Hydrophobic polymers, controlled-release

Another aspect related to control release of drugs concerns the type of structures that currently appear to be working. Not unexpectedly, because of compatibility and degradation purposes, most of the effort on the control release formulations includes polymers that have both a hydrophobic and hydrophilic portion with the material necessarily containing atoms in addition to carbon. Another concern is that the products of degradation are not toxic or do not form toxic materials. It has also been found that amorphous materials appear to be better since they are more flexible and permit more ready entrance of potential degrad-ative compounds. 

The direct reaction of the polyamine with two equivalents of NO under appropriate conditions results in the formation of a stable zwitterionic diazenium-diolate moiety. Upon exposure to aqueous conditions, the diazeniumdiolate spontaneously decomposes into the polyamine precursor and two equivalents of NO. The kinetics of NO release are easily controlled by varying the structure of the amine precursor and/or environment (e.g., pH, temperature, solvent, etc.) In hydrophobic polymers, the NO release is slowed down as diazeniumdiolate decomposition is dependent on water uptake. 

Biostable polymers have been chosen for use in the majority of DES that are marketed or in clinical development. The main attractiveness of biostable polymers is their physical stability, inertness toward the drug, and predictable drug kinetics. In Cypher, a blend of poly(ethylene-co-butyl methacrylate) (PEVAc/PBMA) is used as the drug carrier. This hydrophobic polymer, along with additional polymer process steps, effectively controls the release of sirolimus, eluting 80% of the drug over 30 days after implantation. In the case of Taxus, atri-block copolymer of styrene-isobutylene-styrene (SIBS) is used as the hydrophobic polymer matrix that releases 10% of incorporated paclitaxel in the first 30 days (20). 

The controlled release of macromolecules from non-erodible, hydrophobic polymeric matrices is modelled as a discrete diffusion process with the release of solute occuring through distinct pores in the polymer which are formed as solid particles of molecule dissolve. In order to formulate predictive models of the release behavior of these devices, quantitative information on the microgeometry of the system is required. We present a computer-based system for obtaining estimates of the system porosity, isotropy, particle shape, and particle size distribution from observations on two-dimensional sections from the polymer matrix. 

Polymeric micelles Micelles consisting of amphiphilic polymers Loading hydrophobic drugs in the core for solubilization, targeted delivery, and controlled release 35,36... 

Water-insoluble materials such as hydrophobic polymers can supply hydrophobic interfacial environments. However, molecular assemblies such as micelles and lipid bilayer vesicles are more advantageous, because they supply large surface areas that are in contact with a water phase and more flexible organization. These characteristics are advantageous for substrate incorporation and product release. As explained in Chap. 4, a lipid bilayer provides a more stable hydrophobic environment, while micelles provide more dynamic and less stable assembUes. Structural and orientational control between the... 

In this review dealing with recent advances in membrane science, the term membrane" will be used to indicate any medium which acts as a barrier to transport into or out of a region, provides selective transfer of one species over another or regelates the transport of a material to its environment at a controlled rate. In addition to the common usage of the word membrane" to indicate a dense polymer film, the above definition includes a variety of interesting cases such as highly porous ultrafiltration membranes and hydrophobic liquid membranes with selectivity properties which can be tailored by incorporation of materials which selectively complex with one of the species to be processed. The important topics of controlled release of chemicals from polymeric devices and removal of volatile monomers from addition polymers such as poly (vinyl chloride and poly (acrylonitrile are also treated here. 

Reza MS, Quadir MA, Haider SS. Comparative evaluation of plastic, hydrophobic and hydrophilic polymers as matrices for controlled-release drug delivery. J Pharm Pharm Sci 2003 6(2) 282-291. 

Biopolymers represent an interesting alternative to synthetic polymers in order to be used in the route of structured carriers for controlled release and encapsulation applications. In particular, the ability of these carriers to entrap both hydrophilic and hydrophobic drugs may be very promising for many applications. In addition, the absence of chemical compounds and organic solvents used to produce biopolymeric matrices could be very interesting for some industrial applications. 

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