In Vivo MRI-CEST Tumor pH Imaging Detects Resistance to Proton Pump Inhibitors in Human Prostate Cancer Murine Models

Simple Summary Tumor acidosis plays a major role in tumor aggressiveness, invasion and resistance, and it is considered an important target for novel anticancer strategies. In this work, we investigated the therapeutic efficacy of several proton pump inhibitors (Esomeprazole, Lansoprazole, Amiloride and Cariporide) to alter tumor acidity in prostate murine cancer models. The in vitro results showed a moderate toxicity for Esomeprazole that was selected for the successive in vivo studies. However, the in vivo studies highlighted the lack of response to Esomeprazole treatment in both subcutaneous and orthotopic PC3 prostate cancer models. Overall, MRI-based tumor pH imaging is a powerful tool for monitoring the in vivo response to treatment. The tumor microenvironment acidification confers treatment resistance; therefore, the interference with pH regulating systems is considered a new therapeutic strategy. In this study, two human prostate cancer cell lines, PC3 and LNCaP, have been treated in vitro with proton pump inhibitors (PPIs), namely Lansoprazole, Esomeprazole (V-ATPases-inhibitors), Cariporide, and Amiloride (NHE1-inhibitors). The cell viability and pH were assessed at several drug concentrations either at normoxic or hypoxic conditions. Since Esomeprazole showed the highest toxicity towards the PC3 cancer cells compared to LNCaP ones, athymic nude mice bearing subcutaneous or orthotopic PC3 tumors were treated with Esomeprazole (dose: 2.5 mg/kg body weight) for a period of three weeks-and tumor growth was monitored. MRI-CEST tumor pH imaging with Iopamidol was performed upon treatment at 3 h, 1 week (in combination with FDG-PET), and after 2 weeks for evaluating acute, early, and late responses. Although acute tumor pH changes were observed in vivo, long-term studies on both PC3 prostate cancer models did not provide any significant change in tumor acidosis or tumor growth. In conclusion, this work shows that MRI-CEST tumor pH imaging is a valuable tool for assessing the in vivo treatment response to PPIs.