Drugs and chemicals
Metoprolol and quinidine are gifted by Orchid Health Care, Chennai, India. Hesperetin was purchased from Sigma Chemical Co. (St. Louis, MO). The required analytical grade solvents for this study were purchased from Finar chemical Ltd, India.
Laboratory animals
Animal experiments were carried out following CPCSEA guidelines at KVSR Siddhartha College of Pharmaceutical Sciences ((993/PO/Re/S/06/CPCSEA). Mahaveer Enterprises in Hyderabad, India, provided male Wistar rats weighing 180–220 g and housed in an animal house. For at least one week before the commencement of the studies, the animals were housed in conventional laboratory settings (12/12 h light/darkness, 22 °C, and 50–60% humidity).
Study protocol
The present investigation was divided into a single-dose pharmacokinetic (SDPK) study and a repeated-dose pharmacokinetic (RDPK) study as previously mentioned in rats [19].
Single-dose pharmacokinetic study
Wistar rats were randomly divided into four groups of six animals each in SDPK. Metoprolol and hesperetin were suspended in 1% SCMC for oral administration.
Group I treated with metoprolol (30 mg/kg).
Group II hesperetin (25 mg/kg) and metoprolol (30 mg/kg).
Group III hesperetin (50 mg/kg) and metoprolol (30 mg/kg).
Group IV hesperetin (100 mg/kg) and metoprolol (30 mg/kg). About 100 μL of blood was collected from the tail vein at various times (0.16, 0.33, 0.5, 1, 2, 4, 6, 8, 12, and 24 h) after administration. The plasma was separated and stored at − 20 °C until analysis.
Repeated-dose pharmacokinetic study
In the RDPK study, rats were treated with the above drugs once a day for 15 days. On the 15th day, 100 μL of blood was drawn from the tail vein at various intervals (0.16, 0.33, 0.5, 1, 2, 4, 6, 8, 12, and 24 h). The plasma was separated and stored at − 20 °C until analysis.
Drug absorption study in vitro
Gut sac preparation
The method that was described previously by [20] for the preparation of everted gut sacs of rat ileum was followed for this study also. Pentobarbital sodium 40 mg/kg was used to anesthetize the rats, and the small intestine was removed [21]. The intestinal digesta was removed and cleaned with ice-cold saline, and the distal ileum (about 15 cm each) was extracted and everted using a glass rod.
Influence of hesperetin on the intestinal absorption
Krebs–Ringer bicarbonate (KRB) buffer containing metoprolol 50 µg/mL was filled in everted sacs. Each sac was placed in a 50-mL Erlenmeyer flask containing 30 mL oxygenated (O2/CO2; 95:5) KRB and incubated in a shaker bath at 37 °C for 60 min. At 10, 20, 30, and 60 min, 1 ml of sample was collected from the outer medium and the 1 ml KRB buffer was replaced. The movement of metoprolol from the serosal to the mucosal side was determined using RP-HPLC after centrifugation at 6000 rpm for 10 min. Each experiment was carried out three times. The same study was repeated with and without of quinidine 50 µg/ml and hesperetin 25, 50, and 100 µg/ml.
Analytical methods
The plasma concentrations of metoprolol were determined using a previously described method [22]. The data were collected and processed using LC solution software (Tokyo, Japan). 0.2% in acetonitrile and water (80:20 v/v) WAS the mobile phase. Before use, the prepared mobile phase was ultrasonically degassed and filtered through a 0.45 µm membrane filter. The injection volume was 20 µL, and the effluent was monitored at a flow rate of 1 ml/min at 222 nm with a UV detector. The chromatographic run duration was 5.0 min, while metoprolol was eluted at 3.35 min (Fig. 1). The experiment was conducted out at room temperature.
Extraction of metoprolol from plasma
The metoprolol was extracted from rat plasma using a liquid–liquid extraction technique [22]. 1 mL tert-butyl methyl ether was added to a 50 µL plasma sample, vortex mixed for 5 min, and then centrifuged at 6000 rpm for 5 min. The supernatant (500 µL) was dried at 40 °C under a moderate nitrogen stream. The dried residue was reconstituted in 50 µL of mobile phase (80:20, v/v), and 20 µL of this was used to run the HPLC.
Calculation of PK parameters
Thermo Kinetica (Version 5.1) was used to carry out a non-compartmental PK analysis.
Data analysis
Graph Pad Prism 5.0 was used to calculate all statistics (San Diego, CA). For multiple comparisons, one-way analysis of variance (ANOVA) and two-way ANOVA were applied to compare the PK parameter values and plasma concentrations, respectively. Significant was defined as a p < 0.05.