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Chemistry Essay

2437 words - 10 pages

2. Disposition of BZD:
The disposition of benzodiazepines in humans has received attention due to frequent use of BZD compounds. Generally, BZD are well absorbed from the GIT. The onset of drug action after oral intake depends on the rate of drug absorption from the GIT and then the rate of drug passage from blood to the brain. IM injection of BZD may have unpredicted absorption. On the other hand, intravenous administration of BZD showed rapid drug absorption. Additionally, other routes of drug administration showed variable rates of absorption {S.37}. BZD are metabolized by hepatic biotransformation through phases of drug metabolism either oxidation or conjugation. However, some BZD are metabolized by both phases. Oxidation usually produces active metabolites that prolong the action of the parent compound. Oxidation also is more susceptible to impairment by different factors including clinical status, age, and concomitant administration of drugs that affect the oxidizing ability e.g. cimetidine, isoniazid, ethanol, and phenytoin. Oxidative pathway occurs via microsomal oxidation {8} of the α-carbon to the carbonyl group of 1,4-diazepine ring e.g. temazepam as well as by oxidative N-dealkylation of alkyl group of the ring as in diazepam. Other metabolic pathway is the reductive reaction of the nitro group of benzodiazepine ring as in clonazepam. Generally, the oxidized metabolites underwent phase II conjugation with glucuronic acid to more polar compounds that are excreted in the urine as β-glucuronide metabolites. The triazolo- and imidazo- benzodiazepines such as triazolam showed different metabolic pathway in which a hydroxylation reaction occurs at CH3-group of the imidazole ring (figure 6). BZD with long half-lives as diazepam, chlordiazepoxide, and flurazepam are metabolized to long acting active metabolites e.g. nordiazepam in case of diazepam and chlordiazepoxide, and desalkylflurazepam in case of flurazepam. Due to long half- life of the BZD compound and its metabolite, they may accumulate in body as in elderly patients and in patients with severe liver function impairment {S. 29}. Other BZD are metabolized mainly by glucuronide formation e.g. oxazepam, lorazepam, and temazepam. Most of BZD are metabolized in presence of cytochrome P450 (CYP) 3A4 and CYP2C19 enzymes. Inhibitors of CYP 3A4 enzymes can affect the metabolism of benzodiazepines.
Urinary excretion of β-glucuronides of BZD, is the final step of benzodiazepine’s disposition. Only trace amounts of BZD compounds may appear in the urine unchanged. The duration of drug elimination depends on the compound’s half -life and the formation of the active metabolites during phase Ι.
Fig. 6. Metabolism of triazolam Adapted from (6)

Fig.7 Metabolism of temazepam (Phase I and Phase II) Adapted from (6)
1. Abuse of BZD:
Abuse of BZD either alone or in combination with other substances such as alcohol or opiates is documented. Generally, most of abusers obtained BZD substances...

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