Absorption enhancers characteristics

In order to develop compounds that can selectively target duplex RNA, Sinha et al. [ 194] studied the interaction of berberine with two different conformations of poly(rC) poly(rG) structures. Poly(rC) poly(rG) has been shown [15,215] to exist in two conformations depending on the pH of the solution, the A-form at physiological pH and the protonated form at pH 4.3. These two conformations have been characterized to have clearly defined but distinctly different circular dichroic and absorption spectral characteristics. Both the A-form and the protonated form of the RNA induced moderate hypochromic change and bathochromic shifts in the absorption maxima peaks at 344 nm and 420 nm of the alkaloid with three isosbestic points centered around 357,382 and 448 nm. Binding of berberine to both forms enhanced the fluorescence intensity, which was higher with the protonated form than with the A-from, suggesting clear differences in the nature of orientation... 

F.t arises from absorption of characteristic X-rays emerging from the specimen, F allows for an enhancement of the characteristic X-ray intensity due to fluorescence,... 

Wu, S. H.-W. and W. K. Hopkins (1999). Characteristic efc-tocopheryl PEG 1000 succinate for applications as an absorption enhancer in drug delivery syst ltiarm. Tech, 23 44-58. 

As mentioned above, the rectal route is very attractive for systemic delivery of peptide and protein drugs, but rectal administration of peptides often results in very low bioavailability due to not only poor membrane penetration characteristics (transport barrier) but also due to hydrolysis of peptides by digestive enzymes of the GI tract (enzymatic barrier). Of these two barriers, the latter is of greater importance for certain unstable small peptides, as these peptides, unless they have been degraded by various proteases, can be transported across the intestinal membrane. Therefore, the use of protease inhibitors is one of the most promising approaches to overcome the delivery problems of these peptides and proteins. Many compounds have been used as protease inhibitors for improving the stability of various peptides and proteins. These include aprotinin, trypsin inhibitors, bacitracin, puromycin, bestatin, and bile salts such as NaCC and are frequently used with absorption enhancers for improvement in rectal absorption. 

Figure 19.4C presents data on the phosphorescence decay dynamics on two of the samples in Figure 19.4B, the one without silver and the one having optimum enhancement. These exhibit the characteristic signature of plasmon-enhanced emission, namely that increases in luminescence are accompanied by decreases in excited state lifetime. Ordinarily, radiative rates are fixed by quantum mechanical matrix elements and variation in excited state lifetime is due to changes in nonradiative rates so that increases in lifetime correspond to lower non-radiative rates and increases in luminescence yield. Here, the lifetime is reduced by about a factor of 3 due to increased emissive rate even as the luminescence increases 215 times. These large enhancements cannot be accounted for simply by increase in phosphorescence yield since the yield is greater than 10 % in the absence of silver. It is evident that a substantial fraction of the increase must be accounted for by absorption enhancement. 

It is generally accepted that the bioavailability of most topically applied drugs remains low. Various methods to increase this bioavailability have been used. One of the approaches is the use of absorption enhancers, and over the years, there has been a great interest in new chemical absorption enhancers. An absorption enhancer should be pharmacologically inert, non-toxic, have a rapid and reversible onset of action, be chemically and physically compatible with other formulation compounds, and be cosmetically acceptable. Of course not all absorption enhancers possess all of these characteristics, and a benefit-to-risk evaluation will determine the choice of a molecule as an absorption enhancer. The range of absorption enhancers that has been researched is large. Thus, overview of the most researched compounds is presented. 

From recent topics in pioneering new absorption enhancers with desirable characteristics, chitosan is explained as an example. 

Nasal drug absorption can be accomplished by use of prodrugs, chemical modification of the parent molecule, and use of physical methods of increasing permeability. Special excipient used in the nasal preparations comes into contact with the nasal mucosa and may exert some effect to facihtate the drug transport. The mucosal pores are easier to open than those in the epidermis. The following characteristics should be considered in choosing an absorption enhancer ... 

Wu S and Hopkins W. Characteristics of D-a-Tocopheryl PEG 100 Succinate for Applications as an Absorption Enhancer in Drug Delivery Systems. Pharm Tech 1999 23 52-68. 

There are many factors affecting the effectiveness of absorption enhancers, including the physico-chemical characteristics of the drugs, the administration site of absorption enhancers, and species differences in the effectiveness of absorption enhancers. In this section, we describe those factors that can regulate the effectiveness of various absorption enhancers. 

Research on oral liposomal delivery systems has moved forward with the development of polymer-modified liposomes. For example, targeted PEGylated liposomes furnished with folic acid for oral delivery were promising, showing enhanced permeability of dextran (used as a marker) across Caco-2 cell monolayers (Anderson et al., 1999). PEG and chitosan-coated lipid nanoparticles were constmcted as oral delivery systems for salmon calcitonin (sCT). The PEG-coated nanoparticles did not alter the transepithelial electrical resistance of Caco-2 cell monolayers, while the chitosan-coated nanoparticles showed a dose-dependent increase in the permeability of dextran across the monolayers (Garcia-Fuentes et al., 2005). It demonstrated that the favourable interaction of the chitosan-coated nanoparticles with intestinal mucosa, together with their permeation enhancing characteristics, could improve the oral absorption of sCT. 

In the conversion of net line intensity to analyte concentration, it may be necessary to correct for absorption and/or enhancement effects. Absorption effects include both primary and secondary absorption. Primary absorption occurs because all atoms of the spiecimen matrix absorb photons from the primary source. Since there is competition for these primary photons by the atoms making up the specimen, the intensity — wavelength distribution of the photons available for excitation of a given analyte element may be modified by other matrix elements. Secondary absorption refers to the absorption of characteristic analyte radiation by the specimen matrix. As characteristic... 

Polyimide. Polyimide is a biaxiaHy oriented high performance film that is tough, flexible, and temperature- and combustion-resistant. Its room temperature properties compare to poly(ethylene terephthalate), but it retains these good characteristics at temperatures above 400°C. Its electrical resistance is good and it is dimensionally stable. The principal detriment is fairly high moisture absorbance. The main uses are for electrical insulation, particularly where high temperatures are prevalent or ionizing radiation is a problem. The films may be coated to reduce water absorption and enhance... 

Drugs that are too highly hydrophilic are often absorbed rather poorly from the gastrointestinal tract. It is sometimes possible to circumvent this difficulty by preparing esters of such compounds so as to change their water lipid partition characteristics in order to enhance absorption. Once absorbed, the esters are cleaved by the numerous esterase enzymes in the bloodstream, releasing free drug. 

Generally used with mineral wool products where, in its decorative forms, it gives attractive facings to ceilings and wall tiles and enhances their sound-absorption characteristics. PVC is also used as a vapor control layer facing. 

Glass-fiber tissue or non-woven fabrics are used for decorative purposes on many insulants. They also give improved strength to foam plastics and enhanced sound-absorption characteristics to mineral wools. 

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