 | |
Issue
37 — August 2004 |
| | | |
| Issue
37 |
|
 |
|
|
| |
|
|
|
|
| Category |
|
Title |
|
Author |
| Guest
Article |
|
Strategies
for Skin Penetration Enhancement |
|
Rolf
Daniels |
1
Introduction
Dermatological and cosmetic preparations frequently contain active principles
which can only act when they penetrate at least the outermost layer of the skin.
However, the efficacy of topically applied actives is often suboptimal because
the transport into the skin is slow due to the resistance of the outermost layer
of the skin, the stratum corneum. Most small water-soluble non-electrolytes therefore
diffuse into the systemic circulation a thousand times more rapidly when the horny
layer is absent. Thus, a variety of means have been studied in attempts to overcome
this barrier. Such strategies include physical, biochemical, and chemical methods
(Figure 1).
2 Structure of the skin barrier
The skin is the largest human organ and consists of three functional layers:
epidermis, dermis, and subcutis. It has a wide variety of functions. One major
task of the skin is to protect the organism from water loss and mechanical, chemical,
microbial, and physical influences. The protective properties are provided by
the outermost layer of the skin, the epidermis. Although its thickness measures
on average only 0.1 mm (from 0.02 mm on the face up to 5 mm on the soles of the
feet) it is specially structured to fulfill this challenging task. Out of the
five layers of the epidermis, it is mainly the uppermost layer (horny layer; stratum
corneum) which forms the permeability barrier.
The stratum corneum consists of horny skin cells (corneocytes) which are connected
via desmosomes (protein-rich appendages of the cell membrane). The corneocytes
are embedded in a lipid matrix. Thus the structure of the stratum corneum can
be roughly described by a “brick and mortar” model [1]. The corneocytes
of hydrated keratin comprise the bricks and the epidermal lipids fill the space
between the dead cells like mortar (Figure 2).
| Figure
2: Schematic structure of the stratum corneum according to the
brick and mortar model. The horny cells are embedded in a lamellar structured
lipid matrix
Enlarged version
|
The epidermal lipids comprise 10 to 30 % of the total volume of the stratum corneum.
The major components are: ceramides, fatty acids, cholesterol, and cholesterol
esters [2]. The lipids are organized as multiple lipid bilayers which form regions
of semi-crystalline gel and liquid crystals domains [3].
3 Routes of Penetration
Figure 3 illustrates the possible pathways for a penetrant
to cross the skin barrier. Accordingly, a molecule may use two diffusional routes
to penetrate normal intact human skin: the appendageal route and the transepidermal
route.
The appendageal route comprises transport via the sweat glands and the hair follicles
with their associated sebaceous glands. These routes circumvent penetration through
the stratum corneum and are therefore known as shunt routes. Although these routes
offer high permeability, they are considered to be of minor importance because
of their relatively small area, approximately 0.1% of the total skin area. The
appendageal route seems to be most important for ions and large polar molecules
which hardly permeate through the stratum corneum [4].
| Figure
3: Possible pathways for a penetrant to cross the skin barrier.
(1) across the intact horny layer, (2) through the hair follicles with
the associated sebaceaous glands, or (3) via the sweat glands
Enlarged
version |
Transepidermal transport means that molecules cross the intact horny layer. Two
potential micro-routes of entry exist, the transcellular (or intracellular) and
the intercellular pathways (Figure 4). The principal pathway
taken by a penetrant is decided mainly by the partition coefficient (log K). Hydrophilic
drugs partition preferentially into the intracellular domains, whereas lipophilic
permeants (octanol/water log K > 2) traverse the stratum corneum via the intercellular
route. Most molecules pass the stratum corneum by both routes. However, the tortuous
intercellular pathway is widely considered to provide the principal route and
major barrier to the permeation of most drugs [5].
Figure
4: Schematic diagram of the two microroutes of penetration.
Enlarged
version |
Considering that the skin is such a heterogeneous membrane, it is surprising that
simple diffusion laws can be used to describe the transport through the skin.
For steady-state conditions this can be described with Fick’s first law
of diffusion:
Where J is the flux per unit area, K is the stratum corneum-formulation partition
coefficient of the active, and D is its diffusion coefficient in the stratum corneum
of the thickness h; c0 is the concentration of active substance
applied to the skin surface, and ci is its concentration
inside the skin.
4 Penetration Enhancement
The perfect barrier properties of the epidermis restricts the transport through
the skin to molecules with certain properties such as low molecular weight (<
500 Dalton), moderate lipophilicity (octanol–water partition coefficient
between 10 and 1000), and modest melting point (< 200 °C) correlating with
good solubility. Even when an active substance exhibits such properties, it is
usually necessary to find additional means to increase its transport across the
skin.
4.1 Supersaturation
Supersaturation is a means to increase skin penetration without alteration of
stratum corneum structure [6]. The mechanism of enhancement is based simply on
the increased thermodynamic activity of the drug. This increases the concentration
gradient (c0 – ci) in the
Fick’s law and thus forces the active principle out of the formulation and
into and across the stratum corneum. Several methods can be used to produce supersaturated
systems:
| • |
Heating
and subsequent cooling |
| • |
Removal of a solvent |
| • |
Reaction of two or
more solutes to produce a compound which is less soluble |
| • |
Addition of a substance
to a solution that reduces the solubility of the solute |
However, supersaturated systems are thermodynamically unstable and inherently
tend to recrystallize. Therefore special efforts are necessary to transiently
stabilize the supersaturated system for an appropriate period of time, e.g. addition
of polymers as antinucleant in order to delay recrystallization.
4.2 Water as penetration enhancer
Hydration of the stratum corneum is one of the primary measures to increase the
penetration of most active compounds. Water opens up the compact structure of
the horny layer. The water content of the horny layer can be increased either
by delivering water from the vehicle to the skin or by preventing water loss from
the skin when partially occlusive formulations are applied to the skin.
Table 1 summarizes general effects of carrier systems on
the stratum corneum water content and on the penetration of active ingredients.
Table 1: Effects of carrier systems on
the stratum corneum water content and on the penetration of active ingredients
4.3 Chemical Enhancers
Several excipients are able to promote the transport of an active substance across
the skin barrier by a variety of mechanisms. The most important are [7]:
• Extraction of lipids from the stratum corneum
• Alteration of the vehicle/skin partitioning coefficient
• Disruption of the lipid bilayer structure
• Displacement of bound water
• Loosening of horny cells
• Delamination of stratum corneum
Chemical enhancers can be categorized into different groups (Figure
5). Solvents like alcohols, alkylmethyl sulfoxides, and polyols mainly increase
solubility and improve partitioning coefficient. Moreover, some solvents, e.g.
Dimethylsulphat (DMSO), ethanol, may extract lipids, making the stratum corneum
more permeable. Oleic acid, Azone® (epsilon-Laurocapram), and isopropyl myristate
are typical examples of chemical enhancers which intercalate into the structured
lipids of the horny layer where they disrupt the packing. This effect makes the
regular structure more fluid and thus increases the diffusion coefficient of the
permeant. Ionic surfactants, decylmethyl sulfoxide, DMSO, urea interact with the
keratin structure in the corneocytes. This opens up the tight protein structure
and leads to an increased diffusion coefficient D mainly for those substances
which use the transcellular route.
Figure
5: Chemical structure of typical chemical
penetration enhancers
Enlarged
version |
An unfortunate feature of many potent chemical enhancers is that they irritate
due to their ability to interact effectively with the corneocytes and the intercellular
lipid structure.
4.4 Physical Enhancement Techniques
Hydration of the horny layer and addition of chemical enhancers that temporarily
alter the barrier properties can enhance the flux of active substances. However,
all these principles have clear limitations concerning the delivery of sufficiently
high amounts of ionic molecules, large molecular weight actives and substances
with low potency. These limitations of chemical enhancement can be overcome to
some extent by physical enhancement technologies [8].
4.4.1 Phonophoresis
Phonophoresis ( or sonophoresis) uses ultrasound energy in order to enhance the
skin penetration of active substances [8]. When skin is exposed to ultrasound,
the waves propagate to a certain level and cause several effects that assist skin
penetration. Figure 6 depicts the processes that can contribute
to phonophoresis. One of these effects is the formation and subsequent collapse
of gas bubbles in a liquid called cavitation. The force of cavitation causes the
formation of holes in the corneocytes, enlarging of intercellular spaces, and
perturbation of stratum corneum lipids. Another effect is heating which is mainly
due to the energy loss of the propagating ultrasound wave due to scattering and
absorption effects. The resulting temperature elevation of the skin is typically
in the range of several degrees centigrade. This temperature rise will increase
the fluidity of the stratum corneum lipids as well directly increase the diffusivity
of molecules through the skin barrier. These main effects can be assisted by acoustic
microstreaming caused by the acoustic shear stress which is due to unequal distribution
of pressure forces. In addition, ultrasound can push particles through by pressure
increase in the skin, although only slightly.
Figure
6: Basic principle of phonophoresis. Ultrasound
pulses are passed through the probe into the skin fluidizing the lipid
bilayer by the formation of bubbles caused by cavitation.
Enlarged
version |
4.4.2 Iontophoresis
The basic principle of iontophoresis is that a small electric current is applied
to the skin. This provides the driving force to primarily enable penetration of
charged molecules into the skin. A drug reservoir is placed on the skin under
the active electrode with the same charge as the penetrant. A indifferent counter
electrode is positioned elsewhere on the body. The active electrode effectively
repels the active substance and forces it into the skin (Figure
7). This simple electrorepulsion is known as the main mechanism responsible
for penetration enhancement by iontophoresis. The number of charged molecules
which are moved across the barrier correlates directly to the applied current
and thus can be controlled by the current density. Other factors include the possibility
to increase the permeability of the skin barrier in the presence of a flow of
electric current and electroosmosis. Contrary to electrorepulsion, electroosmosis
can be used to transport uncharged and larger molecules. Electroosmosis results
when an electric field is applied to a charged membrane such as the skin and causes
a solvent flow across this membrane. This stream of solvent carries along with
it dissolved molecules. It enhances the penetration of neutral and especially
polar substances.
Figure
7: Basic principle of iontophoresis. A
current passed between the active electrode and the indifferent electrode
repelling drug away from the active electrode and into the skin.
Enlarged
version |
4.4.3 Electroporation
Electroporation is also based on the application of a voltage to the skin [9].
In contrast to iontophoresis where a low voltage is applied, electroporation requires
a large voltage treatment for a short period of 10 µs to 100 ms. Electroporation
produces transient hydrophilic pores (aqueous pathways) across the skin barrier
(Figure 8). These pores allow the passage of macromolecules
via a combination of diffusion, electrophoresis and electroosmosis.
Figure
8: Basic principle of electroporation.
Short pulses of high voltage current are applied to the skin producing
hydrophilic pores in the intercellular bilayers via momentary realignment
of lipids.
Enlarged
version |
4.4.4 Microneedles
In the last years, several attempts have been made to enhance the transport of
substances across the skin barrier using minimally invasive techniques [10]. The
proper function of an appropriate system requires that the thickness of the stratum
corneum ( 10 to 20 µm) has to be breached. More recent developments focus
on the concept of microneedles. Microneedles are needles that are 10 to 200 µm
in height and 10 to 50 µm in width (Figure 9). They
are solid or hollow and are connected to a reservoir which contains the active
principle.
Figure
9: Basic design of microneedle delivery
devices. Needles of approximately with or without centre hollow channels
are placed onto the skin surface so that they penetrate the stratum corneum
and epidermis without reaching the nerve endings present in the upper
dermis.
Enlarged
version |
Microneedle arrays are applied to the skin surface so that they pierce the upper
epidermis far enough to increase skin permeability and allow drug delivery, but
too short to cause any pain to the receptors in the dermis. Therefore there is
no limitation concerning polarity and molecular weight of the delivered molecules.
The fabrication of such tiny structures became possible with the advent of micromachining
technology which is an essential technology for the microelectronic industry.
It is not difficult to imagine that microneedle systems can be easily combined
with microelectronic elements which can fully control the delivery rate. Furthermore,
this type of system could be linked to a micro sensor system which measures the
actual concentration of an active molecule which then triggers the release. It
can be envisioned that such a “pharmacy on a chip” may be the future
of drug delivery.
4.5 Formulation approaches
Penetration enhancement with special formulation approaches is mainly based on
the usage of colloidal carriers. Submicron sized particles are intended to transport
entrapped active molecules into the skin. Such carriers include liposomes, nanoemulsions,
and solid-lipid nanoparticles (Figure 10) [11]. Most reports
cite a localizing effect whereby the carriers accumulate in stratum corneum or
other upper skin layers. Generally, these colloidal carriers are not expected
to penetrate into viable skin. However, the effectiveness of these carriers is
still under debate.
More recently a new type of liposomes called transferosomes has been introduced
[12, 13]. Transferosomes consist of phospholipids, cholesterol and additional
surfactant molecules such as sodium cholate. The inventors claim that transferesomes
are ultradeformable and squeeze through pores less than one-tenth of their diameter.
Thus 200 to 300 nm sized transfereosmes are claimed to penetrate intact skin.
Penetration of these colloidal particles works best under in vivo conditions and
requires a hydration gradient from the skin surface towards the viable tissues.
Another formulation approach aiming to enhance skin penetration is the preparation
of microemulsions. Such systems consist of water, oil, and amphiphilic compounds
(surfactant and co-surfactant) which yield a transparent, single optically isotropic,
and thermodynamically stable liquid. Microemulsions can be either oil continuous,
water continuous, or bi-continuous. The main difference between macroemulsions
and microemulsions lies in the size of the particles of the dispersed phase: these
are at least an order of magnitude smaller in the case of microemulsions ( 10
– 200 nm) than those of conventional emulsions (1 – 20 µm).
Typical properties of microemulsions include optical transparency, thermodynamic
stability, and solubility of both hydrophobic and hydrophilic components. Penetration
enhancement from microemulsions is mainly due to an increase in drug concentration
which provides a large concentration gradient from the vehicle to the skin. Furthermore
it has been suggested that the surfactants and the oil from the microemulsion
interact with the rigid lipid bilayer structure and acts as a chemical enhancer
[14].
5 Measurement of skin penetration
The penetration behavior of an active ingredient can be evaluated in vitro, ex
vivo, and in vivo.
Most of the data on percutaneous penetration have been gained with in vitro or
ex vivo studies by experiments using a Franz-Diffusion chamber (Figure
11). The donor (formulation) is separated from the acceptor (aqueous buffer
solution) by an appropriate barrier. For in vitro studies this barrier can consist
of an artificial skin construct (ASC). ASC is cultivated from different cell types
and comprises a dermis and a epidermis equivalent [15]. The advantage of ASC is
that the properties are more consistent than in natural skin. However, the barrier
properties of artificial skin are more closely to that of baby skin. This means
it is less restrictive than the skin of adults.
Ex vivo studies use animal or human cadaver skin as the barrier. Due to market
differences in the barrier properties animal skin is not always an accurate predictor
for the situation in human. The cadaver skin can be used in a whole but more frequently
excised skin is taken for the experiments. In this case the stratum corneum is
separated from the rest of the skin by a special preparation technique.
Also very useful for ex vivo studies is the bovine udder skin (BUS) model which
was developed 10 years ago [16]. The udder is from slaughter house material and
can be maintained in culture at high vitality for 8 – 10 hours. A warmed,
oxygen enriched Tyrode solution is pumped through the venous system of the udder.
Test substances are applied topically and the perfusate can be analyzed for the
penetrant (Figure 12). In addition, the BUS model allows
to assess the distribution of a substance in the udder skin from either tape stripping
or punched biopsies. Moreover, the BUS model can be used to measure irritation
caused by a certain formulation.
Figure
12: Scheme of the experimental set up of
the isolated perfused bovine udder skin model
Enlarged
version |
For human in vivo penetration studies the active content in different layers of
the stratum corneum can be determined after tape stripping or with the aid of
some advanced spectroscopic methods, e.g. ATR (attenuated total reflection) spectroscopy.
A more advanced in vivo technique is microdialysis (Figure 13).
For cutaneous microdialysis a small probe equipped with a semipermeable hollow
fiber is inserted superficially in the dermis. The principle of microdialysis
is that a physiological solution pumped through the probe is in equilibrium with
the diffusible molecules in the surrounding tissue. Therefore the concentration
of a solute in the dialysate is proportional to the concentration in the tissue
and allows direct monitoring of the in vivo penetration behavior of a active ingredient.
With such studies the influence of formulation variables as well as skin condition
can be evaluated.
6 Conclusions
The skin has an extremely good barrier function and to improve the penetration
of active ingredients it is frequently necessary to employ enhancement strategies.
The understanding of the barrier architecture and the mechanisms of penetration
has improved and many of the different determinants are understood. This knowledge
enables to develop both passive (chemical) and active (physical) approaches to
facilitate the entry of active molecules into the skin. However, skin penetration
enhancement could destroy the skin barrier formed by the lipid and protein and
thus induce side effects. Such unwanted effects are in most cases directly correlated
to an increase in transepidermal water loss (TEWL). Briefly, high TEWL means high
skin penetration, and high skin penetration means greater skin barrier impairment.
Future strategies should therefore aim to optimize the balance between the TEWL
increase and effectiveness of the penetration enhancement.
7 References
[1] Elias PM. Epidermal lipids, barrier function, and desquamation. J. Invest.
Dermatol. 80, 44-49, 1983
[2] Lampe MA et al. Human stratum corneum lipids: Characterization and regional
differences. J. Lipid Res. 24, 120-130, 1983
[3] Bouwstra JA, Gooris GS, Dubbelaar FE, Ponec M. Phase behavior of lipid mixtures
based on human ceramides: coexistence of crystalline and liquid phases. J. Lipid
Res. 42, 1759-1770, 2001
[4] Williams AC, Barry BW. Skin absorption enhancers. Critical Reviews in Therapeutic
Drug Carrier Systems 9; 305-353, 1992
[5] Fartasch M, Bassukas ID, Dipegen TL. Structural relationship between epidermal
lipid lamellae, lamellar bodies and desmosomes in human epidermis. An ultrastructural
study. Br. J. Dermatol. 128, 1-9, 1993
[6] Pellet M, Raghavan SL, Hadgraft J, Davis A. The Application of Supersaturated
Systems to Percutaneous Drag Delivery. In: Guy RH, Hadgraft J (Eds.) Transdermal
Drug Delivery. 2nd Edition, Marcel Dekker, New York, 2003, pp. 305-326.
[7] Barry BW. Novel mechanism and devices to enable successful transdermal drug
delivery. Eur. J. Pharm. Sci. 14, 101-114, 2001
[8] Cross SE, Roberts MS. Physical enhancement of transdermal drug application:
Is delivery technology keeping up with pharmaceutical development? Current Drug
Delivery 1, 81-92, 2004
[9] Preat V, Vanbever R. Skin Electroporation for Transdermal and Topical Drug
Delivery. In: Guy RH, Hadgraft J (Eds.) Transdermal Drug Delivery. 2nd Edition,
Marcel Dekker, New York, 2003, pp. 227-254.
[10] Down JA, Harvey NG. Minimally invasive Systems for transdermal Drug Delivery.
In: Guy RH, Hadgraft J (Eds.) Transdermal Drug Delivery. 2nd Edition, Marcel Dekker,
New York, 2003, pp. 327-359.
[11] Daniels R. Galenic principles of modern skin care products. Skin Care Forum
Online Issue 25, http://www.scf-online.com/english/25_e/galenic_25_e.htm
[12] Cevc G. Eur Pat Appl 1992; A 61 k 9/50.
[13] Cevc G. Lipidproperties as a basis for the modeling and design of liposome
membranes. In Gregoriadis G (Ed.) Liposome Technology. 2nd Edition, CRC Press,
Boca Raton, 1992, pp. 1-36.
[14] Schmalfuss U, Neubert R, Wohlrab W. Modification of drug penetration into
human skin using microemulsions. J. Control. Release 46, 279-285, 1997
[15] Specht C, Stoye I, Mueller-Goymann CC. Comparative investigations to evaluate
the use of organotypic cultures of transformed and native dermal and epidermal
cells for permeation studies. Eur. J. Pharm. Biopharm. 46, 273-278, 1998
[16] Pittermann W, Jackwerth B, Schmitt M. The isolated perfused Bovine Udder
Skin Model. A new in vitro model for the assessment of skin penetration and irritation.
Toxic. in Vitro 10, 17-21, 1997
Author
Prof. Dr. Rolf Daniels
Prof. Dr. Rolf Daniels has a Ph.D. degree in Pharmaceutics. Before continuing
his academic career, he worked for Pfizer in the department of pharmaceutical
development for 2 years. In 1995 he became Professor of Pharmaceutics in the Institute
of Pharmaceutical Technology at the Technical University of Braunschweig. His
main interests are in the field of surfactant-free emulsions, stability assessment
of semi-solids, and controlled delivery of insect repellents. Since 1997 he has
been head of the department Dermocosmetics of the Society of Dermopharmacy (GD).
