Drug Interactions between nalidixic acid and probenecid

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.