MRI-based characterization of vascular disruption by 5,6-dimethylxanthenone-acetic acid in gliomas

Abstract

The well-vascularized nature of gliomas has generated a lot of interest in antiangiogenic therapies. However, the potential of vascular disrupting agents (VDAs) against gliomas has not been investigated extensively. In this study, we examined the in vivo efficacy of the tumor-VDA 5,6-dimethylxanthenone-4-acetic acid (DMXAA) against gliomas. Contrast-enhanced magnetic resonance imaging (MRI) and diffusion-weighted MRI were used to characterize the vascular and cellular responses of GL261 and U87 gliomas to DMXAA treatment. Therapeutic efficacy was assessed by Kaplan-Meier survival analysis. Before VDA treatment, minimal enhancement was detected within the tumor in both models. Longitudinal relaxation rate (R1=1/T1) maps acquired 24 h after treatment showed marked extravasation and accumulation of the contrast agent in the tumor indicative of treatment-induced vascular disruption. Normalized change in relaxation rate (DeltaR1) values of the tumor showed a significant increase (P<0.01 GL261; P<0.05 U87) after therapy compared with baseline estimates. Mean apparent diffusion coefficient (ADC) values were significantly increased (P=0.015) 72 h after therapy in GL261 but not in U87 gliomas. Vascular disrupting agent therapy resulted in a significant (P<0.01) increase in median survival in both models evaluated. The results highlight the potential of VDAs against gliomas and the utility of MRI in the assessment of glioma response to VDA therapy.

Figure 1

(A) Schematic outline of the study design. Approximately 3 weeks post intracranial implantation of GL261 and U87 cells, tumor-bearing mice were administered a single dose of the VDA DMXAA as described in the Materials and Methods section. CE-MRI was performed prior to (preRx) and 24 hours following treatment to monitor the vascular response of gliomas to DMXAA treatment. Additionally, DW-MRI was performed 72 hours post treatment to examine changes in cellularity following VDA therapy. Long-term survival analysis was performed by observing animals in the control and treatment groups over a 40-day period. (B) T2W MRI of tumor growth. T2-weighted (T2W) MR images of a control C57Bl6 mouse bearing a GL261 glioma and a nude mouse bearing U87 glioma are shown. Tumors appeared as well-defined hyperintense regions at the site of injection as indicated by the arrows.

Figure 2. Contrast-enhanced MRI of Response of GL261 gliomas to VDA therapy

(A) T2W images and corresponding pseudo-colorized R1 maps of a C57Bl6 mouse brain bearing GL261 glioma before treatment and 24 hours post DMXAA treatment. R1 maps calculated from the first (1) and last (7) post contrast scans are shown at both time points. Compared to baseline images, marked extravasation of the contrast agent was observed in post contrast R1 maps indicative of tumor vascular disruption following treatment. (B) Bar graph represents normalized mean ΔR1 values (tumor/muscle) expressed as Gd concentration in GL261 gliomas at baseline and 24 hours post treatment. A statistically significant increase (**p<0.01) 24 hours post VDA therapy was observed compared to baseline estimates as a consequence of DMXAA-induced tumor vascular disruption.

Figure 3. Vascular response of U87 gliomas to DMXAA

(A) Post contrast R1 maps of a nude mouse brain bearing U87 glioma before and after DMXAA treatment showing evidence of vascular disruption by DMXAA. R1 maps were calculated on a pixel-by-pixel basis from the first (1) and last (7) post contrast T1W scans. Corresponding T2W images are also shown. Arrows point to location of the tumor. (B) Concentration of Gd as estimated from normalized mean ΔR1 values (tumor/muscle) of U87 gliomas showing a statistically significant increase (*p<0.05) 24 hours post VDA therapy compared to baseline values.

Figure 4. DW-MRI of GL261 response to DMXAA

(A) Pseudo-colorized ADC maps of a C57Bl6 mouse bearing GL261 glioma overlaid on anatomical T2W scans at baseline and 72 hours post treatment. Enlarged region of interest (ROI, tumor) is also shown. (B) Mean ADC values of the tumor showed a statistically significant increase at 72 hours post treatment compared to baseline values in all 3 animals (p=0.015).

Figure 5. Therapeutic efficacy of DMXAA against gliomas

Kaplan-Meier survival analysis of GL261 (A) and U87 glioma-bearing animals (B) treated with a single dose of DMXAA (30 mg/kg – GL261; 27.5 mg/kg – U87). Differences in median survival between animals in the control and treatment groups were analyzed for statistical significance using the log-rank test. Control vs treatment groups: p=0.0028 (GL261; Controls n=12; DMXAA n=6) and p=0.0005 (U87; Controls n=13; DMXAA n=10).