RAC1mutation is not a predictive biomarker for PI3'-kinase-ß-selective pathway-targeted therapy.

Mutational activation of RAC1 is detected in ~7% of cutaneous melanoma, with the most frequent mutation (RAC1C85T ) encoding for RAC1P29S . RAC1P29S is a fast-cycling GTPase that leads to accumulation of RAC1P29S -GTP, which has potentially pleiotropic regulatory functions in melanoma cell signaling and biology. However, the precise mechanism by which mutationally activated RAC1P29S propagates its pro-tumorigenic effects remains unclear. RAC1-GTP is reported to activate the beta isoform of PI3'-kinase (PIK3CB/PI3Kß) leading to downstream activation of PI3'-lipid signaling. Hence, we employed both genetic and isoform-selective pharmacological inhibitors to test if RAC1P29S propagates its oncogenic signaling in melanoma through PI3Kß. We observed that RAC1P29S -expressing melanoma cells were largely insensitive to inhibitors of PI3Kß. Furthermore, RAC1P29S melanoma cell lines showed variable sensitivity to pan-class 1 (α/ß/γ/δ) PI3'-kinase inhibitors, suggesting that RAC1-mutated melanoma cells may not rely on PI3'-lipid signaling for their proliferation. Lastly, we observed that RAC1P29S -expressing cell lines also showed variable sensitivity to pharmacological inhibition of the RAC1 â†’ PAK1 signaling pathway, questioning the relevance of inhibitors of this pathway for the treatment of patients with RAC1-mutated melanoma.

Inhibition of MEK1/2 Forestalls the Onset of Acquired Resistance to Entrectinib in Multiple Models of NTRK1-Driven Cancer.

NTRK1 gene fusions are actionable drivers of numerous human malignancies. Here, we show that expression of the TPR-NTRK1 fusion kinase in immortalized mouse pancreatic ductal epithelial (IMPE) (pancreas) or mouse lung epithelial (MLE-12) cells is sufficient to promote rapidly growing tumors in mice. Both tumor models are exquisitely sensitive to targeted inhibition with entrectinib, a tropomyosin-related kinase A (TRKA) inhibitor. Initial regression of NTRK1-driven tumors is driven by induced expression of BIM, such that BIM silencing leads to a diminished response to entrectinib in vivo. However, the emergence of drug-resistant disease limits the long-term durability of responses. Based on the reactivation of RAF>MEK>ERK signaling observed in entrectinib-treated tumors, we show that the combination of entrectinib plus the MEK1/2 inhibitor cobimetinib dramatically forestalls the onset of drug resistance in vivo. Collectively, these data provide a mechanistic rationale for rapid clinical deployment of combined inhibition of TRKA plus MEK1/2 in NTRK1-driven cancers.