The phosphatidylinositol 3-kinase (PI3K) pathway is a major determinant of cell cycling and proliferation. Its deregulation, by activation or transforming mutations of the p110alpha subunit, is associated with the development of many cancers. 2-(1H-Indazol-4-yl)-6-(4-methanesulfonyl-piperazin-1-ylmethyl)-4-morpholin-4-yl-thieno[3,2-d]pyrimidine (GDC-0941) is a novel small molecule inhibitor of PI3K currently being evaluated in the clinic as an anticancer agent. The objectives of these studies were to characterize the relationships between GDC-0941 plasma concentrations and tumor reduction in MCF7.1 breast cancer xenografts and to evaluate the association between the tumor pharmacodynamic biomarker [phosphorylated (p) Akt and phosphorylated proline-rich Akt substrate of 40 kDa (pPRAS40)] responses and antitumor efficacy. MCF7.1 tumor-bearing mice were treated for up to 3 weeks with GDC-0941 at various doses (12.5-200 mg/kg) and dosing schedules (daily to weekly). An indirect response model fitted to tumor growth data indicated that the GDC-0941 plasma concentration required for tumor stasis was approximately 0.3 muM. The relationship between GDC-0941 plasma concentrations and inhibition of pAkt and pPRAS40 in tumor was also investigated after a single oral dose of 12.5, 50, or 150 mg/kg. An indirect response model was fitted to the inhibition of Akt and PRAS40 phosphorylation data and provided IC(50) estimates of 0.36 and 0.29 muM for pAkt and pPRAS40, respectively. The relationship between pAkt inhibition and tumor volume was further explored using an integrated pharmacokinetic biomarker tumor growth model, which showed that a pAkt inhibition of at least 30% was required to achieve stasis after GDC-0941 treatment of the MCF7.1 xenograft.