In Vivo pH Imaging with 99mTc-pHLIP

Purpose: A novel molecular imaging agent has been developed recently, which stains tissues of low extracellular pH [pH (low) insertion peptide, pHLIP (R)]. A pH-dependent process of peptide folding and insertion into cell membranes has been found in vitro. Targeting of acidic solid tumours has been demonstrated in vivo using fluorescence and PET labels. Here, we present proof of feasibility studies of pHLIP with a single-photon emission computed tomography (SPECT) label, Tc-99m-AH114567, with focus on preclinical efficacy and imageability. Procedures: Lewis lung carcinoma, lymph node carcinoma of the prostate and prostate adenocarcinoma tumour xenografts were grown in mice and characterised by the angiogenesis marker Tc-99m-NC100692 and by extracellular pH measurements with P-31-MRS of 3-aminopropyl phosphonate. Biodistribution was assessed and CT/SPECT imaging performed. Oral administration of bicarbonate served as control. Results and Conclusion: Tc-AH114567 can be obtained via a robust synthesis with good radiolabelling profile and improved formulation. The tracer retains the pH-dependent ability to insert into membranes and to target tumours with similar pharmacokinetics and efficacy that had been demonstrated earlier for pHLIP with optical or Cu-64 PET labels. Despite the inherent challenges of SPECT compared to optical and PET imaging, e.g., in terms of lower sensitivity, Tc-99m-AH114567 shows adequate image quality and contrast. The main development need for transitioning SPECT labelled pHLIP into the clinic is more rapid background signal reduction, which will be the focus of a subsequent optimisation study.