Protein–protein interactions (PPIs) form the molecular bases of many physiological processes in health and dis- ease. In cancer, PPIs can have important regulatory roles; for example, in the cell division cycle or in cell signalling. The specificity and affinity of these interactions are criti- cally important — changes in them can lead to cellular malfunctions, such as the uncontrolled cell growth that typifies cancer. Thus, it is not surprising that modulators of PPIs, whether as biological agents (such as antibodies) or as small-molecule synthetic compounds, are urgently being sought and developed by the pharmaceutical industry to treat an unmet medical need. This interest in PPIs as drug targets has historically been tempered by the pervasive view that finding mol- ecules, particularly small synthetic molecules (those <500 Da), that will modulate PPIs is irredeemably hampered by the large, flat, featureless areas of protein surfaces that form the interface — an interface that is thought to be ‘undruggable’ (REFS 1–4). This has often been in spite of compelling evidence from molecular, cellular and animal studies that such targets have an excellent potential for drug development. Nevertheless, major inroads into discovering such molecules have been made in recent years, with more than 12 small- molecule PPI modulators currently in clinical develop- ment (TABLE 1) and sales worldwide predicted to be in excess of US$800 million within 5 years 5 . As with other areas of drug discovery, a major chall- enge is to identify and validate physiologically relevant PPIs as cancer-related targets; this task is particularly chall- enging for many PPIs, as they can be transient in nature. The most common tools for identifying PPIs have been genetic approaches, such as yeast two-hybrid, synthetic lethal, systematic RNA interference and co-evolution analyses; cell biology approaches, includ- ing the localization of proteins with fluorescent mark- ers; proteomic approaches, such as affinity purification and mass spectrometry (MS) of complexes and pro- tein arrays; and direct protein co-purification and immunoprecipitation. The past couple of years have also seen the development of highly sophisticated approaches to genome-wide prediction of interacting proteins, including studies that have identified more than 300,000 potential interaction pairs encoded in the human genome 6 . Of particular relevance to can- cer is the genome-wide profiling of tumour samples by initiatives such as The Cancer Genome Atlas and the International Cancer Genome Consortium. Validating the physiological relevance of a PPI to cancer facilitates the initiation of a drug discovery programme. The pathways in a typical drug discov- ery programme are outlined in FIG. 1; the nature of the protein target and the available resources will dictate which pathways are followed. FIGURE 2a illustrates how different approaches are being applied to a single pro- tein target in the hunt for PPI modulators. As well as traditional high-throughput chemical screening (HTS), structure-based discovery approaches are now com- mon. In addition to the challenges described above, these approaches face another challenge that is specific to PPIs. There is a wide gulf between the number of experimentally determined protein structures and the 1 Australian Cancer Research Foundation Rational Drug Discovery Centre and Biota Structural Biology Laboratory, St. Vincent’s Institute of Medical Research, 9 Princes Street, Fitzroy, Victoria 3065, Australia. 2 Department of Medicine, University of Melbourne, 41 Victoria Parade, Fitzroy, Victoria 3065, Australia. 3 Department of Biochemistry and Molecular Biology, Bio21 Molecular Science and Biotechnology Institute, University of Melbourne, Parkville, Victoria 3052, Australia. Correspondence to M.W.P.  e‑mail: mparker@svi.edu.au doi:10.1038/nrc3690 Published online 13 March 2014 High-throughput chemical screening (HTS). A systematic way to measure the modulation of an interaction or a biological function against a given protein target by a large number of individual compounds. Oncogenic protein interfaces: small molecules, big challenges Tracy L. Nero 1 , Craig J. Morton 1 , Jessica K. Holien 1 , Jerome Wielens 1,2 and Michael W. Parker 1,3 Abstract | Historically, targeting protein–protein interactions with small molecules was not thought possible because the corresponding interfaces were considered mostly flat and featureless and therefore ‘undruggable’. Instead, such interactions were targeted with larger molecules, such as peptides and antibodies. However, the past decade has seen encouraging breakthroughs through the refinement of existing techniques and the development of new ones, together with the identification and exploitation of unexpected aspects of protein–protein interaction surfaces. In this Review, we describe some of the latest techniques to discover modulators of protein–protein interactions and how current drug discovery approaches have been adapted to successfully target these interfaces. REVIEWS 248 | APRIL 2014 | VOLUME 14 www.nature.com/reviews/cancer © 2014 Macmillan Publishers Limited. All rights reserved