Drug Interaction Report

MONITOR: Coadministration with dacomitinib may increase the plasma concentrations of drugs that are substrates of the CYP450 2D6 isoenzyme. The mechanism is reduced clearance due to inhibition of CYP450 2D6 by dacomitinib. When dextromethorphan, a probe substrate for CYP450 2D6, was coadministered with a single 45 mg dose of dacomitinib, dextromethorphan peak plasma concentration (Cmax) and systemic exposure (AUC) increased by 9.7- and 9.6-fold, respectively. The interaction may be particularly important for sensitive CYP450 2D6 substrates or those that demonstrate a narrow therapeutic index.

MANAGEMENT: Caution is advised if dacomitinib is used in combination with CYP450 2D6 substrates. Dosage adjustments as well as clinical and laboratory monitoring may be appropriate whenever dacomitinib is added to or withdrawn from therapy. Avoid concomitant use with dacomitinib where minimal increases in concentration of the CYP450 2D6 substrate may lead to serious or life-threatening toxicities.

One study has reported that coadministration of caffeine and aspirin lead to a 25% increase in the rate of appearance and 17% increase in maximum concentration of salicylate in the plasma. A significantly higher area under the plasma concentration time curve of salicylate was also reported when both drugs were administered together. The exact mechanism of this interaction has not been specified. Physicians and patients should be aware that coadministration of aspirin and caffeine may lead to higher salicylate levels faster.

GENERALLY AVOID: Alcohol may have additive CNS- and/or respiratory-depressant effects with propoxyphene. Misuse of propoxyphene, either alone or in combination with other CNS depressants, has been a major cause of drug-related deaths, particularly in patients with a history of emotional disturbances, suicidal ideation, or alcohol and drug abuse.

MANAGEMENT: The use of alcohol during propoxyphene therapy should be avoided. Patients should be warned not to exceed the recommended dosage of propoxyphene and to avoid activities requiring mental alertness until they know how these agents affect them.

GENERALLY AVOID: The concurrent use of aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs) and ethanol may lead to gastrointestinal (GI) blood loss. The mechanism may be due to a combined local effect as well as inhibition of prostaglandins leading to decreased integrity of the GI lining.

MANAGEMENT: Patients should be counseled on this potential interaction and advised to refrain from alcohol consumption while taking aspirin or NSAIDs.

The effect of grapefruit juice on the pharmacologic activity of caffeine is controversial. One report suggests that grapefruit juice increases the effect of caffeine. The proposed mechanism is inhibition of cytochrome P-450 metabolism of caffeine. However, a well-conducted pharmacokinetic/pharmacodynamic study did not demonstrate this effect. The clinical significance of this potential interaction is unknown.

One study has reported that coadministration of caffeine and aspirin lead to a 25% increase in the rate of appearance and 17% increase in maximum concentration of salicylate in the plasma. A significantly higher area under the plasma concentration time curve of salicylate was also reported when both drugs were administered together. The exact mechanism of this interaction has not been specified. Physicians and patients should be aware that coadministration of aspirin and caffeine may lead to higher salicylate levels faster.