CH5132799 (also known as PA-799) is a novel class I PI3K inhibitor, exhibiting potent inhibition of PI3Kα with an IC(50) of 0.014 μM. This compound demonstrates significant antiproliferative effects on human tumor cell lines with an activated PI3K pathway. Administrable orally, CH5132799 shows notable antitumor activity in human cancer xenograft mouse models where the PI3K pathway is activated. It specifically inhibits class I PI3K and PI3Kα mutants, as confirmed by in vitro kinase assays. Tumors harboring the PIK3CA mutation are highly sensitive to CH5132799 in vitro and are significantly regressed in in vivo mouse xenograft models.