Materials and Reagents
Vincristine and posaconazole powders (purity >99%) were purchased from Selleckchem (Houston, TX, USA). Itraconazole was a kind gift from Nifty Labs Pvt Ltd (Hyderabad, India). High-performance liquid chromatography grade methanol and acetonitrile (Fisher Scientific UK Limited, Loughborough, UK), analytical grade potassium dihydrogen orthophosphate (Riedel-de-Haën, Germany), and high-purity distilled water were used. Vinracine® intravenous injection vials labeled to contain 1 mg/mL of VCR sulfate (EIMC United Pharmaceuticals, Cairo, Egypt), Aerraine® isoflurane USP, normal saline, and heparin sodium for injection 5000 U/mL were purchased from the Egyptian local market. Noxafil® oral suspension labeled to contain 40 mg/mL of PSZ (Pathe 103 on Inc., ON, Canada) was purchased from Schering-Plough S.A., Alexandria, Egypt.
Animals and Pre-Experimental Procedures
Experimental protocols involving animals were approved by the Ethics Committee of the Faculty of Pharmacy, Alexandria University. Sprague–Dawley rats were used in the studies. All rats weighed between 150 and 250 g and were housed in temperature-controlled rooms with 12 h of light per day. The animals were fed a standard commercial rat chow containing 4.5% fat (Tanta Oil and Soap JSC, Tanta, Egypt). Free access to food and water was permitted prior to experimentation. Rats were allocated into normolipidemic (NL), intermediate hyperlipidemic (IHL), or extreme hyperlipidemic (HL) groups. Hyperlipidemia was induced by a single intraperitoneal injection of 1 g/kg of P407 (0.13 g/mL solution in normal saline). To ensure the proper injection of P407, the animals were lightly anesthetized using isoflurane, and then allowed to recover. The rats were prepared for intravenous dosing by jugular vein cannulation. For each rat, a single jugular vein was catheterized with Silastic Laboratory Tubing (Dow Corning Corporation, Midland, MI, USA) under isoflurane/O2 anesthesia. The cannula was filled with locking solution containing 100 U/mL of heparin in 0.9% saline. After implantation, the rats were transferred to their regular holding cages and allowed free access to water, but food was withheld overnight so that the drug would be administered in the fasted state. The next morning, NL rats were transferred to the metabolic cages and dosed with PSZ orally; 4 h later, the rats were given VCR intravenously via the cannulated jugular vein. As for the HL and IHL groups, the dosing was started 36 and 72 h after the P407 injection, respectively [6, 22].
Drug Administration and Sample Collection
For the pharmacokinetic study, the rat groups (n = 6–10 each) received PSZ 40 mg/kg by oral gavage. Noxafil® oral suspension (40 mg/mL) was used for the administration of PSZ. Four hours later, the rat groups received 0.15 mg/kg VCR intravenously via the jugular vein cannula, immediately followed by an injection of normal saline. The intravenous solution was prepared by diluting the Vinracine® (1 mg/mL) injection vials with normal saline to reach a final concentration of 0.1 mg/mL. At the time of the first sample withdrawal post-intravenous dosing, the first 0.2 mL of blood was discarded. Food was provided to animals 5 h after the dose administration.
Serial blood samples (0.15–0.25 mL) were collected using the jugular vein cannula at 0.5, 1, 1.5, 2, 4.083, 4.25, 4.5, 5, 6, 8, 10, 24, 48, 72, and 96 h after the PSZ oral dose for each group. Plasma was separated by centrifugation of the blood at 4000g for 5 min and transferred into clean glass test tubes. The samples were kept at −20 °C until assayed.
Protein-Binding Study
The unbound fractions of PSZ and VCR in plasma were determined using ultrafiltration (Centrifree®; Amicon, Beverly, MA, USA). Blood was obtained from NL, IHL, and HL rats by exsanguination via cardiac puncture. For IHL and HL rats, blood was collected 72 and 36 h after intraperitoneal doses of P407, respectively. The blood was centrifuged at 2500g for 10 min. The NL, IHL, and HL rat plasma were spiked with PSZ and VCR methanolic solution to allow for final concentrations of 3.5 and 1 mg/L, respectively. The volume of methanol added to each tube did not exceed 0.05% (v/v). Tubes were incubated for 1 h in a 37 °C water bath shaker [15, 20]. One milliliter of each tube was transferred to an ultrafiltration device (n = 4) (Centrifree®; Millipore, Carrigtwohill, Ireland), which was then placed in a fixed-angle centrifuge rotor and spun at 2000g for 10 min at 37 °C. The samples were then analyzed for PSZ and VCR concentrations. In HL and IHL protein-binding tests, the ultrafiltration devices were pre-incubated with 5% Triton® (X-100) for 12 h to overcome the binding of drugs and lipids to the filter. The devices were then used after being rinsed several times with double-distilled water [16, 22].
Assay
A validated high-performance liquid chromatography assay was used for the measurement of PSZ and VCR concentrations, with itraconazole as an internal standard in plasma, Sorenson sodium phosphate buffer (pH 7.4), liver tissue and lung tissues [21]. The validated lower limit of quantitation was 50 ng/mL based on 0.2 mL of plasma, tissue homogenate, or buffer. For standard curve construction, drug-free plasma, tissue homogenate, and Sorenson sodium phosphate buffer (pH 7.4) were used and spiked with appropriate amounts of PSZ and VCR. The calibration curves relating the peak area ratio to the expected concentration were highly linear from 50 to 5000 ng/mL of PSZ and VCR in NL, IHL, HL plasma, tissue homogenate and buffer (r
2 > 0.997).
Data and Statistical Analysis
Non-compartmental methods were applied to calculate the pharmacokinetic parameters. The elimination rate constant (λz) was calculated by subjecting the plasma concentrations in the terminal phase to linear regression analysis. The terminal phase t½ was calculated using the equation t½ = 0.693/λz. The area under the plasma concentration–time curve from time zero to infinity (AUC0–∞) was calculated using the combined log-linear trapezoidal rule from time zero post-dose to the time of the last measured concentration, plus the quotient of the last measured concentration divided by λz. The concentration at time zero after intravenous dosing was estimated by back extrapolation of the log-linear regression line using the first three measured plasma concentrations. The clearance was calculated as the quotient of dose to AUC0–∞ and the V
d as the quotient of clearance by λz. The V
d of the central compartment is calculated as the quotient of dose by the concentration at time zero. The PSZ maximum plasma concentration and the time at which it occurred were determined by visual examination of the data.
The plasma-unbound fraction was determined by dividing the PSZ or VCR concentration in the filtrate by that measured in the pre-filtered plasma. All compiled data were reported as mean ± standard deviation, unless otherwise indicated. Significance of comparisons of means for the interactions of lipoprotein status, VCR pharmacokinetics, PSZ pharmacokinetics, and protein binding were assessed by two-way analysis of variance (ANOVA), followed by Tukey’s multiple comparison post-hoc test (Prism, Irvine, CA, USA). In all cases, the level of significance was set at p = 0.05.