Lecture 72 - Pharmacology of Opiate Drugs Flashcards
OPIUM CONTAINS TWO TYPES OF ALKALOIDS
Phenanthrenes: Morphine (10%), Codeine (0.5%), Thebaine (0.2%)
Benzylisoquinolines: Noscapine (6%), Papaverine (1%)
Opiates are only those opioids that are naturally occurring
STRUCTURE ACTIVITY RELATIONSHIPS (SAR) OF PHENANTHRENES
3 position substitutions ether or ester produces decreased potency – codeine
6 position increases activity – hydromorphone or hydrocodone (from codeine)
14 position OH has increased potency – oxycodone
N-allyl give antagonist (or mixed antagonists) – naloxone or naltrexone
THE HUMAN GENOME CONTAINS SEVERAL GENES ENCODING ENDOGENOUS OPIOIDS
Peptides active endogenously
Large precursor proteins are cleaved into more opioid subtype selective peptides
Degree of redundancy
1. Pro-opiomelanocortin: beta-endorphin –> Mu opioid
2. Preproenkephalin: Leu-Enkephalin –> delta opioid, Met-Enkephalin = mu and delta
3. Preprodynorphin: dynorphin –> kappa opioid
4. Nociceptin/Orphanin FQ
TYPES OF OPIOID RECEPTORS
G protein-coupled receptor: Family A – peptide receptors; Gi/o-coupled (inhibition of cAMP production); Open GIRK potassium channels; Close calcium channels
Mu (M – morphine): Endogenous opioid = endorphin
Kappa (K – ketocyclazocine): Endogenous opioid = dynorphin
Delta (D – deferens -> where identified): Endogenous opioid = enkephalin
Nociceptin, orphanin FQ receptor: Endogenous opioid = nociceptin
Sigma receptors – not an opioid receptor
OPIOID RECEPTOR SIGNAL TRANSDUCTION
Presynaptic = Inhibit calcium channel (Gi) decrease in neurotransmitter release
Postsynaptic = activate GIRK channel (Gβγ) Efflux of K+ => hyperpolarization
THE MU OPIOID RECEPTOR
Beta-endorphins (endogenous morphine): Pro-opiomelanocortin (POMC); Component of runners high
Therapeutic use: Analgesia - Not as effective for chronic pain; Cancer pain, palliative, PCA
(patient-controlled analgesia)
Sedation
Antitussive: Suppression of cough center in the medulla oblongata - Codeine (mechanism unclear though)
OPIOID INDUCED SIDE EFFECT – ARE MOSTLY ON-TARGET EFFECTS!!
Respiratory depression: Brain stem; pre-Bötzinger complex in the ventrolateral medulla
Constipation: GI tract
Pruritus (itch): Side effect, not allergic response
Addiction
Urinary retention: opioid-induced ADH release
Nausea/vomiting: Chemoreceptor trigger zone - medulla
Miosis: Oculomotor nerve (PAG); Not mepiridine (has anti-cholinergic effects)
KAPPA OPIOID RECEPTOR
Dynorphins natural ligand: Preprodynorphin
Activation is dysphoric, aversive
Potential use for treatment of addiction –> Reduce dopamine release
Counterbalance mu opioid receptor effects
DELTA OPIOID RECEPTOR
Enkephalins are natural ligand: Preproenkephalin
More dynamic expression: Intracellular, “externalized” upon chronic stimuli
Role in hypoxia/ischemia/stroke: Hibernation release of enkephalin like opioid
Reduce anxiety
Reduce depression
Treat alcoholism
Relief hyperalgesia, chronic pain
Side effect: seizures!
No FDA-approved delta opioids
Opioid site of action
ventral tegmental area –> nucleus accumbens
DEPRESSANTS CAN CAUSE DOPAMINE RELEASE JUST LIKE STIMULANTS
- Opioid binds mu receptor
- Gi signaling inhibits neurotransmitter release
- Less GABA to activate GABAA
- Less inhibition of dopamine neuron activity
- Increase dopamine release
- Increased activation of dopamine receptors
ADMINISTRATION ROUTES AND PHARMACOKINETICS OF OPIOIDS
Intravenous
Intra-axial: intrathecal, epidural
Intra muscular
Oral
Topical/transdermal
PHARMACOKINETICS OF MORPHINE/PHENANTHRENES
Metabolism: Readily absorbed; First pass metabolism - Morphine bioavailability 25% Hepatic: CYP2D6, CYP3A4; Genetic differences; Elimination T1/2 increased with liver disease
Glucuronidation at 3’ and 6’ position
Morphine-6-glucuronide (M6G) - Still potent Excretion: Glomerular filtration; 90% excreted in 24h
SOME OPIOID METABOLITES ARE STILL ACTIVE
Heroin, codeine, tramadol ≈ prodrugs
Heroin+ codeine form morphine; tramadol forms O-desmethyltramadol
Fentanyl and methadone do not produce active metabolites
Onset/duration influenced by lipophilicity: Morphine: low lipophilicity, slower passage across BBB, prolonged duration of action; Fentanyl: high lipophilicity, rapid onset, short duration
CYP3A4 (FOUR) -> MAKES OPIOIDS STARTING WITH
NOR