Antidepressant Drugs

Drug Therapy for the Treatment of Depression

Overview of Depression

  • Definition of Depression: A mental disorder characterized by persistent sadness and a lack of interest or pleasure in previously enjoyable activities.

  • Key Neurotransmitter Involved: 5-Hydroxytryptamine (5-HT), commonly known as serotonin.

Epidemiology of Depression

  • Past Year Prevalence of Major Depressive Episode Among U.S. Adults (2020):

    • Overall Prevalence: 17.0%

    • By Sex:

    • Female: 15.9%

    • Male: 10.5%

    • By Age Group:

    • 18-25: 9.5%

    • 26-49: 9.1%

    • 50+: 8.4%

    • By Race/Ethnicity:

    • Hispanic: 6.2%

    • White: 7.0%

    • Black or African American: 5.4%

    • Asian: 4.2%

    • AI/AN: 4.2%

    • 2 or More Races: 6.0%

  • Data Source: SAMHSA and NIMH

Clinical Presentation & Diagnosis

  • Symptoms of Depression:

    • Depressed/sad mood

    • Decreased interest in normal activities

    • Difficulty concentrating

    • Changes in sleep patterns

    • Weight fluctuation or change in appetite

    • Feelings of guilt and/or worthlessness

    • Fatigue and impaired libido

    • Suicidal ideations

  • Diagnostic Criteria (DSM-V):

    • Patient must present with at least one of the first two symptoms plus two additional symptoms.

    • Symptoms should occur most days of the week for at least two weeks.

Etiology: Neurobiology and Chemistry

  • Organic Causes: Depression and anxiety may have organic causes rooted in neurochemistry; a "chemical imbalance" is an oversimplification.

    • Influenced by social factors, stress, life events, and medical issues.

  • Serotonin's Role: Serotonin (5-HT) modulates activity in numerous regions of the limbic system.

  • Involvement of Cortical Regions: Cortical regions contribute to sophisticated executive processes related to mood regulation.

A. The Serotonin System

  • Structures Involved:

    • Neocortex

    • Hypothalamus

    • Temporal lobe

    • Basal ganglia

    • Thalamus

B. The Norepinephrine System

  • Structures Involved:

    • Neocortex

    • Hypothalamus

    • Temporal lobe

    • Locus ceruleus

    • Raphe nuclei

    • Cerebellum

    • Spinal cord

C. Dopamine System

  • Structures Involved:

    • Ventral tegmental area (VTA)

Evidence for the Monoamine Hypothesis

  • Dopamine:

    • Reserpine (VMAT inhibitor) can induce depression-like symptoms; elevating dopamine alleviates symptoms.

  • Norepinephrine:

    • Evidence similar to dopamine.

  • Serotonin:

    • Released in brain regions associated with mood.

    • Depletion can induce symptoms of depression; low levels correlate with suicides.

    • Alterations in serotonin receptors (5-HT1A and 2A) observed in suicide victims.

    • Selective Serotonin Reuptake Inhibitors (SSRIs; e.g., Prozac®) show marked improvement in patients.

Overview of Antidepressant Activity

  • Mechanisms of Action:

    1. Inhibition of neurotransmitter (NT) metabolism (e.g., MAO inhibitors; first-generation).

    2. Prevention of NT reuptake (e.g., transporter inhibitors; first- and second-generation).

    3. Receptor antagonists targeting presynaptic autoreceptors.

  • Long-Term Adaptations: Persistent signaling may lead to long-term adaptive changes in receptor signaling and neurotrophic release.

Reuptake Inhibitors

  • Importance in Therapy: Most antidepressants currently available act through inhibition of monoamine reuptake transporters.

  • Differences in Agents: Variations in chemical structures correspond to different specificities for monoamine transporters.

SSRIs: Mechanism of Action

  • Function: Primarily inhibit presynaptic reuptake of serotonin via the serotonin transporter (SERT).

  • Inhibition Process:

    • Competitive inhibitors bind to the main 5-HT pocket.

    • Binding results in a locked transporter in the outward-facing conformation.

    • SSRIs prevent 5-HT binding but do not induce conformational change or transport function.

  • Common SSRIs: Paxil, Prozac, Luvox, Zoloft.

SSRIs: Pharmacokinetics

  • Key Properties:

    • Lipophilicity: Readily absorbed and bioavailable after oral administration with a large volume of distribution (VD).

    • Metabolism: Through CYP2C9 and 2D6 (Phase I), and glucuronidation (Phase II); can generate active metabolites extending the half-life.

    • Fluoxetine: approx. 2 days half-life and norfluoxetine: approx. 10 days half-life.

  • Characteristics:

    • Large volume of distribution includes CNS.

    • Long half-life.

    • Extensively metabolized (hepatic function and potential drug-drug interactions).

SSRIs: Clinical Lag Time

  • Onset of Action: SSRIs may take several weeks to exhibit therapeutic effects.

    • Initially theorized to be due to pharmacokinetics.

    • More likely relates to long-term changes in 5-HT neurotransmission, including:

    • 5-HT1A autoreceptor desensitization.

    • Brain-derived neurotrophic factor (BDNF) upregulation and induction of neuroadaptive changes.

SSRIs: Clinical Considerations

  • Serotonin Syndrome:

    • Life-threatening medical emergency caused by excess serotonin levels, predictable through drug-drug interactions and/or overdose.

  • Involved Drugs:

    • SSRIs, SNRIs, TCAs, MAO inhibitors, certain hallucinogens (e.g., Ecstasy, LSD), stimulants (e.g., cocaine, amphetamine), St. John's Wort, Dextromethorphan.

  • Additional Indications: Generalized anxiety disorder, obsessive-compulsive disorder, eating disorders, and premature ejaculation.

  • Side Effects:

    • Sexual dysfunction, insomnia, diarrhea, tremors.

  • In Children and Adolescents:

    • Increased risk of suicidal ideation and agitation/hostility reported (nearly doubling incidence according to the FDA).

    • Withdrawal Syndrome: Discontinuation should involve tapering over several weeks to avoid nausea, headaches, dizziness, chills, aches, and insomnia.

SNRIs: Tricyclic Antidepressants (TCAs)

  • Chemical Structure: Dibenzazepine pharmacophore.

  • Mechanism of Action: Inhibition of both SERT and NET, little activity at dopamine transporter (DAT).

  • Side Effects: Potent antihistamine and antimuscarinic effects lead to sedation and anticholinergic side effects (e.g., dry mouth, constipation).

    • Also inhibit rapid sodium and L-type calcium channels, posing a risk of cardiotoxicity.

    • Common TCAs: Norpramin, Anafranil, Elavil.

Clinical Considerations for TCAs

  • Usage Decline: Due to lengthy side effect profiles combined with overdose risks.

  • CNS Effects in Overdose: Hallucinations, muscle twitching, seizures, delirium, coma.

  • Cardiovascular Effects: Orthostatic hypotension, tachycardia, arrhythmias, resulting in Class I-like effects by slowing phase 0 depolarization.

SNRIs: Cyclohexanol Ethylamine

  • Members: Venlafaxine and Desvenlafaxine are distinct from TCAs and aryloxypropanamines.

  • Side Effect Profile: Similar to TCAs, but less severe with lower incidence rates, making them safer.

  • Active Metabolite: Desvenlafaxine is the active form of Venlafaxine (metabolized by CYP2D6).

Bupropion – an NDRI

  • Classification: Member of the phenethylamine class; specifically, a cathinone.

  • Properties: Mild stimulant; serves as an alternative option in ADHD treatment.

  • Mechanism of Action: Reuptake inhibitor for dopamine (DA) and norepinephrine (NE).

  • Usage: Can be combined with SSRIs or SNRIs with a reduced risk of serotonin syndrome.

Additional Antidepressants

  • Serotonin Antagonists and Reuptake Inhibitors (SARIs):

    • Act primarily at 5-HT1A presynaptic autoreceptors functioning in negative feedback.

    • Examples: Nefazodone and Trazodone.

  • Monoamine Oxidase Inhibitors (MAO-Is):

    • Used as alternatives to reuptake inhibitors but are usually not first-line due to potential side effects associated with drug-diet interactions.

    • May cause hypertensive crises and serotonin syndrome.

    • Examples include Pheneizine and Selegiline.

Ubiquity of Antidepressants

  • Top 10 Psychiatric Drugs by Primary Mechanism:

    • SSRI: 66,697,000 prescriptions.

    • SNRI: 31,618,000 prescriptions.

    • Benzodiazepine: 83,906,000 prescriptions.

  • Top 10 Most Prescribed Psychiatric Drugs in the U.S.:

    • Zoloft: 19,500,000 prescriptions.

    • Effexor XR: 14,992,000 prescriptions.

    • Cymbalta: 16,626,000 prescriptions.

    • Xanax: 44,029,000 prescriptions.

  • Overview of Use: Diverse classes of medications reflecting various mechanisms and indications in treating depression and anxiety disorders.