静脈内持続注入から経口投与への移行：換算方法

Extended-release formulations enable safer, more convenient patient transitions from IV to oral therapy while maintaining therapeutic drug levels. This shift improves quality of life by allowing self-administration at home rather than requiring continuous hospital infusions. The conversion preserves efficacy and prevents adverse effects through careful pharmacokinetic management.

Discontinuing IV infusion causes plasma drug concentration to decline following first-order elimination kinetics. Oral dosing must be carefully calculated to counteract this drop and establish a new steady state. Extended-release formulations help by allowing steady drug release over time, mimicking constant levels achieved with IV infusions.

The first method matches the steady-state plasma concentration achieved with IV infusion to the desired average concentration with oral dosing. This calculation incorporates the drug's salt form, fractional bioavailability, clearance, and dosing rate. For example, an IV aminophylline infusion of 34 mg/hour converts to approximately 700 mg daily oral theophylline dose.

The second method directly equates the IV infusion rate to the desired oral dosing rate by multiplying the total daily IV dose by the drug's salt form. While simpler to apply, this approach does not account for differences in bioavailability or dosing schedule effects, potentially leading to inaccuracies in maintaining therapeutic drug levels compared to the first method.

Critical parameters include the drug's salt factor, fractional bioavailability, clearance rate, and desired dosing frequency. For theophylline with 100% bioavailability, a 28.9 mg/hour IV infusion rate yields approximately 700 mg daily oral dose, often divided into two 350 mg controlled-release doses administered every 12 hours to maintain therapeutic levels.

Extended-release formulations enable steady drug release over extended periods, mimicking constant plasma levels achieved with IV infusions. This sustained delivery maintains therapeutic drug concentrations throughout the dosing interval, reducing fluctuations and improving patient compliance compared to immediate-release oral medications requiring frequent dosing.

Bioavailability determines how much oral drug reaches systemic circulation compared to IV administration. The first conversion method accounts for bioavailability differences through the fractional bioavailability factor, ensuring equivalent therapeutic exposure. Ignoring bioavailability, as in the simpler second method, may result in subtherapeutic or toxic drug levels.