1. The first known bifunctional inhibitor for the mechanistic target of rapamycin (mTOR) protein was developed to overcome current known mechanisms of tumor resistance to mTOR inhibitors.
2. Treatment with the novel compound prevented tumor growth in mouse models of drug-resistant tumors.
Evidence Rating Level: 2 (Good)
Study Rundown: mTOR is a protein commonly affected in a variety of cancers, making it a prime target for therapeutics. mTOR inhibitors have been developed and show great efficacy; however, mutations in the protein have rendered some patients unresponsive to these treatments. Specifically, mutations in the kinase domain and FKBP12-rapamycin-binding domain (FRB domain) of mTOR have contributed to the resistance of current mTOR inhibitors. As a result, this study developed the first bivalent inhibitor for a protein kinase that binds to both of these domains.
Three potential drugs were developed that were able to simultaneously bind to both domains. Experiments in drug-sensitive cell lines showed that the compounds inhibited mTOR phosphorylation activity in a dose-dependent manner. The most efficacious drug, RapaLink-1, was then tested in cell cultures and mouse models to determine its effect on cancer cells with drug-resistant mTOR. In vitro, RapaLink-1 inhibited mTOR at low doses compared to rapamycin and a recently developed inhibitor, MLN0128. In mice with xenografts of cancer cells containing the common mutations that confer mTOR inhibitor resistance, only RapaLink-1 consistently limited tumor growth over the course of 60 days.
Further studies will need to be performed to determine the toxicity and potential systemic effects of this drug. However, this study shows the potential of a treatment for patients who are resistant to current mTOR inhibitors. These experiments are also the first to demonstrate the successful production of a pharmaceutically efficacious bivalent protein kinase inhibitor.
In-Depth [animal study]: First, researchers analyzed the crystal structure of rapamycin binding to the FRB and kinase domains of mTOR, the main structural regions containing mutations that lead to mTOR inhibitor resistance. In order to overcome the reduced drug binding conferred by the mutations, bivalent inhibitors were designed using the modeling program Molecular Operating Environment. Three potential drugs were designed, RapaLink-1, -2, and -3, each containing varying lengths of linkers between the two functional parts of the drug.
Each drug was initially tested on drug-sensitive MCF-7 cells, a human breast cancer cell line, to determine its effect on mTOR signaling. Western blotting demonstrated successful inhibition of phosphorylation of mTOR complexes 1 and 2. RapaLink-3, however, was found to have a decreased potency, a characteristic expected due to the shorter linker of the drug.
RapaLink-1 was further tested to assess its efficacy against drug-resistant cancer cells. Cells expressing mutant resistant versions of mTOR were used in culture or xenografted into mice. The efficacy of RapaLink-1 was compared to rapamycin and a recently developed mTOR inhibitor, MLN0128. In vitro, RapaLink-1 was more efficacious at inhibiting mTOR signaling. In vivo, RapaLink-1 was more efficacious at limiting tumor growth over the course of 60 days.
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