Molecular and cellular effects of radiotherapy on tumor microenvironment (TME) can help prime and propagate antitumor immunity. We hypothesized that delivering radiation to all tumor sites could augment response to immunotherapies. We tested an approach to enhance response to immune checkpoint inhibitors (ICIs) by using targeted radionuclide therapy (TRT) to deliver radiation semiselectively to tumors. NM600, an alkylphosphocholine analog that preferentially accumulates in most tumor types, chelates a radioisotope and semiselectively delivers it to the TME for therapeutic or diagnostic applications. Using serial 86Y-NM600 positron emission tomography (PET) imaging, we estimated the dosimetry of 90Y-NM600 in immunologically cold syngeneic murine models that do not respond to ICIs alone. We observed strong therapeutic efficacy and reported optimal dose (2.5 to 5 gray) and sequence for 90Y-NM600 in combination with ICIs. After combined treatment, 45 to 66% of mice exhibited complete response and tumor-specific T cell memory, compared to 0% with 90Y-NM600 or ICI alone. This required expression of STING in tumor cells. Combined TRT and ICI activated production of proinflammatory cytokines in the TME, promoted tumor infiltration by and clonal expansion of CD8+ T cells, and reduced metastases. In mice bearing multiple tumors, combining TRT with moderate-dose (12 gray) external beam radiotherapy (EBRT) targeting a single tumor augmented response to ICIs compared to combination of ICIs with either TRT or EBRT alone. The safety of TRT was confirmed in a companion canine study. Low-dose TRT represents a translatable approach to promote response to ICIs for many tumor types, regardless of location.

Finding improved therapeutic strategies against T-cell Non-Hodgkin’s Lymphoma (NHL) remains an unmet clinical need. We implemented a theranostic approach employing a tumor-targeting alkylphosphocholine (NM600) radiolabeled with 86 Y for positron emission tomography (PET) imaging and 90 Y for targeted radionuclide therapy (TRT) of T-cell NHL. PET imaging and biodistribution performed in mouse models of T-cell NHL showed in vivo selective tumor uptake and retention of 86 Y-NM600. An initial toxicity assessment examining complete blood counts, blood chemistry, and histopathology of major organs established 90 Y-NM600 safety. Mice bearing T-cell NHL tumors treated with 90 Y-NM600 experienced tumor growth inhibition, extended survival, and a high degree of cure with immune memory toward tumor reestablishment. 90 Y-NM600 treatment was also effective against disseminated tumors, improving survival and cure rates. Finally, we observed a key role for the adaptive immune system in potentiating a durable anti-tumor response to TRT, especially in the presence of microscopic disease.

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There is a clinically unmet need for effective treatments for triplenegative breast cancer (TNBC), as it remains the most aggressive subtype of breast cancer. Herein, we demonstrate a promising strategy using a tumor-targeting alkylphosphocholine (NM600) for targeted radionuclide therapy of TNBC. Methods: NM600 was radiolabeled with 86 Y for PET imaging and 177 Lu for targeted radionuclide therapy. 86 Y-NM600 PET imaging was performed on female BALB/C mice bearing syngeneic 4T07 (nonmetastatic) and 4T1 (metastatic) TNBC tumor grafts (n 5 3-5). Quantitative data derived from a PET-image region-of-interest analysis, which was corroborated by ex vivo biodistribution, were used to estimate the dosimetry of 177 Lu-NM600 treatments. Weight measurement, complete blood counts, and histopathology analysis were performed to determine 177 Lu-NM600 toxicity in naïve BALB/C mice administered 9.25 or 18.5 MBq. Groups of mice bearing 4T07 or 4T1 grafts (n 5 5-6) received excipient or 9.25 or 18.5 MBq of 177 Lu-NM600 as a single or fractionated schedule, and tumor growth and overall survival were monitored. Results: Excellent tumor targeting and rapid normal-tissue clearance of 86 Y-NM600 were noted in both 4T07 and 4T1 murine models. Ex vivo biodistribution corroborated the accuracy of the PET data and validated 86 Y-NM600 as a surrogate for 177 Lu-NM600. 177 Lu-NM600 dosimetry showed absorbed doses of 2.04 ± 0.32 and 1.68 ± 0.06 Gy/MBq to 4T07 and 4T1 tumors, respectively, which were larger than those delivered to liver (1.28 ± 0.09 Gy/MBq) and to bone marrow (0.31 ± 0.05 Gy/MBq). The 177 Lu-NM600 injected activities used for treatment were well tolerated and resulted in significant tumor growth inhibition and prolonged overall survival in both tested TNBC models. A complete response was attained in 60% of treated mice bearing 4T07 grafts. Conclusion: Overall, our results suggest that 177 Lu-NM600 targeted radionuclide therapy has potential for TNBC and merits further exploration in a clinical setting.

BackgroundSystemic radiation treatments that preferentially irradiate cancer cells over normal tissue, known as targeted radionuclide therapy (TRT), have shown significant potential for treating metastatic prostate cancer. Preclinical studies have demonstrated the ability of external beam radiation therapy (EBRT) to sensitize tumors to T cell checkpoint blockade. Combining TRT approaches with immunotherapy may be more feasible than combining with EBRT to treat widely metastatic disease, however the effects of TRT on the prostate tumor microenvironment alone and in combinfation with checkpoint blockade have not yet been studied.MethodsC57BL/6 mice-bearing TRAMP-C1 tumors and FVB/NJ mice-bearing Myc-CaP tumors were treated with a single intravenous administration of either low-dose or high-dose 90Y-NM600 TRT, and with or without anti-PD-1 therapy. Groups of mice were followed for tumor growth while others were used for tissue collection and immunophenotyping of the tumors via flow cytometry.Results90Y-NM600 TRT was safe at doses that elicited a moderate antitumor response. TRT had multiple effects on the tumor microenvironment including increasing CD8 +T cell infiltration, increasing checkpoint molecule expression on CD8 +T cells, and increasing PD-L1 expression on myeloid cells. However, PD-1 blockade with TRT treatment did not improve antitumor efficacy. Tregs remained functional up to 1 week following TRT, but CD8 +T cells were not, and the suppressive function of Tregs increased when anti-PD-1 was present in in vitro studies. The combination of anti-PD-1 and TRT was only effective in vivo when Tregs were depleted.ConclusionsOur data suggest that the combination of 90Y-NM600 TRT and PD-1 blockade therapy is ineffective in these prostate cancer models due to the activating effect of anti-PD-1 on Tregs. This finding underscores the importance of thorough understanding of the effects of TRT and immunotherapy combinations on the tumor immune microenvironment prior to clinical investigation.