Background: Plasma and serum biomarkers of angiogenesis and activated endothelial cells were evaluated to assess biological activity of PTK787/ZK 222584 (PTK/ZK), a novel oral angiogenesis inhibitor targeting all known vascular endothelial growth factor (VEGF) receptor tyrosine kinases. Patients and methods: Patients with colorectal cancer (CRC) (n = 63) were enrolled into two phase I/II dose escalation trials of PTK/ZK in 28-day cycles until discontinuation. Patients with stable disease for ≥2 months were categorized as 'non-progressors'. Plasma markers of angiogenesis, VEGF-A and basic fibroblast growth factor (bFGF), and the serum markers of activated endothelial cells, sTIE-2 and sE-Selectin, were assessed at baseline, and pre-dose on days 1, 8, 15, 22 and 28 of every cycle, with additional assessments 10h post-dose on days 1 and 15. The percentage change from baseline was subsequently correlated with AUC and Cmax. of PTK/ZK on day 1, cycle 1 and clinical outcome. Results: A dose-dependent increase in plasma VEGF-A and bFGF was observed in the first cycle of PTK/ZK treatment. The correlation of change in plasma VEGF-A with AUC and Cmax was characterized by an Emax model, suggesting that a change of ≥150% from baseline VEGF-A correlated with non-progressive disease. Change from baseline plasma VEGF-A within the first cycle of treatment was significantly correlated with clinical outcome by logistic regression analysis (P=0.027). Conclusions: In patients with CRC treated with PTK/ZK, changes in plasma VEGF-A and bFGF demonstrate biological activity of PTK/ZK, may help to establish optimal dose and correlate with outcome. © 2005 European Society for Medical Oncology.