Drug Interactions between ampicillin / probenecid and dexamethasone / ketorolac / moxifloxacin

CONTRAINDICATED: The concomitant administration of probenecid and ketorolac may cause ketorolac toxicity. Probenecid decreases the plasma clearance of ketorolac, resulting in a three-fold increase in ketorolac AUC and a two-fold increase in its half-life. Renal failure, GI ulcerations or perforation, and hemorrhage may occur.

MANAGEMENT: The manufacturer considers the concurrent use of probenecid and ketorolac contraindicated.

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MONITOR CLOSELY: Concomitant administration of corticosteroids may potentiate the risk of tendinitis and tendon rupture associated with fluoroquinolone treatment. The mechanism is unknown. Tendinitis and tendon rupture have most frequently involved the Achilles tendon, although cases involving the rotator cuff (the shoulder), the hand, the biceps, and the thumb have also been reported. Some have required surgical repair or resulted in prolonged disability. Tendon rupture can occur during or up to several months after completion of fluoroquinolone therapy.

MANAGEMENT: Caution is recommended if fluoroquinolones are prescribed in combination with corticosteroids, particularly in patients with other concomitant risk factors (e.g., age over 60 years; recipient of kidney, heart, and/or lung transplant). Patients should be advised to stop taking the fluoroquinolone, avoid exercise and use of the affected area, and promptly contact their physician if they experience pain, swelling, or inflammation of a tendon. In general, fluoroquinolones should only be used to treat conditions that are proven or strongly suspected to be caused by bacteria and only if the benefits outweigh the risks.

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MONITOR: The combined use of corticosteroids and nonsteroidal anti-inflammatory drugs (NSAIDs) may increase the potential for serious gastrointestinal (GI) toxicity, including inflammation, bleeding, ulceration, and perforation. In a large, case-control study of elderly patients, those who used corticosteroids and NSAIDs concurrently had an estimated relative risk (RR) for peptic ulcer disease and GI hemorrhage of 14.6 compared to those who used neither. Corticosteroid use was associated with a doubling of the risk (estimated RR = 2.0), but the risk was confined to those who also used NSAIDs. It is possible that both categories of agents are ulcerogenic and have additive effects on the GI mucosa during coadministration. Some investigators have also suggested that the primary effect of corticosteroids in this interaction is to delay healing of erosions caused by NSAIDs rather than cause de novo ulcerations.

MANAGEMENT: Caution is advised if corticosteroids and NSAIDs are used together, especially in patients with a prior history of peptic ulcer disease or GI bleeding and in elderly and debilitated patients. During concomitant therapy, patients should be advised to take the medications with food and to immediately report signs and symptoms of GI ulceration and bleeding such as severe abdominal pain, dizziness, lightheadedness, and the appearance of black, tarry stools. The selective use of prophylactic anti-ulcer therapy (e.g., antacids, H2-antagonists) may be considered.

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MONITOR: Coadministration with nonsteroidal anti-inflammatory drugs (NSAIDs) may potentiate the risk of central nervous system toxicity sometimes associated with fluoroquinolone use. The interaction has been reported most often with enoxacin. It may occur with other fluoroquinolones as well, but is poorly documented. The exact mechanism of interaction is unknown. Some investigators suggest that the piperazine ring of fluoroquinolones may inhibit the binding of gamma-aminobutyric acid (GABA) to brain receptors and that NSAIDs may synergistically add to this effect. Patients with a history of seizures may be at greater risk.

MANAGEMENT: Clinical monitoring for signs of CNS stimulation such as tremors, involuntary muscle movements, hallucinations, or seizures is recommended if fluoroquinolone antibiotics are prescribed in combination with NSAIDs.

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Probenecid may increase the plasma concentrations and half-lives of penicillins. The mechanism is competitive inhibition by probenecid of the renal tubular secretion of penicillins. While this interaction is often exploited to enhance the antibacterial effect of penicillins, toxicity may occur and should be considered if high penicillin dosages are administered intravenously.

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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.

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