Tertiary Oxidation of Deoxycholate Is Predictive of CYP3A Activity in Dogs

A variety of cytochrome P450 enzymes and flavin-containing monooxygenases in dogs and pigs commonly used as preclinical animal models

Drug oxygenation is mainly mediated by cytochromes P450 (P450s, CYPs) and flavin-containing monooxygenases (FMOs). Polymorphic variants of P450s and FMOs are known to influence drug metabolism.　Species differences exist in terms of drug metabolism and can be important when determining the contributions of individual enzymes. The success of research into drug-metabolizing enzymes and their impacts on drug discovery and development has been remarkable. Dogs and pigs are often used as preclinical animal models. This research update provides information on P450 and FMO enzymes in dogs and pigs and makes comparisons with their human enzymes. Newly identified dog CYP3A98, a testosterone 6β- and estradiol 16α-hydroxylase, is abundantly expressed in small intestine and is likely the major CYP3A enzyme in small intestine, whereas dog CYP3A12 is the major CYP3A enzyme in liver. The roles of recently identified dog CYP2J2 and pig CYP2J33/34/35 were investigated. FMOs have been characterized in humans and several other species including dogs and pigs. P450 and FMO family members have been characterized also in cynomolgus macaques and common marmosets. P450s have industrial applications and have been the focus of attention of many pharmaceutical companies. The techniques used to investigate the roles of P450/FMO enzymes in drug oxidation and clinical treatments have not yet reached maturity and require further development. The findings summarized here provide a foundation for understanding individual pharmacokinetic and toxicological results in dogs and pigs as preclinical models and will help to further support understanding of the molecular mechanisms of human P450/FMO functionality.

Separation of bile acid isomer plays a pivotal role in bioequivalence evaluation of ursodeoxycholic acid

Based on our experiences in bile acid profiling, this work developed and validated a liquid chromatography electrospray ionization tandem mass spectrometry method to separate endogenous bile acid isomers and quantitatively determine ursodeoxycholic acid (UDCA), glycoursodeoxycholic acid (GUDCA) and tauroursodeoxycholic acid (TUDCA) in human plasma. The separation was performed on a CORTECS C18 column with the mobile phase consisting of 1.0 mM ammonium acetate and acetonitrile-methanol (80:20, v/v). UDCA, GUDCA and TUDCA were detected in the negative mode on a triple-quadrupole mass spectrometer at the ion transitions of m/z 391 > 391, m/z 448 > 74, m/z 498 > 80, respectively. Phosphate buffer was employed as the surrogate matrix to establish the isotope internal standard corrected calibration curves of analytes. The background-method with a linearity range of 10-200 ng/mL was partially validated to determine the endogenous levels of analytes in blank human plasma, which was incorporated into the validation of bioequivalence-method with a linearity range of 50-10000 ng/mL. The bioequivalence (BE)-method was fully validated with special focus on matrix effects, which have been critically evaluated using the precision and accuracy of quality control samples prepared from the blank human plasma of 12 individuals. It is disclosed for the first time that the BE results of UDCA formulation may yield false results when the method is insufficient to separate UDCA from isoursodeoxycholic acid, a microbial metabolite of both endogenous and exogenous UDCA. The present method has established a milestone for the evaluation of UDCA formulations and is expected to provide a valuable reference for the bioanalytical development of endogenous medicinal products.

Comparison of 1Beta- and 5Beta-hydroxylation of Deoxycholate and Glycodeoxycholate as In Vitro Index Reactions for Cytochrome P450 3A Activities

Cytochrome P450 3A (CYP3A) participates in the metabolism of more than 30% of clinical drugs. The vast intra- and inter-individual variations in CYP3A activity pose great challenges to drug development and personalized medicine. It has been disclosed that human CYP3A4 and CYP3A7 are exclusively responsible for the tertiary oxidations of deoxycholic acid (DCA) and glycodeoxycholic acid (GDCA) regioselectivity at C-1β and C-5β This work aimed to compare the 1β- and 5β-hydroxylation of DCA and GDCA as potential in vitro CYP3A index reactions in both human liver microsomes and recombinant P450 enzymes. The results demonstrated that the metabolic activity of DCA 1β- and 5β-hydroxylation was 5-10 times higher than that of GDCA, suggesting that 1β-hydroxyglycodeoxycholic acid and 5β-hydroxyglycodeoxycholic acid may originate from DCA oxidation followed by conjugation in humans. Metabolic phenotyping data revealed that DCA 1β-hydroxylation, DCA 5β-hydroxylation, and GDCA 5β-hydroxylation were predominantly catalyzed by CYP3A4 (>80%), while GDCA 1β-hydroxylation had approximately equal contributions from CYP3A4 (41%) and 3A7 (58%). Robust Pearson correlation was established for the intrinsic clearance of DCA 1β- and 5β-hydroxylation with midazolam (MDZ) 1'- and 4-hydroxylation in fourteen single donor microsomes. Although DCA 5β-hydroxylation exhibited a stronger correlation with MDZ oxidation, DCA 1β-hydroxylation exhibited higher reactivity than DCA 5β-hydroxylation. It is therefore suggested that DCA 1β- and 5β-hydroxylations may serve as alternatives to T 6β-hydroxylation as in vitro CYP3A index reactions. SIGNIFICANCE STATEMENT: The oxidation of DCA and GDCA is primarily catalyzed by CYP3A4 and CYP3A7. This work compared the 1β- and 5β-hydroxylation of DCA and GDCA as in vitro index reactions to assess CYP3A activities. It was disclosed that the metabolic activity of DCA 1β- and 5β-hydroxylation was 5-10 times higher than that of GDCA. Although DCA 1β-hydroxylation exhibited higher metabolic activity than DCA 5β-hydroxylation, DCA 5β-hydroxylation demonstrated stronger correlation with MDZ oxidation than DCA 1β-hydroxylation in individual liver microsomes.

Cannabidiol and cannabidiolic acid: Preliminary in vitro evaluation of metabolism and drug-drug interactions involving canine cytochrome P-450, UDP-glucuronosyltransferase, and P-glycoprotein

Phytocannabinoid-rich hemp extracts containing cannabidiol (CBD) and cannabidiolic acid (CBDA) are increasingly being used to treat various disorders in dogs. The objectives of this study were to obtain preliminary information regarding the in vitro metabolism of these compounds and their capacity to inhibit canine cytochrome P450 (CYP)-mediated drug metabolism and canine P-glycoprotein-mediated transport. Pure CBD and CBDA, and hemp extracts enriched for CBD and for CBDA were evaluated. Substrate depletion assays using pooled dog liver microsomes showed CYP cofactor-dependent depletion of CBD (but not CBDA) and UDP-glucuronosytransferase cofactor-dependent depletion of CBDA (but not CBD) indicating major roles for CYP and UDP-glucuronosytransferase in the metabolism of these phytocannabinoids, respectively. Further studies using recombinant canine CYPs demonstrated substantial CBD depletion by the major hepatic P450 enzymes CYP1A2 and CYP2C21. These results were confirmed by showing increased CBD depletion by liver microsomes from dogs treated with a known CYP1A2 inducer (β-naphthoflavone) and with a known CYP2C21 inducer (phenobarbital). Cannabinoid-drug inhibition experiments showed inhibition (IC50  = 4.6-8.1 μM) of tramadol metabolism via CYP2B11-mediated N-demethylation (CBD and CBDA) and CYP2D15-mediated O-demethylation (CBDA only) by dog liver microsomes. CBD and CBDA did not inhibit CYP3A12-mediated midazolam 1'-hydroxylation (IC50  > 10 μM). CBD and CBDA were not substrates or competitive inhibitors of canine P-glycoprotein. Results for cannabinoid-enriched hemp extracts were identical to those for pure cannabinoids. These in vitro studies indicate the potential for cannabinoid-drug interactions involving certain CYPs (but not P-glycoprotein). Confirmatory in vivo studies are warranted.

High-fat diets enhance and delay ursodeoxycholic acid absorption but elevate circulating hydrophobic bile salts

Background: Ursodeoxycholic acid (UDCA) is a natural drug essential for the treatment of cholestatic liver diseases. The food effects on the absorption of UDCA and the disposition of circulating bile salts remain unclear despite its widespread global uses. This study aims to investigate the effects of high-fat (HF) diets on the pharmacokinetics of UDCA and disclose how the circulated bile salts were simultaneously perturbed. Methods: After an overnight fast, a cohort of 36 healthy subjects received a single oral dose (500 mg) of UDCA capsules, and another cohort of 31 healthy subjects received the same dose after consuming a 900 kcal HF meal. Blood samples were collected from 48 h pre-dose up to 72 h post-dose for pharmacokinetic assessment and bile acid profiling analysis. Results: The HF diets significantly delayed the absorption of UDCA, with the T_max of UDCA and its major metabolite, glycoursodeoxycholic acid (GUDCA), changing from 3.3 h and 8.0 h in the fasting study to 4.5 h and 10.0 h in the fed study, respectively. The HF diets did not alter the C_max of UDCA and GUDCA but immediately led to a sharp increase in the plasma levels of endogenous bile salts including those hydrophobic ones. The AUC_0-72h of UDCA significantly increased from 25.4 μg h/mL in the fasting study to 30.8 μg h/mL in the fed study, while the AUC_0-72h of GUDCA showed no difference in both studies. As a result, the C_max of total UDCA (the sum of UDCA, GUDCA, and TUDCA) showed a significant elevation, while the AUC_0-72h of total UDCA showed a slight increase without significance in the fed study compared to the fasting study. Conclusion: The HF diets delay UDCA absorption due to the extension of gastric empty time. Although UDCA absorption was slightly enhanced by the HF diets, the beneficial effect may be limited in consideration of the simultaneous elevation of circulating hydrophobic bile salts.

A Comprehensive Investigation of Dog Cytochrome P450 3A (CYP3A) Reveals a Functional Role of Newly Identified CYP3A98 in Small Intestine

Dogs are frequently used in drug metabolism studies, and their important drug-metabolizing enzymes, including cytochromes P450 (P450), have been analyzed. In humans, CYP3A4 is an especially important P450 due to its abundance and major roles in liver and intestine. In the present study, dog CYP3A98 and CYP3A99 were identified and characterized, along with previously identified CYP3A12 and CYP3A26. The dog CYP3A cDNAs contained open reading frames of 503 amino acids and shared high sequence identity (78%-80%) with human CYP3As. Among the dog CYP3A mRNAs, CYP3A98 mRNA was expressed most abundantly in small intestine. In contrast, dog CYP3A12 and CYP3A26 mRNAs were expressed in liver, where CYP3A12 mRNA was the most abundant. The four CYP3A genes had similar gene structures and formed a gene cluster in the dog and human genomes. Metabolic assays of dog CYP3A proteins heterologously expressed in Escherichia coli indicated that the dog CYP3As tested were functional enzymes with respect to typical human CYP3A4 substrates. Dog CYP3A98 efficiently catalyzed oxidations of nifedipine, alprazolam, and midazolam, indicating major roles of CYP3A98 in the small intestine. Dog CYP3A12 and CYP3A26 metabolizing nifedipine and/or midazolam would play roles in these reactions in the liver. In contrast, dog CYP3A99 showed minimal mRNA expression and minimal metabolic activity, and its contribution to overall drug metabolism is, therefore, negligible. These results indicated that newly identified dog CYP3A98, a testosterone 6 β - and estradiol 16 α -hydroxylase, was abundantly expressed in the small intestine and is likely the major CYP3A in the small intestine in combination with liver-specific CYP3A12. SIGNIFICANCE STATEMENT: Novel dog cytochromes P450 3A98 (CYP3A98) and CYP3A99 were identified and characterized to be functional and highly identical to human CYP3A4. Known CYP3A12 and new CYP3A98 efficiently catalyzed estradiol 16α-hydroxylation and midazolam 1'-hydroxylation. CYP3A98 mRNA was expressed in small intestine, whereas CYP3A12 mRNA was predominant in liver. Dog hepatic CYP3A12 and intestinal CYP3A98 are the enzymes likely responsible for the metabolic clearances of orally administered drugs, unlike human CYP3A4/5, which are in both the liver and intestine.

Compensatory Transition of Bile Acid Metabolism from Fecal Disposition of Secondary Bile Acids to Urinary Excretion of Primary Bile Acids Underlies Rifampicin-Induced Cholestasis in Beagle Dogs

Drug induced cholestasis (DIC) is complexly associated with dysbiosis of the host-gut microbial cometabolism of bile acids (BAs). Murine animals are not suitable for transitional studies because the murine BA metabolism is quite different from human metabolism. In this work, the rifampicin (RFP) induced cholestasis was established in beagle dogs that have a humanlike BA profile to disclose how RFP affects the host-gut microbial cometabolism of BAs. The daily excretion of BA metabolites in urine and feces was extensively analyzed during cholestasis by quantitative BA profiling along the primary-secondary-tertiary axis. Oral midazolam clearance was also acquired to monitor the RFP-induced enterohepatic CYP3A activities because CYP3A is exclusively responsible for the tertiary oxidation of hydrophobic secondary BAs. RFP treatments caused a compensatory transition of the BA metabolism from the fecal disposition of secondary BAs to the urinary excretion of primary BAs in dogs, resulting in an infantile BA metabolism pattern recently disclosed in newborns. However, the tertiary BAs consistently constituted limitedly in the daily BA excretion, indicating that the detoxification role of the CYP3A catalyzed tertiary BA metabolism was not as strong as expected in this model. Multiple host-gut microbial factors might have contributed to the transition of the BA metabolism, such as inhibition of BA transporters, induction of liver-kidney interplaying detoxification mechanisms, and elimination of gut bacteria responsible for secondary BA production. Transitional studies involving more cholestatic drugs in preclinical animals with a humanlike BA profile and DIC patients may pave the way for understanding the complex mechanism of DIC in the era of metagenomics.