Affected cytochrome P450 (CYP450) isoenzymes and drug transporters: Cationic proximal renal tubular secretion transport
Cationic drugs that are eliminated by renal tubular secretion theoretically have the potential for interaction with metformin by competing for common renal tubular transport systems. An interaction between metformin and oral cimetidine has been observed, with a 60% increase in peak metformin plasma and whole blood concentrations and a 40% increase in plasma and whole blood metformin AUC, but no change in metformin elimination half-life. Interactions remain theoretical (except for cimetidine), but careful patient monitoring and dose adjustment of metformin and/or the potentially interfering drug is recommended with concurrent use of cationic medications that are excreted via the proximal renal tubular secretory system.
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Route-Specific Pharmacokinetics
Oral Route
• Metformin Solution (Riomet): The rate and extent of absorption is comparable to regular-release tablets. Peak plasma concentrations are achieved at approximately 2.2 hours. The extent of absorption from the oral solution increases when given with food, and while there is a negligible decrease in Cmax, the Tmax increases from 2.5 to approximately 4 hours. Studies indicate that dose proportionality is lacking with increasing doses due to a decrease in the absorption at higher doses.
• Metformin Regular-release tablets: The bioavailability of 500 mg tablets is 50—60% with peak plasma concentrations achieved at approximately 2.5 hours. Food decreases the extent and slightly delays the absorption. Studies indicate that dose proportionality is lacking with increasing doses due to a decrease in the oral absorption at higher doses.
• Metformin Extended-release tablets (Glucophage XR): Using a dual hydrophilic polymer system, metformin is released slowly by diffusion through a gel matrix, allowing for once-daily administration. Peak plasma concentrations are achieved at approximately 7 hours (range, 4—8 hours). While peak plasma concentrations are approximately 20% lower for extended-release tablets versus the same dose of regular-release tablets, the extent of absorption from 2000 mg once-daily of extended- release tablets is similar to the same total daily dose administered as 1000 mg twice-daily of regular-release tablets. Food increases the extent of