Margaret Porter-Scott, Epizyme
Written by Jackie Howard Wednesday, 09 May 2012 09:30
Reducing histone methyl transferase biology to practice: development of physiologically relevant in vitro assays to identify hits and successfully develop leads into candidate molecules
Despite the strong pathobiologic rationale for HMTs as drug targets, initiating a practical drug discovery program has posed many challenges. The first challenge was uncovered when Epizyme scientists engaged in a systematic campaign to understand the enzymology of HMT catalysis in detail. A study of our HMT panel revealed that HMTs generally have relatively low rates of substrate turnover and bind substrates and products with unusually high affinity. These characteristics require assay formats with exquisite sensitivity. A second key challenge for this enzyme class is that many HMTs function under physiological conditions in the context of multi-protein assemblies; indeed, in some cases catalytic activity in vitro is only realized when all components of a multi-protein complex are co-expressed. Moreover, the physiological substrate for HMTs also poses challenges as chromatin is large, heterogeneous and difficult to express recombinantly.
In addition to the challenges of assay development for the HMTs, these enzymes also represent unprecedented drug discovery targets for which conventional diversity libraries may not constitute a rich source of inhibitor starting points. We have committed to a multi-pronged approach to hit identification, using a combination of diversity screening, fragment approaches, mechanism-guided design and structure-guided design. Because of the unprecedented nature of the HMTs as drug targets, all of these hit-finding modalities have been challenged by various forms of false positive hits, such as redox active species and promiscuous inhibitors. Hence, we have established a comprehensive suite of post-hit validation assays to identify bona fide HMT inhibitors with good pharmacological tractability. Through these efforts we have developed a large and growing library of HMT-focused inhibitors that provide starting points for new program initiation within the HMT target class.
Finally, a case study of our DOT1L program will be presented to illustrate how these various aspects of our platform are integrated to form a targeted approach to drug discovery and development.
Margaret Porter Scott, Epizyme