Imipramine

MECHANISM OF ACTION

Imipramine is a tricyclic antidepressant (TCA) that primarily works by inhibiting the reuptake of norepinephrine and serotonin (5-HT) into presynaptic nerve terminals. This leads to increased concentrations of these neurotransmitters in the synaptic cleft, enhancing neurotransmission and improving mood in patients with depression.

PHARMACOKINETICS

Absorption 

Imipramine is well absorbed orally, with peak plasma concentrations typically occurring 2 to 6 hours after administration. The bioavailability ranges from 30% to 60% due to significant first-pass metabolism in the liver.

Distribution

Imipramine is widely distributed throughout the body after absorption. It is highly protein-bound (90–95%) in plasma and readily crosses the blood-brain barrier, which is essential for its central nervous system effects.

Metabolism

Imipramine undergoes extensive hepatic metabolism, primarily via the cytochrome P450 enzyme system, especially CYP2D6. It is metabolized to several active and inactive metabolites, the most notable being desipramine, which itself is a tricyclic antidepressant with noradrenergic activity.

Elimination

Imipramine and its metabolites are primarily excreted via the kidneys. Only a small fraction of the parent drug is eliminated unchanged in the urine; most is excreted as metabolites.

PHARMACODYNAMICS

Imipramine exerts its antidepressant effects by inhibiting the reuptake of norepinephrine and serotonin at presynaptic nerve terminals, leading to increased neurotransmitter levels in the synaptic cleft and enhanced neuronal signaling. This results in improved mood, reduced anxiety, and relief of depressive symptoms.

ADMINISTRATION

Imipramine is administered orally in the form of immediate-release or extended-release tablets. Immediate-release tablets are usually taken 1–3 times daily, with or without food, while extended-release formulations are taken once daily, typically at bedtime, to minimize daytime sedation. Tablets should be swallowed whole, as crushing or chewing extended-release forms can increase the risk of side effects. Dosage adjustments may be necessary for elderly patients or those with hepatic or renal impairment to ensure safe and effective therapy.

DOSAGE AND STRENGTH

Imipramine is typically initiated at 75 mg per day for adults, divided into two or three doses for immediate-release tablets or taken once daily for extended-release formulations. Maintenance doses usually range from 100 to 200 mg per day, with a maximum of 300 mg per day in severe cases. For children, particularly when used for enuresis, doses range from 25 to 75 mg per day depending on age and weight.

DRUG INTERACTIONS

Imipramine has a high potential for drug interactions due to its metabolism by CYP2D6 and central nervous system effects. Concomitant use with MAO inhibitors can lead to severe serotonin syndrome or hypertensive crises, requiring a 14-day washout period. Combining Imipramine with other serotonergic drugs, such as SSRIs, SNRIs, or triptans, also increases the risk of serotonin syndrome. Drugs that inhibit CYP2D6, including fluoxetine, paroxetine, or quinidine, can raise Imipramine plasma levels and increase toxicity risk.

FOOD INTERACTIONS

Imipramine has minimal interactions with food, and its tablets can be taken with or without meals. Taking the medication with food may help reduce gastrointestinal discomfort such as nausea. Unlike MAO inhibitors, Imipramine does not require dietary tyramine restrictions, so typical foods like cheese, cured meats, and fermented products are generally safe. However, alcohol should be avoided or limited, as it can enhance sedation, impair coordination, and increase central nervous system depression.

CONTRAINDICATIONS

Imipramine is contraindicated in patients with known hypersensitivity to tricyclic antidepressants. It should not be used concurrently with monoamine oxidase inhibitors (MAOIs) or within 14 days of MAOI therapy due to the risk of severe hypertensive crises or serotonin syndrome. Imipramine is also contraindicated in individuals with recent myocardial infarction, uncontrolled arrhythmias, or severe cardiovascular disease, as it can exacerbate cardiac conduction abnormalities.

SIDE EFFECTS

• Dry mouth 

• Constipation 

• Blurred vision 

• Drowsiness or sedation 

• Weight gain 

• Urinary retention

OVER DOSAGE

Overdose of Imipramine is a serious and potentially life-threatening condition because it affects the heart, brain, and nervous system. As a member of the Tricyclic antidepressants group, excessive intake can rapidly lead to symptoms such as drowsiness, confusion, fast or irregular heartbeat, vomiting, and agitation, which may progress to seizures, severe cardiac arrhythmias, low blood pressure, coma, and even death.

TOXICITY

Imipramine can be toxic in overdose, with a narrow safety margin typical of tricyclic antidepressants. Acute toxicity primarily affects the cardiovascular and central nervous systems, causing arrhythmias, hypotension, seizures, and CNS depression, which can be life-threatening. Other manifestations may include hyperthermia, respiratory depression, and anticholinergic effects such as confusion and urinary retention. Chronic or cumulative toxicity can occur in patients with hepatic or renal impairment, requiring careful monitoring and dose adjustment.