Quantitative Analysis of Biomarkers Defines an Optimal Biological Dose for Recombinant Human Endostatin in Primary Human Tumors

Purpose: In a recent study, we presented preliminary evidence for biological activity in a Phase I dose-finding study (15-600 mg/m²) of recombinant human endostatin in patients with refractory solid tumors. Here, we conducted additional biomarker analyses to correlate changes in tumor biology with dose. Experimental Design: Excisional tumor biopsies were obtained at baseline and after 56 days of endostatin therapy. Laser scanning cytometry (LSC) was used to quantify biomarker levels in whole tissue sections. Apoptosis in tumor cells (TCs) and tumor-associated endothelial cells (ECs) was quantified by fluorescent three-color anti-CD31/terminal deoxynucleotidyl transferase-mediated nick end labeling staining. Microvessel densities were measured by LSC-guided vessel contouring. Levels of tumor-associated EC BCL-2 and hypoxia-inducible factor 1α were determined by immunofluorescence and LSC quantification. The results, including tumor blood flow measured by positron emission tomography, were analyzed using a quadratic polynomial model. Results: Significant increases in EC death and decreases in tumor microvessel density were observed, with maximal effects of endostatin at a dose of 249 mg/m² (95% confidence interval, 159-338) and 257 mg/m² (95% confidence interval, 183-331), respectively. In contrast, levels of TC death were uniformly low and did not correlate with endostatin dose. Maximal nuclear hypoxia-inducible factor 1α and minimal EC Bcl-2 levels were observed at ∼250 mg/m², although the changes did not reach statistical significance. Conclusions: The data suggest that endostatin had optimal biological activity at doses ∼250 mg/m² in our cohort of patients. Endostatin's failure to induce high levels of TC death may explain its lack of significant clinical activity in this Phase I trial.