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Pharmacology 5: psychoactive drugs, opioids R190,26
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Pharmacology 5: psychoactive drugs, opioids
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  • Course
  • Pharmacology
  • Institution
  • Imperial College London (ICL)

Lecture notes from Imperial College London, Medical Biosciences BSc, 2nd year, Pharmacology module PHAR 5, on opioids: - Structure activity relationships: Predict how the chemical structure of opioids can determine lipid solubility and affinity/efficacy of these drugs. - Absorption: Review...

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Document information

  • September 25, 2023
  • 5
  • 2022/2023
  • Class notes
  • Chris john
  • All classes

Subjects

  • psychoactive
  • opioid
  • solubility
  • affinity
  • efficacy
  • drugs
  • absorption
  • bioavailibility
  • administration
  • metabolism
  • response
  • pharmacodynamics
  • pharmacology
  • analgesia
  • pain
  • euphoria
  • depression
  • toxicity
  • addict
  • ad

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  • Imperial College London (ICL)
  • Unknown
  • Pharmacology
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PD/PK theory: opioids drugs
- opioid = depressant drugs with opiate-like activities
- opiate = naturally plant based alkaloids
=> derived from papaver somniferum plant (“opium poppy”)
- morphine: major alkaloid within opium poppy => opiate + opioid
- codeine: natural opioid + opiate from opium poppy
- heroin: synthetic derivative => opioid (was originally a non-addictive alternative)

Opioid structure
-




=> similarities (phenanthrene ring core + side chains to the left)
- morphine: the OH (alcohol) => 2 OH
- methyl morphine = codeine: the ether => 1 methyl substitution
- diacetyl morphine = heroin: the ester => 2 acetyl substitutions


- lipid solubility: heroin > codeine > morphine (polar)


- morphine-like opioids: tertiary Nitrogen + HO group at position 3 important for receptor affinity
only prerequisite far binding
=> efficacy determined by side chain extending from tertiary N
=> only side chains with < 3C activate receptor (efficacy)

ACTIVE INERT - antagonists have affinity but
no efficacy

blocks specific effects and side-effects
=> used to reverse effects + overdose

, - opioid receptor: important binding sites
=> tertiary N binging site (ionic bond)
=> position 3 HO binding site (H bond)




- codeine & heroin don’t have a position 3 HO group => low affinity: PRO-DRUGS
=> penetrate brain easily + reach receptor (lipid soluble) BUT can’t bind receptor effectively

Pharmacokinetics maximise drug availability at target
- most common administration: intra-venous + oral (also intra-muscular, transdermal, sublingual...)
=> heroin: intravenous / morphine: intravenous or oral / codeine: oral
- weak bases => ionisation of morphine in stomach = 100,000:1 ; blood + ileum = 4:1 ; urine = 1
=> via intravenous route: 20% of 100% injected is unionised (4:1) end small intestine


=> via oral route: small proportion absorbed across small intestine + hepatic first metabolism
=> metabolites can be active/ inert:
- heroin: major metabolite = 6-acetyl-morphine => less lipid soluble BUT affinity for receptor
(still more lipid soluble than morphine)
N
-
~




ACTIVL

=> intravenous injection
=> accumulate in brain + some
metabolised by esterases (in brain)
=> also larger amount metabolised
in liver => to brain + bind receptor
high regional blood
glow
-> metabolism here
alway some
-




W




- codeine: metabolites = norcodeine (inert: no affinity) AND 10% morphine (active: affinity)

=> metabolism in liver:
- CYP3A4: “fast” (norcodeine)
- CYP2A6: “slow” (morphine)
ACTIVE
-




INERT
10 % 90 %

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