Purpose: To determine the maximum tolerated dose (MTD), safety, pharmacokinetics, pharmacodynamics, and preliminary evidence of antitumor activity of the PI3K/mTOR inhibitor PF-04691502, administered orally once daily.
Methods: Escalating doses of PF-04691502 were administered to 23 patients with advanced solid tumors in sequential cohorts across the following dose levels: 2 mg, 4 mg, 8 mg, and 11 mg. 14 additional patients were enrolled in an expansion cohort at the MTD to ensure at least five matched pre- and post-treatment biopsies for biomarkers of PI3K activity.
Results: The MTD of PF-04691502 was 8 mg orally once daily. There were three dose-limiting toxicities: one grade 3 fatigue at 8 mg, one grade 3 rash at 11 mg, and one intolerable grade 2 fatigue at 11 mg. Among 37 patients enrolled, treatment-related adverse events included fatigue, decreased appetite, nausea, hyperglycemia, rash, and vomiting. Across all dose levels, average steady-state plasma PF-04691502 concentrations approximated or exceeded the target concentration of 16.2 ng/mL required for ≥75 % tumor growth inhibition in preclinical models. PF-04691502 resulted in increased mean fasting serum glucose, insulin, and c-peptide levels, and produced partial blockade of PI3K signalling in five paired tumor biopsies, as demonstrated by reductions in phosphorylated Akt, FKHR/FKHRL1, and STAT3. No objective anti-tumor responses were observed.
Conclusions: Daily oral administration of PF-04691502 was tolerable at 8 mg orally once daily, with a safety profile similar to other PI3K/mTOR inhibitors. PF-04691502 demonstrated PI3K pathway inhibition by changing glucose homeostasis, and by decreasing phosphorylation of downstream molecules in tumor tissue.