Drug simple diffusion

As in gastrointestinal absorption, the lipophillic nonionic form of a drug is more susceptible to reabsorption from the renal tubules by simple diffusion. Therefore,... 

Alternate ways to interfere with the orexin system may be via inhibition of dipeptidyl peptidases or proteolysis-resistant peptide analogs as shown for other peptides. This could prolong and boost orexinergic signaling. OX-A but not OX-B can enters the brain by simple diffusion via the blood-brain barrier. Abundance of orexins and their receptors in the olfactory bulb and throughout all parts of the central olfactory system may offer transnasal routes for drug application. 

LBPs are likely to have conventional roles in the energy metabolism and transport of lipids in nematodes for membrane construction, etc. Many parasitic helminths have deficiencies in the synthesis of some lipids and so their lipid acquisition, transport and storage mechanisms clearly need to be specialized and therefore pertinent to the host-parasite relationship (Barrett, 1981). From a practical point of view, lipid transporter proteins may also be important in the delivery of anthelmintic drugs to their target most anthelmintics are hydrophobic and if they do not distribute to their site of action within the parasites by simple diffusion across and along membranes, then the parasite s own carrier proteins may be involved. 

Simple diffusion is another mechanism by which substances cross membranes without the active participation of components in the membranes. Generally, lipid-soluble substances employ this method to enter cells. Both simple diffusion and filtration are dominant factors in most drug absorption, distribution, and elimination. 

This refers to the transport across the epithelial cells, which can occur by passive diffusion, carrier-mediated transport, and/or endocytic processes (e.g., transcytosis). Traditionally, the transcellular route of nasal mucosa has been simply viewed as primarily crossing the lipoidal barrier, in which the absorption of a drug is determined by the magnitude of its partition coefficient and molecular size. However, several investigators have reported the lack of linear correlation between penetrant lipophilicity and permeability [9], which implies that cell membranes of nasal epithelium cannot be regarded as a simple lipoidal barrier. Recently, compounds whose transport could not be fully explained by passive simple diffusion have been investigated to test if they could be utilized as specific substrates for various transporters which have been identified in the... 

Amylose, another natural polysaccharide, prepared under appropriate conditions, is not only able to produce films, but is also found to be resistant to the action of pancreatic a-amylase while remaining vulnerable to the colonic flora [82]. However, incorporation of ethylcellulose was necessary to prevent premature drug release through simple diffusion [83], In vitro release of 5-aminosalicylic acid from pellets coated with a mixture of amylose-ethylcellulose in a ratio of 1 4 was complete after 4 hr in a colonic fermenter. By contrast, it took more than 24 hr to release only 20% of the drug under conditions that mimic that of the stomach and of the small intestine. 

The fate of a drag in vivo is dictated by a variety of physiochemical properties, including size, lipophilicity, and charge. These properties determine how a drag is absorbed into the blood, distributed throughout the body, metabolized, and eventually eliminated. While movement of a drug molecule can occur through simple diffusion, there are many transporter proteins expressed on cell membranes to assist... 

The placental membranes are also lipoidal in nature and lipophilic drugs (also nonlipid soluble drugs to some extent) can easily cross the placental barrier. It is a contact between the foetal blood and the maternal blood. Drugs are transferred through this barrier by simple diffusion method, once across this, drug molecules circulate in the foetal blood before diffusing back. 

Drug transfer by simple diffusion represents the free passage of a drug down its concentration gradient, without the expenditure of... 

The microcap systems showed slower release than the matrices in all three pH values, because release of the drug from the matrix tablets occurs by simple diffusion through the pores existing between poly (DL-lactic acid) plates. 

Oral metronidazole and tinidazole are readily absorbed and permeate all tissues by simple diffusion. Intracellular concentrations rapidly approach extracellular levels. Peak plasma concentrations are reached in 1-3 hours. Protein binding of both drugs is low (10-20%) the half-life of unchanged drug is 7.5 hours for metronidazole and 12-14 hours for tinidazole. Metronidazole and its metabolites are excreted mainly in the urine. Plasma clearance of metronidazole is decreased in patients with impaired liver function. 

Almost all of the ophthalmic drugs that have been studied so far appear to cross the cornea by simple diffusion involving the paracellular and transcellular pathways (Figure 25.3). The paracellular pathway anatomically involves the intercellular space and is the primary route of... 

Once a drug is conjugated to a macromolecule, it cannot enter the cell by simple diffusion but rather requires endocytosis, which could be enhanced in cancerous cells by overexpression of receptors. Therefore, to achieve cellular targeting, macromolecules require access to the interior of a cell through endocytosis, a process in which a portion of the plasma membrane invaginates to create a new intracellular vesicle (Fig. 12.6). Internalization via endocytosis can occur in one of two ways ... 

Most drugs and toxicant pass through membranes by simple diffusion down a concentration gradient. The driving force being the concentration gradient across the membrane. 

Most drugs enter cells by simple diffusion through the hydrophobic region of the cell membrane and consequently uncharged drugs permeate more rapidly... 

The mechanism of the drug release appeared to be simple diffusion from an EC matrix tablet and the data could be adequately described by the Higuchi square root of time relationship for water-soluble pseudophedrine hydrochloride at 12.5-25 % drug loading [309]. Release of the slightly soluble theophylline or the practically insoluble indomethacin from such tablets at 50 % or 25 % drug loading, respectively, was described by diffusion with polymer relaxation and erosion contributions to the release mechanism [313]. 

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