264 - Tumor Immunity Following Radiopharmaceutical + Checkpoint Blockade Varies with Treatment Sequence, Radioisotope and Tumor Immunogenicity

C. Kerr, W. J. Jin, P. Liu, J. J. Grudzinski, C. A. Ferreira, H. Comas Rojas, A. J. Onate, O. Kwon, M. Hyun, M. Bio Idrissou, R. Welch Schwartz, P. A. Clark, M. Takashima, A. G. Shea, A. N. Pinchuk, R. Hernandez, B. Bednarz, I. M. Ong, J. Weichert, and Z. S. Morris; University of Wisconsin School of Medicine and Public Health, Madison, WI

Purpose/Objective(s): Radiopharmaceutical therapy (RPT) may enhance tumor response to immune checkpoint inhibitors (ICI), yet the effects of emitted radiation type, linear energy transfer, treatment timing, and dose rate have not been elucidated. To evaluate these parameters and mechanisms of response to RPT+ICI, we used NM600, an alkylphosphocholine analog that is selectively taken up by most tumors. We hypothesized that ²²⁵Ac-NM600 would enhance therapeutic response to ICI therapy to a greater extent than ⁹⁰Y- or ¹⁷⁷Lu-NM600 in murine tumors. Materials/

Methods: Dosimetry was performed using the Monte Carlo-based RAPID platform utilizing PET/CT or SPECT/CT and/or ex vivo biodistribution. C57BL/6 mice bearing syngeneic B78 melanoma (immunogenic) or MC38 colorectal cancer (poorly immunogenic) received 2 Gy from ⁹⁰Y-, ¹⁷⁷Lu-, or ²²⁵Ac-NM600, or no radiation (RT) on day 1 +/- ICI (anti-CTLA4 + anti-PDL1) on days -3/0/3 (early), 4/7/10 (intermediate), or 11/14/17 (delayed). Mice were monitored for tumor growth and survival. Blood and splenocyte memory cell populations from complete responders were evaluated by flow cytometry. CD8⁺ depletion studies were performed with MC38-bearing mice treated with 2 Gy RPT or no RT on day 1 +/- ICI. B78 melanoma tumors treated with 2 Gy ⁹⁰Y-/¹⁷⁷Lu-/²²⁵Ac-NM600 or external beam radiation therapy + ICI or ICI alone were harvested on day 15 and CD45⁺ cells were isolated by density gradient centrifugation. Single cell RNA-sequencing (scRNA-seq) and T cell receptor sequencing (TCR seq) were performed on these cell populations. Statistical significance was determined by two-way ANOVA (flow) or Kaplan-Meier with log-rank testing (therapy).
Results: 90Y-/¹⁷⁷Lu-/or ²²⁵Ac-NM600 delivering 2 Gy mean tumor dose promoted tumor regression and improved survival when combined with ICIs in syngeneic mice bearing B78 or MC38 tumors. Regardless of the administered isotope, this combination was optimized with early ICI administration (days -3/0/3) relative to day 1 RPT. ⁹⁰Y-NM600+ICI produced the greatest anti-tumor response for MC38, whereas high LET alpha particle radiation from ²²⁵Ac-NM600 was most effective with ICIs against the poorly immunogenic B78 model. Flow cytometry, depletion studies, and scRNA and TCR seq illuminated mechanisms of response to ⁹⁰Y-, ¹⁷⁷Lu-, or ²²⁵Ac-NM600+ICI. CD8⁺ depletion studies confirmed that the MC38 tumor response to RPT+ICI required CD8⁺ T cells. Paired scRNA and TCR seq of B78 tumors elucidated the critical role of CD8⁺ T cells in the anti-tumor response to ⁹⁰Y-, ¹⁷⁷Lu-, and ²²⁵Ac-NM600+ICI, with ²²⁵Ac-NM600+ICI leading to diversified effector T cell responses with key shared clonotypes between CD8⁺ effector and memory cells.
Conclusion: Antitumor immune response can be achieved with appropriate application of a- or ß- emitting RPT in combination with ICIs in diverse murine tumor models.