Pharmacology of antidepressants

This chapter tries to address Section K2(iv) of the 2017 CICM Primary Syllabus, which expects the exam candidate to develop an " understand the pharmacology (including toxicology) of anti-depressant and anti-psychotic drugs". Like with antiepileptics, this is a complex topic, and most people trying to answer an exam question on this topic would usually end up tripping over the pharmacodynamics, even if they oversimplified things down to some truly puerile monamine level. That sort of defensive reaction is probably natural, as in the last century of research we really have not come very far along the road to understanding human mood, or how it gets so broken. 

Two questions from the First Part exam have looked at this:

  • Question 9(p.2) from the first paper of 2009 (compare and contrast classes)
  • Question 3 from the second paper of 2007 (TCA overdose)

Toxicology has subsequently moved into the Fellowship curriculum, which means that probably TCA overdose will never appear again, and it has been over ten years since the last antidepressant class question, which suggests that this whole subject area has been depreciated in the examiners' mind. With that in mind, the reader may wish to limit themselves to the contents of the summary box below:

  • Pharmacokinetics of antidepressants
    • All of these drugs are only available in oral formulation
    • The vast majority of them are well absorbed enterically
      • The exceptions are duloxetine, which is degraded by stomach acid, and sertraline, which is absorbed very slowly
    • Most have excellent oral bioavailability (except agomelatine and selegiline)
    • Most have a wide volume of distribution (except trazodone)
    • Most are highly protein bound (except venlafaxine)
    • All undergo extensive hepatic metabolism
    • Many have active metabolites (selegiline, fluoxetine, citalopram, bupropion, TCAs)
  • Pharmacodynamics
    • MAOIs bind to monoamine oxidase and inhibit the catabolism of monoamines, increasing their synaptic effect
      • This also increases the systemic availability of catecholamines, which can give rise to hypertensive crises
    • SSRIs inhibit SERT, the serotonin reuptake protein
      • This can result in serotonin syndrome in the presence of other serotonergic or monoamine agonist drugs
    • SNRIs inhibit NET, the noradrenaline reuptake protein
    • Mirtazapine and mianserin target presynaptic α-adrenoceptors and histamine receptors in the CNS, increasing the synaptic release of noradrenaline
      • The antihistamine effect leads to sedation
    • TCAs act by at least five different mechanisms, of which two are SNRI and SSRI like effects, and the others consist of the inhibition of α-adrenoceptors, histamine receptors and muscarinic acetylcholine receptors
      • Antihistamine-like sedation, postural hypotension, and anticholinergic side effects, as well as sodium channel blockade in overdose

Chemical classes of antidepressants

"The present antidepressants’ classification does not have a logical and epistemic nomenclature oriented to allow a quick recognition of main adverse drug reactions... and pharmacodynamic interactions", complained Alvano & Zieher (2020)  in one of the best articles on this subject, from which most of what follows has been derived. Without recapitulating the contents of Alvano and Zieher's work, it will suffice to say that the authors, frustrated by the inconsistency and uselessness of the existing systems, proposed their own and then designed a study which demonstrated its superiority on several very logical grounds. The author is also grateful to this paper for introducing him to the term "heuristic fertility", which is supposed to describe something that "serves to achieve or generate knowledge" but instead sounds like a process leading to the proliferation of mental shortcuts (which would also be excellent). Without further ado, here is the Alvano-Zieher system:

Class A I: Monoamine oxidase inhibitors:

  • AIa: Irreversible and nonselective: Tranylcypromine, phenelzine, isocarboxazid
  • AIb: Irreversible and selective: Selegiline (selects for MAO-B)
  • AIc: Reversible selective: Moclobemide

Class A II: Reuptake inhibitors:

  • AIIa1: SSRIs: Fluoxetine, sertraline, citalopram, paroxetine, fluvoxamine
  • AIIb: SSRI/SNRIs: venlafaxine, duloxetine
  • AIIc: Noradrenaline/dopamine reuptake inhibitors: Bupropion

Class A III: α2-receptor antagonists:

  • Mirtazapine

Class A IV: Multimodal drugs

  • AIVa: Serotonergic drugs: Trazodone, vilazodone, vortioxetine
  • AIVb: Noradrenergic drugs: Mianserine
  • AIVc: Noradrenerig/serotonergic/anticholinergic: Amitriptyline, nortriptyline, etc

Class B: Non-monoaminergic drugs:

  • Melatonin receptor agonists: Agomelatine

Class C: Weird unclassifiables:

  • Steroids, eg. dexamethasone
  • Ketamine
  • Amphetamines

To discuss the properties of this many classes with this many drugs in them would certainly be outside the scope of what is necessary to discuss an answer for Question 9(p.2) from the first paper of 2009. However, for some incomprehensible reason, one or two examples from each Alvano-Zieher class has been selected here for a slightly more detailed discussion. 

Pharmacokinetics of antidepressants

Absorption and bioavailability

The route of administration for all antidepressants is essentially just oral. None of these drugs have a parenteral formulation, apart from weird substances with vaguely mood-elevating side effects such as amphetamines and ketamine. In fact many studies complain that they are unable to give a straight answer to the question of bioavailability because the drugs were not available in a parenteral formulation. 

  Oral absorption Bioavailability
Tranylcypromine Rapidly and completely absorbed 50%
Selegiline Well absorbed (even better with food) 10%
Moclobemide Rapidly and completely absorbed 50%
Fluoxetine Rapidly and completely absorbed 90%
Sertraline Complete but slow absorption (6-8 hrs) 100%
Citalopram Complete and rapid absorption 100%
Venlafaxine Well absorbed (92%) 45%
Duloxetine Acid-labile; degraded by stomach acid (thus, requires enteric-coated tablets).  50%
Bupropion Oral absorption is close to 100% 20%
Mirtazapine Rapidly and completely absorbed 50%
Trazodone Rapidly and completely absorbed 63-91%
Mianserine Rapidly and completely absorbed 30%
Amitriptyline Rapidly and completely absorbed 33-62%
Nortriptyline Rapidly and completely absorbed 52%
Agomelatine Well absorbed 78% less than 5%

In general, most of them are absorbed very well after enteral administration, and though some of them undergo considerable first-pass metabolism, it does not play much of a role in their pharmacokinetics (for example, in the case of bupropion, the main metabolite has essentially the same activity as the parent drug). The only outliers are duloxetine (which is degraded extensively by stomach acid and which needs to be enteric-coated) and agomelatine, which has massive first-pass metabolism and a bioavailability of less than 5%.

Distribution

Most of these drugs are widely distributed and highly protein-bound, as these are the sort of properties one might expect from a lipophilic molecule that penetrates the blood-brain barrier. 

  Volume of distribution (L/kg)

Protein binding

Tranylcypromine 1.75 ? high
Selegiline 25 96%
Moclobemide 2 50%
Fluoxetine 14-100 94%
Sertraline 20 99%
Citalopram 12 80%
Venlafaxine 6-7 27%
Duloxetine 25% 90%
Bupropion 19 84%
Mirtazapine 4.8 95%
Trazodone 0.8 95%
Mianserine 15.7 90%
Amitriptyline 19 84%
Nortriptyline 21 93%
Agomelatine 30 95%

The notable exceptions are venlafaxine, which has minimal interest in binding to proteins, and trazodone which seems to be confined to the body water compartment. The rest have volumes of distribution in the 5-20L/kg range, suggesting that they are bound to proteins around the tissues.

Metabolism and clearance of antidepressants

All of these drugs are cleared by the liver in one way or another. Their only real difference is whether or not the metabolites have some sort of activity, and how rapidly they are metabolised (because this has implications for half-life.

Tranylcypromine Extensively metabolised, probably in the liver, into N-acetylated and ring-hydroxylated metabolites, which retain some limited MAO-inhibitory activity Minimum renal excretion (4% as unchanged drug) Half-life is only about 2 hours, but this does not have any relationship to the duration of its effect
Selegiline Hepatic metabolism; rapidly metabolized by the microsomal enzymes to amphetamine, methamphetamine, and desmethyl-deprenyl Renal clearance is important: 86% of the active metabolites are recovered in the urine Elimination half-life of the parent drug is only aout 1.5 hours, but it leaves behind active metabolites with longer periods of activity, and its MAO-I effect is long lasting
Moclobemide Hepatic metabolism; oxidation of the morpholine ring moiety, aromatic hydroxylation and deamination; multiple inactive metabolites Renal clearance accounts for very little total clearance, and appears to be unimportant Half-life is only about 2 hours, but the duration of MAO-I effect is much greater
Fluoxetine Hepatic metabolism into active metabolite norfluoxetine and multiple other (inactive) metabolites All the metabolites are renally excreted Half life is variable and long, 1-4 days
Sertraline Hepatic metabolism, mainly by N-demethylation to from an extremely long-lived metabolite (with about 10% of the potency of the parent drug, and a half-life of about 100 hours) 50% of the metabolites are renally excreted, and 50% of them are eliminated in the faeces, suggesting that there is some biliary excretion. Half life is about 26 hours
Citalopram Hetaptic metabolism by N-demethylation into several active metabolites 50% of the metabolites are renally excreted, and 50% of them are eliminated in the faeces, suggesting that there is some biliary excretion. Half life is about 36 hours
Venlafaxine Extensive hepatic metabolism to the major O-demethyl metabolite and 2 minor metabolites. Elimination of the parent drug is hepatic and of its metabolites is mainly renal (only 4.7% is recovered in the urine as unchanged drug) Half-life is about 4 hours, which called for an extended release formulation
Duloxetine Hepatic metabolism into numerous metabolites, mainly by oxidation in the naphthyl ring followed by further oxidation, methylation and conjugation Elimination of inactive metabolites is mainly renal Half life is about 12 hours
Bupropion Hepatic metabolism by CYP2B6, into an active metabolite (hydroxybupropion) The majority of parent drug and metabolites are eliminated in the urine as glycine conjugates Elimination half-life is about 18 hours
Mirtazapine Hepatic metabolism into largely inactive metabolites, mainly by CYP1A2, CYP2D6, and CYP3A4 All metabolites are renally eliminated Elimination half-life is about 20-40 hours
Trazodone Hepatic metabolism into tow main metabolites, oxotriazolepyridinpropionic acid (TPA) and mCPP, of which mCPP is more active as an inhibitor of SERT than trazodone itself. All metabolites are renally eliminated Elimination half-life is about 7 hours
Mianserine Hepatic metabolism by aromatic hydroxylation, N-oxidation and N-demethylation. 70% of the metabolites are renally excreted; the rest in the bile Elimination half-life 10-17 hrs
Amitriptyline Hepatic metabolism mainly by CYP2D6 into breakdown products which all have some degree of antidepressant activity All metabolites are renally eliminated Elimination half-life of around 20 hours
Nortriptyline Hepatic metabolism mainly by CYP2D6 into breakdown products which all have some degree of antidepressant activity All metabolites are renally eliminated Elimination half-life of around 26 hours
Agomelatine Hepatic metabolism by 7-O-demethylation and hydroxylation; all metabolites are inactive at MT receptors About 80% of the metabolites are eliminated in the urine, and the rest in the bile Elimination half-life is about 2.3 hours; duration of activity is much longer

Pharmacodynamics of antidepressants

A single-sentence summary of course cannot capture the messy confusion of neurochemistry which describes mood regulation, and so what follows is by necessity an oversimplification:

Tranylcypromine By binding to both kinds of monoamine oxidase enzymes, MAOIs increase the availability of catecholamine neurotransmitters (i.e. mainly noradrenaline and dopamine, but also to some extent serotonin)
Selegiline
Moclobemide
Fluoxetine By inhibiting the reuptake of serotonin from the synaptic cleft, SSRIs increase serotonergic neurotransmission, which is thought to be involved in mood regulation.
Sertraline
Citalopram
Venlafaxine By inhibiting the reuptake of serotonin and noradrenaline from the synaptic cleft, SSRI/SNRI drugs increase monoamine neurotransmission, which is thought to be involved in mood regulation.
Duloxetine
Bupropion By inhibiting the reuptake of serotonin and dopamine from the synaptic cleft, SSRI/SDRI drugs increase monoamine neurotransmission, which is thought to be involved in mood regulation.
Mirtazapine The mechanism of the antidepressant effect of mirtazapine is thought to be the inhibition of presynaptic control of noradrenaline release (i.e. it does the opposite of what clonidine does).
Trazodone The direct antagonist effect of trazodone at the 5-HT2 receptor results in increased presynaptic noradrenaline release. That receptor is a presynaptic autoregulatory receptor that normally downregulates noradrenaline release as part of a negative feedback loop.
Mianserine The mechanism of antidepressant effect is thought to be the inhibition of presynaptic control of noradrenaline release. Also, it inhibits reuptake of noradrenaline.
Amitriptyline Multiple mechanisms of effect, mostly related to the inhibition of reuptake of noradrenaline and serotonin by interference with the function of SERT and NET transport proteins. Additionally, tricyclic antidepressants act as antagonists at muscarinic receptors, histamine receptors and α-adrenergic receptors
Nortriptyline Multiple mechanisms of effect, mostly related to the inhibition of reuptake of noradrenaline and serotonin by interference with the function of SERT and NET transport proteins
Agomelatine Mechanism of action is thought to be related to the regulation of dopamine release by melatonin receptor activation (i.e. the effect of agomelatine, as an MT1 and MT2 agonist, is to increase the concentration of dopamine).

Side effects and toxicities

Fortunately, these can be grouped according to class, as all agents from each group tend to have similar side effect profiles. The exception to this rule are the MAOIs, which all have slightly different toxic effects. For example, nothing is quite as good at killing you with hypertensive catastrophes as eating a small amount of blue cheese while on tranylcypromine, but the other two do not seem to have as much of this problem. On the other hand, selegiline is metabolised into amphetamine, which is thought to be responsible for a lot of its positive effects, and that's a unique feature. In contrast, all SSRIs and tricyclics and basically be grouped together, as they all tend to have a fairly stereotypical side effect profile.

Tranylcypromine Hypertensive crises, hepatotoxicity, seizures, hypoglycaemia, mania, serotonin syndrome. Dangerous pharmacodynamic interactions with other antidepressants and monoaminergic drugs, as well as foods that act as catecholamine precursors
Selegiline Euphoria, insomnia, hypertension (i.e. the effects of amphetamine intoxication,it being one of the major metabolites)
Moclobemide Hypertension, restlessness, agitation, nausea, insomnia (i.e. features of sympathomimetic effect)
Fluoxetine

These effects are common to all SSRIs: agitation, diarrhoea, loss or gain of weight, vertigo, and sexual dysfunction; risk of serotonin syndrome with overdose

Sertraline
Citalopram
Venlafaxine
Duloxetine Agitation, diarrhoea, loss or gain of weight, vertigo; risk of serotonin syndrome with overdose (i.e. similar side effect profile to SSRIs). Additionally, duloxetine can sometimes have anticholinergic side effects, such as xerostomia and urinary retention
Bupropion Dry mouth, constipation, headache, nausea, insomnia and weight loss. The most common reason for discontinuation is apparently "an unpleasant state variously described as a crazy feeling".
Mirtazapine Anticholinergic side effects, sedation (antihistamine effect), increased appetite, weight gain
Trazodone Sedation, diarrhoea, loss or gain of weight, vertigo; risk of serotonin syndrome with overdose
 
Mianserine
Amitriptyline Sedation, postural hypotension, anticholinergic side effects, sodium channel blockade in overdose (giving rise to QRS prolongation and ventricular arrhythmias)
 
Nortriptyline
Agomelatine Both sedation and insomnia have been reported as side effects, but generally the incidence of side effects seems to be no greater than placebo

References

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