Design and Synthesis of Novel Macrocyclic Mer Tyrosine Kinase Inhibitors

Abstract

Mer tyrosine kinase (MerTK) is aberrantly elevated in various tumor cells and has a normal anti-inflammatory role in the innate immune system. Inhibition of MerTK may provide dual effects against these MerTK-expressing tumors through reducing cancer cell survival and redirecting the innate immune response. Recently, we have designed novel and potent macrocyclic pyrrolopyrimidines as MerTK inhibitors using a structure-based approach. The most active macrocycles had an EC 50 below 40 nM in a cell-based MerTK phosphor-protein ELISA assay. The X-ray structure of macrocyclic analogue 3 complexed with MerTK was also resolved and demonstrated macrocycles binding in the ATP binding pocket of the MerTK protein as anticipated. In addition, the lead compound 16 (UNC3133) had a 1.6 h half-life and 16% oral bioavailability in a mouse PK study.

The Emerging Role of TYRO3 as a Therapeutic Target in Cancer

“…The demonstrated roles for TYRO3 in tumorigenesis, metastasis and therapeutic resistance in a variety of tumor types and emerging roles in immune suppression in the tumor microenvironment implicate TYRO3 as a therapeutic target in cancer and development of translational agents to target TYRO3 will allow direct testing of this idea. Multi-kinase inhibitors with activity against TYRO3 have been described and are in various stages of development with the intent of targeting other oncogenic kinases (Table 2) [127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145]. In particular, numerous agents with inhibitory activity against MET also have potent activity against TYRO3, including BMS-777607 [127], foretinib [130,132], LDC1267 [133], LY2801653 [134], RXDX-106 [138] and sitravatinib [139].…”

“…All of these agents also target MERTK and/or AXL with similar potencies. More recently, several compounds designed to target MERTK have been described and these have activity against both TYRO3 and AXL at 10–50 fold higher concentrations [135,136,140,141,142,143,144,145].…”

Discovery of Macrocyclic Pyrimidines as MerTK‐Specific Inhibitors

“…This dual inhibitory activity of UNC2025 is desirable for certain diseases such as acute myeloid leukemia (AML); however, inhibition of Flt3 has been associated with hematopoietic toxicity, and is therefore inadvisable for other applications of MerTK inhibitors. The new pyrrolopyrimidine macrocycles that we have developed recently share the same undesired selectivity profile . Herein we present a new type of MerTK‐specific inhibitor: macrocyclic pyrimidines.…”

“…The new pyrrolopyrimidine macrocycles that we have developedrecently share the same undesired selectivity profile. [17] Hereinw epresent an ew type of MerTK-specific inhibitor:macrocyclic pyrimidines.…”

“…In general, most of the analoguesh ad excellent selectivity for MerTKo ver Flt3 and good selectivity over Axl and Tyro3. To evaluate the inhibitory activity of these compounds in ac ell-based assay of tyrosine phosphorylated MerTK (pMerTK), we used a3 84-wellf ormat enzyme-linked immunosorbenta ssay( ELISA), [17] in which HEK293 cells are transfected with acDNA encodingc himericprotein consisting of the extracellular andt ransmembrane domains from the epidermal growth factor receptor (EGFR) andt he MerTK intracellular kinase domain. Activity is then stimulated with EGF.Achimeric protein was used because consistent stimulation of native MerTK kinasew ith the complex, natural ligand (GAS6 plus PtdSer)w as not practical.…”