Time-dependent vascular regression and permeability changes in established human tumor xenografts induced by an anti-vascular endothelial growth factor/vascular permeability factor antibody

Abstract

The hyperpermeability of tumor vessels to macromolecules, compared with normal vessels, is presumably due to vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) released by neoplastic and/or host cells. In addition, VEGF/VPF is a potent angiogenic factor. Removal of this growth factor may reduce the permeability and inhibit tumor angiogenesis. To test these hypotheses, we transplanted a human glioblastoma (U87), a human colon adenocarcinoma (LS174T), and a human melanoma (P-MEL) into two locations in immunodeficient mice: the cranial window and the dorsal skinfold chamber. The mice bearing vascularized tumors were treated with a bolus (0.2 ml) of either a neutralizing antibody (A4.6.1) (492 micrograms/ml) against VEGF/VPF or PBS (control). We found that tumor vascular permeability to albumin in antibody-treated groups was lower than in the matched controls and that the effect of the antibody was time-dependent and influenced by the mode of injection. Tumor vascular permeability did not respond to i.p. injection of the antibody until 4 days posttreatment. However, the permeability was reduced within 6 h after i.v. injection of the same amount of antibody. In addition to the reduction in vascular permeability, the tumor vessels became smaller in diameter and less tortuous after antibody injections and eventually disappeared from the surface after four consecutive treatments in U87 tumors. These results demonstrate that tumor vascular permeability can be reduced by neutralization of endogenous VEGF/ VPF and suggest that angiogenesis and the maintenance of integrity of tumor vessels require the presence of VEGF/VPF in the tissue microenvironment. The latter finding reveals a new mechanism of tumor vessel regression-i.e., blocking the interactions between VEFG/VPF and endothelial cells or inhibiting VEGF/VPF synthesis in solid tumors causes dramatic reduction in vessel diameter, which may block the passage of blood elements and thus lead to vascular regression.

Figures

Figure 1

Changes in vessel diameter of LS174T tumors transplanted in dorsal skinfold chambers in SCID mice. Vessel diameter was measured at various time points post-i.v. bolus injection of the anti-VEGF/VPF antibody (N = 3) or PBS (N = 3). The values of n in the parentheses indicate the number of vessels used to generate the histogram. These results demonstrate that anti-VEGF/VPF treatment significantly reduces tumor vessel diameter.

Figure 2

Diameter of long vessels (>200 μm) decreased significantly in the antibody-treated (solid symbols) LS174T tumors transplanted in dorsal skinfold chambers in SCID mice, compared with the controls (open symbols) (P < 0.0001). Vessel diameter and length were measured at various time points postbolus i.v. injection of the anti-VEGF antibody (N = 3) or PBS (N = 3).

Figure 3

Photographs showing the reduction of vessel volume in LS174T tumors transplanted in dorsal skinfold chambers after the anti-VEGF/VPF treatment. The photos were taken at various time points both before (day 0; A and D) and after bolus i.p. injections of PBS (B and C) or the anti-VEGF/VPF antibody (E and F) on day 3 (B and E) or 7 (C and F). The injections were given at days 0 and 4, respectively. The size of tumors before treatments was ≈4 mm in diameter. There was a significant decrease in vessel volume of the antibody-treated tumor. Small vessels in this tumor were not visible on days 3 (E) or 7 (F) because of the lower magnification of these images.