Rrx-001 Is a Phase 3 Small Molecule Dual Inhibitor of CD47 and Sirpα with Activity in Multiple Myeloma

Background:

CD47 binds to SIRPα on the surface of macrophages delivering a "do not eat" signal to suppress phagocytosis. To evade macrophage-mediated destruction, CD47 is frequently overexpressed in cancers, including multiple myeloma (MM). CD47 inhibition is an area of intense therapeutic interest with three anti-CD47 monoclonal antibodies (mAbs) currently in clinical trials. However, these mAbs have been associated with severe hematologic toxicities. RRx-001 is a minimally toxic small molecule that dually downregulates CD47 on tumor cells and SIRPα on macrophages and triggers tumor associated macrophage phagocytosis of tumor cells in vitro and in vivo. RRx-001 is entering Phase 3 trials for the treatment of multiple cancer indications.

Methods:

The effect of RRx-001 on the expression of CD47 and SIRPα on macrophages was evaluated with Western blotting and flow cytometry. An in vitro phagocytotic assay was used to determine whether RRx-001 promoted engulfment of multiple myeloma cells by macrophages. Transcriptional mRNA profiling in murine tumor associated macrophages (TAMs) was performed to analyze the cytokine profile of TAMs in the presence or absence of RRx-001. Finally, myeloma bearing mice were treated with RRx-001 in the presence or absence or absence of clodronate to determine the effect of macrophage depletion on RRx-001 anticancer activity.

Results:

RRx-001 was shown to downregulate CD47 and SIRPα expression on tumor cells and macrophages, respectively, and to promote the phagocytosis of multiple myeloma tumor cells. RRx-001 also stimulated the production of pro-inflammatory cytokines in TAMs. In tumor bearing mice, depletion of macrophages by clodronate reduced the antitumor effects of RRx-001.

Conclusions:

RRx-001 is a Phase 3-ready small molecule innate immune checkpoint inhibitor, which triggers tumor associated macrophage phagocytosis of high-expressing CD47 tumor cells. Dual downregulation of CD47 and SIRPα by RRx-001 results in TAM repolarization and phagocytosis of tumor cells. Depletion of macrophages by clodronate in tumor-bearing mice reduced the antitumor effects of RRx-001 and further suggests that the target of RRx-001 is the macrophage.