The antidepressant properties of duloxetine are due to blocking the reuptake of serotonin and possibly also noradrenalin within the central nervous system. Major depressive disorder is believed to be due in part to an increase in pro-inflammatory cytokines within the central nervous system. Antidepressants including ones with a similar mechanism of action as duloxetine, i.e. serotonin metabolism inhibition, cause a decrease in proinflammatory cytokine activity and an increase in anti-inflammatory cytokines; this mechanism may apply to duloxetine in its effect on depression but research on cytokines specific to duloxetine therapy is lacking. The analgesic properties of duloxetine in the treatment of diabetic neuropathy and central pain syndromes such as fibromyalgia are believed to be due to sodium ion channel blockade.
It is also considered a less potent inhibitor of dopamine reuptake. However, duloxetine has no significant affinity for dopaminergic, adrenergic, cholinergic, histaminergic, opioid, glutamate, and GABA receptors and can therefore be considered to be a selective reuptake inhibitor at the 5-HT and NA transporters. Duloxetine undergoes extensive metabolism, but the major circulating metabolites do not contribute significantly to the pharmacologic activity.
Duloxetine has an elimination half-life of about 12 hours (range 8 to 17 hours) and its pharmacokinetics are dose proportional over the therapeutic range. Steady-state is usually achieved after 3 days.
Elimination of duloxetine is mainly through hepatic metabolism involving two P450 isozymes, CYP2D6 and CYP1A2.
When orally administered it is well absorbed. There is an average 2-hour lag until absorption begins with maximum plasma concentrations occurring about 6 hours post dose. Food does not affect the Cmax of duloxetine, but delays the time to reach peak concentration from 6 to 10 hours. Duloxetine is highly bound (>90%) to proteins in human plasma, binding primarily to albumin and α1-acid glycoprotein.
Metabolism and Elimination — Only trace amounts (<1%) of unchanged duloxetine are present in the urine and most of the dose (approx. 70%) appears in the urine as metabolites of duloxetine with about 20% excreted in the feces.