Quantifying antivascular effects of monoclonal antibodies to vascular endothelial growth factor: insights from imaging

Abstract

Purpose: Little is known concerning the onset, duration, and magnitude of direct therapeutic effects of anti-vascular endothelial growth factor (VEGF) therapies. Such knowledge would help guide the rational development of targeted therapeutics from bench to bedside and optimize use of imaging technologies that quantify tumor function in early-phase clinical trials.

Experimental design: Preclinical studies were done using ex vivo microcomputed tomography and in vivo ultrasound imaging to characterize tumor vasculature in a human HM-7 colorectal xenograft model treated with the anti-VEGF antibody G6-31. Clinical evaluation was by quantitative magnetic resonance imaging in 10 patients with metastatic colorectal cancer treated with bevacizumab.

Results: Microcomputed tomography experiments showed reduction in perfused vessels within 24 to 48 h of G6-31 drug administration (P <or= 0.005). Ultrasound imaging confirmed reduced tumor blood volume within the same time frame (P = 0.048). Consistent with the preclinical results, reductions in enhancing fraction and fractional plasma volume were detected in patient colorectal cancer metastases within 48 h after a single dose of bevacizumab that persisted throughout one cycle of therapy. These effects were followed by resolution of edema (P = 0.0023) and tumor shrinkage in 9 of 26 tumors at day 12.

Conclusion: These data suggest that VEGF-specific inhibition induces rapid structural and functional effects with downstream significant antitumor activity within one cycle of therapy. This finding has important implications for the design of early-phase clinical trials that incorporate physiologic imaging. The study shows how animal data help interpret clinical imaging data, an important step toward the validation of image biomarkers of tumor structure and function.

Figure 1. Ex-vivo evidence for rapid anti-vascular effects of G6-31

(A) Change in vascular density (VV/TV), as measured by micro-CT at 90 min, 24 hours and 48 hours in control animals and those treated with the anti-VEGF antibody G6-31. (B) Representative micro-CT angiographic data for each treatment group at 90 min and at 24 and 48 hours. Extracted vascular network (red) and entire tumor (gray) are shown. (C) Histological sections stained with MECA-32 for corresponding time points. Note that the brown intravascular pigment is micro-CT lead chromate contrast agent, which only partially fills vessel lumens after histological processing. Vessel area was measured by assessing DAB-stained endothelium at the vessel perimeter. See supplemental Figure 1 for the corresponding segmented images.

Figure 2. In-vivo evidence for reduction in relative blood volume following G6-31

(A) Percentage change in relative blood volume (rBV) and relative blood flow (rBF) in control and treated groups after 48 hours of anti-VEGF treatment. (B) Representative ultrasound perfusion blood volume maps for each treatment group pre-treatment and at 48 hrs post-treatment.

Figure 3. In vivo evidence of the temporal evolution of anti-vascular effects of bevacizumab

Measurements of enhancing fraction (E_F), mean blood plasma volume (v_p) and median volume transfer constant (K^trans) in 26 tumors; error bars indicating the 95% confidence intervals for point estimates of log drops.

Figure 4. Representative MRI parameter maps from one patient

(A) Map of enhancing voxels shows a significant decrease beginning at 48 hours that persists through to day 12. Note that the loss of central enhancement. (B) Maps of fractional blood plasma volume (v_p) also shows a significant decrease, here beginning at 4 hours that persists through to day 12. (C) Maps of the volume transfer constant (K^trans) show reduction in parameter within 4 hours that persist at 48 hours and day 8, but return to baseline levels by day 12. These changes are consistent with reduction in either vessel permeability and/or blood flow. (D) Maps of the longitudinal relaxation time (T₁) measured prior to contrast agent administration show a clear progressive reduction in T₁, a measurement of tumor edema. (E) T₂-weighted anatomical image without contrast (arbitrary signal intensity units). All parameter maps and images were obtained at baseline twice (Base 1 and Base 2) and at 4 hours, 48 hours, 8 days and 12 days after bevacizumab administration. Tumors region of interest are outlined in red and show gradual reduction in tumor size.

Figure 5. Evidence for reduction in tumor size with bevacizumab mono-therapy

(A) Temporal evolution of tumor volume reduction and change in longitudinal relaxation time (T₁) from baseline through to day 12 in 26 tumors. (B) Waterfall plots demonstrate percentage reduction in tumor volume from baseline size at day 12. The dotted lines denote lesions that changed in size by greater than the lower and upper limits of 95% of serial parameter ratios.