inhibitors are highly or extremely lipophilic at pH 7.4 (logD ≥ 2.7). At acidic pH 2.0 PON and ERD are rather lipophilic, NIN is amphiphilic, while KP2692 is highly hydrophilic. All four compounds bind to HSA and AGP. Moderate binding of PON, KP2692 and NIN was found towards albumin (logK' = 4.5-4.7), while their affinity for AGP was about one order of magnitude higher (logK' = 5.2-5.7). ERD shows a larger affinity for both proteins (logK' ≈ 5.2, logK' ≈ 7.0). The computed constants were used to model the distribution of the FGFR inhibitors in blood plasma under physiological and pathological (acute phase) conditions. The changing levels of the two proteins under pathological conditions compensate each other for PON and NIN, so that the free drug fractions do not change considerably. In the case of ERD the higher AGP levels distinctly reduce the free available fraction of the drug. Comparison with clinical pharmacokinetic data indicates that the here presented solution distribution studies can very well predict the conditions in cancer patients.