BIODISTRIBUTION AND PRETHERAPEUTIC DOSIMETRY WITH [44Sc]-PSMA-617 - FIRST RESULTS

Objectives: Endoradiotherapy with 177Lu-labeld PSMA-analoga is of emerging interest. Pretherapeutic testing of PSMA-binding is often performed with 68Ga-labeled PSMA. However, for reliable dosimetric values late measurements (e.g. after 24 h) are necessary. Therefore isotopes with longer half-life than 68Ga are needed for this aim. Here we show the feasibility of pretherapeutic dosimetry with [44Sc]-PSMA-617 (half life 3.9 h) in patients with castration resistant prostate carcinoma (CRPC). Methods: Four patients with CRPC who were scheduled for [177Lu]-PSMA-617 therapy were injected i.v with 40-62MBq (mean±SD; 50.5±9.3 MBq) of [44Sc] Sc-44-PSMA-617. Dynamic PET data of the abdomen were acquired in list mode for 30 minutes on a Siemens Biograph 2 PET/CT scanner. Static PET/CT data (skull to mid-thigh) were acquired 45 min, 120 min and about 18-20 h post injection. Organ volumes were drawn on CT images and counts/ccm were estimated in 6 frames each 300 s in dynamic as well as in static PET images. Time dependent changes of activity in organs showing high accumulation were determined. Urine samples were collected at about 2 h p.i.. Blood based approach was used for calculation of bone marrow. Mean organ absorbed doses and effective doses were calculated using OLINDA/EXM. Results: A mean whole body dose of 0.090 mSv/MBq (range: 0.019-0.284 mSv/MBq) injected activity was found. Highest radiation absorbed dose of 0.354 mSv/MBq (range 0.180-0.488 mSv/MBq) was seen in the kidneys followed by urinary bladder wall (0.263 mSv/MBq; range 0.121-0.605 mSv/MBq), Spleen (0.177 mSv/MBq; range 0.064-0.248 mSv/MBq) and liver (0.110 mSv/MBq; range 0.030-0.229 mSv/MBq). Red marrow dose was found to be 0.033 mSv/MBq with a range between 0.025 and 0.037 mSv/MBq. No adverse pharamacological effects were seen. Conclusion: [44Sc]-PSMA-617 was shown to be feasible for dosimetric calculations for absorbed organ doses in four patients with CRPC. No adverse side effects were seen and absorbed doses of the PET/CT examination were found to be equal or less than for [68Ga]PSMA (1). The clinical benefit of the longer half-life time must be analyzed in further studies.