Targeting the Angiopoietin-2/Tie-2 axis in conjunction with VEGF signal interference

Abstract

Anti-angiogenic therapies target the tumor vasculature, impairing its development and growth. It was hypothesized over 40 years ago by the late Judah Folkman and Julie Denekamp that depriving a tumor of oxygen and nutrients, by targeting the tumor vasculature, could have therapeutic benefits. Identification of growth factors and signaling pathways important in angiogenesis subsequently led to the development of a series of anti-angiogenic agents that over the past decade have become part of the standard of care in several disease settings. Unfortunately not all patients respond to the currently available anti-angiogenic therapies while others become resistant to these agents following prolonged exposure. Identification of new pathways that may drive angiogenesis led to the development of second-generation anti-angiogenic agents such as those targeting the Ang-2/Tie2 axis. Recently, it has become clear that combination of first and second generation agents targeting the blood vessel network can lead to outcomes superior to those using either agent alone. The present review focuses on the current status of VEGF and Ang-2 targeted agents and the potential utility of using them in combination to impair tumor angiogenesis.

Keywords: Ang-2 targeting; Angiogenesis; Anti-angiogenic therapy; Combination therapy; VEGF targeting.

Fig. 1

The Angiopoietin/Tie2 and VEGF/VEGFR signaling axis in angiogenesis. (A) Ang-1/Tie2 interaction maintains normal-quiescent vasculature with tight endothelial– endothelial and endothelial–smooth muscle cell/pericyte contacts as demonstrated by an in vitro, unstimulated, co-culture endothelial and smooth muscle cell sphere and in vivo normal vasculature with smooth muscle cell coverage. (B) Ang-2/Tie2 interaction with Tie2 homodimers or heterodimers results in loss of endothelial–endothelial and endothelial–smooth muscle cell contacts (i.e. destabilization of the vasculature) as demonstrated by an in vitro, PMA (stimulant of Ang-2 secretion) stimulated, co-culture sphere and in vivo vasculature from tumor tissue with loss of smooth muscle cell coverage. (C) VEGF/VEGFR-2 interaction stimulates endothelial cell survival, proliferation and sprouting of new vessels. Endothelial cell sprouting is illustrated by the collagen embedded 3D HUVEC spheres (image taken at 24 hrs) and tube formation by matrigel plated HUVEC (image taken at 12 hrs). EC: endothelial cell (HUVEC); VSMC: vascular smooth muscle cell (HUASMC); MECA: endothelial cell marker; NG2: peri-endothelial cell marker (ex. smooth muscle cell, pericyte); DAPI: nuclear marker (modified from Microvasc. Res. 2012: 83 (3): 290–297. Molnar N. and Siemann D.W., fig. 3D [22]; and Biel N.M. and Siemann D.W., unpublished results).

Fig. 2

VEGF axis targeting in angiogenesis. Interference of the VEGF/VEGFR-2 signaling axis leads to inhibition of human umbilical vein endothelial cell (HUVEC) proliferation. (A) Transwell migration at 24 hrs (B) and tube formation on matrigel at 24 hrs ± VEGFi (Cediranib/AZD2171, AstraZeneca) (C) (Biel N.M. and Siemann D.W., unpublished results).

Fig. 3

Ang-2 axis targeting in angiogenesis. Interference with Ang-2/Tie2 interaction inhibits the Ang-2 dependent loss of endothelial and smooth muscle cell contacts. Red: human umbilical vein endothelial cells (HUVEC); Green: human umbilical artery smooth muscle cells (HUASMC); Ang2i: MEDI3617 (MedImmune) (modified from Microvasc. Res. 2012: 83 (3): 290–297. Molnar N. and Siemann D.W., fig. 2B [22]). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

Combination of Ang-2 and VEGF targeting in vivo. Both Ang-2 and VEGF targeting agents inhibit tumor angiogenesis and reduce tumor growth but the combination of these therapies results in greater anti-angiogenic and anti-tumor effects. Ang2i: MEDI3617; VEGFi: Sunitinib (modified from J. Cancer Ther. 2013: 4: 1–6. Molnar N. and Siemann D.W., fig. 2A [115]).