Opioid Analgesics (Narcotic Anlagesics)
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Opioid Analgesics (Narcotic Anlagesics ) Analgsia : Unpleasant sensation evoked by external or internal noxious stimuli. Analgesics : The drugs that relives the pain by acting in the CNS or on Pheripheral pain mechanism. It divide into Opioid analgesics and non-opioid (NSAIDs) analgesics. Classification of Opiod analgesics present in papaver somniferum 1. Phenanthrene derivatives: Morphine (10%), Codeine (0.5%), Thebaine (0.2%) 2. Benzisoquinoline derivatives: Papavarine (1%), Noscapine (6%) (Narcotine)
Demethylation of Codeine to Morphine Classification: 1. Narcotics agonist analgesics A. Phenanthrenes: Natural opium alkaloids: Morphine, Codeine Semi-synthetic derivatives: Morphine derivative: Hydroxymorphone, Oxymorphone Codeine Derivatives: Hydrocodone, Oxycodone B. Methadones: Methadone, Propoxyphene C. Morphinan : Levorphenol D. Phenyl piperidens (Meperidine): Pethidine, Fentanyl, Alfentanil, Sufentanil 2. Narcotic agonist-antagonist analgesics A. Phenanthrenes – Buprenorphine, Nalbuphine B. Morphinan – Butorphanol C. Benzomorphan – Phenazocine, Pentazocine 3. Narcotic antagonist: Naloxone, Naltraxone
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Opioid Receptors: Endogenous Partial Receptors Agonist Antagonist Substance agonist Buprenorphine Nalorphine µ receptors Endorphin Morphine Butorphanol Naltrexone Butorphanol Naltrexone κ receptors Dynorphin A, B Nalorphine --- Naloxone Pentazocine Morphine Naltrexone (Weak) δ receptors Enkephalins ---- (Weak) Naloxone (Weak)
MOA of Opioids:
Fig. 1 Action of Opioid Explanation of Fig. 1: Opioids excites neurons from periaqueductal gray matter (PAG) and nucleus reticularis paragigantocellularis (NRPG). which project to retroventral medulla which contain nucleus raphe magnus (NRM) 5-HT and enkephalin containing neurons
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runs towards dorsal horn and exert an inhibitory influence on transmission (Substance P). Opioids directly act on dorsal horn as well as peripheral terminal of nociceptive afferent neurons. Upon activations of opioids receptors (µ and δ) decreases c-AMP formation Opens K + channels hyperpolarisation Decrease in release of Substance P (neurotransmitter) On activation of κ receptors Suppresses N type Ca ++ ion channels inhibition of Ca ++ influx Hyperpolarisation. SAR of Morphine:
Modification Interpretation rd Replacement of OCH 3 at 3 Position Codeine – decreased analgesic activity rd Replacement of OC 2H5 at 3 Position Ethylmorphine – little analgesic activty rd Replacement of -O-CO -CH 3 at 3 Position Heroin – increases analgesic but also and 6 th postion increases addiction effect. Dihydromorphinone or dihydrocodeine – 14-Hydroxy dihydromorphine – more potent Oxidation at C-6 analgesics.
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addition of OH at C-14 Bridging at C-6 and C-14 through ethylene Etorphine – 200 times more potent than linkage morphine Modification at tertiary nitrogen (N-R) Potent Antagonistic activity Replacement of R with methyl, n-phenyl, n- hexyl, allyl etcs. Thenaine, Oripavine Increased activity, selectivity, and decreased toxicity. Ethorphine 10000 time more potent than morphine but no. of side effects, use as a sedative in veterinary medicines.
SAR of Meperidine Class:
Removal of B,C, D rings
Morphine Pethidine
Modification Interpretation 4-phenyl group with hydrogen, alkyl, Reduces Activity aroalkyl or hetrocyclic group
Replacement of –COOC 2H5 by –COC 2H5 Prodine - increases analgesic activity
Replacement of N-CH 3 by various aralkyl e.g. Piminodine – Increased analgesic group activity Piperidine ring enlarge to 7 - membered e.g. Proheptazine – Active and potent azepine ring analgesic
Methadone Class:
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Methadone series • Unlike mepridine series insertion of m -hydroxyl group in one of the phenyl ring of methadone series causes a marked decreased in activity. • Methadone is more potent than isomethadone
Methadone Isomethadone
• Replacement of propionyl group (COC 2H5) with hydrogen, hydroxyl, acetoloxy decreases in activity.
• Replacement of propionyl group (COC 2H5) with amide group Racemoramide (More active than methadone)
Racemoramide • Removal of any phenyl ring lead to decreases in activity. Morphinanes
• Does not process C4 –C5 ether linkage. • N-Methyl Morphinan (20% of activity) • Levo form possesses greater activity , Levorphenol shows 5% more potent than morphine
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• dextro form of recemorphan is used as cough suppressant (Dextromorphan)
Benzomorphan series:
Compound R1 R2 R3 R4
Benzomorphan OH CH 3 H H
Pentazocine OH CH 2CH=C(CH 3)2 CH 3 CH 3
Phenazocine OH CH 2-CH 2-C6H5 CH 3 CH 3
Metazocine OH CH 3 CH 3 CH 3
Cyclazocine OH CH 3 CH 3
• Trimethyl compounds is 3 time more potent than dimethyl amalogous • N-phenethyl derivatives possess 20 times greater potency than N -methyl analogous. • Pentazocine and cyclazocine are classic antagonist. (Pentazocine - High analgesia less addiction) Narcotic antagonist: Replacement of N-methyl group in morphine by larger alkyl group lowers activity and act as antagonist. antagonist activity increases in following order:
C2H5 < C3H7 < CH 2CH=CH 2 < e.g. Nalorphine, Naloxone, Levallorphan
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