ZSTK474, a specific phosphatidylinositol 3-kinase inhibitor, induces G1 arrest of the cell cycle in vivo.

Phosphatidylinositol 3-kinase (PI3K) is regarded as a promising therapeutic target because it is often activated in cancer. We previously reported that ZSTK474, a specific PI3K inhibitor, inhibits tumour cell proliferation via G1 arrest of the cell cycle without inducing apoptosis in vitro. However, it remained unclear whether ZSTK474 induces G1 arrest to exert antitumour efficacy in vivo. We recently developed a live imaging system, named Fluorescent Ubiquitination-based Cell Cycle Indicator (Fucci), to visualise cell cycle distribution. Here, by using this system, we tested whether ZSTK474 induces G1 arrest in tumour cells in vivo, as well as in vitro. Fucci-introduced human breast cancer MCF-7 cells and cervical cancer HeLa cells were subcutaneously xenografted in nude mice. ZSTK474 was administered to the tumour-bearing mice for 5 days, and the cell cycle distribution in the xenografted tumours were analysed by monitoring fluorescence in live mice. We demonstrate that ZSTK474 induces G1arrest along with tumour suppression in vivo. Moreover, we show that ZSTK474 suppresses the tumour growth without inducing apoptosis. Interestingly, such increase in G1 cells and tumour suppression was maintained during long-term (3-month) administration of ZSTK474. These results suggest that ZSTK474 exerts its in vivo antitumour efficacy via G1 arrest but not via apoptosis as long as it is administered, and could be used for months as maintenance therapy for patients with advanced cancers.