Brain metastases develop in over 60% of advanced
melanoma patients and negatively impact quality of life and prognosis. In a murine
melanoma model, we previously showed that an in situ vaccination (ISV) regimen, combining
radiation treatment and intratumoral (IT) injection of immunocytokine (IC: anti-GD2 antibody fused to
IL2), along with the
immune checkpoint inhibitor anti-CTLA-4, robustly eliminates peripheral flank
tumors but only has modest effects on co-occurring intracranial
tumors. In this study, we investigated the ability of low-dose radiation to the brain to potentiate anti-
tumor immunity against a
brain tumor when combined with ISV + anti-CTLA-4. B78 (GD2+, immunologically "cold")
melanoma tumor cells were implanted into the flank and the right striatum of the brain in C57BL/6 mice. Flank
tumors (50-150 mm3) were treated following a previously optimized ISV regimen [radiation (12 Gy × 1, treatment day 1), IT-IC (50 µg daily, treatment days 6-10), and anti-CTLA-4 (100 µg, treatment days 3, 6, 9)]. Mice that additionally received whole-brain
radiation treatment (WBRT, 4 Gy × 1) on day 15 demonstrated significantly increased survival compared to animals that received ISV + anti-CTLA-4 alone, WBRT alone or no treatment (control) (P < 0.001, log-rank test). Timing of WBRT was critical, as WBRT administration on day 1 did not significantly enhance survival compared to ISV + anti-CTLA-4, suggesting that the effect of WBRT on survival might be mediated through immune modulation and not just direct
tumor cell cytotoxicity. Modest increases in T cells (CD8+ and CD4+) and monocytes/macrophages (F4/80+) but no changes in FOXP3+ regulatory T cells (Tregs), were observed in brain
melanoma tumors with addition of WBRT (on day 15) to ISV + anti-CTLA-4.
Cytokine multiplex immunoassay revealed distinct changes in both intracranial
melanoma and contralateral normal brain with addition of WBRT (day 15) to ISV + anti-CTLA-4, with notable significant changes in pro-inflammatory (e.g., IFNγ, TNFα and LIX/CXCL5) and suppressive (e.g.,
IL10,
IL13)
cytokines as well as
chemokines (e.g., IP-10/CXCL10 and MIG/CXCL9). We tested the ability of the alkylphosphocholine analog, NM600, to deliver immunomodulatory radiation to
melanoma brain tumors as a targeted
radionuclide therapy (TRT).
Yttrium-86 (86Y) chelated to NM600 was delivered intravenously by tail vein to mice harboring flank and brain
melanoma tumors, and PET imaging demonstrated specific accumulation up to 72 h at each
tumor site (∼12:1
brain tumor/brain and ∼8:1 flank
tumor/muscle). When NM600 was chelated to therapeutic β-particle-emitting 90Y and administered on treatment day 13, T-cell infiltration and
cytokine profiles were altered in
melanoma brain tumor, like that observed for WBRT. Overall, our results demonstrate that addition of low-dose radiation, timed appropriately with ISV administration to
tumors outside the brain, significantly increases survival in animals co-harboring
melanoma brain tumors. This observation has potentially important translational implications as a treatment strategy for increasing the response of
tumors in the brain to systemically administered
immunotherapies.