Imaging biomarkers to monitor response to the hypoxia-activated prodrug TH-302 in the MiaPaCa2 flank xenograft model

Abstract

TH-302, a hypoxia-activated anticancer prodrug, was evaluated for antitumor activity and changes in dynamic contrast-enhanced (DCE) and diffusion-weighted (DW) magnetic resonance imaging (MRI) in a mouse model of pancreatic cancer. TH-302 monotherapy resulted in a significant delay in tumor growth compared to vehicle-treated controls. TH-302 treatment was also associated with a significant decrease in the volume transfer constant (K(trans)) compared to vehicle-treated controls 1 day following the first dose measured using DCE-MRI. This early decrease in K(trans) following the first dose as measured is consistent with selective killing of the hypoxic fraction of cells which are associated with enhanced expression of hypoxia inducible transcription factor-1 alpha that regulates expression of permeability and perfusion factors including vascular endothelial growth factor-A. No changes were observed in DW-MRI following treatment with TH-302, which may indicate that this technique is not sensitive enough to detect changes in small hypoxic fractions of the tumor targeted by TH-302. These results suggest that changes in tumor permeability and/or perfusion may be an early imaging biomarker for response to TH-302 therapy.

Figure 1

Immunohistochemical staining for: A) hematoxylin and eosin (H&E), B) Carbonic Anhydrase IX (CA IX), C) Vascular endothelial growth factor A (VEGF-A), and D) pimonidazole adducts, which is a biomarker for hypoxia, in MiaPaCa2 xenograft tumor tissue.

Figure 2

Effect of treatment with TH-302 on MiaPaCa2 flank xenograft tumor model. The errors bars show the standard error of the mean (Control, N=10 and TH-302, N=9).

Figure 3

The effect of TH-302 therapy on K^trans measured with DCE-MRI. A) An anatomical image shows the location of the tumor. Dark bands below the tumor were caused by excitation of orthogonal slices that imaged the renal artery. B) After injecting the agent, a strong change in MRI signal was observed in the tumor. C) The Arterial Input Function (AIF) from the femoral artery showed less variability than the renal AIF. D) The parametric map of K^trans values showed good spatial homogeneity.

Figure 4. The effect of treatment on vascular permeability (K^trans)

The average K^trans of the tumor was measured with Dynamic Contrast Enhancement MRI. Error bars represent the standard deviation of cohorts of 5 to 7 mice. Statistically significant differences relative to the same cohort at Day -1 are indicated by asterisks (*, p< 0.05; **, p< 0.01). A) The median value of K^trans and B) the relative changes in median K^trans values showed that TH-302 caused a significant decrease in K^trans relative to vehicle after one day of treatment, but not after eight days of treatment.

Figure 5

The effect of TH-302 therapy on ADC measured with DW-MRI. A) The parametric map of ADC values showed good spatial homogeneity, especially relative to the torso. The anatomical image that corresponds to this ADC map is shown in Figure 2A. B) The median ADC did not change following TH-302 therapy. Error bars represent the standard deviation of each cohort. All median ADC values were statistically equivalent (p> 0.05).