Taking Aim at the Undruggable.

The term "undruggable" is used to describe a protein that is not pharmacologically capable of being targeted; recently, however, substantial efforts have been made to turn these proteins into "druggable" targets. Thus, "difficult to drug" or "yet to be drugged" are perhaps more appropriate terms. In cancer, a number of elusive targets fall into this category, including transcription factors such as STAT3, TP53, and MYC. Pharmacologically targeting these intractable proteins is now a key challenge of modern drug development, requiring innovation and the development of new technologies. In this article, we discuss some of the recent technologic and pharmacologic advances that have underpinned the erosion of the concept of undruggability. We describe recent successes in drugging the undruggable RAS (KRAS G12C and HRAS), and discuss the advances that have led to the validation of further targets previously believed to be undruggable, such as HIF-2α, BCL-2, MDM2, and MLL. Finally, we look to the future and describe important advances that are likely to have a major impact on targeting undruggable targets, such as the advent of proteolysis-targeting chimeras and protein-protein modulators, which are leading to considerable excitement surrounding the development of cancer targets.