Discussion
To gain insight into the non-clinical development program for cell-based
products we analysed the non-clinical data package of 86 different
cell-based products based on scientific advices provided by the EMA
between January 2013 and June 2018. A 5-year time period was chosen to
ensure that our data contained a sufficient number and diversity of
cell-based therapies to allow for a meaningful analysis of the
non-clinical development program. In comparison, a previous analysis of
the non-clinical program in EMA advices on ATMPs contained 54 ATMPs of
which thirteen were somatic cell therapy and eight were tissue
engineered products[22].Within the 86 products analysed, a variety
of autologous or allogenic cell-based products with and without
genetically modifications were included, however the class of products
containing genetically modified allogenic cells was only represented by
six products.
A non-clinical development program consists of in vitro andin vivo studies designed to provide a clinical treatment
rationale (PoC) and to gain insight into the safety profile of a
medicinal product. For cell-based therapies translation of animal data
towards the human situation has significant limitations. When testing a
human cell-based product in an animal (heterologous model) limitations
include possible differences in cell size or cell metabolic rate
[23], immunogenicity
(xeno-reactions can occur extremely rapid and vigorous) [24] and
potential species specificities in
cell-cell or cell-environment interactions [25]. These limitations
all need to be considered when interpreting study results. A homologous
model in which the animal equivalent of the human cell-based product is
tested, could be considered as an alternative in vivo testing
strategy. However, interpretation
of results from such a model may be complicated by differences in
manufacturing processes of the animal equivalent, deviation of animal
response from the human response or absence of an appropriate animal
equivalent of the human cells[25].
Interestingly, while the translation of animal data to the human
situation has significant limitations for cell-based products, only for
six products animal studies were not performed or proposed by the
sponsors nor requested by the regulators.
These products contained cell
types, which have been studied in humans for a relatively long time,
such as dendritic cells [26], antigen-specific T-cells [27] and
mesenchymal stromal cells (MSC) [28]. Apparently, for these products
it was considered that additional animal studies would not provide new
insights on these types of products. As the clinical experience with
cell-based products is increasing, the knowledge on the (potential)
effects of the studied cell types is also increasing. This may result
into a reduction or even omission for the need of in vivo animal
studies for certain types of cell-based therapies.
Still for most products the sponsors considered data from animal studies
of value for the development of their product. This was particularly the
case for studies on PoC as in vivo pharmacology studies were
performed for almost all the products. Also safety was studied in
vivo in animals for the majority of the cell-based products, albeit
more often in combination with other endpoints e.g. by also addressing
the cellular biodistribution or by including the safety endpoints in
pharmacology studies. Thus, it seems that the focus of the non-clinical
development program for cell-based products is more on pharmacology/PoC
than on safety. This is different from what we see for more
‘conventional’ medicinal products, where safety is an important
component of the non-clinical in vivo studies and almost
exclusively evaluated in dedicated toxicity studies.
The fact that pharmacology is most often studied in a dedicated study
could be explained by a difference in timing of the studies, as
non-clinical data are mostly intended to support the rationale for
further development before safety studies are performed to ensure the
safety of the first clinical trial subjects. Pharmacology studies do not
require GLP compliance, in contrast to safety studies. This may have
contributed to the higher number of products with pharmacology studies.
Notably, although GLP-compliance is formally required for non-clinical
safety studies for human medicinal products, for cell-based therapies it
has been acknowledged that GLP compliance of in vivo safety
studies may not always be feasible [29].
In our analysis we particularly focussed on the need for and the type of
biodistribution and/or tumourigenicity studies since these aspects are
very difficult to study in humans. For more conventional medicinal
products, the kinetics of the product and its safety profile is studied
in animals. Interestingly, in our analysis in vivo animal studies
on biodistribution and/or tumourigenicity were not considered necessary
by both sponsors and regulators for approximately one third of the
cell-based products irrespective of product type. This observation
suggests that for cell-based products a more tailored non-clinical
development program is necessary. Thereby sponsors should focus on the
need to better characterise the properties of the cell-based product and
consider the (im)possibilities of studying a products behaviour in an
animal model.
Even though a considerable number of cell-based product in our database
are lacking an in vivo study on biodistribution, still this type
of study was performed for two-thirds of the products and considered to
be of value for understanding the PoC and to gain insight into the
safety profile. For biodistribution most often a heterologous animal
model was used. This is remarkable as cell migration is expected to be
dependent on chemotactic signals and cell-cell interactions which might
be species-specific and may thus better be studied in a homologous
model. Next to distribution to target and non-target tissue, also
persistence of the cells was (proposed to be) evaluated for
approximately two third of the products, suggesting that sponsors
already acknowledge persistence as an important biodistribution endpoint
to be investigated. Detection of administered cells in target and
non-target tissues was most commonly done by tissue sampling followed by
(semi)quantitative detection of these cells with various sensitive
techniques. Notably, more sophisticated techniques based on PET, SPECT,
and MRI imaging are being developed, which can trace living cellsin vivo [14,20], allowing for serial sampling and whole-body
scanning. However, these techniques were rarely used to study the
biodistribution of the products in our database. Possibly these
techniques were still not sufficiently developed at the time of the
design of the non-clinical studies for our analysed products. It is also
possible that these techniques were not (yet) attractive due to high
costs, limited access and/or the perception that these techniques are
too novel and have not yet proven their value to ensure their
acceptability by regulators. Notably, these newer techniques of cell
tracking may also become suitable for the evaluation of biodistribution
of cellular therapeutics in humans, thus in the clinical setting.
Clinical biodistribution studies would be of much more relevance as the
therapeutic product can be tested directly in the target ‘species’. The
hope is that these more advanced methods analysing distribution of
cellular therapies, will be implemented in due time and that they could
possibly obviate the need for extensive evaluation of in vivobiodistribution of human cell-based products in animals.
As tumourigenicity is a concern associated with cell-based products, it
is not surprising that for most products experimental data was used to
address the potential for tumourigenicity. For half of the products this
was planned or performed to be addressed by means of in vivoanimal studies. For four-fifths of these products, in vitrostudies supplemented the tumourigenicity evaluation. One-fifth thus
leaned on in vivo studies only. The design of these in
vivo studies appeared rather similar across the various products, and
all tended to follow the WHO guidance [16] on the assessment of
tumourigenicity of mammalian cells (i.e. 1x106 cells,
subcutaneous administration, 3 to6 months in duration), except that
intravenous administration was often used as well to better reflect the
clinical RoA. Interestingly, this WHO guidance was developed for
characterisation of a master or a working cell bank (MCB, WCB) [16],
and not to address the tumourigenic potential of human cell-based
products. The suitability of this study design for evaluation of the
tumorigenic risk of cell-based products has not been established.
Recently, the added value of in vivo tumourigenicity studies has
been questioned [21]. With respect to this, it is interesting that
only for a minority of products the in vivo study was considered
to have some relevance by the regulators or experts. Despite the limited
relevance, the non-clinical data package on the tumourigenic potential
was considered sufficiently informative for most of the products. This
implies that while limited value was given to the in vivostudies, the value of in vitro studies for the assessment of
tumourigenicity is recognized. Notably, an in vitro -only
evaluation of tumourigenic risk was accepted for two types of products,
i.e. for autologous cells and for genetically modified products. For the
genetically modified products in vitro evaluation of insertion
site analyses (ISA) seem to be sufficient for addressing the
tumourigenic risk. Possibly, the need for in vivo animal studies
for the safety assessment of cell-based products may be further reduced
and more sensitive and standardised in vitro assays may be
developed to characterise the risk for tumourigenicity [18,21,30].
To conclude, our analysis has given insight into the non-clinical
development program for cell-based products, specifically on the number
and types of studies performed for the various types of products. It
appears not possible to define a common route to-be-followed for the
non-clinical development program of cell-based therapies. While it is
clear that during development studies on the pharmacology,
biodistribution, general safety and tumourigenicity should be
considered, it is not evident that for all these aspects animal studies
need to be performed or how they should be designed. Not only because
the variety of products prohibits a-one-size–fits-all approach for
non-clinical development, but also because insights are changing and
clinical experience is growing, which can affect the need for animal
studies for future products. Moreover, technical possibilities to study
certain aspects may change in time as well. Therefore, the
recommendation remains to tailor the non-clinical development program
for cell-based products to the specific need for information, depending
on the characteristics of the product and take into consideration
available knowledge and relevance of in vivo animal studies. To
this end, early dialogue between sponsors and regulators will remain
utterly valuable.
Acknowledgement
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