High-Throughput and High-Content Screening for Huntington’s Disease Therapeutics

Since the isolation of the gene and mutation that cause Huntington’s disease (HD) more than a decade ago, there has been optimism that this knowledge would lead to the rapid discovery of therapeutic agents for this fatal and incurable disease [1]. In fact, considerable effort has been invested in HD drug discovery, predominantly in academic environments but also in the biopharmaceutical industry to some certain extent. Historically, however, interest in HD research and development in large pharmaceutical firms has been limited by the relatively small size of the HD patient population (~30,000 affected persons in United States), which has led to the perception that the market size for HD might be too small to justify the investment of substantial resources necessary to bring a new drug to clinical trials. However, there is a growing appreciation for the actual market size of “first in class” drugs for otherwise unaddressable diseases, as well as for the idea that HD may prove to be a paradigmatic disease for other, much more prevalent neurodegenerative diseases, such as Alzheimer’s disease and Parkinson’s disease [2]. One hurdle in drug discovery for most neurodegenerative disorders is their incompletely understood, multigenic, and multifactorial etiology. Thus, animal models of these diseases may often recapitulate only some aspects of such diseases and rarely reproduce the full pathophysiology of the human diseases [3], [4]. In contrast, HD is caused by a well-defined mutation in a single autosomal gene that has a dominant and fully penetrant phenotype. This simplicity in the genetics of HD has allowed the creation of a variety of cells cultures and transgenic animal models for HD in which there can be greater confidence that these models do capture important aspects of human disease initiation and progression [5]. This chapter will focus on the development and implementation of and assays for the discovery of HD therapeutics. High-throughput screening (HTS) of small molecules allows the rapid interrogation of the effects of thousands to hundreds of thousands of small molecules in a variety of and cell-based assays, whereas high-content screening (HCS) approaches may sacrifice some of these high-throughput capabilities in return for great biological and phenotypic complexity in the assay endpoints used. Why do we need such high-throughput and high-content methods? Simply because we do not currently have sufficient knowledge of the molecular targets and pathways that may be therapeutic for HD nor do we know how to design custom small molecule compounds that will be guaranteed to have the desired effect on such biological targets. Hence rapid testing of many tens of thousands or more drug molecule candidates in HD models offers the potential for systematized serendipity, that we will encounter effective compounds in such discovery campaigns that will prove to be clinically relevant, using controllable and predictable screening processes. Active compounds emerging from HTS and HCS screens—termed “hits”—are the templates on which eventually drug “leads” are developed through further combinatorial and medicinal chemistry efforts Figure 5.1. Such efforts in the field have identified a number of hits that are being pursued as drug leads (see Chapters 8 and 12, this volume). In the following sections we will describe different strategies and approaches in the design and implementation of HTS and HCS screens for HD, and will also discuss the development of prioritization schemes for potential drug leads identified, future screening approaches, and the use of these compounds for gaining new insights into mechanisms underlying HD.