Temozolomide arrests glioma growth and normalizes intratumoral extracellular pH

Gliomas maintain an acidic extracellular pH (pH_e), which promotes tumor growth and builds resistance to therapy. Given evidence that acidic pH_e beyond the tumor core indicates infiltration, we hypothesized that imaging the intratumoral pH_e in relation to the peritumoral pH_e can provide a novel readout of therapeutic influence on the tumor microenvironment. We used Biosensor Imaging of Redundant Deviation in Shifts (BIRDS), which utilizes chemical shifts of non-exchangeable protons from macrocyclic chelates (e.g., DOTP^8-) complexed with paramagnetic thulium (Tm³⁺), to generate pH_e maps in rat brains bearing U251 tumors. Following TmDOTP^5- infusion, T₂-weighted MRI provided delineation of the tumor boundary and BIRDS was used to image the pH_e gradient between intratumoral and peritumoral regions (ΔpH_e) in both untreated and temozolomide treated (40 mg/kg) rats bearing U251 tumors. Treated rats had reduced tumor volume (p < 0.01), reduced proliferation (Ki-67 staining; p < 0.03) and apoptosis induction (cleaved Caspase-3 staining; p < 0.001) when compared to untreated rats. The ΔpH_e was significantly higher in untreated compared to treated rats (p < 0.002), suggesting that temozolomide, which induces apoptosis and hinders proliferation, also normalizes intratumoral pH_e. Thus, BIRDS can be used to map the ΔpH_e in gliomas and provide a physiological readout of the therapeutic response on the tumor microenvironment.