End-to-end in vitro and in vivo evaluation of radiopharmaceuticals in patient-derived models

Background & Rationale Targeted radionuclide therapy (TRT) using alpha- and beta-emitting agents offers a precision approach to cancers treatments, with improving outcomes while minimizing systemic toxicity. The development and clinical translation of novel radiopharmaceuticals depend on the integration of robust in vitro and in vivo assays throughout the drug discovery process, critical for assessing target specificity, pharmacokinetics, biodistribution, drug resistance mechanism and therapeutic index. 3D cell lines and cell line-derived xenograft (CDX) provide fast and cost-effective means to assess drug effects, whereas patient-derived (xenograft) organoid (PD(X)O) and patient-derived xenograft (PDX) models preserve heterogeneity, tumour architecture, and enable more clinically predictive assessment of therapeutic efficacy across diverse patient populations. In this study, the theranostic pairing of [ 68 Ga]Ga-PSMA-617 and [¹⁷⁷Lu]Lu-PSMA-617 were prepared and tested in 2D and 3D in vitro assays using CDX and PDXO, followed by in vivo testing in PDX models to evaluate target expression, therapeutic efficacy, and translational relevance across heterogeneous PSMA expression profiles. Methods PSMA-617 was radiolabelled with 68 Ga and 177 Lu using standard manual procedures. The RCP was confirmed by RP-HPLC and TLC analysis. Viability was measured on prostate cancer models with differential PSMA expression levels upon treatment with [ 177 Lu]Lu-PSMA-617. DNA damage (gammaH2AX) was assessed by immunofluorescence to evaluate radiobiological effects. PDX mouse models were imaged with [ 68 Ga]Ga-PSMA-617 PET to assess target density and heterogeneity prior to therapy and monitor treatment response. Prostate PDX models received a single dose of 50 MBq [ 177 Lu]Lu-PSMA-617. Subcutaneous tumour growth, survival and body weight were monitored over time. CBC (WBC, RBC, platelets, neutrophils, lymphocytes) was analysed prior to treatment and regularly after treatment to assess bone marrow toxicity. Results Radiolabelling of PSMA-617 with 68 Ga and 177 Lu yielded products with an RCP≥95%, as determined by HPLC analysis, with no further purification. The specific activities of [ 68 Ga]Ga-PSMA-617 and [ 177 Lu]Lu-PSMA-617 were 10 and 17.5 MBq/nmol, respectively. Both in vitro and in vivo findings demonstrate distinct responses to [^177Lu]Lu-PSMA-617 treatment, consistent with varying PSMA expression levels, evidenced by diagnostic PET imaging and comprehensive PDX models characterization. Conclusions Our results highlight the importance of integrating radiochemistry expertise with clinically relevant patient-derived models to thoroughly characterize novel radiopharmaceuticals. This holistic approach enhances the predictive accuracy of preclinical studies and ensures that emerging therapeutic strategies are closely aligned with clinical demand.