Hit compound

A lead is a hit compound that displays specificity and potency against a target in a library screen and continues to show the initial positive dose-dependent response in more complex models such as cells and animals. 

As the availability of crystal structures increased in the early 1990s, a number of experimental and computational methods were developed to use the structure of the protein target as a route to discover novel hit compounds. The methods include de novo design, virtual screening, and fragment-based discovery. These developments are covered in more detail in the later chapters of this book, but their main features can be summarized as follows. 

In the right-hand section, N-ethylation of the aniline nitrogen was optimal, while shifting the methyl to the 4-position increased activity. This, combined with fluorination at the m-position, afforded (23b) as the most potent compound of this series. Since the pharmacokinetic properties of (23b) were not disclosed, it is not clear if manipulation of the hit compound led, apart from an increase in potency, to a corresponding improvement of the pharmacological profile. 

In a HTS campaign, typically hit compounds are ranked as a function of the number of initial positives (varying from 0.1 to 0.001% of the screened compounds), followed by those for which dose responses could be obtained (generally only 10% or less of the original hits). Of the few remaining compounds (if any),... 

Fig. 1 Different sources of hits typically provide different distributions of number of hit compounds and amount known (and relevant) about those compounds, each of which contributes to the decision making process of hit selection and hit triage...

These specific and general data inputs serve as the basis for making judgments on how substantially the structure of the hit compound(s) will need to change as the potency, ADME, and safety properties are concurrently optimized. These judgments form the basis of the lead optimization effort. As such, an important consideration is the selection of compounds to define the boundaries of the structure-ADME relationship of a series. In some cases, the compounds that are selected for defining the ADME relationships will not be the same set of compounds that have the most potent primary pharmacology. 

Table 1 Common in vitro assays to assess ADME properties of hit compounds Clearance...

In a hit triage decision making process that blends the use of experimental data with expected general property trends and principles, there are situations where it is not feasible to obtain sufficient data to identify experimentally property trends for ADME or safety endpoints (either due to a small number of hit compounds in a series, or due to limited experimental capacity). Computational models for these parameters may provide some useful information when integrated with other known information [101],... 

The development of maraviroc (21), much like other chemokine receptor antagonists, started with a high-throughput screen employing a competition binding assay and led to the hit compound UK-107,543. Chemical optimization of this compound led to the development candidate UK-427,857 during this optimization phase, a parallel characterization of the compounds was performed... 

The validated HTS assay was then employed to screen a 100,000-compound library against SARS CoV. The hit rate for the library in a single-dose format was determined to be approximately 0.8%. Screening of the three libraries resulted in the identification of several novel compounds that effectively inhibited the CPE of SARS CoV in vitro. Three hit compounds, shown below, were identified as promising lead candidates for further evaluation. 

Partly due to the limited throughput of Caco-2 permeability measurements, the structure-activity evaluation of compounds tested in the hit-to-lead phase is done with minimal permeability information, at best. Given the importance of membrane permeability in drug absorption, early consideration of the permeability characteristics of hit compounds would enhance the drug-like quality, and ultimately the probability of success, of selected lead candidates. To incorporate permeability information into the hit-to-lead phase of the drug discovery process it is necessary that permeability measurements be made quickly and with small amounts of material. Thus, efforts have been made to automate and miniaturize the Caco-2 permeability assay. 

A number of criteria are essential for the decision to take up a hit compound singleton or compound series to the hit-to-lead stage. The criteria. 

Large-scale production of selected hit compounds e.g. large-scale isolation or synthesis... 

As HTS still remains the method of choice to discover novel hit compounds, researchers focused their attention to the design and development of appropriate tools to reduce the size of the chemical libraries to be tested while increasing the quality of the compounds, what could maximize the chances of identifying hit compounds amenable to the subsequent lead optimization stages... 

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