Similarly, gene expression studies of clinical samples have also defined molecularly defined subgroups within a number

of tumour types [26, 27 and 28•]. It is entirely plausible that these molecularly defined subgroups will exhibit different biological characteristics including drug response and therefore any screen that utilises cancer cell lines must be of sufficient scale to capture both the tissue-type and genetic diversity of human cancers. Only in this way will it be possible to accurately model the effect of cancer mutations on drug response. One of the first systematic efforts to use cancer cell lines to identify biomarkers of drug sensitivity was the NCI-60 panel at the National Cancer Institute in 1990 [29••] (http://dtp.nci.nih.gov/branches/btb/ivclsp.html). Although these 60 cell lines have now been screened against many thousands of chemical agents, it has become increasingly check details clear that much larger numbers of cell lines are required to capture the genetic diversity of human cancer. It is now clear from next-generation sequencing studies that cancers are remarkably

heterogeneous and many cancer genes are present in only a fraction of any tumour type. It is therefore likely that hundreds of cancer cell lines would be required to capture this landscape of cancer gene mutations. To address this need, a Wellcome Trust Sanger Institute and Massachusetts General Hospital collaboration was established in 2009 to screen PI3K inhibitor drugs >1000 cancer cell lines against 400 cancer drugs and to make that data publicly accessible (pharmacologic profiles of 142 cancer drugs screened across 668 cell lines are currently available) (http://www.cancerrxgene.org/) (Figure 1). A similar initiative funded by the pharmaceutical company Novartis at the Broad Institute has profiled 24 cancer drugs across 504 cell lines (http://www.broadinstitute.org/ccle/home). A key element of both endeavours is the detailed genomic, epigenetic ever and transcriptomic characterisation that has been made possible for these cancer cell

lines by advances in next-generation sequencing, such that multi-dimensional signatures of drug response can be derived from such screens and that could be used to stratify patients for clinical trial recruitment or treatment in the clinic. Landmark papers by both these groups recently demonstrated the power of these large screens to identify both novel and previously documented biomarkers of drug response in a completely unbiased fashion [18•• and 30••]. It is now feasible to consider profiling all new experimental oncology compounds in such screens in order to develop hypotheses as to mechanisms of activity as well as insights into patient subgroups that may be most likely to respond to treatment in the clinic.