Vascular toxicity risk assessment of MC18 and MC70, novel potential diagnostic tools for in vivo PET studies

Background: The P-glycoprotein (P-gp) inhibitor MC18 has recently been proposed as a valuable PET tracer to measure P-gp expression in vivo [1]. The aim of this study was to evaluate the toxic hazard towards the vasculature of MC18 along with the structurally related and more potent P-gp inhibitor MC70 [2]. Methods: Their effects on A7r5 and human endothelial EA.hy926 cell viability, on the mechanical activity of fresh and cultured rat aorta rings, as well as on CaV1.2 channel current (ICa1.2) of A7r5 cells were analysed [3]. Results: At concentrations > 10 μM, MC18 and MC70 decreased cell viability causing evident morphological changes. In fresh rat aorta rings, both compounds antagonized phenylephrine-induced contractions in a concentration-dependent manner with IC50 values in the range 2.44–14.5 µM, whereas only MC18 caused a concentrationdependent decrease of the responses induced by 60 mM K+ (K60). In rings cultured for 7 days in the presence of tested compounds, 10 µM MC70 significantly reduced, while 10 µM MC18 completely prevented the contractile response to both phenylephrine and K60. MC18 and MC70 inhibited ICa1.2 in a concentration-dependent manner with IC50 values of 16.81 and 32.13 µM, respectively. The effects of the two compounds on the induction of endothelial-mesenchymal transition in EA.hy926 cells are currently under investigation. Discussion: These findings demonstrate that MC18-induced vascular effects take place at concentrations that are at least three orders of magnitude higher than those allowing in vivo measurement of P-gp expression (≤ 10 nM). Thus, MC18, and possibly MC70, can be considered promising PET tools for in vivo P-gp quantification. Acknowledgements: This work was supported by the Italian Ministry for Instruction, Universities and Research (Futuro in Ricerca 2012, RBFR12SOQ1 to S. S.)