Influence of grapefruit juice on the pharmacokinetics of diltiazem in Wistar rats upon single and multiple dosage regimens

Drug efflux by intestinal P-glycoprotein (P-gp) is recognized as a significant biochemical barrier affecting oral absorption for a number of drugs apart from the cytochrome P450 3A enzyme. Various conflicting reports have been published regarding the effects of grapefruit juice (GFJ) on P-gp mediated drug efflux, in which GFJ has been shown to have no effect, as an inhibitor effect or activation of the enzyme. Therefore the present study's objective was to provide clarification of previous findings, adopting a two-way approach, involving both single dose and multiple dosage regimens. Diltiazem (DTZ) 15 mg/kg was administered concomitantly with 5 ml/kg of GFJ to one group (n = 6) of male Wistar rats and another group (n = 6) of animals were provided distilled water with DTZ (the control). A third group of rats was administered GFJ orally for six days and on seventh day GFJ and DTZ were administered concomitantly. The Cmax and AUC of DTZ were decreased significantly in the presence of multiple dose treatment of GFJ. These data were also decreased in presence of simultaneous treatment of single dose GFJ. In vitro metabolism studies and gut sac experiments were conducted in order to understand the mechanism involved. In the liver S9 fraction prepared from the rats treated with multiple doses of GFJ, DTZ metabolism was significantly increased compared to the control. Furthermore, the amount of drug transported from the duodenum was reduced in GFJ treated rats compared to that of the control (1581.0 +/- 7.8 nM vs 1084.81 +/- 6.1 nM, respectively). Grapefruit juice was also reported to inhibit the organic anion transporting polypeptide (OATP), an influx transporter thus reducing the blood levels of OATP substrates which was evident from the in vitro studies. The amount of drug transported from the duodenum was reduced in the presence of pravastatin, a specific OATP inhibitor (1581.0 +/- 7.8 nM to 1265.0 +/- 5.5 nM). Oral single dose exposure to GFJ showed no effect on P-gp, whereas multiple dose administration of GFJ resulted in increased levels of P-gp expression and decreased levels of OATP, thus showing a varied effect on intestinal absorption, and therefore overcoming the inhibition of DTZ metabolism in rats.

Effect of pentoxifylline on the pharmacokinetics of rosiglitazone in Wistar rats

The aim of this study was to investigate the effect of pentoxifylline (PTX), an antiplatelet agent, on the pharmacokinetics of rosiglitazone (RSG) in rats. Pharmacokinetic parameters of RSG were determined in rats after oral administration (3 mg/kg/day) in the presence and absence of PTX (10 mg/kg) 3 times daily. Compared to control animals, rats pretreated with PTX for 7 days had a decrease in RSG peak plasma concentration (Cmax) of 19% with no change in the values of the area under the concentration-time curve (AUC). Alternatively, rats coadministered single-dose PTX did not show any differences from control with regard to RSG Cmax and AUC parameters. The time to peak concentration (tmax) of RSG was significantly increased in rats pretreated with PTX under both single- and multiple-dose conditions, whereas the elimination half-life (t1/2) of RSG was increased only with multiple-dose PTX pretreatment. In conclusion, the presence of PTX was found to cause a slight decrease in the oral exposure of RSG in rats. Concurrent use of PTX with RSG therefore needs to be appropriately evaluated for proper dose adjustments in humans.

Studies on the signal cascade mechanism mediating the cardioprotective actions of bradykinin

The cardioprotective involvement of bradykinin was evaluated using the ACE inhibitor, lisinopril, and the APP inhibitor, 2-mercaptoethanol alone and in combination in rats with experimental myocardial infarction. The signal cascade mechanism mediating the cardioprotective actions of bradykinin was evaluated by administering aspirin and methylene blue prior to lisinopril and 2-mercaptoethanol combined treatment. Myocardial infarction was produced by occlusion of the left anterior descending coronary artery for 30 min followed by 4 h of reperfusion. Infarct size was measured by the TTC stain method. Serum free radical levels were estimated by the method developed by Yagi. A lead II electrocardiogram was monitored throughout the experiment. With the combined inhibition of both the enzymes ACE and APP, a better cardioprotection was observed. The observed cardioprotection was decreased with the prior administration of aspirin and methylene blue. The results suggest the cardioprotective role of bradykinin during experimental myocardial infarction. The results are further suggesting the involvement of both prostaglandins and nitric oxide pathways in the cardioprotective actions of bradykinin.