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 (Tm(3+)), to generate pH(e) maps in rat brains bearing U251 tumors. Following TmDOTP(5−) infusion, T(2)-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.