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Created February 9, 2021, last updated almost 4 years ago.
Collection: 141, Score: 1064, Trend score: 0, Read count: 1482, Articles count: 4, Created: 2021-02-09 21:06:48 UTC. Updated: 2021-02-09 21:31:31 UTC.Notes
Morphine is one of the most commonly used opioids worldwide. First isolated in 1803 and commercially marketed by Merck in 1827.
A. Physiochemistry
- Natural phenathrene opioid - plant, animal and even human synthesis identified.
- Synthesized by mammalian cells from dopamine, although exact role unclear.
- pKa - 7.9 (20% nonionised @ 7.4)
- Octanol water coefficient - 1.4 (relatively low lipid solubility compared with other opioids)
- 3 rings attached to piperidine ring with a tertiary amine.
B. Pharmacokinetics
- Dose - 50 mcg/kg IV
- analgesia @ [plasma] 0.05 mcg/mL
- epidural: 10-20 mcg/mL
- PCA adult: 50 mg in 50 mL; 1 mL (1 mg) bolus 5 min lockout, commonly used.
- PCA paeds: 1 mg/kg in 50 mL; 1 mL (20 mcg/kg) bolus; background 0.5-1 mL/h (10-20 mcg/kg/h).
- Absorption - IV, IM, s/c, po (3x dose as HER 0.69)
- Distribution - Vdcc 0.3, Vdss 3.5 L/kg
- Protein binding - 30% (albumin)
- Onset: peak onset at 20 min when given parenteral, 60 min orally; Offset 4 h
- Metabolism - t½α 10-20 min, t½ß 2-4 h
- 75% metabolised by conjugation → 90% morphine-3-glucuronide (no activity)
- 10% morphine-6-gluc (13x potency of morphine). MAOIs inhibit glucuronidation.
- Clearance - 15 mL/kg/min
C. Pharmacodynamics
- Mech - mu, kappa, delta agonist. (GI linked). Effective against visceral, skeletal & joint pain.
- CNS - little CNS penetration (cf. heroin, which readily crosses BBB), although alkalisation (⇣pCO2) ⇡ non-ionised fraction, and ⇡pCO2 ⇡CBF. Both ⇡ cerebral morphine concentration.
- 'Ceiling effect' on EEG reaching high voltage, slow frequency (delta 2-4 Hz) waves.
- ⇣ CMRO2 & ⇣ ICP.
- ⇡ cortical stimulation of Edinger-Westphal nucleus → miosis.
- CVS - ⇣ SNS & ⇡ PNS tone. Bradycardia, venodilation, histamine release (causes ⇣ MAP). Orthostatic hypotension due to depression of SNS responses. Direct depressant effect on SA node, slowing conduction (⇡ VF risk).
- Administration with N2O results in CVS depression.
- Resp - Respiratory depression & response to CO2 & hypoxia (shift pCO2/VA curve to right).
- Bronchoconstriction due to histamine release (similar with pethidine). Depresses airway reflexes & ciliary reflexes.
- Renal - diuresis (kappa receptors → ADH release)
- GIT - Nausea & vomiting due to stimulation of CTZ (30-40% of subjets); ileus; constipation; sphincter of Oddi spasm.
- Pruritus
Daniel Jolley
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Collected Articles
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Morphine is a potent opioid analgesic widely used for the treatment of acute pain and for long-term treatment of severe pain. Morphine is a member of the morphinan-framed alkaloids, which are present in the poppy plant. The drug is soluble in water, but its solubility in lipids is poor. ⋯ M3G exhibits no analgesic effect after ICV or IT administration. Some studies do, however, indicate that M3G may cause non-opioid mediated hyperalgesia/allodynia and convulsions after IT administration in rats. These observations led to the hypothesis that M3G might be responsible for side-effects, hyperalgesia/allodynia and myoclonus seen after high-dose morphine treatment.
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Review Historical Article
Endogenous morphine and its metabolites in mammals: history, synthesis, localization and perspectives.
Morphine derived from Papaver somniferum is commonly used as an analgesic compound for pain relief. It is now accepted that endogenous morphine, structurally identical to vegetal morphine-alkaloid, is synthesized by mammalian cells from dopamine. ⋯ However, the exact role of these compounds is a matter of debate although different links with infection, sepsis, inflammation, as well as major neurological pathologies (Parkinson's disease, schizophrenia) have been proposed. The present review describes endogenous morphine and morphine derivative discovery, synthesis, localization and potential implications in physiological and pathological processes.
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The well established use of oral morphine in the treatment of chronic cancer pain has developed empirically and a knowledge of its pharmacokinetics is not necessary in order to use the drug effectively. However recent information about the pharmacokinetics of morphine may help resolve the controversy about oral to parenteral relative potency ratios, and may also in the future shed some light on the problem of patients whose pain does not respond to morphine.
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The metabolism of morphine was studied in seven fullterm neonates and five infants receiving a continuous infusion of morphine. All the patients had detectable plasma concentrations of morphine 3-glucuronide (M3G) and 10 had detectable concentrations of morphine 6-glucuronide (M6G). The mean plasma clearance of morphine was 20.1 ml min-1 kg-1 in neonates and 23.4 ml min-1 kg-1 in the group as a whole. The M3G/morphine ratio (7.3) was higher than that previously reported for preterm neonates (5.0) but lower than that reported for children (23.9).
- Natural phenathrene opioid - plant, animal and even human synthesis identified.