Microphysiological Systems Emerging as Powerful Tools for Drug Discovery

Abstract

Given that only one in ten preclinical candidates in Phase I trials are likely to gain market approval, better predictors of clinical success are urgently needed (1). Translating findings from in vitro studies to in vivo settings remains a challenge because of species differences in pharmacokinetics and pharmacodynamics (PKPD), oversimplified in vitro models, and an insufficient understanding of the underlying pathophysiology. Termination is often attributed to safety issues identified in animal studies, which could be minimised with more accurate predictions of absorption, distribution, metabolism, and excretion (ADME) profiles (2). While 2D monolayer cell culture experiments and animal models are deeply embedded in the pharmaceutical infrastructure, clear gaps, inefficiencies, and inaccuracies exist, creating a need for new alternative and complementary research models (3). At the cross-section of bioengineering and cell biology lies a new approach to drug discovery and development, which is being pursued to help overcome the notoriously low clinical success rate. Microphysiological systems (MPS) are an emerging class of in vitro models expected to expedite drug discovery by providing robust physiologically relevant data at critical stages of R&D (4).

Full text

Improving Drug Discovery Models
A new class of
in vitro
model is expected to assist drug discovery by providing robust
physiologically relevant data at critical stages of R&D
Microphysiological
Systems Emerging as
Powerful Tools for
Drug Discovery
48
Given that only one in ten preclinical candidates in Phase I
trials are likely to gain market approval, better predictors of
clinical success are urgently needed (1). Translating findings
from in vitro studies to in vivo settings remains a challenge
because of species differences in pharmacokinetics and
pharmacodynamics (PKPD), oversimplified in vitro models,
and an insufficient understanding of the underlying
pathophysiology. Termination is often attributed to safety
issues identified in animal studies, which could be
minimised with more accurate predictions of absorption,
distribution, metabolism, and excretion (ADME) profiles (2).
While 2D monolayer cell culture experiments and animal
models are deeply embedded in the pharmaceutical
infrastructure, clear gaps, inefficiencies, and inaccuracies
exist, creating a need for new alternative and complementary
research models (3). At the cross-section of bioengineering
and cell biology lies a new approach to drug discovery and
development, which is being pursued to help overcome
the notoriously low clinical success rate. Microphysiological
systems (MPS) are an emerging class of in vitro models
expected to expedite drug discovery by providing robust
physiologically relevant data at critical stages of R&D (4).
Micro-Engineering Meets Physiology
MPS encompass a range of platforms that mimic aspects of
organ function by using microengineering technology, often
combined with 3D microenvironments (5). Such systems
have been reported as 3D spheroids, organoids, organ-
on-a-chip, multi-organs on chips, static micropatterned
technologies, and physiome-on-a-chip models (5-6). In
these platforms, living cells and microfluidic technologies are
combined with some form of drug delivery, stimulation, and/
or sensing tools. Organ-on-a-chip (OOC) models can exist as
single systems or as connected units that simulate elements
of organ crosstalk. MPS build on concepts harnessed by
traditional two-dimensional assays and include design
features that improve physiological relevance, such as (2):
• A 3D microenvironment within a biopolymer or tissue-
derived matrix
• Mechanical cues that mimic those found in vivo, such as
stretch cues and perfusion, to provide shear stress
• Multiple cell types
• The ability to introduce a concentration gradient
A diverse array of design and manufacturing approaches lie
behind these compact and adaptable systems. Computer-
aided design tools are used to generate digital 3D designs
for microfluidic and microelectronic systems, which can be
imported into 3D printing software (also known as ‘additive
manufacturing’). A range of 3D printing methods exist in
the production of tissue-engineering scaffolds; extrusion-
based 3D printing is an established method that directly
deposits thermoplastic or thermosetting materials using a
layer-by-layer process (7-8). In contrast, stereolithography
is employed to print entire microfluidicsystems and
Dr Audrey Dubourg at CN Bio
48 European Biopharmaceutical Review | July 2020

49
harnesses light and
photoreactive material to
cause spatially-controlled
photopolymerisation.
A diverse array of intricate
components may be
incorporated among
microfluidic channels in an
MPS, such as micropumping
systems, mixing chambers,
synthetic matrices, sensors
(that may be integrated to
online data recorders), valves,
and independently controllable
pneumatic lines (3, 6).
Pathways for cell crosstalk
must be established for
multi-organ MPS, which
may involve soluble factors
or cell migration across
matrices. Tunable flow rates, intra- and inter-MPS mixing and
distribution, and adjustable oxygenation levels provide a high
level of flexibility for researchers to optimise cell viability or ask
experimental questions.
Single and Multi-Organ MPS in ADME Profiling
The applications of OOC models and other MPS are diverse
– just like their manufacturing and design approaches.
Organoids, on-chip models, and other MPS have been
developed for most tissues, and provide unprecedented
opportunities for toxicity testing, personalised medicine,
and studies of PKPD and disease mechanisms. Given their
importance in drug development, significant efforts have been
directed at developing models for absorption and metabolism.
Assays for intestinal drug absorption typically employ colon
adenocarcinoma (Caco-2) cells in a static, 2D monoculture.
Despite their popularity, Caco-2 assays suffer from inherent
limitations that result in a severe under-prediction of
paracellular drug transport (9). Innovative organ-on-a-chip
technologies provide opportunities to overcome this, as in
vivo conditions can be replicated more closely. Improving the
gut MPS epithelial barrier integrity is a high priority, and this
can be assessed by measuring the transepithelial electrical
resistance (10). Towards this goal, Caco-2 cells have been
co-cultured with other intestinal cells, such as goblet-like
mucosal cells, to provide further complexity and complement
dynamically perfused models (5, 10).
The value of perfused hepatocytes in a single-organ MPS was
demonstrated in a toxicology study that captured the effects of
a well-characterised hepatotoxin and revealed novel insights
into the (previously underestimated) toxicity of its analogue (11).
Metabolites formed in a dose-dependent manner, similar to
those seen in a patient overdose, and albumin secretion and
glutathione depletion were measured to assess hepatocyte
function and toxicity, respectively. While the capacity to assess
drug metabolism and toxicological response is a significant
step forward, researchers are aware that an MPS comprised
of a single cell type will not offer a complete solution for
all metabolism studies. To provide access to organ-like
models that more closely reflect the complexity of the liver
microarchitecture in vivo, co-culture models have been
created using multiple cell types
Single- and multi-organ MPS technologies are designed to mimic
specific aspects of organ function and/or crosstalk, rather than
reproduce an entire organ or human body (10). For example,
fully capturing the complexity of kidney function may be out of
reach for renal excretion-related studies, yet progress has been
made towards developing on-chip models and primary kidney
tubule epithelial organoids for studies of specific aspects of
kidney physiology (12-13). Multi-organ MPS can provide insights
on interactions between organs and enable different processes
to be studied simultaneously; incorporating liver tissue or other
organs susceptible to toxicity provides a unique opportunity to
study efficacy and toxicity concurrently (3-4).
Particular emphasis has been placed on mimicking gut-
liver interactions, which are critical to the prediction of
drug disposition, efficacy, toxicity, and the elucidation of
pathophysiological mechanisms. The simulation of gut-
liver crosstalk has been achieved to some degree in MPS,
as evidenced by gut-mediated suppression of hepatic
CYP7A1, an enzyme central to bile acid synthesis (5).
Interconnected MPS that incorporate multiple cell types can
help fill in the gaps in ADME profiling. For example, data on
drug distribution can be obtained indirectly by combining
observations from studies of intestinal permeability, hepatic
metabolism, drug vehicles, carrier proteins, and efflux/influx
membrane pumps (14).
Figure 1: The future of MPS technologies; from single-organ to patient-on-a-chip for personalised medicine
Source: Modied from
Edington
et al,
2018
Oral
dose
IV
dose Gut
Brain
Transwell
Liver scaffold
European Biopharmaceutical Review | July 2020

To support PKPD modelling in multi-MPS, an experimental
design approach known as quantitative systems
pharmacology (QSP) is applied. QSP uses computational
models to describe the complex interactions within multi-
MPS, while considering both physical dynamics (e.g., media
volumes and flow rates) and biological dynamics (e.g.,
hormone production and release) (6). The ability to measure
aspects of ADME in MPS has created many opportunities
across drug development, from disease modelling through to
drug repurposing (see Figure 2).
MPS in Drug Discovery: Finding a Niche
Although safety assessment and ADME profiling is the
primary context for MPS technologies, there are a number
of other ways in which these research models could improve
the efficiency of drug development. To ensure that MPS
development aligns with the greatest needs in the industry,
these opportunities have been considered in depth.
Ultimately, MPS technology innovators aim to improve the
predictivity of drug efficacy and safety for new and existing
drugs (drug repurposing). In turn, this improves the clinical
success rate and accelerates drug development, alleviates
costs associated with drug failures, and reduces the risk to
clinical trial participants. MPS have the potential to greatly
benefit the health sector and identifying specific gaps in
current preclinical research is critical to realising this goal.
Opportunities for MPS applications lie in disease modelling
and phenotypic screening, to help identify and rank new and
known (including orphan drugs and failed compounds that
are available for repurposing) compound candidates (2).
Improved models are being sought for conditions that are
poorly served by animal models (e.g., hepatitis B infection)
and to enable host genetic studies, modelling of drug
treatment responses, and the identification of biomarkers that
could be used to monitor drug treatments (15). Advanced
in vitro models are under development to support studies of
highly prevalent diseases with well-established impacts on
public health, such as non-alcoholic steatohepatitis (NASH).
Microtissue models of human NASH can demonstrate key
hallmarks of disease, providing opportunities to elucidate
pathophysiological mechanisms on a cellular level (16).
By improving predictivity to identify risk over standard tools,
or by providing a more comprehensive model that is not
otherwise available, MPS are expected to fill many gaps. The
ability to reveal toxicities that would otherwise go undetected,
or reveal changes in cell function that precede adverse drug
events would be of significant value (2). However, to maximise
the potential of MPS, insights gathered from these advanced
in vitro models should be considered alongside in vivo data
for comparison. MPS could complement in vivo models by (2):
• Clarifying mechanisms underlying apparent adverse events
revealed by animal models
• Providing confidence to data derived from human-based
MPS, using animal-based chips
• Settling differences found in animal models, e.g.,
when preclinical lesions are found in one of two
preclinical species
50
50
Figure 2: Current opportunities for MPS across the drug discovery process
Source: Modied from Hughes et al, 2017
Discovery Development
Disease modelling Drug metabolism and
pharmacokinetics safety
Clinical trial design and stratification, precision medicine
Target
discovery
Lead
discovery
Lead
optimisation
Pre-clinical Ph I Ph II Ph III Approved
Short-medium duration (2 days to 2 weeks)
cultures of human primary cells or stem cells
under well controlled conditions to simulate
tissue homeostasis or tissue remodelling
Interconnected
MPS
Continuous feeds/sampling for temporal control of
culture conditions e.g., to test complex dosing regimes
or circadian cycles
Extended human MPs cultures
(1+ months) and multi-organ studies
e.g., for investigational toxicology
Fluidic Human
Human specific modalities
Immunotherapy Viral vector
drug delivery
Gene therapies
(RNAL, CRISPR)
Novel
biological
insights
Inter-organ cross talk
Adaptive immune
responses
Combinations
therapies
Target organ for
drug efficacy…
…linked to target
organ for toxicity
Temporal
PK/PD
Female
reproductive cycle
Stem cell
differentiation
Drug
repurposing
Post-approval studies
Investigational
toxicology Drug rescue
Fluidic and mechanical
stimulation of disease
states
Cue-signal-
response
Basic biological
research and target ID Inflammatory
diseases Metabolic syndrome
and NASH
European Biopharmaceutical Review | July 2020

Organoids are being actively pursued for use in cancer
research, where the consideration of inter- and intra-patient
heterogeneity is critical to therapy development. Equally,
there could be benefits to reducing variability in certain
settings, by using cells from the same individual to create
MPS for studies of multiple doses, drugs, and timepoints (2).
Looking Ahead: What is Needed for the
Successful Implementation of MPS?
Establishing translational relevance will be critical to the
successful integration of MPS in preclinical research.
Developers and researchers must clearly demonstrate
benefits over existing models, while communicating
effectively with other stakeholders to identify and navigate
challenges, needs, and approaches to validation (2).
Technology needs to be developed with the end-user
in mind, with a design that maximises usability and
reproducibility. Offering high-throughput capabilities that
are compatible with automation provides an incentive to
researchers who would benefit from efficiency gains and
reduced labour costs. In some cases, MPS could also
reduce the cost of animal trials, cells, and reagents, as
smaller volumes are needed for many microfluidic devices.
Significant efforts have been directed at extending the
lifespan of MPS models to provide a greater window for
long-term experiments, enabling compound dosing and
observations of disease progression; in vitro models of
intestinal barrier function and liver disease models have
been maintained for a number of weeks (5, 16).
After finding their way into research settings across
the globe, single- and multi-organ MPS are emerging
as powerful tools for drug discovery and development,
from disease modelling to drug repurposing. To improve
the chances of clinical success, the pharma industry is
currently evaluating and adopting these technologies, while
technology developers continue in their pursuit of advancing
MPS for use in drug discovery.
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Dr Audrey Dubourg is CN Bio’s Product Manager for their
PhysioMimix™ Organ-On-Chip lab benchtop platform, which enables
researchers to model human biology in the lab through rapid and
predictive 3D tissue-based studies harnessing microfluidic technology.
Audrey has an extensive research background in microbiology and
parasitology, with significant experience in molecular biology techniques
and 3D mammalian cell culture employing MPS technologies. Audrey
gained her PhD at the University of East Anglia, UK and carried out a
postdoctoral scholarship at the University of California, US.
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European Biopharmaceutical Review | July 2020