Metabolic transformations half-life

After oral administration, BPA is rapidly and efficiently absorbed from the gastrointestinal tract and undergoes extensive first-pass metabolism in the gut wall and hver. BPA is mainly transformed into BPA-glucuronide and to a lesser extent into BPA-sulfate. These highly water-soluble metabolites are rapidly cleared from blood by the kidneys and excreted with urine [205]. Volkel et al. determined that the terminal half-life of BPA is less than 6 h in humans [206]. 

The introduction of fluorine atoms in a molecule can be used to modify the processes and the rates of metabolism of the drug, in order to extend the plasma half-life or to avoid the formation of toxic metabolites. Because of the properties of fluorine atom, in particular its electronic effects, it may interact differently on the bio-transformation steps, according to the type of processes involved... 

Metabolic transformation to more water-soluble metabolites is necessary for clearance of sedative-hypnotics from the body. The microsomal drug-metabolizing enzyme systems of the liver are most important in this regard, so elimination half-life of these drugs depends mainly on the rate of their metabolic transformation. 

After intravenous administration heroin is very quickly metabolized (half-life is about 3 min) into 6-MAM and morphine. The 6-MAM reaches its maximum concentration in 6 min, then transforms itself into morphine, which reaches its peak in 20 min. In the liver and kidneys, deacetylation occurs, but in the liver other reactions also occur N-demethylahon that leads to normorphine, methylation that leads to codeine and conjugation with glucuronic acid. 

The rate and extent of pesticide metabolism can vary dramatically, depending on chemical stmcture, the number of specific pesticide-degrading microoiganisms present and their affinity for the pesticide, and environmental parameters. The extent of metabolism can vary from relatively minor transformations which do not significandy alter the chemical or toxicological properties of the pesticide, to mineralization, ie, degradation to CO2, H2O, NH, Q, etc. The rate of metabolism can vary from extremely slow (half-life of years) to rapid (half-life of days). 

Uptake of TNT in fish and invertebrates resulted in substantial bioaccumulation of nonidentified extractable and nonextractable compounds. Contrasting to the parent compound, metabolically formed transformation products of TNT appear to be eliminated at much slower rates. Neither the biological half-life nor the chemical nature of nonextractable transformation products of TNT in organisms has been investigated to date. Investigations on the biotransformation of explosives other than TNT in aquatic animals were not found in the available literature and are therefore warranted. Further studies of the fate of explosives in aquatic animals are necessary to reveal the identity of their transformation products present in the tissues of exposed organisms, to further characterize species-specific differences in the bioconcentration of transformation products, and to elucidate the mechanism of toxic action. 

PROBABLE FATE photolysis, could be important, direct photolysis does not occui below the ozone layer, if released to air, the transformation process in the troposphere is reactior with hydroxyl radicals, with an estimated half-life of 6.65 months oxidation, could occur hydrolysis too slow to be important volatilization volatilization has been demonstrated, probabl> an important transport mechanism, if released to water or soil, volatilization will be the dominani fate process, volatilization half-life from rivers and streams 33 min to 12 days with a typical half-life of 35 hrs sorption no information, but adsorption onto activated carbon has been demonstrated biological processes moderate potential for bioaccumulation, metabolization by some aquatic life is known to occur, anaerobic biodegradation may be the major removal process in aquatic regions where volatilization is not possible other reactions/interactions may be formed by a haloform reaction following chlorination of drinking water if sufficient bromide is present... 

The kinetics of excretion are a direct consequence of the kinetics of metabolic transformations. The faster a drug is metabolized, the faster its elimination can be expected. In accordance with this assertion, rats given R,S( ), S(-t-), and R(-)-amphetamine, were found to excrete less (-t-)-p-hydroxy-amphetamine than its (- )-isomer this may be the basic explanation of the more pronounced pharmacological properties of the dextro-, compared with the levo-amphe-tamine. For the hypnotic agent hexobarbital, the elimination half-life in man is about three times longer for the... 

Administered as a once-daily, 20-pg SC injection in the thigh or abdominal wall, teriparatide is a clear, colorless liquid that is available as a 750 pg/3 ml, prefilled, disposable pen that requires refrigeration. Concurrent calcium (1,000 mg) and vitamin D (400 lU) supplementation is recommended. Treatment for longer than 2 years is not recommended. Teriparatide is rapidly absorbed, demonstrates 95% bioavailability, and is quickly eliminated via both hepatic and extrahepatic routes. The half life is 1 hour when administered SC. Metabolic studies have not been performed on teriparatide however, the entire PTH preprohormone has been shown to undergo enzyme-mediated transformations in the liver. Dizziness and leg cramps are the most commonly reported adverse side effects. 

The elimination of volatile substances by the lungs can also be a slow process. By this route of elimination ethanol would have a half-life of 7.4 days [7]. The point in dwelling on these aspects is to emphasize the importance of metabolic transformation of drugs in the termination of the action of most drugs. 

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