| Indication | For the relief of symptoms of depression and as temporary adjunctive therapy in reducing enuresis in children aged 6 years and older. May also be used to manage panic disorders, with or without agoraphobia, as a second line agent in ADHD, management of eating disorders, for short-term management of acute depressive episodes in bipolar disorder and schizophrenia, and for symptomatic treatment of postherpetic neuralgia. |
| Pharmacodynamics | Imipramine is a tricyclic antidepressant with general pharmacological properties similar to those of structurally related tricyclic antidepressant drugs such as amitriptyline and doxepin. A tertiary amine, imipramine inhibits the reuptake of serotonin more so than most secondary amine tricyclics, meaning that it blocks the reuptake of neurotransmitters serotonin and noradrenaline almost equally. With chronic use, imipramine also down-regulates cerebral cortical β-adrenergic receptors and sensitizes post-synaptic sertonergic receptors, which also contributes to increased serotonergic transmission. It takes approximately 2 - 4 weeks for antidepressants effects to occur. The onset of action may be longer, up to 8 weeks, in some individuals. It is also effective in migraine prophylaxis, but not in abortion of acute migraine attack. |
| Mechanism of action | Imipramine works by inhibiting the neuronal reuptake of the neurotransmitters norepinephrine and serotonin. It binds the sodium-dependent serotonin transporter and sodium-dependent norepinephrine transporter preventing or reducing the reuptake of norepinephrine and serotonin by nerve cells. Depression has been linked to a lack of stimulation of the post-synaptic neuron by norepinephrine and serotonin. Slowing the reuptake of these neurotransmitters increases their concentration in the synaptic cleft, which is thought to contribute to relieving symptoms of depression. In addition to acutely inhibiting neurotransmitter re-uptake, imipramine causes down-regulation of cerebral cortical beta-adrenergic receptors and sensitization of post-synaptic serotonergic receptors with chronic use. This leads to enhanced serotonergic transmission. |
| Absorption | Rapidly and well absorbed after oral administration. Bioavailability is approximately 43%. Peak plasma concentrations usually attained 1 - 2 hours following oral administration. Absorption is unaffected by food. |
| Volume of distribution | Not Available |
| Protein binding | 60-95% |
| Metabolism | Exclusively metabolized by the liver. Imipramine is converted in the liver by various CYP isoenzymes (e.g. CYP1A2, CYP2D6, CYP3A4, CYP2C9) to active metabolites desipramine and 2-hydroxydesipramine. |
| Route of elimination | Approximately 40% of an orally administered dose is eliminated in urine within 24 hours, 70% in 72 hours. Small amounts are eliminated in feces via the biliary elimination. |
| Half life | Imipramine - 8-20 hours; Desipramine (active metabolite) - up to 125 hours |
| Clearance | Not Available |
| Toxicity | Oral, rat LD50: 355 to 682 mg/kg. Toxic signs proceed progressively from depression, irregular respiration and ataxia to convulsions and death. Antagonism of the histamine H1 and α1 receptors can lead to sedation and hypotension. Antimuscarinic and anticholinergic side effects such as blurred vision, dry mouth, constipation and urine retention may occur. Cardiotoxicity may occur with high doses of imipramine. Cardiovascular side effects in postural hypotension, tachycardia, hypertension, ECG changes and congestive heart failure. Psychotoxic effects include impaired memory and delirium. Induction of hypomanic or manic episodes may occur in patients with a history of bipolar disorder. Withdrawal symptoms include GI disturbances (e.g. nausea, vomiting, abdominal pain, diarrhea), anxiety, insomnia, nervousness, headache and malaise. |