mTOR Inhibitors Synergize on Regression, Reversal of Gene Expression, and Autophagy in Hepatocellular Carcinoma

Combination therapy causes gene reprogramming, autophagy, and tumor regression in a mouse model approximating human HCC. Bridging the Generation Gap Kids of every generation disdain their elders—who clearly don’t understand them and are stuck in the past. Newer is better, after all. But sometimes, a blend of the old and new may be exactly what’s needed to solve a particular problem. Thomas et al. set out to see whether the new—the phosphatidylinositol 3-kinase/mammalian target of rapamycin (mTOR) adenosine triphosphate–site competitive inhibitor BEZ235—was better than the old—the U.S. Food and Drug Administration–approved mTOR-allosteric inhibitor RAD001. What they instead found was that these two drugs worked together to treat hepatocellular carcinoma (HCC). mTOR signaling is up-regulated in about 50% of HCCs. When the authors tested two mTOR-targeting drugs, BEZ235 and RAD001, on cultured HCC cells, they unexpectedly found that the drugs acted synergistically. In a mouse model that mimics human HCC, the two drugs induced a marked regression in tumor burden through a mechanism that involved down-regulation of genes involved in autophagy—where the cell degrades its own components. In patients with HCC, dysregulation of autophagy genes correlated with poor prognosis. The authors are now taking this observation into clinical trials to determine whether it holds true in people. By working together, old and new mTOR inhibitors may provide a new therapeutic option for HCC. Hepatocellular carcinoma (HCC) affects more than half a million people worldwide and is the third most common cause of cancer deaths. Because mammalian target of rapamycin (mTOR) signaling is up-regulated in 50% of HCCs, we compared the effects of the U.S. Food and Drug Administration–approved mTOR-allosteric inhibitor, RAD001, with a new-generation phosphatidylinositol 3-kinase/mTOR adenosine triphosphate–site competitive inhibitor, BEZ235. Unexpectedly, the two drugs acted synergistically in inhibiting the proliferation of cultured HCC cells. The synergistic effect closely paralleled eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) dephosphorylation, which is implicated in the suppression of tumor cell proliferation. In a mouse model approximating human HCC, the drugs in combination, but not singly, induced a marked regression in tumor burden. However, in the tumor, BEZ235 alone was as effective as the combination in inhibiting 4E-BP1 phosphorylation, which suggests that additional target(s) may also be involved. Microarray analyses revealed a large number of genes that reverted to normal liver tissue expression in mice treated with both drugs, but not either drug alone. These analyses also revealed the down-regulation of autophagy genes in tumors compared to normal liver. Moreover, in HCC patients, altered expression of autophagy genes was associated with poor prognosis. Consistent with these findings, the drug combination had a profound effect on UNC51-like kinase 1 (ULK1) dephosphorylation and autophagy in culture, independent of 4E-BP1, and in parallel induced tumor mitophagy, a tumor suppressor process in liver. These observations have led to an investigator-initiated phase 1B-2 dose escalation trial with RAD001 combined with BEZ235 in patients with HCC and other advanced solid tumors.