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Drug resistance often critically limits the efficacy of molecular targeted drugs. Although pharmacological inhibition of phosphatidylinositol 3‐kinase (PI3K) is an attractive therapeutic strategy for cancer therapy, molecular determinants for efficacy of PI3K inhibitors (PI3Kis) remain unclear. We previously identified that overexpression of insulin‐like growth factor 1 receptor (IGF1R) contributed to the development of drug resistance after long‐term exposure to PI3Kis. In this study, we examined the involvement of basal IGF1R expression in intrinsic resistance of drug‐naïve cancer cells to PI3Kis and whether inhibition of IGF1R overcomes the resistance. We found that cancer cells highly expressing IGF1R showed resistance to dephosphorylation of Akt and subsequent antitumor effect by ZSTK474 treatment. Knockdown of IGF1R by siRNAs facilitated the dephosphorylation and enhanced the drug efficacy. These cells expressed tyrosine‐phosphorylated insulin receptor substrate 1 at high levels, which was dependent on basal IGF1R expression. In these cells, the efficacy of ZSTK474 in vitro and in vivo was improved by its combination with the IGF1R inhibitor OSI‐906. Finally, we found a significant correlation between the basal expression level of IGF1R and the inefficacy of ZSTK474 in an in vivo human cancer panel, as well as in vitro. These results suggest that basal IGF1R expression affects intrinsic resistance of cancer cells to ZSTK474, and IGF1R is a promising target to improve the therapeutic efficacy. The current results provide evidence of combination therapy of PI3Kis with IGF1R inhibitors for treating IGF1R‐positive human cancers. Cancer cells highly expressing insulin‐like growth factor 1 receptor (IGF1R) exhibited resistance to a PI3K inhibitor ZSTK474, and the resistance was diminished after expression knockdown of the IGF1R gene. Moreover, the combination therapy of ZSTK474 with an IGF1R inhibitor exerted a synergistic effect on cancer cells highly expressing IGF1R both in vitro and in vivo. These observations indicated that basal expression of IGF1R in cancer cells confers intrinsic resistance to ZSTK474, and a combination therapy of ZSTK474 with an IGF1R inhibitor would be a promising therapeutic strategy for treating IGF1R‐positive human tumors.

Acquired resistance is a major obstacle for conventional cancer chemotherapy, and also for some of the targeted therapies approved to date. Long‐term treatment using protein tyrosine kinase inhibitors (TKIs), such as gefitinib and imatinib, gives rise to resistant cancer cells carrying a drug‐resistant gatekeeper mutation in the kinase domain of the respective target genes, EGFR and BCR–ABL. As for the phosphatidylinositol 3‐kinase inhibitors (PI3Kis), little is known about their acquired resistance, although some are undergoing clinical trials. To address this issue, we exposed 11 human cancer cell lines to ZSTK474, a PI3Ki we developed previously, for a period of more than 1 year in vitro. Consequently, we established ZSTK474‐resistant cells from four of the 11 cancer cell lines tested. The acquired resistance was not only to ZSTK474 but also to other PI3Kis. None of the PI3Ki‐resistant cells, however, contained any mutation in the kinase domain of the PIK3CA gene. Instead, we found that insulin‐like growth factor 1 receptor (IGF1R) was overexpressed in all four resistant cells. Interestingly, targeted knockdown of IGF1R expression using specific siRNAs or inhibition of IGF1R using IGF1R‐TKIs reversed the acquired PI3Ki resistance. These results suggest that long‐term treatment with PI3Kis may cause acquired resistance, and targeting IGF1R is a promising strategy to overcome the resistance.