In vitro and in vivo characterization of small molecule inhibitors of the anti-inflammatory TAM receptor MerTK

MerTK, a TAM (Tyro3, Axl, MerTK) family RTK, is expressed on phagocytic cells. Its normal function is to dampen innate immune responses to self-antigens. MerTK is an indirect phosphatidylserine (PtdSer) receptor: PtdSer-binding TAM ligands (Gas6 or Protein S) bridge interactions between MerTK and PtdSer externalized on apoptotic cells (ACs), resulting in AC internalization (efferocytosis). Ensuing MerTK signaling leads to anti-inflammatory M2 macrophage polarization, suppression of pro-inflammatory cytokine production, and a tolerogenic outcome. Tumors are rich in ACs and TAM ligands. Syngeneic tumors implanted in MerTK −/− mice exhibit impaired growth and metastasis compared with those implanted in WT mice. Moreover, MerTK aberrantly expressed on hematological and epithelial malignancies promotes survival and chemoresistance. Thus, pharmacological inhibition of MerTK may have clinical benefit by increasing availability of dead tumor cell antigens, blocking tumor-induced immunosuppression, or blocking tumor cell survival. Here we describe novel and potent MerTK-selective and Mer-Axl small molecule inhibitors that block both MerTK in vitro kinase activity and MerTK autophosphorylation and downstream signaling in cells. Moreover, compounds were able to inhibit phagocytosis of ACs by MerTK-expressing human primary macrophages, and they block activation of MerTK in vivo . The MerTK inhibitors were not overtly cytotoxic or antiproliferative, and did not block the activity of TLR or T cell immune effector pathways. Limited off-target activity was observed in an in vitro kinase panel . Activity of these novel MerTK inhibitors in tumor models is currently under investigation.