Inhibitors of pan-PI3K Signaling Synergize with BRAF or MEK Inhibitors to Prevent BRAF-Mutant Melanoma Cell Growth

Abstract

BRAF and MEK inhibitors have improved outcomes for patients with BRAF-mutant melanoma, but their efficacy is limited by both intrinsic and acquired resistances. Activation of the PI3K pathway can mediate resistance to these agents, providing a strong rationale for combination therapy in melanoma. Here, a panel of nine low-passage human metastatic melanoma cell lines with BRAF mutations was tested in cell proliferation and protein expression assays for sensitivity to inhibitors of MEK (selumetinib) and BRAF (vemurafenib) as single agents and in combination with inhibitors of pan-PI3K (ZSTK474), pan-PI3K/mTOR (BEZ235), individual PI3K isoforms (p110α, A66; p110β, TGX-221; p110γ, AS-252424; p110δ, idelalisib), or mTORC1/2 (KU-0063794). Selumetinib and vemurafenib potently inhibited cell proliferation in all cell lines, especially in those that expressed low levels of phosphorylated AKT (pAKT). ZSTK474 and BEZ235 also inhibited cell proliferation in all cell lines and enhanced the antitumor activity of selumetinib and vemurafenib in the majority of lines by either interacting synergistically or additively to increase potency or by inducing cytotoxicity by significantly increasing the magnitude of cell growth inhibition. Furthermore, ZSTK474 or BEZ235 combined with selumetinib to produce robust inhibition of pERK, pAKT, and pS6 expression and synergistic inhibition of NZM20 tumor growth. The inhibitors of individual PI3K isoforms or mTORC1/2 were less effective at inhibiting cell proliferation either as single agents or in combination with selumetinib or vemurafenib, although KU-0063794 synergistically interacted with vemurafenib and increased the magnitude of cell growth inhibition with selumetinib or vemurafenib in certain cell lines. Overall, these results suggest that the sensitivity of BRAF-mutant melanoma cells to BRAF or MEK inhibitors is at least partly mediated by activation of the PI3K pathway and can be enhanced by combined inhibition of the BRAF/MEK and PI3K/mTOR signaling pathways.

Identification of NRAS isoform 2 overexpression as a mechanism facilitating BRAF inhibitor resistance in malignant melanoma

“…We theorize that isoform 2 may enhance resistance to vemurafenib by activating the PI3K pathway and downregulating the MAPK pathway. This theory is in line with a recent publication which found that pan-PI3K inhibition was able to synergize with BRAF inhibitors to prevent BRAF mutant melanoma cell growth (17). Additionally, studies with the combination of vemurafenib and the MEK inhibitor selumetinib showed that combination with different PI3K pathway inhibitors led to synergistic reduction of cell viability and enhanced apoptosis in vitro (18).…”

Overcoming resistance to BRAF inhibitors

“…While in vitro studies clearly show the potential of using AKT, pan-PI3K or dual PI3K/mTOR inhibitors in combination with BRAF/MEK inhibitor, the translation into the clinical setting is troublesome due to enhanced systemic toxicities (124)(125)(126)(127)(128). The PI3K family of proteins is relevant in many physiological processes and malignant conditions but nevertheless this interest on the pathway is producing encouraging data advancing in the understanding of melanoma specific PI3K-signaling (129) and lately PI3Kβ isoform specific inhibitors are being developed and trialed in the context of PTEN mutant melanoma patients (130)(131)(132).…”

The Combination of Vemurafenib and Procaspase-3 Activation Is Synergistic in Mutant BRAF Melanomas

“…(10) Given the current clinical limitations of existing therapies, novel and rationally-designed combination studies with other kinase inhibitors are being explored. (11,12) Despite all efforts to date, the development of resistance to targeted V600E BRAF therapies emerges in virtually 100% of patients treated; acquired drug resistance to this class of agents remains a significant obstacle to dramatically enhanced survival benefits for metastatic melanoma patients.…”