High-throughput screening devices such as optical analyzers and microplates are important tools in the pharmaceutical research industry, particularly for the discovery and development of new drugs.
The “drug discovery process” involves the synthesis and testing, or screening, of candidate drug compounds against a target. The candidate drug compounds are molecules or other species that might mediate a disease by affecting a target. The target is a biological (or biologically active) molecule, such as an enzyme, receptor, or other protein or nucleic acid, that is believed to play a role in the onset, progression, and/or symptoms of a disease.
FIG. 1 shows stages of the drug discovery process, which include (A) target identification, (B) compound selection, (C) assay development, (D) screening (including primary screening for hits, secondary screening for lead compounds, and optimization (or tertiary screening) of lead compounds), and (E) clinical evaluation.
A. Target Identification
A target is identified based on its anticipated role in the prevention or progression of a disease. Until recently, scientists using conventional methods had identified only a few hundred targets, many of which have not been comprehensively screened. However, recent developments in molecular biology and genomics have led to a dramatic increase in the number of targets available for drug discovery research.
B. Compound Selection
A library of compounds is selected, following target identification, to screen against the target. Compounds historically have been synthesized one at a time or obtained from natural sources. Pharmaceutical companies, using conventional synthetic techniques over decades, have compiled compound libraries of hundreds to thousands of compounds. However, recent advances in combinatorial chemistry and other chemical synthetic techniques, as well as licensing arrangements, have enabled industrial and academic groups to increase greatly the supply and diversity of compounds available for screening against targets. As a result, many researchers are gaining access to libraries of hundreds of thousands of compounds, in months rather than years.
C. Assay Development
A biological test or assay is developed, after selection of a target, but before screening compounds against the target, to measure the effect(s) of compounds on the activity of the target. Assay development involves selection and optimization of an assay that will measure performance of a compound against the selected target.
Assays may be classified broadly as either biochemical or cellular. Biochemical assays usually are performed with purified molecular targets, which generally have certain advantages, such as speed, convenience, simplicity, and specificity. Cellular assays are performed with living cells, which may sacrifice speed and simplicity, but which may provide information that is more relevant biologically. Researchers use both biochemical and cellular assays in drug discovery research.
Biochemical and cellular assays may use a variety of detection modalities, including photoluminescence, chemiluminescence, absorbance, and/or scattering. Photoluminescence and chemiluminescence assays typically involve determining the amount of light that is emitted from excited electronic states arising from the absorption of light or a chemical reaction, respectively. Absorbance and scattering assays typically involve determining the amount of light that is transmitted through or scattered from a composition relative to the amount of light incident on the composition, respectively.
These different detection modalities each may use a variety of equipment. For example, photoluminescence assays typically employ at least a light source, detector, and filter; absorbance and scattering assays typically employ at least a light source and detector; and chemiluminescence assays typically employ at least a detector. Moreover, the type of light source, detector, and/or filter employed typically varies even within a single detection modality. For example, among photoluminescence assays, photoluminescence intensity and steady-state photoluminescence polarization assays may use a continuous light source, and time-resolved photoluminescence polarization assays may use a time-varying light source.
Adding to this variability, the types of assays that are desired for high-throughput screening are evolving constantly. In particular, as new assays are developed in research laboratories, tested, and then published in literature and/or presented at scientific conferences, these new assays become popular, and many become available commercially. New or adaptable analytical equipment may be required to support the most popular commercially available assays.
D. Screening/Clinical Evaluation
Following selection of a target, compound library, and assay, the library is “screened” by running assays to determine the effects of compounds in the library on the target, if any. Compounds that have an effect on the target are termed “hits.” The statistical probability of obtaining hits may be increased by increasing the number of compounds screened against the target. Once a compound is identified as a hit, a number of secondary screens are performed to evaluate its potency and specificity for the intended target. This cycle of repeated screening continues until a small number of lead compounds are selected. The lead compounds are optimized by further screening. Optimized lead compounds with the greatest therapeutic potential may be selected for clinical evaluation.
E. Shortcomings
Due to the recent dramatic increase in the number of available compounds and targets, a bottleneck has resulted at the screening stage of the drug discovery process. Historically, screening has been a manual, time-consuming process. Recently, screening has become more automated, and standard high-density containers known as microplates have been developed to facilitate automated screening. Microplates are multiwell sample plates that include a plurality of sample wells for containing a plurality of samples. Microplates with 96, 384, and 768 wells have been developed for use in the high-throughput screening industry. In addition, some high-throughput screening laboratories are experimenting with higher-density microplates, with 1536, 3456, and even 9600 wells.
FIG. 2 shows a stack of overlapping microplates with various well densities. Plate 30 has 96 wells. Plate 32 has 384 wells. Plate 34 has 1536 wells. Plate 36 has 3456 wells. Plate 38 has 9600 wells. FIG. 2 illustrates the substantial differences in well dimensions and densities that may be used in high-throughput screening assays. Many analyzers are not flexible enough to read microplates having different numbers of wells, such that it currently may be necessary to provide different analyzers for different modes of analysis. Moreover, many analyzers are not sensitive or accurate enough to read results from the smaller wells associated with the higher-density microplates. Inadequate sensitivity may result in missed hits, limited research capabilities, increased costs of compounds, assays, and reagents, and lower throughput.
Screening an increasing number of compounds against an increasing number of targets requires a system that can operate with a high degree of automation, analytical flexibility, and speed. In particular, because high-throughput applications may involve repeating the same operations hundreds of thousands of times, even the smallest shortcomings are greatly magnified. Current screening systems operate with various degrees of automation. Automation, from sample dispensing to data collection, enables round-the-clock operation, thereby increasing the screening rate. Automated high-throughput screening systems usually include combinations of assay analyzers, liquid handling systems, robotics, computers for data management, reagents and assay kits, and microplates.
Most analyzers in use today are not designed specifically for high-throughput screening purposes. They are difficult and expensive to integrate into a high-throughput screening environment. Moreover, even after the analyzer is integrated into the high-throughput screening environment, there often are many problems, including increased probability of system failures, loss of data, time delays, and loss of costly compounds and reagents.
Most analyzers in use today also offer only a single assay modality, such as absorbance or chemiluminescence, or a limited set of modalities with non-optimum performance. To perform assays using different detection modes, researchers generally must switch single-mode analyzers and reconfigure the high-throughput screening line. Alternatively, researchers may set up the high-throughput screening line with multiple single-mode analyzers, which often results in critical space constraints.
Thus, prior sample analysis systems generally have not recognized the need to provide analytic flexibility and high performance for assay development as well as ease of use and smooth automation interface for the high-throughput screening laboratory. A real need exists for a versatile, sensitive, high-throughput screening apparatus and multiwell sample holders that can handle multiple detection modalities and wide ranges of sample volumes and variations in container material, geometry, size, and density format while reliably maintaining a high level of sensitivity.