A moderately decreased expression of the proliferation marker Ki-67 was detected check details in tumor xenografts of mice challenged with CysLT1R antagonists after tumor appearance compared to tumor xenografts of mice challenged with DMSO. At the experimental endpoint, we were unable to detect any statistically significant changes in angiogenesis as determined by immunostaining of CD31. Animals receiving HCT-116 colon cancer cells pretreated with ZM198,615 before transplantation exhibited tumors of markedly reduced volume and weight. This finding could implicate an important role of CysLT1R in the initiation stage of colon cancer. The fact that mice receiving HCT-116 colon cancer cells pretreated with Montelukast did not exhibit any tumors could be due to the potency of this drug.

Reported drug potencies, which are the half-maximal inhibitory concentrations for ZM198.615 and Montelukast in terms of inhibiting the binding of [3H]-LTD4 to guinea lung membranes, are 2.66 nM and 0.64 nM, respectively [40], [41]. The ZM198,615 pre-treated group exhibited tumors with moderately decreased proliferative ability compared to DMSO-treated animals. In contrast to animals receiving ZM198,615 treatment first after tumor appearance, animals from the ZM198,615 pretreated group had significantly smaller vessel formation in their tumors compared to DMSO-treated animals, as determined by CD31 immunostaining. In addition to a significant decrease in vascular size in xenograft tumors of animals transplanted with ZM198,615 pretreated cells, a significant decrease in the expression levels of VEGF was also detected by immunoblotting.

A significant decrease in VEGF expression was also observed in tumor samples from the Montelukast-treated group compared to DMSO II tumor samples. Thus, we postulate that CysLT1R antagonists impair angiogenesis Drug_discovery in colon cancer xenografts, inhibiting their growth. The effects of CysLTs on vascular responses, which are related to vascular permeability and subsequent plasma extravasation, are mediated via CysLT1R [42]. CysLT1R antagonists Pranlukast and Montelukast have been shown to reduce vascular permeability by regulating VEGF expression in the lungs of mice with allergen-induced asthma [43]. These antagonists have also been shown to inhibit tumor metastasis in a Lewis lung carcinoma metastasis model by inhibiting the permeability of peripheral capillaries [44]. Interestingly, Montelukast has also been shown to reduce LTD4-induced migration of the endothelial cell line EA.hy926 mediated by CysLT1Rs via the Erk1/2 pathway [45]. Both proliferation and migration of endothelial cells are needed to form new vessels.