Unusually High Affinity of the PLK Inhibitors RO3280 and GSK461364 to HSA and Its Possible Pharmacokinetic Implications

The binding processes of two Polo-like kinase inhibitors, RO3280 and GSK461364, to the human serum albumin (HSA) protein as well as the protonation equilibria of both compounds have been studied combining absorbance and fluorescence spectroscopy experiments together with density functional theory calculations. We found that the charge states of RO3280 and GSK461364 are +2 and +1, respectively, at the physiological pH. Nevertheless, RO3280 binds to HSA in the charge state +1 prior to a deprotonation pre-equilibrium. Binding constants to site I of HSA of 2.23 x 10(6) and 8.80 x 10(4) M-1 were determined for RO3280 and GSK461364, respectively, at 310 K. The binding processes of RO3280 and GSK461364 to HSA are entropy- and enthalpy-driven, respectively. The positive enthalpy found for the RO3280-HSA complex formation could be related to a proton pre-equilibrium of RO3280.

Automated summary

This study investigated the binding processes of two Polo-like kinase inhibitors, RO3280 and GSK461364, to the human serum albumin (HSA) protein and the protonation equilibria of both compounds. The charge states of RO3280 and GSK461364 were found to be +2 and +1, respectively, at physiological pH. The binding constants to site I of HSA were determined to be 2.23 x 10(6) M-1 and 8.80 x 10(4) M-1 for RO3280 and GSK461364, respectively. The binding processes of RO3280 and GSK461364 to HSA were driven by entropy and enthalpy, respectively. The positive enthalpy observed for the formation of the RO3280-HSA complex could be attributed to the proton pre-equilibrium of RO3280.