RRx-001, an epigenetic-based radio- and chemosensitizer, has vascular normalizing effects on SCCVII and U87 tumors

Abstract

Background: The tumor-specific microregional effects of the anticancer agent RRx-001, a novel epigenetic-based radio/chemosensitizer with nitrogen oxide-donating properties in phase II clinical trials, were investigated with whole tissue section quantitative immunohistological staining in mouse SCCVII and human U87 tumors.

Results: SCCVII tumors exhibited regions of intermittent perfusion exemplified by co-localization of vessels with the hypoxia marker pimonidazole commonly occurring throughout the tissue. A moderate increase in perfusion (21 to 28 %) was observed after a bolus dose of the perivascular stain DiOC7(3), however, with the absence of an increase in tissue oxygenation. U87 tumors showed an absence of blood flow over large areas of treated tumors after dosing with RRx-001. However, these areas did not become necrotic and returned to near normal levels after 12 h. No significant change in tumor hypoxia was seen at 90 min or 12 h. For both tumor types, RRx-001 treatment resulted in the loss of perfusion in the large regions of the tumor; however, at the 12-h time point, both tumor types showed an increase in vessel perfusion but no significant decrease in hypoxia.

Conclusions: These data suggest a redistribution of blood flow within the tumor for both tumor types akin to vascular normalization. Differences between the tumors were related to tumor architecture and distribution of alpha-smooth muscle actin (α-SMA). RRx-001 shows promise for short-term blood flow redistribution in tumors with a pericyte- and α-SMA-rich vasculature. Expression of α-SMA in tumor vasculature could therefore be useful for predicting tumor response to RRx-001.

Keywords: Chemosensitizer; Epigenetic agent; RRx-001; Radiosensitizer; Tumor; Vascular normalization.

Fig. 1

Chemical structure of RRx-001 (1-bromoacetyl-3,3-dinitroazetidine)

Fig. 2

Immunohistochemistry study design. ABDNAZ = RRx-001

Fig. 3

a Effect of RRx-001 (ABDNAZ) treatment on blood flow and tissue hypoxia in SCCVII tumors. b Comparison of RRx-001 (ABDNAZ) effect on tumor architecture, blood flow, hypoxia, and HIF1-α status in SCCVII tumors

Fig. 4

Percent of vessels positive in SCCVII tumors for the perfusion marker DiOC₇(3). Error bars depict ±SD, N = 6 for each group, p < 0.05 between control and 12 h

Fig. 5

Extent of hypoxia by average tissue pimonidazole intensity staining for SCCVII tumors. Error bars depict ±SD, N = 6 for each group (not statistically significant)

Fig. 6

Percent tissue HIF1-α positive in SCCVII tumors. Error bars depict ±SD, N = 6 for each group (p < 0.05 between control and 12 h)

Fig. 7

a Effect of RRx-001 (ABDNAZ) treatment on blood flow and tissue hypoxia in U87 tumors. b Comparison of RRx-001 effect on tumor architecture, blood flow, and hypoxia in U87 tumor

Fig. 8

Percent of vessels positive in U87 tumors for the perfusion marker DiOC₇(3). Error bars depict ± SD, N = 6 for each group (not statistically significant)

Fig. 9

Percent tissue area exhibiting gross vascular shutdown. Regions of shutdown and necrosis were identified as a fraction of whole tissue area. Percent tumor area in U87 tumors that is necrotic (blue square) or unperfused (red square). Error bars depict ±SD, N = 6 for each group (p < 0.05 between control and 90 min for unperfused area, other data not significant)

Fig. 10

Tumor hypoxia in U87 tumors by average tissue pimonidazole intensity staining. Error bars depict ±SD, N = 6 for each group (not statistically significant)