Drug Interaction Report

GENERALLY AVOID: Coadministration with probenecid may reduce the efficacy of nalidixic acid in the treatment of urinary tract infections while increasing the risk of systemic side effects. The mechanism is probenecid inhibition of the urinary excretion of nalidixic acid via renal tubular secretion, resulting in decreased antibiotic concentration in the urine and increased concentration in the plasma. In a study consisting of two healthy volunteers, coadministration of a single 500 mg dose of nalidixic acid with a single 500 mg dose of probenecid resulted in a threefold increase in peak serum nalidixic acid levels at 8 hours postdose compared to administration of nalidixic acid alone. High plasma levels of nalidixic acid has been associated with metabolic acidosis and may cause central nervous system adverse effects such as drowsiness, weakness, dizziness, vertigo, visual disturbances, convulsions, increased intracranial pressure, and toxic psychosis.

MANAGEMENT: Alternatives to nalidixic acid should preferably be considered for the treatment of urinary tract infections in patients receiving probenecid. Nitrofurantoin and sulfamethoxazole-trimethoprim have not been reported to interact with probenecid and may be suitable choices depending on organism susceptibility.

GENERALLY AVOID: Coadministration with grapefruit juice may increase the serum concentrations of colchicine. Clinical toxicity including myopathy, neuropathy, multiorgan failure, and pancytopenia may occur. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism and P-glycoprotein efflux in the gut wall by certain compounds present in grapefruits. A published case report describes an eight-year-old patient with familial Mediterranean fever who developed acute clinical colchicine intoxication after ingesting approximately one liter of grapefruit juice per day for two months prior to hospital admission while being treated with colchicine 2 mg/day. Her condition progressed to circulatory shock and multiorgan failure, but she recovered with supportive therapy after 24 days in the hospital. In a study of 21 healthy volunteers, administration of 240 mL grapefruit juice twice a day for 4 days was found to have no significant effect on the pharmacokinetics of a single 0.6 mg dose of colchicine. However, significant interactions have been reported with other CYP450 3A4 inhibitors such as clarithromycin, diltiazem, erythromycin, ketoconazole, ritonavir, and verapamil.

MANAGEMENT: Patients treated with colchicine should be advised to avoid the consumption of grapefruit and grapefruit juice, and to contact their physician if they experience symptoms of colchicine toxicity such as abdominal pain, nausea, vomiting, diarrhea, fatigue, myalgia, asthenia, hyporeflexia, paresthesia, and numbness.

ADJUST DOSING INTERVAL: Oral preparations that contain magnesium, aluminum, or calcium may significantly decrease the gastrointestinal absorption of nalidixic acid. Absorption may also be reduced by sucralfate, which contains aluminum, as well as other polyvalent cations such as iron and zinc. The mechanism is chelation of nalidixic acid by polyvalent cations, forming a complex that is poorly absorbed from the gastrointestinal tract.

MANAGEMENT: When coadministration cannot be avoided, nalidixic acid should be dosed at least 2 hours before or 2 hours after polyvalent cation-containing products to minimize the potential for interaction.

MONITOR: Coadministration with certain quinolones may increase the plasma concentrations and pharmacologic effects of caffeine due to inhibition of the CYP450 1A2 metabolism of caffeine. Quinolones that may inhibit CYP450 1A2 include ciprofloxacin, enoxacin, grepafloxacin, nalidixic acid, norfloxacin, pipemidic acid, and pefloxacin (not all commercially available). In healthy volunteers, enoxacin (100 to 400 mg twice daily) increased systemic exposure (AUC) of caffeine by 2- to 5-fold and reduced its clearance by approximately 80%. Pipemidic acid (400 to 800 mg twice daily) increased AUC of caffeine by 2- to 3-fold and reduced its clearance by approximately 60%. Ciprofloxacin (250 to 750 mg twice daily) increased AUC and elimination half-life of caffeine by 50% to over 100%, and reduced its clearance by 30% to 50%. Norfloxacin 400 mg twice daily increased caffeine AUC by 16%, while 800 mg twice daily increased caffeine AUC by 52% and reduced its clearance by 35%. Pefloxacin (400 mg twice daily) has been shown to reduce caffeine clearance by 47%.

MANAGEMENT: Patients using caffeine-containing products should be advised that increased adverse effects such as headache, tremor, restlessness, nervousness, insomnia, tachycardia, and blood pressure increases may occur during coadministration with quinolones that inhibit CYP450 1A2. Caffeine intake should be limited when taking high dosages of these quinolones. If an interaction is suspected, other quinolones such as gatifloxacin, gemifloxacin, levofloxacin, lomefloxacin, moxifloxacin, and ofloxacin may be considered, since they are generally believed to have little or no effect on CYP450 1A2 or have been shown not to interact with caffeine.