Sci. Trans. Med. 11, eaaw8412 (2019)

There is growing evidence that the failure of clinical trials for new drugs may be due to off-target effects. To determine whether this is a general phenomenon, Lin et al. performed a systematic CRISPR–Cas9-based competition screen generating null mutations in the known targets for ten cancer drugs. They found that loss of these cellular targets did not impair cancer cell growth, which differed from previous findings using RNA interference. The authors suggested that the previous RNA interference findings were a result of off-target toxicity. In addition, inhibitors against these five cellular targets were responsive in cells deficient for the respective targets, confirming that the cytotoxicity of these inhibitors was due to off-target inhibition. Lin et al. further investigated OTS964, a reported inhibitor of PBK, and identified OTS964-resistant clones. Sequencing analysis revealed missense mutations in the cyclin-dependent kinase member CDK11B that conferred resistance to OTS964. Loss of CDK11B function was found to prevent mitotic progression, suggesting that CDK11B may be the critical target of OTS964. Overall, the findings by Lin et al. stress the importance of target validation of cancer drugs before progressing to clinical trials.