Design and synthesis of pyrrolo[3,2-d]pyrimidine human epidermal growth factor receptor 2 (HER2)/epidermal growth factor receptor (EGFR) dual inhibitors: exploration of novel back-pocket binders.

To develop novel human epidermal growth factor receptor 2 (HER2)/epidermal growth factor receptor (EGFR) kinase inhibitors, we explored pyrrolo[3,2-d]pyrimidine derivatives bearing bicyclic fused rings designed to fit the back pocket of the HER2/EGFR proteins. Among them, the 1,2-benzisothiazole (42m) ring was selected as a suitable back pocket binder because of its potent HER2/EGFR binding and cell growth inhibitory (GI) activities and pseudoirreversibility (PI) profile as well as good bioavailability (BA). Ultimately, we arrived at our preclinical candidate 51m by optimization of the N-5 side chain to improve CYP inhibition and metabolic stability profiles without a loss of potency (HER2/EGFR inhibitory activity, IC(50), 0.98/2.5 nM; and GI activity BT-474 cells, GI(50), 2.0 nM). Reflecting the strong in vitro activities, 51m exhibited potent tumor regressive efficacy against both HER2- and EGFR-overexpressing tumor (4-1ST and CAL27) xenograft models in mice at oral doses of 50 mg/kg and 100 mg/kg.

Design and synthesis of novel human epidermal growth factor receptor 2 (HER2)/epidermal growth factor receptor (EGFR) dual inhibitors bearing a pyrrolo[3,2-d]pyrimidine scaffold.

Dual inhibitors of human epidermal growth factor receptor 2 (HER2) and epidermal growth factor receptor (EGFR) have been investigated for breast, lung, gastric, prostate, and other cancers; one, lapatinib, is currently approved for breast cancer. To develop novel HER2/EGFR dual kinase inhibitors, we designed and synthesized pyrrolo[3,2-d]pyrimidine derivatives capable of fitting into the receptors' ATP binding site. Among the prepared compounds, 34e showed potent HER2 and EGFR (HER1) inhibitory activities as well as tumor growth inhibitory activity. The X-ray cocrystal structures of 34e with both HER2 and EGFR demonstrated that 34e interacts with the expected residues in their respective ATP pockets. Furthermore, reflecting its good oral bioavailability, 34e exhibited potent in vivo efficacy in HER2-overexpressing tumor xenograft models. On the basis of these findings, we report 34e (TAK-285) as a promising candidate for clinical development as a novel HER2/EGFR dual kinase inhibitor.

Structural analysis of the mechanism of inhibition and allosteric activation of the kinase domain of HER2 protein.

Aberrant signaling of ErbB family members human epidermal growth factor 2 (HER2) and epidermal growth factor receptor (EGFR) is implicated in many human cancers, and HER2 expression is predictive of human disease recurrence and prognosis. Small molecule kinase inhibitors of EGFR and of both HER2 and EGFR have received approval for the treatment of cancer. We present the first high resolution crystal structure of the kinase domain of HER2 in complex with a selective inhibitor to understand protein activation, inhibition, and function at the molecular level. HER2 kinase domain crystallizes as a dimer and suggests evidence for an allosteric mechanism of activation comparable with previously reported activation mechanisms for EGFR and HER4. A unique Gly-rich region in HER2 following the α-helix C is responsible for increased conformational flexibility within the active site and could explain the low intrinsic catalytic activity previously reported for HER2. In addition, we solved the crystal structure of the kinase domain of EGFR in complex with a HER2/EGFR dual inhibitor (TAK-285). Comparison with previously reported inactive and active EGFR kinase domain structures gave insight into the mechanism of HER2 and EGFR inhibition and may help guide the design and development of new cancer drugs with improved potency and selectivity.