Targeting the MAPK pathway in melanoma: why some approaches succeed and other fail

Abstract

The Mitogen Activated Protein Kinase (MAPK) pathway plays a key role in melanoma development making it an important therapeutic target. In normal cells, the tightly regulated pathway relays extracellular signals from cell membrane to nucleus via a cascade of phosphorylation events. In melanomas, dysregulation of the MAPK pathway occurs frequently due to activating mutations in the B-RAF and RAS genes or other genetic or epigenetic modifications, leading to increased signaling activity promoting cell proliferation, invasion, metastasis, migration, survival and angiogenesis. However, identification of ideal pathway member to therapeutically target for maximal clinical benefit to melanoma patients remains a challenge. This review provides an overview of the obstacles faced targeting the MAPK pathway and why certain therapeutic approaches succeed while others fail. The review summarizes the roles played by the proteins, therapeutic potential and the drugs available to target each member of the pathway as well as concerns related to each. Potential for targeting multiple points and inhibiting other pathways along with MAPK inhibition for optimal efficacy are discussed along with explanations for development of drug resistance, which includes discussions related to cross-talk between pathways, RAF kinase isoform switching and phosphatase deregulation. Finally, the use of nanotechnology is reviewed as an approach to target the MAPK pathway using both genetic and pharmacological agents simultaneously targeting multiple points in the pathway or in combination with other cascades.

Figure 1. The MAP and PI3 kinase pathways in melanoma

Cell surface receptor tyrosine kinases are activated by mitogens, which in turn activate phosphoinositide 3-kinase (PI3K) (effectively controlled by the PTEN tumor suppressor gene). PI3K phosphorylates AKT3 thus activating many oncogenic pathways such as CREB, IKK, Cyclin D1 and PRAS40. Activation of receptor tyrosine kinases simultaneously stimulates RAS/RAF/MEK/ERK which can also activate Cyclin D1 and NFkB. All these cascades essentially promote proliferation, cell survival, angiogenesis, and metastasis.

Figure 2. Inhibitors of the MAPK pathway

Schematic representation of key targets and pharmacological agents that inhibit MAPK signaling proteins. Imitanib, dastinib and sorafenib inhibit receptor tyrosine kinases thereby preventing activation of mitogen dependent activation in the MAPK pathway. Tipifarnib, R115777, BMS-214662 and L-778123 are selective to RAS while PLX4032, RAF-265 and XL281 specifically target activated B-RAF and are at various stages of clinical development. AZD6244, U0126, PD0325901, CI-1040, XL518, AZD8330, ARRY-162 and ARRY-300 inhibit MEK 1/2 while curcumin, plumbagin and DHMEQ target IKKβand NFκB.

Figure 3. Schematic depiction of B-RAF showing the various domains of the protein

B-RAF is one of the three RAF kinase isoforms. Structurally, B-RAF contains three highly conserved regions called, CR1, CR2 & CR3. B-RAF activity is regulated primarily by phosphorylation of key serine and threonine residues. Whereas phosphorylation of threonine 598 and serine 601 at the C-terminal kinase domain triggers kinase activity, the phosphorylation of serine 364, serine 428 and threonine 439 by AKT3 inhibits the enzymatic activity.

Figure 4. Structures of selected drugs used for targeting MAPK pathway in melanoma

BMS214662, SCH 66336, L-778123 and farnesyl thiosalicylic acid inhibit Ras activity; Sorafenib (RAF kinase inhibitor) and PLX4032 (a selective ^V600EB-RAF inhibitor); MEK inhibitors U0126, CI-1040 (the first MEK inhibitor in clinical trials); ARRY-142886 (AZD6244) and PD-0325901 (highly selective MEK inhibitor) are shown.