The drug delivery. In the USA (the most

The unique molecular transport and barrier characteristics of human skin,
which provide our protection against most toxic substances in the environment
and have frustrated efforts to use the surface of the body as a route of entry
of drugs for disease treatment, remain incompletely understood1, The skin is the body’s largest organ. It
serves many important functions, including protecting the body against trauma, regulating
body temperature, maintaining water and electrolyte balance, sensing painful
and pleasant stimuli, and Participating in vitamin D Synthesis.
The skin keeps vital chemicals and nutrients in the body while providing a
barrier against dangerous substances from entering the body and provides a
shield from the harmful effects of ultraviolet radiation emitted by the sun2. The skin’s barrier function
is important both to the transdermal delivery of drugs and to risk assessment
following dermal exposure to toxic chemicals.

The delivery of drugs into and
through the skin (skin permeation) has been an important area of research for
many years. With the relatively recent introduction of transdermal drug
delivery systems, interest has paralleled real and potential profits from
marketing of such delivery systems. In vitro skin permeation experiments are performed
at the beginning of most studies involving percutaneous absorption3. Recently, the transdermal
route has vied with oral treatment as the most successful innovative research
area in drug delivery. In the USA (the most important clinical market), out of
129 drug delivery candidate products under clinical evaluation, 51 are
transdermal or dermal systems; 30% of 77 candidate products in preclinical
development represent such drug delivery. The worldwide transdermal patch
market approaches $3 billion, yet is based on only 30 drugs, Scopolamine
(Hyoscine), Nitroglycerine Clonidine, Estradiol (With and Without
Norethisterone Or Levonorgestrel), Testosterone, Fentanyl, Nicotine, Alidocaine,
Methyl Phenindate, Diclofenac and Ritigotine, etc. The fundamental reason for
such few transdermal drugs is that highly impermeable human skin limits daily
drug dosage, delivered from an acceptable sized patch, to about10 mg.

In general, the epidermis (speci?cally,
the stratum corneum) provides the major control element, most small
water-soluble non-electrolyte diffuse into the systemic circulation a thousand

times more rapidly when the SC
layer is absent. Thus, to maximize drug ?ux we usually try to reduce this
barrier’s hindrance although sometimes the follicular route may also be
important.

We employed the principles of membrane permeation and a simplistic
two-phase representation of skin microstructure to evolve a model of the
transdermal permeation process which has to be rational and useful for
predicting the permeability of skin to various
micromolecular substances. we examine another property of skin affecting its
permeation behavior, its tendency to absorb and bind substances during the
process of permeation.

The extent and nature of the binding phenomenon is of great practical
importance in determining the unsteady state kinetics of transdermal mass
transport and the efficiency with which a substance delivered to the skin
surface will be released into the systemic circulation. In this, we have to found
a model of the sorption process, which invokes the coexistence of dissolved and
mobile absorbed molecules in equilibrium with site bound and immobile molecules
within the membrane, quite accurately correlates experimental absorption data
and transient transport measurements.

A significant database of
transdermal delivery rates and associated physical properties has been compiled
for a broad range of compounds. These data should, in theory, provide the means
to develop predictive models of percutaneous absorption. Furthermore, such
predictive models should be consistent with transport mechanisms. Till now,
skin transport data have often been explained using a model with distinct
permeation behavior for compounds of specific polarities. For example, it has been
postulated that very polar compounds traverse the lipophilic stratum corneum (SC)
via “aqueous pores,” while the transport of very nonpolar compounds is
limited by their transfer from the SC into the aqueous, sub adjacent tissue. For
compounds of intermediate polarity, SC permeability appears to be linearly related
to the membrane/water partition coefficient of the permeant.

Many biological processes involve molecular transport across biomembranes.
Molecules can permeate these biomembranes using a variety of mechanisms, some
of which are rather complex. It is important to understand these transport
mechanisms in biomembranes not only for their fundamental interest, but also
for rational design of drug delivery systems, as well as for developing more
efficient separation processes.

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