Permeability to macromolecular contrast media quantified by dynamic MRI correlates with tumor tissue assays of vascular endothelial growth factor (VEGF)

Abstract

Purpose: To correlate dynamic MRI assays of macromolecular endothelial permeability with microscopic area-density measurements of vascular endothelial growth factor (VEGF) in tumors.

Methods and material: This study compared tumor xenografts from two different human cancer cell lines, MDA-MB-231 tumors (n=5), and MDA-MB-435 (n=8), reported to express respectively higher and lower levels of VEGF. Dynamic MRI was enhanced by a prototype macromolecular contrast medium (MMCM), albumin-(Gd-DTPA)35. Quantitative estimates of tumor microvascular permeability (K(PS); μl/min × 100 cm(3)), obtained using a two-compartment kinetic model, were correlated with immunohistochemical measurements of VEGF in each tumor.

Results: Mean K(PS) was 2.4 times greater in MDA-MB-231 tumors (K(PS)=58 ± 30.9 μl/min × 100 cm(3)) than in MDA-MB-435 tumors (K(PS)=24 ± 8.4 μl/min × 100 cm(3)) (p<0.05). Correspondingly, the area-density of VEGF in MDA-MB-231 tumors was 2.6 times greater (27.3 ± 2.2%, p<0.05) than in MDA-MB-435 cancers (10.5 ± 0.5%, p<0.05). Considering all tumors without regard to cell type, a significant positive correlation (r=0.67, p<0.05) was observed between MRI-estimated endothelial permeability and VEGF immunoreactivity.

Conclusion: Correlation of MRI assays of endothelial permeability to a MMCM and VEGF immunoreactivity of tumors support the hypothesis that VEGF is a major contributor to increased macromolecular permeability in cancers. When applied clinically, the MMCM-enhanced MRI approach could help to optimize the appropriate application of VEGF-inhibiting therapy on an individual patient basis.

Figure 1

Schematic structure of albumin-(Gd-DTPA)₃₅. The belt-like random coil represents the human albumin core structure with amino-containing side chains of the lysine residues (small sticks) to which Gd-DTPA (grey ovals) are covalently attached. The actual molecular weight of this macromolecule is 92 kDa, while its effective molecular weight is approximately 180 kDa with a measured hydrodynamic diameter of approximately 6 nanometers.

Figure 2

Two-compartment kinetic model used to describe the kinetics of contrast media transport from the plasma space into the interstitial fluid. The endothelial transfer coefficient K^PS [μL/(min · 100 cm³)] denotes the clearance of contrast medium from plasma to interstitial water. Potential reflux of contrast media molecules from interstitial fluid back to plasma, denoted as the rate constant k (min⁻¹), was not resolvable in the short, one hour time course of these experiments and was therefore set to zero. The box around the plasma compartment denotes a forcing function representing the mono-exponential disappearance of MMCM from the blood. The combined kinetics of both compartments, when considered together, reflect the dynamic response of the entire tissue/tumor to contrast medium enhancement.

Figure 3

Representative T1-weighted spoiled gradient refocused (SPGR) images, precontrast and at 5 min, 15 min, 30 min and 55 min post injection of albumin-(Gd-DTPA)₃₅ which were applied for the calculation of the endothelial transfer coefficient (K^PS). Note the moderate tumor enhancement, most prominent in the rim in the MDA-MB-435 tumor (a) and in the MDA-MB-231 (b) tumor. The blood enhancement seen clearly in the inferior vena cava (white arrow) and nearby hepatic vein, persisted over the 55-minute course of data acquisition. The highly vascularized liver enhances substantially.

Figure 3

Representative T1-weighted spoiled gradient refocused (SPGR) images, precontrast and at 5 min, 15 min, 30 min and 55 min post injection of albumin-(Gd-DTPA)₃₅ which were applied for the calculation of the endothelial transfer coefficient (K^PS). Note the moderate tumor enhancement, most prominent in the rim in the MDA-MB-435 tumor (a) and in the MDA-MB-231 (b) tumor. The blood enhancement seen clearly in the inferior vena cava (white arrow) and nearby hepatic vein, persisted over the 55-minute course of data acquisition. The highly vascularized liver enhances substantially.

Figure 4

Representative fit (solid lines) of model to delta R1 data from blood (△) and tumor (○) using this MMCM-enhanced protocol. Notice the trend toward convergence of the blood and tumor plots on this semi-logarithmic graph indicative of a gradual leak of the MMCM from the intravascular space into the interstitial compartment of this tumor; the more the convergence, the larger is the leak. No convergence, i.e., parallel, gradually decreasing behavior on this semi-logarithmic graph implies the lack of a leak.

Figure 5

Confocal microscopic images of representative MDA-MB-435 (left) and MDA-MB-231 (right) tumors, specifically-stained for VEGF (red) with blood vessels shown as green/yellow. Note the relatively lower level of red-staining VEGF in MDA-MB-435 tumor and the much higher expression of VEGF in MDA-MB-231 tumor. White scale bar: 120 μm.

Figure 6

Correlation of VEGF area-density measurements with K^PS determinations based upon 13 tumors. MDA-MB-435 tumors are depicted by triangles and MDA-MB-231 tumors by squares. Solid line denotes the best fit linear correlation line. The correlation (r=0.67) is statistically significant (p<0.05), with some overlap in the K^PS determinations from the two tumor groups.