Pharmacology of antidepressants

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The

glutamate receptor — produces rapid (within 2 hours), robust and sustained (lasting for up to a fortnight) antidepressant effects.[3] Monoamine precursor depletion also fails to alter mood.[4][5][6] To overcome these flaws with the monoamine hypothesis a number of alternative hypotheses have been proposed, including the glutamate, neurogenic, epigenetic, cortisol hypersecretion and inflammatory hypotheses.[2][3][7][8]
Another hypothesis that has been proposed which would explain the delay is the hypothesis that monoamines don't directly influence mood, but influence emotional perception biases.[9]

Neurogenic adaptations

The neurogenic hypothesis states that molecular and cellular mechanisms underlying the regulation of adult neurogenesis is required for remission from depression and that neurogenesis is mediated by the action of antidepressants.[10] Chronic use of antidepressant increased neurogenesis in the hippocampus of rats.[11][12][13] Other animal research suggests that long term drug-induced antidepressants effects modulate the expression of genes mediated by clock genes, possibly by regulating the expression of a second set of genes (i.e. clock-controlled genes).[14]

The delayed onset of clinical effects from antidepressants indicates involvement of adaptive changes in antidepressant effects. Rodent studies have consistently shown upregulation of the 3, 5-cyclic adenosine monophosphate (cAMP) system induced by different types of chronic but not acute antidepressant treatment, including serotonin and norepinephrine uptake inhibitors, monoamine oxidase inhibitors, tricyclic antidepressants, lithium and electroconvulsions. cAMP is synthesized from adenosine 5-triphosphate (ATP) by adenylyl cyclase and metabolized by cyclic nucleotide phosphodiesterases (PDEs).[15]

Hypothalamic-pituitary-adrenal axis

One manifestation of depression is an altered

stress, that of increased cortisol production and a subsequent impaired negative feedback mechanism. It is not known whether this HPA axis dysregulation is reactive or causative for depression. This briefing suggests that the mode of action of antidepressants may be in regulating HPA axis function.[16]

Monoamine hypothesis

In 1965, Joseph Schildkraut postulated the

Monoamine Hypothesis when he posited an association between low levels of neurotransmitters and depression.[17]
By 1985, the monoamine hypothesis was mostly dismissed until it was revived with the introduction of SSRIs through the successful direct-to-consumer advertising, often revolving around the claim that SSRIs correct a chemical imbalance caused by a lack of serotonin within the brain.

Serotonin levels in the human brain is measured indirectly by sampling cerebrospinal fluid for its main metabolite, 5-hydroxyindole-acetic acid, or by measuring the serotonin precursor, tryptophan. In one placebo controlled study funded by the National Institute of Health, tryptophan depletion was achieved, but they did not observe the anticipated depressive response.[18] Similar studies aimed at increasing serotonin levels did not relieve symptoms of depression. At this time, decreased serotonin levels in the brain and symptoms of depression have not been linked[19]

Although there is evidence that antidepressants inhibit the reuptake of serotonin,[20] norepinephrine, and to a lesser extent dopamine, the significance of this phenomenon in the amelioration of psychiatric symptoms is not known. Given the low overall response rates of antidepressants,[21] and the poorly understood causes of depression, it is premature to assume a putative mechanism of action of antidepressants.

While MAOIs, TCAs and SSRIs increase serotonin levels, others prevent serotonin from binding to 5-HT2Areceptors, suggesting it is too simplistic to say serotonin is a "happy neurotransmitter". In fact, when the former antidepressants build up in the bloodstream and the serotonin level is increased, it is common for the patient to feel worse for the first weeks of treatment. One explanation of this is that 5-HT2A receptors evolved as a saturation signal (people who use 5-HT2A antagonists often gain weight), telling the animal to stop searching for food, a mate, etc., and to start looking for predators. In a threatening situation it is beneficial for the animal not to feel hungry even if it needs to eat. Stimulation of 5-HT2A receptors will achieve that. But if the threat is long lasting the animal needs to start eating and mating again - the fact that it survived shows that the threat was not so dangerous as the animal felt. So the number of 5-HT2A receptors decreases through a process known as

downregulation and the animal goes back to its normal behavior. This suggests that there are two ways to relieve anxiety in humans with serotonergic drugs: by blocking stimulation of 5-HT2A receptors or by overstimulating them until they decrease via tolerance.[medical citation needed
]

Receptor affinity

A variety of monoaminergic antidepressants have been compared below:[1][22][23][24][25][26]

Compound SERT NET DAT H1 mACh α1 α2
5-HT1A
5-HT2A
5-HT2C
D2
MT1A MT1B
Agomelatine ? ? ? ? ? ? ? ? ? 631 ? 0.1 0.12
Amitriptyline 3.13 22.4 5380 1.1 18 24 690 450 4.3 6.15 1460 ? ?
Amoxapine 58 16 4310 25 1000 50 2600 ? 0.5 2 20.8 ? ?
Atomoxetine 43 3.5 1270 5500 2060 3800 8800 10900 1000 940 >35000 ? ?
Bupropion 9100 52600 526 6700 40000 4550 >35000 >35000 >10000 >35000 >35000 ? ?
Buspirone ? ? ? ? ? 138 ? 5.7 138 174 362 ? ?
Butriptyline 1360 5100 3940 ? ? ? ? ? ? ? ? ? ?
Citalopram 1.38 5100 28000 380 1800 1550 >10000 >10000 >10000 617 ? ? ?
Clomipramine 0.14 45.9 2605 31.2 37 39 525 >10000 35.5 64.6 119.8 ? ?
Desipramine 17.6 0.83 3190 110 196 100 5500 >10000 113.5 496 1561 ? ?
Dosulepin 8.6 46 5310 4 26 419 12 4004 152 ? ? ? ?
Doxepin 68 29.5 12100 0.24 83.3 23.5 1270 276 26 8.8 360 ? ?
Duloxetine 0.8 5.9 278 2300 3000 8300 8600 5000 504 916 >10000 ? ?
Escitalopram 0.8-1.1 7800 27400 2000 1240 3900 >1000 >1000 >1000 2500 >1000 ? ?
Etoperidone 890 20000 52000 3100 >35000 38 570 85 36 36 2300 ? ?
Femoxetine 11 760 2050 4200 184 650 1970 2285 130 1905 590 ? ?
Fluoxetine 1.0 660 4176 6250 2000 5900 13900 32400 197 255 12000 ? ?
Fluvoxamine 1.95 1892 >10000 >10000 240000 1288 1900 >10000 >10000 6700 >10000 ? ?
Imipramine 1.4 37 8300 37 46 32 3100 >10000 119 120 726 ? ?
Lofepramine 70 5.4 18000 360 67 100 2700 4600 200 ? 2000 ? ?
Maprotiline 5800 11.1 1000 1.7 560 91 9400 ? 51 122 665 ? ?
Mazindol 100 1.2 19.7 600 ? ? ? ? ? ? ? ? ?
Mianserin 4000 71 9400 1.0 500 74 31.5 1495 3.21 2.59 2052 ? ?
Milnacipran 94.1 111 >10000 ? ? ? ? ? ? ? ? ? ?
Mirtazapine >10000 4600 >10000 0.14 794 608 20 18 69 39 5454 ? ?
Nefazodone 400 490 360 24000 11000 48 640 80 8.6 72 910 ? ?
Nisoxetine 610 5.1 382 ? 5000 ? ? ? 620 ? ? ? ?
Nomifensine 2941 22.3 41.1 2700 >10000 1200 6744 1183 937 >10000 >10000 ? ?
Nortriptyline 16.5 4.37 3100 15.1 37 55 2030 294 5 8.5 2570 ? ?
Oxaprotiline 3900 4.9 4340 ? ? ? ? ? ? ? ? ? ?
Paroxetine 0.08 56.7 574 22000 108 4600 >10000 >35000 >10000 19000 32000 ? ?
Protriptyline 19.6 1.41 2100 60 25 130 6600 ? 26 ? ? ? ?
Quetiapine >10,000 >10,000 >10,000 7 ? 22 3,630 376 99 2502 245 ? ?
Reboxetine 274 13.4 11500 312 6700 11900 >10000 >10000 >10000 457 >10000 ? ?
Sertraline 0.21 667 25.5 24000 625 370 4100 >35000 1000 1000 10700 ? ?
Trazodone 367 >10000 >10000 220 >35000 42 320 118 35.8 224 4142 ? ?
Trimipramine 149 2450 3780 1.4 58 24 680 ? ? ? ? ? ?
Venlafaxine 7.7 2753 8474 >35000 >35000 >35000 >35000 >35000 >35000 >10000 >35000 ? ?
Vilazodone 0.1 ? ? ? ? ? ? 2.3 ? ? ? ? ?
Viloxazine 17300 155 >100000 ? ? ? ? ? ? ? ? ? ?
Vortioxetine 1.6 113 >1000 ? ? ? ? 15 (Agonist) ? 180 ? ? ?
Zimelidine 152 9400 11700 ? ? ? ? ? ? ? ? ? ?

The values above are expressed as equilibrium dissociation constants in nanomoles/liter. A smaller dissociation constant indicates more affinity. SERT, NET, and DAT correspond to the abilities of the compounds to inhibit the reuptake of serotonin, norepinephrine, and dopamine, respectively. The other values correspond to their affinity for various receptors.

Anti-inflammatory and immunomodulation

Recent studies show

clinical depression, mania and bipolar disorder, and it is possible that symptoms of these conditions are attenuated by the pharmacological effect of antidepressants on the immune system.[27][28][29][30][31]

Studies also show that the chronic secretion of

interleukin-6 (IL-6). Antidepressants have also been shown to suppress TH1 upregulation.[32][33][34][35][36]

Antidepressants, specifically TCAs and SNRIs (or SSRI-NRI combinations), have also shown analgesic properties.[37][38]

These studies warrant investigation for antidepressants for use in both psychiatric and non-psychiatric illness and that a psycho-neuroimmunological approach may be required for optimal pharmacotherapy.[39] Future antidepressants may be made to specifically target the immune system by either blocking the actions of pro-inflammatory cytokines or increasing the production of anti-inflammatory cytokines.[40]

Pharmacokinetics

Sources:[41][42][43][44]

Drug Bioavailability t1/2 (hr) for parent drug (active metabolite) Vd (L/kg unless otherwise specified) Cp (ng/mL) parent drug (active metabolite) Tmax Protein binding Parent drug (active metabolite(s)) Excretion Enzymes responsible for metabolism Enzymes inhibited[45]
Tricyclic antidepressant (TCAs)
Amitriptyline 30–60% 9–27 (26–30) ? 100–250 4 hr >90% (93–95%) Urine (18%) ?
Amoxapine ? 8 (30) 0.9–1.2 200–500 90 mins 90% Urine (60%), faeces (18%) ? ?
Clomipramine 50% 32 (70) 17 100–250 (230–550) 2–6 hr 97–98% Urine (60%), faeces (32%) CYP2D6 ?
Desipramine ? 30 ? 125–300 4–6 hr ? Urine (70%) CYP2D6 ?
Doxepin ? 18 (30) 11930 150–250 2 hr 80% Urine ?
Imipramine High 12 (30) 18 175–300 1–2 hr 90% Urine ?
Lofepramine 7% 1.7–2.5 (12–24) ? 30–50 (100–150) 1 hr 99% (92%) Urine CYP450 ?
Maprotiline High 48 ? 200–400 8–24 hr 88% Urine (70%); faeces (30%) ? ?
Nortriptyline ? 28–31 21 50–150 7–8.5 hr 93–95% Urine, faeces CYP2D6 ?
Protriptyline High 80 ? 100–150 24–30 hr 92% Urine ? ?
Tianeptine 99% 2.5–3 0.5–1 ? 1–2 hr 95–96% Urine (65%) ? ?
Trimipramine 41% 23–24 (30) 17–48 100–300 2 hr 94.9% Urine ? ?
Monoamine oxidase inhibitors
(MAOIs)
Moclobemide 55–95% 2 ? ? 1–2 hr 50% Urine, faeces (<5%) ? MAOA
Phenelzine ? 11.6 ? ? 43 mins ? Urine MAOA MAO
Tranylcypromine ? 1.5–3 3.09 ? 1.5–2 hr ? Urine MAO MAO
Selective serotonin reuptake inhibitors
(SSRIs)
Citalopram 80% 35–36 12 75–150 2–4 hr 80% Urine (15%) CYP1A2 (weak)
Escitalopram 80% 27–32 20 40–80 3.5–6.5 hr 56% Urine (8%) CYP2D6 (weak)
Fluoxetine 72% 24–72 (single doses), 96–144 (repeated dosing) 12–43 100–500 6–8 hr 95% Urine (15%) CYP2D6
Fluvoxamine 53% 18 25 100–200 3–8 hr 80% Urine (85%)
Paroxetine ? 17 8.7 30–100 5.2–8.1 (IR); 6–10 hr (CR) 93–95% Urine (64%), faeces (36%) CYP2D6
Sertraline 44% 23–26 (66) ? 25–50 4.5–8.4 hr 98% Urine (12–14% unchanged), faeces (40–45%)
Serotonin-norepinephrine reuptake inhibitors
(SNRIs)
Desvenlafaxine 80% 11 3.4 ? 7.5 hr 30% Urine (69%) CYP3A4 CYP2D6 (weak)
Duloxetine High 11–12 3.4 ? 6 hr (empty stomach), 10 hr (with food) >90% Urine (70%; <1% unchanged), faeces (20%) CYP2D6 (moderate)
Levomilnacipran 92% 12 387–473 L ? 6–8 hr 22% Urine (76%; 58% as unchanged drug & 18% as N-desmethyl metabolite) ?
Milnacipran 85-90% 6-8 (L-isomer), 8-10 (D-isomer) 400 L ? 2–4 hr 13% Urine (55%) ? ?
Venlafaxine 45% 5 (11) 7.5 ? 2-3 hr (IR), 5.5–9 hr (XR) 27–30% (30%) Urine (87%) CYP2D6 CYP2D6 (weak)
Others
Agomelatine ≥80% 1–2 hr 35 L ? 1–2 hr 95% Urine (80%) ?
Bupropion ? 8–24 (IR; 20, 30, 37), 21±7 (XR) 20–47 75–100 2 hr (IR), 3 hr (XR) 84% Urine (87%), faeces (10%) CYP2B6 CYP2D6 (moderate)
Mianserin 20-30% 21–61 ? ? 3 hr 95% Faeces (14–28%), urine (4–7%) CYP2D6 ?
Mirtazapine 50% 20–40 4.5 ? 2 hr 85% Urine (75%), faeces (15%) ?
Nefazodone 20% (decreased by food) 2–4 0.22–0.87 ? 1 hr >99% Urine (55%), faeces (20–30%) CYP3A4 ?
Reboxetine 94% 12–13 26 L (R,R diastereomer), 63 L (S,S diastereomer) ? 2 hr 97% Urine (78%; 10% as unchanged) CYP3A4 ?
Trazodone ? 6–10 ? 800–1600 1 hr (without food), 2.5 hr (with food) 85–95% Urine (75%), faeces (25%) CYP2D6 ?
Vilazodone 72% (with food) 25 ? ? 4–5 hr 96–99% Faeces (2% unchanged), urine (1% unchanged) ?
Vortioxetine ? 66 2600 L ? 7–11 hr 98% Urine (59%), faeces (26%) ?

See also

References