Tuesday, April 19, 2011

Opioid analgesics : Pharmacynetic

Most narcotic/opioid analgesics are well and rapidly absorbed from the gastrointestinal tract. However, the pronounced effect of the first passage through the liver, where much of the drug is inactivated (derived mainly the formation of glucuronides), which greatly reduces its bioavailability. For example, morphine oral bioavailability is about 30%. In this regard, morphine and other opiates are introduced mainly parenterally, usually by intramuscular injection, although it may be subcutaneous and intravenous injections.

Oral opioid analgesics prescribed to patients with chronic pain syndrome, in this case it is expedient to use long-acting drugs (eg, morphine sulfate drug MST-kontinus, or tramadol). Opioids are well absorbed as from the nasal mucosa and oral cavity, fentanyl is able to soak through the skin, making it possible to use data and routes of administration. For example, nasal spray used butorphanol, developed sublingual form of buprenorphine and transdermal forms of buprenorphine and fentanyl (Dyurogezik) give the most prolonged effect (up to 72 hours). Opiates quickly leave the blood and distributed in a well supplying the organs and tissues, the most lipophilic compounds such as fentanyl, are rapidly moving from the plasma in the fatty tissue, resulting in short duration of their action. However, opioids poorly penetrate the blood-brain barrier and produce low concentrations in the CNS (in the brain tissue gets about 1% of the administered dose of morphine). Therefore, in the case of a subdural injection is used very low doses of morphine or fentanyl. Infusion largely metabolized in the liver (eg, morphine is metabolized to 90%). Metabolites and the remaining intact portion of the drug is primarily excreted by the kidneys [2].

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