Growth reduction of a xenotransplanted human soft tissue sarcoma by MDM2 antisense therapy via implanted osmotic minipumps

The MDM2 oncogene plays an important role in tumorigenesis and especially in soft tissue sarcomas (STS). Overexpression of the MDM2 protein is associated with a poorer prognosis for STS patients. An MDM2 antisense approach to reduce MDM2 protein levels has been successfully applied on several carcinomas in vitro and used on a few in vivo cases. However, antisense treatment not only resulted in an MDM2 protein reduction but also in a wild-type P53 gene (wt-P53) mediated tumor growth suppression due to its genetic wt-P53 status. In this study, we used a clinically relevant xenotransplanted STS model with a mutated P53 gene (mt-P53) in order to exclude the influence of wt-P53. The human STSs were surgically implanted and one week later osmotic pumps were implanted intraperitoneally into nude rats releasing MDM2-antisense ODNs (AS ODNs) continuously for one week. As controls MDM2-sense ODN (SE ODN) or a 0.9% NaCl solution (saline solution) were administered. After one week animals treated with MDM2-AS ODN (100 or 200 μg) showed a reduction in tumor mass in comparison to animals treated with MDM2-SE ODN. The reduction in tumor mass was significant in animals treated with MDM2-AS ODNs in comparison to the saline solution treated ones (p=0.018 or p=0.007). Furthermore, a significant reduction in macroscopically visible tumor number with MDM2-AS ODN treatments (100 or 200 μg) in comparison to MDM2-SE ODN or saline solution treatment (both p=0.009) was observed for the first time. As expected a reduction in MDM2 protein expression in the MDM2-AS ODN treated tumors when compared to the MDM2-SE ODN or saline solution treated tumors was detected in Western blot analyses and immunohistochemically. In addition, an unexpected reduction in mt-P53 protein expression after AS ODN therapy was also observed. In short, we have demonstrated in vivo for the first time that MDM2-AS ODN treatment of xenotransplanted STS (mt-P53) reduces tumor mass, tumor number, MDM2 protein and mt-P53 protein expression. Our findings support the hypothesis that MDM2-AS ODN treatment may exert a tumor inhibiting effect on all MDM2 expressing tumors regardless of the P53-status, this in turn may be of general importance in gene therapy of cancer.