UHPLC-MS/MS Determination of Ramelteon in Rat Plasma and Its Altered Pharmacokinetics Under Simulated High Altitude:...

High-altitude hypoxia alters drug-metabolizing enzymes, potentially affecting the pharmacokinetics of sedative-hypnotics. Ramelteon, a melatonin receptor agonist for insomnia, is primarily eliminated via CYP1A2-mediated first-pass metabolism. A sensitive, ICH M10-compliant UHPLC-MS/MS method was developed for ramelteon quantification in rat plasma using only 50 μL of sample and a 3.0-min gradient elution on a CORTECS T3 column. The method was linear over 0.5-250.0 μg/L (LLOQ 0.5 μg/L, R² > 0.995). Intraday and interday accuracy and precision were within ± 15%, and matrix effects were consistent (IS-normalized factor 89.5-95.7%, CV ≤ 7.8%). The method was applied to rats exposed to simulated high altitude (6500 m, 72 h). Compared with the local altitude group (1500 m), high-altitude exposure significantly increased AUC_0-∞ (from 247.9 to 555.8 μg · h/L) and C_max (from 214.0 to 693.2 μg/L), while oral clearance (CL/F) was reduced by more than half (from 21.6 to 9.5 L/h/kg). The terminal half-life remained unchanged. Together with a proportional decrease in volume of distribution, these findings suggest that reduced first-pass metabolism and altered distribution collectively lead to an unchanged half-life. The elevated exposure indicates that a dose reduction may be warranted when ramelteon is used for high-altitude sleep disorders.