Targeting the "undruggable" RAS - new strategies - new hope?

Abstract

K-RAS is the most frequently mutated oncogene in solid tumors, such as pancreatic, colon or lung cancer. The GTPase K-RAS can either be in an active (GTP-loaded) or inactive (GDP-loaded) form. In its active form K-RAS forwards signals from growth factors, cytokines or hormones to the nucleus, regulating essential pathways, such as cell proliferation and differentiation. In turn, activating somatic mutations of this proto-oncogene deregulate the complex interplay between GAP (GTPase-activating) - and GEF (Guanine nucleotide exchange factor) - proteins, driving neoplastic transformation. Due to a rather shallow surface, K-RAS lacks proper binding pockets for small molecules, hindering drug development over the past thirty years. This review summarizes recent progress in the development of low molecular antagonists and further shows insights of a newly described interaction between mutant K-RAS signaling and PD-L1 induced immunosuppression, giving new hope for future treatments of K-RAS mutated cancer.

Keywords: K-RAS; PD-1; PD-L1; immune checkpoints; small molecules.

Figure 1

Amino acid sequences of RAS isoforms. The sequences of the RAS isoforms are compared and non-identical areas are marked by dashed rectangles. Amino acids 1-166 constitute the catalytic domain and amino acids 167-188/189 represent the hypervariable region. The amino acids are coloured based on their physico-chemical properties. Hydrophobic (black), polar/neutral (green), acidic (red) und basic (blue)

Figure 2

Crystal structure of K-RAS 4B GDP in ribbon representation (4EPY). The α-helices (α1-α5), β-sheets (β1-β6), the magnesium-ion (Mg²⁺), and the GDP nucleotide are labeled in black. The loops and switch regions are color-coded as follows: P-loop (red); switch I - SW-I (green); G3-loop (blue); switch II - SW-II (yellow); G4-loop (magenta); G5-loop (orange)

Figure 3

Major effector pathways of K-RAS and potential interactors. Mutations in K-RAS genes lead to constitutive GTP bound K-RAS proteins that activate downstream phosphorylation cascades. Major effector pathways dysregulated in K-RAS mut cancers are the MAPK (RAF/MEK/ERK) and the PI3K/AKT pathway. Transcription factors regulated by these pathways, as well as STAT3, can bind to the PD-L1 gene, enhancing the transcription rate. TPP destabilizes PD-L1 mRNA at AREs on the 3’UTR region and can be inhibited via MEK signaling in K-RAS mut cancers, prolonging the PD-L1 mRNA’s half-life. This figure was partly created using SMART Servier Medical Art. ARE: AU-rich elements; CR: cytokine receptor; RTK: receptor tyrosine kinase; TTP: tristetraprolin

Figure 4

Low-molecular weight compounds targeting the RAS hydrophobic binding pocket

Figure 5

Ligand orientation in the hydrophobic binding pocket of K-RAS 4B. Co-crystal structure of (2) with K-RAS G12V (aa 1-169) [4EPY] in surface projection. The co-crystal structure of (1) [4DST] and (6) [6FA4] as well as (3) docked to [4DSO] have been aligned to 4EPY. The compounds are color-coded as follows: (1) blue; (2) green; (3) magenta; (6) red. The switch regions adjacent to the hydrophobic binding pocket are labeled switch I SW-I (yellow) and switch II SW-II (orange)