Psychiatry: Pharmacology - Antidepressants Flashcards
Describe the monoamine hypothesis of depression. What is this model based on?
Decreased functional amine-dependent synaptic transmission in depression
Based on the observation that all currently available antidepressants enhance synaptic availability of 5HT, NA, DA, or a combination
Describe the neurotrophic hypothesis of depression. What is this model based on?
Nerve growth factors are critical in regulation of neuroplasticity, resilience and neurogenesis: depression caused by loss of neurotrophic support
Based on observation that brain-derived neurotrophic factor (BDNF) is upregulated by effective antidepressants
Describe the neuroendocrine hypothesis of depression
HPA axis dysregulation occurs in depression (e.g. upregulation of stress response: increased cortisol, ACTH)
Describe the integrated model of depression (monoamine, neuroendocrine and neurotrophic hypotheses)
Monoamines and hormones probably upregulate BDNF and other neurotrophic factors, explaining the delay in response to treatment
Six classes of antidepressants with examples
- SSRIs (e.g. fluoxetine, sertraline, citalopram)
- SNRIs (e.g. venlafaxine, duloxetine)
- TCAs (e.g. imipramine, amitriptyline, nortriptyline)
- MAOIs (e.g. moclobemide, seleginine)
- 5HT2 antagonists (e.g. trazodone, nefazodone)
- Tetracyclics and unicyclics (e.g. bupropion, mirtazapine)
Five shared pharmacokinetic characteristics of antidepressants
- Relatively rapid oral absorption
- Achieve peak plasma levels within 2-3hrs
- Highly protein-bound
- Hepatically metabolised
- Renally cleared
Five indications for SSRIs
- Depression
- Anxiety
- OCD
- PMDD
- Bulimia nervosa
Mechanism of action of SSRIs
Allosterically inhibit SERT (bind at site other than 5HT binding site) with ~80% of activity inhibited at therapeutic doses
Pharmacokinetics of SSRIs (using fluoxetine as an example)
Absorption: oral bioavailability of fluoxetine 78%
Distribution: high lipid solubility, crosses BBB, high Vd, high protein binding
Metabolism: fluoxetine unique from other SSRIs -> is hepatically metabolised to active metabolite norfluoxetine which has a long t1/2 (7-9 days)
Excretion: 80% in urine
Seven adverse effects of SSRIs
- GI upset (most common)
- Loss of libido and sexual dysfunction
- Headaches
- Insomnia or hypersomnia
- Weight gain (especially with paroxetine)
- Discontinuation syndrome seen in drugs with short t1/2 (e.g. paroxetine, sertraline): paraesthesias, dizziness
- Risk of serotonin syndrome in combination with MAOI (or other drugs that increase synaptic 5HT)
What is the clinical relevance of fluoxetine’s active metabolite norfluoxetine?
Long t1/2 (7-9 days)
Fluoxetine must be discontinued >/= 4 weeks prior to commencing MAOI to minimise risk of serotonin syndrome
What are some of the drug interactions seen with SSRIs?
Inhibits CYP2D6 (paroxetine, fluoxetine): increases TCAs, fleicanide, risperidone, B-blockers
Inhibits CYP3A4 (fluvoxamine): increases TCAs, glucocorticoids, carbamazepine, CCBs
In combination with MAOI can cause serotonin syndrome
Which SSRIs are not enzyme inhibitors?
Citalopram and escitalopram
For what disorders are SNRIs indicated?
- Depression
- Pain disorders (e.g. neuropathies, fibromyalgia)
- GAD
- Stress urinary incontinence
- Vasomotor symptoms or menopause
What is the mechanism of action of SNRIs?
Bind SERT and NET to inhibit both 5HT and NA reuptake