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Clinical Research
Dermatological Models:
A Sound Basis for Early
Decision-Making
Whereas the early phase of drug
development for systemically acting
drugs is mostly driven by the presumed
relationship between safety/efficacy and
concentration of drug in the systemic
circulation, this relationship does not exist
for topically applied drugs intended for
local or regional action. For these topical
drugs, systemic absorption is not desired
and systemic exposure can often be
kept to very low or even negligible levels
by controlling the size of the treatment
area. In early clinical development this
ability to limit systemic exposure often
allows for initial assessment of efficacy
in innovative models before investment
in costly safety studies.
In dermatological models, discrete
evaluation of treatment effects may
be possible following simultaneous
application of multiple formulations/
actives to small treatment areas in parallel
in one individual, without risk of crossover effects. By making intra-individual
comparisons
between
treatments,
the inter-subject variability is reduced,
allowing meaningful interpretation of
results with smaller panel sizes. Further,
in these models non-invasive biophysical
measurement and imaging methods
to monitor skin function and structure
provide alternative objective endpoints
to support clinical evaluation, delivering
additional information on structural and
functional changes in the skin.
The following provides a brief overview
of several established dermatological
models and how they can be applied in
clinical testing.
Psoriasis Plaque Test
In the psoriasis plaque test, also known
as the psoriasis microplaque assay, antipsoriatic efficacy of multiple formulations
is assessed in parallel in small test areas
located on stable psoriatic plaques in
patients with psoriasis vulgaris. It may
even be possible to test efficacy in this
design as the first Phase I study, analog
to an early Phase I study under an
exploratory IND.
76 INTERNATIONAL PHARMACEUTICAL INDUSTRY
In the original test design described
by Dumas and Scholtz in 19721, the
efficacy of corticosteroids for treatment
of plaque-type psoriasis was evaluated
clinically
following
standardised
occlusive application over several days.
Test fields were graded as follows:
unchanged or less than full involution (0)
or complete return of epidermis to normal
(+). Whereas this design is well suited
for testing of strong anti-psoriatics such
as potent corticosteroids, the sensitivity
may not be sufficient for assessment of
weaker formulations or determination of
slight differences between formulations.
Meanwhile biophysical measurement
methods have been used in the PPT to
objectively measure the inflammatory
alterations accompanying psoriasis,
greatly improving the sensitivity of the
test. Since one of the most important
clinical endpoints in psoriasis is the
extent of the psoriatic infiltrate, objective
measurement of the infiltrate depth with
20 MHz sonography is probably the most
relevant outcome to use as a marker
for treatment effects. Other methods
that have been used in the plaque test
include measurement of intensity of
erythema by colorimetry, skin blood flow
by laser-Doppler flowmetry, skin surface
characteristics by profilometry, and skin
temperature2-5.
In a typical psoriasis plaque test
design, patients (n=15-20) suffering from
psoriasis vulgaris with stable plaques
already existing for several months or
years are suitable for inclusion. Plaques
exhibiting spontaneous regression as
well as exacerbation of psoriasis are
excluded. Small test fields (e.g. 12 mm)
Figure 1
are treated over a 12-28 day period. The
proposed mechanism of action of the
active pharmaceutical ingredient (API)
should be taken into consideration to
determine the length of treatment. Ideally
all test fields are located on a single
psoriatic plaque or on two plaques
located in a similar body area and of
comparable severity. Prior to baseline
measurements and the first treatment,
scales are removed from plaques. This
is necessary since it is not possible to
obtain sonographic images of good
quality if images are taken through
thick scales. A bandage is attached to
the plaque in which the test fields have
been punched out (figure 1). Treatments
are preferably performed in an occluded
manner once daily. More rarely treatments
are performed in an open manner once
or twice daily. In general, occlusion is
preferred as this maximises absorption
of drug, shortening the necessary
treatment period. The primary variable
is the depth of the psoriatic infiltrate
measured in sonographic images taken
at defined intervals during the treatment
period. The echo-poor region directly
underneath the entrance echo mainly
represents the inflammatory infiltrate
and is readily demarcated in good
quality images (figure 2). Alternatively,
full skin thickness (infiltrate plus dermis)
can be measured to evaluate changes
in severity of psoriasis, however this
method is generally less sensitive than
measurement of the psoriatic infiltrate
alone. Clinical improvement is assessed
as a secondary variable, but it is not
posssible to make the fine distinctions
between treatments that are possible with
Figure 2
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Clinical Research
measurements of sonographic images.
The intensity of erythema can also be
measured in the test fields by colorimetry.
Even though this measurement has
proven particularly useful for the
evaluation of corticosteroids where
vasoconstriction plays a prominent role
in the measurable effect, the overall
relevance of additional assessment of
redness is questionable for other drug
classes. Redness is the last symptom
to clear in psoriasis and some residual
erythema may remain after all other skin
alterations have cleared1.
The psoriasis plaque test has
been used to investigate efficacy of
corticosteroids, vitamin D analogs,
retinoids,
oligonucleotides,
and
immunmodulators, among others. The
test is very well suited for comparison of
developmental candidates, alternative
formulations or initial studies of dose
response. Due to the maximised
conditions it is to be expected that
efficacy will be found if a formulation
indeed possesses antipsoriatic activity.
Irritant Dermatitis Model
Acute or chronic eczematous skin
reactions can be simulated by
standardised repeated washing with the
anionic surfactant sodium lauryl sulfate
(SLS)6,7. The clinical appearance of the
skin (erythema, scaling, fissuring) and
biophysical parameters of skin condition
(transepidermal water loss (TEWL),
stratum corneum hydration, skin colour/
erythema) show similarities between the
irritation induced with SLS and irritant
dermatitis.
In one such model used at bioskin
comparable localised skin lesions are
created on the forearms of healthy
or atopic subjects using a highly
standardised open washing procedure
once daily for six days. Following
this washing procedure, test fields
are delineated by application of a
bandage system with punched holes
(figure 3). Treatments are then applied
once or twice daily for up to four days. In
a modification of this design the washing
procedure may be continued during
the treatment phase to better simulate
chronic irritation.
This model is mainly suited for
screening of barrier repair formulations
or other anti-inflammatory formulations
intended for treatment of conditions
such as hand eczema. It may also be
78 INTERNATIONAL PHARMACEUTICAL INDUSTRY
very useful for screening of suitable
moisturisers and lubricating vehicles.
UV-Induced Erythema Test
The UV-induced erythema test is an
experimental model to evaluate antiinflammatory efficacy of steroidal and
non-steroidal topical formulations8,9.
The model has the advantage that it
is performed in healthy volunteers as
opposed to patients with inflammatory
skin disease. UV-induced inflammation
is mediated by several possible
mechanisms and involves the generation
of a variety of inflammatory mediators
such as prostaglandins, histamine,
bradykinin, serotonin and leucotrienes.
In this model, a typical sample size
is 20 to 40 subjects with Fitzpatrick skin
type II to III. Since efficacy is correlated
with the severity of the inflammatory
response, success of the test is
dependent on controlling the extent of
inflammation, even in individuals with
different inherent sensitivities. This is
achieved by irradiating with a defined
UVB dose which is a multiple of the
Minimal Erythemal Dose (MED). The
MED is the smallest amount of UVB light
producing distinct erythema, and differs
from individual to individual. The MED
for each subject must be determined
beforehand by parallel exposition of
small fields to graduated UVB dosages.
After determination of the individual
MED, test fields on the back are
irradiated with the desired UVB dosages.
The degree of inflammation can be
varied by irradiating separate test fields
with different UVB dosages, e.g. 1.25,
1.6 and 2 MED, in a single panel.
Immediately after irradiation the test
fields are treated for the first time with
the test formulations. Since treatments
are performed after irradiation, a sun
protective effect can be excluded. Test
products can be applied in an occlusive
or open manner, however in the case
of weak anti-inflammatory drugs, e.g.
hydrocortisone, it may only be possible
to measure an anti-inflammatory
effect under occlusive conditions. The
frequency of application (single or
repeated applications) and the length of
the treatment period (eight to 48 hours)
can be varied.
The endpoint is the degree of erythema
in the irradiated test fields. Diminished
erythema compared to irradiated,
untreated fields or vehicle-treated fields
is indicative of anti-inflammatory efficacy.
Objective measurements of erythema
are done using colorimetry.
Thermal Sensory Assessment
Alterations in pain perception can be
quantified using a thermal sensory
analyser. Using this technique a thermal
diode is placed on the subject’s skin to
heat or cool the skin. The subject is then
asked to respond to the temperature
stimuli by pressing a button when pain is
first perceived or becomes intolerable.
To take this methodology one step
further, thermal hyperalgesia can first
be induced in test fields by setting
an inflammation with UVB light to
increase pain sensitivity for testing
antihyperalgesic drugs10,11. To measure
the heat pain threshold, the skin
temperature is linearly increased using
the thermode, and subjects are advised
to stop the heat by pressing a button as
soon as the heat becomes painful. The
threshold temperature is recorded. The
threshold is assessed prior to irradiation
and at intervals beginning six to 12 hours
following irradiation. Antihyperalgesic
drugs lower the heat pain threshold.
Wound Healing Models
Aside from patients with wounds
resulting from accidents or diseases,
or wounds resulting from diagnostic
or therapeutic measures (cryosurgical
removal of skin lesions, punch biopsies
Figure 3
Figure 4
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Clinical Research
and post-operative wound care), there
are few ethical models to examine wound
healing in humans. The production
of wounds is inevitably an invasive
procedure that may be associated with
pain, bleeding, wound infection and
scarring.
There are however two good models
for induction of superficial wounds in
which the promotion of wound healing
during the early phase of re-epithelisation
can be measured.
The first is a recently developed
model in which standardised abrasive
wounds are induced which closely mimic
the clinical picture of bagatelle abrasive
wounds12. Multiple wounds are induced
on the forearm by scrubbing the skin
with a hard brush until punctate bleeding
occurs. Healing and re-epithelisation is
then followed for up to 14 days (figure 4).
Panel sizes range from 10-30 subjects.
An alternative model involves
making epidermal defects of defined
size by raising suction blisters using
negative pressure over a surgical pump.
Following removal of the blister roof
there is a large increase in water loss
as a result of the missing epidermal
barrier. It is well established that the
transepidermal water loss (TEWL) is
correlated with the degree of epidermal
damage. The highest TEWL values
are measured in fresh wounds with a
continual decline until values from intact
skin are reached at the end of healing.
Therefore the measurement of TEWL is
a suitable parameter to determine the
degree of re-epithelisation. The critical
phase of epithelial regeneration already
occurs during the first days following
experimental wound induction in this
model.
Expanded Flora Test
Bactericidal
efficacy
of
topical
preparations can be investigated in the
expanded flora test13. In this model the
bacteria on the surface of the skin are
multipied by occluding test fields located
on the back with a plastic covering.
In this way the resident cutaneous
microflora can be expanded from its
normal low density of about 10² colony
forming units to 105-106 after 24-48
hours of occlusion14,15. Modifications of
the original model allow for treatments
before or after expansion of the skin’s
bacteria to assess the antibacterial
activity.
80 INTERNATIONAL PHARMACEUTICAL INDUSTRY
Conclusion
These as well as other “custommade” models can help to make the
all-important go/no go decision early
in a topical product development
programme. Further, test model
results are valuable for planning of
later therapeutic trials, for instance for
sample size calculations, dose finding
and ranking with comparators. These
advantages should be exploited to the
fullest to optimise clinical development
programmes n
References:
1. Dumas KJ, Scholtz JR. The psoriasis
bio-assay for topical corticosteroid
activity.
Acta
Dermatovenerol
(Stockholm) 1972; 52: 43-8.
2. Bangha E, Elsner P. Evaluation of
topical antipsoriatic treatment by
chromametry, visiometry and 20-Mhz
ultrasound in the psoriasis plaque test.
Skin Pharmacol 1996; 9: 298-306.
3. Remitz A, Reitamo S, Erkko P et
al. Tacrolimus ointment improves
psoriasis in a microplaque assay. Br J
Dermatol 1999; 141: 103-7.
4. Harrison DK, Abbot NC, Beck
JS, et al. Laser doppler perfusion
imaging compared with light-guide
laser doppler flowmetry, dynamic
thermographic imaging and tissue
spectrophotometry for investigating
blood flow in human skin. Adv Exp
Med Biol 1994; 345: 853-9.
5. Wolff HH, Kreusch JF, Wilhelm KP,
Klaus S. The psoriasis plaque test and
topical corticosteroids: evaluation by
computerized laser profilometry. Curr
Probl Dermatol 1993; 21: 107-13.
6. Lee CH, Maibach HI. The sodium lauryl
sulfate model: an overview. Contact
Dermatitis 1995; 33: 1-7.
7. Gehring W, Gloor M, Kleesz P. Predictive
washing test for evaluation of individual
eczema risk. Contact Dermatitis 1998;
39: 8-13.
8. Hughes-Formella BJ, Bohnsack K,
Rippke F, et al. Anti-Inflammatory
Effect of Hamamelis Lotion in a UVB
Erythema Test. Dermatology 1998;
196: 316-22.
9. Jocher A, Kessler S, Hornstein S, et
al. The UV erythema test as a model
to investigate the anti-inflammatory
potency of topical preparations –
reevaluation and optimization of the
method. Skin Pharmacol Physiol 2005;
18: 234-240.
10. K
oppert W, Likar R, Geisslinger G,
et al. Peripheral antihyperalgesic
effect of morphine to heat, but
not mechanical, stimulation in
healthy volunteers after ultraviolet-B
irradiation. Anesth Analg 1999; 88:
117-22.
11. B
ickel A, Dorfs S, Schmelz M, et al.
Effects of antihyperalgesic drugs on
experimentally induced hyperalgesia
in man. Pain 1998; 76: 317-25.
12. W
igger-Alberti W, Kuhlmann M,
Ekanayake S, Wilhelm D, Buettner
H, Callaghan T, Wilhelm KP. Using
a novel wound model to investigate
the healing properties of products for
superficial wounds. J Wound Care
2009; 18: 123-128, 131.
13. M
arples RR, Kligman AM. Methods
for evaluating topical antibacterial
agents on human skin. Antimicrob
Agents Chemother 1974; 5: 323-329
14. L eyden JJ, Stewart R, Kligman AM.
Updated in vivo methods evaluating
topical antimicrobial agents on
human skin. J Invest Dermatol 1979;
72: 165-170.
15. L eyden JJ, McGinley KJ, Foglia
AN, Wahrman JE, Gropper, CN,
Vowels BR. A new method for in vivo
evaluation of antimicrobial agents
by translocation fo complex dense
populations of cutaneous bacteria.
Skin Pharmacology 1996; 9: 60-68.
Betsy HughesFormella
is
Director
of
B u s i n e s s
Development
and Consulting
at
bioskin
GmbH. She received her MS and
PhD in Physiology from the University
of Georgia in Athens before moving
to Germany to join a research team
at the University Medical Center
in Hamburg. In 1992 Dr. HughesFormella joined bioskin where she
is responsible for the coordination
of business development activities
and is a consultant and advisor for
dermatological product development.
Email: [email protected]
Volume 3 Issue 2