KRAS, the most frequently mutated oncogene, plays a predominant
role in driving initiation and progression of cancers. Decades of
effort to target KRAS using small molecules has been unsuccessful,
causing KRAS to be considered an “undruggable” cancer
target. However, this view began to change recently, as drug discovery
techniques have developed several KRAS G12C allosteric inhibitors
that are currently being evaluated in clinical trials. Herein we provide
an in-depth analysis of the structure and binding pockets of KRAS,
medicinal chemistry optimization processes, and the biological characterization
of small-molecule inhibitors that directly target KRAS, including
covalent allosteric inhibitors specific for the G12C mutant, GTP-competitive
inhibitors targeting the nucleotide-binding site, and protein–protein
interaction inhibitors that bind in the switch I/II pocket or the
A59 site. Additionally, we propose potential challenges faced by these
new classes of KRAS inhibitors under clinical evaluation.