Drug Interaction Report

Probenecid has been shown to interfere with the urinary excretion of certain quinolone antibiotics, resulting in increased plasma quinolone concentrations in some cases. The clinical relevance of this interaction is unknown but may be greater for quinolones that undergo significant tubular secretion via the renal anion transporter system such as cinoxacin, ciprofloxacin, enoxacin, gemifloxacin, nalidixic acid, and norfloxacin. In one study, serum levels of cinoxacin reportedly doubled in the presence of probenecid, while urinary recovery in a 7-hour period was reduced by 32%. In another study (n=12), mean systemic exposure (AUC) of a 200 mg IV dose of ciprofloxacin increased by 75% and renal clearance decreased by 64% following pretreatment with multiple-dose probenecid. Likewise, probenecid has been found to reduce renal clearance of enoxacin and gemifloxacin by approximately 50%. Another study reported a threefold increase in the peak serum nalidixic acid level of two volunteers 8 hours following coadministration of a 500 mg dose of nalidixic acid and a 500 mg dose of probenecid. Also, a 1 gram dose of probenecid reduced the 12-hour urinary recovery of a single 200 mg dose of norfloxacin by about one-half in five study subjects, although serum concentrations were not significantly changed. In contrast, probenecid appears to have no clinically significant effect on the pharmacokinetics of moxifloxacin, ofloxacin, or sparfloxacin. In general, no precautions appear to be necessary during coadministration of most quinolones and probenecid. However, in the treatment of urinary tract infections, clinicians should consider the possibility of reduced antibacterial efficacy due to decreased quinolone excretion into the urine.

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: Food may reduce the oral absorption and bioavailability of levofloxacin. According to the drug product labeling, administration of levofloxacin 500 mg with food prolonged the time to peak concentration by 1 hour and decreased the Cmax decreased by 25% following administration of the oral solution and by 14% following administration of the oral tablet.

MANAGEMENT: To ensure maximal and consistent oral absorption, levofloxacin oral solution should be taken at least one hour before or two hours after meals. For administration of the oral solution with continuous enteral nutrition, some experts recommend that the tube feeding should be interrupted for one hour before and two hours after the dose of levofloxacin. The oral tablets may be taken without regard to food.