The Microsponge Drug Delivery System : For Delivering an Active Ingredient by Controlled Time Release

John I. D’souza

The expanding arena of emerging drugs, increased sensitivity to clinical
outcomes and healthcare costs are driving the need for alternative drug delivery
methods and devices.

The drug delivery technology landscape has become highly competitive and
rapidly evolving. More and more developments in delivery systems are being
integrated to optimize the efficacy and cost-effectiveness of the therapy.
New classes of pharmaceuticals, biopharmaceuticals (peptides, proteins and
DNA-based therapeutics) are fueling the rapid evolution of drug delivery technology.
These new drugs typically cannot be effectively delivered by conventional
means. The benefits from targeted, localized delivery of therapeutic agents
are other driving forces for the market.

Drug delivery systems (DDS) that can precisely
control the release rates or target drugs to a specific body site have had an
enormous impact on the health care system. Carrier technology offers an
intelligent approach for drug delivery by coupling the drug to a carrier
particle such as microspheres, nanoparticles, liposomes, etc. which
modulates the release and absorption characteristics of the drug. Microspheres
constitute an important part of these particulate DDS by virtue of their small
size and efficient carrier characteristics. ¹

The Microsponge Delivery System

To control the delivery rate of active agents to a predetermined site in
human body has been one of the biggest challenges faced by drug industry.
Several predictable and reliable systems were developed for systemic drugs
under the heading of transdermal delivery system (TDS) using the skin
as portal of entry.² It has improved the efficacy and safety of
many drugs that may be better administered through skin. But TDS is not practical
for delivery of materials whose final target is skin itself.

Controlled release of drugs onto the epidermis with assurance
that the drug remains primarily localized and does not enter the systemic
circulation in significant amounts is an area of research that has only recently
been addressed with success. No efficient vehicles have been developed for
controlled and localized delivery of drugs into the stratum corneum and underlying
skin layers and not beyond the epidermis. Application of topical drugs suffers
many problems such as ointments, which are often aesthetically unappealing,
greasiness, stickiness etc. that often results into lack of patient compliance.
These vehicles require high concentrations of active agents for effective
therapy because of their low efficiency of delivery system, resulting into
irritation and allergic reactions in significant users. Other drawbacks of
topical formulations are uncontrolled evaporation of active ingredient, unpleasant
odour and potential incompatibility of drugs with the vehicles.

Thus the need exists for system to maximize amount of time that an
active ingredient is present either on skin surface or with in the epidermis,
while minimizing its transdermal penetration into the body. The microsponge
delivery system fulfills these requirements.

A Microsponge^® Delivery System (MDS) is “Patented, highly cross-linked,
porous, polymeric microspheres polymeric system consisting of porous microspheres
that can entrap wide range of actives and then release them onto the skin
over a time and in response to trigger”. ³ It is a unique
technology for the controlled release of topical agents and consists of microporous
beads, typically 10-25 microns in diameter, loaded with active agent. When
applied to the skin, the MDS releases its active ingredient on a time mode
and also in response to other stimuli (rubbing, temperature, pH, etc). MDS
technology is being used in cosmetics, over-the-counter (OTC) skin care, sunscreens
and prescription products.

Delivery system comprised of a polymeric bead having network of pores with
an active ingredient held within was developed to provide controlled release
of the active ingredients whose final target is skin itself.⁴ The
system was employed for the improvement of performance of topically applied
drugs.^{5, 6, 7} The common methods of formulation remains same; the
incorporation of the active substance at its maximum thermodynamic activity
in an optimized vehicle and the reduction of the resistance to the diffusion
of the stratum corneum.

The MDS has advantages over other technologies like microencapsulation
and liposomes. Microcapsules cannot usually control the release rate of actives.
Once the wall is ruptured the actives contained with in microcapsules will
be released. Liposomes suffer from lower payload, difficult formulation, limited
chemical stability and microbial instability. While microsponge system in
contrast to the above systems are stable over range of pH 1 to 11, temperature
up to 130^oC; compatible with most vehicles and ingredients; self
sterilizing as average pore size is 0.25μm where bacteria cannot penetrate;
higher payload (50 to 60%), still free flowing and can be cost effective.

Most liquid or soluble ingredients can be entrapped in the particles. Actives
that can be entrapped in microsponges must meet following requirements,

1. It should be either fully miscible in monomer or capable of being
   made miscible by addition of small amount of a water immiscible solvent.
2. It should be water immiscible or at most only slightly soluble.
3. It should be inert to monomers.
4. It should be stable in contact with polymerization catalyst and
   conditions of polymerization.

Active following these
criteria serves as porogen or pore forming agent. Such drugs can be entrapped
while polymerization takes place by one-step process. While when the material
is sensitive to the polymerization conditions, polymerization is performed
using substitute porogen. The porogen is then removed and replaced by contact
absorption assisted by solvents to enhance absorption rate.

Release can be controlled
through diffusion or other triggers such as moisture, pH, friction, or
temperature. This release technology is available for absorbent materials or to
enhance product aesthetics. Microsponge delivery system can be incorporated
into conventional dosage forms such as creams, lotions, gels, ointments, and
powder and share a broad package of benefits. Systems can and improve its
formulation flexibility.

Preparation of Microsponges

Drug loading in microsponges can take place in two ways,
one-step process or by two-step process; based on physico-chemical properties
of drug to be loaded. If the drug is typically an inert non-polar material,
will create the porous structure it is called porogen. Porogen drug, which
neither hinders the polymerization nor become activated by it and stable to
free radicals is entrapped with one-step process.

Liquid-liquid suspension polymerization:

Microsponges
are conveniently prepared by liquid-liquid suspension polymerization.
Polymerization of styrene or methyl methacrylate is carried out in round bottom
flask. A solution of non-polar drug is made in the monomer, to which aqueous
phase, usually containing surfactant and dispersant to promote suspension is
added. Polymerization is effected, once suspension with the discrete droplets
of the desired size is established; by activating the monomers either by
catalysis or increased temperature.

Figure 1: Reaction vessel for microsponge preparation by liquid-liquid
suspension polymerization

When the drug is sensitive to the polymerization conditions, two-step process
is used. The polymerization is performed using substitute porogen and is replaced
by the functional substance under mild experimental conditions ⁸.

Quasi-emulsion solvent diffusion

As explained in Figure 2 the microsponges can also be prepared by quasi-emulsion
solvent diffusion method using the different polymer amounts. The processing
flow chart is presented in Fig. 1a. To prepare the inner phase, Eudragit RS
100 was dissolved in ethyl alcohol. Then, drug can be then added to solution
and dissolved under ultrasonication at 35 ^oC. The inner phase was
poured into the PVA solution in water (outer phase). Following 60 min of stirring,
the mixture is filtered to separate the microsponges. The microsponges are dried
in an air-heated oven at 40 ^oC for 12 h and weighed to determine
production yield (PY). ⁹

Figure 2: Preparation of microsponges by quasi emulsion solvent diffusion
method

Pharmaceutical Considerations of Microsponges

Physical characterization of microsponges

Particle size determination ¹⁰

Free-flowing
powders with fine aesthetic attributes are possible to obtain by controlling
the size of particles during polymerization. Particle size analysis of loaded
and unloaded microsponges can be performed by laser light diffractometry
or any other suitable method. The
values (d₅₀) can be expressed for all formulations as mean
size range. Cumulative percentage drug
release from microsponges of different particle size will be plotted against
time to study effect of particle size on drug release. Particles larger than 30
μm can impart gritty feeling and hence particles of sizes between 10 and
25 μm are preferred to use in final topical formulation.

Morphology and Surface topography of microsponges

For morphology and surface topography,
prepared microsponges can be coated with gold–palladium under an argon
atmosphere at room temperature and then the surface morphology of the
microsponges can be studied by scanning electron microscopy (SEM). SEM of a fractured microsponge particle can also be
taken to illustrate its ultrastructure ¹¹.

Determination of loading efficiency and production yield

The loading efficiency (%) of the microsponges can be
calculated according to the following equation:

The production yield of the microparticles can be determined by calculating
accurately the initial weight of the raw materials and the last weight of the
microsponge obtained.¹²

Determination of true density

The true density of microparticles and BPO was
measured using an ultra-pycnometer under helium gas and was calculated from a
mean of repeated determinations.

Characterization of pore structure

Pore volume and diameter are vital in controlling the intensity and duration
of effectiveness of the active ingredient. Pore diameter also affects the migration
of active ingredients from microsponges into the vehicle in which the material
is dispersed. Mercury intrusion porosimetry can be employed to study effect
of pore diameter and volume with rate of drug release from microsponges. ¹³

Porosity parameters of microsponges such as intrusion–extrusion isotherms,
pore size distribution, total pore surface area, average pore diameters, shape
and morphology of the pores, bulk and apparent density can be determined by
using mercury intrusion porosimetry. Incremental intrusion volumes can be plotted
against pore diameters that represented pore size distributions. The pore diameter
of microsponges can be calculated by using Washburn equation.¹⁴

Where D is
the pore diameter (μm); γ the
surface tension of mercury (485 dyn cm⁻¹); θ the contact angle (130^o);
and P is the pressure (psia).

Total pore area (A_tot) was calculated by using equation,

Where P is
the pressure (psia); V the
intrusion volume (mL g⁻¹); V_tot is the total specific intrusion volume (mL
g−1).

The average pore diameter (Dm) was calculated by using equation,

Envelope (bulk) density (ρ_se) of the microsponges was calculated by using
equation,

Where Ws
is the weight of the microsponge sample (g); V_p the empty penetrometer (mL); V_Hg is the volume of
mercury (mL).

Absolute (skeletal) density (ρ_sa) of microsponges was calculated by using
equation,

Where V_se
is the volume of the penetrometer minus the volume of the mercury (mL).

Finally, the percent porosity of the sample was found
from equation,

Pore morphology can be characterized from the intrusion–extrusion profiles
of mercury in the microsponges as described by Orr. ¹⁵

Compatibility studies

Compatibility of drug with reaction adjuncts can be studied by thin layer chromatography
(TLC) and Fourier Transform Infra-red spectroscopy (FT-IR). Effect of polymerization
on crystallinity of the drug can be studied by powder X-ray diffraction (XRD)
and Differential Scanning Colorimetry (DSC). ^{16, 17, 18} For DSC approximately
5 mg samples can be accurately weighed into aluminum pans and sealed and can
be run at a heating rate of 15^oC/min over a temperature range 25–430^oC
in atmosphere of nitrogen.

Polymer/ Monomer composition

Factors such
as microsphere size, drug loading, and polymer composition govern the drug
release from microspheres. ^{19, 20} Polymer composition of the MDS can affect
partition coefficient of the entrapped drug between the vehicle and the
microsponge system and hence have direct influence on the release rate of
entrapped drug. Release of drug from microsponge systems of different polymer
compositions can be studied by plotting cumulative % drug release against time.
Release rate and total amount of drug released from the system composed of
methyl methacrylate/ ethylene glycol dimethacrylate is slower than styrene/
divinyl benzene system.

Selection of monomer is dictated both by characteristics of active ingredient
ultimately to be entrapped and by the vehicle into which it will be dispersed.
Polymers with varying electrical charges or degrees of hydrophobicity or lipophilicity
may be prepared to provide flexibility in the release of active ingredients.
Various monomer combinations will be screened for their suitability with the
drugs by studying their drug release profile.

Resiliency

Resiliency (viscoelastic
properties) of microsponges can be modified to produce beadlets that is softer
or firmer according to the needs of the final formulation. Increased
cross-linking tends to slow down the rate of release. Hence resiliency of
microsponges will be studied and optimized as per the requirement by
considering release as a function of cross-linking with time.

Release evaluations

Dissolution tests

Dissolution profile of microsponges can be studied by use of dissolution apparatus
USP XXIII with a modified basket consisted of 5μm stainless steel mesh.
The speed of the rotation is 150 rpm. The dissolution medium is selected while
considering solubility of actives to ensure sink conditions. Samples from the
dissolution medium can be analysed by suitable analytical method at various
intervals.

Release mechanisms ²¹

By proper manipulation of the
aforementioned programmable parameters, microsponges can be designed to release
given amount of active ingredients over time in response to one or more
external triggers.

1. Pressure: Rubbing/ pressure applied can
release active ingredient from microsponges onto skin.

2. Temperature change: Some entrapped actives
can be too viscous at room temperature to flow spontaneously from microsponges
onto the skin. Increased in skin temperature can result in an increased flow
rate and hence release.

3. Solubility: Microsponges loaded with
water-soluble ingredients like anti-prespirants and antiseptics will release
the ingredient in the presence of water. The release can also be activated by
diffusion taking into consideration the partition coefficient of the ingredient
between the microsponges and the outside system.

Sustained release microsponges can also be developed. Various factors
that are to be considered during development of such formulations includes,

1.
Physical and
chemical properties of entrapped actives.

2.
Physical
properties of microsponge system like pore diameter, pore volume, resiliency
etc.

3.
Properties of
vehicle in which the microsponges are finally dispersed.

Particle size, pore characteristics, resiliency and monomer
compositions can be considered as programmable parameters and microsponges can
be designed to release given amount of actives in response to one or more
external triggers like; pressure, temperature and solubility of actives.

Safety considerations ^{22, 23}

Safety
substantiation of microsponges can be confirmed by skin irritation studies in
rabbits; eye irritation studies in rabbits; oral toxicity studies in rats;
mutagenicity in bacteria and allergenicity in guinea pigs.

Formulation Considerations

Actives entrapped in MDS can then be incorporated
into many products such as creams, lotions, powders and soaps. When formulating
the vehicle, certain considerations are taken into account in order to achieve
desired product characteristics.

1. The solubility of actives in the vehicle must be limited. Otherwise
   the vehicle will deplete the microsponges before the application.
2. To avoid cosmetic problems; not more than 10 to 12% w/w microsponges
   must be incorporated into the vehicle.
3. Polymer design and payload of the microsponges for the active must
   be optimized for required release rate for given time period.

There remains equilibrium between microsponge and
vehicle and microsponge releases drug in response to the depletion of drug
concentration in the vehicle. Drug concentration in the vehicle is depleted by
absorption of the drug into skin. Hence
continuous and steady release of actives onto the skin is accomplished with
this system.

Drug release from the topical semisolid formulation can be studied by using
Franz-type static diffusion cells. ²⁴

Examples of enhanced product performance

• Oil control: Microsponge can absorb oil up to 6 times
  its weight without drying.
• Extended release
• Reduced irritation and hence improved patient
  compliance
• Improved product elegancy

Examples of improved formulation flexibility

• Improved thermal, physical, and chemical stability
• Incorporation of immiscibles
• Liquids can be converted in to powders improving
  material processing
• Flexibility to develop novel product forms

Applications of microsponge systems

Microsponges are porous, polymeric microspheres that
are used mostly for topical and recently for oral administration. It offers the
formulator a range of alternatives to develop drug and cosmetic products.
Microsponges are designed to deliver a pharmaceutical active ingredient
efficiently at the minimum dose and also to enhance stability, reduce side
effects and modify drug release.

The system can have following applications ²⁵,

Overview of research work published

The Microsponge as Programmable Topical Delivery

The Microsponge systems are based on microscopic,
polymer-based microspheres that can bind, suspend or entrap a wide variety of
substances and then be incorporated into a formulated product, such as a gel,
cream, liquid or powder. A single Microsponge is as tiny as a particle of
talcum powder, measuring less than one-thousandth of an inch in diameter. Like a true sponge, each microsphere consists
of a myriad of interconnecting voids within a non-collapsible structure that
can accept a wide variety of substances.
The outer surface is typically porous, allowing the controlled flow of
substances into and out of the sphere. Several primary characteristics, or parameters, of the
Microsponge system can be
defined during the production
phase to obtain spheres that are
tailored to specific
product applications
and vehicle compatibility.

Microsponge systems are made of biologically inert polymers. Extensive safety
studies have demonstrated that the polymers are non-irritating, non-mutagenic,
non-allergenic, non-toxic and non-biodegradable. As a result, the human body
cannot convert them into other substances or break them down. Furthermore, although
they are microscopic in size, these systems are too large to pass through the
stratum corneum when incorporated into topical products.

Benzoyl peroxide (BPO) is commonly used in topical formulations for the treatment
of acne, with skin irritation as a common side effect. It has been shown that
controlled release of BPO from a delivery system to the skin could reduce the
side effect while reducing percutaneous absorption. Therefore, microspongic
delivery of Benzoyl peroxide was developed using an emulsion solvent diffusion
method by adding an organic internal phase containing benzoyl peroxide, ethyl
cellulose and dichloromethane into a stirred aqueous phase containing polyvinyl
alcohol ²⁶ and by suspension polymerization of styrene and divinyl
benzene.^{27, 28} The prepared microsponges were dispersed in gel base
and microspongic gels are evaluated for anti-bacterial and skin irritancy. The
entrapped system released the drug at slower rate than the system containing
free BPO. Topical delivery system with reduced irritancy were successfully developed.²⁹

Hydroquinone (HQ) bleaching creams are considered the gold standard for treating
hyperpigmentation. A new formulation of HQ 4% with retinol 0.15% entrapped in
microsponge reservoirs was developed to release HQ gradually to prolong exposure
to treatment and to minimize skin irritation. The safety and efficacy of this
product were evaluated in a 12-week open-label study. A total of 28 patients
were enrolled, and 25 completed the study. Study end points included disease
severity, pigmentation intensity, lesion area, and colorimetry assessments.
Adverse events also were recorded. Patients applied the microentrapped HQ 4%
formulation to the full face twice daily (morning and evening). A broad-spectrum
sunscreen was applied once in the morning, 15 minutes after application of the
test product. Patients were evaluated at baseline and at 4, 8, and 12 weeks.
The microentrapped HQ 4%/retinol 0.15% formulation produced improvement at all
study end points. Improvement in disease severity and pigmentation intensity
was statistically significant at weeks 4, 8, and 12 compared with baseline (P<0.001).
Lesion area and colorimetry measurements also were significantly improved at
each visit (P<0.001). Microentrapped HQ 4% was well tolerated, with only
one patient discontinuing because of an allergic reaction, which was not considered
serious. In this open-label study, microentrapped HQ 4% with retinol 0.15% was
safe and effective.¹²

Fluconazole is an active agent against yeasts, yeast-like fungi and dimorphic
fungi, with possible drawback of itching in topical therapy. Microspongic drug
delivery system of fluconazole with an appropriate drug release profile and
to bring remarkable decrease in frequently appearing irritation was attempted.
Microsponges were prepared by liquid-liquid suspension polymerization of styrene
and methyl methacrylate. Compatibility studies were carried out using TLC-FTIR,
DSC and XRD. The prepared microsponges were evaluated for polymer composition,
particle size (microscopy), surface topography (SEM), pore diameter, drug content
(HPLC) and drug release. Microsponges were dispersed in gel prepared by using
carbopol 940 and evaluated for drug release using Franz diffusion cell.
Free flowing powder with size distribution (30 to 107 μm) was obtained.
The average drug release from the gels containing microspongic fluconazole was
67.81 % in 12 h. Drug release from the gels containing microsponge loaded fluconazole
and marketed formulations has followed zero order kinetics (r = 0.973,
0.988 respectively). Drug diffusion study reveals extended drug release, in
comparison with marketed formulations containing un-entrapped fluconazole. Microspongic
system for topical delivery of fluconazole was observed potential in extending
the release.³¹

An MDS system for retinoic acid was developed and tested for drug release and
anti-acne efficacy. Statistically significant greater reductions in inflammatory
and non-inflammatory lesions were obtained with entrapped tretinoin in the MDS.³²

The Microsponge for Oral Delivery

A Microsponge system offers the potential to hold active
ingredients in a protected environment and provide controlled delivery of oral
medication to the lower gastrointestinal (GI) tract, where it will be released
upon exposure to specific enzymes in the colon.
This approach if successful should open up entirely new opportunities
for MDS.

In oral applications, the Microsponge system has
been shown to increase the rate of solubilization of poorly water-soluble drugs
by entrapping such drugs in the Microsponge system's pores. Because these pores
are very small, the drug is in effect reduced to microscopic particles and the
significantly increased surface area thus greatly increases the rate of
solubilization. An added benefit is that the time it takes the Microsponge
system to traverse the small and large intestine is significantly increased
thus maximizing the amount of drug that is absorbed.

Bioerodible
Systems based on new polymers for the
delivery of small and
large molecule drugs, including proteins and peptides, can
also be developed which, if successful open up new fields of opportunity in systemic
drug delivery arenas.

Kawashima
et al. have described methods for the preparation of hollow microspheres
('microballoons') with the drug dispersed in the sphere's shell, and also highly
porous matrix-type microspheres (‘microsponge’). The microsponges were prepared
by dissolving the drug and polymer in ethanol. On addition to water, the ethanol
diffused from the emulsion droplets to leave a highly porous particle. Variation
of the ratios of drug and polymer in the ethanol solution gave control over
the porosity of the particle, and the drug release properties were fitted to
the Higuchi
model. ^{33, 34} An approach to evaluate the loading capacity
of these Microsponge^® delivery systems has been developed utilizing
the relative inter-particulate friction sensing capability of the Hausner ratio
(tap density/apparent density) and comparing it to a more conventional flowability
test.³⁵

To determine if coated microsponges are viable for the slow release of chlorpheniramine
maleate (CPM), cellulose (Cellurofine) microparticles were loaded with CPM and
coated with Eudragit RS to form powder coated microsponges. These microsponges
were dispersed in wax matrix granules and compared with microparticles with
wax matrix only. The dissolution profile of CPM consisted of a fast release
phase and slow release phase. The dissolution rates for fast and slow release
phases of powder coated microsponge-wax matrix granules were 8.92 and 0117 per
h, respectively and were lower than those of the uncoated granules. In dogs,
the powder-coated granules demonstrated lower C_max and longer T_max
than CPM alone following oral administration.³⁶

Ketoprofen was used as a model drug for systemic drug delivery of microsponges
in the study. Ketoprofen microsponges were prepared by quasi-emulsion solvent
diffusion method with Eudragit RS 100 and afterwards tablets of microsponges
were prepared by direct compression method. Different pressure values were applied
to the tablet powder mass in order to determine the optimum pressure value for
compression of the tablets. Results indicated that compressibility was much
improved over the physical mixture of the drug and polymer; due to the plastic
deformation of sponge-like structure microsponges produce mechanically strong
tablets. ³⁷

Colon specific drug delivery system containing flurbiprofen (FLB) microsponges
was designed. Microsponges containing FLB and Eudragit RS100 were prepared by
quasi-emulsion solvent diffusion method. Additionally, FLB was entrapped into
a commercial Microsponge^® 5640 system using entrapment method. The
microsponges were spherical in shape, between 30.7 and 94.5μm in diameter
and showed high porosity values (61–72%). Mechanically strong tablets prepared
for colon specific drug delivery were obtained owing to the plastic deformation
of sponge-like structure of microsponges. In vitro studies exhibited that compression
coated colon specific tablet formulations started to release the drug at the
8^th hour corresponding to the proximal colon arrival time due to
the addition of enzyme, following a modified release pattern while the drug
release from the colon specific formulations prepared by pore plugging the microsponges
showed an increase at the 8^th hour which was the time point that
the enzyme addition made. ^{38, 39}

Bone-substitute compounds were obtained by mixing pre-polymerised powders of
polymethylmethacrylate and liquid methylmethacrylate monomer with two aqueous
dispersions of a-tricalcium phosphate (a-TCP) grains and calcium-deficient hydroxyapatite
(CDHA) powders. The final composites appeared to be porous. The total open porosity
was a function of the amount of water added. The water, which was the pore-forming
agent, vapourised after the polymerisation process, leaving behind empty spaces
in the polymeric matrix. The inorganic powders placed inside the polymeric matrix
were shown to act as local microsponges. The water capacity of these microsponges
can be determined by a centrifugation step carried out on aqueous dispersion
of a-TCP and/or CDHA powders that occur before any reaction with the organic
compound. The relationship between the total open porosity of the composites
and the amount of water trapped inside the inorganic agglomerates proved to
be almost linear. The effect of the chemical composition of the powder on the
total open porosity is not too great, provided that the two kinds of pellets
are prepared with the same amount of water. Both the permeability and shape
of the pores proved to be a function of the total open porosity. An increase
of the latter parameter produces an increase in permeability and a decrease
in tortuosity. Osteoconductivity and osteoinductivity of the final composites
were tested in vivo by implantation in rabbits. Formation of new trabecular
bone was observed inside the pores where the inorganic powders had been placed.
The material produced shows a good level of biocompatibility, good osteointegration
rate and osteogenetic properties.⁴²

The Microsponge in Delivery of biopharmaceuticals

The MDS is employed for the delivery of biopharmaceuticals and in tissue engineering
also. Newton D. W. has overviewed tissue targeted biopharmaceuticals delivery
through microsponges.^{40, 41}

Storage and release of endogenous growth factors by the extracellular matrix
(ECM) are important biological events that control tissue homeostasis and regeneration.
The interaction between basic fibroblast growth factor (bFGF) and heparan sulfate
proteoglycans has been extensively studied and used as a prototype model of
such a system, while the lower affinity of fibrillar type I collagen for bFGF
has generally been considered biologically insignificant. bFGF spontaneously
interacts with type I collagen solution and sponges under in vitro and in vivo
physiological conditions, and is protected from the proteolytic environment
by the collagen. bFGF incorporated in a collagen sponge sheet was sustained
released in the mouse sub-cutis according to the biodegradation of the sponge
matrix, and exhibited local angiogenic activity in a dose-dependent manner.
Intra-muscular injection of collagen microsponges incorporating bFGF induced
a significant increase in the blood flow in the murine ischemic hind limb, which
could never have been attained by bolus injection of bFGF. These results suggest
the significance and therapeutic utility of type I collagen as a reservoir of
bFGF.⁴³

Biodegradable materials with autologous cell seeding have attracted much interest
as potential cardiovascular grafts. However, pretreatment of these materials
requires a complicated and invasive procedure that carries the risk of infection.
To avoid these problems, we sought to develop a biodegradable graft material
containing collagen microsponge that would permit the regeneration of autologous
vessel tissue. The ability of this material to accelerate in situ cellularization
with autologous endothelial and smooth muscle cells was tested with and without
pre-cellularization. Poly (lactic-co-glycolic acid) as a biodegradable scaffold
was compounded with collagen microsponge to form a vascular patch material.
The poly (lactic-co-glycolic acid)–collagen patches with or without autologous
vessel cellularization were used to patch the canine pulmonary artery trunk.
Histologic and biochemical assessments were performed 2 and 6 months after the
implantation. There was no thrombus formation in either group, and the poly
(lactic-co-glycolic acid) scaffold was almost completely absorbed in both groups.
Histologic results showed the formation of an endothelial cell monolayer, a
parallel alignment of smooth muscle cells, and reconstructed vessel wall with
elastin and collagen fibers. The cellular and extra-cellular components in the
patch had increased to levels similar to those in native tissue at 6 months.
The poly (lactic-co-glycolic acid) collagen microsponge patch with and without
pre-cellularization showed good histologic findings and durability. This patch
shows promise as a bioengineered material for promoting in situ cellularization
and the regeneration of autologous tissue in cardiovascular surgery.⁴⁴

A thin biodegradable hybrid mesh of synthetic poly (DL-lactic-co-glycolic acid)
(PLGA) and naturally derived collagen was used for three-dimensional culture
of human skin fibroblasts. The hybrid mesh was constructed by forming web-like
collagen microsponges in the openings of a PLGA knitted mesh. The behaviors
of the fibroblasts on the hybrid mesh and PLGA knitted mesh were compared. The
efficiency of cell seeding was much higher and the cells grew more quickly in
the hybrid mesh than in the PLGA mesh. The fibroblasts in the PLGA mesh grew
from the peripheral PLGA fibers toward the centers of the openings, while those
in the hybrid mesh also grew from the collagen microsponges in the openings
of the mesh resulting in a more homogenous growth. The proliferated cells and
secreted extracellular matrices were more uniformly distributed in the hybrid
mesh than in the PLGA mesh. Histological staining of in vitro cultured fibroblast/mesh
implants indicated that the fibroblasts were distributed throughout the hybrid
mesh and formed a uniform layer of dermal tissue having almost the same thickness
as that of the hybrid mesh. However, the tissue formed in the PLGA mesh was
thick adjacent to the PLGA fibers and thin in the center of the openings. Fibroblasts
cultured in the hybrid mesh were implanted in the back of nude mouse. Dermal
tissues were formed after 2 weeks and became epithelialized after 4 weeks. The
results indicate that the web-like collagen microsponges formed in the openings
of the PLGA knitted mesh increased the efficiency of cell seeding, improved
cell distribution, and therefore facilitated rapid formation of dermal tissue
having a uniform thickness. PLGA–collagen hybrid mesh may be useful for skin
tissue engineering. Human skin fibroblasts were cultured in a thin biodegradable
mesh having a hybrid structure with web-like collagen microsponges formed in
the openings of a PLGA knitted mesh. More fibroblasts adhered and proliferated
more quickly in the hybrid mesh than in the PLGA knitted mesh. The collagen
microsponges in the hybrid mesh facilitated cell seeding, uniform cell distribution
and, therefore, the formation of homogenous dermis tissue. The PLGA knitted
mesh served as a skeleton, reinforced the hybrid mesh, maintained the integrity
of the forming tissue, and resulted in easy handling. PLGA–collagen hybrid mesh
could be a useful candidate as a porous scaffold for skin tissue engineering.⁴⁵

To solve several problems with artificial grafts, a novel bioengineered material
that can promote tissue regeneration without ex vivo cell seeding and that has
sufficient durability to be used for artery reconstruction was developed. It
was tested whether this biodegradable material could accelerate the in situ
regeneration of autologous cardiovascular tissue, especially of the arterial
wall, in various models of cardiovascular surgeries. The tissue-engineered patch
was fabricated by compounding a collagen-microsponge with a biodegradable polymeric
scaffold composed of polyglycolic acid knitted mesh, reinforced on the outside
with woven polylactic acid. Tissue-engineered patches without precellularization
were grafted into the porcine descending aorta (n=5), the porcine pulmonary
arterial trunk (n=8), or the canine right ventricular outflow tract (as the
large graft model; n=4). Histologic and biochemical assessments were performed
1, 2, and 6 months after the implantation. There was no thrombus formation in
any animal. Two months after grafting, all the grafts showed good in situ cellularization
by hematoxylin/eosin and immunostaining. The quantification of the cell population
by polymerase chain reaction showed a large number of endothelial and smooth
muscle cells 2 months after implantation. In the large graft model, the architecture
of the patch was similar to that of native tissue 6 months after implantation.
A tissue-engineered patch made of our biodegradable polymer and collagen-microsponge
provided good in situ regeneration at both the venous and arterial wall, suggesting
that this patch can be used as a novel surgical material for the repair of the
cardiovascular system.⁴⁶

Patent information of Microsponge Products

In September 1, 1987, Won; Richard (Palo Alto, CA) of
Advanced Polymer Systems, Inc. (Redwood City, CA) received US patent for
developing Method for delivering an active ingredient by controlled time
release utilizing a novel delivery vehicle which can be prepared by a process
utilizing the active ingredient as a porogen (United States Patent 4,690,825).

September 8, 1992 , Won; Richard (Palo
Alto, CA) of Advanced Polymer Systems, Inc. ( Redwood City ,
CA ) received US patent for developing Two-step
method for preparation of controlled release formulations (United States Patent
5,145,675).

Advanced Polymer Systems, Inc. and subsidiaries ("APS" or the "Company")
is using its patented Microsponge(R) delivery systems and related proprietary
technologies to enhance the safety, effectiveness and aesthetic quality of topical
prescription, over-the-counter ("OTC") and personal care products
like tretinoin, 5-fluorouracil and Vitamin-A etc. As on July
23, 2006 , the Company has a total of 10 issued U.S.
patents and an additional 92 issued foreign patents. 21 patent applications
are pending worldwide.

Dean, Jr. et al received US patent no. 4863856 for the development of weighted
collagen microsponges having a highly cross-linked collagen matrix are described
suitable for use in culturing organisms in motive reactor systems. The microsponges
have an open to the surface pore structure, pore sizes and volumes suitable
for immobilizing a variety of bioactive materials.⁴⁷

Marketed Formulation Using the MDS

Microsponge delivery systems are used to enhance the
safety, effectiveness and aesthetic quality of topical prescription,
over-the-counter ("OTC") and personal care products. Products under
development or in the marketplace
utilize the Topical Microsponge systems in three primary ways;

1.
As reservoirs releasing
active ingredients over an
extended period of time,

2.
As receptacles
for absorbing undesirable
substances, such as excess skin
oils, or

3.
As closed containers
holding ingredients away from the skin for superficial action.

The resulting benefits include extended efficacy, reduced
skin irritation, cosmetic elegance, formulation flexibility and improved
product stability.

The fundamental appeal of the Microsponge technology stems from the difficulty
experienced with conventional topical formulations in releasing active
ingredients over an extended period of time. Cosmetics and skin care
preparations are intended to work only on the outer layers of the skin. Yet,
the typical active ingredient in conventional products is present in a relatively
high concentration and, when applied to the skin, may be rapidly absorbed.
The common result is over-medication, followed by a period of under-medication
until the next application. Rashes and more serious side effects can occur
when the active ingredients rapidly penetrate below the skin's surface. Microsponge
technology is designed to allow a prolonged rate of release of the active
ingredients, thereby offering potential reduction in the side effects
while maintaining the therapeutic efficacy.

Marketed formulation using the MDS includes Ethical
Dermatological products (APS defined ethical dermatology products as
prescription and non-prescription drugs that are promoted primarily through the
medical profession for the prevention and treatment of skin problems or
diseases). Several ethical dermatology products approved by US FDA, OTC and
personal care products are sold in the United States. Results from
various human clinical
studies reaffirmed that
the technology offers the potential to reduce the drug side
effects, maintain the therapeutic efficacy and potentially increase
patient compliance with
the treatment regimen.

Ethical dermatology products have been developed or are
under development includes,

Tretinoin
Acne Medication: In February
1997, the FDA approved for the first ethical pharmaceutical product
based on patented
Microsponge technology;
Retin-A-Micro^(TM), which
has been licensed
to Ortho-McNeil Pharmaceutical Corporation. This product was
launched in March 1997. However, skin irritation among sensitive individuals
can limit patient compliance with the prescribed therapy. The Company believes its patented approach to
drug delivery reduces the potentially irritating side effects of
tretinoin. Ortho Dermatological began
marketing this product in March 1997.

5-Fluorouracil
(5-FU):
5-FU is an effective
chemotherapeutic agent for
treating actinic keratosis,
a pre-cancerous,
hardened-skin condition caused by
excessive exposure to sunlight.
However, patient compliance with the treatment regimen is poor, due to
significant, adverse side effects.
Microsponge-enhanced
topical formulation that potentially offers a less irritating
solution for treating actinic keratosis is sold under the brand of Carac.

Tretinoin
Photo-damage Treatment: Microsponge system
product for the treatment of photo-damage, which contributes to the premature
aging of skin and has been implicated in skin cancer.

Cosmeceutical
Products Retinol: Retinol is a highly pure form of vitamin A
which has demonstrated a remarkable
ability for maintaining the skin's youthful appearance. However, it has been available only on a
limited basis because it becomes unstable when mixed with other
ingredients. Stabilized retinol in a
formulation which is cosmetically elegant and which has a low potential for
skin irritation were successfully developed and marketed.

Personal Care and OTC Products: MDS is ideal for skin and personal care products. They can retain several times their weight in
liquids, respond to a variety of release
stimuli, and absorb large amounts of excess skin oil, all while retaining an
elegant feel on the skin's surface. The technology is currently employed in
almost number of products sold by major cosmetic and toiletry companies
worldwide. Among these products are skin cleansers, conditioners, oil control
lotions, moisturizers, deodorants, razors, lipstick, makeup, powders, and eye
shadows; which offers several advantages, including improved physical and
chemical stability, greater available concentrations, controlled release of the
active ingredients, reduced skin irritation and sensitization, and unique
tactile qualities.

Table 2: List of marketed products using microsponge drug delivery
system

APS developed microsphere precursors to the Microsponge for
use as a testing standard for gauging the purity of municipal drinking water.
Marketed nationwide, these microspheres are suspended in pure water to form an
accurate, stable, reproducible turbidity standard for the calibration of
turbidimeters used to test water purity. The technology can have much broader
applications than testing the turbidity of water and can even be used for the
calibration of spectrophotometers and colorimeters.

Summary

The MDS which was originally developed for topical
delivery of drugs can also be used for controlled oral delivery of drugs using
bioerodible polymers, especially for colon specific delivery. It provides a
wide range of formulating advantages. Liquids can be transformed into free
flowing powders. Formulations can be developed with otherwise incompatible
ingredients with prolonged stability without use of preservatives. Safety of
the irritating and sensitizing drugs can be increased and programmed release
can control the amount of drug release to the targeted site.

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About Authors:

John I. D’souza
Bharati Vidyapeeth College of Pharmacy, Near Chitranagari, Kolhapur 416013,
(M.S.), India
Email: johnsir4u@gmail.com

Harinath N. More
More is presently working as Professor in Pharmaceutical Chemistry and Principal
at Bharati Vidyapeeth College of Pharmacy, Kolhapur.