Evaluation of a Radio-IMmunoStimulant (RIMS) in a Syngeneic Model of Murine Prostate Cancer and ImmunoPET Analysis of T-cell Distribution.

An immunosuppressive tumor microenvironment and tumor heterogeneity have led to the resilience of metastatic castrate resistant prostate cancer (mCRPC) to current treatments. To address these challenges, we developed and evaluated a new drug paradigm, Radio-IMmunostimulant (RIMS), in a syngeneic model of murine prostate cancer. RIMS-1 was generated using a convergent synthesis employing solid phase peptide and solution chemistries. The prostate-specific membrane antigen (PSMA) inhibitory constant for nat Lu-RIMS-1 was determined, and radiolabeling with 177 Lu generated 177 Lu-RIMS-1. The TLR 7/8 agonist payload release from nat Lu-RIMS-1 was determined using a cathepsin B assay. The biodistribution of 177 Lu-RIMS-1 was evaluated in a bilateral xenograft model in NCru nude mice bearing PSMA(+) (PC3-PiP) and PSMA(-) (PC3-Flu) tumors at 2, 24, and 72 h. The therapeutic effect of 177 Lu-RIMS-1 was evaluated in C57BL/6J mice bearing RM1-PGLS (PSMA-positive, green fluorescent protein-positive, and luciferase-positive) tumors and compared to that of 177 Lu-PSMA-617 at the same total administered radioactivity of 57 MBq and molar activity of 5.18 MBq/nmol. nat Lu-RIMS-1 and vehicle were evaluated as the controls. Immuno-positron emission tomography (PET) using 89 Zr-DFO-anti-CD3 was used to visualize T-cell distribution during treatment. 177 Lu-RIMS-1 was quantitatively radiolabeled at >99% radiochemical purity and maintained a high affinity toward PSMA ( K i = 3.77 ± 0.5 nM). Cathepsin B efficiently released the entire immunostimulant payload in 17.6 h. 177 Lu-RIMS-1 displayed a sustained uptake in PSMA(+) tumor tissue up to 72 h (2.65 ± 1.03% ID/g) and was not statistically different ( P = 0.1936) compared to 177 Lu-PSMA-617 (3.65 ± 0.59% ID/g). All animals treated with 177 Lu-RIMS-1 displayed tumor growth suppression and provided a median survival of 30 days ( P = 0.0007) while 177 Lu-PSMA-617 provided a median survival of 15 days, which was not statistically significant ( P = 0.3548) compared to the vehicle group (14 days). ImmunoPET analysis revealed 2-fold more tumor infiltrating T-cells in 177 Lu-RIMS-1-treated animals compared to 177 Lu-PSMA-617-treated animals; 177 Lu-RIMS-1 improves therapeutic outcomes in a syngeneic model of mouse prostate cancer and elicits greater T-cell infiltration to the tumor compared to 177 Lu-PSMA-617. These results support further investigation of the RIMS paradigm as the first example of a single molecular entity combining radiotherapy and immunostimulation.