Bioavailability local

Most trace metals may be precipitated with phosphate into insoluble metal phosphates (Table 7.5). Most metal phosphates have low solubility. High localization of phosphates reduces the bioavailability of Zn in arid soils. The banded application of P near the seeds depresses Zn uptake by com (Adriano and Murphy, 1970 Grant and Bailey, 1993), causing Zn deficiency. However, both N and P fertilizers increase Cd concentration in plants. Cadmium and Zn are antagonistic in root uptake and distribution within plants. 

Chemical properties of carotenoids play an important role in carotenoid micellarization and, therefore, bioavailability. Apolar carotenoids (carotenes) are generally incorporated in the central region, which is highly hydrophobic, of the oil droplets, whereas polar carotenoids (xanthophylls) are localized on the surface, and therefore xanthophylls are more easily micellarized and absorbed than carotenes (Borel and others 1996). van het Hof and others (2000) found in humans that lutein is five times more bioavailable than (3-carotene. 

After oral administration, drug and metabolite concentrations in blood, urine, and feces can easily be monitored. In contrast, topical application to the skin usually aims at a local treatment. Therefore, the main interest lies in determining the drug level within the skin, in order to evaluate the dermal bioavailability of compounds or assess the bioequivalence between different formulations. In the following sections, appropriate analytical techniques will be presented. 

In contrast to P-gp and the MRP proteins, the breast cancer resistance protein (BCRP) contains six transmembrane domains and only one ATP-binding domain. It was first cloned from the breast cancer cell line MCF-7 selected in doxombicin, in the presence of the P-gp inhibitor verapamil. It is found in many human tissues, such as the placenta, small intestine, colon, and liver [133], It is localized to the apical membrane of epithelial cells of the small intestine and colon and to the bile canalicular membrane in the liver and is involved in reducing intestinal uptake, increasing hepatobiliary excretion, etc., leading to diminished oral bioavailability. cDNA sequences identical to BCRP and named MXR and ABCP, respectively, were independently isolated from human colon carcinoma cells and human placenta [134], BCRP requires... 

The ocular route is used mainly for the local treatment of eye pathologies. Absorption of drugs administered by conventional eyedrops can result in poor ocular bioavailabilities (2-10%). This is due to the limited area of absorption, the lipophilic character of the corneal epithelium, and a series of elimination factors that reduce the contact time of the medication with the comeal surface, such as drainage of instilled solutions, lacrimation, and tear turnover and tear evaporation [56]. 

Adhesive gels may provide greater bioavailability by allowing a longer residence time than solutions. Adhesive gels may be preferred for local therapy [54,55]. 

Buccal dosage forms can be of the tablet, patch, gel, or ointment type and can be employed for local or systemic delivery. For local deliveiy, conventional dosage forms such as solutions and various types of tablets (immediate release, effervescent, etc.) are more suitable. These forms generally have uncontrolled drug release with subsequent variable absorption and short residence times, and may not provide sufficient bioavailability. Novel dosage forms such as adhesive tablets, patches, gels, and... 

Compound Selectivity ISA Local anaesthetic Half-life Bioavailability (in %) Remarks... 

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