How it works
Product labeling notes that olanzapine is an atypical antipsychotic, that its efficacy in schizophrenia or bipolar I disorder could be mediated through a combination of dopamine and serotonin type 2 (5HT2) antagonism, and that the mechanism of action of samidorphan could be mediated through opioid receptor antagonism.2
The pharmacodynamic profile of olanzapine is complex.2 It binds with high affinity to the following receptors: serotonin 5HT2A/2C, 5HT6 (Ki = 4, 11, and 5 nM, respectively), dopamine D1-4 (Ki = 11-31 nM), histamine H1 (Ki = 7 nM), and adrenergic alpha-1 receptors (Ki = 19 nM). Olanzapine is an antagonist with moderate affinity binding for serotonin 5HT3 (Ki = 57 nM) and muscarinic M1-5 (Ki = 73, 96, 132, 32, and 48 nM, respectively). Olanzapine binds with low affinity to gamma aminobutyric acid type A (GABA-A), benzodiazepine, and beta-adrenergic receptors (Ki >10 µM). Olanzapine’s muscarinic receptor affinity can explain why olanzapine can be associated with constipation, dry mouth, and tachycardia, all adverse reactions possibly related to cholinergic antagonism. Thus, OSC should be used with caution in patients with a current diagnosis or prior history of urinary retention, clinically significant prostatic hypertrophy, constipation, or a history of paralytic ileus or related conditions; a potential drug-drug interaction can be anticipated with concomitant use of anticholinergic medications.2 Other pharmacodynamic drug-drug interactions that can occur with the olanzapine component of OSC include the possibility that diazepam, alcohol, or other CNS-acting drugs may potentiate orthostatic hypotension, and there may be a need to reduce the dosage of concomitantly prescribed antihypertensive drugs in patients being treated for hypertension. Moreover, OSC is not recommended in patients receiving levodopa and dopamine agonists.
Samidorphan binds to the mu-, kappa-, and delta-opioid receptors (Ki = .052, .23, and 2.7 nM, respectively).2 Samidorphan is an antagonist at the mu-opioid receptors with partial agonist activity at kappa- and delta-opioid receptors. A major human metabolite of samidorphan (N-dealkylated) binds to the mu-, kappa-, and delta-opioid receptors (Ki = .26, 23, and 56 nM, respectively), and functions as a mu-opioid receptor agonist. The N-oxide major human metabolite binds to mu-, kappa-, and delta-opioid receptors (Ki = 8, 110, and 280 nM, respectively) and functions as a mu-opioid receptor antagonist. This profile differs from that of other opioid antagonists such as naltrexone.15,16
OSC is not a scheduled drug subject to the Controlled Substances Act. Because samidorphan functions as an opioid antagonist, OSC is contraindicated in patients using opioids or undergoing acute opioid withdrawal.2 To avoid precipitating opioid withdrawal, there should be at least a 7-day opioid-free interval from the last use of short-acting opioids, and at least a 14-day opioid-free interval from the last use of long-acting opioids, before initiating OSC. In emergency situations when an opioid is required, OSC should be discontinued. Patients who attempt to overcome opioid blockade while receiving OSC by using high or repeated doses of exogenous opioids could experience life-threatening or fatal opioid intoxication. Likewise, patients may have decreased tolerance to opioids if OSC therapy is interrupted or discontinued.
Regarding cardiac electrophysiology, OSC was not observed to prolong the electrocardiogram QTc interval to any clinically relevant extent when tested at doses up to 30 mg/30 mg (1.5 times and 3 times the maximum recommended daily dosage of olanzapine and samidorphan, respectively).17
Clinical pharmacokinetics
The pharmacokinetics of both olanzapine and samidorphan are linear over the clinical dose range and there is no pharmacokinetic interaction between olanzapine and samidorphan after oral administration of OSC.2 Coadministration of OSC with lithium or valproate does not have a clinically significant effect on systemic exposure of lithium or valproate.13 OSC steady-state concentrations of olanzapine and samidorphan are reached within 7 days, with accumulation at steady state being 2-fold for olanzapine and 1.3-fold for samidorphan (at 5 days). Elimination half-life for olanzapine is 35 to 52 hours, and for samidorphan, 7 to 11 hours. Olanzapine is metabolized primarily via UGT1A4 and CYP1A2, whereas samidorphan is primarily metabolized by CYP3A4. Consequently, concomitant use of OSC with strong CYP3A4 inducers is not recommended. The recommendation regarding CYP1A2 modulators and OSC are similar to those for olanzapine2,4: consider reducing the dosage of the olanzapine component in OSC when used concomitantly with strong CYP1A2 inhibitors, and consider increasing the dosage of the olanzapine component in OSC when used concomitantly with CYP1A2 inducers. Because cigarette smoke contains polycyclic aromatic hydrocarbons that act as CYP1A2 inducers,18 olanzapine clearance is much higher in smokers than in nonsmokers.2 This translates to potentially clinically relevant differences when optimizing the dose. In a study of patients with schizophrenia, olanzapine concentrations were lower in self-reported smokers (16.5, 34.2, and 60.9 ng/mL) than in self-reported nonsmokers (25.6, 43.4, and 113.2 ng/mL) for dosages of 10, 20, and 40 mg/d, respectively.19 In contrast, samidorphan pharmacokinetics are not affected by smoking status.2
No dose adjustment of OSC is needed in patients with hepatic or renal impairment; however, OSC is not recommended for patients with end-stage renal disease because this has not been specifically studied.2
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