Colony stimulating factor receptor CSF R or cFMS is a

Colony stimulating factor-1 receptor (CSF-1R or cFMS) is a type III receptor tyrosine kinase. It is activated by binding with CSF-1 and IL-34, which stimulates differentiation, proliferation, survival and migration of monocyte-macrophage lineage cells., , , Also, macrophages produce inflammatory mediators such as interleukin and lymphokine, and lead to differentiation, proliferation and activation of a variety of immune cells. In addition, the signaling promotes differentiation of osteoclastic precursors into mature osteoclasts, which promote bone destruction and bone absorption. Therefore, inhibition of CSF-1R signaling is expected to be therapeutic in rheumatoid arthritis (RA). Indeed, a number of CSF-1R inhibitors with anti-inflammatory efficacy have been reported in the literature, including Ki20227, GW2580,, , JNJ-28312141, Arry382, BLZ945, AZD7507, and PLX-3397. Many kinase inhibitors act by binding to the ATP-binding site, but because all kinases have an ATP-binding site, kinase selectivity is difficult to achieve, leading to concerns about toxicology due to low kinase selectivity. Type II inhibitors (e.g., imatinib and sorafenib), which induce the inactive DFG(Asp-Phe-Gly)-out conformation and also occupy an additional Cell Counting Kit-8 australia pocket created by this rearrangement, are well known as a solution to obtain increased kinase selectivity. Therefore, we explored scaffolds for Type II kinase inhibitors, and consequently obtained a novel and promising azetidine scaffold shown in , as we have reported. Herein, we report our development of the azetidine scaffold to the clinical candidate () () which showed high kinase selectivity, high cellular potency, and a good pharmacokinetic profile. At this study, we optimized substituents of R on the benzyloxy functionality and R of the azetidine scaffold (), in an effort to improve cellular activity and efficacy. Especially, we explored various characters of hydrophilic substituents on R while maintaining R as the ethyl, cyclopropyl, and methoxy groups. Our docking model of compound (R = OMe, R = H, respectively of ) as shown in ,, which we reported previously, led us to two factors that guided our work. Firstly, we predicted that we could not change the substituents at R (ethyl (Et), cyclopropyl (cPr), and methoxy (OMe) groups) to other groups as well as introduce substituents at other positions on phenyl ring, because the phenyl ring and R binding region is narrow and lipophilic. Therefore, ethyl, cyclopropyl, and methoxy groups at R were maintained. Secondly, we predicted that we could control the physical properties of compounds by using various substituents at R. Although the 4- and 5-positions on the pyridine ring are directed outward from the protein, compound with ethylene glycol at the 4-position on pyridine ring (IC = 9.1 nM), displayed higher biochemical activity than its 5-position counterpart (IC = 25 nM). Therefore, further optimization of the substituents on pyridine ring was focused at 4-position (R).