Development of a novel intestinal and vascular access port (IVAP) rabbit model to study regiospecific oral absorption pharmacokinetics.

BACKGROUND AND PURPOSE: The limited availability and cost of many drugs prohibits routine use of the previously developed intestinal and vascular access port (IVAP) canine model by our group. A lower animal species model such as the rabbit is suitable for implanting intestinal and vascular access ports for investigating regiospecific intestinal absorption and hepatic elimination while requiring significantly lower doses of drugs. In addition, expression of certain cytochrome P450 enzymes and apical secretory and absorptive transporters in rabbit intestine is similar to that in humans making the rabbit a suitable model. METHODS: Individual 5-F Silastic catheters were placed in the proximal or distal portion of the small intestine or the colon of subject animals, while a 5-F Heparin Coated Polyurethane (HCP) catheter was implanted in the portal vein of each subject. The catheters were tunneled out of the abdomen and attached to separate subcutaneous access ports along the spine. The animals were allowed a two-week minimum recovery period prior to being used in pharmacokinetic studies. RESULTS AND DISCUSSION: After some initial difficulties, rabbits with IVAP implants proved to be an efficient and dependable model for investigating intestinal and hepatic extraction of drugs. Fluoroscopic visualization of intestinal and portal venous catheters indicated that surgically implanted catheters did not interfere with gastrointestinal motility or blood flow into the liver, respectively. Acute pH studies in the proximal portion of the small intestine were consistent with normal GI motility patterns.

Differentiation of gut and hepatic first-pass effect of drugs: 1. Studies of verapamil in ported dogs.

PURPOSE: To investigate the relative contributions of the gut and liver to the first-pass loss of verapamil (VL) using an in vivo intestinal-vascular access port (IVAP) dog model. METHODS: Basic pharmacokinetics of VL were determined after intravenous (IV: 0.5 mg/kg), portal venous (PV: 2 mg/kg), and duodenal (ID: 2 mg/kg) administration in IVAP dogs. Serial blood samples were collected for 8 h after dosing, and plasma was analyzed for unchanged drug by a high-performance liquid chromatography-fluorescence method. Extraction ratios in the liver and intestinal tract were determined from the area under the concentration-time curves for ID, PV, and IV administration. The functional role of CYP450 or secretory transporters such as P-gp on the gut and liver first-pass loss of VL was further studied using ritonavir, a known substrate or inhibitor of these processes. RESULTS: The liver had a high intrinsic capacity for clearing VL because the absolute bioavailability (BA) of VL was 21.7% after PV administration. The BA of VL after ID administration was 23.5%; therefore, intestinal absorption was complete and intestinal extraction was negligible (ER(GI) approximately 0). The BA of VL increased from 23.5% to 66.2% in the presence of ritonavir primarily due to a reduction in hepatic extraction. CONCLUSIONS: Although the liver had a high intrinsic capacity for extracting VL, the contribution of gut to the first-pass loss of VL was negligible. Because of the additive effects of intestinal CYP3A-mediated metabolism and secretory transport, a significant gut first-pass effect was expected, but not observed in dogs. These studies demonstrate the utility of the in vivo IVAP dog model for evaluating the relative contribution of the gut and liver to the first-pass loss of drugs and for characterizing the functional role that CYP450 metabolism and/or secretory transporters play in drug-drug interactions and reduced oral bioavailability.

Active efflux kinetics of etoposide from rabbit small intestine and colon.

The aim of the present study was to investigate the directional transport kinetics of etoposide in rabbit intestinal tissues using side-by-side diffusion chambers. Etoposide is a routinely used mixed-mechanism 'efflux' inhibitor; however, its absorptive and secretory transport kinetics in rabbit intestinal tissues, a commonly used animal model, have not yet been reported. Kinetic studies revealed that the apical (AP) to basolateral (BL) (i.e. absorptive) transport of etoposide was not apparently mediated by specialized transporters, whereas secretion (i.e. BL to AP transport) by intestinal tissues was concentration dependent and saturable. Half-saturation constant values (K(m), mean+/-standard deviation (S.D.)) ranged from 53.6+/-35.8 microM to 168.7+/-127.3 microM, consistent with previous results from our group in intestinal tissues from other species and Caco-2 cell monolayers. Secretory permeability was greatest in the ileum, whereas values in the upper small intestine and colon were approximately equal, and represented only 50% of the value in the ileum. The ileal secretory transport of etoposide was temperature dependent, with the activation energy (E(a)) >4 kCal/mole at 5 microM, suggesting the involvement of the active, energy dependent mechanism. Etoposide inhibition by verapamil and saquinavir, known inhibitors of intestinal secretion, was characterized as competitive with K(i)'s equal to 193.0+/-164.4 microM and 72.6+/-53.5 microM, respectively. The current results demonstrate that the absorptive transport of etoposide in rabbit tissue was not mediated by specialized carriers, and that secretory transport was regionally dependent, mediated by a transporter or transporters, the K(m)'s were in the micromolar range, and involved the energy dependent mechanism(s). The relatively low k(m) of etoposide compared with its aqueous solubility (0.25-0.34 mM, pH 5-6.5, 25 degrees C) makes it the excellent mixed-mechanism competitive inhibitor for determining the secretory transport properties of putative drug substrates. Understanding the in vitro secretory transport kinetics of etoposide provides a mechanistic basis for ongoing studies exploring the functional role of 'efflux' in vivo.

Effect of menthol on permeability of an optically active and racemic propranolol across guinea pig skin.

The effect of menthol on the percutaneous penetration of S(-)-propranolol (SPL) and racemic form of propranolol (RSPL) was investigated in vitro using excised abdominal skin of guinea pig. In the presence of menthol, the permeability coefficient of SPL was high compared with that of RSPL. The enhancement factors for SPL and RSPL were 2.12 and 0.85, respectively. The lag times for SPL and RSPL were reduced considerably in the presence of menthol compared to those for control (without enhancer). The present findings suggest the enantio-selective permeation of SPL across the guinea pig skin in the presence of menthol.

Effect of menthol and related terpenes on the percutaneous absorption of propranolol across excised hairless mouse skin.

The potential use of terpenes/terpenoids as penetration enhancers in the transdermal delivery of propranolol hydrochloride (PL) was investigated. PL was chosen for the reasons of its extensive first-pass metabolism and short elimination half-life. The terpenes studied included L-menthol, (+)-limonene, (+/-)-linalool, and carvacrol at 1%, 5%, and 10% w/v concentrations. The diffusion of PL across excised hairless mouse skin was determined using side-by-side diffusion cells. Flux, permeability coefficient (Pm), and lag time (tL) were calculated. PL showed comparable lag times with menthol at all three concentration levels. At a 1% level of carvacrol, PL exhibited a 2.4- and 2.2-fold increase in lag time compared with 5 and 10% levels of enhancer, respectively. In the presence of limonene, PL had shown maximum lag time (between 3.0 and 3.3 h) at all three levels. In the case of linalool, the lag times for PL with 5 and 10% levels of enhancer were 7.0- and 5.2-fold less compared with 1% level. A significant (p < 0.05) concentration effect was observed only with linalool. Hydrogel-based patches were formulated with or without menthol as enhancer. Release profiles from the hydrogel formulations obeyed zero-order kinetics. The permeability of propranolol was significantly higher (p < 0.05) from the test patch than the control (no enhancer) patch across the mouse skin. The mechanism of permeation enhancement of menthol could involve its distribution preferentially into the intercellular spaces of stratum corneum and the possible reversible disruption of the intercellular lipid domain. The results suggest the potential use of menthol as effective penetration enhancer in the delivery of significant amounts of PL through skin.