ABSTRACT:As a direct-acting inhibitor of CYP2C19 in vitro, lansoprazole is more potent than omeprazole and other proton pump inhibitors (PPIs), but lansoprazole does not cause clinically significant inhibition of CYP2C19 whereas omeprazole does. To investigate this apparent paradox, we evaluated omeprazole, esomeprazole, R-omeprazole, lansoprazole, and pantoprazole for their ability to function as directacting and metabolism-dependent inhibitors (

ABSTRACT:Metabolism-dependent inhibition (MDI) of cytochrome P450 is usually assessed in vitro by examining whether the inhibitory potency of a drug candidate increases after a 30-min incubation with human liver microsomes (HLMs). To augment the IC 50 shift, many researchers incorporate a dilution step whereby the samples, after being preincubated for 30 min with a high concentration of HLMs (with and without NADPH), are diluted before measuring P450 activity. In the present study, we show that the greater IC 50

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...

The ability of a drug to cause clinically significant drug-drug interactions due to direct or metabolism-dependent inhibition of cytochrome P450 (CYP) can generally be predicted from in vitro studies with human liver microsomes (HLM) or recombinant CYP enzymes, as recommended by the FDA and other regulatory agencies. This review highlights some examples of system-dependent inhibition of CYP and uridine diphosphate glucuronosyltransferase (UGT) enzymes. In the case of CYP enzymes, examples are presented where in vitro studies with HLM under-predict or over-predict the degree of inhibition observed in the clinic and where the correct prediction comes from studies with human hepatocytes. Studies with HLM under-predict the ability of gemfibrozil and bupropion to cause clinically significant inhibition of CYP2C8 and CYP2D6, respectively, and over-predict the ability of ezetimibe to cause clinically significant inhibition of CYP3A4. Gemfibrozil and bupropion represent examples of glucuronidation-dependent and reduction-dependent activation to metabolites that inhibit CYP2C8 and CYP2D6, respectively, whereas ezetimibe represents an example of glucuronidation-dependent protection against metabolism-dependent inhibition of CYP3A4. This article illustrates why, when drug candidates are extensively metabolized by non-CYP enzymes, it would be prudent to use human hepatocytes in addition to HLM or recombinant enzymes to evaluate their ability to inhibit CYP enzymes.

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