Pharmacology Flashcards
Levidopa/Carbidopa
Used in Parkison’s.
Levodopa = metabolic precursor of dopamine.
Carbidopa = peripheral decarboxylase inhibitor, which prevents peripheral breakdown to increase CNS levels.
Selegline
MAO-B inhibitor
Used in early Parkinson’s as breaks down dopamine selectively.
Caution with SSRIs.
Entacapone
Used in Parkinson’s to increase the half-life of L-dopa so there are less motor fluctuations and dyskinesia.
Donepezil
Selective reversible AChE inhibitor.
Binds at peripheral site.
Given once daily.
Dose 5-10mg.
ADRs: N+V, insomnia, diarrhoea, bradycardia.
Rivastigmine
Reversible AChE inhibitor. Also acts on butylcholinesterase.
Given BD.
Dose 1.5-6mg BD.
Also available as transdermal patch.
Galantamine
Selective competitive AChE inhibitor. Also acts as antagonist at nicotinic receptors.
Given BD.
Dose 12mg BD.
ADRs: N+V.
Not subsidised in NZ.
Memantine
NMDA-receptor antagonist.
Blocks excitotoxic glutamate activity.
Indicated in moderate/severe dementia.
Acetylcholinesterase inhibitors
Indications: Alzheimer’s, Lewy body, Parkinson’s, vascular dementia.
Better evidence in mild/mod than severe.
Contraindications: FTD, pure alcoholic dementia, large strokes, MCI.
Stabilise symptoms for 6-12 months, with subtle improvement in 50% (sx improved include apathy, spontaneity, delusions/hallucinations).
Bupropion
Dopamine + NA reuptake inhibitor.
Indicated in smoking cessation (reduces withdrawals, post-cessation weight gain) and depression.
ADRs: psychiatric sx, seizures
Varenicline
Nicotine α4β2 partial receptor agonist
Indicated in smoking cessation
ADRs: nausea, dry mouth, depression/SI
Disulfiram
Prevents ETOH breakdown by aldehyde dehydrogenase.
Indicated in alcohol abuse as negative reinforcement.
ADRs: can affect liver and thyroid
Naltrexone
Opiate receptor antagonist
Used in alcohol dependence as reduces pleasure, no withdrawal, no negative effects, minimal ADRs/interactions
But high relapse rate, withdrawal sx if using opiates
Acamprosate
GABA analogue, acts on GABA and glutamate systems, allosteric modulator of GABA-A
Used in alcohol dependence
Dose: <60kg = 333mg TDS, >60kg = 666mg TDS
Lofexidine
α2A adrenergic receptor agonist
Can be used in opiate withdrawal to reduce sx
Methadone
Full opioid agonist
Start low and give incremental doses as per withdrawal scale
Usual dose 60-120mg, can take 6 weeks to stabilise
Need to monitor LFTs and QTc
Advantages: well absorbed, long half-life (OD dosing, in urine for 3 days), used in pregnancy
Suboxone
Buprenorphine (long acting partial mu agonist, weak kappa agonist, delta agonist) + naloxone (opioid antagonist).
Diminishes cravings and helps with withdrawal.
Safe in overdose due to ceiling effect. Blocks other opioid agonists in dose-dependent fashion. Can precipitate withdrawal so first dose given when already in withdrawal.
Usual dose 2-8mg. Half-life 24-37 hours.
Advantages: milder withdrawal, alleviates cravings, long-acting, greater blockade effect
Caution with CYP3A4 inhibitors (inducer).
First generation antipsychotics
Predominantly dopamine blockers in mesolimbic pathway (to reduce psychotic sx), but not selective, so also act on mesocortical (secondary negative sx, depression, cognitive dysfunction), nigrostriatal (EPSE) and tuberoinfundibular (raised PL) pathways.
Second generation antipsychotics
Combination of D2 blockade plus 5HT-2 blockade, which causes reciprocal increase in dopamine, so effects more specific to mesolimbic pathway, with reduced side effects.
Variable in how tightly they bind D2 and how quickly they dissociate.
If used in high doses, dopamine blockade exceeds 60-80%, and they start to act as typical antipsychotics.
Benztropine
Anticholinergic and antihistamine
Used in movement disorders, such as parkinsonism and dystonia, and EPSE, such as akathisia.
Not useful in tardive dyskinesia.
ADRs: dry mouth, blurry vision, nausea, constipation
Benztropine overdose presents with confusion, trouble swallowing, hot and dry skin, dilated pupils, tachycardia, urinary retention, irregular pulse, fainting or seizures.
Antidote = physostigmine
Tetrabenazine
Inhibits vesicular monoamine transporter and depletes presynaptic dopamine.
Used for treatment of hyperkinetic movement disorders, such as tardive dyskinesia, hemiballismus, Huntington’s chorea, tics.
Depletes dopamine so can lead to depression/SI.
Bromocriptine
Partial D2 agonist
Used in NMS, as well as hyperprolactinaemia and acromegaly
Aripiprazole
Partial D2 agonist, 5HT-2A antagonist, partial 5HT-1A agonist.
High affinity for receptor but low intrinsic activity (Goldilock’s effect).
Can be used as augmentation to reduce side effects.
Highly protein bound (99%).
Long half-life (94 hours).
CYP3A4 substrate.
Amisulpride
Selective D2/D3 antagonist.
D3 are presynaptic autoreceptors in frontal lobe so antagonism increases dopamine.
At <400mg, amisulpride enhances dopamine transmission.
At higher doses, it blocks D2 and can cause raised PL.
Purely excreted renally. Can be used in hepatic disease. Avoid in renal impairment.
Given BD at doses >300mg to increase therapeutic effect and reduce adverse effects.
Chlorpromazine
D1/D2/D3/D4 antagonist
Also acts as antagonist of 5-HT, H1, alpha1-2, M1-M2
Highest risk of sunburn due to increased skin photosensivity
Clozapine - Mechanism + Metabolism
D2 and 5HT-2 antagonist with hit-and-run mechanism (highest dissociation rate)
Highest affinity for D4
Potent anticholinergic
CYP1A2 - so caffeine increases levels and smoking reduces levels
CYP3A4 responsible for 35% of clozapine metabolism - so drug interactions
Clozapine - Side effects
- Most epileptogenic antipsychotic
- Agranulocytosis (usually first 18 weeks, contraindicated with carbamazepine)
- Myocarditis (usually first 8 weeks)
- Cardiomyopathy
- VTE and PE
- Constipation
- Tachycardia
Flupenthixol
D2 antagonist, also acts on 5HT
Typical antipsychotic
Antidepressant effect at lower doses
Available as depot
Haloperidol
D2 antagonist
Typical antipsychotic
Can be used in renal and hepatic impairment, and in pregnancy (1st line for mania)
Indicated for tics in Tourette;s
Available as depot
Olanzapine
D2 and 5HT-2 antagonist, H1 blockade (sedating)
Can be used in renal impairment.
CYP1A2 involved (nicotine, caffeine).
Half-life 30 hours.
Highest passage across placenta of SGAs, associated with high birth weight, macrosomia, increased ICU admissions.
Need to monitor for post-injection syndrome.
Paliperidone
D2 and 5HT-2 antagonist
Major active metabolite of risperidone
Low potential for drug interactions as limited hepatic metabolism
Half-life 24-28 hours
Quetiapine
D2 (higher dissociation so lower risk of EPSE) and 5HT-2 antagonist, H1 blockade (sedating)
CYP3A4 involved
Half-life 6-12 hours so multiple daily dosing
Risperidone
D2 and 5HT-2 antagonist
At higher doses, binds selectively to D2 so more risk of EPSE and high PL.
Potent alpha-1 blocker so first dose hypotensive effect.
Extensive first pass metabolism by CYP2D6 to 9-OH risperidone.
Half-life of metabolite = 24 hours.
Second highest passage across placenta.
Sertindole
Strong D2 and 5HT-2 antagonist
Potent alpha-1 blocker
Highest potential for QTc prolongation
Sulpride
Dose-related antagonist at presynaptic D3 (<800mg/day) and postsynaptic D2 (>800mg/day).
Purely renally excreted. Avoid in renal impairment. Recommended in hepatic disease.
Trifluoperazine
D1/D2 antagonist
Typical antipsychotic
Thioridazine
Not available due to highest risk of prolonged QTc
Ziprasidone
D2 and 5HT-2 antagonist, 5HT-1A agonist, monoamine reuptake inhibitor
Acts as antidepressant at low doses (due to MAOI effect).
Reduces weight through 5HT agonism.
High risk for prolonged QTc.
CYP3A4 involved.
Half-life = 7-12 hours.
Zuclopenthixol
D1/D2 antagonist, 5HT-2 antagonist, alpha-1 adrenergic
Typical antipsychotic
Available as decanoate (depot), acetate (accuphase, effect peaks at 48-72 hours) and oral.
Interacts with paroxetine due to shared hepatic metabolism.
Lurasidone
D2/D3 antagonist, 5HT-2A/5HT-7 antagonist, 5HT-1A partial agonist, alpha-2c antagonist
Atypical antipsychotic
Used in bipolar depression, alone or as an adjunct.
Low risk of weight gain.
SSRIs - Mechanism
Block SERT (serotonin reuptake transporter) so serotonin remains in synaptic cleft and can bind to various serotonin receptors.
Initially, there is an increase in signaling across synapses that use 5HT as main neurotransmitter.
With time, increased occupancy of post-synaptic 5HT receptor downregulates release of 5HT from presynaptic neurons, which ultimately leads to downregulation of postsynaptic 5HT receptors.
Serotonin receptors involved in SSRIs
5HT1A = presynaptic autoreceptor, acts as brake.
5HT-2A = postsynaptic receptor, effects include anxiety, akathisia, emotional blunting, insomnia.
5HT-2C = postsynaptic, can lead to sexual dysfunction and anorexia.
5HT-3 = postsynaptic, can lead to nausea and diarrhoea, ligand gated.
5HT-7 = circadian rhythms.
SSRIs - side effects
Initial anxiety
Sleep disturbance
GI upset
Headaches
Hyponatraemia
GI bleeding
Sexual dysfunction
Side effects dominate when SSRI first started, but they should reduce as receptors down regulate.
Paroxetine has most weight gain and sexual side effects.
SSRIs - Interactions
Enzyme inhibitors so can increase levels of clozapine, olanzapine, benzos, TCAs, methadone.
Do not prescribe with MAOIs due to risk of serotonin syndrome.
Mirtazapine - Mechanism
NASSA (NA serotonin specific antagonist)
At doses of 15-30mg, it is primarily a serotonin antagonist, so effects are:
- 5HT-2A = antianxiety, treats akathisia, improves slow wave sleep
- 5HT-2C = increased appetite, no sexual effects
- anti-H1 = increased appetite, sedation
At doses of 45-60mg, it primarily has a noradrenergic dopaminergic effect, by antagonising alpha-2 presynaptic receptor, increasing NA and dopamine => broad spectrum antidepressant.
Mirtazapine - Side effects
Sedation at low doses (H1).
Weight gain, increased appetite.
Agranulocytosis.
Caution in epilepsy.
No sexual dysfunction.
Minimal risk of serotonin syndrome.
SNRIs - Mechanism
SNRIs = venlafaxine, duloxetine.
Serotonin noradrenergic reuptake inhibitors.
- Block SERT to increase serotonin.
- Block NET to increase NA and dopamine.
Broad spectrum effect.
Half-life 4-5 hours.
Venlafaxine predominantly acts as SSRI up to 225mg, then has SNRI effects above that.
SNRIs - Side effects
Side effects of both SSRIs and dopamine-related effects (activation, agitation).
Sexual dysfunction.
Hypertension at doses >225mg.
Greater risk of discontinuation syndrome.
Caution in epilepsy (seizure risk).
Agomelatine - Mechanism
Melatonin agonist (M1/M2) = improves slow wave sleep.
5HT-2C antagonist
Also increases frontal lobe dopamine and NA.
Agomelatine - Side effects
LFT derangement
No risk of serotonin syndrome
No sexual dysfunction