VAX-IP MINICELLS, A NOVEL TREATMENT, DELAY TUMOR FORMATION IN MOUSE MODEL OF COLON CANCER

Engineered bacterial minicells (VAX-IP) are non-replicating, nano-sized bacteria that are capable of targeting specific tumor cell-surface proteins and delivering a membrane pore-forming toxin that rapidly kills the cells. Minicell-based therapeutics are highly effective at tumor stabilization and regression in some animal models, but no study on colorectal cancer (CRC) has been reported. In this study, a colon cancer mouse model is being used to evaluate the therapeutic potential of VAX-IP in CRC. By conditionally-deleting the tumor suppressor Apc gene only in colon, tumors develop but leave the immune system intact. VAX-IP treatment of mice (1.5x109 cells/ml, 1 dose/week) from 8-13 weeks of age, a time when lesions are only in the earliest stages, significantly decreases tumor number in mice harvested at 6 months of age (p=0.04). Similarly, when animals were treated later during the lesion development stage (age 14-19 weeks), a more significant decrease in tumor number (p=0.007) was observed. Studies have shown that inflammatory mediators significantly contribute to tumor progression in CRC. To study whether VAX-IP might reduce tumor load by modulating the colonic environment and mucosal immunity, Chloracetate Esterase (CAE) stains for inflammatory granulocytes and mast cells were used. Numbers of CAE+ cells in normal or tumor-adjacent tissue were significantly lower in VAX-IP-treated mice in comparison to PBS-treated controls (p=0.0002). Immunohistochemistry staining for inflammatory, myeloid-lineage cells using the marker CD11b also showed significantly decreased numbers in the VAX-IP animals treated at 14-19 weeks of age (p=0.003) but not in 8-13 weeks (p=0.4) or parental control (p=0.6) groups. Because macrophages also express CD11b, the F4/80 marker was analyzed on these mouse tissues, but no significant differences were found. The three stains together suggest the difference in CD11b and CAE positive cells is in granulocytic cells and a granulocyte-specific marker Ly6G is being used to confirm this hypothesis. Together, these data strongly support the conclusion that VAX-IP don’t affect the colon immune environment in the absence of tumors but when tumors are present, VAX-IP reduce inflammatory granulocytic infiltration into the colonic environment, which could contribute to the reduced tumor load in this mouse model of CRC. Tumor cell proliferation and apoptosis assays are being used to evaluate the direct killing ability of VAX-IP on colon tumor cells, to determine if this also contributes to reduced tumor load in this mouse model of CRC.