Target to Candidate – one phase or two – where to make the split?
The organization of processes in the Lead Discovery and Lead Optimization stages may vary from company to company, especially around the milestone that signifies the end of screening activities and the initiation of work on optimizing chemical series towards a clinical candidate. Invariably the differences reduce to the distinction between a hit from screening, a tractable hit series (a chemical series that merits further investigation, e.g. in additional biochemical and biological assays), and a lead series (a chemical series that the project team believes has the potential of being optimized into a clinical candidate). It is usually the point when medicinal chemistry commits to working on a chemical series that the milestone decision is based upon, e.g. Target to Lead and Lead to Candidate. With the exception of chemistry, however, for the other disciplines engaged in Discovery, there are merits in considering both phases together as Target to Candidate.