Effects of recombinant human endostatin and its synergy with cisplatin on circulating endothelial cells and tumor vascular normalization in A549 xenograft murine model.

Purpose: Endostatin can normalize the tumor vasculature to some extent. However, exact length of its time window and corresponding markers for tumor vascular normalization are needed to be explored.

Methods: The A549 lung adenocarcinoma xenograft murine model was treated with recombinant human endostatin (rh-endostatin) for 14 days. Cisplatin was combined in different schedules. The effects of rh-endostatin on circulating endothelial cells (CECs) by flow cytometry, tumor vasculature and angiogenesis-related factors by confocal microscope and immunohistochemistry, and anti-tumor efficacy of cytotoxic drugs were observed.

Results: The activated CECs (aCECs) were increased on day 7 and decreased on day 10, and apoptotic CECs were increased on day 10. Tumor vasculature was transiently normalized with increased collagen coverage, decreased vessel permeability, intratumoral hypoxia, and microvascular density from day 7 to 10 after rh-endostatin administration. Extracellular matrix metalloproteinase inducer, vascular endothelial growth factor, matrix metalloproteinase (MMP)-2, and MMP-9 were transiently decreased by rh-endostatin from day 4 to 10, whereas the opposite effects were observed with tissue inhibitors of matrix metalloproteinase (TIMP)-1 and TIMP-2. The maximal anti-tumor effects of cisplatin were observed on administration from day 5 to 9 after rh-endostatin initial administration.

Conclusions: Rh-endostatin could transiently normalize tumor vasculature, probably via regulation of both pro- and anti-angiogenesis factors. The synergistic efficacy of anti-angiogenesis and chemotherapy was found during "the normalization window". CEC could be a feasible blood biomarker for defining "vascular normalization window" and providing the evidence to make an optimizing combination therapeutic schedule in human tumor.