Abstract Background Acidic pH values of the tumor microenvironment (TME) have crucial effects on metastatic behavior, host defense, immune regulation and cellular metabolism. Several studies have shown that the acidity of the interstitial space in the TME influences the functions of cancer and stromal cells, particularly regarding immune effects. Changing intratumoral pH might therefore be a potential target for therapy, and pH imaging might guide further developments. We describe radiopharmaceutical probes for positron emission tomography (PET) that exploit the concept of pH-dependent intratumoral hydrolysis of glycosylamine bonds of PET-tracers ( 18 FFDG-4-methoxybenzylamine ( 18 FFDG-4MBA) and 18 FFDG-benzylamine ( 18 FFDG-BA)) to release 18 FFDG as functional moiety with the aim to non-invasively image pH changes. Results Nuclear magnetic resonance (NMR) spectroscopy demonstrated hydrolysis at pseudo first order and showed pH dependent hydrolysis time, at which 50% of 19 FFDG-4-methoxybenzylamine ( 19 FFDG-4MBA) was cleaved, ranging from 3 h at pH 7.4 over 60 min at pH 6.9 to 45 min at pH 6.5. Hydrolysis of 18 FFDG-4MBA in human serum at pH 7,6 was similar to comparable pH without serum (below 10% FDG release after 2 h). The glycosylamines 18 FFDG-BA and 18 FFDG-4MBA were relatively stable in vitro at pH 8.3 with less than 15% FDG release from 18 FFDG-4MBA and less than 10% FDG release from 18 FFDG-BA within 60 min. Hydrolysis at acidic pH led to 18 FFDG release at pH 6.0 of 71% from 18 FFDG-4MBA and 40% from 18 FFDG-BA at 60 min. In vitro uptake into B16F10 or MC38 cells was pH dependent in contrast to FDG uptake. In a preclinical model bearing the two different acidic subcutaneous tumors (B16F10 and MC38), pH differences in the acidic TME were better discriminated with 18 FFDG-4MBA than with 18 FFDG-BA. In vivo neutralization of the acidic extracellular tumor pH prevented pH-dependent cleavage of 18 FFDG-4MBA resulting in decrease of PET signal. Conclusion The determination of pH differences by glycosylamines radiotracers and PET imaging in acidic TME may serve as a novel marker for various questions such as interaction of pH regulation and response to immunotherapies. Notably, even small pH differences in the acidic TME of different tumors, in the same in vivo model, could be discriminated. Graphical abstract
Brück et al. (Thu,) studied this question.