Pharmaceutical High Throughput Screening (HTS) programs are under pressure to process an ever-increasing number of samples in ever-shorter time frames. At the same time, there is demand for better performance and lower costs. Fluorescence resonance energy transfer (FRET) is a phenomenon that can be exploited to achieve these objectives. When two suitably chosen fluorophores are brought in close proximity to one another, one (the donor) can transfer its excitation energy, through a radiationless long-range dipole-dipole interaction, to the other (the acceptor). The efficiency of the FRET process is a steep function of the donor-acceptor intermolecular distance. Thus, FRET offers a general, non-destructive experimental spectroscopic approach to monitor the dynamic association of molecular partners in living cells or in vitro with high temporal and spatial resolution. In terms of HTS applications FRET has the significant advantage that it enables a homogeneous assay format, which avoids separation steps like filtration and washing which can be time consuming and labor intensive and are the cause of some of the major bottlenecks in current HTS systems.

The energy transfer efficiency is defined as the ratio of the energy transfer rate to the sum of the rates of all donor de-excitation processes; hence transfer efficiency can be determined experimentally from the relative quantum yield of the donor determined in the presence and absence of the acceptor. Since it is not practical to measure quantum yields, generally the transfer efficiency is calculated from the relative fluorescence intensity of the donor determined in the presence and absence of the acceptor (alternatively, if the acceptor is fluorescent, sensitized acceptor fluorescence intensity may be monitored). While these approaches may be adequate in a research setting, with stringently controlled samples, intensity measurements can be affected by many factors, and become subject to many errors in "real world" settings, such as in an automated cell-based HTS system. In particular, intensity-based systems are subject to errors introduced by unavoidable photobleaching of the fluorophores, uncertainties in fluorophore concentration, changes in sample turbidity or optical density, and any instrumental factors that affect the excitation intensity or optical collection efficiency. A more robust approach is to determine transfer efficiency from the lifetimes of the donor determined in the presence and absence of acceptor.

Ciencia is currently developing novel nuclear reporter gene assays and ligand binding assays targeted to cell-based HTS applications based on the use of fluorescent fusion proteins, FRET and lifetime sensing.