Screening
improving pharmaceutical productivity. Thus, as the industry begins to adopt phenotypic approach- es in drug discovery, it will be important to take into consideration that successful phenotypic pro- grammes will also require validation to ensure that the assays used effectively recapitulate the dis- ease biology of interest. The need for better under- standing of disease biology remains true whether target-based or phenotypic approaches are taken.
Designing physiologically- relevant phenotypic assays
There have been several recent perspectives written about the design of physiologically-relevant assays for phenotypic drug discovery. In 2015, Vincent and co-workers at Pfizer4 published their ‘rule of 3’ covering: (1 Selecting physiologically-relevant cell types and formats; (2) Choosing assay stimuli that optimise disease relevance; and (3) Using assay endpoints that are proximal to clinical endpoints. Horvath and colleagues5 described a set of 17 prin- ciples to be considered when designing physiologi- cally-relevant assays encompassing assay objectives (context of use), cell types and media selection, as well as micro-physiological characteristics and selection of clinically-relevant doses. Most of the efforts in phenotypic discovery are focused on in vitro systems rather than animal models, as in vitro assays provide higher throughput and can be devel- oped with human cell types, improving translation. Incorporating greater physiological relevance through the use of human iPSC-derived cells or pri- mary cells in phenotypic screens has been made possible through the development of methods to culture and differentiate these cells. While primary human cells are considered the gold standard, iPSC or stem cell-derived cell types are highly attractive due to the potential for genetic screening or gene editing through new technologies such as Cas9/CRISPR6. Differentiation protocols to pro- duce more adult-like phenotypes continue to evolve7. The use of co-cultures, incorporating more than one cell type, is another option to improve physiological relevance within the con- fines of high throughput screening paradigm. Co- culture systems, if appropriately designed, have been shown to increase the biological relevance and detection of disease-relevant mechanisms with minimal impact on assay performance8. While developing assays with high physiological relevance is a goal, there can be practical consider- ations for high throughput primary screening that limit assay design choices. For example, the use of patient-derived cells may not be feasible due to excessive variance caused by patients being at dif-
Drug Discovery World Fall 2017
ferent stages of disease or having different drug exposures. Throughput is also a consideration. Although advanced cellular systems with complex formats such as organs-on-a-chip, bio-printed tis- sues and organoids have been proposed as a solu- tion for their perceived greater physiological rele- vance5,9, many lack the requisite throughput required for large-scale screening. Indeed, initial studies using these systems have generated much excitement10, even though validation of these for many contexts of use remains to be established. The validation process for complex in vitro sys- tems to establish reproducibility and disease rele- vance is not trivial. As the complexity of an assay system increases, so do the number of variables and the potential for reproducibility problems. Validation requires sufficient throughput to per- form the extensive evaluation needed to fully char- acterise the biology represented by the system and clinical relevance11. Since sampling the largest expanse of chemical matter is the goal in primary screening, it is preferable to employ the simplest assay that covers the desired biological mechanism. Thus, the most valuable application of these highly complex, physiologically-relevant systems may be in basic research, building our understanding of disease biology through testing known drugs or genetic modifications, for new target identifica- tion, or for characterising small numbers of lead candidates at later stages of discovery.
Phenotypic drug discovery – practical lessons learned
As adoption of phenotypic approaches in pharma- ceutical discovery research has expanded, reports of successes as well as lessons learned have begun to emerge. A recent perspective9 written by inves- tigators from Genentech, Pfizer, Eli Lilly, Novartis and Roche emphasised the need to have sufficient understanding of disease biology to develop a suc- cessful screen. They propose a ‘chain of translata- bility’ to build confidence in the assay system to ensure effective modelling of the disease of interest. This chain of translatability connects the endpoints measured in the assay system through to the clini- cal outcome, a framework that is in some ways analogous to the adverse outcome pathway con- cept being developed for chemical toxicity risk assessments12. The strength and confidence in this connection is important given that the ability of the screening assay to predict clinical therapeutic response is a fundamental determinant of success. This is more straightforward for some therapeutic areas (eg infectious diseases where the infectious agent itself is causative) than for others (eg chronic
27
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72
