Methylprednisolone drug interactions

Since erythromycin complexes and inactivates drug oxidizing systems such as cytochrome P-450, it has the potential to alter the metabolism of other drugs. The metabolism and excretion of theophylline, warfarin, carbamaz-epine, and methylprednisolone are inhibited by erythromycin [283-286]. As a potent antibiotic, it can also affect metabolism by gut micro-organisms of drugs such as digoxin. At least some of the newer derivatives may cause fewer drug interactions and thus may be better tolerated if co-administered with medications for other illnesses [287-289]. 

A retrospective analysis of 3995 patients treated with azithromycin did not show any pharmacokinetic interactions in patients who were also taking various other drugs, including methylprednisolone (1,45). 

Co-administration of diltiazem with methylprednisolone increased plasma concentrations of methylprednisolone and its adrenal suppressant effects in nine healthy volunteers (29). Care should be taken when these two drugs are co-administered for a long period, even if the clinical relevance of this pharmacokinetic interaction still needs to be evaluated. 

After absorption, aprepitant is bound extensively to plasma proteins (>95%) it is extensively metabolized, primarily by hepatic CYP3A4, and is excreted in the stool its t is 9—13 hours. Aprepitant has the potential to interact with other substrates of CYP3A4, requiring adjustment of other drugs, including dexamethasone, methylprednisolone (whose dose may need to be reduced by 50%), and warfarin. Aprepitant is contraindicated in patients on cisapride (see above) or pimozide, in whom life-threatening ventricular tachyarrthmias has been reported. 

One UK manufacturer warns that drugs affecting electrolyte balance, such as corticosteroids, may alter lithium excretion and should therefore be avoided, but other manufaeturers do not appear to mention this potential interaction. An early study in rats reported increased lithium clearance with methylprednisolone. The available evidence is insufficient to recommend routine monitoring. However, it may be prudent to consider monitoring lithium effects in patients with renal impairment, or other conditions pre-disposing to lithium toxicity, taking levels if early symptoms suggest a potential problem. 

Ketoconazole can lead to reduced clearance of CS and a mild increase in levels (62,66,67). In addition, patients with significant liver or renal failure, age older than 65 years (68), or women receiving exogenous estrogen have increased unbound concentrations of prednisolone (69). Conversely, hyperthyroid patients have lower levels (70). There is one case report of an increased prothrombin time occurring in a patient on a stable dose of coumadin who subsequently received prednisone (71). This interaction seems rare and may be overshadowed by other drugs and events in the patient. Troleandomycin, a macrolide, has been shown to reduce clearance of methylprednisolone, but not prednisone (72). 

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