In Vitro Inhibition and Induction of Human Liver Cytochrome P450 Enzymes by Milnacipran

Milnacipran (Savella) inhibits both norepinephrine and serotonin reuptake and is distinguished by a nearly 3-fold greater potency in inhibiting norepinephrine reuptake in vitro compared with serotonin. We evaluated the ability of milnacipran to inhibit and induce human cytochrome P450 enzymes in vitro. In human liver microsomes, milnacipran did not inhibit CYP1A2, 2B6, 2C8, 2C9, 2C19, or 2D6 (IC50 ≥ 100 μM); whereas, a comparator with dual reuptake properties [duloxetine (Cymbalta)] inhibited CYP2D6 (IC50 = 7 μM) and CYP2B6 (IC50 = 15 μM) with a relatively high potency. Milnacipran inhibited CYP3A4/5 in a substrate-dependent manner (i.e., midazolam 1′-hydroxylation IC50 ≈ 30 μM; testosterone 6β-hydroxylation IC50 ≈ 100 μM); whereas, duloxetine inhibited both CYP3A4/5 activities with equal potency (IC50 = 37 and 38 μM, respectively). Milnacipran produced no time-dependent inhibition (10 times plasma Cmax) produced 2.6- and 2.2-fold increases in CYP2B6 and CYP3A4/5 activity (making it 26 and 34% as effective as phenobarbital and rifampin, respectively). Given these results, milnacipran is not expected to cause clinically significant P450 inhibition or induction.