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

Abstract

ABSTRACT: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 (IC 50 > 100 M); whereas, a comparator with dual reuptake properties [duloxetine (Cymbalta)] inhibited CYP2D6 (IC 50 ‫؍‬ 7 M) and CYP2B6 (IC 50 ‫؍‬ 15 M) with a relatively high potency. Milnacipran inhibited CYP3A4/5 in a substrate-dependent manner (i.e., midazolam 1-hydroxylation IC 50 Ϸ 30 M; testosterone 6␤-hydroxylation IC 50 Ϸ 100 M); whereas, duloxetine inhibited both CYP3A4/5 activities with equal potency (IC 50 ‫؍‬ 37 and 38 M, respectively). Milnacipran produced no time-dependent inhibition (<10%) of P450 activity, whereas duloxetine produced time-dependent inhibition of CYP1A2, 2B6, 2C19, and 3A4/5. To evaluate P450 induction, freshly isolated human hepatocytes (n ‫؍‬ 3) were cultured and treated once daily for 3 days with milnacipran (3, 10, and 30 M), after which microsomal P450 activities were measured. Whereas positive controls (omeprazole, phenobarbital, and rifampin) caused anticipated P450 induction, milnacipran had minimal effect on CYP1A2, 2C8, 2C9, or 2C19 activity. The highest concentration of milnacipran (30 M; >10 times plasma C max ) 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.Milnacipran (Savella), which was recently approved for the treatment of fibromyalgia, is a dual reuptake inhibitor of norepinephrine and serotonin, which is distinguished by an approximately 3-fold greater potency in inhibiting norepinephrine reuptake in vitro compared with serotonin reuptake (Vaishnavi et al., 2004). These two neurotransmitters have been shown to exert significant modulatory effects on peripheral and central pain processing (Dubner and Hargreaves, 1989). Selective serotonin-norepinephrine reuptake inhibitors (SNRIs) such as duloxetine and venlafaxine are more potent inhibitors of serotonin reuptake than norepinephrine reuptake, whereas the converse is true of milnacipran (Vaishnavi et al., 2004).Milnacipran is well absorbed (85-90%) after oral administration and has linear pharmacokinetics (PK) over the therapeutic dose range (Delini-Stula, 2000). The terminal elimination half-life in plasma is 6 to 8 h, and steady-state levels can be predicted from single-dose PK data, indicating the absence of autoinhibition or autoinduction. Milnacipran is eliminated primarily by renal excretion of the unchanged drug (50 -60%), conjugation to form a carbamoyl glucuronide (ϳ20%), and N-dealkylation by cytochrome P450 (mainly CYP3A4) to N-desethyl milnacipran (ϳ8%) (Puozzo and Leonard, 1996; DeliniStula, 20...

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