Bioavailability solution

Co-administration of ofloxacin and chitosan in eyedrops increased the bioavailabUity of the antibiotic [290]. Trimethyl chitosan was more effective because of its solubility (plain chitosan precipitates at the pH of the tear fluid). On the other hand, N-carboxymethyl chitosan did not enhance the corneal permeability nevertheless it mediated zero-order ofloxacin absorption, leading to a time-constant effective antibiotic concentration [291]. Also W,0-carboxymethyl chitosan is suitable as an excipient in ophthalmic formulations to improve the retention and the bioavailability of drugs such as pilocarpine, timolol maleate, neomycin sulfate, and ephedrine. Most of the drugs are sensitive to pH, and the composition should have an acidic pH, to enhance stability of the drug. The delivery should be made through an anion exchange resin that adjusts the pH at around 7 [292]. Chitosan solutions do not lend themselves to thermal sterilization. A chitosan suspension, however. 

L d Agay-Abensour, A Fjellestad-Paulsen, P Hoglund, Y Ngo, O Paulsen, JC Rambaud. Absolute bioavailability of an aqueous solution of l-deamino-8-D-argi-nine vasopressin from different regions of the gastrointestinal tract in man. Eur J Clin Pharmacol 44 473-476, 1993. 

Whenever a drug is administered by an extra-vascular route, there is a danger that part of the dose may not reach the blood (i.e., absorption may not be complete). When the intravenous route is used, the drug is placed directly in the blood therefore an IV injection is, by definition, 100% absorbed. The absolute bioavailability of an extravascular dosage form is defined relative to an IV injection. If IV data are not available, the relative bioavailability may be defined relative to a standard dosage form. For example, the bioavailability of a tablet may be defined relative to an oral solution of the drug. 

Another new development has been the application of oral absorption promoters. These materials are designed to enhance the oral bioavailability of many compounds and improve variable absorption. However, many of these compounds are hydrophobic in nature and cause difficulty during tableting itself. The challenge for formulators is to arrive at clever solutions to the process problems while retaining material performance. 

Bioavailability depends not only on having the drug in solution, but also on the drug s permeability. A jejunal permeability of at least 2-4 x 10 4cm/s, measured in human subjects by intubation, is considered high [97]. For many drugs and other substances, this permeability corresponds to a fraction absorbed of 90% or better. Amidon et al. [97] thus proposed a Biopharmaceutics Classification System (BCS) for drugs based on the above definitions of these two parameters. Table 3 defines the BCS and includes some drugs representative of each class. 

Comparative bioavailability data are discussed where a number of different dosage forms/routes of administration have been used during the development process, e.g., tablets, capsules, oral solutions, granules, and injections. 

A number of oral solution or suspension products are included in the EPARs. Apart from the usual points of consideration for active ingredients and excipients, particular mention is made of possible precipitation of active ingredient when a solution is in use, the inclusion of excipients having a major impact on bioavailability, the need for flavoring to mask the taste of the active ingredient, relative potency compared with other routes of administration, preservation issues, dosing devices and the precision and accuracy of the dose delivered, and bioequivalence where formulations have been modified during the development process. 

The total metal concentration in a solution can be easily determined using methods such as atomic absorption spectroscopy (AAS) however, the bioavailability of different metal species likely varies. In addition, much of the original concentration may have speciated into insoluble precipitates. Therefore, the concentration of some bioavailable species may be extremely low, perhaps even within or below the nanomolar range.99 Ion-selective electrodes are useful for measuring the bioavailable concentration of a metal because they measure only the free, ionic species, which is often most prevalent.102... 

Other studies use soil or sediment samples for a more accurate indication of microbial activity in natural environments. In these samples, organic matter and clay particles play a role in metal toxicity.76112113 Both organic material and clay particles in soil can bind metals and reduce their bioavailability. For example, Pardue et al.87 demonstrated that much less solution-phase cadmium was required to inhibit trichloroaniline (TCA) dechlorination in a mineral-based soil than in a soil containing a higher concentration of organic matter. Other studies have shown that adding clay minerals to a medium mitigates toxicity. Clay minerals, such as kaolinite, montmorillonite, bentonite, and vermiculite, can bind to metals to decrease the amount that is bioavailable.112 115... 

Although buffer components are often present at the highest concentration in a medium, metals can also bind to inorganic ligands in solution. Ligands such as CL, OHand SO can form soluble complexes with many metals.127 These complexes remain in solution, but are considered to be less bioavailable than the free, ionic species under most conditions.97 Even though other soluble metal species are considered less bioavailable, they may play roles in metal toxicity. 

Pm)- Many scientists have tried to relate these Pe or PM values for solutes to parameters which are determined from in vivo studies (e.g., oral bioavailability). In vitro-in vivo correlations should be performed judiciously with full appreciation of the nature of the parameters being measured. 

GM Grass, JR Robinson. (1984). Relationship of chemical structure to corneal penetration and influence of low viscosity solution on ocular bioavailability. J Pharm Sci 73 1021-1027. 

The book focuses on the biogeochemistry of trace elements in arid and semiarid zone soils and includes an introductory chapter on the nature and properties of arid zone soils. It presents an updated overview and a comprehensive coverage of the major aspects of trace elements and heavy metals that are of most concern in the world s arid and semi-arid soils. These include the content and distribution of trace elements in arid soils, their solution chemistry, their solid-phase chemistry, selective sequential dissolution techniques for trace elements in arid soils, the bioavailability of trace elements, and the pollution and remediation of contaminated arid soils. A comprehensive and focused case study on transfer fluxes of trace elements in Israeli arid and semi-arid soils is presented. The book concludes with a discussion of a quantitative global perspective on anthropogenic interferences in the natural trace elements distributions. The elements discussed in this book include Cd, Cu, Cr, Ni, Pb, Zn, Hg, As, Se, Co, B, Mo and others. This book is an excellent reference for students and professionals in the environmental, ecological, agricultural and geological sciences. 

Soil solution is the aqueous phase of soil. It is in the pore space of soils and includes soil water and soluble constituents, such as dissolved inorganic ions and dissolved organic solutes. Soil solution accommodates and nourishes many surface and solution reactions and soil processes, such as soil formation and decomposition of organic matter. Soil solution provides the source and a channel for movement and transport of nutrients and trace elements and regulates their bioavailability in soils to plants. Trace element uptake by organisms and transport in natural systems typically occurs through the solution phase (Traina and Laperche, 1999). 

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