Factors controlling bioavailability

Polymorphs and solvated crystals is generally observed in pharmacentical indnstry [1], The bioavailability, stability, solnbility, and morphology of the pharmacentical products are very influenced by polymorphs [2-7], therefore the control of the polymorphic crystallization is very important. The crystallization process of polymorphs and solvated crystals is composed of competitive nucleation, growth, and transformation from a meta-stable form to a stable form [4], Furthermore, the crystallization behavior is influenced by various controlling factors such as temperature, supersaturation, additives and solvents [8], In order to perform the selective crystallization of the polymorphs, the mechanism of each elementary step in the crystallization process and the key controlling factor needs to be elucidated [8], On the other hand, we reported for L-Glutamic acid and L-Histidine system previously [4] that the nucleation and transformation behaviors of polymorphs depend on the molecular stractures. If the relationship between molecular stmcture and polymorphic crystallization behavior is known, the prediction of the polymorphism may become to be possible for the related compound. However, detail in such relationship is not clearly understood. 

Many studies have shown that marine organisms can accumulate PAHs from the environment. It is not so much a question of if an animal will accumulate PAHs but rather how much it will accumulate given the environmental concentrations, controlling factors of bioavailability, time... 

GIT, is considered to be lost from the absorption site, as is metabolic clearance and sequestration in various cell types and membranes (72,14). It is clear from Scheme I that the relative rates of the various processes will define the bioavailable fraction of the dose and understanding those factors which control pulmonary absorption kinetics is obviously the key to enhancing bioavailability via the lung. In a recent book (75) the molecular dependence of lung binding and metabolism was considered alongside the parallel processes of absorption, clearance and dissolution in the lung (14). Some key features of this work will be repeated as it relates to the systemic delivery of polypeptides. 

High hydrophobicity of pyrethroids is likely to result in their bioconcentration in biota which is mostly controlled by lipid content. On the other hand, the high hydrophobicity would reduce bioavailability of pyrethroids due to association with DOM and/or adsorption to suspended and bottom sediments [10]. The bioconcentration factor (BCF) from water to organisms is conveniently defined by... 

Leaner, J. J. and Mason, R. P. (2002). Factors controlling the bioavailability of ingested methylmercury to channel catfish and atlantic sturgeon, Environ. Sci. Technol., 36, 5124-5129. 

Early optimism about the possibility of in vitro-in vivo correlation was tempered by the need for a performance test that would yield reproducible results (10). Even though not necessarily correlated to bioavailability, dissolution requirements were seen as useful in controlling variables in formulation or processing. Thus, from the start, sources of variability in the results were seen as factors to be minimized in any proposed compendial method. 

The bioavailability of trace elements is further complicated by differences in the factors controlling transport to plant roots. These are ... 

Temperature is a factor that often limits bioremediation. The first cold-temperature groundwater bioremediations are in progress. In soil treatment, heat generation during composting may overcome the temperature limitations. Bioavailability is another factor controlling bioremediation and strongly affects the residual concentrations achieved. 

Dissolution of a drug substance is controlled by several physicochemical properties, including solubility, surface area, and wetting properties. For insoluble compounds, dissolution is often the rate-limiting step in the absorption process. Knowledge ofthe dissolution rate of a drug substance is therefore very useful for formulation development. The appropriate dissolution experiments can help to identify factors that contribute to bioavailability problems, and also assist in the selection of the appropriate crystal form and/or salt form. Dissolution tests are also used for other purposes such as quality control and assisting with the determination of bioequivalence (Dressman et al., 1998). 

It is generally accepted that free ionic forms of heavy metals are generally more toxic to biota than chelated or precipitated forms. Several factors control metal bioavailability and, thus, toxicity in environmental samples. These factors include pH, redox potential, alkalinity, hardness, adsorption to suspended solids, cations and anions, as well as interaction with organic compounds (Kong et al., 1995). 

We have addressed the topic of metal bioavailability and metal toxicity in environmental samples. Traditionally, metal availability is investigated using a chemical approach. Afterwards, the concept of Water Effect Ratio (WER) was proposed by the U.S. EPA and employed bioassays (e.g., fish and invertebrate tests) to assess metal bioavailability and toxicity. In the HMBC approach discussed in this review, we have made use of a bacterial assay that is specific for metal toxicity to achieve this goal. This is only a preliminary survey of the potential applications of the HMBC concept. Some preliminary results on the use of MetPLATE for the fractionation of HMBC to obtain information on the factor(s) that control metal bioavailability in environmental samples were also presented. Using MetPLATE eliminates or diminishes the confounding factor represented by the presence of organic toxicants in a given sample. Further work is needed to refine the fractionation scheme. 

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