INDUCTION OF APOPTOSIS AND CELL-CYCLE ARREST BY CP-358,774, AN INHIBITOR OF EPIDERMAL GROWTH-FACTOR RECEPTOR TYROSINE KINASE

The epidermal growth factor receptor (EGFR) is overexpressed in a sign ificant percentage of carcinomas and contributes to the malignant phen otype. CP-358,774 is a directly acting inhibitor of human EGFR tyrosin e kinase with an IC50 of 2 nM and reduces EGFR autophosphorylation in intact tumor cells with an IC50 of 20 nM. This inhibition is selective for EGFR tyrosine kinase relative to other tyrosine kinases we have e xamined, both in assays of isolated kinases and whole cells. At doses of 100 mg/kg, CP-358,774 completely prevents EGF-induced autophosphory lation of EGFR in human HN5 tumors growing as xenografts in athymic mi ce and of the hepatic EGFR of the treated mice. CP-358,774 inhibits th e proliferation of DiFi human colon tumor cells at submicromolar conce ntrations in cell culture and blocks cell cycle progression at the G(1 ) phase. This inhibitor produces a marked accumulation of retinoblasto ma protein in its underphosphorylated form and accumulation of p27(KIP 1) in DiFi cells, which may contribute to the cell cycle block. Inhibi tion of the EGFR also triggers apoptosis in these cells as determined by formation of DNA fragments and other criteria. These results indica te that CP-358,774 has potential for the treatment of tumors that are dependent on the EGFR pathway for proliferation or survival.