In collaboration with LASP, we conducted an in vivo study of trametinib and ganitumab in mouse xenograft models of Ras-mutated rhabdomyosarcoma. We have found that the combination of trametinib and ganitumab provided a survival advantage compared to either agent alone, and also compared to a standard of care chemotherapeutic, vincristine. We also identified that this combination provided the expected pharmacodynamic effects within the tumors, i.e. ERK phosphorylation and IGF1R expression was decreased in tumors that were treated with the combination of trametinib and ganitumab as compared to controls. We also identified that the human Ras-mutated rhabdomyosarcoma cell line, SMS-CTR, is an exceptional responder to the combination of trametinib and ganitumab. In an effort to identify the reason why SMS-CTR responds so well to this combination, we showed that of the Ras-mutated rhabdomyosarcoma cell lines, SMS-CTR has the highest expression of IGF1R.
We aim ed to identify mechanisms of co-targeting the RAS-RAF-MEK-ERK pathways and the IGF1R-IRS1/2-PI3K-AKT-mTOR pathways that would be effective in cell lines that do not have high IGF1R expression, including monoclonal antibodies specific for the ligand of IGF1R, IGF1/2, and an inhibitor of IRS1/2. This work is currently ongoing. In the course of the animal study of the trametinib and ganitumab combination, we created an SMS-CTR cell line that is resistant to trametinib. We are currently in the process of identifying the mechanism of resistance to trametinib in this cell line. We have identified that one mechanism that explains the synergy between trametinib and ganitumab is that ganitumab blocks the rebound ERK phosphorylation and increase in AKT phosphorylation induced by trametinib monotherapy. However, trametinib and ganitumab induce metabolic, epigenetic and gene expression changes related to the changes in cell signaling, which we are in the process of investigating. Finally, we are in discussions with CTEP regarding the initiation of a phase I/II clinical trial of trametinib and ganitumab in pediatric patients.
