Circulating tumor and endothelial cells as pharmacodynamic biomarkers in a phase I clinical trial of intravenous bevacizumab in combination with escalating doses of oral cediranib for patients with advanced malignancies.

3525 Background: Rare circulating tumor cells (CTCs) and endothelial cells (CECs) offer a feasible approach for studying the pharmacodynamic effects of investigational agents. We investigated the effects of bevacizumab (B) and cediranib (C) on inhibition of the VEGFR pathway and correlated these changes with dose and clinical response. METHODS Peripheral blood was obtained at baseline, 24hrs and at C2D26-30 post-treatment from patients (n=14) undergoing dose escalation of intravenous B and oral C. CTCs and CECs (CD31+ or CD105+) were isolated and immunofluorescently stained. Laser scanning cytometry (LSC) was used to quantify phosphorylated and total-VEGFR2 (pVEGFR2/VEGFR2), pERK/ERK, and apoptosis in each phenotype. Changes in each biomarker were correlated with partial response (PR) or stable disease and progression > 2 months, evaluated using RECIST. RESULTS Overall, immature CECs (CD105+) enumerated by CellSearch™ revealed a dose-dependent significant decrease (p=0.0001). A 3-fold induction in apoptosis was observed at 24 hrs compared to baseline in the CD105+ CECs. Mature CD31+ cells assessed for VEGFR2 activity revealed an 83% and 1.9% significant (p=0.019) inhibition in pVEGFR2 expression at low (B;3mg/kg) and high (B;5mg/kg) doses, respectively. In the non-responders, mature CECs revealed a dose-dependent significant increase (-6.8% to 63%;p=0.031) in pERK/ERK expression levels. No significant changes were observed in CTC enumeration by CellSearch™. LSC-mediated CTC enumeration revealed a 4.77 % and 2.33% increase in CTCs following treatment in the non- responders and responder (p=0.809), respectively. Analysis of pVEGFR2 in CTCs revealed a 58% inhibition in the responder versus a 163% increase in expression in the non-responders (p=0.63). CONCLUSIONS Inhibition of pVEGFR2 and induction of apoptosis in CECs confirmed the target therapy. An increase in CD105+ CECs is consistent with the hypothesis that anti-angiogenic efficacy induces endothelial cell shedding. Assessment of CECs indicates that B and C are more biologically active at lower doses, and resistance may be attributed to ERK activity. Support: UO1 CA062461 (RK) No significant financial relationships to disclose.