Tuesday, 18 July 2017

A Review on Transdermal Spray: Formulation Aspect

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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