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Part II
Experimental Techniques
5Differences between Human and Animal
Discs: Pros and Cons of Current Animal
Models for Preclinical Development of
Biological Therapies for Low Back Pain 38
6 Grading Scales for Disc Degeneration
and Regeneration: Clinical and
Experimental 48
7 Disc Regeneration: In Vitro Approaches
and Experimental Results 54
8 Intervertebral Disc Whole Organ
Cultures 66
II
Differences between Human and Animal Discs
5Differences between Human and Animal Discs: Pros and
Cons of Current Animal Models for Preclinical Development
of Biological Therapies for Low Back Pain
Jeffrey C. Lotz
Abstract
Animal models are an important component of preclinical therapy
development for disc degeneration and discogenic pain. These
models demonstrate the biological response and functional performance within context of in situ stressors that include load, host
cells, and transport. Unfortunately, there is no consensus on the
preferred model for preclinical purposes. Animal model factors
that are commonly debat ed include disc size, cellular content, tissue composition, and in vivo loading. This review summarizes
current literature regarding these factors and discusses how they
should be integra ted into the decision making for those developing biological therapies for the disc. Ultimately, the most significant barrier for identifying the ideal model is ambiguity regarding
disc pain mechanisms in humans, and the lack of validated pain
measures in animals that forecast clinical trial outcomes.
Keywords: animal models, low back pain, spinal disc
degeneration
5.1 Introduction
Animal models allow investigators to collect data on physiological and pathological conditions that form the basis for developing clinical interventions. These models also provide a critical
therapy development tool, and form a central component of preclinical evaluation of biological treatments for disc degeneration–related back pain. In situ factors such as physical stress,
transport, immunity, and tissue spatial relationships will
undoubtedly influence the retention and activity of regenerative
products. Equally important, the consequences of therapy activity such as paracrine communication with host cells and new
matrix formation will mature over time, and therefore the longitudinal assessment in vivo is required to judge the ultimate quality and safety of the desired clinical disease-modifying activity.
These therapy aspects cannot be assessed purely with in vitro or
computer models, and consequently, animal models are a necessary bridge to human clinical studies and provide the framework
for submission, review, and regulatory approval.
A primary obstacle for disc regenerative therapies is whether
the product (cells, matrices, and/or growth factors) can stimulate a desirable disease-modifying activity within the inhospitable niche of the degenerating human intervertebral disc
1
This fact underscores the importance that the chosen
(IVD).
animal model mimics the human situation relative to the salient technical and clinical features. The larger the discrepancy
between the animal model and human, the greater the risk that
the therapy will be ineffectual or even harmful.
Many animal models have been investigated in attempts to
clarify disease mechanisms.
sarily appropriate for judging the quality of a regenerative
2,3,4,5
However, these are not neces-
response. Similarly, there is a diversity of animal models used
for preclinical testing of bioactive disc therapies.
cant gaps between outcome measures in animals and pain
mechanisms in humans muddle decision making when designing preclinical studies. The purpose of this review is to summarize factors that should be weighed when choosing an animal
model for therapy development and validation.
6
Yet, signifi-
5.2 Anatomy
Across the various species used for spine research, the IVD comprises the same basic subtissues: nucleus pulposus (NP), annulus fibrosus (AF), and cartilage end plate (▶ Fig. 5.1, ▶ Fig. 5.2,
▶ Fig. 5.3). This is not surprising given the similar develop-
mental origins and functional requirements to facilitate spine
flexibility while supporting load. However, species diversity,
principally in size and bipedalism, leads to differences that can
become significant in the context of disc tissue engineering.
5.2.1 Size
There is a tremendous range in disc size across species. For
example, disc height can range between 0.25 mm for a mouse
and up to 11 mm or more for a human (▶ Fig. 5.1; ▶Table 5.1).
However, when disc height is normalized by width as a measure of shape, the differences between animal and human are
less extreme, and range between 12% for mouse and 31% for
7
Yet, these size differences have important biomechani-
sheep.
cal and biological consequences. Three centuries ago Galileo
proposed the square-cube law, which states that as a structure’s
size increases, its volume (proportional to length cubed) grows
faster than its surface area (proportional to length squared). For
example, if an animal’s disc size is doubled, then the surface
area is increased by a factor of 4, whereas the volume is
increased by a factor of 8. In this illustration, the square-cube
law indicates that the larger disc may carry four times as much
stress and nutritionally support 8 times as much tissue. Assuming that the same biomechanical and biological principles apply
across species, then discs need to be redesigned and/or the animal behavior needs to be modified for tissue homeostasis as
the animal becomes larger.
5.2.2 Biochemical Composition
Biomechanical properties of spinal discs can be linked to their
biochemical composition and size.
constituent of the disc nucleus and, due to their fixed negative
charge, provide the disc the capacity to osmotically swell to
resist compressive mechanical forces.
cate that nuclear proteoglycan and water contents are relatively
similar between species (▶ Table 5.1), with the exception of
8,9
Proteoglycans are a major
10
Comparative tests indi-
38
Differences between Human and Animal Discs
Fig. 5.1 Safranin-O stained mid-sagittal sections of various animal spinal discs. Scale bars indicate the range of disc sizes. In addition to size, images
demonstrate diversit y in several features, including nucleus/annulus distinction and presence of a vertebral growth plate.
rattail, which is significantly less, and baboon disc, which is sig-
11
nificantly more.
Interestingly, despite the range of measured
values, proteoglycan and water contents do not correlate with
compressive properties across species.
The annulus is reinforced with collagen fibers that are ori-
ented to support disc compression, bending, and axial tor-
12,13
sion.
Disc collagen within the nucleus is similar between
baboon, goat, and sheep disc, whereas that from pig is less than
the polar moment of inertia) of discs from goat and mouse (tail
and lumbar) were within 10% of human, whereas calf and
14
bovine discs were within 25%.
In this case, however, discs
from pig and sheep were approximately 3 times stiffer, perhaps
due to differences in annular material properties.
15
together, these data indicate that the biomechanical effects of
regenerative therapies as measured in animals may be scaled to
human as an indicator of anticipated clinical effect size.
half and that from calf and cow is more than 3 times greater
14
(▶ Table 5.1).
goat had 48% less and calf had 45% more than human. Yet, like
proteoglycan and water composition, collagen content of annulus does not correlate strongly with disc torsional properties.
Similar variations are noted for the AF, where
5.4 Transport
Because the disc is avascular, disc cells rely on transport to and
from capillaries at the outer annulus and vertebral end plate.
This constraint creates opposing gradients of nutritional factors
5.3 Biomechanics
Functionally, disc size, shape, and biochemical constituents
affect biomechanical properties, which in turn, can influence
how the regenerative effects of biological therapies observed in
animals are scaled to judge an effect size for humans. Axial
compressive stiffness varies between 13 N/mm in mouse and
2,491 N/mm in calf spines.
by normalizing by a ratio of disc height to area, in which case
disc stiffnesses from calf, cow, pig, baboon, sheep, rabbit, and
rat are not statistically different from human.
have been reported for discs tested in axial torsion, where the
normalized torsional stiffness (normalized by height divided by
11
These differences can be narrowed
11
Similar results
(e.g., glucose and oxygen) and products of cell metabolism (e.g.,
17
lactate
) from the disc periphery to the disc center. The
square-cube law makes clear that the surface area for transport
becomes disproportionately small compared with tissue volume as the disc size increases. Another consequence of size differences between species is the disc tolerance for metabolic
demand. Computational studies demonstrate that the kinetics
of solute transport into the disc significantly vary between spe-
18
cies in relation to disc size (▶ Fig. 5.4).
By analogy, diffusion of
disc metabolic products out of the disc will have a similar size
dependency. These transport dynamics limit the extent of metabolic activity that can be supported within a disc of a particular size. This principle is beautifully depicted by experimental
Taken
16
39
Differences between Human and Animal Discs
40
Fig. 5.2 High power view of Safranin-O–stained disc sections highlighting the interface between the nucleus, cartilage end plate, and subchondral
bone. Species differences in end plate cartilage thickness are apparent.
data for nucleus cell density versus disc height across species
19
(▶ Fig. 5.5).
Importantly, square-cube law scaling effects also
explain why smaller discs have a greater healing capacity, as
wound-site transport is better able to supply signals and a
regenerative milieu, as seen in species-dependent healing of
long bone fractures (▶ Fig. 5.6).
20
5.5 Surgical Considerations
Variations of disc size across species have additional practical
implications. A major disadvantage of small animals (mouse,
rat, and to some extent rabbit) is that surgical techniques are
limited, and it may not be meaningful or practical to study
therapies that involve implants. It may also be difficult in small
discs to reliably inject cells that may include viscous carriers
11
(e.g., the mouse nucleus is approximately 0.13 μL
). Injecting
through small-gage needles can damage cells due to high shear
stress, as lower cell viability is associated with small needle
21
diameter and long needle lengths.
Also, working with small
volumes can lead to proportionately greater errors in the administered dose due to cell adherence within the syringe.
Size constraints are also a factor when attempting to implant
tissues, scaffolds, and devices that are part of the regenerative
therapy. Scaling to constructs appropriate for small animals
Differences between Human and Animal Discs
Fig. 5.3 High power view of Safranin-O–stained disc sections highlighting the cellularity of the nucleus pulposus. Species differences in cell type,
density, and matrix homogeneity are notable.
may require significant deviation from the formulation and
configuration of implants intended to be used in the clinical
situation. For this reason, primates have been a model of
choice for more traditional spinal impla nts and, more
recently, disc arthrop lasty devices.
are more similar to human and hence more accommod ating
for implantation. Primates may also be valuable for refining
the surgical approach and associated instrumentation.
Another factor is the availability of imaging to facilitatelongitudinal studies. The use of magnetic resonance imaging (MRI)
to assess disc healing longitudinally can help increase statistical
power and reduce cost by using each animal as its own control.
However, high field strength is needed to achieve a reasonable
22
Here, disc dimensions
resolution in small animals (up to 7 T) and access to imaging
resources may be limited for large animals.
5.6 Bipedalism
In addition to differences in metabolic stress between species,
there may also be differences in physical loading. Although the
common notion is that quadrupeds have lower spinal forces
than humans due to their horizontal spine orientation, research
in evolutionary biology indicates that in vivo biomechanical
stresses are generally independent of animal size, and that tissues across species seem to be operating with a similar factor of
41
Differences between Human and Animal Discs
Table 5.1 Representative properties of animal model discs used for biological research, with human lumbar disc included for comparison
NP AF NP AF NP AF
Animal Disc height
Human 10.91 1925 9.95 0.087 466 161 81 72 16 103
Calf 6.09 1100 12.72 0.108 384 66 80 69 60 149
Pig 5.46 872 15.77 0.403 379 72 83 59 6 122
Baboon 5.97 808 9.36 0.127 971 333 80 66 19 110
Goat 4.28 670 7.2 0.084 335 25 84 66 19 53
Sheep 3.4 511 9.78 0.356 547 133 75 57 20 107
Rabbit 2.4 90 10.44 0.152 579 160 82 62 78
Rat lumbar 0.77 11.85 5.09 0.04 384 47 82 65
Mouse lumbar
Cow tail 9.18 857 8.84 0.068 548 112 83 69 43 107
Rat tail 0.94 12.86 4.19 0.015 95 20.5 75 48
Mouse tail 0.095
Source: Adapted from
Abbreviations: AF, annulus fibrosus; NP, nucleus pulposus.
(mm)
0.31 1.61 2.93 0.083
7,11,14
Disc area
2
(mm
)
Normalized
compressive
stiffness
(MPa)
Normalized
Torsional
Stiffness
(N-m/deg)
GAG per
Dry
Weight
(ug/mg)
GAG per
Dry
Weight
(ug/mg)
Water
Content
(%)
Water
Content
(%)
Collagen
Content
per Dry
Weight
(ug/mg)
Collagen
Content
per Dry
Weight
(ug/mg)
42
Fig. 5.4 Computational predictions of the time course of glucosamine
uptake within the intervertebral disc after intravenous administration.
Data indicate that disc size has a large effect on the postinjection
concentration profiles. Adapted from Motaghinasab et al.
18
safety (ratio of failure load to typical functional load)—of
between 3 and 5.
23
All animal spines are loaded by ligament and muscle forces
developed during movement, and during maintenance of posture against gravitational loading. Even though the spines of
quadrupeds are aligned parallel to gravitational forces, significant muscle forces are generated to support bending and tor-
24
sional movements required for locomotion.
24
that vertebral strength
and normalized disc stiffness (cited
The observation
Fig. 5.5 Disc cell density as a function of disc size across several
species. Red line indicates a curve fit proportional to disc height cubed,
supporting that square-cube law scaling influences disc cell homeostasis. Adapted from Stairmand et al.
19
above) in animals are comparable to, if not greater than human,
also supports the notion that quadruped discs are subjected to
significant in vivo force.
Some direct in vivo measurements have been made. Spine
forces in baboon are generally proportionate (by body weight)
Fig. 5.6 Comparison of the time course of long bone fracture healing
in mouse, rat, sheep, and human. Adapted from Garcia et al.
20
Differences between Human and Animal Discs
Table 5.2 Important biological milestones of animals used in disc re-
search
Animal Age at skele-
Human 20 years 10 years 30–50 years
Chondrodystrophoid Dog
Nonchondrodystrophoid Dog
Sheep 20 months > 2 years not reported
Rabbit 6 months 6 months 1 year
Cow 3 years 12 months not reported
Rat 2 months 12 months 12 months
Horse 5 years Birth 6 years
Source: Adapted from
tal maturity
12 months 6 months 4 years
12 months 5 years 7 years
28,86,87,88
Age at loss of
notochordal
cells
Age at onset of
disc degeneration
to human,25whereas those in sheep are approximately 50%.
Yet, the intradiscal pressures in sheep can be significantly
26
higher than human depending on the activity.
Similar to
baboon, bovine measurements show that loads and pressures
are comparable to human during static postures, whereas
dynamic pressures tend to be higher.
27
These studies demonstrate that quadrupeds can generate significant spinal forces and pressures that are in the human range.
Importantly, they indicate that animals with upright postures
(e.g., primates) are not necessarily required to approximate
human biomechanical conditions on tissue engineered constructs in vivo.
5.7 Cells
Cells are the heart of biological approaches for spinal disc
repair. Although the therapeutic strategy may include delivery
of donor cells, a bioactive agent that stimulates host cells, or a
combination of both, the ultimate goal is to exploit a cellular
disease-modifying activity. Several animal model features can
impact the behavior of such strategies, potentially in ways that
differ significantly from the human situation.
One main consideration is the fact that nucleus cells in animals may be notochordal, and have behaviors that differ from
adult human NP cells. During embryonic development of the
spinal column, the NP is formed by notochordal cells (NCs) that
are sequestered from the developing vertebral bodies and AF.
Rodents, rabbits, and other nonchondrodystrophoid species
maintain a high NC number throughout their lifetime, whereas
in other animals such as bovine, goat, and sheep, and in humans
these cells disappear early in life
NCs are histologically and functionally distinct from human
NP cells, appearing physaliferous or vacuolated. NCs express
phenotypic markers such as CD44 s, galectin 3, vimentin, cytokeratins 8 and 19, chondroitin-sulfate proteoglycan (CSPG), and
29,30,31
(▶ Table 5.2).
26
collagen type IIA.
29
Molecular and immunologic studies show
that NCs are more metabolically active than NP cells, and have
been shown to produce significantly more lactate and proteo-
32
glycan than bovine NP cells.
Additionally, NCs have a number
of beneficial activities: They can promote chondrogenesis of
33,34,35
mesenchymal stem cells (MSCs),
36,37
apoptosis,
increase glycosaminoglycan (GAG) production,
protect NP cells from
and secretion of regenerative factors.39Importantly, these therapeutic cell behaviors may vary by species, for example, porcine
and canine NCs appear more potent than human.
40
NP cells in degenerated human discs become senescent, display
41
a catabolic phenotype,
respond synthetically to mechanical load
43
agents.
and have diminished capacity to
42
and anabolic
A second important species difference that affects disc cell
function is related to disc size, which influences transport and
cell density. Therapies that aim to increase disc cell number
and/or stimulate disc cell anabolic activity will, by definition,
44
increase metabolic demand,
centrations to below critical levels.
which can reduce nutrient con-
45
Increased metabolic activity in the context of restricted glucose and oxygen can increase
lactate production, which has known catabolic effects on disc
46
cells.
Consequently, given species differences in cell activity, transport, and cell density, the rate and magnitude of a biologically
relevant response will likely be overestimated in studies performed on small animals and may overpredict clinical effects in
28
humans. Therefore, treatments showing efficacy in smaller ani-
3
mals should have safety and efficacy profiles confirmed in
larger animal models, where appropriate.
6
5.8 The Degenerative Phenotype
As mentioned above, the unique microenvironment within the
degenerated and painful disc presents an obstacle to biological
38
By contrast,
43
Differences between Human and Animal Discs
manipulation for therapeutic purposes. It is becoming increasingly clear that symptomatic human discs have elevated levels
of inflammatory cytokines, including tumor necrosis factor-α
(TNF-α) and interleukins (ILs),
47,48,49,50
and other catabolic factors that can frustrate disc regeneration strategies. Inflammation is a normal response to tissue injury, but because the disc
51
is avascular the response to inflammation is unique.
Within
the disc, accumulation of metabolic waste products, matrix
fragments, and cytokines inhibit matrix production and pro-
52,53,54
mote degeneration.
expression of matrix-degrading enzymes,
For example, IL-1 exposure increases
55
and when com-
bined with TNF-α, can stimulate nerve growth factor produc-
56
tion in human IVD cells,
which can sensitize nociceptors and
stimulate further cytokine production. Furthermore, IL-1β and
TNF-α have been shown to inhibit chondrogenesis of human
57
MSCs,
and prolonged exposure to an ischemic, nutrient-
deprived microenvironment that resembles that of the degen-
58,59
erated IVD can lead to stem cell death
60,61,62
inflammation.
Although cytokines secreted by the disc
and potentiate
cells can stimulate wound healing, at the same time they can
cause damage to transplanted cells. For example, TNF-α is a
potent pro-death factor for MSCs.
63
Many animal models of disc degeneration have been
3,4,64,65
described in the literature.
These have been classified as
either spontaneous or experimentally induced. Spontaneous
degeneration models include certain rodents (sand rats and
66
), chondrodystrophoid breeds of dogs,67and primates.
mice
Alternatively, degeneration can be produced by mechanical (in
vivo compression, surgical instability), chemical (enzymatic
5
treatment), or surgical (annular stab injury) means.
For the
most part, these have been used to study degeneration mechanisms, and demonstrate many clinically relevant architectural,
biochemical, and cellular changes—such as fibrous tissue formation in the nucleus, annular disorganization, loss of disc height,
and decreases in nucleus cellularity.
Whether degeneration in animals mimics the inflammatory
profile of painful human discs is less clear. Disc injury and herniation in animals can lead to a transient inflammation that
lasts from days in mice and rats, to weeks in rabbits, and up to
5
months in large animals.
However, the inflammation in surgically injured discs is primarily centered at the injury site and
the granulation t issue that forms at the periphery of the wound
69
tract.
Additionally, whether there is a clinically relevant
inflammatory response to degeneration in spontaneous animal
models has not been described. As a result, inflammation
mechanisms, potential species differences, and their practical
impact on regenerative therapies are yet to be disentangled.
5.9 Pain-Related Behavior
It may sound obvious, but low back patients seek clinical care
because of pain and disabilit y, not because of disc degeneration
per se. Although in general, subjective self-ratings of back pain
using questionnaires and scales are reliable,
between disc pathology and pain is variable.
because chronic back pain in humans is commonly comorbid
with a wide range of complex clinical conditions, all contribu-
72
ting to reductions in quality of life.
This fact is reinforced by
the many studies that demonstrate disc degeneration in
70
the linkage
71
This may be
73,74
asymptomatic volunteers,
and has significant implications
for the preclinical development of back pain therapies and diagnostic tools that reliably identify discs appropriate for treatment. Specifically, if disc degeneration is necessary but not
sufficient to cause back pain, then therapies targeting degeneration may not necessarily be clinically effective, particularly if it
is difficult to distinguish degenerated/asymptomatic from
degenerated/painful discs in the clinical setting.
Pain is a subjective experience associated with actual or
75
impending tissue damage.
In humans, pain can have effects
on behavior such as complaints of pain, reductions in activity,
increased medication intake, or alterations in facial expression
75
and body posture.
Although animals can’t tell us about their
pain, they may experience simple reflexive or innate pain
behaviors such as withdrawal from a stimulus (hot, cold, or von
Frey fibers) or other self-protective actions such as licking, biting, scratching, limited mobility, and vocalization. These behaviors may lack clinical face validity, but they can be reliably and
objectively scored. Other proxy measures, such as gene expression, imaging, and neural activity have also been reported.
Generally, the most used animals in pain research are mice
and rats, and these have been most effective in backward validation—confirming activity of therapies already known to be
effective in humans—rather than forward validation during
72
therapy development.
Because an injured disc can chemically
and mechanically irritate adjacent nerve roots, an early model
68
for disc injury involved applying NP directly onto the nerve root
64
in rats.
More recently, several groups have reported painrelated behaviors in rats secondary to surgical or chemical disc
injury.
76,77,78,79
However, similar associations are not reliably
reported for larger animals, such as rabbits, which make them
less suitable to assess therapy efficacy for discogenic pain.
5.10 Conclusion
In general, animal models are critical for evaluating the biological response and simulating the functional performance of new
therapies. However, there is no consensus regarding the preferred animal model for regenerative therapies for spinal discs.
Although there are often-debated species-specific considerations—size, tissue composition, and cellular content as
described above—the primary barrier preventing progress is
the uncertain pathogenesis for axial back pain in humans. If we
don’t know what causes the pain that brings patients to the
clinic, then it is unreasonable to expect consensus on preclinical
models to establish treatment efficacy.
Consequently, the logical first step toward animal model
standardization is development and validation of diagnostic
tools that localize spinal pathologies that associate with axial
pain in human clinical studies.
apy development by providing evidence for biologically plausible disease-modifying activities. Without this, even promising
therapies judged using standardized models may not be clinically effective. For example, there is an existing, generally
accepted model for bioactive therapies for back pain—the rabbit
model for intertransverse process spine fusion.
model, critical variables of disease-modifying activity are bony
bridging and mechanical stability of the treated spinal level.
Therapies tested in this model have been shown to be effective
80
Such studies will inform ther-
81,82
72
77
In this
44
Differences between Human and Animal Discs
for many spinal indications, such as degenerative spondylolis-
83
thesis.
Yet, despite the availability of this standardized model
and the generally agreed-upon mechanism of action (eliminating mechanical instability), the successful clinical application of
spine fusion for axial back pain is not consistently demon-
80,84
strated.
So, the lack of clinical success for new chronic low
back pain therapies can’t be blamed solely on the absence of an
accepted animal model. The primary dilemma is our inability to
reliably identify the origin of pain in this difficult patient
population.
So, how should we proceed? Given the growing global bur-
85
den of low back pain
we need to be aggressively advancing in
parallel both improved diagnostic tools and therapies, given the
circular reliance between proper diagnosis and efficacious
treatment. First, it is critical that new therapies be grounded in
current basic and clinical science, with mechanisms of action
that are biologically plausible and clinically relevant. Scientists
need to be working closely with diagnosticians and clinicians to
understand how the proposed therapy would fit within the
current diagnostic and patient management paradigms. How
would patients and affected spinal levels for treatment be identified, and how would the outcomes be monitored? Second,
biological plausibility of the desired disease-modifying activity
should be established in models of increasing complexity, starting with cells in vitro and moving to small, and perhaps large
animals. Given the absence of validated pain models, it will be
important to push toward well-designed Phase I clinical studies
once safety has been appropriately established. Incremental
progress toward adoption of new low back pain therapies will
come from an iterative and integrated development of both
improved diagnostic algorithms plus promising therapeutics
with clinically relevant mechanisms of action.
Still, animal models will continue to have a central role in
preclinical development of biologics for low back pain—even in
the absence of consensus on pain mechanisms. Investigators
will need to select the model that best balances the therapyspecific constraints on how the treatment is delivered and what
it is intended to accomplish. In this selection, the most critical
variables to justify are size (relative to transport and surgical
considerations) and host cell activity (e.g., notochordal versus
adult disc cells). Other important model factors are reproducibility, and standardization of spinal level, therapy dose, choice
of control, postoperative survival period, and postmortem evaluation metrics. Although some could contend that primates
best approximate the human situation, there are significant
ethical concerns, costs, and logistical constraints that limit their
applicability. Moral considerations have appropriately led to
86
significant oversight on primate studies,
and justification that
this species is required is difficult to reconcile given the ambiguities inherent in the disc regeneration field summarized here.
Arguably, perhaps the best current model from these perspectives is the rabbit, in which the disc stab injury has been shown
to reliably create a degenerative phenotype and reproducibly
87
simulate a therapeutic response.
Models of spontaneous
degeneration may be argued to be more clinically relevant,
but come with practical challenges for conducting comprehensive studies with consistent starting points. Large animals with
non-notochordal disc cells are similarly problematic due to cost,
which creates difficulties in designing and justifying studies
with sufficient statistical power, particularly given the lack of
relevant pain outcomes. Consequently, the combination of factors that include (1) good reproducibility and low cost of rabbits
and (2) the push to Phase I human clinical studies to address
pain outcomes once safety has been established, indicate that
governing bodies, such as the US Food and Drug Administration
(FDA), may accept smaller animals for therapies that aren’tprimarily structural, such as injectable biologics as contrasted with
tissue engineered constructs for total disc replacement or annular repair in postdiscectomy patients. This clearly requires discussion with FDA to confirm that the chosen model meets the
agency's qualification criteria.
This review highlights the pressing need for large animal
models of discogenic pain that reliably forecast clinical trial outcomes. The ideal animal model for preclinical development of
biological therapies for discogenic pain would (1) be of similar
size to human so as to match in situ stressors such as load,
transport, and cytokines; (2) include cells with similar activity
to adult human; and (3) have a validated degenerative phenotype with relevant pain-related behavior.
Given the current ambiguity regarding disc degeneration and
back pain mechanisms in humans, validated large animal models won’t likely be available anytime soon.
88,89,90
5.11 References
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