Combination of Targeted Radionuclide Therapy and Checkpoint Blockade Augments Therapeutic Response in a Syngeneic Murine Model of Melanoma

Objectives: Immune checkpoint blockade (CB) using monoclonal antibodies (mAb) against PD-1 and CTLA-4 improves survival for patients with metastatic melanoma, but durable responses are achieved in only a fraction of the cases. Preclinical studies show that combining local external beam radiation therapy (EBRT) with systemic CB results in a synergistic interaction; however, early reports of prospective clinical studies have not demonstrated response rates greater than anticipated with CB alone. We hypothesize that this reflects a need to address further the suppressive immune microenvironment in distant tumors not targeted by EBRT. The goal of our study is to evaluate whether the response to CB can be enhanced when combined with a radiolabeled alkylphosphocholine analog (⁹⁰Y-NM600) that is capable of delivering tumor-selective targeted radionuclide therapy (TRT) to all tumors in the setting of metastatic disease.

Methods: C57BL/6 mice bearing syngeneic B78 melanoma were administered 9.25 MBq of ⁸⁶Y-NM600, and sequential PET/CT scans were acquired at 3, 21, and 48 h post injection (p.i.). ⁸⁶Y-NM600 time-activity curves were determined via region-of-interest (ROI) analysis of the images and reported as %ID/g. The integral Gy/MBq delivered to the tumor and normal tissues by the therapeutic analog ⁹⁰Y-NM600 were estimated based on ⁸⁶Y-NM600 in vivo biodistribution using a Monte Carlo voxel-based dosimetry platform. For TRT studies, groups of mice bearing B78 tumors (75-150 mm³; n=5-6) were administered IV excipient or a ⁹⁰Y-NM600 dose prescription of approximately 3, 9, or 15 Gy to B78 tumors (1.85, 5.55 or 9.25 MBq), and tumor growth was monitored via caliper measurements. Complete blood count (CBC) analysis was performed twice a week for 4 weeks in naïve C57BL/6 mice administered ⁹⁰Y-NM600 at each therapeutic dose level. Treatment combination groups (n=6) received a 3 Gy TRT indication followed by IP injections of an anti-CTLA-4 mAb (10 mg/kg) at days 4, 7, and 11. Results: PET/CT imaging unveiled a selective and persistent accumulation of ⁸⁶Y-NM600 in B78 tumors (5.57 %ID/g at 48 h p.i.) and evidenced a hepatobiliary excretion route of the tracer. Mouse-specific dosimetry estimated a cumulative dose to the tumor and whole-body of 1.58 and 0.55 Gy/MBq, respectively. B78 tumor-bearing mice treated with ⁹⁰Y-NM600 alone or anti-CTLA-4 mAb alone showed a dose-dependent decrease in the rate of tumor progression, but tumor regression was not observed in this radio-resistant tumor line. A 3 Gy tumor dose prescription was selected for TRT + CB studies since no systemic lymphopenia or leukopenia was observed at this dose level. Transient lympho/leukopenia (day 10 nadir) was detected at higher ⁹⁰Y-NM600 (9.25 MBq) injected activity, which fully resolved by 3 weeks. Notably, mice treated with the combination of ⁹⁰Y-NM600 and anti-CTLA-4 showed tumor regression and improved survival compared to other treatment groups, with 66% of mice exhibiting a durable complete tumor response.

Conclusions: In a preclinical syngeneic murine melanoma model, we demonstrate a cooperative therapeutic effect with combined TRT and CB. This confirms that systemic administration of ⁹⁰Y-NM600 can deliver an immunomodulatory radiation dose to the tumor microenvironment without inducing systemic immunosuppression. We are now testing this approach in similar preclinical studies in combination with other immunotherapies and in the setting of disseminated metastatic disease.

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