Aristolochic acid I (AAI) is widely recognized as a genotoxic and cytotoxic compound. To rationally propose detoxification strategies, it is essential to fully elucidate the in vivo disposition of AAI. Nevertheless, the toxicokinetic characteristics of AAI, particularly the possible involvement of the recirculation process, remain incompletely understood. In this research, toxicokinetics of AAI was studied following a single oral administration of AAI in Fisher rats (10, 30 and 100 mg/kg, n = 6). A method of ultra-performance liquid chromatography coupled with triple quadrupole tandem mass spectrometry (UPLC-QQQ-MS/MS) was developed to achieve the quantitation of AAI in rat plasma. Plasma concentration–time profiles and kinetic parameters were analyzed to characterize the toxicokinetic behavior of AAI. A secondary elevation was observed in the plasma concentration–time profiles of AAI, suggesting the existence of AAI reabsorption. The non-linear elimination kinetics of AAI might be attributed to capacity-limited excretion via bile. Additionally, the biliary excretion of AAI and several key metabolites was also explored through qualitative analysis of bile samples. For the first time, AAI-O-glucuronide was identified in bile, providing further support for enterohepatic recirculation (EHR)-mediated reabsorption of AAI. In conclusion, these findings provided solid evidence for EHR-mediated reabsorption of AAI in rats. The recirculation process might be a key mechanism responsible for the prolonged retention of AAI. In the future, detoxification strategies targeting the EHR process could be effective approaches to minimize the systemic exposure of AAI.

Background/Objectives: As a monocyclic β-lactam antibiotic, aztreonam has regained attention recently because combining it with β-lactamase inhibitors helps fight drug-resistant bacteria. This study aimed to systematically characterize the plasma and tissue concentration-time profiles of aztreonam in rats, mice, dogs, monkeys, and humans by developing a multi-species, physiologically based pharmacokinetic (PBPK) model. Methods: A rat PBPK model was optimized and validated using plasma concentration-time curves determined by liquid chromatography–tandem mass spectrometry (LC-MS/MS) following intravenous administration, with reliability confirmed through another dose experiment. The rat model characteristics, modeling experience, ADMET Predictor (11.0) software prediction results, and allometric scaling were used to extrapolate to mouse, human, dog, and monkey models. The tissue-to-plasma partition coefficients (K_p values) were predicted using GastroPlus (9.0) software, and the sensitivity analyses of key parameters were evaluated. Finally, the cross-species validation was performed using the average fold error (AFE) and absolute relative error (ARE). Results: The cross-species validation showed that the model predictions were highly consistent with the experimental data (AFE < 2, ARE < 30%), but the deviation of the volume of distribution (V_ss) in dogs and monkeys suggested the need to supplement the species-specific parameters to optimize the prediction accuracy. The K_p values revealed a high distribution of aztreonam in the kidneys (K_p = 2.0–3.0), which was consistent with its clearance mechanism dominated by renal excretion. Conclusions: The PBPK model developed in this study can be used to predict aztreonam pharmacokinetics across species, elucidating its renal-targeted distribution and providing key theoretical support for the clinical dose optimization of aztreonam, the assessment of target tissue exposure in drug-resistant bacterial infections, and the development of combination therapy strategies.