1. INTRODUCTION:
The Main problem associated with the oral drug delivery
include an uneven bio distribution, lack of drug targeting specificity, the
necessity of large doses to achieve local concentration and adverse side
effects due to such high dose. Hence novel drug delivery methods are of vital
interest to pharmaceutical industries. The development of transdermal drug
delivery systems (TDDS) is one of fastest developing field. TDDS is
particularly desirable for drugs that need prolonged administration at
controlled plasma level. As most drugs show inadequate skin permeability,
attempts are on to develop iontophoretic systems. Iontophoresis needs active
energy supplies in the form of low intensity electric current but can enhance
the skin permeation to a significant degree for ionic drugs.
As the medication becomes the integral part of life,
the success of the therapy depends on patient compliance too. Since transdermal
dosage forms can minimize the fluctuations of plasma level and increase patient
compliance, the development of the transdermal dosage form for this has become
research interest of late.
Diabetes was the sixteenth leading cause of global
mortality in 1990, accounting for 571,000 deaths.1 Type 2 diabetes represents about 98% of all
diabetes cases among persons older than 45 years of age. Medication
related problems leading to morbidity and mortality is quite common with the
disease.2
For diabetic patients, medication becomes an integral
part of life and noncompliance of therapy may lead to chronic complication.
As transdermal delivery can reduce the fluctuation of plasma level and bypass
the first pass elimination3 attempts have already been undertaken
for this category of drugs to develop transdermal systems.
Glimepiride is an oral hypoglycemic agent, used for the
treatment of non-insulin dependent diabetes mellitus.4,5 The drug
has plasma half-life 3-5 hrs4,6 and needs frequent administration.
Moreover its oral use is associated with severe and some time fatal
hypoglycemic symptoms like nausea, vomiting, heartburn, anorexia and increase
in appetite5 In 1997, Takahshi et al, had investigated the
sulfonylureas for transdermal administration and reported promising results.7
Later, Mutalik et al had studied permeability of this drug in mouse
skin with the objective of its transdermal development.8 The present
work investigates the iontophoretic permeability of the drug to assess its
potential for patient controlled active transdermal system.
2. MATERIALS AND METHODS:
2.1 Materials:
UV Visible Spectrophotometer SHIMADZU
UV-1700 PC, Shimadzu Corporation, Japan, was used. Iontophoretic DC source
(digital display, current 0-10 mA, voltage 0-25 V) was purchased from C-tech
Psu-2510/lab Mumbai; India. Glimepiride was a gift sample from West-Coast
Pharmaceutical Works, Gota, Ahmedabad, India; Silver plates (purity
99.99%, 5 mm diameters) were obtained from a local goldsmith shop. Ethanol,
Methanol, Sodium hydroxide, Potassium dihydrogen orthophosphate (KH2PO4),
Octanol and hydrochloric acid were obtained from SD Fine-Chem, Mumbai India.
Silver/silver chloride electrode was prepared as per the standard procedure.9
Silver wire (99.99% pure, 1.0 mm thickness) was used as connecting wire. All
the reagents/chemicals used were analytical grade.
2.2 Preparation of skin membrane:
From a local
abattoir, ear was obtained from freshly slaughtered pigs. The skin was removed
carefully from the outer regions of the ear and separated from the underlying
cartilage with a scalpel. After separating the full thickness skin, the fat
adhering to the dermis side was removed using a scalpel and isopropyl alcohol.
Finally the skin was washed with tap water and stored at refrigerator in
aluminum foil packing and was used within two days.10
Fig. 1: Effect of β- cyclodextrin on solubility of
glimepiride
2.3 Procedure of
passive permeation:
The in vitro passive
permeation studies were conducted using vertical type Franz diffusion cell
having a receptor compartment capacity of 10 ml. The excised skin was mounted
between the half-cells with the dermis in contact with receptor fluid
(phosphate buffer pH 6.8) and was equilibrated for 1 h. The area available for
diffusion was about 1.21 cm2. The donor cell was covered with an
aluminum foil to prevent the evaporation of vehicle. The fluid in the receptor
compartment was maintained at 37 ± 0.5°C. Under these conditions, the temperature
at the skin surface was approximately 32°C. 3 ml of the glimepiride suspension having
concentration 0.0720 mmol/ ml was placed in
the donor compartment. The entire assembly was kept on a magnetic stirrer and
the solution in the receiver compartment was stirred continuously using a
magnetic bead. The sample solution was withdrawn from the receptor compartment
at regular intervals and assayed for drug content. 5
2.4 Procedure of
iontophoretic diffusion:
For iontophoresis diffusion
cell was modified as suggested by Glikfield et al. 11 The apparatus
essentially consisted of a glass molded large receiving chamber provided with
two parallel ports on the topside and a sampling port on the side. Two upper
chambers are made from open-ended cylindrical glass tubes, the outer diameters
of which were equivalent to the inner diameter of the parallel ports. After the
skin is tied at this constricted end, the effective diameter increases and
becomes exactly equal to inner diameter of extended ports. Once slipped into
parallel ports they stay attached by glass joints forming two separate chambers
with skin at the base. Both the skin touches the receptor solution at the same
depth and each chamber houses one electrode. Once the battery is switched on
current flows through the skin placed in anodal compartment into receiving
solution below and reaches the cathodal electrode through the skin tied to
cathodal end. Donor solution was filled in one of the top chambers depending on the polarity of the drug and the other
serve as return electrode chamber. For our study, silver/silver chloride
electrode was inserted into the donor compartment whereas silver plate was
inserted into anodal chamber as return electrode. Direct current (0.5 mA cm-2)
was used throughout experiment. The receptor fluid (5 ml) was withdrawn at
regular intervals and replaced with fresh buffer to maintain sink condition.
The samples were assayed by UV spectrophotometer at 226 nm.
2.5 Solubility determination:
Solubility measurements were carried out
according to the method of Higuchi and Connors.12
An excess of glimepiride was added to phosphate buffer solutions (pH 6.8)
containing different concentrations of cyclodextrins. The suspensions were
shaken at 25 °C for 72 h and then filtered through a whatman filter
paper no. 40. An aliquot portion of the filtrate was analyzed for its drug
content by measuring its extinction at 226 nm against blank solution
containing the same concentration of cyclodextrin.
2.6 Data analysis:
The cumulative amount permeated was plotted against
time, and the slope of the linear portion of the plot was estimated as the
steady state flux. Permeability coefficient was calculated using following
formulas:
KP = JSS / Cd ………….…….
(1)
Where, Kp represents permeability
coefficient, Jss steady-state flux, Cd concentration of drug in donor
compartment , Flux enhancements were calculated by dividing iontophoretic
steady state flux to the corresponding passive steady state flux.
3. RESULTS AND DISCUSSION:
Though it is hypothesized that skin is permeable
to the lipophilic moieties of low molecular weight, in reality the extent of
transdermal permeation is a composite parameter influenced by a number of
physiochemical and biological factors. In addition to molecular weight (MW),
partition co-efficient and solubility, the pKa value, which determines the extent of ionization,
is of prime importance. According to Doh et al drug candidates for transdermal
delivery should have MW around 200~500 Da 13.
Glimepiride having a MW of around 490.617 Da
fits into the category. However two of its physiochemical properties,
solubility and pKa are not favorable for transdermal permeation. As
glimepiride is poorly water soluble, the rationale of this study was to improve
the 
solubility of
this drug by help of β-
cyclodextrin (β-CyD). The
solubility was increased, as increasing the concentration of β-cyclodextrin in
media phosphate buffer pH 6.8. (Fig.1)
At pH 7 mammalian skins are negatively charged and glimepiride being an
acidic drug it is largely ionized,14 which reduces its natural
affinity towards the skin. 15 In our initial study,
intrinsic permeability was found to be low when the drug was delivered from
aqueous saturated solution (data not shown). Because of low aqueous solubility
a high enough concentration gradient could not be developed which is the
driving force of passive permeation. Hence Cyclodextrins and iontophoresis was
attempted to enhance the solubility and permeation of the drug. To simulate the
physiological condition the diffusion cell was modified where both the
electrodes are placed on the same side of skin. Cyclodextrins
are known to solubilize lipophilic entities through molecular
encapsulation. Our solubility results are comparable to the data of other
authors. [16,17,18,19,20] regarding improvement of the solubility of
other sulfonylurea drugs as gliquidone, gliclazide,
glibenclamide, tolbutamide and acetohexamide
by cyclodextrins.
At pH 6.8 acquires a negative charge due to ionization
of sulfonyl group and was delivered from cathodal chamber. In this study the
drug was delivered as suspension. For drugs of low solubility this is
particularly necessary as the amount required for prolonged maintenance often
exceeds the limits of solubility. In suspension, the loss due to permeation is
supplemented by the presence of solid drug in the reservoir and thermodynamic
activity is maintained constant. Moreover, thermodynamic activity is a
function of percent saturation in the vehicle and high thermodynamic activity
results in higher partitioning into the stratum corneum. [21] Hence maximum
fluxes can be achieved from suspension, which represents the highest saturation
level.
Fig 2 show comparison of passive and iontophoretic
permeation profiles of glimepiride at chosen donor concentration. At all time
points iontophoresis considerably increased the permeation rate compared to
passive controls.
Finally to analyze the net benefit of electrical
energy, the active fluxes of drug is compared with the corresponding passive
value. (Table-1)
From an economic point of view, low oral
bioavailability results in wasting of a large portion of an oral dose and adds
to the cost of drug therapy, especially when the drug is an expensive one.[22]
This approach of enhanced solubility by Cyclodextrins and enhanced permeation
by iontophoresis would offer a promising drug delivery with reducing the dose
and other side effects.
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