Preclinical Proof-of-Concept of a Novel mRNA-Based Therapeutic Combination of an FcRH5 Targeted NK Cell Engager and an IL-15 Superagonist for the Treatment of Multiple Myeloma

Introduction: Despite recent advancements, multiple myeloma (MM) patients inevitably relapse and develop resistance to available therapies, thus new therapeutic approaches are needed. Here, we describe a novel combination of two systemically administered mRNA immunotherapies for the treatment of MM: mRNA-NKE and mRNA-IL15. mRNA-NKE encodes a secreted NK cell engager (NKE) protein which binds to validated MM tumor associated antigen (TAA) FcRH5 and to CD16 on NK cells. mRNA-IL15 encodes a half-life extended (HLE) superagonist IL-15 developed to boost activity of mRNA-NKE and other therapies by activating and expanding natural killer (NK) and cytotoxic T cells.

Methods: mRNA-NKE is an LNP-encapsulated mRNA encoding a bispecific protein consisting of antibody binders against human CD16 and FcRH5, and a functionally silenced Fc fragment. In vitro binding affinity to FcRH5 and CD16 and cytotoxicity against FcRH5⁺ MM cell lines and MM patient cells was assessed with recombinant FcRH5xCD16 protein. mRNA-IL15 is an LNP-encapsulated mRNA encoding an IL-15 cytokine fused to the sushi domain + hinge region of IL-15Rα and to human serum albumin (HSA-sIL-15). The potency of recombinant HSA-sIL-15 at inducing the proliferation and activation of rodent, cynomolgus, and human effector cells was evaluated in vitro by flow cytometry. In vivo expression and target cell depletion of intravenously (IV) administered mRNA-NKE or mRNA-IL15 was assessed in naïve and humanized mouse models and cynomolgus monkeys. The combination of these two therapeutics was also evaluated in cytotoxicity assays against FcRH5⁺ MM cell lines and primary MM patient samples and in humanized xenograft mice.

Results: FcRH5xCD16 protein binds to its target receptors FcRH5 and CD16a with low nanomolar affinity. In cell lines, FcRH5xCD16 induced NK-mediated cytotoxicity of FcRH5⁺ MM cells with an EC₅₀ in the picomolar range. Robust cytotoxic response was observed ex vivo with human NK cells on FcRH5⁺ MM cell lines and MM patient cells with autologous NK cells. In vivo administration of mRNA-NKE IV in mice and cynomologus monkeys resulted in expression of FcRH5xCD16 protein and was well-tolerated. mRNA-NKE demonstrated clear pharmacodynamic effects based on reduction in circulating FcRH5⁺ B cells, most notably among memory B cells with the highest FcRH5 expression levels.

HSA-sIL-15 protein potently induced activation and proliferation of NK and other effector cells across multiple healthy human and cynomolgus donors and mouse cells ex vivo. In rodents this regimen resulted in dose-dependent expression of HSA-sIL-15 with corresponding increase in activation and proliferation markers on NK and T cells. In cynomolgus monkeys, weekly dosing of mRNA-IL15 for 5 weeks was well-tolerated and achieved dose-dependent HSA-sIL-15. Robust pharmacodynamic responses to a single dose of mRNA-IL15 were demonstrated based on increases in proliferation, assessed by Ki-67, and NK and T cell numbers.

The combination of FcRH5xCD16 and HSA-sIL-15 led to synergistic enhancement of NK cell-mediated cytotoxicity against FcRH5⁺ cell lines in vitro. Addition of HSA-sIL-15 to FcRH5xCD16 led to increased NK cell proliferation, higher maximum killing, and a decrease in the EC₅₀ of cytotoxicity, indicating improved potency over FcRH5xCD16 alone. In MM patient-derived samples, treatment with HSA-sIL-15 protein led to dose-dependent proliferation and activation of NK cells and depletion of MM blasts was observed only with the combination of NKE and HSA-sIL-15. This trend was also observed in immunodeficient mice implanted with an FcRH5⁺ MM cell line and human NK cells, where intravenous administration of mRNA-NKE and mRNA-IL15, but neither component alone, led to a decrease in MM cells in the spleen.

Conclusions: These preclinical data demonstrate potent activity of the combination of mRNA-NKE, an mRNA-encoded FcRH5xCD16 NK cell engager, with mRNA-IL15, an mRNA-encoded HSA-sIL-15, against FcRH5+ MM. The mRNA platform can deliver rational combination immunotherapies through the administration of mRNA-encoded immune cell engaging bispecific antibodies and agonistic cytokines.