The European Federation for Medicinal Chemistry and Chemical Biology (EFMC) Best Practice Initiative: Hit to Lead

The Hit to Lead (H2L) process is an integral part of contemporary drug discovery, encompassing the optimisation of validated Hit structures into Lead molecules. High quality leads build confidence, through activity and property profiles as well as preliminary biological data, which might include validating pharmacologic hypotheses along the way, indicating that further investment in the structure(s) and target would be worthwhile. Leads have line of sight to a development candidate and bring an understanding of what priorities Lead Optimisation should address. In this set of best practices, we detail the essential criteria that characterise a good lead, which include establishing SAR from analogues and assessing preliminary DMPK indicators, selectivity and early safety parameters. We highlight the importance of identifying liabilities of the lead series and demonstrating that each can be individually modulated whilst maintaining on target potency. We make the case for having physicochemical properties as critical optimisation parameters and how ligand efficiency metrics can enable this. Then we go over general tactics that can be used to convert hits into a lead series. These include essential steps that, when performed early, increase the chance of success such as deconstructive SAR, pharmacophore and bioactive conformation determination and scaffold optimisation. Finally, we suggest decision-making criteria to substantiate confidence in further investment or, as importantly, making a recommendation to cease further work on a series.