KRAS Mutant Pancreatic Cancer: No Lone Path to an Effective Treatment

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is among the deadliest cancers with a dismal 7% 5-year survival rate and is projected to become the second leading cause of cancer-related deaths by 2020. KRAS is mutated in 95% of PDACs and is a well-validated driver of PDAC growth and maintenance. However, despite comprehensive efforts, an effective anti-RAS drug has yet to reach the clinic. Different paths to inhibiting RAS signaling are currently under investigation in the hope of finding a successful treatment. Recently, direct RAS binding molecules have been discovered, challenging the perception that RAS is an "undruggable" protein. Other strategies currently being pursued take an indirect approach, targeting proteins that facilitate RAS membrane association or downstream effector signaling. Unbiased genetic screens have identified synthetic lethal interactors of mutant RAS. Most recently, metabolic targets in pathways related to glycolytic signaling, glutamine utilization, autophagy, and macropinocytosis are also being explored. Harnessing the patient's immune system to fight their cancer is an additional exciting route that is being considered. The "best" path to inhibiting KRAS has yet to be determined, with each having promise as well as potential pitfalls. We will summarize the state-of-the-art for each direction, focusing on efforts directed toward the development of therapeutics for pancreatic cancer patients with mutated KRAS.

Keywords: RAS; cancer; pancreatic; therapeutics.

Figure 1

Human RAS proteins are composed two functional domains, the G domain and the membrane targeting domain. The G domain spans residues 1–164 and includes the regions of the protein responsible for binding and hydrolyzing GTP. Specifically, residues in the switch I (SI = amino acids 30–38) region and switch II (SII = amino acids 60–76) region experience a conformational change during GDP-GTP cycling. The membrane targeting domain is comprised of the remaining 24/25 C-terminal residues. The first 20–21 amino acids are referred to as the hypervariable region and this is where the three RAS isoforms exhibit the greatest diversity in protein sequence. The hypervariable region contains elements important for membrane association including cysteines (blue, underlined) that are covalently modified by the addition of a palmitate fatty acid, and stretches of polybasic amino acids. Additionally KRAS4B contains a serine (181) that can be phosphorylated and regulates the association of this protein with the plasma membrane or endomembranes. The four most C-terminal residues of the membrane-targeting domain comprise the CAAX motif, where C = cysteine, A = any aliphatic residue, and X = the terminal amino acid. A C15 farnesyl group is covalently attached to the cysteine residue by farnesyltransferases and this lipid moiety aids in membrane association.

Figure 2

Mutant KRAS is continuously in a GTP-bound, active state. Wild-type KRAS cycles between an active, GTP-bound and an inactive, GDP-bound state, and it exists largely in an inactive state in non-dividing cells. Upon growth factor stimulation, normal KRAS is activated by RAS guanine nucleotide exchange factors (RASGEFs), which facilitate the binding of GTP to KRAS. KRAS-GTP then binds downstream effectors. This signaling is attenuated due to the action of RAS GTPase-activating proteins (RASGAPs), which promote the hydrolysis of the bound GTP to GDP and hence formation of inactive KRAS-GDP. Mutation of residues G12, G13 or Q61 constitutively activates KRAS by preventing the formation of van der Waals interactions between RAS and RASGAPs [20] and interfering with the position of a water molecule necessary for GTP hydrolysis [21], respectively. The arrow thickness and relative size of the symbols for GEFs and GAPs indicate the level of signaling.

Figure 3

The current paths in the pursuit of an anti-KRAS therapy. There have been past and ongoing efforts to synthesize molecules that bind directly to the RAS protein and inhibit its GDP-GTP regulation or effector signaling. Disrupting RAS membrane localization by inhibiting farnesylation showed promising preclinical effects but no anti-tumor activity in clinical trials. Attempts to inhibit downstream effector signaling have generated a large number of inhibitors currently under clinical evaluation. Unbiased genetic functional RNAi screens have identified genes that may act as synthetic lethal interactors. However, these studies have been limited by reproducibility or the transition of hits to a therapeutic strategy. The broken line represents the functional relationship in the absence of a linkage via a specific signaling network. The elucidation of the many metabolic processes that KRAS regulates may result in new therapies for patients with PDAC. Likewise, the discovery of ways to degrade the dense stroma associated with PDAC tumors and employ the immune response may lead to novel therapies for PDAC.