Drug Interactions between dexamethasone / moxifloxacin and thiotepa

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.

MONITOR: Coadministration with CYP450 3A4 and/or 2B6 inducers may decrease plasma concentrations of thiotepa and increase concentrations of its active metabolite triethylenephosphoramide (TEPA). Thiotepa is a prodrug that is primarily converted to TEPA by these isoenzymes. In a study involving a 42-year-old male with relapsing germ-cell cancer, the pharmacokinetics of thiotepa and its active metabolite (TEPA) were assessed during two high-dose chemotherapy courses (cyclophosphamide 1500 mg/m2/day, thiotepa 120 mg/m2/day, and carboplatin), with phenytoin initiated five days before the second course for seizure management. In the second course, TEPA exposure increased by 115% and thiotepa exposure decreased by 29%, resulting in a thiotepa dose reduction of nearly 40% on day 3 due to the increased risk of toxicity from higher TEPA exposure. Clinical data for thiotepa use in combination with other less potent CYP450 3A4 inducers or with CYP450 2B6 inducers are not available.

MANAGEMENT: Caution and closer monitoring for adverse effects is advised when thiotepa is used concurrently with CYP450 3A4 and/or 2B6 inducers. Patients should be more closely monitored for thiotepa-related toxicities such as myelosuppression, cutaneous toxicity, and neurotoxicity. A dosage reduction of thiotepa may be necessary. Pretreatment and subsequent blood counts may be used to guide dose adjustments in accordance with product labeling.

Limited data suggest that chemotherapy with antineoplastic agents may reduce the plasma concentrations of oral quinolone antibiotics. The proposed mechanism is decreased quinolone absorption secondary to alteration of intestinal mucosa by cancer chemotherapy. In six patients with newly diagnosed hematologic malignancy, treatment with various antineoplastic agents (cyclophosphamide, cytarabine, daunorubicin, doxorubicin, mitoxantrone, prednisolone, vincristine) decreased the mean peak serum concentration (Cmax) and area under the concentration-time curve (AUC 0 to 4 hours) of ciprofloxacin by approximately 46% each. Data are not available for other quinolone antibiotics.