Download A novel adult human ventricle slice preparation for cardiac drug

EDITORIAL
Cardiovascular Research (2012) 93, 10–11
doi:10.1093/cvr/cvr293
A novel adult human ventricle slice preparation for
cardiac drug discovery and safety pharmacology
Michael J. Curtis*
Cardiovascular Division, King’s College London, Rayne Institute, St Thomas’ Hospital, London SE17EH, UK
Online publish-ahead-of-print 2 November 2011
The editorial refers to ‘Organotypic slice culture from human
adult ventricular myocardium’ by M. Brandenburger et al., pp.
50–59, this issue.
Cardiac drug discovery and cardiac safety pharmacology assessment
both require new models/methods to advance strategy.1 – 6 Ventricular arrhythmias are no longer a major focus for therapeutic drug discovery, and to become so again, a range of new preclinical models and
a novel experimental strategy will be required for detecting desirable
effects on cardiac electrophysiology and rhythm.2,3 In the field of
cardiac safety pharmacology, new chemical entities that possess the
desired target specificity are entered into high throughput screens
(HTS) for safety pharmacology frontloading, including assessment of
torsades de pointes liability.4 – 6 An integrated safety assessment strategy has appeared during the past 10 years, and optimization of the deployment of the numerous available constituent methods continues to
evolve.4 Part of this involves the search for more inexpensive, more
predictive medium-throughput screens that fit somewhere between
the crude HTS methods, and the expensive and technically demanding
late-stage safety screens.4 – 6
In cardiac safety and discovery, the headline goals are the same: to
determine what a drug does to cardiac ion channels and how this
manifests as an integrated outcome (i.e. effect on action potential configuration, conduction and refractoriness, rhythm and function).1 – 6
The human ventricular slice preparation described by Brandenburger
et al.7 may have a role both in antiarrhythmic drug discovery and in
safety assessment going forward. Its potential value arises from
several of its aspects.
First, it is a human tissue preparation. In research, we justify the use
of animals on the basis of the utility of the readout vs. ethical considerations. Readout utility, in drug discovery and safety assessment,
means clinical relevance. We can never know the clinical relevance
of any method, model, or screen until we have a complete validation
data set.3,4 This means a portfolio of evidence that shows that drugs
that alter the designated readout of the screen do so in a way that
predicts, with precision and accuracy, the effects in humans. For
example, if the model is an HTS channel screen, for the screen to
be validated we need to know that all drugs that block the channel,
and only drugs that block the channel, produce the effect of interest
in humans (the effect of interest being a good effect if this is a discovery screen, and an adverse effect if this is a safety screen). Of course,
there are few if any screens (for discovery or safety) that fully comply
with this requirement for a variety of reasons, the most poignant
being that we may not yet have a range of drugs effective in
humans (a template) with which to validate the model. This is poignant because models are most needed for diseases that are the most
poorly treated and, hence, have the fewest positive control drugs to
construct a response template (this is known as the model validation
paradox).4 There is no way of ensuring that any new model is valid
until it is validated. However, in the absence of proof, the likelihood
is improved if the model is a human tissue model, through the
process of positive reinforcement.3
The second reason why the new model may have advantages in discovery and safety is that it is a slice preparation rather than a single
cell preparation, and it is an adult heart cell preparation. The syncytium is electrophysiologically complete in two dimensions in an organotypic thin-slice culture, but it is absent in the next preparation
‘down’—an isolated adult myocyte—and unavailable in adult form in
myocyte/fibroblast cell co-culture. The combination of adult human
and syncytium gives potential added value greater than the sum of
the parts.
The third reason why the new model may have advantages is that
the thin slice means that cells remain viable in superfusion culture for
long periods,7 allowing for complex experimentation as well as
routine investigation. In addition, and connected with this, the preparation may be used for contractile function assessment (by standard
one-dimensional analysis) providing a separate readout for viability
plus the possibility of simultaneous measurement of drug effects on
electrical and mechanical readout in human ventricle.7 The value of
the viability is not so much the prospect of allowing for long experiments, but that it allows for harvested heart tissue to be stored for
later use. This is extremely important because the availability of
(fresh human) tissue is likely to be a critical issue in most research
centres. The present study shows that after 28 days storage in
culture, 67% of preparations met electrophysiological study entry criteria.7 The hit rate was less (33%) for contractility criteria, and the
function readout did run down markedly over 28 days (both the
The opinions expressed in this article are not necessarily those of the Editors of Cardiovascular Research or of the European Society of Cardiology.
* Corresponding author. Tel: +44 2071881095; fax: +44 2071883902, Email: [email protected]
Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2011. For permissions please email: [email protected].
11
Editorial
inotropic capacity and the lusitropic phenotype, especially relating to
elastance),7 but this nevertheless represents value, especially given
that b-agonist responses (fold changes in force and EC50) were undiminished even after 28 days.7 The contractile rundown was attributable to remodelling typical in unloaded heart tissue; it was not caused
by necrosis.7 Viability validation was substantiated by extensive and
detailed histological, biochemical, and other assessments, including
ion channel mRNA expression and action potential duration responsiveness to dofetilide and rilmakalim.7 Conservation of the latter
drugs’ effects is of particular relevance to readers interested in
cardiac safety pharmacology. These are all benefits of the thin-slice
preparation and are important in that they represent their first realization in an adult human ventricle.
The question that needs to be addressed is whether these potential advantages give rise to more effective translational readout. To be
effective, the model needs to be available on demand. This requires a
reliable supply of viable human tissue. This potential limitation is certainly obviated to a great extent by the storable properties of the
preparation and the associated longevity, but the practicality will
need to be fully explored in any centre wishing to use the preparation. Moreover, to have advantage, the model needs to give
readout that is more clinically relevant than readout that could have
been derived from slices of animal ventricle (dog, or the less expensive and less ethically nuanced guinea pig, for example) or from other
human or animal cardiac preparations (isolated hearts, cells, or membrane preparations). It is implicit that if the present model does
provide readout that is more clinically relevant than that from
other models, the readout must be different in some way. For
example, the model must detect an effect of at least one drug or
drug class that is missed or is qualitatively or quantitatively different
(in a substantial and relevant way) when evaluated in all other
models. The real value of the new model therefore requires that it
be shown to be different, and that the difference is shown to be
uniquely predictive of effects in intact humans. This may seem to
be a harsh stricture, but it applies to any new model. It should be
considered not only when a human tissue model is proposed, but
also when a transgenic mouse or zebrafish model is introduced.
The difference, of course, is that inexpensive and/or ethically unchallenging models are used with fewer pauses for thought than human
tissue models, and this includes pause for thought about the validity
of the readout.
All these factors considered should prompt investigators to examine
the potential value of the new model7 with an unbiased approach.
Aside from the obvious questions that I have raised, and that cannot
yet be answered, one may consider whether the longevity of the preparation confers any advantage in addition to the one noted above. In drug
screening, one drug will require one preparation, meaning that extended
longevity has no real value in that context. Perhaps longevity may allow
the study of electrical or mechanical remodelling, but this will require validation (especially with respect to time-matching, a key issue noted by the
inventors).7 Does the fact that contractile activity can be measured
confer any advantage? It could be argued that we recognize that contractile function is subtle, meaning that its evaluation requires readout that can
be obtained only at the whole organ level. Is it possible to measure inotropy and lusitropy in a meaningful way in the slice preparation? If not, the
benefit obtained from the mechanical function readout may be limited.
However, these are questions for the future. Presently, we have the
makings of a valuable new model that will succeed or fail on the
strength of evolving validation research.
Conflict of interest: none declared.
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