Mechanistic Study on the Antitumor Potential of the Endocannabinoid Reuptake Inhibitor OMDM-2

Abstract

Introduction: Endocannabinoid signalling has been shown to be enhanced in several cancer tissues. Previous studies propose that this up-regulation contributes to the inhibition of the cancer cell proliferation by affecting different hallmarks of cancer. However, this anti-proliferative activity is suppressed by the rapid uptake and metabolism of endocannabinoids. An inhibition of endocannabinoid metabolism at the site of tumor growth might prolong their selective antitumor activity and avoid the side effects of direct cannabinoid agonists. The present study was undertaken to investigate the anti-tumor activity of OMDM-2, an endocannabinoid reuptake inhibitor, <em>in-vivo</em> and<em> in-vitro</em>. The molecular mechanisms involved in this activity were investigated. In addition we investigated the possible optimization of the treatment by a combination of OMDM-2 with natural or benchmark chemotherapeutics. <br /> Methods: Ehrlich ascites carcinoma bearing mice were used for the <em>in-vivo</em> experiments (n= 287). In these experiments we evaluated the effect of a systemic administration of OMDM-2 on tumor volume, mean survival time and increase in the life span. Time course effects of OMDM-2 on tumor weights, serum transforming growth factor beta-1, and the intratumoral expression of the endoglin receptor were observed. Also the effect of OMDM-2 on angiogenesis and hematological parameters were investigated. <br /> The <em>in-vitro</em> anti-proliferative activity of OMDM-2 was analyzed against human breast cancer (MCF-7) and glioma cells (U-87). The involvement of the CB-1 and vanilloid (TPRV1) receptors were evaluated by applying adequate pharmacological inhibitors. The type of cell death was investigated by ELISA, DNA electrophoresis, and morphological observations. Signaling pathways were analyzed by western blots for the MAP-kinases ERK 1 and 2 as well as for AKT. The combination of OMDM-2 with curcumin or temozolomide (TMZ) or paclitaxel was evaluated by isobole, combination index, sensitization factor, and dose reduction index methods.<br /> Results: OMDM-2 showed in our <em>in-vivo</em> study promising anticancer and antiangiogenic activities. Both activities were not mediated by the CB-1 receptor. The TGF-B₁/endoglin system appears to be involved in the mediation of the anti-tumor activity of OMDM-2 at early stages of tumor development. OMDM-2 showed significant anti-proliferative activity against both - MCF-7 and U-87 <em>in-vitro</em>. The receptors, the mechanisms of action, and the signaling pathways involved in this activity differ according to the type of cancer. Co-incubation of curcumin with OMDM-2 showed potent synergistic activity against MCF-7 while this drug combination may be synergistic or antagonistic against U-87 according to the ratio. Combinations of OMDM-2 with TMZ or paclitaxel were able to decrease their individual doses and to increase the sensitivity of cancer cells. <br /> Conclusions: We provided for the first time data on the possible use of OMDM-2 as an anti-cancer drug to be an alternative to direct cannabinoid receptor agonists to avoid their psychotropic and immunosuppressive side effects. In particular, combination therapies utilizing both - molecules targeting the endocannabinoid system and natural anticancer agents or benchmark chemotherapeutic agents- are a worthwhile option to be further systematically explored in the treatment of cancer.