Saturated absorption process

Pharmacokinetics and absorption of the drug should be linear. If the pharmacokinetic processes are dependent on the fraction of dose reaching the systemic blood flow (or of the dose administered) or on the rate of absorption, comparison between formulation and simulation cannot be made. This non-linearity may be owing to saturable absorption processes (active absorption), induction or inhibition of the metabolism, the first past effect, which is rate/absorption dependent, etc. Those points must be studied before any attempt to establish an IVIVC. 

In this paper, we present the consistent theoretical investigation of the saturation absorption processes in a CNT on the basis of the formalism of kinetic equations for density matrix of 7t-electrons. 

Reverse saturable absorption is an increase in the absorption coefficient of a material that is proportional to pump intensity. This phenomenon typically involves the population of a strongly absorbing excited state and is the basis of optical limiters or sensor protection elements. A variety of electronic and molecular reorientation processes can give rise to reverse saturable absorption many materials exhibit this phenomenon, including fuUerenes, phthalocyanine compounds (qv), and organometaUic complexes. 

Fig. 10 Urinary excretion of riboflavin (A, B) and ascorbic acid (C, D) in humans as a function of oral dose. Graphs A and C illustrate the nonlinear dependence of absorption on dose, which is suggestive of a saturable specialized absorption process. Graphs B and D represent an alternative graph of the same data and illustrate the reduced absorption efficiency as the dose increases. (Graphs A and C based on data in Ref. 39 and graphs B and D based on data in Ref. 40.)...

The CAT model estimates not only the extent of drug absorption, but also the rate of drug absorption that makes it possible to couple the CAT model to pharmacokinetic models to estimate plasma concentration profiles. The CAT model has been used to estimate the rate of absorption for saturable and region-depen-dent drugs, such as cefatrizine [67], In this case, the model simultaneously considers passive diffusion, saturable absorption, GI degradation, and transit. The mass balance equation, Eq. (51), needs to be rewritten to include all these processes ... 

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. 

The total fluorescence intensity saturated around a few hundreds of mJ/cm2 which corresponds to the irradiation condition where the new plasma-like emission was observed. Above this value fluorescence intensity decreased, which is accompanied with the recovery of the relative intensity of excimer emissions. This means that a quite efficient deactivation channel of excitation intensity opens in this energy range, and the contribution of Si -Si annihilation is depressed. This suggests that fragmentation reactions to diatomic radicals are not induced by the annihilation process. Multi-photon absorption processes via the Si states and chemical intermediates should be involved, although no direct experimental result has as yet been obtained. 

Facilitated Diffusion. Temporary combination of the chemical with some form of carrier occurs in the gut wall, facilitating the transfer of the toxicant across the membranes. This process is also dependent on the concentration gradient across the membrane, and there is no energy utilization in making the translocation. In some intoxications, the carrier may become saturated, making this the rate-limiting step in the absorption process. 

Molecules in crystals or dispersed in host lattices are often present in a range of environments, and this results in a broadening of the electronic absorption spectrum. Such an inhomogeneously broadened absorption band (envelope of transitions) may be considered as a superposition of several distinguishable sites. A narrow line laser can saturate one of the transitions under the envelope and the corresponding molecules will no longer take part in the absorption process. This phenomenon is referred to as hole... 

The above discussion of the Q-switching process immediately gives some requirements for a good Q-switching dye. First, this dye must exhibit a saturable absorption at the laser wavelength. Second, the residual losses by excited state absorption should be as low as possible. Third, the photochemical stability (and, of course, also the chemical stability in the dark) should be as high as possible. Fourth, the... 

Thus, a large dose may be ineffectively distributed and remain at the site of administration as a depot. A large dose of a compound given orally, for instance, may not be all absorbed, depending on the rates of absorption and transit time within the gut. Saturable active absorption processes would be particularly prone to dose effects, which could result in unexpected dose-response relationships. 

One important process for organics that affords optical limiting behavior is reverse saturable absorption (RSA). If a substantial proportion of the... 

At laser intensities sufficient to saturate the le-2e transition, the stimulated emission and absorption processes which couple the levels are fast relative to collisional transfer processes, and a quasi-equilibrium balance (e)/Np(e)] is quickly established. If the total population of levels le and 2e is approximately constant during the laser pulse, the upper level population 112(e) can be reliably related to Np(e)0 using an analysis similar to a two level atomic model (1, 2, 3). ... 

The first step in this direction is to correlate the permeability values obtained in the Caco-2 cell lines (apical-basolateral direction, Pay) with the fraction of dose absorbed in vivo in rat For this proposal eight fluoroquinolones were assayed and the results found with Caco-2 cell lines were compared with those obtained in vivo in rat In the Caco-2 cell lines the permeability of the quinolones was evaluated at different initial concentration, in order to test for no linearities in the absorption process. For some of them, it was observed that a secretion system worked in the opposite direction to passive diffusion for this reason the permeability value used for the correlations, in the case of secretion, was the one obtained at the highest concentration of the quinolone, which corresponds to saturation of the secretion process. 

Absorption Process with Solvent Pre-Saturation, U.S. Patent 5,687,584 Nov, 18, 1997. 

For saturation of absorption to be used for dose selection, information that the absorption process has been saturated using the intended route of administration is necessary. These data can usually be obtained during well-designed pharmacokinetic studies that evaluate linearity of absorption and dose proportionality using the route and frequency of dosing projected for human clinical studies. 

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