Transdermal spray offers numerous advantages over the other conventional transdermal drug delivery forms such as gel,
ointment and patches, in terms of its cosmeceutical appearance, ready availability for application, flexibility in dosage
design, less occurrence of skin irritation and faster drying rate from the application site due to the use of volatile solvent.
However, compared to other transdermal drug delivery dosage forms, transdermal spray has the least and limited number
of products approved for marketing. Among the drugs are, Evamist®, an estradiol formulation approved in 2007 by
the FDA followed by Axiron® a non-spray solution to treat low testosterone in men and Recuvyra®, a pain reliever solution
indicated for dogs. This review article focuses current status on the formulation and evaluation of transdermal spray in
the background of the role and effects of its composition specially the selection of drugs, volatile solvents, penetration
enhancers and film forming polymer, etc. The limitation of transdermal spray highlighted in this review is the concern of
its use, especially, the third party exposure particularly for endocrinology indication. Moreover, transdermal spray is also
restricted in drugs with large doses due to the limited diffusivity into the skin. The difficulty of exploiting hydrophilic drugs
like peptides, macromolecules and new genetic treatments using DNA or small-interfering RNA (siRNA) into transdermal
spray formulations is also a limitation that needs to be explored in depth.
Transdermal drug delivery (TDD) is one of the alternative
modes of drug administration preferred over oral and injections.
This is due to its distinct advantages such as avoidance
of first pass metabolism relating to oral administration, provision
of steady state drug-plasma concentration, improvement
of patient adherence, prevention of potential gastrointestinal
(GI) adverse effects and reduction of medical waste of hypodermic
needles in low resource settings. The skin provides
a large surface area suitable for absorption and the noninvasive
procedure for the transdermal drug system such as a
patch that enables a continuous intervention with the applied
medication. The amount of drug delivered through the
skin and the obtained therapeutic effect depends on the ability
of the drug to permeate through the skin. The permeation
of the drug into the skin is restricted by the stratum corneum
(SC), the outermost layer of the skin, which is surrounded by
a lipid region. Numerous approaches like iontophoresis,
sonophoresis, electroporation, use of chemical permeation
enhancers (PE), microniddle, and the use of lipid vesicles have
been studied for the last 30 years to break the barrier properties
of SC and some of them have produced commercial success.
The development of the transdermal delivery system can be
classified into three generations. The first generation consists
of low-molecular weight, lipophilic, and low dose drugs. The
second generation uses permeation enhancement methods
such as conventional chemical PEs, iontophoresis, and non- cavitational ultrasound in order to increase the drug permeability
through the SC. The selection of PE was carried out
carefully according to several criteria; i) enhanced permeation
ability without causing permanent disruption in the structure
of the stratum corneum, ii) ability to enhance transdermal flux
in maximum amount iii) permeation ability without causing
any injury to the deeper tissues. The third generation focuses
more on giving the effects on the stratum corneum by incorporating
microneedles, thermal ablation, microdermabrasion,
electroporation and cavitational ultrasound.
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