Cyclosporine metabolism by P450IIIA in rat enterocytes--another determinant of oral bioavailability?

Extracts of Immature Orange (Aurantii fructus immaturus) and Citrus Unshiu Peel (Citri unshiu pericarpium) Induce P-Glycoprotein and Cytochrome P450 3A4 Expression via Upregulation of Pregnane X Receptor

P-glycoprotein (P-gp) and cytochrome P450 3A4 (CYP3A4) are expressed in the intestine and are associated with drug absorption and metabolism. Pregnane X receptor (PXR) is the key molecule that regulates the expression of P-gp and CYP3A4. Given that PXR activity is regulated by a variety of compounds, it is possible that unknown PXR activators exist among known medicines. Kampo is a Japanese traditional medicine composed of various natural compounds. In particular, immature orange [Aurantii fructus immaturus (IO)] and citrus unshiu peel [Citri unshiu pericarpium (CP)] are common ingredients of kampo. A previous study reported that kampo containing IO or CP decreased the blood concentration of concomitant drugs via upregulation of CYP3A4 although the mechanism was unclear. Some flavonoids are indicated to alter P-gp and CYP3A4 activity via changes in PXR activity. Because IO and CP include various flavonoids, we speculated that the activity of P-gp and CYP3A4 in the intestine may be altered via changes in PXR activity when IO or CP is administered. We tested this hypothesis by using LS180 intestinal epithelial cells. The ethanol extract of IO contained narirutin and naringin, and that of CP contained narirutin and hesperidin. Ethanol extracts of IO and CP induced P-gp, CYP3A4, and PXR expression. The increase of P-gp and CYP3A4 expression by the IO and CP ethanol extracts was inhibited by ketoconazole, an inhibitor of PXR activation. The ethanol extract of IO and CP decreased the intracellular concentration of digoxin, a P-gp substrate, and this decrease was inhibited by cyclosporine A, a P-gp inhibitor. In contrast, CP, but not IO, stimulated the metabolism of testosterone, a CYP3A4 substrate, and this was inhibited by a CYP3A4 inhibitor. These findings indicate that the ethanol extract of IO and CP increased P-gp and CYP3A4 expression via induction of PXR protein. Moreover, this induction decreased the intracellular substrate concentration.

CYP2S1 depletion enhances colorectal cell proliferation is associated with PGE2-mediated activation of β-catenin signaling

Colorectal epithelial cancer is one of the most common cancers in the world and its 5-year survival rate is still relatively low. Cytochrome P450 (CYP) enzymes in epithelial cells lining the alimentary tract play an important role in the oxidative metabolism of a wide range of xenobiotics, including (pro-)carcinogens and endogenous compounds. Although CYP2S1, a member of CYP family, strongly expressed in many extrahepatic tissues, the role of CYP2S1 in cancer remains unclear. To investigate whether CYP2S1 involves in colorectal carcinogenesis, cell proliferation was analyzed in HCT116 cells depleted of CYP2S1 using small hairpin interfering RNA. Our data show that CYP2S1 knockdown promotes cell proliferation through increasing the level of endogenous prostaglandin E2(PGE2). PGE2, in turn, reduces phosphorylation of β-catenin and activates β-catenin signaling, which contributes to the cell proliferation. Furthermore, CYP2S1 knockdown increase tumor growth in xenograft mouse model. In brief, these results demonstrate that CYP2S1 regulates colorectal cancer growth through associated with PGE2-mediated activation of β-catenin signaling.

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.

Role of efflux pumps and metabolising enzymes in drug delivery

The impact of efflux pumps and metabolic enzymes on the therapeutic activity of various drugs has been well established. The presence of efflux pumps on various tissues and tumours has been shown to regulate the intracellular concentration needed to achieve therapeutic activity. The notable members of efflux proteins include P-glycoprotein, multi-drug resistance protein and breast cancer resistance protein. These efflux pumps play a pivotal role not only in extruding xenobiotics but also in maintaining the body's homeostasis by their ubiquitous presence and ability to coordinate among themselves. In this review, the role of efflux pumps in drug delivery and the importance of their tissue distribution is discussed in detail. To improve pharmacokinetic parameters of substrates, various strategies that modulate the activity of efflux proteins are also described. Drug metabolising enzymes mainly include the cytochrome P450 family of enzymes. Extensive drug metabolism due to the this family of enzymes is the leading cause of therapeutic inactivity. Therefore, the role of metabolising enzymes in drug delivery and disposition is extensively discussed in this review. The synergistic relationship between metabolising enzymes and efflux proteins is also described in detail. In summary, this review emphasises the urgent need to make changes in drug discovery and drug delivery as efflux pumps and metabolising enzymes play an important role in drug delivery and disposition.

Concomitant Clotrimazole Therapy More Than Doubles the Relative Oral Bioavailability of Tacrolimus

The purpose of this pharmacokinetic study was to determine whether the relative oral bioavailability of tacrolimus is increased with concomitant administration of clotrimazole. Pharmacokinetic studies were conducted in 6 adult kidney transplant patients receiving tacrolimus therapy. Pharmacokinetic profiling was performed by blood sampling over 12 hours before and after the administration of a 5-day course of clotrimazole. Tacrolimus whole-blood concentrations were determined by microparticle enzyme immunoassay. Noncompartmental pharmacokinetic analysis was conducted using WinNonLin, Standard Edition, Version 1.1. Concomitant administration of clotrimazole more than doubled the relative oral bioavailability of tacrolimus. The mean AUC0-12 of tacrolimus was increased 250% with clotrimazole (467.0 +/- 170.0 ng.h/mL versus 188.7 +/- 50.2 ng.h/mL; P = 0.002). Tacrolimus blood trough concentrations also more than doubled with coadministration of clotrimazole (27.7 +/- 10.4 ng/mL versus 11.6 +/- 4.0 ng/mL; P = 0.003). Mean Cmax was significantly increased with clotrimazole (70.7 +/- 34.7 ng/mL versus 27.4 +/- 11.1 ng/mL, P = 0.01). Tmax decreased from 3.2 +/- 1.6 hours to 1.9 +/- 1.0 hours (P = NS). In addition, the apparent oral clearance decreased 60% with coadministration of clotrimazole (median oral clearance 0.16 L/h/kg versus 0.40 L/h/kg; P = 0.03). Thus, clotrimazole causes a significant increase in the relative oral bioavailability, Tmax, and trough concentration of tacrolimus. Tacrolimus levels should be monitored following initiation or discontinuation of clotrimazole to minimize toxicity or precipitation of an acute rejection episode due to subtherapeutic levels.

Effect of probucol on the oral bioavailability of cyclosporine A

We have previously reported the reduction in oral bioavailability of cyclosporine A (CsA) by probucol, a lipid-lowering drug. To evaluate the mechanism, we examined the effect of probucol on the transport of CsA across Caco-2 cell monolayers, on the influence of CsA pharmacokinetics in rats, and on the change of ultraviolet-absorption spectrum of the drug. Pretreatment with probucol (50 microM) inhibited (P < 0.001) both the apical-to-basal (-73.1%) and basal-to-apical (-77.8%) fluxes of [3H]-CsA. In rats, probucol orally given 6 h after, but not simultaneous with CsA did not decrease peak CsA concentration or area under the blood CsA concentration-time curve following a single oral dosing of CsA after the pretreatment with probucol for 7 days. These data indicate that P-glycoprotein-mediated active transport from intracellular to apical is not involved in the mechanism of probucol-CsA interaction, and absorption of CsA decreases in the presence of probucol in the gastrointestinal tract. In difference spectral analysis, probucol reduced the absorption peak of CsA in a concentration-dependent manner, indicating that probucol could form a complex with CsA. These results suggest that probucol interferes with CsA absorption probably by physicochemical mechanism such as complex formation, but not the enhancement of P-glycoprotein function.

Synergistic effects of RAD and Neoral in inhibition of host-vs.-graft and graft-vs.-host immune responses in rat small-bowel transplantation

The combined effects of RAD and Neoral were tested in a rat orthotopic small-bowel transplantation model. Seven groups (n = 6) were involved in this study, and each one was included in three rejection models for the evaluation of host-vs.-graft disease (HVG) (LBN-F1 to LEW), graft-vs.-host disease (GVH) (LEW to LBN-F1), and combined HVG and GVH immune responses (BN to LEW). Both drugs were administered orally throughout the study. Low doses of RAD (1.0-2.5 mg/kg/day) combined with Neoral (2.0-5.0 mg/kg/day) produced strong synergistic effects in the prolongation of small-bowel graft survival in HVG (combination index, CI = 0.095, 0.1212), GVH (CI = 0.027, 0.020), and combined HVG and GVH immune responses (CI = 0.070, 0.301). The combination therapy of RAD and Neoral produces a strong synergistic effect toward the inhibition of HVG, GVH, and combined HVG and GVH immune responses in a rat small-bowel transplantation model.

Pharmacokinetic interactions between sirolimus and microemulsion cyclosporine when orally administered jointly and 4 hours apart in healthy volunteers

Sirolimus (RAPA) and cyclosporine (CsA) are immunosuppressive compounds that are being used concomitantly in renal transplant patients. Both drugs are dosed orally, have common intestinal and hepatic metabolism and intestinal transport mechanisms, and thus offer potential for pharmacokinetic drug interactions. A single-dose, open-label, four-period, four-treatment, randomized crossover study was completed in 15 male and 6 female volunteers. Each subject received a 10-mg oral dose of RAPA alone (Rapamune Oral Solution), a 300-mg oral dose of CsA alone (3 x 100-mg Neoral Soft Gelatin Capsules), RAPA and CsA jointly, and CsA followed by RAPA delayed by 4 hours. Blood samples were collected for either 144 hours (RAPA) or 48 hours (CsA) and analyzed by either liquid chromatography/tandem mass spectrometry (RAPA) or radioimmunoassay (CsA). RAPA bioavailability was markedly increased by CsA when given jointly, with Cmax,tmax, and AUC being increased 116%, 92%, and 230%, respectively. However, when RAPA was administered 4 hours after CsA, increases in RAPA Cmax, tmax, and AUC were only 37%, 58%, and 80%, respectively. CsA did not affect t1/2 or mean residence time (MRT) by either mode of combined administration. RAPA did not significantly affect CsA bioavailability after either joint or delayed combined administrations. It was concluded that CsA markedly increases the bioavailability of RAPA, which may be attributed to a large intestinal and hepatic first-pass effect, rather than altered elimination. RAPA did not affect the bioavailability of CsA.

The site-specific transport and metabolism of tacrolimus in rat small intestine

The objective of this study was to evaluate the absorption of tacrolimus by means of simultaneous perfusion of intestinal lumen and blood vessels in rats. In our previous report, the permeability of tacrolimus was found to be higher in the jejunum than in the ileum or colon, suggesting the site-dependent absorption after oral administration. However, in this article, simultaneous perfusion technique revealed that the extent of absorption into blood vessels was similar in the jejunum and the ileum regardless of the site difference in permeability as the absorption rate. In addition to the multidrug resistance-associated protein-mediated efflux, cytochrome P450 (P450)-mediated metabolism could be a possible mechanism to explain the inconsistencies in the site dependence of tacrolimus absorption. Two enzyme inhibitors, ketoconazole and midazolam, were coperfused in rat intestinal lumen with tacrolimus to specify the effect of P-gp and P450. In the jejunum, both inhibitors significantly enhanced the absorbed amount of tacrolimus, whereas the permeability was not affected. It was suggested that both inhibitors mainly suppress P450-mediated metabolism in the upper region of the intestine. In contrast, in the ileum, ketoconazole significantly enhanced both the absorbed amount and the permeability of tacrolimus. However, midazolam failed to enhance the absorption of tacrolimus, indicating the dominant role of P-glycoprotein (P-gp)-mediated efflux in the lower region. From these findings, it is concluded that the site-dependent differences in P-gp and/or P450 activity could be the prime cause of large intra- and interindividual variability in the oral absorption of tacrolimus.

Cyclosporine A and tacrolimus: in vitro investigations on the differential interactions with the cytochrome P450 system in rat and human liver

Species differences in the interactions of cyclosporine A (CSA) and tacrolimus (TAC) with the cytochrome P450 (CYP) system in male rat and human liver were investigated in vitro by assessing effects on a series of model reactions for different CYP isoforms. CSA and TAC concentration dependently inhibited ethoxyresorufin O-deethylation, ethoxycoumarin O-deethylation and pentoxyresorufin O-depentylation and 7alpha- and 17-testosterone hydroxylation (TH) activities in both species. In rat liver no effect of CSA was seen on ethylmorphine N-demethylation and 2alpha- and 6beta-TH activities, but an inhibition due to TAC. Both CSA and TAC, however, distinctly decreased ethylmorphine N-demethylation and 2beta- and 6beta-TH activities in human liver. The same results were seen with 14alpha- and 15beta-TH activities. 2alpha-, 16alpha- and 16beta-TH activities were only inhibited in human liver with TAC, whereas only in this case 6alpha-TH activity was left unaffected. p-Nitrophenol hydroxylase activity was not influenced by either substance in both species. Thus, CSA mainly interacts in rat with the CYP isoforms 1A, 2A and 2B and in man with the CYP subtypes 1A, 2A, 2B, 2C and 3A. TAC seems to interfere predominantly in rat with the CYP isoforms 2A, 2B, 2C and 3A and in man with the CYP subtypes 1A, 2B, 2C and 3A. In summary, our results point to distinct species differences in the interactions with the CYP system with both substances, and although from literature CSA and TAC are known to be metabolized mainly by CYP 3A, according to our findings in rat liver CSA seems not to interact with this CYP subtype.

Drug–phytochemical interactions

Changes in dietary habits favouring diets rich in fruits and vegetables, and a meteoric rise in the consumption of dietary supplements and herbal products have substantially increased human exposure to phytochemicals. It is, therefore, not surprising that diet and herbal remedies can modulate drug-metabolising enzyme systems, such as cytochromes P450, leading to clinically relevant drug-phytochemical interactions. Phytochemicals have the potential to both elevate and suppress cytochrome P450 activity. Such effects are more likely to occur in the intestine, where high concentrations of phytochemicals may be achieved, and alteration in cytochrome P450 activity will influence, in particular, the fate of drugs that are subject to extensive first-pass metabolism as a result of intestinal cytochrome P450-mediated biotransformation. Moreover, it is becoming increasingly apparent that phytochemicals can also influence the pharmacological activity of drugs by modifying their absorption characteristics through interaction with drug transporters. Clearly, phytochemicals have the potential to alter the effectiveness of drugs, either impairing or exaggerating their pharmacological activity.

Identification of a novel route of extraction of sirolimus in human small intestine: roles of metabolism and secretion

Using Caco-2 cell monolayers expressing CYP3A4, we investigated the interplay between metabolism and transport on the first-pass intestinal extraction of the immunosuppressant sirolimus, a CYP3A4/P-glycoprotein (P-gp) substrate. Modified Caco-2 cells metabolized [(14)C]sirolimus to the predicted amounts of CYP3A4-mediated products based on CYP3A4 content, which was approximately 20% of that measured in human small intestinal mucosal homogenate. [(14)C]Sirolimus also degraded to the known ring-opened product, seco-rapamycin. Unexpectedly, a ring-opened dihydro metabolite (M2) was the major product detected in cells at all sirolimus concentrations examined (2-100 microM). Greater M2 formation after apical versus basolateral dosing (1.6-fold) was explained by higher intracellular content of sirolimus after apical dosing. M2 was not detected in incubations with human liver and intestinal microsomes but was readily detected with corresponding homogenates. M2 formation was NADPH-dependent but unaffected by the CYP3A4 inhibitors ketoconazole and troleandomycin. Although M2 was formed from purified seco-rapamycin (20 microM) in the homogenates, it was not detected in cells when seco-rapamycin was added to the apical compartment, because seco-rapamycin was essentially impermeable to the apical membrane. Sirolimus, seco-rapamycin (basolaterally dosed), and M2 were all actively secreted across the apical membrane, and secretion of each was inhibited by the P-gp inhibitor LY335979 [(2R)-anti-5-[3-[4-(10,11-difluoromethanodibenzo-suber-5-yl)piperazin-1-yl]-2-hydroxypropoxy]quinoline trihydrochloride]. Along with CYP3A4-mediated metabolism and P-gp-mediated secretion, we conclude that the following novel pathway, which occurs at least in the intestine, may contribute significantly to the first-pass extraction of sirolimus in humans: intracellular degradation of sirolimus to seco-rapamycin, metabolism of seco-rapamycin to M2 by an unidentified non-microsomal enzyme, and P-gp-mediated secretion of M2 and seco-rapamycin.

The fallacy of using adrenochrome reaction for measurement of reactive oxygen species formed during cytochrome p450-mediated metabolism of xenobiotics

The adrenochrome reaction (oxidation of epinephrine to adrenochrome) has been widely employed as a standard assay for reactive oxygen species, produced under a variety of conditions, including those produced during cytochrome P450 (CYP)-mediated oxidation of substrates such as cyclosporine. However, it has been reported that epinephrine and adrenochrome can be metabolized by hepatic microsomes and that adrenochrome can also be metabolized by NADPH-CYP reductase. Thus, in the present report, we provide evidence that measurement of adrenochrome cannot be used as an index of reactive oxygen species generated during CYP-mediated metabolism of xenobiotics because adrenochrome and its precursor, epinephrine, interact with the CYP enzyme system as substrates and inhibitors. Our results indicated that adrenochrome was moderately stable in phosphate buffer but degraded rapidly (over 50% consumed in less than 2 min) by (cloned and expressed) CYP3A4 and CYP reductase in the presence of NADPH. Furthermore, both epinephrine and adrenochrome were found to be inhibitors of CYP3A4-mediated oxidation of testosterone. Together, these results lead to the conclusion that the use of adrenochrome reaction for measurement of reactive oxygen species formed during CYP3A4-mediated metabolism of xenobiotics is inappropriate.

Peppermint oil enhances cyclosporine oral bioavailability in rats: comparison with D-alpha-tocopheryl poly(ethylene glycol 1000) succinate (TPGS) and ketoconazole

Peppermint oil inhibits cyclosporine metabolism in vitro. The current work compared the effects of peppermint oil, ketoconazole, and D-alpha-tocopheryl poly(ethylene glycol 1000) succinate (TPGS) on cyclosporine oral bioavailability. Male Sprague-Dawley rats were administered cyclosporine (25 mg/kg) as the Sandimmune formulation. Peppermint oil (100 mg/kg) tripled the mean cyclosporine maximum concentration (C(max)) from 0.60 to 1.6 microg/mL and increased the area under the concentration versus time curve (AUC(0-infinity)) from 8.3 to 24.3 microg x h/mL. The median time to reach C(max) (t(max)) was increased from 2 to 6 h. Terminal half-life (10 h) and mean residence time (MRT; 15 h) were unaffected. Coadministration of TPGS (50 mg/kg) with cyclosporine in a saline vehicle doubled cyclosporine C(max) from 1.3 to 2.9 microg/mL and increased AUC(0-infinity) from 28.5 to 59.7 microg x h/mL. The t(max) was unchanged (3 h). Terminal half-life and MRT were increased by 44% (15.4 versus 10.7 h) and 24% (19.9 versus 16.0 h), respectively. Cyclosporine pharmacokinetics were not altered when corn oil was used instead of saline as a gavage vehicle, however the TPGS effect was abolished. Ketoconazole (10 and 20 mg/kg) had no effect on cyclosporine absorption. The lack of a significant ketoconazole effect may reflect poor metabolism of cyclosporine in rat intestinal tissue and suggests that inhibition of cytochrome P450 3A is not the only means by which peppermint oil enhances cyclosporine oral bioavailability.

Expression of multiple cytochrome p450 enzymes and multidrug resistance-associated transport proteins in human skin keratinocytes

Cytochrome P450 enzymes metabolize various endogenous and exogenous small molecular weight compounds. Transport-associated proteins, such as P-glycoprotein, multidrug resistance-associated protein and lung resistance protein are overexpressed in drug-resistant cell lines, as well as in human tumors from various histologic origins, including malignant melanoma. Little is known about the expression and function of cytochrome enzymes and multidrug resistance-associated transport proteins in human skin; therefore, the aim of this study was to analyze the expression pattern of cytochrome enzymes and multidrug resistance-associated transport proteins in proliferating human epidermal keratinocytes under constitutive conditions and after induction with various inducers. Reverse transcription-polymerase chain reaction revealed constitutive expression of cytochromes 1A1, 1B1, 2B6, 2E1, and 3A5 in keratinocytes and showed expression of cytochrome 3A4 after incubation with dexamethasone. The expression of cytochrome 1A1 was enhanced on the mRNA level after induction with benzanthracene. Reverse transcription-polymerase chain reaction analysis of the multidrug resistance-associated transport proteins revealed constitutive expression of multidrug resistance-associated proteins 1 and 3-6, and lung resistance protein in human epithelial keratinocytes and was negative for multidrug resistance 1 and 2. Expression of 1 was seen after induction with dexamethasone. Reverse transcription-polymerase chain reaction results were confirmed by immunoblots which showed expression of cytochromes 1A1, 2B6, 2E1, and 3A, multidrug resistance-associated proteins 1, 3, and 5 as well as multidrug resistance 1 after induction with dexamethasone. Immunohistology showed positive immunofluorescence in skin specimens for cytochromes 1A1, 2B6, 2E1, and 3A and multidrug resistance-associated protein 1 and multidrug resistance 1. Constitutive activity of cytochrome 1A1, 2B, 2E1, and 3A enzymes was measured by catalytic assays. These results show that keratinocytes of the human skin express various transport-associated enzymes and detoxifying metabolic enzymes. Previous studies have revealed that cytochrome enzymes and transport-associated proteins play complementary parts in drug disposition by biotransformation (phase I) and anti-transport (phase III) and act synergistically as a drug bioavailability barrier.

Effect of grapefruit juice on Sandimmun Neoral absorption among stable renal allograft recipients

BACKGROUND

The oral formulation of cyclosporin A (CsA)-Sandimmun-has a highly variable absorption. The development of a CsA microemulsion-Sandimmun Neoral-resulted in increased bioavailability, and decreased variability of absorption. The first oral formulation (Sandimmun) interacted with numerous other drugs and grapefruit juice. Several of these interactions might be explained by decreased pre-systemic metabolism by a cytochrome-enzyme (e.g. CYP3A4) located in the enteral mucosa, and/or via the P-glycoprotein-mediated decreased transport of CsA back from enterocytes into the gut lumen. The purpose of this pharmacokinetic study was to investigate the interaction between Sandimmun Neoral and grapefruit juice.

METHOD

Eight stable renal transplant recipients were studied during two 8-h sessions in a randomized cross-over design with 4 weeks interval. Following an overnight fast the patients ingested their habitual morning dose of Neoral either with water or with grapefruit juice. During the 8-h study period 10 blood samples were taken for determination of CsA concentration. These results formed the basis for calculation of area under curve (AUC), and half-life (t(1/2)). Maximum concentration (C(max)) and time until C(max) (t(max)) were obtained from the concentration-time profile.

RESULTS

The median AUC increased by 38% (12-194%) (P<0.05) following co-administration of Neoral with grapefruit juice. There were no significant changes in C(max), t(max), and t((1/2)).

CONCLUSION

Co-administration of Sandimmun Neoral with grapefruit juice resulted in an increased bioavailability of CsA, indicating unchanged pre-systemic enterocyte first-pass metabolism as compared to Sandimmun. There was no impact of an oral grapefruit juice load on systemic clearance of CsA. It seems prudent to advise renal allograft recipients treated with Sandimmun Neoral not to ingest their medication with grapefruit juice.

Region-dependent disappearance of vinblastine in rat small intestine and characterization of its P-glycoprotein-mediated efflux system

This study was aimed to characterize the absorption behavior of vinblastine (VLB), a well-known substrate of P-glycoprotein (P-gp), from rat small intestine, especially focusing on the regional-dependence of its efflux mediated by P-gp. VLB disappeared from duodenal and ileal loops of male Wistar rats fairly rapidly (30-60% in 30 min). In contrast, its disappearance from the jejunal loop was almost negligible and in some rats >100% of the jejunal dose was recovered. The radioactivity derived from [3H]VLB, which was absorbed from duodenum and ileum, was detected in the jejunal region. The jejunal appearance of radioactivity was increased when unlabeled VLB was present in the region in advance. The basolateral-to-apical transport of [3H]VLB across Caco-2 cell monolayers was greater when unlabeled VLB was added to the apical medium than when VLB-free buffer was applied to the apical side. When verapamil or cyclosporin A, potent modulators of P-gp, was added to the apical medium together with unlabeled VLB, enhanced basolateral-to-apical transport of [3H]VLB was disappeared. It is suggested that VLB absorption is strongly restricted by P-gp, especially in the jejunal region of the rat small intestine, and that the secretory transport via intestinal P-gp may be subject to trans-stimulation. Moreover, intravenously administered methylprednisolone and intramuscularly administered progesterone significantly enhanced the absorption of VLB, suggesting that parenterally administered P-gp modulators could influence the intestinal absorption of P-gp substrates.

Role of CYP3A in haloperidol N-dealkylation and pharmacokinetics in rats

Haloperidol (HP), an antipsychotic drug, is N-dealkylated by cytochrome P450 (CYP) to 4-fluorobenzoylpropionic acid (FBPA). The purpose of this study was to identify whether CYP3A metabolizes HP to FBPA in hepatic microsomes of rats and to investigate whether an inhibitor or an inducer of CYP3A affects HP pharmacokinetics in rats. The rate of FBPA formation was determined in hepatic microsomes from 8-week-old male Sprague-Dawley rats. Among several specific CYP isozyme inhibitors including troleandomycin (TAO), diethyldithiocarbamate, furafylline and quinine, only TAO showed marked inhibition of FBPA formation. Anti-rat CYP3A serum inhibited FBPA formation by 76.4%, while other anti-rat CYP sera (1A1, 1A2, 2B1, 2C11, 2E1) only slightly did. In a pharmacokinetic study, 8-week-old male Sprague-Dawley rats were given 0.5 mg/kg HP intravenously after treatment with 100 mg/kg erythromycin, a CYP3A inhibitor, or 80 mg/kg dexamethasone, a CYP3A inducer, intraperitoneally once a day for 7 days or 2 days, respectively or untreated. HP half-life was prolongated to 171% of the average control value by erythromycin and shortened to 49% of control by dexamethasone. HP clearance was reduced to 63% of control by erythromycin and was increased to 167% of control by dexamethasone. These results suggested that CYP3A mainly catalyzed HP to FBPA in rats, and the modification of this enzyme activity would affect the pharmacokinetics of HP.

Coadministration of neoral and the novel rapamycin analog, SDZ RAD, to rat lung allograft recipients: potentiation of immunosuppressive efficacy and improvement of tolerability of staggered versus simultaneous treatment

BACKGROUND

Neoral and rapamycin derivative (RAD) have complementary mechanisms for inhibition of lymphocyte activation and are substrates for the same pathways of drug metabolism. Therefore, we investigated treatment regimens designed to minimize pharmacokinetic interactions and to potentiate immunosuppressive efficacy in a highly stringent rat lung allograft model.

METHODS

Lewis recipients of Brown Norway lungs received the following daily oral doses: (A) RAD at 2.5 mg/kg (n=9); (B) Neoral at 7.5 mg/kg (n=8); (C) RAD at 2.5 mg/kg + Neoral at 7.5 mg/kg simultaneously (n=8); or (D) RAD at 2.5 mg/kg + Neoral at 7.5 mg/kg (n=6) staggered 6 hr apart. Rats were assessed by daily weights, chest radiographs, drug trough levels (high-performance liquid chromatography/mass spectrometry), and blinded scoring of graft histology at death (day 21).

RESULTS

Radiographs were completely opacified in all grafts of control and RAD monotherapy groups on days 7 and 14, respectively. Grafts were mildly opacified (Neoral monotherapy) and completely clear (both RAD + Neoral groups) on day 21. Simultaneous or staggered combined treatment dramatically reduced histologic rejection compared with treatment with either drug alone. Simultaneous treatment caused poor tolerability (poor grooming, lethargy) and significantly higher day-14 RAD and cyclosporine (CsA) trough levels (49+/-5 and 638+/-106 ng/ml; P<0.04) than in the staggered group (28+/-3 and 318+/-25 ng/ml) in which all animals were clinically normal. RAD and CsA day-14 trough levels in the staggered group were the same or lower than trough levels in animals treated with either drug alone (RAD 27+/-3/Neoral 815+/-67 ng/ml).

CONCLUSIONS

(1) Administration of RAD + Neoral suppressed lung rejection more effectively than treatment with either drug alone. (2) Trough levels did not differ between monotherapy and staggered combination therapy for RAD but were lower for CsA. These results suggested that pharmacological, rather than pharmacokinetic, interactions between the parent drugs were responsible for the potentiation of immunosuppression when these drugs were coadministered. 3) Staggered administration of RAD+Neoral avoided the pharmacokinetic interactions that caused the elevated drug blood levels and poor tolerability caused by simultaneous administration. Thus, we could potentiate efficacy and improve tolerability by staggering administration of RAD and Neoral.

LC/ESI-MS allows simultaneous and specific quantification of SDZ RAD and cyclosporine, including groups of their metabolites in human blood

An analytic technique using liquid chromatography (LC) coupled with electrospray-mass spectrometry (ESI-MS) has been developed for the simultaneous determination of the new immunosuppressant SDZ RAD (40-O-[2-hydroxy)ethylrapamycin) and cyclosporine (Cs), including their metabolites in blood. With the time-sparing, automated on-line extraction technique, the recovery of SDZ RAD averaged 95% and that of Cs, 94%. The calibration lines were linear from 0.5 to 100 microg/L (r2 = 0.99) for SDZ RAD and from 10 to 1,000 microg/L (r2 = 0.99) for Cs. The method has been tested on blood samples from renal transplant recipients taken between 1 and 5 hours after oral SDZ RAD and Cs administration. In blood, we found the following metabolites: Hydroxy-SDZ RAD, dihydroxy-SDZ RAD, demethyl-SDZ RAD, and the ring-opened form of SDZ RAD. The main metabolite of SDZ RAD in blood was hydroxy-SDZ RAD. This novel LC/ESI-MS technique provided an excellent method for simultaneous quantitative monitoring of SDZ RAD and Cs, including their relevant groups of metabolites in patients treated simultaneously with these immunosuppressants.

Suppression of acute rejection in allogeneic rat lung transplantation: a study of the efficacy and pharmacokinetics of rapamycin derivative (SDZ RAD) used alone and in combination with a microemulsion formulation of cyclosporine

BACKGROUND

The novel immunosuppressant SDZ RAD, 40-0 (2-hydroxyethyl)rapamycin, is an orally active rapamycin analogue developed for use in combination with cyclosporine (Neoral). The present study was designed to evaluate the efficacy of SDZ RAD, Neoral, or a combination of both drugs for suppression of acute rejection in an allogeneic, unilateral rat lung transplant model.

METHODS

Brown-Norway (RT1n) donor lungs were implanted into Lewis (RT1l) recipients that were observed for 21 days. Postoperative evaluation included daily weights, serial chest radiographs, drug trough levels, and histology scores of the transplanted lung on the day of sacrifice. Treatment groups were comprised of rats treated orally with the RAD vehicle as controls (n = 6); SDZ RAD 2.5 mg/kg/day (n = 9); Neoral 7.5 mg/kg/day (n = 8); Neoral 2.5 mg/kg/day (n = 6); SDZ RAD 2.5 mg/kg/day plus Neoral 7.5 mg/kg/day (n = 7); and Neoral 2.5 mg/kg/day plus SDZ RAD 2.5 mg/kg/day (n = 6).

RESULTS

The results of this study showed that neither monotherapy with 2.5 mg/kg/day of Neoral, nor 2.5 mg/kg/day of SDZ RAD prevented severe acute rejection in unilateral lung transplant recipients. Furthermore, despite high dose (7.5 mg/kg/day) Neoral treatment, graft histology showed moderate rejection. However, addition of 2.5 mg/kg/day of SDZ RAD to 7.5 mg/kg/day of Neoral completely prevented histologic rejection in four of seven grafts, although the remaining 3 grafts showed minimal rejection. This combination resulted in significantly higher RAD trough levels when compared to SDZ RAD treatment alone. Combining a lower dose of Neoral (2.5 mg/ kg/day) with 2.5 mg/kg/day of SDZ RAD resulted in less weight loss and improved animal health; however, the histology of lung grafts in these rats showed mild rejection.

CONCLUSIONS

This is the first study on the efficacy of the novel rapamycin derivative SDZ RAD for the control of acute lung allograft rejection. Results showed that acute unilateral rat lung allograft rejection is refractory to monotherapy with either high dose Neoral or SDZ RAD. The two regimens of combined treatment with Neoral plus SDZ RAD used in these studies produced either minimal rejection and reduced tolerability or mild rejection and better tolerability and showed potentiation of immunosuppression when both drugs were used together. Additional investigation of these two drugs is needed, however, to devise regimens that produce both high immunosuppressive efficacy and good tolerability.

Simultaneous on-line extraction and analysis of sirolimus (rapamycin) and ciclosporin in blood by liquid chromatography-electrospray mass spectrometry

We developed a sensitive and specific semi-automated liquid chromatography-electrospray mass spectrometric (HPLC-ESI-MS) assay for the simultaneous quantification of sirolimus and ciclosporin in blood. Following a simple protein precipitation step, the supernatants were injected into the HPLC system and extracted on-line. After column switching, the analytes were backflushed from the extraction column onto the analytical narrow-bore column and eluted into the ESI-MS system. The assay was linear from 0.4 to 100 microg/l sirolimus and from 2 to 1500 microg/l ciclosporin. The mean recoveries of sirolimus and ciclosporin were 98 and 96%, respectively. The mean interday precision/accuracy was 8.6%/-4.8% for sirolimus and 9.3%/-2.9% for ciclosporin.

Effects of glucocorticoids on pharmacokinetics and pharmacodynamics of midazolam in rats

We investigated the effect of dexamethasone (80 mg/kg per day for 2 days) and prednisolone (600 mg/kg per day for 2 days, equivalent to dexamethasone for glucocorticoid (GC) potency) on both pharmacokinetics and pharmacodynamics of midazolam (MDZ), a substrate for cytochrome P450 (CYP) 3A, in 8-week-old male Sprague-Dawley rats. Animals received a single injection of MDZ (pharmacokinetic study, 10 mg/kg; pharmacodynamic study, 55.5 mg/kg) in the tail vein 24 h after the last dose of GC or placebo. The elimination half-life (t(1/2)) and the area under the concentration-time curve of MDZ were significantly reduced by pretreatment with dexamethasone to 58.9% and 44.7% of the control value, respectively, and the clearance of MDZ was significantly increased by dexamethasone. Similar changes observed by prednisolone pretreatment did not reach significance. The t(1/2) of the dexamethasone pretreatment group (14.4+/-0.7 min) was significantly shorter than that of the prednisolone group (20.9+/-1.5 min). The amount of CYP3A2 protein and the activity of erythromycin N-demethylase were significantly increased by dexamethasone and prednisolone pretreatments, but dexamethasone showed a greater effect than prednisolone. Sleeping time was significantly shortened by dexamethasone and prednisolone pretreatment to 38.7% and 57.1% of control value, respectively. The current study demonstrates that the anesthetic effect of MDZ would be reduced in patients treated with dexamethasone or prednisolone, and that the CYP3A induction was greater by dexamethasone than by prednisolone, implying that the potency of CYP3A induction may differ among GCs even when GC activity is the same.

Metabolism and drug interactions of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors in transplant patients: are the statins mechanistically similar?

3-Hydroxy-3-methylglutaryl coenzyme A reductase (EC 1.1.1.88) inhibitors are the most effective drugs to lower cholesterol in transplant patients. However, immunosuppressants and several other drugs used after organ transplantation are cytochrome P4503A (CYP3A, EC 1.14.14.1) substrates. Pharmacokinetic interaction with some of the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, specifically lovastatin and simvastatin, leads to an increased incidence of muscle skeletal toxicity in transplant patients. It is our objective to review the role of drug metabolism and drug interactions of lovastatin, simvastatin, pravastatin, fluvastatin, atorvastatin, and cerivastatin. In the treatment of transplant patients, from a drug interaction perspective, pravastatin, which is not significantly metabolized by CYP enzymes, and fluvastatin, presumably a CYP2C9 substrate, compare favorably with the other statins for which the major metabolic pathways are catalyzed by CYP3A.

Effects of intestinal and hepatic metabolism on the bioavailability of tacrolimus in rats

PURPOSE

Tacrolimus, an immunosuppressive agent, has poor and variable bioavailability following oral administration in clinical use. We investigated the contribution of intestinal metabolism to the first pass effect of tacrolimus in rats.

METHODS

Tacrolimus was administered intravenously, intraportally or intraintestinally to rats. Blood samples were collected over a 240-min period, and blood tacrolimus concentrations were measured. The extraction ratios of tacrolimus in the intestine and liver were investigated. In addition, the metabolism of tacrolimus in the everted sacs of the small intestine was examined.

RESULTS

The rate of absorption of tacrolimus in the intestine was rapid, and tacrolimus was almost completely absorbed after intestinal administration. The bioavailability of tacrolimus was about 40% and 25% after intraportal and intraintestinal administration, respectively. indicating that tacrolimus is metabolized in both the intestine and the liver. In addition, tacrolimus was significantly metabolized in the everted sacs of the rat intestine.

CONCLUSIONS

The present study suggested that the metabolism of tacrolimus in the intestine contributes to its extensive and variable first pass metabolism following the oral administration.

Possibility of partial absorption of nicardipine by routes other than the hepato-portal system after oral administration in rats

The systemic availability of nicardipine after different routes of administration has been examined in rats, with particular attention to differentiating oral absorption from intestinal and hepatic metabolism. The quantities of nicardipine and its metabolite were determined by capillary column gas chromatography. A linear relationship was shown between the hepatic first-pass effect and dose after hepato-portal administration of nicardipine; the hepatic first-pass effect was calculated to be approximately 80%. However, the availability after oral and rectal administration was found to be more than twice that observed after hepato-portal administration. Partial avoidance of the hepatic first-pass effect after oral and rectal administration are estimated to be 37.3% and 35.2%, respectively, assuming that all absorbed molecules pass through the liver. These findings suggest that the absorption of nicardipine after oral administration also occurs by routes other than the hepato-portal system.

Accumulation of lovastatin, but not pravastatin, in the blood of cyclosporine-treated kidney graft patients after multiple doses

OBJECTIVES

To study pravastatin and lovastatin pharmacokinetic and pharmacodynamic effects and their interactions with cydosporine (INN, ciclosporin) in kidney transplant patients after single and multiple doses.

SUBJECTS AND METHODS

The pharmacokinetic and pharmacodynamic effects of administration of 20 mg/day oral pravastatin and lovastatin for 28 days and their interactions with cyclosporine (2 to 6 mg/kg/day) were studied in a double-blind, double-dummy, randomized, parallel-group multicenter trial in 44 stable kidney graft recipients.

RESULTS

The median area under the curve [AUC(0-24)] of pravastatin was 249 microg x hr/L (range, 104 to 1026 microg x hr/L) after a single dose (day 1) and 241 microg x hr/L (114 to 969 microg x hr/L) after multiple doses (day 28) and was fivefold higher than values reported in the absence of cyclosporine. The median AUC(0-24) of lovastatin was 243 microg x hr/L (105 to 858 microg x hr/L) on day 1 and 459 microg x hr/L (140 to 1508 microg x hr/L) on day 28. Besides a significant accumulation during the study period (p < 0.001), the lovastatin AUC(0-24) values were twentyfold higher than values reported without cyclosporine. Coadministration of pravastatin or lovastatin did not alter cyclosporine pharmacokinetics. In this study, 20 mg/day doses of both drugs resulted in a significant improvement of the lipid profile and were well tolerated.

CONCLUSIONS

In contrast to lovastatin, pravastatin did not accumulate over the study period, which is probably one of the reasons rhabdomyolysis has been reported in lovastatin-treated but not pravastatin-treated transplant patients receiving cyclosporine immunosuppression.

Tacrolimus oral bioavailability doubles with coadministration of ketoconazole

OBJECTIVE

To quantitate the effect of ketoconazole, an azole antifungal agent and potent inhibitor of CYP3A4 and P-glycoprotein, on the bioavailability of tacrolimus, a substrate of the CYP3A system and of P-glycoprotein.

SUBJECTS AND METHODS

The pharmacokinetics of tacrolimus were studied in six healthy volunteers (two women and four men) in a four-dose study after each received single doses of tacrolimus alone (0.1 mg/kg orally and 0.025 mg/kg intravenously) and with coadministered ketoconazole (200 mg orally at bedtime for 12 days). The dose of tacrolimus was reduced during the ketoconazole phase (0.04 mg/kg orally; 0.01 mg/kg intravenously). Ketoconazole and tacrolimus doses were separated by approximately 10 hours. Whole blood tacrolimus concentrations were determined by enzyme-linked immunosorbent assay. Estimated pharmacokinetic parameters in whole blood (mean +/- SD) before and with ketoconazole were calculated with noncompartmental techniques.

RESULTS

Coadministration of ketoconazole did not consistently affect tacrolimus clearance (55.6 +/- 16.7 ml/hr/kg versus 42.5 +/- 7.6 ml/hr/kg), and steady-state volume of distribution was unchanged (0.99 +/- 0.26 L/kg versus 0.93 +/- 0.25 L/kg). However, a significant increase in tacrolimus bioavailability (14% +/- 5% versus 30% +/- 8%; p < 0.01) was observed with coadministered ketoconazole. Hepatic bioavailability was unchanged by the presence of ketoconazole (96% +/- 1% versus 97% +/- 1%).

CONCLUSIONS

Because ketoconazole did not alter hepatic bioavailability and because 10 hours separated administration times of the drugs, it appears that the marked increase in tacrolimus bioavailability can be explained by ketoconazole having a local inhibitory effect on tacrolimus gut metabolism or on intestinal P-glycoprotein activity.

Pharmacokinetics of sirolimus in stable renal transplant patients after multiple oral dose administration

In this 2-week, ascending dose study, the pharmacokinetic activity of sirolimus was examined in 40 stable renal transplant patients treated with cyclosporine and prednisone. Nine dose levels (range, 0.5-6.5 mg/m2/12 hr) of sirolimus were studied in a parallel design. Mean values for the pharmacokinetic parameters of sirolimus calculated in all dose groups were as follows: time to peak blood concentration, 1.4 +/- 1.2 hours; terminal half-life, 62 +/- 16 hours; oral dose clearance, 208 +/- 95 mL/h/kg; apparent oral steady-state volume of distribution, 12 +/- 5 L/kg; and blood/plasma ratio, 38 +/- 13. The intersubject variabilities in dose clearance, steady-state volume of distribution, and blood/plasma ratio were 4.5-fold. Preliminary assessments suggests linear dose proportionality. An excellent correlation existed between area under the concentration-time curve and trough blood concentration at steady state. Sirolimus did not produce any significant changes in area under the concentration-time curve of cyclosporine. Preliminary analysis suggested that values for the pharmacokinetic parameters of sirolimus vary among races (black versus nonblack) but not among genders.

Ascorbic acid protects guinea pigs from acute aflatoxin toxicity

These studies were conducted to investigate whether ascorbic acid protected guinea pigs from aflatoxin B1 (AFB1) toxicity. Young guinea pigs, fed either 0 (AA) or 25 mg (25 AA) or gavaged 300 mg ascorbic acid (300 AA) per day for 21 days, were gavaged with the LD50 dose of AFB1 on the 22nd day. Seven out of 10 animals in the AA group died within 72 hr of AFB1 administration. The livers of the animals showed regional massive necrosis and multilobular degeneration. There was no mortality in the 25 AA group. Their livers, however, showed changes similar to those seen in AA group. Serum alanine amino transferase (ALAT) and aspartate amino transferase (ASAT) levels were elevated. There was neither mortality nor pathological changes in livers in the 300 AA group. Their ALAT and ASAT levels were unaffected. In vitro production of AFM1 by liver microsomes tended to be higher than that in the other two groups. Three animals saved from the 300 AA group and continued with their supplementation were administered a second, intraperitoneal (ip) LD50 dose of AFB1 1 month after the first AFB1 dose. One animal died. Livers of the animals showed centrilobular degeneration and moderate necrosis in scattered hepatocytes. Liver microsomal cytochrome P450 and cytosolic glutathione S-transferase (GST) levels and AFM1 production were drastically reduced. ALAT and ASAT activities were raised. The results indicated that intake of 300 mg of ascorbic acid almost protected the animals from acute toxicity of AFB1 when given by gavage, but not when administered as a second dose ip.

Distribution of sirolimus in rat tissue

OBJECTIVES

To examine the distribution of sirolimus (SRL, rapamycin), an immunosuppressive macrolide antibiotic, in the tissues of adult male Wistar-Furth rats following continuous intravenous infusion (CIVI) and repeated daily peroral gavage (PO).

DESIGN AND METHODS

Animals received 14-day courses of SRL by either CIVI (0.04-0.4 mg/kg/day) or PO (0.4-1.6 mg/kg/day) administration. Samples of whole blood and homogenates of five solid organs (heart, kidney, liver, lung and spleen), and portions of intestinal, muscle and testicular tissues were prepared on day 13 of CIVI treatment or 24 hours after administration of the 14th PO dose. SRL concentrations were determined by high performance liquid chromatography with reference to calibration curves produced from SRL-spiked whole blood or tissue homogenates prepared from drug-free animals.

RESULTS

Following PO but not CIVI administration, SRL concentrations in whole blood and all tissues increased linearly in relation to dose. SRL was extensively distributed among most tissues tested (tissue partitions coefficients of > 40 were observed in some cases). Comparatively, SRL whole blood concentrations were low. The ratio between the SRL whole blood concentrations after PO versus after CIVI administration (at like doses of 0.4 mg/kg/day) was 0.04. Therefore, we inferred that the oral bioavailability of SRL was low.

CONCLUSIONS

The linear relationships between PO dose and SRL concentrations in whole blood and tissues may be attributed to the low oral bioavailability of SRL, which is indicated by the low levels of SRL observed in whole blood and tissues after PO administration. The nonlinear relationships between CIVI dose and SRL concentrations in whole blood and tissues may result because although whole blood depots may be saturated with SRL, the tissues continue to absorb SRL as the dose of SRL increases. Thus, because a high percentage of SRL is widely distributed into tissues stores, caution must be used when administering this drug in humans.

Effect of grapefruit juice on the pharmacokinetics and pharmacodynamics of quinidine in healthy volunteers

A study was conducted to examine the effect of grapefruit juice on the disposition of quinidine sulfate and changes of QT intervals after oral administration to twelve healthy male volunteers. Participants received two oral doses of quinidine sulfate tablets (400 mg) with 240 mL of water or grapefruit juice, each separated by a 1-week washout period. Plasma samples for analysis of quinidine and its major metabolite, 3-hydroxyquinidine, were collected for a 24-hour period and analyzed by a high-performance liquid chromatography method. For pharmacodynamic data, the electrocardiograms (ECGs) were performed for 12 hours, and the recordings were marked for ECG interval at all blood collection time periods. There was no significant difference in pharmacokinetic parameters of quinidine when administered with grapefruit juice or water, except for time to maximum concentration (tmax), which was 1.6 hours after administration with water and 3.3 hours after administration with grapefruit juice. Administration with grapefruit juice also resulted in a 33% decrease in the area under the concentration-time curve (AUC) of 3-hydroxyquinidine compared with water, but did not increase the AUC of quinidine or change the ratio of AUC of 3-hydroxyquinidine to the AUC of quinidine. Pharmacodynamic parameters, including changes in the rate-corrected QT (QTc) interval, closely paralleled the pharmacokinetic data, in that administration with grapefruit juice led to delayed maximal effect on QTc and reduction in maximal effect. Administration with grapefruit juice therefore delays the absorption of quinidine and inhibits the metabolism of quinidine to 3-hydroxyquinidine.

Suppression of intestinal and hepatic cytochrome P4503A in murine Toxoplasma infection. Effects of N-acetylcysteine and N(G)-monomethyl-L-arginine on the hepatic suppression

1. Cytochrome P4503A (CYP3A) expression was studied in a murine model of infection. Mice were infected with a cystogenic strain of Toxoplasma gondii and microsomes were prepared for liver homogenates and jejunum villus tip enterocytes on day 10 postinfection. Total cytochrome P450 (CYP) and CYP3A were quantitated, and CYP3A activity was determined. 2. In the infected mouse, total CYP and CYP3A contents fell in the liver (-39 and - 49% respectively) and intestine (-43 and - 48 % respectively), as did the rate of metabolism of erythromycin (Ery) and cyclosporine A (CyA), two markers of CYP3A activity (-36 and -26% in the liver, -35 and -58% in the intestine). 3. To determine the mechanism(s) involved in the depression of hepatic CYP3A, infected mice were treated on day 7.5 post-infection with a monoclonal antibody raised against interferon-gamma (anti-IFN-gamma, or from days 7.5 to 10 post-infection with either N(G)-monomethyl-L-arginine (NMMA), an inhibitor of reactive nitrogen intermediates (RNI) production, or N-acetylcysteine (NAC), a reactive oxygen intermediates (ROI) scavenger. 4. Total CYP content was restored in the liver of infected mice treated with anti-IFN-gamma, but with marked interindividual variability. NAC treatment led to a recovery in the liver of total CYP content (+35 %), CYP3A content (total recovery), and the rates of Ery (+59%) and CyA (+87%) metabolism, whereas inconsistent results were obtained with NMMA. These results suggest that NAC, but probably not NMMA, partially protects hepatic CYP3A from Toxoplasma-mediated suppression in mouse.

Metabolism of the macrolide immunosuppressant, tacrolimus, by the pig gut mucosa in the Ussing chamber

1. The macrolide tacrolimus (FK506), used as an immunosuppressant, is a cytochrome P450 (CYP) 3A substrate in the liver. The metabolism of tacrolimus and the transport of its metabolites in the pig gut was studied in the Ussing chamber. Tacrolimus and its metabolites were quantified by h.p.l.c./mass spectrometry. 2. In the Ussing chamber, demethyl, didemethyl, hydroxy and hydroxy-demethyl tacrolimus were generated. Their formation was concentration- and time-dependent. The metabolite pattern was not different from that after incubation of tacrolimus with human small intestinal microsomes. 3. The metabolite formation was highest in the duodenum and declined in the order duodenum > jejunum > ileum > colon > stomach. 4. Since tacrolimus metabolism was inhibited by the specific CYP3A inhibitors, troleandomycin and ketoconazole, we concluded that these enzymes are involved in intestinal metabolism of tacrolimus. 5. Tacrolimus metabolites re-entered the mucosa chamber (> 90%) and passed through the small intestinal preparation into the serosa chamber. 6. It is concluded that tacrolimus is metabolized in the intestine, that the metabolites are able to re-enter the gut lumen and also enter into the portal vein and that small intestinal metabolism and transport is at least in part responsible for the low oral bioavailability of tacrolimus.

Decreased intestinal CYP3A in celiac disease: reversal after successful gluten-free diet: a potential source of interindividual variability in first-pass drug metabolism

BACKGROUND

Cytochrome P450 (CYP) 3A is constitutively expressed in human intestinal villi and may account for significant "first-pass" prehepatic metabolism of a number of drugs in the intestine. Celiac disease results in small intestinal atrophy. We hypothesized that this would result in a loss of CYP3A.

METHODS

Formalin-fixed jejunal biopsy specimens taken from nine patients with celiac disease at variable times before and after treatment with a gluten-free diet were immunoperoxidase stained after incubation with anti-CYP3A4 rabbit antisera (1:2000 dilution). The amount of immunoreactive CYP3A was determined by two observers who were blinded to the treatment states of the patients. Staining intensity was graded on a numerical scale from 1 to 4+ on the basis of intensity of staining in individual enterocytes, as well as the total number of enterocytes stained.

RESULTS

Slides of biopsy specimens from all nine untreated patients with celiac disease were graded 1. Treatment with a gluten-free diet was associated with a significant increase in CYP3A immunoreactive protein in small bowel biopsy specimens (p < 0.05, Wilcoxon signed-rank test).

CONCLUSIONS

We conclude that patients with celiac disease have low intestinal CYP3A immunoreactivity and that treatment with a gluten-free diet is associated with an increase in intestinal CYP3A immunoreactive protein. Our findings suggest that intestinal disease and variability in intestinal CYP3A activity might be an unexamined variable that may contribute to interindividual variability in drug disposition.

Multiple drug interactions with cyclosporine in a heart transplant patient

OBJECTIVE

To report multiple drug interactions with cyclosporine in a heart transplant recipient.

CASE SUMMARY

A 53-year-old man underwent heart transplantation in December 1990. Immunosuppression therapy consisted of prednisone, azathioprine, and cyclosporine 300 mg/d. For 5 months, the trough specific cyclosporine (parent compound) concentration was stable (range 211-226 ng/mL). More recently, he developed a productive cough accompanied by high fever, chills, and weakness and was admitted to a hospital near his home. Antituberculosis therapy was advised including rifampin and isoniazid. After a week, erythromycin 3.6 g/d i.v. was added. After 10 days of the combined therapy he was transferred to our hospital, where the first cyclosporine blood concentrations measured were 77 and 238 ng/mL for specific and total cyclosporine (parent drug + metabolites). Because of the low cyclosporine blood concentration, the dose was increased to 400 mg/d. In light of negative sputum smears for acid-fast bacilli and culture, the rifampin/isoniazid therapy was withdrawn; the erythromycin was continued. At this time, the specific cyclosporine blood concentration rose to 934 ng/mL and the total cyclosporine concentration reached 1503 ng/mL. High cyclosporine blood concentrations were measured during the intravenous erythromycin treatment period, even though the cyclosporine dose had been decreased to 150 mg/d. A further increase in cyclosporine concentration was observed when erythromycin was given orally (4.0 g/d). The cyclosporine dose was then discontinued for 2 days and started again at 50 mg/d until the end of the erythromycin treatment period. The patient recovered, the cyclosporine dose was increased to 100 mg/d, and on regular monitoring the cyclosporine blood concentrations were within the therapeutic range (100-400 ng/mL for specific and 250-1000 ng/mL for total cyclosporine).

DISCUSSION

Cyclosporine is metabolized almost completely in the liver by the cytochrome P-450IIIA enzyme system. Drugs such as rifampin and erythromycin, which are known to be inducers or substrates of cytochrome P-450IIIA, have the potential to alter cyclosporine blood concentrations. The present case shows a multiple drug interaction with cyclosporine. Coadministration of rifampin/isoniazid and cyclosporine for a week, and erythromycin for the last 4 days, resulted in low cyclosporine blood concentrations, probably because of microsomal induction by rifampin. When the rifampin/isoniazid treatment was discontinued, the cyclosporine blood concentrations rose, indicating the interacting effect of intravenous erythromycin. This effect was even more pronounced when therapy was changed from intravenous to oral administration. Erythromycin, a substrate that is metabolized with great affinity by the cytochrome P-450IIIA enzyme, prolonged the elimination of cyclosporine by competing for the same site of metabolism.

CONCLUSIONS

Awareness of potential cyclosporine drug interactions in organ transplant patients of great clinical importance. Regular monitoring of cyclosporine blood concentrations and renal function are essential to detect such interactions, to allow adjustment of drug dosage, and to reduce toxicity and enhance therapeutic effect, in particular in patients coadministered the many drugs known to have pharmacokinetic interactions with cyclosporine.

Augmented arterial pressure responses to cyclosporine in spontaneously hypertensive rats. Role of cytochrome P-450 3A

Evidence to support a hypertensinogenic role of family 3A cytochrome P-450 (CYP3A) activity is that troleandomycin, a selective inhibitor of CYP3A, decreases both blood pressure and in vivo corticosterone 6 beta-hydroxylation in spontaneously hypertensive rats (SHR). Renal CYP3A activity is markedly increased in SHR compared with Wistar-Kyoto (WKY) rats. Cyclosporine acutely increases both systolic blood pressure and renal total cytochrome P-450 in SHR. We tested the hypothesis that the augmentation of blood pressure by cyclosporine is mediated by a further increase in renal CYP3A activity. Accordingly, we assessed the effect of troleandomycin administration on cyclosporine-induced systolic blood pressure increase and renal and hepatic microsomal CYP3A activity in SHR. Cyclosporine (5 mg/kg SC) given daily in 11-week-old SHR resulted in substantial augmentation of blood pressure after 6 days. This blood pressure increase was attenuated by troleandomycin (40 mg/kg) given either during or after development of hypertension. Cyclosporine increased renal (60%) but decreased hepatic (25%) microsomal CYP3A activity in SHR. In contrast, cyclosporine failed to produce any detectable increase in either blood pressure or renal CYP3A activity in WKY rats. Troleandomycin completely inhibited renal CYP3A activity measured after cyclosporine treatment of SHR, which correlated with its attenuation of the cyclosporine-induced blood pressure increase. These findings suggest that renal CYP3A could play an important role in acute cyclosporine-induced hypertension.

Intestinal clearance of H2-antagonists

Jejunal perfusion of cimetidine resulted in the appearance of lumenal cimetidine sulfoxide in both rats and humans. In the rat, ileal perfusion yielded negligible sulfoxide metabolite as compared with that of the jejunum. Jejunal co-perfusion of an anionic-exchange inhibitor, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, blocked the appearance of drug sulfoxide, and methionine co-perfusion yielded concentration-dependent inhibition of lumenal cimetidine sulfoxide. Intravenous injection of high concentrations of cimetidine sulfoxide did not produce detectable lumenal metabolite levels during jejunal perfusion of drug-free buffer, providing in situ evidence that lumenal metabolite is generated by the small intestine. The extent of the appearance of lumenal sulfoxide was significantly greater for cimetidine than for the other three marketed H2-antagonists in rat jejunum. Variable intestinal clearance of this extensively prescribed class of therapeutic agents may contribute to their absorption variability.

Aflatoxin B1-adduct formation in rat and human small bowel enterocytes

BACKGROUND/AIMS

Hepatic CYP3A enzymes have been implicated in the bioactivation of aflatoxin B1 (AFB1) to DNA binding metabolites. CYP3A enzymes are also abundant in the small bowel, and we therefore examined the ability of this tissue to form intracellular AFB1 adducts.

METHODS

Immunohistochemistry using a antibody to the stable AFB1-DNA adduct was performed on small bowel sections obtained from rats orally gavaged with AFB1 and on human small bowel biopsy specimens maintained in explant culture. 3H-AFB1 was instilled into a loop of small bowel of untreated rats and rats pretreated with the CYP3A inducer dexamethasone during vivisection. DNA was isolated from the loop 2 hours later and assayed for specific activity.

RESULTS

In both rats and humans, AFB1-adducts were detected exclusively in mature enterocytes in a pattern similar to the distribution of CYP3A enzymes. Induction of enterocyte CYP3A in rats resulted in an increase in enterocyte immunoreactive AFB1 adducts and in a 1.8-fold increase in 3H-AFB1-nucleic acid adducts (P = 0.01).

CONCLUSIONS

Intracellular AFB1 adducts are formed in the small intestine, and this reflects, at least in part, the catalytic activity of CYP3A enzymes. Because these AFB1 adducts should ultimately pass in stool, enterocyte CYP3A may represent a regulatable barrier to dietary aflatoxins.

Pharmacokinetics of orally and intravenously administered cyclosporine in pre-kidney transplant patients

The pharmacokinetics of cyclosporine (CSA) and four metabolites were evaluated in eight hemodialysis subjects awaiting renal transplantation to compare metabolic patterns with those observed in post-transplant patients and normal volunteers. Each subject received a single 4-mg/kg intravenous and a single 10-mg/kg oral dose separated by a 1-week washout period. Blood samples were collected before and at .5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 10, 12, 14, and 24 hours after CSA dosing. Cyclosporine blood, plasma, and metabolite (M17, M1, M18, M21) levels were determined by high-pressure liquid chromatography. Mean (+/- standard deviation) CSA blood clearance was .47 +/- .15 L/hour/kg, steady-state volume of distribution (Vss) was 1.9 +/- .5 L/kg, and mean residence time (MRT) was 4.4 +/- 1.8 hours after intravenous dosing. With plasma, mean clearance was .70 +/- .31 L/hour/kg, Vss was 2.4 +/- 1.2 L/kg, and MRT was 3.7 +/- 2.2 hours. Cyclosporine bioavailability (F) averaged 24 +/- 11 and 24 +/- 15%, using blood and plasma, respectively. Values for clearance and Vss were approximately 30 to 100% greater than comparable estimates in healthy volunteers, but F and MRT were not altered to this extent. These changes might be explained on the basis of decreased protein binding in uremic patients. The area under the curve ratio for M17 and M1 to CSA increased an average of 1.7- and 3.9-fold, respectively, after oral dosing compared with intravenous administration, indicating increased conversion during first-pass metabolism.

Metabolism of cyclosporine after orthotopic liver transplantation

The aim of this work was to determine whether the extensive metabolism of cyclosporine, acquired in a donor by treatment with an inducer of cytochrome P450 3A (P450 3A) (cyclosporine oxidase), was transmissible to the recipient by orthotopic liver transplantation. For this purpose, male Wistar rats were divided into five groups including: control animals (group C), animals treated with dexamethasone (an inducer of P450 3A, 50 or 300 mg/kg/day, for 4 days, group D), animals transplanted with the livers of control rats (group G) or with the livers of dexamethasone-induced rats (group GD), and animals treated with beta-naphthoflavone (an inducer of P450 1A, group B). All animals received a single i.v. dose of 10 mg/kg cyclosporine 24 hr after either the last dose of inducer or the transplantation. For each group of animals, the area under the curve (AUC) of cyclosporine was calculated from the curves of blood cyclosporine levels (by radioimmunoassay) against time; liver microsomes were assayed for cyclosporine oxidase activity by HPLC, erythromycin demethylase and P450 3A level by western blot with specific anti-P450 3A antibodies. The decrease in the AUC in groups D and GD with respect to C and G was correlated with increased level of P450 3A (4-5-fold with respect to control) as well as of microsomal cyclosporine oxidase. In addition, cyclosporine oxidase activity of liver microsomes was specifically inhibited by anti-P450 3A antibodies and troleandomycin. The animals in group B did not exhibit increased metabolism of cyclosporine either in vivo or in vitro. We conclude that: (1) cyclosporine is predominantly oxidized in the rat liver by a form of P450 from the 3A subfamily; (2) the extensive metabolism of cyclosporine acquired by donor rats after treatment with dexamethasone is transmissible to the recipients through orthotopic liver transplantation.

Cultured adult rat jejunal explants as a model for studying regulation of CYP3A

Enzymes within the CYP3A subfamily are major Phase I drug-metabolizing enzymes present in hepatocytes and small bowel enterocytes. These enzymes are highly inducible in the liver by many structurally diverse compounds, including a number of commonly used medications. Studies indicate that CYP3A enzymes present in small bowel enterocytes are also inducible. However, the regulation of CYP3A enzymes in this tissue has not been well characterized, in part because in vivo studies are difficult, especially in humans. Our goals was to develop an in vitro model to study the regulation of CYP3A in enterocytes. To this end, we defined culture conditions under which adult rat jejunal explants maintained viable appearing villi for 21 hr. When dexamethasone, the prototypical inducer of CYP3A1 in rat hepatocytes, was added to the culture medium, there was a time-dependent induction of CYP3A1 mRNA and CYP3A protein in explant enterocytes which was essentially indistinguishable from the time course of induction of CYP3A1 mRNA and protein in enterocytes in vivo. This effect of dexamethasone appeared to be specific since dexamethasone had no consistent effect on the explant concentration of another enterocyte specific mRNA, intestinal fatty acid binding protein. Using this explant culture model, we found that CYP3A1 mRNA was also inducible by clotrimazole but we were unable to detect induction by rifampicin or troleandomycin. Our observations suggest that jejunal explants may provide an appropriate model for the study of the regulation of CYP3A and other drug-metabolizing enzymes.

Cyclosporin metabolism in transplant patients

The immunosuppressant cyclosporin, a cyclic undecapeptide, is metabolized to more than 30 metabolites. Cytochrome P450IIIA enzymes located in liver and small intestine are responsible for the biotransformation of cyclosporin and its metabolites and are the site of several drug interactions. It is still under discussion, whether the cyclosporin metabolites are involved in the immunosuppressive and/or toxic activities of cyclosporin. While isolated metabolites show not more than 10-20% of the activity of the mother compound in vitro, metabolite combinations have additive and synergistic effects. Isolated metabolites show no toxic effects in rat models while there is an association between metabolite blood concentrations and cyclosporin toxicity in several clinical studies. Possible mechanisms for the toxic effect of cyclosporin metabolites are covalent binding to macromolecules in liver and kidney, alteration of the cytochrome P450 pattern in liver and kidney, increased endothelin production in the kidney and synergistic effects of cyclosporin combinations on mesangial cells. Liver dysfunction leads to an alteration of the metabolite patterns and to increased concentrations of cyclosporin metabolites in blood. In conclusion there is evidence that cyclosporin metabolites may contribute to cyclosporin toxicity and high metabolite blood concentrations in patients should not be tolerated.

Identification of rifampin-inducible P450IIIA4 (CYP3A4) in human small bowel enterocytes

Enzymes within the P450IIIA (CYP3A) subfamily appear to account for significant "first pass" metabolism of some drugs in the intestine. To identify which of the known P450IIIA genes are expressed in intestine, enterocyte RNA was hybridized on Northern blots with synthetic oligonucleotides complementary to hypervariable regions of hepatic P450IIIA4, P450IIIA5, and P450IIIA7 cDNAs. Hybridization was detected only with the P450IIIA4-specific oligonucleotide. The identity of the hybridizing mRNA was confirmed to be P450IIIA4 by direct sequencing of a DNA fragment amplified from enterocyte cDNA by the polymerase chain reaction. To determine if enterocyte P450IIIA4 is inducible, biopsies of small bowel mucosa were obtained from five volunteers before and after they received 7d of treatment with rifampin, a known inducer of P450IIIA4 in liver. Rifampin treatment resulted in a five- or eightfold mean increase (P < 0.05) in the biopsy concentration of P450IIIA4 mRNA when normalized for content of sucrase isomaltase or intestinal fatty acid binding protein mRNAs, respectively. Rifampin also induced P450IIIA immunoreactive protein in enterocytes in each of the subjects, as judged by immunohistochemistry, and resulted in a 10-fold increase in P450IIIA4-specific catalytic activity (erythromycin N-demethylation) in the one patient studied. Our identification of inducible P450IIIA4 in enterocytes may in part account for drug interactions characteristic of P450IIIA4 substrates and suggests a strategy for controlling entry into the body of a major class of xenobiotics.

First-pass metabolism of cyclosporin by the gut

Cyclosporin is thought to be exclusively metabolised in the liver. We instilled cyclosporin into the small bowel of 2 patients during the anhepatic phase of liver transplantation; cyclosporin metabolites were readily detected in portal venous blood. Our findings indicate that the small intestine is a major site of cyclosporin breakdown: such intestinal metabolism might help to explain the poor oral bioavailability and drug interactions of cyclosporin.