INTRODUCTION

Isothermal titration calorimetry (ITC) has gained wide acceptance in drug discovery and development laboratories throughout the world. Every major pharmaceutical company and most biopharmaceutical companies and major research institutions now use ITC. Modern, highly sensitive ITC instruments are being applied in the drug discovery and development process for applications such as: (a) selection of small-molecule “hits” following primary and secondary screening of chemical libraries against protein targets of interest, (b) optimization of smallmolecule leads based on elucidation of the binding mechanism and binding characteristics and development of structure–activity relationships (SAR) used in the optimization process, and (c) selection and optimization of therapeutic protein variants.

Because ITC directly measures the heat released or absorbed during a biomolecular binding event, it is the only technique that allows simultaneous determination of all thermodynamic binding parameters (n, KA, ΔH and ΔS) in a single experiment. The ITC technique provides this unique capability in an experimental environment that is label-free, in solution, and that does not require immobilization of either the target macromolecule or ligand. Modern instruments are highly sensitive, accurate, and reproducible, and unlike spectroscopic methods, the degree of optical clarity of the solutions is unimportant. As modern ITC instrumentation has evolved, these instruments have become more sensitive, faster, and easier to use. Binding parameters determined by ITC are often referred to as “gold standard” values and are frequently used as reference standard values for other techniques.

Despite the fundamental advantages of this technique and the advances in instrumentation, use of ITC in the early, critical decision-making stages of drug discovery is often limited.