FDA’s draft guidance on Enrichment Strategies raises many intriguing possibilities to support approval of personalised medicines. One such example is defining the responsive subgroup on the basis of a biomarker observed as a response to treatment; examples being positron emission tomography (PET) response observed rapidly after therapy or the presence of a side-effect such as rash.

There are quite different challenges when the responding population can only be identified on the basis of a PTB especially if it represents only a small proportion of patients. This presentation will explore whether or when a treatment could be licensed on the basis of a PTB. It will review the potential pitfalls in hypothesising the presence of a PTB. It will also present challenges in trial design required to confirm and license on the basis of a PTB: what’s the control population?; could there be a detriment to non-responders by exposure to the new treatment?; can responders be identified rapidly?; could prior exposure to the new treatment adversely affect performance of the control in responders?. Nevertheless, if the patients to be treated could be rapidly identified after prior exposure to treatment, and without harm to non-responders, in appropriately designed and analysed trials, may be more personalised medicines could be made available to patients.