The purpose of this study was to investigate the pharmacokinetics and metabolism of streptochlorin and its derivative 5-hydroxy-2′-isobutyl streptochlorin (HIS) in mice. Plasma concentration of streptochlorin declined rapidly resulting in a high sustemic plasma clearance (CL p ) (5.8 1.7 L/h/kg), a large volume of distribution (V ss ) (1.4 0.9 L/kg) and a short half-life (t 1/2 ) (0.4 0.1 h) after a single intravenous administration (5 mg/kg). Oral bioavailability (F) was 10.3 3.4% after a single oral administration (10 mg/kg). HIS also showed a rapid plasma decline with a high CL p (11.3 8.8 L/h/kg), a high V ss (0.8 1.0 L/kg) and a short t 1/2 (0.070 0.004 h) following intravenous administration. It was not detected in plasma after oral administration. Metabolic stability studies using mouse liver microsomes and S9 fractions predicted a high hepatic clearance for both compounds, consistent with the in vivo data. Metabolite identification studies revealed three metabolic pathways for streptochlorin: monooxygenation, glucuronidation of the indole moiety and oxidative opening of the 4-chlorooxazole ring. HIS was metabolized via monooxygenation of the isobutyl chain and glucuronidation of the indole ring. These results may aid in structural optimization to mitigate the metabolic liability of streptochlorin.Key words streptochlorin; 5-hydroxy-2′-isobutyl streptochlorin; pharmacokinetics; metabolism Terrestrial organisms have been a rich source of natural products for drug discovery and development for thousands of years. Since the mid-twentieth century, studies have shown that marine creatures, especially microbes, have provided many bioactive compounds with novel structures. These include compounds such as ziconotide which is an analgesic agent for severe and chronic pain treatment, trabectedin, an anticancer drug, and eribulin for metastatic breast cancer and liposarcoma treatment.

Nontargeted analysis can be used for the rapid screening and confirmatory analysis of veterinary drugs and their metabolites, which are important for the comprehensive safety evaluation of animal-derived foods. Here, a novel nontargeted screening approach based on liquid chromatography coupled with electrospray ionization–high-resolution mass spectrometry (LC/ESI–HR-MS) was developed to determine erythromycin, clarithromycin, and their metabolites in chicken liver microsomes. Erythromycin and clarithromycin were incubated in vitro in the presence of NADPH for 60 min to generate metabolites in chicken liver microsomes. After the incubation, the supernatant was extracted using ultrasonic shaking, orbital shaking, and centrifugation before analysis using LC/ESI-HR-MS in positive ion mode on an Agilent Eclipse Plus C18 column (100 mm × 2.1 mm; i.d. 3.5 µm) with 0.1 percent formic acid-water and acetonitrile as the mobile phases for gradient elution at 0.4 mL/min. The results show that erythromycin can produce N-desmethyl-erythromycin A in chicken liver microsomes, but clarithromycin cannot produce N-desmethyl-clarithromycin in chicken liver microsomes. The N-desmethyl-erythromycin A and N-desmethyl-clarithromycin were tentatively identified in chicken liver microsomes using the established quick analytic method, which will provide a theoretical foundation for future research on pharmacokinetics and drug elimination in poultry.