In vitro and in vivo drug interactions involving human CYP3A

Evidence of CYP3A allosterism in vivo: analysis of interaction between fluconazole and midazolam

The allosteric effect of fluconazole (effector) on the formation of 1’-hydroxymidazolam (1’-OH-MDZ) and 4-hydroxymidazolam (4-OH-MDZ) from the CYP3A4/5 substrate, midazolam (MDZ), was examined in healthy volunteers. Following pre-treatment of fluconazole, AUC_4-OH/AUC_MDZ increased 35–62%, while AUC_1’-OH/AUC_MDZ decreased 5–37%; AUC_1’-OH/AUC_4-OH ratio decreased 46–58% by fluconazole and had no association with CYP3A5 genotype. 1’-OH-MDZ formation in vitro was more susceptible than 4-OH-MDZ formation to inhibition by fluconazole. Fluconazole decreased the intrinsic formation clearance ratio of 1’-OH-MDZ/4-OH-MDZ to an extent that was quantitatively comparable to in vivo observations. The elimination clearance of midazolam metabolites appeared unaffected by fluconazole. This study demonstrated that fluconazole alters midazolam product formation both in vivo and in vitro in a manner consistent with an allosteric interaction. The 1'-OH-MDZ/4-OH-MDZ ratio may serve as a biomarker of such interactions between midazolam, CYP3A4/5 and other putative effectors.

Pain, analgesia and genetics

OBJECTIVES

In the clinical setting, there is marked intersubject variability in the intensity of pain reported by patients with apparently similar pain states, as well as widely differing analgesic dosing requirements between individuals to produce satisfactory pain relief with tolerable side-effects. Genetic and environmental factors as well as their interaction are implicated, and these are discussed in this review.

KEY FINDINGS

Pioneering work undertaken in mice more than a decade ago, showed a strong genetic contribution to levels of nociception/hypersensitivity as well as levels of antinociception produced by commonly available analgesic agents. To date more than 300 candidate 'pain' genes have been identified as potentially contributing to heritable differences in pain sensitivity and analgesic responsiveness in animals and humans, with this information available in a publicly accessible database http://www.jbldesign.com/jmogil/enter.html. Since then, many genetic association studies have been conducted in humans to investigate the possibility that single nucleotide polymorphisms (SNPs) in an individual gene may explain drug inefficacy or excessive toxicity experienced by a small subset of the whole population who have the rare allele for a particular SNP.

SUMMARY

Despite the fact that SNPs in more than 20 genes that affect pain sensitivity or contribute to interindividual variability in responses to analgesic medications have been identified in the human genome, much of the data is conflicting. Apart from deficiencies in the design and conduct of human genetic association studies, recent research from other fields has implicated epigenetic mechanisms that facilitate dynamic gene-environment communication, as a possible explanation.

The effect of age on the discriminative stimulus effects of ethanol and its GABA(A) receptor mediation in cynomolgus monkeys

RATIONALE

Excessive alcohol consumption is less common among aged compared to young adults, with aged adults showing greater sensitivity to many behavioral effects of ethanol.

OBJECTIVES

This study compared the discriminative stimulus effects of ethanol in young and middle-aged adult cynomolgus monkeys (Macaca fascicularis) and its γ-aminobutyric acid (GABA)(A) receptor mediation.

METHODS

Two male and two female monkeys trained to discriminate ethanol (1.0 g/kg, i.g.; 60-min pre-treatment interval) from water at 5-6 years of age (Grant et al. in Psychopharmacology 152:181-188, 2000) were re-trained in the current study more than a decade later (19.3 ± 1.0 years of age) for a within-subjects comparison. Also, four experimentally naïve middle-aged (mean ± SEM, 17.0 ± 1.5 years of age) female monkeys were trained to discriminate ethanol for between-subjects comparison with published data from young adult naïve monkeys.

RESULTS

Two of the naïve middle-aged monkeys attained criterion performance, with weak stimulus control and few discrimination tests, despite greater blood-ethanol concentration 60 min after 1.0 g/kg ethanol in middle-aged compared to young adult female monkeys (Green et al. in Alcohol Clin Exp Res 23:611-616, 1999). The efficacy of the GABA(A) receptor positive modulators pentobarbital, midazolam, allopregnanolone, pregnanolone, and androsterone to substitute for the discriminative stimulus effects of 1.0 g/kg ethanol was maintained from young adulthood to middle age.

CONCLUSIONS

The data suggest that 1.0 g/kg ethanol is a weak discriminative stimulus in naive middle-aged monkeys. Nevertheless, the GABA(A) receptor mechanisms mediating the discriminative stimulus effects of ethanol, when learned as a young adult, appear stable across one third of the primate lifespan.

CYP2D plays a major role in berberine metabolism in liver of mice and humans

Berberine is a widely used plant extract for gastrointestinal infections, and is reported to have potential benefits in treatment for diabetes and hypercholesterolemia. It has been suggested that interactions between berberine-containing products and cytochromes P450 (CYPs) exist, but little is known about which CYPs mediate the metabolism of berberine in vivo. In this study, berberine metabolites in urine and feces of mice were analyzed, and the role that CYPs play in producing these metabolites were characterized in liver microsomes from mice (MLM) and humans (HLM), as well as recombinant human CYPs. Eleven berberine metabolites were identified in mice, including 5 unconjugated metabolites, mainly in feces, and 6 glucuronide and sulfate conjugates, predominantly in urine. Three novel berberine metabolites were observed. Three unconjugated metabolites of berberine were produced by MLM, HLM, and recombinant human CYPs. CYP2D6 was the primary recombinant human CYP producing these metabolites, followed by CYP1A2, 3A4, 2E1 and CYP2C19. The metabolism of berberine in MLM and HLM was decreased the most by a CYP2D inhibitor, and moderately by inhibitors of CYP1A and 3A. CYP2D plays a major role in berberine biotransformation, therefore, CYP2D6 pharmacogenetics and potential drug-drug interactions should be considered when berberine is used.

Integrated transcriptional and proteomic analysis with in vitro biochemical assay reveal the important role of CYP3A46 in T-2 toxin hydroxylation in porcine primary hepatocytes

Both T-2 toxin and its metabolites are highly potent mycotoxins that can cause severe human and animal diseases upon exposure. Understanding the toxic mechanism and biotransformation process of T-2 toxin at a cellular level is essential for the development of counter-measures. We investigated the effect of T-2 toxin in porcine primary hepatocytes using porcine genome array and two-dimensional difference gel electrophoresis with matrix-assisted laser desorption/ionization tandem time of flight mass spectrometry. Integrated transcriptional and proteomic analysis demonstrated that T-2 toxin adversely affected porcine hepatocytes by initiating lipid metabolism disorder, oxidative stress response, and apoptosis. In addition, xenobiotic metabolism genes, including cytochrome P450 3As (CYP3A46 and CYP3A39), carboxylesterase 1Cs (CES1C4 and CES1C5), and epoxide hydrolase (EPHX1), increased in T-2 toxin treatment cells. Using HepG2 cells to over-express the recombinant xenobiotic metabolism genes above and rapid resolution liquid chromatography/tandem mass spectrometry to detect metabolites of T-2 toxin, we determined that porcine CYP3A46 mainly catalyzed T-2 to form 3'-hydroxy-T-2, which was further confirmed by purified CYP3A46 protein. However, recombinant porcine CES1C5 and EPHX1 did not enhance hydrolysis and de-epoxidation of T-2 implying that other esterases and epoxide hydrolases may play dominant roles in those reactions.

Pharmacokinetics of verapamil in diabetic rats induced by combination of high-fat diet and streptozotocin injection

The aim of this study was to investigate effects of type 2 diabetes on the pharmacokinetics of verapamil after intravenous administration. Diabetes mellitus (DM) rats were induced by combination of high-fat diet (HFD) and streptozotocin. Plasma concentrations of verapamil in DM rats, rats fed with HFD, and control (CON) rats were measured after intravenous administration of 1 mg/kg verapamil and corresponding pharmacokinetic parameters were estimated. Area under the plasma concentration in DM rats was significantly smaller than that in CON rats. In vitro microsomal study showed that intrinsic clearance of verapamil in DM rats was significantly higher than those in CON rats. Compared to CON rats, higher intrinsic clearance was also observed in HFD rats. Western blot results demonstrated higher levels of CYP3A2 in DM and HFD rats, which was in line to activity of CYP3A. All the results gave a conclusion that diabetes may enhance metabolism of verapamil in rat, and the enhancement may partly result from induction of CYP3A.

A critical analysis of the interplay between cytochrome P450 3A and P-glycoprotein: recent insights from knockout and transgenic mice

CYP3A is one of the most important drug-metabolizing enzymes, determining the first-pass metabolism, oral bioavailability, and elimination of many drugs. It is also an important determinant of variable drug exposure and is involved in many drug-drug interactions. Recent studies with CYP3A knockout and transgenic mice have yielded a number of key insights that are important to consider during drug discovery and development. For instance, studies with tissue-specific CYP3A-transgenic mice have highlighted the importance of intestinal CYP3A-dependent metabolism. They also revealed that intestinal CYP3A plays an important role in the regulation of various drug-handling systems in the liver. Intestinal CYP3A activity can thus have far-reaching pharmacological effects. Besides CYP3A, the active drug efflux transporter P-glycoprotein also has a strong effect on the pharmacokinetics of numerous drugs. CYP3A and P-glycoprotein have an extensive overlap in their substrate spectrum. It has been hypothesized that for many drugs, the combined activity of CYP3A and P-glycoprotein makes for efficient intestinal first-pass metabolism of orally administered drugs as a result of a potentially synergistic collaboration. However, there is only limited in vitro and in vivo evidence for this hypothesis. There has also been some confusion in the field about what synergy actually means in this case. Our recent studies with Cyp3a/P-glycoprotein combination knockout mice have provided further insights into the CYP3A-P-glycoprotein interplay. We here present our view of the status of the synergy hypothesis and an attempt to clarify the existing confusion about synergy. We hope that this will facilitate further critical testing of the hypothesis and improve communication among researchers. Above all, the recent findings and insights into the interplay between CYP3A and P-glycoprotein may have implications for improving oral drug bioavailability and reducing adverse side effects.

Understanding the determinants of selectivity in drug metabolism through modeling of dextromethorphan oxidation by cytochrome P450

Cytochrome P450 enzymes play key roles in the metabolism of the majority of drugs. Improved models for prediction of likely metabolites will contribute to drug development. In this work, two possible metabolic routes (aromatic carbon oxidation and O-demethylation) of dextromethorphan are compared using molecular dynamics (MD) simulations and density functional theory (DFT). The DFT results on a small active site model suggest that both reactions might occur competitively. Docking and MD studies of dextromethorphan in the active site of P450 2D6 show that the dextromethorphan is located close to heme oxygen in a geometry apparently consistent with competitive metabolism. In contrast, calculations of the reaction path in a large protein model [using a hybrid quantum mechanical-molecular mechanics (QM/MM) method] show a very strong preference for O-demethylation, in accordance with experimental results. The aromatic carbon oxidation reaction is predicted to have a high activation energy, due to the active site preventing formation of a favorable transition-state structure. Hence, the QM/MM calculations demonstrate a crucial role of many active site residues in determining reactivity of dextromethorphan in P450 2D6. Beyond substrate binding orientation and reactivity of Compound I, successful metabolite predictions must take into account the detailed mechanism of oxidation in the protein. These results demonstrate the potential of QM/MM methods to investigate specificity in drug metabolism.

Metabolic intermediate complex formation of human cytochrome P450 3A4 by lapatinib

Lapatinib, an oral breast cancer drug, has recently been reported to be a mechanism-based inactivator of cytochrome P450 (P450) 3A4 and also an idiosyncratic hepatotoxicant. It was suggested that formation of a reactive quinoneimine metabolite was involved in mechanism-based inactivation (MBI) and/or hepatotoxicity. We investigated the mechanism of MBI of P450 3A4 by lapatinib. Liquid chromatography-mass spectrometry analysis of P450 3A4 after incubation with lapatinib did not show any peak corresponding to irreversible modifications. The enzymatic activity inactivated by lapatinib was completely restored by the addition of potassium ferricyanide. These results indicate that the mechanism of MBI by lapatinib is quasi-irreversible and mediated via metabolic intermediate complex (MI complex) formation. This finding was verified by the increase in a signature Soret absorbance at approximately 455 nm. Two amine oxidation products of the metabolism of lapatinib by P450 3A4 were characterized: N-hydroxy lapatinib (M3) and the oxime form of N-dealkylated lapatinib (M2), suggesting that a nitroso or another related intermediate generated from M3 is involved in MI complex formation. In contrast, P450 3A5 was much less susceptible to MBI by lapatinib via MI complex formation than P450 3A4. In addition, P450 3A5 had a significantly lower ability than 3A4 to generate M3, consistent with N-hydroxylation as the initial step in the pathway to MI complex formation. In conclusion, our results demonstrate that the primary mechanism for MBI of P450 3A4 by lapatinib is not irreversible modification by the quinoneimine metabolite, but quasi-irreversible MI complex formation mediated via oxidation of the secondary amine group of lapatinib.

Genetic polymorphisms of drug-metabolizing phase I enzymes CYP2E1, CYP2A6 and CYP3A5 in South Indian population

CYP2E1, CYP2A6 and CYP3A5 enzymes belong to phase I group of drug-metabolizing enzymes, which are involved in the metabolism of various compounds and xenobiotics. Presence of polymorphisms in the genes coding for these enzymes results in interindividual variations in drug metabolism, therapeutic response and susceptibility towards various diseases. The frequencies of these variants in genes differ considerably between ethnic groups. This study was carried out to estimate the allele and genotype frequencies of common variants in CYP2E1, CYP2A6 and CYP3A5 in South Indian population. Six hundred and fifty-two unrelated healthy volunteers of South Indian origin (Andhra Pradesh, Karnataka, Kerala and Tamil Nadu) were included in this study. Polymerase chain reaction-restriction fragment length polymorphism, allele-specific PCR, real-time PCR, SNaPshot and gene sequencing methods were used for the identification of gene polymorphisms. The frequencies of CYP2E1*1B, CYP2E1*5B and CYP2E1*6 alleles in South Indian population were 14.3, 1.3 and 22.4%, respectively. The frequencies of CYP2A6*2, CYP2A6*4A and CYP2A6*5 alleles were found to be 1, 8.9 and 0.7%, respectively. The distribution of CYP3A5*3 allele was 63.5%. There were no variant alleles of CYP3A5*2, CYP3A5*4 and CYP3A5*6 in South Indian population. The frequencies of CYP2E1, CYP2A6 and CYP3A5 in the South Indian population are distinct from Caucasians, Chinese, Japanese, African Americans and other compared populations. This is the first study conducted in the South Indian population with a larger sample size. The findings of our study provide the basic genetic information for further pharmacogenomic investigations in the population.

Regional distribution of drug-metabolizing enzyme activities in the liver and small intestine of cynomolgus monkeys

The cynomolgus monkey is an animal species widely used to study drug metabolism because of its evolutionary closeness to humans. However, drug-metabolizing enzyme activities have not been compared in various parts of the liver and small intestine in cynomolgus monkeys. In this study, therefore, drug-metabolizing enzyme activities were analyzed in the liver (the five lobes) and small intestine (six sections from the duodenum to the distal ileum). 7-Ethoxyresorufin O-deethylation, coumarin 7-hydroxylation, paclitaxel 6α-hydroxylation, diclofenac 4'-hydroxylation, tolbutamide methylhydroxylation, S-mephenytoin 4'-hydroxylation, bufuralol 1'-hydroxylation, chlorzoxazone 6-hydroxylation, midazolam 1'-hydroxylation, and testosterone 6β-, 16α-, 16β-, and 2α-hydroxylation were used as the probe reactions for this investigation. In liver, all probe reactions were detected and enzyme activity levels were similar in all lobes, whereas, in the small intestine, all enzyme activities were detected (except for coumarin 7-hydroxylase and testosterone 16α-hydroxylase activity), but from jejunum to ileum there was a decrease in the level of enzyme activity. This includes midazolam 1'-hydroxylation and testosterone 6β-hydroxylation, which are catalyzed by cynomolgus monkey cytochrome P450 (CYP) 3A4/5, orthologs of human CYP3A4/5, which are important drug-metabolizing enzymes. The data presented in this study are expected to facilitate the use of cynomolgus monkeys in drug metabolism studies.

Enhancement of GABAergic activity: neuropharmacological effects of benzodiazepines and therapeutic use in anesthesiology

GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). The type A GABA receptor (GABA(A)R) system is the primary pharmacological target for many drugs used in clinical anesthesia. The α1, β2, and γ2 subunit-containing GABA(A)Rs located in the various parts of CNS are thought to be involved in versatile effects caused by inhaled anesthetics and classic benzodiazepines (BZD), both of which are widely used in clinical anesthesiology. During the past decade, the emergence of tonic inhibitory conductance in extrasynaptic GABA(A)Rs has coincided with evidence showing that these receptors are highly sensitive to the sedatives and hypnotics used in anesthesia. Anesthetic enhancement of tonic GABAergic inhibition seems to be preferentially increased in regions shown to be important in controlling memory, awareness, and sleep. This review focuses on the physiology of the GABA(A)Rs and the pharmacological properties of clinically used BZDs. Although classic BZDs are widely used in anesthesiological practice, there is a constant need for new drugs with more favorable pharmacokinetic and pharmacodynamic effects and fewer side effects. New hypnotics are currently developed, and promising results for one of these, the GABA(A)R agonist remimazolam, have recently been published.

The design of in vitro liver sinusoid mimics using chitosan-hyaluronic acid polyelectrolyte multilayers

Interactions between hepatocytes and liver sinusoidal endothelial cells (LSECs) are essential for the development and maintenance of hepatic phenotypic functions. We report the assembly of three-dimensional liver sinusoidal mimics comprised of primary rat hepatocytes, LSECs, and an intermediate chitosan-hyaluronic acid polyelectrolyte multilayer (PEM). The height of the PEMs ranged from 30 to 55 nm and exhibited a shear modulus of approximately 100 kPa. Hepatocyte-PEM cellular constructs exhibited stable urea and albumin production over a 7-day period, and these values were either higher or similar to cells cultured in a collagen sandwich. This is of significance because the thickness of a collagen gel is approximately 1000-fold higher than the height of the chitosan-hyaluronic acid PEM. In the hepatocyte-PEM-LSEC liver-mimetic cellular constructs, LSEC phenotype was maintained, and these cultures exhibited stable urea and albumin production. CYP1A1/2 activity measured over a 7-day period was significantly higher in the hepatocyte-PEM-LSEC constructs than in collagen sandwich cultures. A 16-fold increase in CYP1A1/2 activity was observed for hepatocyte-PEM-10,000 LSEC samples, thereby suggesting that interactions between hepatocytes and LSECs are critical in enhancing the detoxification capability in hepatic cultures in vitro.

Effect of danshen extract on the activity of CYP3A4 in healthy volunteers

AIMS

To assess the effect of danshen extract on CYP3A4 activity using midazolam as an in vivo probe.

METHODS

A sequential, open-label, two-period pharmacokinetic interaction study design was used to compare midazolam pharmacokinetic parameters before and after 14 days of administration of danshen tablets. Twelve healthy volunteers received a single oral dose (15 mg) of midazolam followed by danshen tablets (four tablets orally, three times a day) for 14 days. On the last day of the study they received four danshen tablets with a 15 mg midazolam tablet and plasma concentrations of midazolam and its corresponding metabolite 1-hydroxylmidazolam were measured prior to and after the administration of danshen tablets periodically for 12 h.

RESULTS

The 90% confidence intervals of C(max,)t(1/2), CL/F and AUC(0,infinity) of midazolam before and after administration of danshen tablets were (0.559, 0.849), (0.908, 1.142), (1.086, 1.688) and (0.592, 0.921), respectively; and those of C(max), t(1/2) and AUC(0,infinity) of 1-hydroxylmidazolam after vs. before administration of danshen tablets were (0.633, 0.923), (0.801, 1.210) and (0.573, 0.980), respectively. Ratios of geometric LS means of C(max(1OHMid)) : C(max(Mid)) and AUC(max(1OHMid)) : AUC(max(Mid)) (after vs. before 14-day danshen) were 1.072 and 1.035, respectively.

CONCLUSIONS

Our findings suggest that multiple dose administration of danshen tablets may induce CYP3A4 in the gut. Accordingly, caution should be taken when danshen products are used in combination with therapeutic drugs metabolized by CYP3A.

3D hepatic cultures simultaneously maintain primary hepatocyte and liver sinusoidal endothelial cell phenotypes

Developing in vitro engineered hepatic tissues that exhibit stable phenotype is a major challenge in the field of hepatic tissue engineering. However, the rapid dedifferentiation of hepatic parenchymal (hepatocytes) and non-parenchymal (liver sinusoidal endothelial, LSEC) cell types when removed from their natural environment in vivo remains a major obstacle. The primary goal of this study was to demonstrate that hepatic cells cultured in layered architectures could preserve or potentially enhance liver-specific behavior of both cell types. Primary rat hepatocytes and rat LSECs (rLSECs) were cultured in a layered three-dimensional (3D) configuration. The cell layers were separated by a chitosan-hyaluronic acid polyelectrolyte multilayer (PEM), which served to mimic the Space of Disse. Hepatocytes and rLSECs exhibited several key phenotypic characteristics over a twelve day culture period. Immunostaining for the sinusoidal endothelial 1 antibody (SE-1) demonstrated that rLSECs cultured in the 3D hepatic model maintained this unique feature over twelve days. In contrast, rLSECs cultured in monolayers lost their phenotype within three days. The unique stratified structure of the 3D culture resulted in enhanced heterotypic cell-cell interactions, which led to improvements in hepatocyte functions. Albumin production increased three to six fold in the rLSEC-PEM-Hepatocyte cultures. Only rLSEC-PEM-Hepatocyte cultures exhibited increasing CYP1A1/2 and CYP3A activity. Well-defined bile canaliculi were observed only in the rLSEC-PEM-Hepatocyte cultures. Together, these data suggest that rLSEC-PEM-Hepatocyte cultures are highly suitable models to monitor the transformation of toxins in the liver and their transport out of this organ. In summary, these results indicate that the layered rLSEC-PEM-hepatocyte model, which recapitulates key features of hepatic sinusoids, is a potentially powerful medium for obtaining comprehensive knowledge on liver metabolism, detoxification and signaling pathways in vitro.

Repeated administration of oxycodone modifies the gene expression of several drug metabolising enzymes in the hepatic tissue of male Sprague-Dawley rats, including glutathione S-transferase A-5 (rGSTA5) and CYP3A2

OBJECTIVES

Clinical use and illicit abuse of the potent opioid agonist oxycodone has dramatically increased over the past decade. Yet oxycodone remains one of the least studied opioids, particularly its interactions on the genomic level. The aim of this study was to examine potential alterations in gene expression of drug metabolising enzymes in the liver tissue of male Sprague-Dawley rats chronically treated with oxycodone.

METHODS

Rats were administered saline or oxycodone 15 mg/kg i.p. twice a day for 8 days. Changes in RNA levels were detected using microarray analysis validated by quantitative real-time PCR; consequent changes in protein expression and functionality were further assessed by Western blotting and activity assays.

KEY FINDINGS

The expression of several drug metabolising enzymes was modulated by oxycodone treatment: cytochrome P450 (CYP) 2B2, CYP2C13, CYP17A1, epoxide hydrolase 2, carboxylesterase 2, flavin-containing monooxygenase 1, glutathione S-transferase alpha 5 (rGSTA5) and CYP3A2. In particular, the mRNA level of rGSTA5 (formerly GSTYc(2)) was up-regulated by approximately 6.5 fold and CYP3A2 was down-regulated by approximately 7.0 fold. Immunoblotting assays demonstrated a corresponding significant elevation of rGSTA5 protein and repression of CYP3A2 protein. The apparent cytosolic GST activity towards 1-chloro-2,4-dinitrobenzene conjugation and reduction of cumene hydroperoxide were significantly higher in liver from oxycodone-treated rats than that of saline-treated rats. In addition, the microsomal activity of CYP3A2, measured via 6beta-hydroxylation of testosterone, was significantly decreased in oxycodone-treated rats.

CONCLUSIONS

Repeated oxycodone administration is associated with a significant up-regulation of rGSTA5 and concomitant down-regulation of CYP3A2 mRNA, protein expression and functionality. These results support further in-vivo studies into the clinical impact of our findings.

Activation of CYP3A-mediated testosterone 6β-hydroxylation by tanshinone IIA and midazolam 1-hydroxylation by cryptotanshinone in human liver microsomes

This study evaluated the in vitro activation of CYP3A-mediated midazolam 1-hydroxylation and testosterone 6β-hydroxylation by tanshinone I, tanshinone IIA, and cryptotanshinone. The abilities of tanshinones to activate CYP3A-mediated midazolam 1-hydroxylation and testosterone 6β-hydroxylation in human liver microsomes (HLMs) were tested. Substrate- and effector-dependent activation of CYP3A by tanshinones were both observed. Cryptotanshinone was shown to activate CYP3A-mediated midazolam 1-hydroxylation in a concentration-dependent manner. In contrast, tanshinone IIA and tanshinone I did not activate this hydroxylation reaction. In addition, tanshinone IIA activated CYP3A-mediated testosterone 6β-hydroxylation, whereas cryptotanshinone and tanshinone I did not. The results from our study enhance the understanding of CYP3A activation by tanshinone IIA and cryptotanshinone in HLMs. Additionally, these data allow for an accurate prediction of the magnitude and likelihood of Danshen-drug interactions.

Regional distribution of cytochrome p450 mRNA expression in the liver and small intestine of cynomolgus monkeys

The cynomolgus monkey is used to study drug metabolism because of its evolutionary closeness to humans. Despite their importance, regional distribution of cytochrome P450 (CYP) enzymes including CYP3As in the liver and small intestine, the major sites of drug metabolism, has not been fully investigated in cynomolgus monkeys. In this study, we measured mRNA expression levels of 14 CYPs in the CYP1, 2, and 3 subfamilies, including orthologs of human CYP3A4 and CYP3A5, in the liver and small intestine of cynomolgus monkeys. Expression levels of each CYP mRNA in various regions of the liver were quantified and comparisons were made between the right lobe, quadrate lobe, left medial lobe, left lateral lobe, and caudate lobe and with four different sections of the right lobe. In the small intestine, the same mRNAs were measured in the duodenum and six different sections from the proximal jejunum to the distal ileum. Expression levels of the CYP mRNAs were not substantially different between liver samples, but varied between the different sections of the small intestine, including CYP3A4. These results suggest that analysis of distinct sections is required for a better understanding of cynomolgus monkey CYPs in the small intestine.

Azole antimycotics and drug interactions in the perioperative period

PURPOSE OF REVIEW

A beneficial effect of antifungal prophylaxis on the prevention of invasive fungal infections has increased the use of azole antimycotics in intensive care and during the perioperative period. At the same time more severe illnesses are treated and multiple drug therapies are employed. Thus, the potential for severe drug-drug interactions has increased. Previous studies have shown that azole antimycotics increase the risk of many clinically significant drug interactions with potentially hazardous consequences.

RECENT FINDINGS

A recent pharmacoepidemiological study has found a more than five-fold incidence ratio in the adjusted rate of sudden death from cardiac diseases among those patients who were given simultaneously inhibitors of cytochrome P450 (CYP) enzymes and their substrates. Although new triazole antifungals are well tolerated, they still cause significant inhibition of CYP enzymes.

SUMMARY

This review focuses on azole antimycotics and anesthetic drugs being used during the perioperative period and discusses the possible clinically significant drug-drug interactions. Azole antimycotics are amongst the strongest inhibitors of CYP-mediated drug metabolism. Anesthesiologists must be aware of the interaction potential of azole antimycotics to be able to adjust their perioperative strategies according to the patient's condition and concomitant medication.

Effects of cytochrome P450 3A (CYP3A) and the drug transporters P-glycoprotein (MDR1/ABCB1) and MRP2 (ABCC2) on the pharmacokinetics of lopinavir

BACKGROUND AND PURPOSE

Lopinavir is extensively metabolized by cytochrome P450 3A (CYP3A) and is considered to be a substrate for the drug transporters ABCB1 (P-glycoprotein) and ABCC2 (MRP2). Here, we have assessed the individual and combined effects of CYP3A, ABCB1 and ABCC2 on the pharmacokinetics of lopinavir and the relative importance of intestinal and hepatic metabolism. We also evaluated whether ritonavir increases lopinavir oral bioavailability by inhibition of CYP3A, ABCB1 and/or ABCC2.

EXPERIMENTAL APPROACH

Lopinavir transport was measured in Madin-Darby canine kidney cells expressing ABCB1 or ABCC2. Oral lopinavir kinetics (+/- ritonavir) was studied in mice with genetic deletions of Cyp3a, Abcb1a/b and/or Abcc2, or in transgenic mice expressing human CYP3A4 exclusively in the liver and/or intestine.

KEY RESULTS

Lopinavir was transported by ABCB1 but not by ABCC2 in vitro. Lopinavir area under the plasma concentration - time curve (AUC)(oral) was increased in Abcb1a/b(-/-) mice (approximately ninefold vs. wild-type) but not in Abcc2(-/-) mice. Increased lopinavir AUC(oral) (>2000-fold) was observed in cytochrome P450 3A knockout (Cyp3a(-/-)) mice compared with wild-type mice. No difference in AUC(oral) between Cyp3a(-/-) and Cyp3a/Abcb1a/b/Abcc2(-/-) mice was observed. CYP3A4 activity in intestine or liver, separately, reduced lopinavir AUC(oral) (>100-fold), compared with Cyp3a(-/-) mice. Ritonavir markedly increased lopinavir AUC(oral) in all CYP3A-containing mouse strains.

CONCLUSIONS AND IMPLICATIONS

CYP3A was the major determinant of lopinavir pharmacokinetics, far more than Abcb1a/b. Both intestinal and hepatic CYP3A activity contributed importantly to low oral bioavailability of lopinavir. Ritonavir increased lopinavir bioavailability primarily by inhibiting CYP3A. Effects of Abcb1a/b were only detectable in the presence of CYP3A, suggesting saturation of Abcb1a/b in the absence of CYP3A activity.

Using drug probes to monitor hepatic drug metabolism in critically ill patients: midazolam, a flawed but useful tool for clinical investigation of CYP3A activity?

IMPORTANCE OF THE FIELD

In the UK, acute kidney injury (AKI) occurs in 25% of patients admitted to intensive care. Outcome is worsened in the presence of AKI for reasons not easily explained. AKI unpredictably affects the pharmacokinetics and pharmacodynamics of drugs and dosing in patients with AKI is largely based on data from chronic kidney disease patients, but how appropriately is unknown.

AREAS COVERED IN THIS REVIEW

Midazolam as a drug probe of CYP3A activity is reviewed, with discussion of its limitations and alternatives in critically ill patients. Pharmacogenetics of CYP3A enzymes and their significance are discussed and emerging evidence that AKI affects liver metabolism is reviewed.

WHAT THE READER WILL GAIN

The aim is to give the reader insight into the complexities of in vivo research in critically ill patient with discussion of interaction between the kidney and liver. We explain the use of midazolam as a drug probe for the investigation of the effect of AKI on hepatic function.

TAKE HOME MESSAGE

Critically ill patients are difficult to manage but methods are now available for investigation of complex interrelationships that complicate the care and management of these patients with the potential to improve safety, efficacy and outcome, particularly for drug administration.

Development of stably transfected human and rat hepatoma cell lines for the species-specific assessment of xenobiotic response enhancer module (XREM)-dependent induction of drug metabolism

Based on our current knowledge, PXR holds a key position in the induction of a selective battery of enzymes and transporters of drug metabolism. In order to prevent serious adverse drug effects or unpredicted drug-drug interactions (DDI), it is compulsory to investigate the possible inducing potency of drugs under development. Furthermore, analysis of the inducing potency of environmental pollutants and new or manufactured chemicals is part of toxicological risk assessment. In non-transfected human HepG2 and rat H4IIE hepatoma cells, we examined the characteristics of expression of 45 genes involved in drug metabolism. A few gene products such as CYP2B6 or CYP3A4 mRNA were prominent in HepG2 cells while their major rat counterparts were, e.g., CYP2B3 or CYP3A1/3A3. Furthermore, a number of xenobiotic receptors including PXR were expressed in both cell lines. A number of genes were regulated in a cell type and species-specific manner after incubation with the prototypical PXR agonists rifampicin or dexamethasone, respectively. Then, we established cell-based reporter gene assays for screening for PXR-dependent induction of drug metabolism. HepG2 and H4IIE cells were stably transfected with a reporter gene containing PXR responsive elements (XREMs) which mediate the induction of PXR target genes such as CYP3A enzymes. With both stable cell lines the CYP inducers clotrimazole, dexamethasone, omeprazole, phenobarbital, rifampicin, as well as the drug candidate EMD 392949 and the brominated flame retardants hexabromocylododecane (HBCD) and a pentabromodiphenyl ether (pentaBDE) mixture were screened. In the human HepG2-XREM3 and rat H4IIE-XREM3 cells, clotrimazole and HBCD were found as common activators of the human and rat PXR whereas pentaBDE was more effective with the human cell system. Omeprazole and phenobarbital did not induce the rat PXR-dependent reporter gene expression in H4IIE-XREM3 cells, while a moderate increase was found in HepG2-XREM3 cells. EMD 392949 also acted as inducer in human but not in rat cells confirming in vivo observations. In summary, the established PXR-dependent in vitro system allows the simultaneous, fast, and species-specific screening of chemicals, environmental contaminants, food ingredients and drugs for CYP3A induction in cells of human and rat origin.

A novel framework for predicting in vivo toxicities from in vitro data using optimal methods for dense and sparse matrix reordering and logistic regression

In this work, we combine the strengths of mixed-integer linear optimization (MILP) and logistic regression for predicting the in vivo toxicity of chemicals using only their measured in vitro assay data. The proposed approach utilizes a biclustering method based on iterative optimal reordering (DiMaggio, P. A., McAllister, S. R., Floudas, C. A., Feng, X. J., Rabinowitz, J. D., and Rabitz, H. A. (2008). Biclustering via optimal re-ordering of data matrices in systems biology: rigorous methods and comparative studies. BMC Bioinformatics 9, 458-474.; DiMaggio, P. A., McAllister, S. R., Floudas, C. A., Feng, X. J., Rabinowitz, J. D., and Rabitz, H. A. (2010b). A network flow model for biclustering via optimal re-ordering of data matrices. J. Global. Optim. 47, 343-354.) to identify biclusters corresponding to subsets of chemicals that have similar responses over distinct subsets of the in vitro assays. The biclustering of the in vitro assays is shown to result in significant clustering based on assay target (e.g., cytochrome P450 [CYP] and nuclear receptors) and type (e.g., downregulated BioMAP and biochemical high-throughput screening protein kinase activity assays). An optimal method based on mixed-integer linear optimization for reordering sparse data matrices (DiMaggio, P. A., McAllister, S. R., Floudas, C. A., Feng, X. J., Li, G. Y., Rabinowitz, J. D., and Rabitz, H. A. (2010a). Enhancing molecular discovery using descriptor-free rearrangement clustering techniques for sparse data sets. AIChE J. 56, 405-418.; McAllister, S. R., DiMaggio, P. A., and Floudas, C. A. (2009). Mathematical modeling and efficient optimization methods for the distance-dependent rearrangement clustering problem. J. Global. Optim. 45, 111-129) is then applied to the in vivo data set (21.7% sparse) in order to cluster end points that have similar lowest effect level (LEL) values, where it is observed that the end points are effectively clustered according to (1) animal species (i.e., the chronic mouse and chronic rat end points were clearly separated) and (2) similar physiological attributes (i.e., liver- and reproductive-related end points were found to separately cluster together). As the liver and reproductive end points exhibited the largest degree of correlation, we further analyzed them using regularized logistic regression in a rank-and-drop framework to identify which subset of in vitro features could be utilized for in vivo toxicity prediction. It was observed that the in vivo end points that had similar LEL responses over the 309 chemicals (as determined by the sparse clustering results) also shared a significant subset of selected in vitro descriptors. Comparing the significant descriptors between the two different categories of end points revealed a specificity of the CYP assays for the liver end points and preferential selection of the estrogen/androgen nuclear receptors by the reproductive end points.

Gene expression of P-glycoprotein and cytochrome P450 3A4 in peripheral blood mononuclear cells and correlation with expression in liver

Long-term immunosuppression in renal transplant recipients generally includes calcineurin inhibitors (CNIs), which demonstrate marked interindividual diversity and a narrow therapeutic range. In the clinical setting, it is important to reach therapeutic drug concentrations to prevent allograft rejection. The same immunosuppressant dosage leads to different drug concentrations. Therefore, we investigated factors that influence the metabolism of immunosuppressant agents. The CNIs are substrates of cytochrome P450 (CYP450) and P-glycoprotein. The CYP450 3A genotype significantly influences CNI concentration. Differences in expression of these proteins may explain interindividual pharmacokinetic variations. However, it is risky and impractical to obtain specimens from the liver in renal transplant recipients. Therefore, we investigated the correlation of gene expression between peripheral blood mononuclear cells (PBMCs) and liver parenchyma. We observed that the correlation of relative P-glycoprotein gene expression between PBMCs and liver is not significant (r2=0.03; P=.65). In addition, the correlation of CYP450 3A4 gene expression between PBMCs and liver is not strong (r2=0.23; P=.42). The expression level of CYP450 3A5 is too low to be detected in the sample from PBMCs.

Maternal ethnicity influences on neonatal respiratory outcomes after antenatal corticosteroid use for anticipated preterm delivery

OBJECTIVE

To explore the influence of maternal ethnicity on neonatal outcomes after antenatal corticosteroid administration.

METHODS

A retrospective review of ethnicity, maternal factors, and neonatal birth outcomes was performed for preterm births at a single institution. Cases were limited to women who received antenatal corticosteroids. The impact of ethnicity on specific neonatal respiratory outcomes and mortality was analyzed by bivariate comparisons and by logistic regression analysis.

RESULTS

Complete ethnicity data were obtained for 548 women. Controlling for gestational age at delivery, diabetes, whether the subject completed a course of steroids, and the dosing of the steroids, logistic regression demonstrated that ethnicity was independently associated with respiratory distress syndrome (compared to Caucasians: African-Americans OR 0.49 (95% CI 0.29-0.85); Filipinos OR 0.45 (95% CI 0.21-0.96).

CONCLUSIONS

Ethnicity is independently associated with neonatal respiratory outcomes after antenatal corticosteroid use. Perhaps individualized dosing of antenatal corticosteroids is needed to further improve neonatal outcomes.

Update information on drug metabolism systems--2009, part I

The field of drug metabolism has changed dramatically in the past generation. Two of the driving forces are the advances in analytical chemistry and our understanding of the biological systems. With regard to the former, the advances in liquid chromatography-mass spectrometry (LC-MS) are extremely impressive, and the speed of analysis has been increased even more with the recent developments in ultraperformance LC (UPLC). NMR analysis on a sub-microgram scale is not unusual, and the 2-dimensional methods are also very impressive. The biology underlying gene regulation is highly developed, and the recombinant methods have greatly facilitated progress in the field. Today it is common to design discovery and development efforts focused on critical human phenomena from the very start, with animal studies supporting the efficacy and safety efforts.

A comparative study of genome-wide transcriptional profiles of primary hepatocytes in collagen sandwich and monolayer cultures

Two commonly used culture systems in hepatic tissue engineering are the collagen sandwich (CS) and monolayers of cells. In this study, genome-wide gene expression profiles of primary hepatocytes were measured over an 8-day period for each cell culture system using Affymetrix GeneChips and compared via gene set enrichment analysis to elicit biologically meaningful information at the level of gene sets. Our results demonstrate that gene expression in hepatocytes in CS cultures steadily and comprehensively diverges from that in monolayer cultures. Gene sets up-regulated in CS cultures include several associated with liver metabolic and synthesis functions, such as metabolism of lipids, amino acids, carbohydrates, and alcohol, and synthesis of bile acids. Monooxygenases such as Cytochrome-P450 enzymes do not show any change between the culture systems after 1 day, but exhibit significant up-regulation in CS cultures after 3 days in comparison to hepatocyte monolayers. These data provide insights into the up- and down-regulation of several liver-critical gene sets and their subsequent effects on liver-specific functions. These results provide a baseline for further explorations into the systems biology of engineered liver mimics.

Vitamin E supplementation and hepatic drug metabolism in humans

Meta-analyses studies suggest that high-dose vitamin E may be associated with increased mortality in some populations. Vitamin E may increase the production of CYP3A4 in the liver, and this could lead to an increase in drug metabolism, potentially lowering the efficacy of therapeutic drugs. We hypothesized that upregulation of CYP3A4 by alpha-tocopherol (alpha-TOH) would decrease the plasma concentration of the CYP3A4 substrate midazolam. Baseline metabolism of midazolam (1 mg intravenously) was determined in 12 healthy subjects before randomization into 2 groups of 6 to receive either RRR-alpha-TOH (750 IU/d) or placebo for 3 weeks. At completion, subjects were given an additional 1 mg intravenous bolus of midazolam. Plasma midazolam, 1-hydroxy-midazolam, and urinary alpha-TOH metabolite excretion were measured using gas chromatography mass spectrometry. Serum alpha-TOH was measured using high performance liquid chromatography with electrochemical detection. Serum alpha-TOH increased by 100% (P = 0.002) and urinary alpha-TOH metabolite excretion increased 20-fold in the treatment group versus placebo (P = 0.001). There was no effect on the area under time curve of midazolam in subjects taking alpha-TOH compared with placebo. These findings do not support the hypothesis that alpha-TOH supplementation interferes with hepatic CYP3A4-mediated drug metabolism.

Hepatic OATP1B transporters and nuclear receptors PXR and CAR: interplay, regulation of drug disposition genes, and single nucleotide polymorphisms

Drug uptake transporters are now increasingly recognized as clinically relevant determinants of variable drug responsiveness and unexpected drug-drug interactions. Emerging evidence strongly suggests members of the organic anion transporting polypeptide (OATP) family appear to be particularly important to the disposition of many drugs in clinical use today. Specifically, the liver-enriched OATP1B subfamily members OATP1B1 and OATP1B3 exhibit broad substrate specificity and the ability to transport drugs which are ligands for xenobiotic sensing nuclear receptors such as the pregnane X receptor (PXR) and the constitutive androstane receptor (CAR). Accordingly, OATP1B transporters may indirectly regulate expression of drug metabolism genes via modulation of the intracellular concentration of PXR and CAR ligands. Moreover, a number of functionally important single nucleotide polymorphisms (SNPs) in OATP1B transporters have been described. In this review, a brief summary of known SNPs in PXR and CAR will be followed by an in-depth outline of OATP1B1 and OATP1B3 transporters particularly in relation to the known SNPs in these OATPs and the interplay between OATP1B transporters with PXR and CAR, both in vitro and in vivo.

Pharmacogenomics of CYP3A: considerations for HIV treatment

The understanding of the cytochrome P450 3A SNP in antiretroviral therapy is important, because it is highly inducible, extremely polymorphic and metabolizes many of the drugs that are key components of highly active antiretroviral therapy regimens. This enzyme is prolific and promiscuous towards drug and xenobiotic substrate selection and it is also unpredictable among individuals, having a 5- to 20-fold variability in its ability to contribute to drug clearance. The importance of human CYP3A pharmacogenetics is also gaining attention in other established areas of pharmacotherapy as it may contribute to the goal of predicting efficacy and/or toxicity, specifically with the discovery of null allele CYP3A4*20. This review summarizes the current understanding, implications of genetic variation in the CYP3A enzymes, the central role of CYP3A in linking human genetics, the pharmacokinetics and resulting pharmacodynamic responses to certain antiretroviral drugs, and their eventual place in applied clinical pharmacotherapy.

Regulation of CYP3A4 by pregnane X receptor: The role of nuclear receptors competing for response element binding

Induction of the major drug metabolizing enzyme CYP3A4 by xenobiotics contributes to the pronounced interindividual variability of its expression and often results in clinically relevant drug-drug interactions. It is mainly mediated by PXR, which regulates CYP3A4 expression by binding to several specific elements in the 5' upstream regulatory region of the gene. Induction itself shows a marked interindividual variability, whose underlying determinants are only partly understood. In this study, we investigated the role of nuclear receptor binding to PXR response elements in CYP3A4, as a potential non-genetic mechanism contributing to interindividual variability of induction. By in vitro DNA binding experiments, we showed that several nuclear receptors bind efficiently to the proximal promoter ER6 and distal xenobiotic-responsive enhancer module DR3 motifs. TRalpha1, TRbeta1, COUP-TFI, and COUP-TFII further demonstrated dose-dependent repression of PXR-mediated CYP3A4 enhancer/promoter reporter activity in transient transfection in the presence and absence of the PXR inducer rifampin, while VDR showed this effect only in the absence of treatment. By combining functional in vitro characterization with hepatic expression analysis, we predict that TRalpha1, TRbeta1, COUP-TFI, and COUP-TFII show a strong potential for the repression of PXR-mediated activation of CYP3A4 in vivo. In summary, our results demonstrate that nuclear receptor binding to PXR response elements interferes with PXR-mediated expression and induction of CYP3A4 and thereby contributes to the interindividual variability of induction.

Sex-dependent differences in cytochrome P450 3A activity as assessed by midazolam disposition in humans: a meta-analysis

Controversy exists concerning the sex-dependent differences in cytochrome P450 3A activity in humans. Meta-analysis of selected studies may address this question. Meta-analysis was performed on published or unpublished data in terms of sex-dependent differences in midazolam (MDZ) disposition in humans. The following pharmacokinetic parameters were included for the analysis: MDZ oral and systemic clearance, area under the concentration-time curve (AUC) of oral and intravenous MDZ, MDZ oral bioavailability (F), and MDZ gastrointestinal extraction (E(G)). Ten studies including 409 healthy volunteers were identified. Women exhibited 16% higher weight-corrected MDZ oral clearance (P < 0.001) and 20% higher systemic clearance (P = 0.002) than men. No significant difference in the AUC after oral dosing of MDZ was noted between sexes. Women showed lower AUC of intravenous MDZ than men (P = 0.02). No sex-dependent differences were observed in F and E(G). In conclusion, women showed significantly greater hepatic CYP3A activity than men, whereas no sex-dependent difference in intestinal CYP3A activity was observed.

Identification of polymorphisms in the 3'-untranslated region of the human pregnane X receptor (PXR) gene associated with variability in cytochrome P450 3A (CYP3A) metabolism

Single nucleotide polymorphisms in the 3'-untranslated region (3'UTR) of the human pregnane X receptor (PXR) gene might contribute to interindividual variability in cytochrome P450 3A (CYP3A) activity. Genotype-phenotype associations involving PXR-3'UTR single nucleotide polymorphisms were investigated through in vitro (53 human livers from primarily White donors) and in vivo (26 mainly White or African-American volunteers) studies using midazolam 1'-hydroxylation and midazolam apparent oral clearance (CL/F), respectively, as CYP3A-specific probes. PXR-3'UTR resequencing identified twelve single nucleotide polymorphisms, including two that were novel. Although none of the single nucleotide polymorphisms evaluated were associated with altered midazolam 1'-hydroxylation in the liver bank, both rs3732359 homozygotes and rs3732360 carriers showed 80% higher (p < 0.05) CL/F compared with homozygous reference individuals. These differences in CL/F were even larger (100% and 120% higher, respectively; p < 0.01) when only African-American subjects (n = 14) were considered. Five major haplotypes were identified containing the PXR-3'UTR single nucleotide polymorphisms and previously identified intron single nucleotide polymorphisms. Although CL/F differences were not statistically significant within the entire study cohort, African-American carriers of Haplotype-1 (which includes both rs3732359 and rs3732360 variants) exhibited 70% higher median CL/F compared with African-American non-carriers (p = 0.036). The results identify rs3732359 and rs3732360 as PXR-3'UTR single nucleotide polymorphisms associated with higher CYP3A activity in vivo in African-Americans.

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

Single-molecule fluorescence spectroscopy using phospholipid bilayer nanodiscs

Nanodiscs are a new class of model membranes that are being used to solubilize and study a range of integral membrane proteins and membrane-associated proteins. Unlike other model membranes, the Nanodisc bilayer is bounded by a scaffold protein coat that confers enhanced stability and a narrow particle size distribution. The bilayer diameter can be precisely controlled by changing the diameter of the protein coat. All these properties make Nanodiscs excellent model membranes for single-molecule fluorescence applications. In this chapter, we describe our work using Nanodiscs to apply total internal reflection fluorescence microscopy (TIRFM), fluorescence correlation spectroscopy (FCS), and Förster resonance energy transfer (FRET) to study the integral membrane protein cytochrome P450 3A4 and the peripheral membrane-binding proteins islet amyloid polypeptide (IAPP) and alpha-synuclein, respectively. The monodisperse size distribution of Nanodiscs enhances control over the oligomeric state of the membrane protein of interest, and facilitates accurate solution-based measurements as well. Nanodiscs also comprise an excellent system to stably immobilize integral membrane proteins in a bilayer without covalent modification, enabling a range of surface-based experiments where accurate localization of the protein of interest is required.