Absorption mechanism

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

Absorption. An absorption mechanism capable of handling between 1.5 to 3.0 pg of vitamin responsible for most of the intestinal... 

Various kinetic models on particle formation were proposed by different researchers. These may be classified as follows (1) radical absorption mechanisms by Gardon [28-34] and Fisch and Tsai [13], (2) micellar nucleation newer models by Nomura et al. [35,36] and by Hansen and Ugelstad [37], (3) homogeneous nucleation by Fistch and coworkers [13,38,39]. 

Under the present reaction conditions, we observed the formation of succinic anhydride almost simultaneously together with the formation of GBL. The hydrogaiation of maleic anhydride yields succinic anhydride, and the subsequent hydrogenation of succinic anhydride produces GBL. The rate of hydrogenation of maleic anhydride to succinic anhydride was very fast compare to that of succinic anhydride to GBL. When the reaction was CEuried out wifliout solvent, tetrahydrofiiran was not producal. The above results indicate that the Pd-Mo-Ni/SiOz catalyst under our experimental conditions played an important role for the selective formation of GBL. Therefore, it is inferred that the catalyst composition may influence the route by which tetrahydrofiiran was formed, probably due to the different absorption mechanism of maleic anhydride, succinic anhydride, and GBL. 

Liposomes, which are lipid bilayer vesicles prepared from mixtures of lipids, also provide a useful tool for studying passive permeability of molecules through lipid. This system has, for example, been used to demonstrate the passive nature of the absorption mechanism of monocarboxylic acids [131]. Liposome partitioning of... 

Since many essential nutrients (e.g., monosaccharides, amino acids, and vitamins) are water-soluble, they have low oil/water partition coefficients, which would suggest poor absorption from the GIT. However, to ensure adequate uptake of these materials from food, the intestine has developed specialized absorption mechanisms that depend on membrane participation and require the compound to have a specific chemical structure. Since these processes are discussed in Chapter 4, we will not dwell on them here. This carrier transport mechanism is illustrated in Fig. 9C. Absorption by a specialized carrier mechanism (from the rat intestine) has been shown to exist for several agents used in cancer chemotherapy (5-fluorouracil and 5-bromouracil) [37,38], which may be considered false nutrients in that their chemical structures are very similar to essential nutrients for which the intestine has a specialized transport mechanism. It would be instructive to examine some studies concerned with riboflavin and ascorbic acid absorption in humans, as these illustrate how one may treat urine data to explore the mechanism of absorption. If a compound is... 

The decline in activity at the longer chain lengths (Fig. 6) also has a plausible explanation. Two factors are reasoned to be operative here declining solubilities coupled with changes in the absorption mechanism associated with stratification of the barrier. Aqueous solubilities of homologs exponentially decline as alkyl... 

In conclusion, it should be apparent from this discussion of the absorption mechanisms that, although the major features influencing drug absorption are well known, implementation of a coherent delivery strategy is highly specific for any compound, and many variables need to be adjusted for their significant influence on absorption and more importantly, on bioavailability. In addition, from the suggestion of the role of... 

Enalaprilat and SQ27,519 are angiotensin-converting enzyme (ACE) inhibitors with poor oral absorption. Enalapril and fosinopril are dipeptide and amino acid derivatives of enalaprilat and SQ27,519, respectively [51] (Fig. 10). Both prodrugs are converted via deesterification to the active drug by hepatic biotransformation. In situ rat perfusion of enalapril indicated a nonpassive absorption mechanism via the small peptide carrier-mediated transport system. In contrast to the active parent, enalapril renders enalaprilat more peptide-like, with higher apparent affinity for the peptide carrier. The absorption of fosinopril was predominantly passive. Carrier-mediated transport was not demonstrated, but neither was its existence ruled out. 

The mass balance approach was also applied to amoxicillin data. Amoxicillin is a broad-spectrum bacterial antibiotic administered orally for the treatment of various gram-positive and gram-negative infections. The dose ranged from 250 to 3000 mg with 200 mL of water. The corresponding dose concentration varied from 1.25 to 15 mg/mL. Low solubility (6 mg/mL) and a nonpassive absorption mechanism makes estimation or prediction of absorption more diffi-... 

The syngas resulted is mainly composed of CO ranging from 15 to 40%v/v, and H2 at elevated pressure from which C02 can be easily separated by a physical absorption mechanism and then C02 can be easily released by simply dropping pressure. 

Numerous observations of non-linear relationships between PbB concentration and lead intake in humans provide further support for the existence of a saturable absorption mechanism or some other capacity limited process in the distribution of lead in humans (Pocock et al. 1983 Sherlock et al. 1984, 1986). However, in immature swine that received oral doses of lead in soil, lead dose-blood lead relationships were non-linear whereas, dose-tissue lead relationships for bone, kidney and liver were linear. The same pattern (nonlinearity for PbB and linearity for tissues) was observed in swine administered lead acetate intravenously (Casteel et al. 1997). These results suggest that the non-linearity in the lead dose-PbB relationship may derive from an effect of lead dose on some aspect of the biokinetics of lead other than absorption. Evidence from mechanistic studies for capacity-limited processes at the level of the intestinal epithelium is compelling, which would suggest that the intake-uptake relationship for lead is likely to be non-linear these studies are discussed in greater detail in Section 2.4.1. 

It is important to recognize that the in vitro permeability obtained in cell mono-layers (such as Caco-2 models) should be considered as a qualitative rather than quantitative value. Especially poor are predictions of fraction dose absorbed for carrier-mediated drugs with low Caco-2 permeability and predictions of high fraction dose absorbed in humans [7, 20, 42, 48, 51]. However, it is possible to establish a reasonably good IVIVC correlation when passive diffusion is the dominating absorption mechanism. 

Kurosaki, Y., Nishimura, H., Terao, K., Nakayama, T., Kimura, T., Existence of a specialised absorption mechanism for cefadroxil, an amino-cephalosprorin antibiotic, in the human oral cavity, Int. J. Pharm. 1992, 82, 165-169. 

This equation presumes that each photon absorbed creates an electron-hole pair if there are other absorption mechanisms, the right-hand side must be multiplied by a quantum efficiency. The total rate of minority carrier generation is obtained by integrating over the space-charge region ... 

Figure 8. The bioconcentration ratios of chlorophenols in goldfish at 1 h exposure to their media as a function of the difference between pH and pKa. 4-CP 4-chlorophenol, 2,5-DCP 2,5-dichlorophenol, 3,5-DCP 3,5-dichlorophenol, 2,4,6-TCP 2,4,6-trichlorophenol, PCP pentachlorophenol, 2-CP 2-chlorophenol, 3-CP 3-chlorophenol, 2,4-DCP 2,4-dichlorophenol, 2,3-DCP 2,3-dichlorophenol, 2,4,5-TCP 2,4,5-trichlorophenol, 2,3,4,6-TCP 2,3,4,6-tetrachlorophenol, 2,6-DCP 2,6-dichlorophenol. Reprinted from [185] Water Res., 29, Kishino, T. and Kobayashi, K. Relation between toxicity and accumulation of chlorophenols at various pH, and their absorption mechanism in fish , pp. 431-442. Copyright (1995), with permission from Elsevier...

Kishino, T. and Kobayashi, K. (1995). Relation between toxicity and accumulation of chlorophenols at various pH, and their absorption mechanism in fish, Water Res., 29, 431-442. 

Fig. 1. Adsorption vs absorption mechanisms of organic pollutants in aqueous media...

Clinically relevant drug concentrations Linear absorption characteristics at clinically relevant concentrations should be demonstrated. Lack of effect of concentration on Peff determined in a range of physiologically relevant concentrations can be used to demonstrate lack of a saturable absorption mechanism (i.e. carrier mediated). Concentrations used in experiment should represent clinically relevant concentrations [132]... 

---