Assessment of quality control and effect of radiolysis on the in vivo biodistribution of [¹⁷⁷Lu]Lu-PSMA-I&T

Background: One of the challenges in optimizing PSMA-based therapies for the treatment of prostate cancer is the stability of the radiopharmaceutical, defined by the degradation of radiolabeled compounds due to exposure to ionizing radiation (radiolysis). Radiolysis of a radiopharmaceutical is difficult to measure and can influence the biodistrubion of radiopharmaceuticals in vivo, possibly leading to toxicity or suboptimal outcomes. Therefore, an inter-laboratory study was performed to confirm accurate detection of radiolysed [¹⁷⁷Lu]Lu-PSMA-I&T, and the biodistribution of radiolysed radiopharmaceutical was investigated in vivo, and HPLC methods are compared. Results: Multiple HPLC-methods were compared to evaluate the analysis of degraded radiopharmaceutical and an inter-laboratory comparison was conducted to evaluate the consistency of current analytical methods across various institutions and with current methods from literature. Additionally, for the biodistribution experiments, a radiolabeling of [¹⁷⁷Lu]Lu-PSMA-I&T was adjusted to achieve a radiochemical purity of 50%, 70%, and 97% of the final product. A PC3-PIP patient-derived xenograft mouse model was used to investigate in vivo behavior of the formed impurity, followed by SPECT/CT imaging and biodistribution. When evaluating radio-HPLC methods for the impurity detection, the European Pharmacopoeia method (with a phosphate buffered eluent) shows no significant difference in radiochemical purity, an increased peak separation but decreased recovery (< 75%). The inter-laboratory comparison demonstrated the ability to measure the radiolysed radiopharmaceutical across multiple hospitals, with a maximum standard deviation of 6.6%. The in vivo SPECT/CT and biodistribution results show that as ingrowth of impurities caused by radiolysis increasing the tumor-to-kidney ratio is decreasing (97%/50% RCP, p < 0.05). Conclusion: The inter-laboratory study confirms that impurities caused by radiolysis can be measured accurately across different institutions, ensuring consistency and reliability in clinical diagnostics and therapy, contributing to improved patient safety. Phosphate buffered HPLC-methods showed no significant benefits in quality control. Increased impurities caused by radiolysis lead to lower tumor-to-kidney ratios, with a decrease in specific tumor binding.