On-target toxicities from 211At MM4 shows a quick recovery timeline

Introduction: Poly(ADP-ribose) polymerase 1 (PARP-1) is a unique target for alpha-particle therapy with robust evidence for potent anti-tumor efficacy using an astatinated PARP inhibitor radiotherapeutic, [²¹¹At]MM4. However, there is concern for on-target toxicity in normal tissues with high PARP-1 expression, especially marrow and lymphocytes. Therefore, we evaluated the direct toxicity of [²¹¹At]MM4 to marrow and blood.

Methods: Immunocompetent C57BL6 mice were used in all studies in accordance with University of Pennsylvania guidelines for animal care. Mice were administered [²¹¹At]MM4 twice weekly for a total of 4 doses at either 975 µCi/kg, 1300 µCi/kg, or 1625 µCi/kg to determine the cumulative maximum tolerated dose (cMTD). All subsequent studies were performed at the MTD to determine on-target toxicities to marrow and blood. Cumulative toxicity was evaluated after 1, 2, or 3 doses. Recovery of toxicity was evaluated for cumulative doses at 72 hour, 1, 2, and 4 week time points. For all mice, the red marrow-containing femur was harvested and analyzed by a colony formation assay (CFA). Peripheral blood was collected and analyzed by a complete blood count (CBC). Femurs, spleens, kidneys, and livers were harvested for histopathology and immunohistochemistry (IHC).

Results: The cumulative MTD of [²¹¹At]MM4 was 3.9 mCi/kg. We observed a dose dependent decrease in colony formation of bone marrow progenitor cells, however severe thrombocytopenia and leukopenia was observed at the cMTD. Decreased colony formation of bone marrow progenitors was evident after 1 week following either 1 or 2 doses of 975 µCi/kg (MTD) and complete or near-complete recovery of marrow progenitor colony formation occurred 2 weeks post-therapy. Acute thrombocytopenia and leukopenia were observed at the 72-hour time-point post administration of 1 or 2 doses with recovery by 2 weeks. Delayed hematological recovery was observed in mice administered 3 doses with full recovery evident by 4 weeks post-therapy. Histopathology and IHC of the spleens from the MTD group were observed to have a dose dependent increase in markers of DNA damage (γH2AX) and apoptosis (cleaved PARP).Conclusion: Overall, these data suggest that acute hematological toxicity is not caused by marrow suppression. This is supported by direct hematological toxicities observed at 72 hours after a single dose, but delayed marrow toxicity observed at 1 week. Furthermore, there was a dose dependent toxicity to the marrow with greater marrow suppression at higher dose levels. However, myelogenous tissue was also highly sensitive with equivalent toxicity evident at 975 µCi/kg and at higher dose levels. We conclude there is a differential sensitivity to marrow and myelogenous tissue, despite both showing direct toxicity. While two doses allow faster recovery to the marrow compartment there is a possibility that marrow suppression might provide additional therapeutic effects in the tumor microenvironment. Future work will be directed to study how [²¹¹At]MM4 therapy effects the tumor microenvironment as a function of marrow suppression and tumor response.