Calorimetric Drug Screening
Robert David and Prof. Ian Hunter
Objective: To build a high throughput microcalorimeter suitable for use in early
drug development.
Background: High throughput
screening is currently dominated by fluorescence- and radioactivity-based
assays. While these formats provide
excellent sensitivity, the chemical labels may interfere with the natural
activity of the protein and ligand, and development time for each assay is
typically several months. Non-labelling
technology is therefore desirable.
The gold standard of
non-labelling methods is calorimetry, since it allows direct thermodynamic
characterization of the protein-ligand interaction with the reactants in their
native state (no assay customization, labelling, or immobilization is required). However, current microcalorimeters lack the
sensitivity and throughput necessary to achieve wider application in drug
screening.
Accomplishments/Results: We have
built a single-well microcalorimeter based on a liquid expansion
thermometer. The expansion of the liquid
is measured optically by an interferometer (see Figure 1). This sensing method offers lower theoretical
resolution in a parallel configuration than thermopiles or thermistors.
The first version of
the instrument was a thermometer that measured changes in ambient temperature,
or applied Joule heating. Temperature
resolution (three standard deviations) was ~1 μK in a 0.01-0.3 Hz bandwidth,
and ~0.2 μK in a 0.1-0.3 Hz bandwidth, lower than any known thermometer near room
temperature.
The second version
of the instrument is a microcalorimeter.
It has been used to measure the heat of dilution of 1 μL of sulfuric
acid in 1 μL of water. The reaction drops
are placed on top of the liquid expansion thermometer and physically mixed
using a pipette tip mounted on a stepping motor. Resolution of ~7 μJ has been attained thus
far (see Figure 2).
