Abstract
1255
Objectives: The salivary glands are one of the dose limiting organs for using prostate specific-membrane antigen (PSMA) targeted radionuclide therapies. Most patients treated with PSMA-targeted radiotherapeutic agents report xerostomia. Salivary hypofunction is likely due to the radiation dose received by the salivary epithelia and resultant damage. This radiation-induced salivary hypofunction is likely irreversible thereby compromising the patient’s quality of life post-therapy. Furthermore, this limits the dose of PSMA targeted radiotherapeutic agents that can be prescribed to the patient. Thus, there is an urgent need to identify methods that can minimize damage to the salivary gland during PSMA-targeting radionuclide therapy. Thus, we aimed to identify an appropriate animal model with PSMA expression comparable to human salivary glands. Briefly, in our experiments, we investigated the expression level of PSMA on the major salivary glands (submandibular, SM; Sublingual, SL; and parotid, PR) of rats and mice by using a well-characterized 18F labeled small molecule inhibitor of PSM and compared them to PSMA levels in minor human salivary glands. Materials: 18F-DcFPYL (18F-DCF) was synthesized by labeling a PSMA targeted small molecule inhibitor DcFPYL (2-(3-{1-Carboxy-5-[(6-fluoro-pyridine-3-carbonyl)-amino]-pentyl}-ureido)-pentanedioic acid; DCF) with 18F. Biodistributions were done in rats (F344/SAS Fischer, female) and mice (NCR-nu/nu athymic, female), respectively. Approximately 100 µCi and 50 µCi of 18F-DCF was administered IV either in the presence or absence of excess DCF (3.5 nmoles, mice; 40 nmoles, rats; 500x). 1 h post-injection the animals were euthanized and imaging and biodistribution studies were performed to determine the uptake (%ID/g) of 18F-DCF in organs. In vitro autoradiography of rat and mice SM were performed using 18F-DCF and quantitated (PSL/mm2). PSMA IF staining was performed on rat, mice, and human salivary gland sections.
Results: The biodistribution study demonstrated that there was a difference in the PSMA expression level in salivary glands of rats and mice. The %ID/g for rat SM, SL, and PR was found to be 0.005, 0.005, and 0.007 respectively. Whereas the 18F-DCF uptake (%ID/g) for mice SM, SL, and PR were 0.367, 0.307, and 0.459 respectively. The tissue to muscle ratio indicated that the PSMA expression level of in mice SM and SL is 2 to 3-fold higher than in rats. These results were consistent with the imaging results in which mice salivary glands were easily discernable compared to rats. The blocking study indicated that the uptake of 18F-DCF in SM, SL, and PR could be competed with DCF and thus confirming that the uptake of the radioactive agent was PSMA mediated in rats and mice. In addition to the salivary glands, the kidneys in both rats and mice showed uptake of 18F-DCF. The uptake of 18F-DCF in kidneys was blocked by the administration of DCF indicating specific PSMA binding as expected. In vitro autoradiography result similarly exhibited higher PSMA levels in mice (374 PSL/mm2) compared to rats (32 PSL/mm2) although the magnitude of the increase in mice was greater than that observed in vivo. The autoradiography results were comparable to IF across species with PSMA mouse expression levels approximating minor salivary glands. In summary, the higher expression of PSMA in salivary glands of mice suggests they are a better animal model for screening prevention strategies involving the salivary glands compared to the rats. Conclusion: These studies demonstrate that mice are a more suitable model to develop methods for preservation of salivary gland function from PSMA-targeted radionuclide therapy in human patients. Further the use of xenograft mouse tumor models can be used to ensure that the efficacy of PSMA-targeted radionuclide therapeutic agent has not been altered due to preventive interventions.