So many targets, so little time...

In the post-genomic era of drug discovery, HTS is proving a vital tool in identifying new drug leads

HTS (high throughput screening) combines modern robotics and specialised laboratory hardware that allow the scientist to conduct hundreds of investigative experiments at once. HTS collects a large amount of experimental data, typically about how a biological entity reacts to exposure to various chemical compounds, in a relatively short time and is an important tool for drug discovery.

In HTS, plates containing the biological components are incubated with a range of compounds and observed to see whether the compounds have absorbed, bound to or reacted in any way with the drug target. If any compounds provide such ‘hits’, further investigation can be carried out. This allows the collection of data on this narrowed set, confirming and refining observations. Automation is the key element in the usefulness of HTS in the lab.

HTS is a relatively recent innovation in the drug discovery field and has come about as a result of the advances in computing, assay and robotic technology. It does, however, involve a fair amount of expense to set up and run HTS operations.

The analytical and screening team at the Cancer Research UK Centre for Cancer Therapeutics, based at the Institute of Cancer Research’s site in Surrey, discover and develop small molecule drugs that act on molecular targets responsible for the growth of cancer cells. They are also developing a gene-directed enzyme pro-drug therapy (GDEPT). To achieve these aims the centre has a multidisciplinary team structure and a range of modern technologies, (including combinatorial chemistry, gene expression microarrays, high throughput pharmacokinetic analysis and HTS), to accelerate the drug discovery process.

HTS, used to identify potential new drug leads, is carried out by the Analytical Technology and Screening (ATS) Team. The team’s main focus is to run high throughput screens for the identification of mechanism-based cancer drug target inhibitors. The pharmacological properties of compounds identified this way are then optimised using medicinal chemistry, often supported by structure-based design, and the mechanisms of action and antitumour activity investigated. The overall aim is to develop compounds for proof of principle clinical trials, which are carried out in collaboration with the Royal Marsden Hospital and Cancer Research UK.

 HTS id used by the analyticaltechnology and screening team to identify potential new drug leads.

Dr Wynne Aherne, ATS Team Leader, explains: “Recent advances in understanding the molecular mechanisms that underlie the malignant phenotype, including the identification of oncogenes, tumour suppressors, overexpression and constitutive activity of signalling pathways etc, backed up by information from the Human Genome and Cancer Genome projects, have provided the opportunity to selectively target tumour cells.

“Previously developed anticancer agents relied to a great extent on targeting DNA synthesis, DNA integrity and the mechanics of cell division, resulting in agents with poor selectivity for tumour cells versus normal cells. Targeting the molecular basis of malignancy will lead to much more selective agents, reducing the effects on normal cells. HTS, complemented by rational design, virtual screening and structure-based design, is one tool for identifying molecules that inhibit or modulate these novel drug targets. There is now a real possibility of finding not one, but a series of genome targeted ‘magic bullets’ for cancer treatment. And it is not unreasonable to forecast that in the next ten years or so patients will be treated with therapies that are tailored to interact with molecular aberrations that drive their own individual tumours.”

HTS has had a vital role in the initial identification of many of the small molecules that are currently being evaluated as inhibitors of cancer-causing pathways and will continue to be a key step for finding the small molecules that will moderate the post-genomic stream of novel drug targets. Previously, the identification of cancer drugs relied almost exclusively on screening for cytotoxic effects against certain cancer cell lines grown in animals or tissue culture. Typically the cell lines used in this ‘black box’ screening were, in many cases, not of human origin.

With the advent of new genome-based targets, HTS has become one of the major influences on the way drug discovery has been conducted over the last decade. As with other diseases, the number of targets identified for cancer drug discovery is too great for them all to be pursued economically. A robust validation and selection of targets is required before undertaking a screen.

Dr Aherne explains: “HTS is used because the number of potential targets for molecular mechanism-based drug discovery is now too high to rely on traditional drug discovery processes. We aim to run at least six different target screens a year. HTS uses chemical diversity to find hits which can then be developed further. High throughput increases the numbers of targets that can be evaluated, although it is important to carefully select the target with respect to certain criteria and to be critical on selecting compounds for further investigation.”

The success of HTS depends to a large extent on the quality of the compounds screened. Inclusion of molecules that conform to Lipinski’s rule of five (Lipinski's rule-of-five analysis helped to raise awareness about properties and structural features that make molecules more or less drug-like), use of appropriate libraries and exclusion of highly reactive molecules from compound collections have improved the quality of hits obtained.

Dr Aherne adds: “High throughput screening has overcome the bottleneck of finding compounds active against the many new targets identified in this post-genomic era. It does not necessarily accelerate the process, and it is important to ensure bottlenecks do not subsequently occur downstream of HTS. This is why a managed, integrated project approach is important.”

The team has been successful in meeting it’s goal of running at least six HTS screens each year – both biochemical and cell-based – and this process is filling the Centre’s drug discovery pipeline.

Dr Aherne concludes: “The ATS team aims to use the best technology available to continue finding hits against emerging molecular targets important in cancer and, within the Centre as a whole and in collaboration with our colleagues and partners, to develop these compounds towards the clinic for patient benefit.”  

By Caroline King