Bioavailability physicochemical processes

Finally, it is clear that much more research needs to be performed to connect the disciplines of colloid science and nutrition, especially in the application areas of functional foods and delivery systems. The combination of information from both in vitro and in vivo studies is evidently required in order to be able to understand properly the complex physicochemical processes that can occur during the digestion of food colloids. Our ultimate objective must be to make use of this knowledge to produce effective strategies for the formulation of a healthier diet with optimized bioavailability of the most desirable ingredients. 

An excipient is defined as a material that is deliberately incorporated into the formulation to aid some physicochemical process, for example for a tablet, integrity, dissolution, bioavailability or taste excipients are typically chosen from among many compounds without pharmacological properties (e.g. lactose), although there are examples where pharmacokinetics change with the excipient used. There are specialized examples of excipients, for example propellants are excipients that assist in the delivery of inhaled drugs to the respiratory tract. For intravenous infusions or ophthalmic products, the excipients are usually pH buffers or... 

The release of a compound from the food matrix in which it is incorporated is a determining process for its bioavailability and is largely influenced by the physicochemical characteristics of the compound, the type of food matrix, the subcellular location of the compound in plant tissues, and the food processing. The, food matrix type greatly influences the compound bioaccessibility. 

In reviewing the pH-partition hypothesis, it is apparent that it is an oversimplification of a very complex process. It does not consider one of the critical physicochemical factors, solubility. Low aqueous solubility is often the cause of the low bioavailability. To address this issue, Dressman et al. [28] developed an absorption potential concept that takes into account not only the partition coefficient but also solubility and dose. Using a dimensional analysis approach, the following simple equation was proposed ... 

The present volume of the series focuses on the interplay between organisms and the physical chemistry of the environmental media in which they live. It critically discusses the different physicochemical and biophysical features of the kinetics of processes at the biointerface, with special attention given to aspects such as bioavailability of chemical species, analysis of the necessary mass transfer towards/from the biointerface, routes of transfer through the biomembrane, etc. This volume was realised within the framework of the activities of the former IUPAC Commission on Fundamental Environmental Chemistry of the Division of Chemistry and the Environment. We thank the IUPAC officers responsible, especially the executive director, Dr John Jost, for their support and assistance. We also thank the International Council for Science (ICSU) for financial support of the work of the Commission. This enabled us to organise the discussion meeting of the full team of chapter authors (in Diibendorf, Switzerland, 2001) which formed such an essential step in the preparation and harmonisation of the various chapters of this book. 

The chemical form of arsenic in marine environmental samples is of interest from several standpoints. Marine organisms show widely varying concentrations of arsenic [4-6] and knowledge of the chemical forms in which the element occurs in tissues is relevant to the interpretation of these variable degrees of bioaccumulation and to an understanding of the biochemical mechanisms involved. Different arsenic species have different levels of toxicity [7] and bioavailability [8] and this is important in food chain processes, while physicochemical behaviour in processes such as adsorption onto sediments also varies with the species involved [9]. It has... 

Two of the recognized limitations of in situ technologies are (1) physicochemical restraints (e.g., bioavailability, desorption kinetics), and (2) a need for extended treatment time as compared to ex situ biotreatment approaches. Inherent geological parameters such as permeability, vertical and horizontal conductivity, and water depth can also represent constraints that are critically important to recognize and appreciate (Norris et al., 1993 Norris Falotico, 1994). Another widely recognized limitation inherent to in situ processes is that the systems are difficult to monitor and thus effective and complete treatment is difficult toascertain and validate. 

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). 

Oral administration is the most popular route due to ease of ingestion, pain avoidance, versatility, (to accommodate various types of dmg candidates), and, most importantly, patient compliance [119]. In addition, solid oral delivery systems do not require sterile conditions and are, therefore, less expensive to manufacture. Orally delivered pharmacologically active compounds must have favorable absorption and clearance properties, and satisfactory metabolic stability to provide adequate systemic exposure to elicit a pharmacodynamic response. If the compounds possess reasonable physicochemical properties have low to intermediate clearance and reasonable absorption, adequate oral bioavailabdity may be achieved [120]. Indeed, oral bioavailability, defined as the rate and extent to which the active dmg is absorbed from a pharmaceutical form and becomes available at the site of dmg action [121], is influenced by several factors including solubility, permeability, intestinal and liver metabolism, rapid biliary and other efflux pump-mediated excretion, and conditions in the gastrointestinal milieu [122,123]. Thus, both absorption and elimination processes determine the oral bioavailability F of a given dmg. Accordingly, F can be estimated as... 

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