AKT-independent signaling in PIK3CA-mutant thyroid cancer mediates resistance to dual SRC and MEK1/2 inhibition.

Anaplastic thyroid cancer (ATC) is a rare and aggressive disease with 90% of patients succumbing to this disease 1 year after diagnosis. The approval of the combination therapy of a BRAF inhibitor dabrafenib with the MEK1/2 inhibitor trametinib has improved the overall survival of ATC patients. However, resistance to therapy remains a major problem. We have previously demonstrated combined inhibition of Src with dasatinib and MEK1/2 with trametinib synergistically inhibits growth and induces apoptosis in BRAF- and RAS-mutant thyroid cancer cells, however PIK3CA-mutant cells exhibit a mixed response. Herein, we determined that AKT is not a major mediator of sensitivity and instead PIK3CA-mutants that are resistant to combined dasatinib and trametinib have sustained activation of PDK1 signaling. Furthermore, combined inhibition of PDK1 and MEK1/2 was sufficient to reduce cell viability. These data indicate PDK1 inhibition is a therapeutic option for PIK3CA mutations that do not respond to combined Src and MEK1/2 inhibition.

Targeting CDK4/6 Represents a Therapeutic Vulnerability in Acquired BRAF/MEK Inhibitor-Resistant Melanoma.

There is a clear need to identify targetable drivers of resistance and potential biomarkers for salvage therapy for patients with melanoma refractory to the combination of BRAF and MEK inhibition. In this study, we performed whole-exome sequencing on BRAF-V600E-mutant melanoma patient tumors refractory to the combination of BRAF/MEK inhibition and identified acquired oncogenic mutations in NRAS and loss of the tumor suppressor gene CDKN2A We hypothesized the acquired resistance mechanisms to BRAF/MEK inhibition were reactivation of the MAPK pathway and activation of the cell-cycle pathway, which can both be targeted pharmacologically with the combination of a MEK inhibitor (trametinib) and a CDK4/6 inhibitor (palbociclib). In vivo, we found that combination of CDK4/6 and MEK inhibition significantly decreased tumor growth in two BRAF/MEK inhibitor-resistant patient-derived xenograft models. In vitro, we observed that the combination of CDK4/6 and MEK inhibition resulted in synergy and significantly reduced cellular growth, promoted cell-cycle arrest, and effectively inhibited downstream signaling of MAPK and cell-cycle pathways in BRAF inhibitor-resistant cell lines. Knockdown of CDKN2A in BRAF inhibitor-resistant cells increased sensitivity to CDK4/6 inhibition alone and in combination with MEK inhibition. A key implication of our study is that the combination of CDK4/6 and MEK inhibitors overcomes acquired resistance to BRAF/MEK inhibitors, and loss of CDKN2A may represent a biomarker of response to the combination. Inhibition of the cell-cycle and MAPK pathway represents a promising strategy for patients with metastatic melanoma who are refractory to BRAF/MEK inhibitor therapy.

The mutational landscape of mucosal melanoma.

Mucosal melanoma is a rare and aggressive subtype of melanoma that has a less favorable prognosis due to the lack of understanding and identification of oncogenic drivers. Recently, whole genome and whole exome sequencing have unveiled the molecular landscape and potential oncogenic drivers of mucosal melanoma, which remains distinct from cutaneous melanoma. In this review, we provide an overview of the genomic landscape of mucosal melanoma, with a focus on molecular studies identifying potential oncogenic drivers allowing for a better mechanistic understanding of the biology of mucosal melanoma. We summarized the published genomics and clinical data supporting the observations that mucosal melanoma harbors distinct genetic alterations and oncogenic drivers from cutaneous melanoma, and thus should be treated accordingly. The common drivers (BRAF and NRAS) found in cutaneous melanoma have lower mutation rate in mucosal melanoma. In contrast, SF3B1 and KIT have higher mutation rate in mucosal melanoma as compared to cutaneous melanoma. From the meta-analysis, we also observed that the mutational profiles are slightly different between the "upper" and "lower" regions of mucosal melanoma, providing new insights and therapeutic options for the mucosal melanoma patients. Mutations identified in mucosal melanoma should be incorporated into routine clinical testing, as there are targeted therapies already developed for treating patients with these mutations in the precision medicine era.

Src-mediated regulation of the PI3K pathway in advanced papillary and anaplastic thyroid cancer.

Advanced stages of papillary and anaplastic thyroid cancer continue to be plagued by a dismal prognosis, which is a result of limited effective therapies for these cancers. Due to the high proportion of thyroid cancers harboring mutations in the MAPK pathway, the MAPK pathway has become a focal point for therapeutic intervention in thyroid cancer. Unfortunately, unlike melanoma, a similar responsiveness to MAPK pathway inhibition has yet to be observed in thyroid cancer patients. To address this issue, we have focused on targeting the non-receptor tyrosine kinase, Src, and we and others have demonstrated that targeting Src results in inhibition of growth, invasion, and migration both in vitro and in vivo, which can be enhanced through the combined inhibition of Src and the MAPK pathway. Therefore, we examined the efficacy of the combination therapy across a panel of thyroid cancer cell lines representing common oncogenic drivers (BRAF, RAS, and PIK3CA). Interestingly, combined inhibition of Src and the MAPK pathway overcomes intrinsic dasatinib resistance in cell lines where both the MAPK and PI3K pathways are inhibited, which we show is likely due to the regulation of the PI3K pathway by Src in these responsive cells. Interestingly, we have mapped downstream phosphorylation of rpS6 as a key biomarker of response, and cells that maintain rpS6 phosphorylation likely represent drug tolerant persisters. Altogether, the combined inhibition of Src and the MAPK pathway holds great promise for improving the overall survival of advanced thyroid cancer patients with BRAF and RAS mutations, and activation of the PI3K pathway and rpS6 phosphorylation represent important biomarkers of response for patients treated with this therapy.