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Molecular Modeling Studies of Polymeric Transdermal Adhesives: Structure and Transport Mechanisms

Dr Solomon Jacobson of National Starch has used Accelrys software to develop a method to simulate the diffusion of drugs through amorphous polymers.

His work revealed the manner in which drug molecules are transported through acrylic polymers, and has applications for commercial transdermal drug delivery systems.

Transdermal Adhesives are amorphous, rubbery materials that exhibit permeabilities in a range that is useful for drug delivery systems. Drug delivery rates from these polymers depend on polymer thickness, drug solubility, and drug diffusivity. Molecular modeling lets scientists examine and visualize molecular structures and dynamics, helping them to understand the relationship between atomic structure and drug transport.

Dr Solomon Jacobson of National Starch performed molecular modeling studies of three acrylic copolymer adhesives used in transdermal drug delivery systems.

Dr Jacobson created virtual amorphous polymer structures using the Amorphous Cell and Discover programs in Accelrys' Insight II (now available in Materials Studio). The first polymer considered was Duro-Tak (3)87-2353, an acrylic, pressure-sensitive adhesive for use in transdermal drug delivery systems. The polymer structure was investigated by creating virtual X-ray diffraction patterns using Accelrys' Cerius2 (now also available in Materials Studio), revealing that the polmer has an irregular free volume, with many voids and channels.

In the next step, a drug molecule (estradiol) was placed at a random location in the amorphous model. The structures were minimized, and a force was applied to the estradiol molecule. The drug diffusion constant was determined by dividing the observed drug velocity by the applied force. Constant temperature - constant volume molecular dynamics simulations were performed to study the diffusion. This process was repeated using sodium salicylate as the drug molecule.

The resulting molecular dynamics trajectories showed that the drug moved by a jump-diffusion mechanism. Drug molecules were trapped within the voids for a period of time before moving to an adjacent void. Drug transport involved drug flexibility and motion of the polymer backbone as well as specific molecular interactions between the drug and the polymer matrix.

These studies demonstrate that molecular modeling can be used to study diffusion processes of a drug in a particular polymer and to design and modify adhesives for optimal delvery of specific drug.

References

  1. Jacobson, S. H., "Molecular Modeling Studies of Polymeric Transdermal Adhesives: Structure and Transport Mechanisms", Pharmaceutical Technology, 122-130, September 1999.

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Duro-Tak is a registered trademark of National Starch and Chemical Co.

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