Abstract 5466: Clinically relevant plasma protein binding of RO4929097, a gamma-secretase inhibitor targeting notch signaling
Authors: Wu Jianmei , LoRusso Patricia , Li Jing 
Published: June 2012

Abstract
Purpose. Aberrant activation of the Notch pathway contributes to tumor initiation and/or progression. RO4929097 (RO) is an orally bioavailable, small-molecule inhibitor of γ-secretase, which inhibits Notch signaling in tumor cells. RO is being evaluated in phase I/II clinical trials as monotherapy or in combination with other anticancer agents (e.g., Hedgehog inhibitor GDC-0449). RO exhibits large inter-individual pharmacokinetic (PK) variability, which may contribute to difference in treatment outcome. Unbound drug plasma concentration is believed to be more relevant to pharmacological and toxicological effects than total drug. Thus, it is desirable to determine RO binding in plasma and factors influencing this process. The objective of this study was to investigate RO binding to plasma proteins and blood cells, and to examine the influence of GDC-0449 on RO plasma protein binding and PK in cancer patients.
Method. An equilibrium dialysis method using 96-well microdialysis plates was optimized and validated for determining RO fraction unbound (fu) in human plasma. RO binding in plasma from four different species and in isolated plasma protein solutions, as well as drug partitioning in human blood cells were investigated. The PK of total and unbound RO was evaluated in 5 cancer patients who received daily oral RO 20 mg alone or in combination with daily oral GDC-0449 150 mg.
Results. RO4929097 was extensively bound in human, dog, rat, and mouse plasma, with a mean fu value of 4.7%, 2.1%, 10.6%, and 1.5%, respectively. In human whole blood, the blood-to-plasma concentration ratio was 0.73. In isolated protein solutions, ∼ 88% and 95% of RO4929097 was bound to human serum albumin (HSA, 40 mg/ml) and alpha1-acid glycoprotein (AAG, 1.4 mg/ml), with a binding constant of 1.8 × 104 M−1 and 1.0 × 106 M−1, respectively. The presence of 25 μM of GDC-0449 increased RO fu by 8.4 and 1.1 folds in AAG (1.4 mg/ml) and HSA (40 mg/ml) solution, respectively. In cancer patients, RO fu exhibited a large intra- and inter-individual variability (range, 0.2 – 3.0%; mean, 1.0 ± 0.7%); co-administration of GDC-0449 increased RO fu by 3.7 to 8.5 folds. Co-administration of GDC-0449 significantly decreased the plasma exposure to total RO, but did not have apparent influence on the unbound RO exposure.
Conclusion. RO is highly bound to plasma proteins (mainly to AAG), with a large inter-species and inter-individual variability. Consideration of protein binding is important in the scale-up of RO PK and pharmacodynamic parameters from animal models to humans. Changes in protein binding caused by drug-drug interactions (such as by GDC-0449) or disease states (e.g., increased AAG concentration in cancer patients) can influence the plasma exposure to the total but not unbound RO. Measurement of unbound RO plasma concentration is recommended to avoid misleading conclusions or unadvisedly adjusting RO dose based on total plasma levels.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5466. doi:10.1158/1538-7445.AM2011-5466