Clinical Cancer Research: Regulation of TORC1 by MAPK Signaling Determines Sensitivity and Acquired Resistance to Trametinib in Pediatric BRAFV600E Brain Tumor Models (Houghton & Kurmasheva)

We investigated why three patient-derived xenografts (PDX) childhood BRAF^V600E-mutant brain tumor models are highly sensitive to trametinib. Mechanisms of acquired resistance selected in situ and approaches to prevent resistance were also examined, which may translate to both low-grade gliomas (LGG) molecular subtypes.

Sensitivity to trametinib [MEK inhibitor (MEKi)] alone or in combination with rapamycin (TORC1 inhibitor), was evaluated in pediatric PDX models. The effect of combined treatment of trametinib with rapamycin on the development of trametinib resistance in vivo was examined. PDX tissue and tumor cells from trametinib-resistant xenografts were characterized.

In pediatric models, TORC1 is activated through ERK-mediated inactivation of the tuberous sclerosis complex (TSC): consequently, inhibition of MEK also suppresses TORC1 signaling. Trametinib-induced tumor regression correlated with dual inhibition of MAPK/TORC1 signaling and decoupling TORC1 regulation from BRAF/MAPK control conferred trametinib resistance. In mice, acquired resistance to trametinib developed within three cycles of therapy in all three PDX models. Resistance to trametinib developed in situ is tumor-cell–intrinsic, and the mechanism was tumor line specific. Rapamycin retarded or blocked the development of resistance.

In these three pediatric BRAF-mutant brain tumors, TORC1 signaling is controlled by the MAPK cascade. Trametinib suppressed both MAPK/TORC1 pathways leading to tumor regression. While low-dose intermittent rapamycin to enhance inhibition of TORC1 only modestly enhanced the antitumor activity of trametinib, it prevented or retarded the development of trametinib resistance, suggesting future therapeutic approaches using rapamycin analogs in combination with MEKis that may be therapeutically beneficial in both KIAA1549::BRAF- and BRAF^V600E-driven gliomas.

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