S-222611, a potent, orally active small molecule inhibitor of EGFR and HER2: in vitro kinase inhibition and antitumor activity

Abstract

Receptor tyrosine kinases such as the epidermal growth factor receptor (EGFR) and human EGFR2 (HER2) plays central roles in tumor formulation including proliferation and metastatic spread. EGFR and HER2 are upregulated in a number of malignancies and their overexpression or constitutive activation is associated with poor prognosis. A dual tyrosine kinase inhibitor of EGFR and HER2, lapatinib, is known to be effective in patients with breast cancer overexpressing HER2 and other several cancers overexpressing EGFR/HER2. Recently, we found S-222611 as a new antitumor drug which is an orally active dual tyrosine kinase inhibitor of EGFR and HER2. In the present study, we for the first time demonstrate the in vitro kinase inhibition and antitumor activity of S-222611. In in vitro kinase assays, S-222611 showed highly selective inhibitory activity against EGFR and HER2 with IC50 values of 1.48 and 7.15 nM, and the other kinases tested were not inhibited by more than 50% at 10 μM except for HER4. Interestingly, it showed very slow dissociation rate from purified EGFR although the inhibition was reversible. S-222611 inhibited the proliferation of human breast cancer cell lines (BT-474, SK-BR-3, MDA-MB-175VII, MDA-MB-453) and human gastric cancer cell line (NCI-N87) expressing EGFR and/or HER2 with IC50 values ranging from 8.3 to 48.6 nM, but very weekly suppressed that of non-cancer cell lines with IC50 values of approximately 5,000 nM. The antiproliferative effect of S-222611 against cancer cells was 3 to 5-fold more potent than that of lapatinib. Consistently, phosphorylation of EGFR and HER2 in NCI-N87 was suppressed by 24-hr treatment with S-222611 and lapatinib with IC50 values of 4.5 and 1.6 nM for S-222611 and 11.0 and 5.4 nM for lapatinib, respectively. The IC50 value of S-222611 for growth inhibition of NCI-N87 was slightly increased from 10 nM to 35 nM (x3.5) and 31 nM (x3.1) when 0.08% α-Glycoprotein or 2% human serum albumin was added to the medium, respectively, and the increase in the IC50 values was less than that of lapatinib. In NCI-N87 bearing mouse model, once daily oral administration of S-222611 and lapatinib significantly reduced the tumor volume at doses ranging from 6.25 to 50 mg/kg and from 50 to 200 mg/kg, and the ED50 values were 10.2 and 57.7 mg/kg, respectively, and the maximum effect of S-222611 was much stronger than lapatinib. The phosphorylation of EGFR and HER2 in NCI-N87 tumor was inhibited by single oral administration of either S-222611 or lapatinib when assessed at 6 hrs post-dosing, and the inhibitory activity of lapatinib was reduced but that of S-222611 was sustained at 24hrs post-dosing. Taken together, S-222611 is expected to show more potent antitumor activity than lapatinib probably through not only stronger antiproliferative activity but also sustained tyrosine kinase inhibition.