Low-dose [212Pb]VMT01 targeted alpha-particle therapy cooperates with immune checkpoint inhibitors to induce robust tumor responses in a heterogeneous melanoma model in mice.

Introduction: Intertumoral heterogeneity represents a significant underpinning of low response rate to immunotherapies in metastatic melanoma. Within this context, targeted α-particle radionuclide therapies (α-TRT) have been increasingly recognized as an ideal avenue for delivery of immunogenic alpha-radiation to multiple, potentially all, lesions and cooperates with immunotherapies. In this study, we aimed to determine the efficacy of [²¹²Pb]VMT01, an α-TRT targeting melanocortin 1 receptor (MC1R), in combination with immune checkpoint inhibitors (ICIs) in heterogeneous murine melanoma models.

Methods: Biodistribution of [²⁰³ Pb]VMT01 surrogate was performed in female C57Bl6 albino mice (n=3-4) bearing syngeneic murine melanoma B16-F10 (high MC1R, melanotic, BRAF^WT, radioresistant), YUMM-D3 (mid MC1R, amelanotic, BRAF^V600E, radiosensitive), and YUMMwt (no MC1R, amelanotic, BRAF^V600E, radiosensitive). Tumor dosimetry of [²¹² Pb]VMT01 in three allografts were calculated. Therapy studies were conducted in female C57Bl6 albino mice bearing either single allografts (B16-F10, YUMM-D3, YUMMwt; n=6-8) or contralateral heterogeneous allografts (B16-F10/YUMM-D3, B16-F10/YUMMwt; n=6-10). ICIs treatment (10 mg kg^-1 α-PD-1 and α-CTLA-4 mAbs; IP; BIW) was initiated concurrently with single injection of increasing activity of [²¹²Pb]VMT01 (0-10 Gy alpha-radiation in tumor). Tumor growth, body weight, and wellness were monitored 3-5 times a week.

Results: Using in vivo biodistribution data, dose deposition from [²¹² Pb]VMT01 in B16-F10 (0.2 Gy/37 kBq – "hot" tumor), YUMM-D3 (0.1 Gy/37 kBq – "warm" tumor) and YUMMwt (nearly 0 Gy/37 kBq – "cold" tumor ) was calculated. Using a B16F10 allograft model, the most effective combination regimen was identified that included single IV injection of 1.5-1.8 MBq [²¹²Pb]VMT01 (i.e., 8-10 Gy in tumor) with dual ICIs (i.e., a-PD-1+ a-CTLA-4). Robust cooperative combination of [²¹²Pb]VMT01 with ICIs was even found at low-dose level of [²¹²Pb]VMT01(i.e., 2 Gy in tumor). No benefit from this combination regimen was observed in MC1R-negative YUMMwt allograft, confirming that the cooperative effect between [²¹²Pb]VMT01 and ICIs relies on deposition of alpha-radiation in tumor. In mice bearing heterogeneous contralateral allografts of B16-F10/YUMM-D3, single injection of 1.5 MBq [²¹²Pb]VMT01 (8 Gy in B16F10, 4 Gy in YUMM-D3) led to strong cooperation with a-PD-1/a-CTLA-4 and significant anti-tumor effect in both tumors, resulting in complete remission of both tumors in 50% treated mice. In mice bearing B16-F10/YUMMwt allografts, despite the presence of YUMMwt "cold" tumor, the anti-tumor effect was not compromised in B16-F10 "hot" tumor, resulting in 100% tumor remission. On the other hand, compared with control animals, the growth of YUMMwt in treated mice was slightly delayed, presumably due to the presence of distal "hot" B16-F10 tumor, indicating the potential of abscopal-like effects.

Conclusions: Strong cooperative anti-tumor effect between [²¹²Pb]VMT01 α-TRT and ICIs was observed in single and heterogeneous murine melanoma allograft models. Induction of this cooperative effect can be observed with 2 Gy of alpha-ration in tumor. Ongoing studies are focused on analyzing dynamic changes in innate and adaptive immunity, as well as T-cell populations in response to [²¹²Pb]VMT01 treatments. These data, including induction of complete remissions in a model that is resistant to checkpoint inhibitors alone provide a rationale for advancing that the combination of [²¹²Pb]VMT01 α-TRT and ICIs to clinical investigations.

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