Preclinical evaluation of ⁶⁸Ga-labelled Pt(IV)-DFO complexes as PET-imaging agents for osteosarcoma

Pt (IV) complexes have emerged as promising redox-activated prodrugs with improved pharmacological properties compared to Pt(II) and the potential for functionalising additional binding sites. In this study, four Pt(IV)-deferoxamine (DFO) complexes (A-D) were evaluated as gallium-68 radiolabelled imaging agents in healthy mice and in a murine osteosarcoma model using positron emission tomography (PET). All complexes presented quantitative ⁶⁸Ga-labelling yields with high radiochemical purity (>95%) and excellent stability in aqueous buffer and human serum after 1h incubation. In vivo evaluation in naïve mice demonstrated that pharmacokinetics of the complexes were largely governed by their hydrophilicity and lipophilicity, displaying predominant renal or hepatobiliary clearance, respectively. In the orthotopic osteosarcoma model, the complexes showed tumour-to-healthy tissue ratios ranging from 1.1 ± 0.7 to 4.5 ± 3.2 at 55 min post-injection. Ex vivo biodistribution confirmed relatively low tumour uptake, ranging from 0.3 ± 0.1 to 1.1 ± 0.9%IA/g at 60 min post-injection. Radio-metabolite studies revealed in vivo formation of chelator [⁶⁸Ga]Ga-1 (DFO-succinic acid), consistent with ester bond hydrolysis, with variable systemic stability across all complexes. Cellular radioFlow analysis of [⁶⁸Ga]Ga-D accumulation in tumours confirmed predominant retention in tumour and stromal cells. Among the investigated complexes, [⁶⁸Ga]Ga-D provided the highest tumour-to-background contrast with the most desirable pharmacokinetic profile and in vivo stability, making it the most promising candidate for future studies. Overall, these results demonstrate that Pt(IV)-DFO complexes retain their in vivo tumour-targeting capabilities, supporting their potential use as PET-based imaging agents for osteosarcoma.