Drug Interaction Report

MONITOR: The concomitant use of corticosteroids and agents that deplete potassium (e.g., potassium-wasting diuretics, amphotericin B, cation exchange resins) may result in increased risk of hypokalemia. Corticosteroids can produce hypokalemia and other electrolyte disturbances via mineralocorticoid effects, the degree of which varies with the agent (from most to least potent: fludrocortisone - cortisone/hydrocortisone - prednisolone/prednisone - other glucocorticoids) and route of administration (i.e. systemic vs. local). However, large systemic doses of any corticosteroid can demonstrate these effects, particularly if given for longer than brief periods. When used pharmacologically, adrenocorticotropic agents such as corticotropin have similar mineralocorticoid activities as cortisone and hydrocortisone.

MANAGEMENT: Patients receiving potassium-depleting agents with corticosteroids should be monitored closely for development of hypokalemia, particularly if fludrocortisone or large doses of another corticosteroid or adrenocorticotropic agent is given. Potassium supplementation may be necessary. Patients should be advised to notify their physician if they experience signs of electrolyte disturbances such as weakness, lethargy, and muscle pains or cramps.

MONITOR: Forced diuresis during administration of radiocontrast agents may increase the risk of renal impairment in patients who are at high risk for contrast-induced nephropathy. Patients considered at high risk include those with diabetes (especially diabetic nephropathy), preexisting renal insufficiency (serum creatinine >1.5 mg/dL or GFR <60 mL/min/1.73 m2), volume depletion, advanced age (>70 years), congestive heart failure, and/or concurrent use of nephrotoxic drugs (e.g., NSAIDs). Diuretics have been studied for use in the prevention of contrast-induced nephropathy because investigators theorized that they may reduce medullary ischemia by decreasing oxygen demands. In published studies, however, maintenance intravenous fluids plus forced diuresis with furosemide, mannitol, or a combination of both given at the time of radiocontrast exposure generally produced similar or even higher rates of nephropathy compared with intravenous fluids alone. Meta-analyses of published data suggest that furosemide-based interventions significantly increase the risk of contrast-induced nephropathy compared with hydration alone, and one study found that hospitalization for all patients who developed contrast-induced nephropathy was increased by 4 days in those who received concomitant diuretic therapy. Contrast-induced nephropathy is most commonly defined as an increase in serum creatinine >=0.5 mg/dL or 25% from baseline within 48 to 72 hours of intravascular contrast administration in the absence of alternative etiologies, although nephropathy may occur up to a week after contrast exposure. While the condition is usually transient and asymptomatic, it can be associated with increased risk of renal failure, dialysis, prolonged hospitalization, significant long-term morbidity, and mortality.

MANAGEMENT: Whenever possible, alternative imaging techniques should be considered in patients who are at high risk for contrast-induced nephropathy. Otherwise, some experts recommend discontinuing diuretics 1 to 2 days before administration of contrast media, depending on the clinical feasibility of doing so. The smallest effective dose of a nonionic, hypo- or iso-osmolar contrast medium (e.g., iohexol, iodixanol, iopamidol) should be used, since the risk of nephropathy is increased with increasing contrast dose and/or osmolarity. Repeat procedures with contrast media, if necessary, should not occur until at least 72 hours after the previous contrast exposure and renal function has fully recovered. Although it is not necessary to measure the serum creatinine levels of every patient before contrast administration, measurements should generally be made in patients receiving contrast agent by intraarterial administration (which is associated with increased risk of nephropathy relative to intravenous administration) and patients with a history of kidney disease, proteinuria, kidney surgery, diabetes, hypertension, gout, or other risk factors for nephropathy. Creatinine measurements should be continued for 24 to 48 hours after administration of contrast medium. It is important that patients be adequately hydrated with either saline or sodium bicarbonate.

MONITOR: Grapefruit and grapefruit juice may increase the plasma concentrations of lidocaine, which is primarily metabolized by the CYP450 3A4 and 1A2 isoenzymes to active metabolites (monoethylglycinexylidide (MEGX) and glycinexylidide). The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit. Inhibition of hepatic CYP450 3A4 may also contribute. The interaction has not been studied with grapefruit juice but has been reported with oral and/or intravenous lidocaine and potent CYP450 3A4 inhibitor, itraconazole, as well as moderate CYP450 3A4 inhibitor, erythromycin. A pharmacokinetic study of 9 healthy volunteers showed that the administration of lidocaine oral (1 mg/kg single dose) with itraconazole (200 mg daily) increased lidocaine systemic exposure (AUC) and peak plasma concentration (Cmax) by 75% and 55%, respectively. However, no changes were observed in the pharmacokinetics of the active metabolite MEGX. In the same study, when the moderate CYP450 3A4 inhibitor erythromycin (500 mg three times a day) was administered, lidocaine AUC and Cmax increased by 60% and 40%, respectively. By contrast, when intravenous lidocaine (1.5 mg/kg infusion over 60 minutes) was administered on the fourth day of treatment with itraconazole (200 mg once a day) no changes in lidocaine AUC or Cmax were observed. However, when lidocaine (1.5 mg/kg infusion over 60 minutes) was coadministered with erythromycin (500 mg three times a day) in the same study, the AUC and Cmax of the active metabolite MEGX significantly increased by 45-60% and 40%, respectively. The observed differences between oral and intravenous lidocaine when coadministered with CYP450 3A4 inhibitors may be attributed to inhibition of CYP450 3A4 in both the gastrointestinal tract and liver affecting oral lidocaine to a greater extent than intravenous lidocaine. In general, the effects of grapefruit products are concentration-, dose- and preparation-dependent, and can vary widely among brands. Certain preparations of grapefruit (e.g., high dose, double strength) have sometimes demonstrated potent inhibition of CYP450 3A4, while other preparations (e.g., low dose, single strength) have typically demonstrated moderate inhibition. While the clinical significance of this interaction is unknown, increased exposure to lidocaine may lead to serious and/or life-threatening reactions including respiratory depression, convulsions, bradycardia, hypotension, arrhythmias, and cardiovascular collapse.

MONITOR: Certain foods and behaviors that induce CYP450 1A2 may reduce the plasma concentrations of lidocaine. The proposed mechanism is induction of hepatic CYP450 1A2, one of the isoenzymes responsible for the metabolic clearance of lidocaine. Cigarette smoking is known to be a CYP450 1A2 inducer. In one pharmacokinetic study of 4 smokers and 5 non-smokers who received 2 doses of lidocaine (100 mg IV followed by 100 mg orally after a 2-day washout period), the smokers' systemic exposure (AUC) of oral lidocaine was 68% lower than non-smokers. The AUC of IV lidocaine was only 9% lower in smokers compared with non-smokers. Other CYP450 1A2 inducers include cruciferous vegetables (e.g., broccoli, brussels sprouts) and char-grilled meat. Therefore, eating large or variable amounts of these foods could also reduce lidocaine exposure. The clinical impact of smoking and/or the ingestion of foods that induce CYP450 1A2 on lidocaine have not been studied, however, a loss of efficacy may occur.

MANAGEMENT: Caution is recommended if lidocaine is to be used in combination with grapefruit and grapefruit juice. Monitoring for lidocaine toxicity and plasma lidocaine levels may also be advised, and the lidocaine dosage adjusted as necessary. Patients who smoke and/or consume cruciferous vegetables may be monitored for reduced lidocaine efficacy.

MONITOR: Smoking cessation may lead to elevated plasma concentrations and enhanced pharmacologic effects of drugs that are substrates of CYP450 1A2 (and possibly CYP450 1A1) and/or certain drugs with a narrow therapeutic index (e.g., flecainide, pentazocine). One proposed mechanism is related to the loss of CYP450 1A2 and 1A1 induction by polycyclic aromatic hydrocarbons in tobacco smoke; when smoking cessation agents are initiated and smoking stops, the metabolism of certain drugs may decrease leading to increased plasma concentrations. The mechanism by which smoking cessation affects narrow therapeutic index drugs that are not known substrates of CYP450 1A2 or 1A1 is unknown. The clinical significance of this interaction is unknown as clinical data are lacking.

MANAGEMENT: Until more information is available, caution is advisable if smoking cessation agents are used concomitantly with drugs that are substrates of CYP450 1A2 or 1A1 and/or those with a narrow therapeutic range. Patients receiving smoking cessation agents may require periodic dose adjustments and closer clinical and laboratory monitoring of medications that are substrates of CYP450 1A2 or 1A1.