DTPA-Receptor – A novel reporter gene system for the specific and sensitive PET imaging of CAR-T cells and AAV transduced cells

Advanced Therapy Medicinal Products (ATMPs), such as cell and gene therapies, necessitate a reliable diagnostic method for quantitative monitoring. We developed a novel reporter gene system for PET imaging consisting of a membrane-anchored Anticalin protein (DTPA-R) that acts as a high-affinity receptor for the radioligand [¹⁸F]F-DTPA•Tb^III. The reporter protein shows high cell surface expression of up to ~1×10⁶ receptors per cell. After systemic administration, the pharmacologically inert radioligand rapidly clears via the renal route and, at t=90 min, generates a strong signal of 22.1 %ID/g for DTPA-R-expressing PC3 cells compared to 0.2 %ID/g for DTPA-R-negative controls (ratio: 125). The detection limit for Jurkat^DTPA-R cells was 500 cells in a PET phantom ex vivo and 8,000 if subcutaneously injected. In vivo expansion and migration of CD19-CAR-T^DTPA-R cells was successfully monitored over four weeks with a linear relationship between PET signal and CAR-T cell number. Furthermore, our reporter system allowed quantitative and longitudinal imaging of AAV9 viral vectors with a linear dose-to-signal relation. In summary, DTPA-R shows high potential for in vivo monitoring of ATMP-based therapies.