Emerging data support the rationale of combined
therapies in advanced
melanoma. Specifically, the combined use of drugs with different mechanisms of action can reduce the probability of selecting resistant clones. To identify agents active against
melanoma cells, we screened a library of 349 anti-
cancer compounds, currently in clinical use or trials, and selected
PIK-75, an inhibitor of the
phosphatidylinositol 3-kinase/
protein kinase B (PI3K/AKT) pathway, as the 'top active'
drug.
PIK-75 was then used alone or in combination with
vemurafenib, the first BRAF inhibitor approved for patients with
melanoma harboring BRAF mutations. We identified a combined dose of
PIK-75 and
vemurafenib that inhibited both the PI3K/AKT and
mitogen-activated protein kinase pathways, thereby overcoming any compensatory activation. In view of the important
tumor suppressor function induced by restoring expression of
microRNA (miR)-126 in metastatic
melanoma cells, we examined whether miR-126 has a synergistic role when included in a triple combination alongside
PIK-75 and
vemurafenib. We found that enforced expression of miR-126 (which alone can reduce tumorigenicity) significantly increased
PIK-75 activity when used as either a single agent or in combination with
vemurafenib. Interestingly,
PIK-75 proved to be effective against early passage cell lines derived from patients' biopsies and on
melanoma cell lines resistant to either
vemurafenib or
dabrafenib, thus suggesting that it potentially has the capability to overcome drug resistance. Finally, the synergistic role played by miR-126 in combination with
vemurafenib and/or
PIK-75 was demonstrated in vivo in mouse xenograft models, in which
tumor growth inhibition was associated with increased apoptosis. These results not only show the efficacy of
PIK-75 and
vemurafenib co-treatment but also indicate that restoration of miR-126 expression in advanced
melanoma can enhance their antitumor activity, which may possibly allow
dose reduction to decrease adverse events without reducing the therapeutic benefits.