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

Canine orosomucoid (alpha-1 acid glycoprotein) variants and their influence on drug plasma protein binding

Orosomucoid polymorphisms influence plasma drug binding in humans; however, canine variants and their effect on drug plasma protein binding have not yet been reported. In this study, the orosomucoid gene (ORM1) was sequenced in 100 dogs to identify the most common variant and its allele frequency determined in 1,464 dogs (from 64 breeds and mixed-breed dogs). Plasma protein binding extent of amitriptyline, indinavir, verapamil, and lidocaine were evaluated by equilibrium dialysis using plasma from ORM1 genotyped dogs (n = 12). Free and total drug plasma concentrations were quantified by liquid chromatography-mass spectrometry. From the five polymorphisms identified in canine ORM1, two were nonsynonymous. The most common was c.70G>A (p.Ala24Thr) with an allele frequency of 11.2% (n = 1464). Variant allele frequencies varied by breed, reaching 74% in Shetland Sheepdogs (n = 21). Free drug fractions did not differ significantly (p > .05; Mann-Whitney U) between plasma collected from dogs with c.70AA (n = 4) and those with c.70GG (n = 8) genotypes. While c.70G>A did not affect the extent of plasma protein binding in our study, the potential biological and pharmacological implication of this newly discovered ORM1 variant in dogs should be further investigated.

Jugular vascular access port implantation for frequent, long-term blood sampling in cats: methodology, assessment, and comparison with jugular catheters

Long-term, frequent venous access for diagnostic, therapeutic, or research purposes in cats is problematic. Frequent blood sampling over extended periods is necessary for some therapeutic regimes and often required for clinical research in veterinary science. In this paper, we describe the implantation of vascular access ports (VAPs) and assess their use for repeated blood sampling over 16 weeks and 38 weeks, as well as the use of jugular catheters for one week. The VAP placement procedure was well-tolerated with few minor complications (minor swelling, contusion, or superficial dermatitis from self-trauma), which were not observed when neck bandages were applied immediately after surgery. Thromboembolism occurred in two cats, but did not occur after switching to a smaller catheter with a rounded tip and taurolidine-citrate locking solution. Although duration of access was much longer with VAPs compared to jugular catheters, patency rates were similar (89% (n=28) to 92% (n=12) after 16 weeks and 75% (n=12) after 38 weeks for VAPs; 88% (n=49) after one week for jugular catheters). Behavioural reactions to blood collection from 30 cats-assessed over 16 weeks and comprising 378 collections-were absent or minor in 99% of collections. These findings indicate that VAPs offer a viable alternative to jugular catheters for studies requiring frequent blood sampling and lasting more than 2 weeks.

In vitro and in vivo small intestinal metabolism of CYP3A and UGT substrates in preclinical animals species and humans: species differences

Intestinal first-pass metabolism has a great impact on the bioavailability of cytochrome P450 3A4 (CYP3A) and/or uridine 5'-diphosphate (UDP)-glucoronosyltranferase (UGT) substrates in humans. In vitro and in vivo intestinal metabolism studies are essential for clarifying pharmacokinetics in animal species and for predicting the effects of human intestinal metabolism. We review species differences in intestinal metabolism both in vitro and in vivo. Based on mRNA expression levels, the major intestinal CYP3A isoform is CYP3A4 for humans, CYP3A4 (3A8) for monkeys, CYP3A9 for rats, cyp3a13 for mice, and CYP3A12 for dogs. Additionally, the intestinal-specific UGT would be UGT1A10 for humans, UGT1A8 for monkeys, and UGT1A7 for rats. In vitro and in vivo intestinal metabolism of CYP3A substrates were larger in monkeys than in humans, although a correlation in intestinal availability between monkeys and humans has been reported. Little information is available regarding species differences in in vitro and in vivo UGT activities; however, UGT-mediated in vivo intestinal metabolism has been demonstrated for raloxifene in humans and for baicalein in rats. Further assessment of intestinal metabolism, particularly for UGT substrates, is required to clarify the entire picture of species differences.

Involvement of cytochrome P450 3A4 and P-glycoprotein in first-pass intestinal extraction of omeprazole in rabbits

AIM

To quantitatively evaluate in vivo first-pass intestinal extraction of omeprazole and to investigate the possible involvement of cytochrome P450 3A4 (CYP3A4) and P-glycoprotein (P-gp) in this process in rabbits.

METHODS

Pharmacokinetic parameters were examined after intraduodenal (id), intraportal venous (ipv), and intravenous (iv) administration of omeprazole at various doses to intestinal and vascular access-ported rabbits. Extraction ratios in the liver and intestinal tract were determined from the area under the plasma concentration-time curve (AUC). In addition, omeprazole was administered by id or iv to rabbits alone or 30 min after the id administration of CYP3A4 or P-gp inhibitors (ketoconazole or verapamil, respectively).

RESULTS

Pharmacokinetic parameters of omeprazole were dose-dependent after id, ipv, and iv administration at various doses. After id administration of 3 mg/kg omeprazole, the hepatic and intestinal extraction ratio was 57.18%+/-2.73% and 54.94%+/-1.85%, while the value was 59.29%+/-3.14% and 54.20%+/-1.53% after given 6 mg/kg, respectively. Compared with the control group, the presence of ketoconazole (60 mg/kg) or verapamil (9 mg/kg) significantly increased the area under the plasma concentration time curve (AUC) and the peak concentration (C(max)) of id-administered omeprazole, while it had no significant effect on omeprazole administered by iv.

CONCLUSION

Oral omeprazole undergoes marked extraction in the small intestine, and increased bioavailability of the drug after id administration of ketoconazole and verapamil suggests that this increase results from inhibition of CYP3A4 and P-gp function in the intestine rather than the liver.

Species Differences in In Vitro and In Vivo Small Intestinal Metabolism of CYP3A Substrates

Intestinal first-pass metabolism has a great impact on the bioavailability of CYP3A substrates in humans, and the in vivo impact has quantitatively been evaluated using CYP3A inhibitors or inducers. In vitro and in vivo preclinical investigations for intestinal metabolism are essential in clarifying pharmacokinetic behavior in animal species and predicting the effect of intestinal metabolism in the human. In this review, we will discuss species differences in intestinal CYP3A enzymes, and CYP3A-mdediated intestinal elimination. Identical CYP3A4 enzyme is expressed in human intestine and liver, but different CYP3A enzymes in both tissues of the mouse and rat are found, that is, respective intestinal enzyme is considered as cyp3a13 and CYP3A62. There is little information on CYP3A enzymes in the monkey and dog intestine, unlike the liver. In vitro metabolic activities of midazolam and nisoldipine are higher in the human and monkey than in the rat. In vivo assessment of cyclosporine, midazolam, nifedipine, tacrolimus, and verapamil has been reported in various species (monkey, rat, mouse, and/or dog) including the human. For midazolam, the monkey shows significant in vivo intestinal metabolism, as evidenced in the human. The monkey might be an appropriate animal model for evaluating small intestinal first-pass metabolism of CYP3A substrates.

New surgical technique for portal venous port system in Swine

The need for serial blood sampling and continuous infusion using the portal vein for experimental surgery and pharmacological studies is evident. As suitable animal models are rare, we established a new technique to implant a permanent vascular access device to the portal vein in swine. All operations in study animals (n=8) were technically feasible due to a regularly detected side branch of the portal vein within the inferior pancreatic region; complications did not occur. Mean operation time was 48+/-17 min. This study clearly shows that the portal venous access is feasible and can be achieved safely using the inferior pancreatic vein for a permanent vascular access port system.

DIFFERENTIATION OF GUT AND HEPATIC FIRST-PASS LOSS OF VERAPAMIL IN INTESTINAL AND VASCULAR ACCESS-PORTED (IVAP) RABBITS

Low and varied oral bioavailability (BA) of some drugs has been attributed to extraction by the intestine and liver. However, the role of the intestine is difficult to directly assess. We recently developed an in vivo intestinal and vascular access-ported (IVAP) rabbit model that allows for a direct assessment of the contributions of the gut and the liver to the first-pass loss of drugs. The current studies validate the utility of the IVAP rabbit model using verapamil (VL). VL pharmacokinetics (PK) were determined after intravenous (i.v.), portal venous (PV), and upper small intestinal (USI) administration. In the i.v. dose range studied, VL exhibited linear PK. The PV concentration of VL was significantly lower than systemic concentrations after i.v. administration, suggesting significant intestinal second-pass extraction. The intestinal and hepatic extraction of VL, calculated directly from area under the curve measurements, were 79% and 92%, respectively, and are in contrast to our previous dog results that showed VL intestinal extraction to be negligible. Assessing the role of intestinal extraction using an "indirect" method was not predictive, further showing the utility of this direct measurement model. The BA of VL after USI administration was 1.65%, much lower than that reported for rats, dogs, or humans. However, humans and rabbits behave similarly in that the contribution of intestinal extraction for VL is high. In conclusion, the current results demonstrate the utility of the rabbit IVAP model in studying the first- and second-pass intestinal and hepatic loss of drugs and other xenobiotics.