UCL Therapeutic Innovation Networks


Lead Discovery (Small Molecules)

The Lead Discovery stage of the small molecules translational pathway comes after target identification and assay development, and involves searching for compounds that can interact with the target.

small molecules roadmap

screening icon

Once a target is known and an assay for activity has been developed, the medicinal chemist search for compounds that can potentially interact with the target. As better compounds are identified they will then under go testing in various biochemical or/and cell based assays. The first step is to find a ‘hit’, which means poorly optimised compounds that nevertheless specifically interacts with the target.

The ways of finding Hit compounds are:

High Throughput Screen (HTS)  - In a HTS large numbers of compounds (sometimes many millions) are tested at a single concentration, searching for anything that appears to interact with the target protein. The HTS assay will require optimisation and miniaturisation as HTS screens are very expensive. Typical screening plates now have 1536 wells per plate. This means HTS allows for a high speed evaluation of many compounds usually in a highly automated fashion. Once the ‘hit compounds’ are found, their activity will be checked at multiple concentrations, a dose response. Activity may be confirmed in an orthogonal assay to eliminate false positive compounds that might have been identified as a result of interfering with the screening technology rather than directly modulating the function of the protein. The generation of dose-response curves allows rank ordering of hits through the estimation of half maximal inhibitory concentration (IC50) in case of inhibitor and half maximal effective concentration (EC50) for measurement of other effects.

Fragment-Based Screening - This is an alternative way of screening. Here libraries of very small molecule (molecular weight typically below 300) are evaluated at high concentrations using highly sensitive biophysical technologies and the hits are identified, even if they bind very weakly to the target.

Virtual Screen - If HTS or fragment screens are expensive to run an alternative approach could be a virtual screen. A virtual library of compounds can be docked in silico to a 3D model of the target to prioritise a small set of compounds for subsequent screening in a biochemical/biological assay.

Hit Confirmation - Once a hit is identified, routinely a counter screening, target engagement assay is developed and executed. This confirms the ability of the compound to directly interact with the target protein. Hits may be further triaged to remove examples that interact with closely related biological targets and whose binding might cause confounding biological consequences. Usually the range of potency of the hit against the target is 1µM to 50µM (100mM-5mM if a fragment).

lab test tube

Hits compounds are refined into a short-list of Lead compounds, depending on the following factors:

  • Synthetic accessibility
  • Potency — Dose response
  • Specificity
  • Selectivity
  • Non-toxic
  • Drug-like structure
  • Physical properties — Solubility — Lipophilicity
  • Preliminary Structure Activity Relationship (SAR)

UCL Support:

The Translational Research Office Drug Discovery Group (TRO DDG) has the expertise to bridge the development gap between target identification and robust lead molecules that are suitable to form the foundation of successful drug discovery projects. The team can advise on designing screening cascades (biochemistry and cell based) for drug discovery projects and help optimise those assays or source CROs capable of generating them.

For more information about how the TRO Drug Discovery can support the Hit to Lead process, visit: https://www.ucl.ac.uk/translational-research/hit-lead