P-glycoprotein mediates brain-to-blood efflux transport of buprenorphine across the blood–brain barrier

Oxycodone, an opioid like the others?

The over-prescription of opioid analgesics is a growing problem in the field of addiction, which has reached epidemic-like proportions in North America. Over the past decade, oxycodone has gained attention as the leading opioid responsible for the North America opioid crisis. Oxycodone is the most incriminated drug in the early years of the epidemic of opioid use disorder in USA (roughly 1999-2016). The number of preclinical articles on oxycodone is rapidly increasing. Several publications have already compared oxycodone with other opioids, focusing mainly on their analgesic properties. The aim of this review is to focus on the genomic and epigenetic regulatory features of oxycodone compared with other opioid agonists. Our aim is to initiate a discussion of perceptible differences in the pharmacological response observed with these various opioids, particularly after repeated administration in preclinical models commonly used to study drug dependence potential.

An evaluation method for developing abuse-deterrent opioid formulations with agonist and antagonist combinations using conditioned place preference

Although opioids are useful narcotic analgesics in clinical settings, their misuse and addiction in the United States of America and other countries are rapidly increasing. Therefore, the development of abuse-deterrent formulations is an urgent issue. We herein investigated how to select the ratio of an opioid and the opioid receptor antagonist, naloxone in abuse-deterrent formulations for mice. The conditioned place preference (CPP) test was used to evaluate the rewarding effects of abused drugs. The opioids morphine (30 μmol/kg), oxycodone (3 μmol/kg), fentanyl (0.4 μmol/kg), and buprenorphine (0.5 μmol/kg) significantly induced place preference in mice. We also examined the optimal ratio of naloxone and opioids to inhibit the rewarding effects of the latter. Naloxone (3-5 μmol/kg) effectively inhibited place preference induced by the opioids tested. We calculated theoretical drug doses that exerted the same pharmacodynamic effects based on two parameters: μ-opioid receptor binding affinity and blood-brain barrier (BBB) permeability. Theoretical doses were very close to the drug doses at which mice showed place preference. Therefore, the CPP test is useful as a behavioral method for evaluating abuse-deterrent formulations of opioids mixed with an antagonist. The ratio of naloxone with opioids, at which mice did not show place preference, may be an effective index for developing abuse-deterrent formulations. Ratios may be calculated for other opioids based on μ-opioid receptor binding affinity and BBB permeability.

The Impact of P-Glycoprotein on Opioid Analgesics: What's the Real Meaning in Pain Management and Palliative Care?

Opioids are widely used in cancer and non-cancer pain management. However, many transporters at the blood-brain barrier (BBB), such as P-glycoprotein (P-gp, ABCB1/MDR1), may impair their delivery to the brain, thus leading to opioid tolerance. Nonetheless, opioids may regulate P-gp expression, thus altering the transport of other compounds, namely chemotherapeutic agents, resulting in pharmacoresistance. Other kinds of painkillers (e.g., acetaminophen, dexamethasone) and adjuvant drugs used for neuropathic pain may act as P-gp substrates and modulate its expression, thus making pain management challenging. Inflammatory conditions are also believed to upregulate P-gp. The role of P-gp in drug-drug interactions is currently under investigation, since many P-gp substrates may also act as substrates for the cytochrome P450 enzymes, which metabolize a wide range of xenobiotics and endobiotics. Genetic variability of the ABCB1/MDR1 gene may be accountable for inter-individual variation in opioid-induced analgesia. P-gp also plays a role in the management of opioid-induced adverse effects, such as constipation. Peripherally acting mu-opioid receptors antagonists (PAMORAs), such as naloxegol and naldemedine, are substrates of P-gp, which prevent their penetration in the central nervous system. In our review, we explore the interactions between P-gp and opioidergic drugs, with their implications in clinical practice.

Central Nervous System Distribution of Buprenorphine in Pregnant Sheep, Fetuses and Newborn Lambs After Continuous Transdermal and Single Subcutaneous Extended-Release Dosing

Buprenorphine is used during pregnancy for the treatment of opioid use disorder. Limited data exist on the central nervous system (CNS) permeation and distribution, and on the fetal exposure to buprenorphine. The aim of our study was to determine the extent of buprenorphine distribution to CNS in the pregnant sheep, and their fetus at steady-state, and their newborn lambs postdelivery, using three different dosing regimens. Twenty-eight pregnant ewes in late gestation received buprenorphine via 7-day transdermal patch releasing buprenorphine 20 µg/h (n=9) or 40 µg/h (n=11), or an extended-release 8 mg/week subcutaneous injection (n=8). Plasma, cerebrospinal fluid, and CNS tissue samples were collected at steady-state from ewes and fetuses, and from lambs 0.33 - 45 hours after delivery. High accumulation of buprenorphine was observed in all CNS tissues. The median CNS/plasma concentration -ratios of buprenorphine in different CNS areas ranged between 13 and 50 in the ewes, and between 26 and 198 in the fetuses. In the ewes the CNS/plasma -ratios were similar after the three dosing regimens, but higher in the fetuses in the 40 µg/h dosing group, medians 65 - 122, than in the 20 µg/h group, medians 26 - 54. The subcutaneous injection (theoretical release rate 47.6 µg/h) produced higher concentrations than observed after 40 µg/h transdermal patch dosing. The median fetal/maternal concentration -ratios in different dosing groups ranged between 0.21 and 0.54 in plasma, and between 0.38 and 1.3 in CNS tissues, respectively, with the highest ratios observed in the spinal cord. Buprenorphine concentrations in the cerebrospinal fluid were 8 - 13 % of the concurrent plasma concentration in the ewes and 28 % in the fetuses. Buprenorphine was quantifiable in the newborn lambs' plasma and CNS tissues two days postdelivery. Norbuprenorphine was analyzed from all plasma, cerebrospinal fluid, and CNS tissue samples but was nondetectable or below the LLOQ in most. The current study demonstrates that buprenorphine accumulates into CNS tissues at much higher concentrations than in plasma in pregnant sheep, fetuses, and their newborn lambs even 45 hours after delivery.

Alcohol and Cocaine Exposure Modulates ABCB1 and ABCG2 Transporters in Male Alcohol-Preferring Rats

Two efflux transporters, ATP-binding cassettes B1 (ABCB1) and G2 (ABCG2), are highly expressed in the endothelial cells of the brain, where they regulate the bioavailability and distribution of several endogenous and xenobiotic compounds. However, whether ABCB1 or ABCG2 has any link with drug dependence, drug withdrawal effects, or the incidence of adverse effects in drug abuser is not known. In this study, we determined the effects of voluntary ethanol consumption following repeated exposure to cocaine or vehicle on the relative mRNA and protein expression of Abcg2/ABCG2 and Abcb1/ABCB1 in the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) of male alcohol-preferring (P) rats. Male P rats were allowed free choice access to ethanol (15 and 30% v/v) and water for 5 weeks to establish baseline drinking behavior. The following week, rats were either injected with 20 mg/kg i.p. of cocaine or saline, once a day, for 7 days. The relative mRNA and protein expression of Abcb1/ABCB1 and Abcg2/ABCG2 in the NAc and mPFC were significantly decreased in ethanol-saline- and ethanol-cocaine-exposed rats compared to control rats that received neither ethanol nor cocaine. Thus, prolonged exposure to commonly abused drugs, ethanol and cocaine, alters the expression of Abcb1/ABCB1 and Abcg2/ABCG2 mRNA and protein levels in brain areas that play a role in drug dependence.

Elimination of substances from the brain parenchyma: efflux via perivascular pathways and via the blood-brain barrier

This review considers efflux of substances from brain parenchyma quantified as values of clearances (CL, stated in µL g-1 min-1). Total clearance of a substance is the sum of clearance values for all available routes including perivascular pathways and the blood-brain barrier. Perivascular efflux contributes to the clearance of all water-soluble substances. Substances leaving via the perivascular routes may enter cerebrospinal fluid (CSF) or lymph. These routes are also involved in entry to the parenchyma from CSF. However, evidence demonstrating net fluid flow inwards along arteries and then outwards along veins (the glymphatic hypothesis) is still lacking. CLperivascular, that via perivascular routes, has been measured by following the fate of exogenously applied labelled tracer amounts of sucrose, inulin or serum albumin, which are not metabolized or eliminated across the blood-brain barrier. With these substances values of total CL ≅ 1 have been measured. Substances that are eliminated at least partly by other routes, i.e. across the blood-brain barrier, have higher total CL values. Substances crossing the blood-brain barrier may do so by passive, non-specific means with CLblood-brain barrier values ranging from < 0.01 for inulin to > 1000 for water and CO2. CLblood-brain barrier values for many small solutes are predictable from their oil/water partition and molecular weight. Transporters specific for glucose, lactate and many polar substrates facilitate efflux across the blood-brain barrier producing CLblood-brain barrier values > 50. The principal route for movement of Na+ and Cl- ions across the blood-brain barrier is probably paracellular through tight junctions between the brain endothelial cells producing CLblood-brain barrier values ~ 1. There are large fluxes of amino acids into and out of the brain across the blood-brain barrier but only small net fluxes have been observed suggesting substantial reuse of essential amino acids and α-ketoacids within the brain. Amyloid-β efflux, which is measurably faster than efflux of inulin, is primarily across the blood-brain barrier. Amyloid-β also leaves the brain parenchyma via perivascular efflux and this may be important as the route by which amyloid-β reaches arterial walls resulting in cerebral amyloid angiopathy.

Modulation of Opioid Transport at the Blood-Brain Barrier by Altered ATP-Binding Cassette (ABC) Transporter Expression and Activity

Opioids are highly effective analgesics that have a serious potential for adverse drug reactions and for development of addiction and tolerance. Since the use of opioids has escalated in recent years, it is increasingly important to understand biological mechanisms that can increase the probability of opioid-associated adverse events occurring in patient populations. This is emphasized by the current opioid epidemic in the United States where opioid analgesics are frequently abused and misused. It has been established that the effectiveness of opioids is maximized when these drugs readily access opioid receptors in the central nervous system (CNS). Indeed, opioid delivery to the brain is significantly influenced by the blood-brain barrier (BBB). In particular, ATP-binding cassette (ABC) transporters that are endogenously expressed at the BBB are critical determinants of CNS opioid penetration. In this review, we will discuss current knowledge on the transport of opioid analgesic drugs by ABC transporters at the BBB. We will also examine how expression and trafficking of ABC transporters can be modified by pain and/or opioid pharmacotherapy, a novel mechanism that can promote opioid-associated adverse drug events and development of addiction and tolerance.

Opioids and the Blood-Brain Barrier: A Dynamic Interaction with Consequences on Drug Disposition in Brain

Background:

Opioids are widely used in pain management, acting via opioid receptors and/or Toll-like receptors (TLR) present at the central nervous system (CNS). At the blood-brain barrier (BBB), several influx and efflux transporters, such as the ATP-binding cassette (ABC)

P-glycoprotein (P-gp, ABCB1), Breast Cancer Resistance Protein (BCRP, ABCG2) and multidrug resistance-associated proteins (MRP, ABCC) transporters, and solute carrier transporters (SLC), are responsible for the transport of xenobiotics from the brain into the bloodstream or vice versa.

Objective:

ABC transporters export several clinically employed opioids, altering their neuro-

pharmacokinetics and CNS effects. In this review, we explore the interactions between opioids

and ABC transporters, and decipher the molecular mechanisms by which opioids can modify their expression at the BBB.

Results:

P-gp is largely implicated in the brain-to-blood efflux of opioids, namely morphine and oxycodone. Long-term ex-posure to morphine and oxycodone has proven to up-regulate the expression of ABC transporters, such as P-gp, BCRP and MRPs, at the BBB, which may lead to increased tolerance to the antinociceptive effects of such drugs. Recent studies uncov-er two mechanisms by which morphine may up-regulate P-gp and BCRP at the BBB: 1) via a glutamate, NMDA-receptor and COX-2 signaling cascade, and 2) via TLR4 activation, subsequent development of neuro-

inflammation, and activation of NF-κB, presumably via glial cells.

Conclusion:

The BBB-opioid interaction can culminate in bilateral consequences, since ABC transporters condition the brain disposition of opioids, while opioids also affect the expression of ABC transporters at the BBB, which may result in increased CNS drug pharmacoresistance.

P-Glycoprotein in skin contributes to transdermal absorption of topical corticosteroids

ATP binding cassette transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP), are expressed in skin, but their involvement in transdermal absorption of clinically used drugs remains unknown. Here, we examined their role in transdermal absorption of corticosteroids. Skin and plasma concentrations of dexamethasone after dermal application were reduced in P-gp and BCRP triple-knockout (Mdr1a/1b/Bcrp^-/-) mice. The skin concentration in Mdr1a/1b/Bcrp^-/- mice was reduced in the dermis, but not in the epidermis, indicating that functional expression of these transporters in skin is compartmentalized. Involvement of these transporters in dermal transport of dexamethasone was also supported by the observation of a higher epidermal concentration in Mdr1a/1b/Bcrp^-/- than wild-type mice during intravenous infusion. Transdermal absorption after dermal application of prednisolone, but not methylprednisolone or ethinyl estradiol, was also lower in Mdr1a/1b/Bcrp^-/- than in wild-type mice. Transport studies in epithelial cell lines transfected with P-gp or BCRP showed that dexamethasone and prednisolone are substrates of P-gp, but are minimally transported by BCRP. Thus, our findings suggest that P-gp is involved in transdermal absorption of at least some corticosteroids in vivo. P-gp might be available as a target for inhibition in order to deliver topically applied drugs and cosmetics in a manner that minimizes systemic exposure.

Transporter-Mediated Disposition of Opioids: Implications for Clinical Drug Interactions

Opioid-related deaths, abuse, and drug interactions are growing epidemic problems that have medical, social, and economic implications. Drug transporters play a major role in the disposition of many drugs, including opioids; hence they can modulate their pharmacokinetics, pharmacodynamics and their associated drug-drug interactions (DDIs). Our understanding of the interaction of transporters with many therapeutic agents is improving; however, investigating such interactions with opioids is progressing relatively slowly despite the alarming number of opioids-mediated DDIs that may be related to transporters. This review presents a comprehensive report of the current literature relating to opioids and their drug transporter interactions. Additionally, it highlights the emergence of transporters that are yet to be fully identified but may play prominent roles in the disposition of opioids, the growing interest in transporter genomics for opioids, and the potential implications of opioid-drug transporter interactions for cancer treatments. A better understanding of drug transporters interactions with opioids will provide greater insight into potential clinical DDIs and could help improve opioids safety and efficacy.

P-glycoprotein interactions of novel psychoactive substances - stimulation of ATP consumption and transport across Caco-2 monolayers

In contrast to drugs for therapeutic use, there are only few data available concerning interactions between P-glycoprotein (P-gp) and drugs of abuse (DOA). In this work, interactions between structurally diverse DOA and P-gp were investigated using different strategies. First, the effect on the P-gp ATPase activity was studied by monitoring of ATP consumption after addition to recombinant, human P-gp. Second, DOA showing an increased ATP consumption were further characterized regarding their transport across filter grown Caco-2- monolayers. Analyses were performed by luminescence and liquid chromatography-mass spectrometry, respectively. Among the nine DOA initially screened, benzedrone, diclofensine, glaucine, JWH-200, MDBC, WIN-55,212-2 showed an increase of ATP consumption in the ATPase stimulation assay. In Caco-2 transport studies, Glaucine, JWH-200, mitragynine, WIN-55,212-2 could moreover be identified as non-transported substrates, but inhibitors of P-gp activity. Thus, drug-drug or drug-food interactions should be very likely for these compounds.

Opioids and efflux transporters. Part 4: influence of N-substitution on P-glycoprotein substrate activity of noroxymorphone analogues

The efflux transporter protein P-glycoprotein (P-gp) is capable of affecting the central distribution of diverse neurotherapeutics, including opioid analgesics, through their active removal from the brain. P-gp located at the blood brain barrier has been implicated in the development of tolerance to opioids and demonstrated to be up-regulated in rats tolerant to morphine and oxycodone. We have previously examined the influence of hydrogen-bonding oxo-substitutents on the P-gp-mediated efflux of 4,5-epoxymorphinan analgesics, as well as that of N-substituted analogues of meperidine. Structure-activity relationships (SAR) governing N-substituent effects on opioid efficacy is well-established, however the influence of such structural modifications on P-gp-mediated efflux is unknown. Here, we present SAR describing P-gp recognition of a short series of N-modified 4,5-epoxymorphinans. Oxymorphone, naloxone, naltrexone, and nalmexone all failed to demonstrate P-gp substrate activity, indicating these opioid scaffolds contain structural features that preclude recognition by the transporter. These results are examined using mathematical molecular modeling and discussed in comparison to other opioid scaffolds bearing similar N-substituents.

Decreased analgesic effect of morphine, but not buprenorphine, in patients with advanced P-glycoprotein(+) cancers

BACKGROUND

P-glycoprotein (P-gp) is expressed on the blood-brain barrier (BBB) and acts as a transporter regulating the analgesic effect of morphine. The P-gp is also expressed by different types of tumors. The aim of this study was to determine the potential association of the P-gp expression in malignant tumors with analgesic effects in patients.

METHODS

The P-gp expression in 120 malignant tumors was examined by immunohistochemistry. The analgesic responses of individual patients to morphine and buprenorphine (BNP) were evaluated by visual analog scale (VAS). The levels of plasma morphine and BNP were determined by HPLC.

RESULTS

We found that there was no significant difference in the values of VAS between patients with P-gp(+) and P-gp(-) malignant tumors in responses to 0.000025 g x kg(-2) of BNP administered by patient-controlled intravenous analgesia (PCIA), accompanied by similar levels of plasma BNP in those patients. In contrast, the values of VAS in response to 0.00075 g x kg(-2) of morphine in patients with P-gp(+) tumors were significantly greater than those in the patients with P-gp(-) tumors, although similar levels of plasma morphine were detected in both groups of patients. Furthermore, treatment with a higher dose (0.0011 g x kg(-2)) of morphine effectively controlled pain in those with P-gp(+) tumors.

CONCLUSION

Our data indicated that patients with P-gp(+) tumors required a higher dose of morphine to achieve an analgesic effect and that the P-gp expression in tumors may be valuable for predicting the analgesic responses of patients with severe pain to morphine.

Respiratory toxicity of buprenorphine results from the blockage of P-glycoprotein-mediated efflux of norbuprenorphine at the blood-brain barrier in mice

OBJECTIVES

Deaths due to asphyxia as well as following acute poisoning with severe respiratory depression have been attributed to buprenorphine in opioid abusers. However, in human and animal studies, buprenorphine exhibited ceiling respiratory effects, whereas its metabolite, norbuprenorphine, was assessed as being a potent respiratory depressor in rodents. Recently, norbuprenorphine, in contrast to buprenorphine, was shown in vitro to be a substrate of human P-glycoprotein, a drug-transporter involved in all steps of pharmacokinetics including transport at the blood-brain barrier. Our objectives were to assess P-glycoprotein involvement in norbuprenorphine transport in vivo and study its role in the modulation of buprenorphine-related respiratory effects in mice.

SETTING

University-affiliated research laboratory, INSERM U705, Paris, France.

SUBJECTS

Wild-type and P-glycoprotein knockout female Friend virus B-type mice.

INTERVENTIONS

Respiratory effects were studied using plethysmography and the P-glycoprotein role at the blood-brain barrier using in situ brain perfusion.

MEASUREMENTS AND MAIN RESULTS

Norbuprenorphine(≥ 1 mg/kg) and to a lesser extent buprenorphine (≥ 10 mg/kg) were responsible for dose-dependent respiratory depression combining increased inspiratory (TI) and expiratory times (TE). PSC833, a powerful P-glycoprotein inhibitor, significantly enhanced buprenorphine-related effects on TI (p < .01) and TE (p < .05) and norbuprenorphine-related effects on minute volume (VE, p < .05), TI, and TE (p < .001). In P-glycoprotein-knockout mice, buprenorphine-related effects on VE (p < .01), TE (p < .001), and TI (p < .05) and norbuprenorphine-related effects on VE (p < .05) and TI (p < .001) were significantly enhanced. Plasma norbuprenorphine concentrations were significantly increased in PSC833-treated mice (p < .001), supporting a P-glycoprotein role in norbuprenorphine pharmacokinetics. Brain norbuprenorphine efflux was significantly reduced in PSC833-treated and P-glycoprotein-knockout mice (p < .001), supporting P-glycoprotein-mediated norbuprenorphine transport at the blood-brain barrier.

CONCLUSIONS

P-glycoprotein plays a key-protective role in buprenorphine-related respiratory effects, by allowing norbuprenorphine efflux at the blood-brain barrier. Our findings suggest a major role for drug-drug interactions that lead to P-glycoprotein inhibition in buprenorphine-associated fatalities and respiratory depression.

Brain efflux index to investigate the influence of active efflux on brain distribution of pemetrexed and methotrexate

Antifolates, in particular methotrexate (MTX), have been widely used in the treatment of primary and secondary tumors of the central nervous system (CNS). Pemetrexed (PMX) is a novel antifolate that also exhibits potent antitumor activity against CNS malignancies. Studies have shown that brain distribution of both antifolates is significantly restricted, possible due to active efflux transport at the blood-brain barrier (BBB). This study characterizes the brain-to-blood transport of PMX and MTX and examines the role of several efflux transporters in brain distribution of the antifolates by use of the intracerebral microinjection technique (brain efflux index). The results from this study show that both PMX and MTX undergo saturable efflux transport across the BBB, with elimination half-lives of approximately 39 minutes and 29 minutes, respectively. Of the various efflux transporters this study investigated, multidrug resistance-associated protein 2 (Mrp2) does not play an important role in the brain distribution of the two antifolate drugs. Interestingly, breast-cancer resistance protein (Bcrp) makes a significant contribution to the brain elimination of MTX but not PMX. In addition, the brain-to-blood transport of both antifolates was inhibited by probenecid and benzylpenicillin, suggesting the involvement of organic anion transporters in the efflux of these compounds from the brain, with organic anion transporter 3 (Oat3) being a possibility. Our results suggest that one of the underlying mechanisms behind the limited brain distribution of PMX and MTX is active efflux transport processes at the BBB, including a benzylpenicillin-sensitive transport system and/or the active transporter Bcrp.

(R)- and (S)-methadone and buprenorphine concentration ratios in maternal and umbilical cord plasma following chronic maintenance dosing in pregnancy

AIMS

The aim of this study was to compare the transfer of buprenorphine and methadone between maternal and cord blood in women under chronic dosing conditions and to determine if differences exist in the transfer of the two methadone enantiomers.

METHODS

Maternal and cord blood samples were collected at delivery from women maintained on methadone (35, 25-140 mg day⁻¹) (median; range) or buprenorphine (6.00, 2-20 mg day⁻¹) during pregnancy. Plasma concentration ratios are presented as an indicator of foetal exposure relative to the mother.

RESULTS

Methadone was quantified in all samples, with cord : maternal plasma methadone concentration ratios (n= 15 mother-infant pairs) being significantly higher (P < 0.0001; mean difference (MD) 0.07; 95% confidence interval (CI) 0.048, 0.092) for the active (R)-methadone enantiomer (0.41; 0.19, 0.56) (median; range) compared with (S)-methadone (0.36; 0.15, 0.53). (R)- : (S)-methadone concentration ratios were also significantly higher (P < 0.0001; MD 0.24 95% CI 0.300, 0.180) for cord (1.40; 0.95, 1.67) compared with maternal plasma (1.16; 0.81, 1.38). Half the infant buprenorphine samples were below the assay lower limit of quantification (LLOQ) (0.125 ng ml⁻¹). The latter was four-fold lower than the LLOQ for methadone (0.50 ng ml⁻¹). The cord : maternal plasma buprenorphine concentration ratio (n= 9 mother-infant pairs) was 0.35; 0.14, 0.47 and for norbuprenorphine 0.49; 0.24, 0.91.

CONCLUSIONS

The transfer of the individual methadone enantiomers to the foetal circulation is stereoselective. Infants born to buprenorphine maintained women are not exposed to a greater proportion of the maternal dose compared with methadone and may be exposed to relatively less of the maternal dose compared with infants born to women maintained on methadone during pregnancy.

Blood-brain barrier transport of naloxone does not involve P-glycoprotein-mediated efflux

The blood-brain barrier (BBB) transport of naloxone, a potent and specific opioid antagonist, was investigated in rats using the brain uptake index method and the brain efflux index method. The apparent influx clearance of [(3)H]naloxone across the BBB was 0.305 mL/min/g brain. [(3)H]naloxone was eliminated from the brain with an apparent elimination half-life of 15.1 min after microinjection into the parietal cortex area 2 regions of the rat brain. The apparent efflux clearance of [(3)H]naloxone across the BBB was 0.152 mL/min/g brain, which was calculated from the elimination rate constant (4.79 x 10(-2) min(-1)) and the distribution volume in the brain (3.18 mL/g brain). The influx clearance across the BBB was two times greater than the efflux clearance. The elimination of [(3)H]naloxone from the brain was not inhibited in the presence of the typical P-glycoprotein (P-gp) inhibitors such as quinidine, verapamil, vinblastine, and vincristine, indicating that naloxone is not a P-gp substrate in the rat. In vitro experiments by using human multidrug resistance 1 (MDR1)/P-gp overexpressing HeLa cells showed that the uptake of naloxone by the cells did not change in the presence of the P-gp inhibitors. In conclusion, the present results obtained from in vivo and in vitro studies suggest that P-gp is not involved in the BBB transport of naloxone.

Efflux transporters- and cytochrome P-450-mediated interactions between drugs of abuse and antiretrovirals

Multidrug regimens and corresponding drug interactions cause many adverse reactions and treatment failures. Drug efflux transporters: P-gp, MRP, BCRP in conjunction with metabolizing enzymes (CYPs) are major factors in such interactions. Most effective combination antiretrovirals (ARV) therapy includes a PI or a NNRTI or two NRTI. Coadministration of such ARV may induce efflux transporters and/or CYP3A4 resulting in sub-therapeutic blood levels and therapeutic failure due to reduced absorption and/or increased metabolism. A similar prognosis is true for ARV-compounds and drugs of abuse combinations. Morphine and nicotine enhance CYP3A4 and MDR1 expression in vitro. A 2.5 fold rise of cortisol metabolite was evident in smokers relative to nonsmokers. Altered functions of efflux transporters and CYPs in response to ARV and drugs of abuse may result in altered drug absorption and metabolism. Appropriate in vitro models can be employed to predict such interactions. Influence of genetic polymorphism, SNP and inter-individual variation in drug response has been discussed. Complexity underlying the relationship between efflux transporters and CYP makes it difficult to predict the outcome of HAART as such, particularly when HIV patients taking drugs of abuse do not adhere to HAART regimens. HIV(+) pregnant women on HAART medications, indulging in drugs of abuse, may develop higher viral load due to such interactions and lead to increase in mother to child transmission of HIV. A multidisciplinary approach with clear understanding of mechanism of interactions may allow proper selection of regimens so that desired therapeutic outcome of HAART can be reached without any side effects.

Interaction of drugs of abuse and maintenance treatments with human P-glycoprotein (ABCB1) and breast cancer resistance protein (ABCG2)

Drug interaction with P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) may influence its tissue disposition including blood-brain barrier transport and result in potent drug-drug interactions. The limited data obtained using in-vitro models indicate that methadone, buprenorphine, and cannabinoids may interact with human P-gp; but almost nothing is known about drugs of abuse and BCRP. We used in vitro P-gp and BCRP inhibition flow cytometric assays with hMDR1- and hBCRP-transfected HEK293 cells to test 14 compounds or metabolites frequently involved in addiction, including buprenorphine, norbuprenorphine, methadone, ibogaine, cocaine, cocaethylene, amphetamine, N-methyl-3,4-methylenedioxyamphetamine, 3,4-methylenedioxyamphetamine, nicotine, ketamine, Delta9-tetrahydrocannabinol (THC), naloxone, and morphine. Drugs that in vitro inhibited P-gp or BCRP were tested in hMDR1- and hBCRP-MDCKII bidirectional transport studies. Human P-gp was significantly inhibited in a concentration-dependent manner by norbuprenorphine>buprenorphine>methadone>ibogaine and THC. Similarly, BCRP was inhibited by buprenorphine>norbuprenorphine>ibogaine and THC. None of the other tested compounds inhibited either transporter, even at high concentration (100 microm). Norbuprenorphine (transport efflux ratio approoximately 11) and methadone (transport efflux ratio approoximately 1.9) transport was P-gp-mediated; however, with no significant stereo-selectivity regarding methadone enantiomers. BCRP did not transport any of the tested compounds. However, the clinical significance of the interaction of norbuprenorphine with P-gp remains to be evaluated.

Interaction between topically and systemically coadministered P-glycoprotein substrates/inhibitors: effect on vitreal kinetics

The objective of the present study was to investigate the effect of topically coadministered P-glycoprotein (P-gp) substrates/inhibitors on the vitreal kinetics of a systemically administered P-gp substrate. Anesthetized male rabbits were used in these studies. The concentration-time profile of quinidine in the vitreous humor, after intravenous administration, was determined alone and in the presence of topically coadministered verapamil, prednisolone sodium phosphate (PP), and erythromycin. The vitreal pharmacokinetic parameters of quinidine in the presence of verapamil [apparent elimination rate constant (λ(z)), 0.0027 ± 0.0002 min(-1); clearance (CL_F), 131 ± 21 ml/min; area under the curve (AUC(0-∞)), 39 ± 7.0 μg · min/ml; and mean residence time, 435 ± 20 min] were significantly different from those of the control (0.0058 ± 0.0006 min(-1), 296 ± 46 ml/min, 17 ± 3 μg · min/ml, and 232 ± 20 min, respectively). A 1.7-fold decrease in the vitreal λ(z) and a 1.5-fold increase in the vitreal AUC of quinidine were observed in the presence of topical PP. Statistically significant differences between the vitreal profiles of the control and erythromycin-treated group were also observed. Plasma concentration-time profiles of quinidine, alone or in the presence of the topically instilled compounds, remained unchanged, indicating uniform systemic quinidine exposure across groups. This study demonstrates an interaction between topically and systemically coadministered P-gp substrates, probably through the modulation of P-gp on the basolateral membrane of the retinal pigmented epithelium, leading to changes in the vitreal kinetics of the systemically administered agent.

Differential involvement of P-glycoprotein (ABCB1) in permeability, tissue distribution, and antinociceptive activity of methadone, buprenorphine, and diprenorphine: in vitro and in vivo evaluation

Conclusions based on either in vitro or in vivo approach to evaluate the P-gp affinity status of opioids may be misleading. For example, in vitro studies indicated that fentanyl is a P-gp inhibitor while in vivo studies indicated that it is a P-gp substrate. Quite the opposite was evident for meperidine. The objective of this study was to evaluate the P-gp affinity status of methadone, buprenorphine and diprenorphine to predict P-gp-mediated drug-drug interactions and to determine a better candidate for management of opioid dependence. Two in vitro (P-gp ATPase and monolayer efflux) assays and two in vivo (tissue distribution and antinociceptive evaluation in mdr1a/b (-/-) mice) assays were used. Methadone stimulated the P-gp ATPase activity only at higher concentrations, while verapamil and GF120918 inhibited its efflux (p < 0.05). The brain distribution and antinociceptive activity of methadone were enhanced (p < 0.05) in P-gp knockout mice. Conversely, buprenorphine and diprenorphine were negative in all assays. P-gp can affect the PK/PD of methadone, but not buprenorphine or diprenorphine. Our report is in favor of buprenorphine over methadone for management of opioid dependence. Buprenorphine most likely is not a P-gp substrate and concerns regarding P-gp-mediated drug-drug interaction are not expected.

The challenge of pain management in patients with myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disorder of the neuromuscular junction. The complexity of the disease and its treatments make MG patients particularly susceptible to adverse effects of drugs. MG is not a painful condition; however, as pain management armamentarium includes drugs from diverse pharmacological groups and with potential for drug-drug interactions, managing pain in patients with MG can be challenging. The underlying disease and the concomitant medications of each patient must be considered and the analgesic treatment individualized. This review presents an update on the various aspects of pain pharmacotherapy in patients with MG, focusing primarily on medications used to treat chronic pain. Drugs discussed are opioids, nonsteroidal anti-inflammatory drugs, antidepressants, anticonvulsants, muscle relaxants, benzodiazepines, intravenous magnesium, and local anesthetics. Drug interactions with agents used for MG treatment (acethylcholinesterase inhibitors, corticosteroids, immunosuppressants) and plasmapheresis are discussed. The clinical usefulness and limitations of each of the drug classes and agents are described.

Imaging of cyclosporine inhibition of P-glycoprotein activity using 11C-verapamil in the brain: studies of healthy humans

The multiple-drug resistance (MDR) transporter P-glycoprotein (P-gp) is highly expressed at the human blood-brain barrier (BBB). P-gp actively effluxes a wide variety of drugs from the central nervous system, including anticancer drugs. We have previously demonstrated P-gp activity at the human BBB using PET of (11)C-verapamil distribution into the brain in the absence and presence of the P-gp inhibitor cyclosporine-A (CsA). Here we extend the initial noncompartmental analysis of these data and apply compartmental modeling to these human verapamil imaging studies.

METHODS

Healthy volunteers were injected with (15)O-water to assess blood flow, followed by (11)C-verapamil to assess BBB P-gp activity. Arterial blood samples and PET images were obtained at frequent intervals for 5 and 45 min, respectively, after injection. After a 60-min infusion of CsA (intravenously, 2.5 mg/kg/h) to inhibit P-gp, a second set of water and verapamil PET studies was conducted, followed by (11)C-CO imaging to measure regional blood volume. Blood flow was estimated using dynamic (15)O-water data and a flow-dispersion model. Dynamic (11)C-verapamil data were assessed by a 2-tissue-compartment (2C) model of delivery and retention and a 1-tissue-compartment model using the first 10 min of data (1C(10)).

RESULTS

The 2C model was able to fit the full dataset both before and during P-pg inhibition. CsA modulation of P-gp increased blood-brain transfer (K(1)) of verapamil into the brain by 73% (range, 30%-118%; n = 12). This increase was significantly greater than changes in blood flow (13%; range, 12%-49%; n = 12, P < 0.001). Estimates of K(1) from the 1C(10) model correlated to estimates from the 2C model (r = 0.99, n = 12), indicating that a short study could effectively estimate P-gp activity.

CONCLUSION

(11)C-verapamil and compartmental analysis can estimate P-gp activity at the BBB by imaging before and during P-gp inhibition by CsA, indicated by a change in verapamil transport (K(1)). Inhibition of P-gp unmasks verapamil trapping in brain tissue that requires a 2C model for long imaging times; however, transport can be effectively measured using a short scan time with a 1C(10) model, avoiding complications with labeled metabolites and tracer retention.

Possible multiple transporters were involved in hepatobiliary excretion of antofloxacin in rats

1. The aim of the current study was to investigate the characteristics of biliary excretion of antofloxacin (ATFX) in rats. Rats received a bolus intravenous injection followed by a constant-rate infusion of ATFX. When plasma concentrations of ATFX reached steady state, cyclosporin A, erythromycin, probenecid, cimetidine and diclofenac were administered intravenously to the rats. Samples of blood and bile were collected and the concentrations of ATFX were measured and the corresponding pharmacokinetic parameters were estimated. 2. Biliary excretion of ATFX was observed in rats subjected to CCl(4)-induced experimental hepatic injury for 24 h (CCl(4)-EHI(24h)). Steady state concentrations of ATFX were attained at 60 min following infusion. 3. A slight increase in concentration of ATFX in plasma was observed after cyclosporin A, erythromycin, probenecid and cimetidine treatment. Significant increases in ATFX plasma levels were found in rats treated with diclofenac. Cyclosporin A, erythromycin, probenecid, cimetidine and diclofenac treatment significantly decreased the steady state biliary clearance of ATFX to 55, 68, 54, 53 and 56% of control values, respectively. 4. Cyclosprin A, probenecid, erythromycin and cimetidine also inhibited the biliary excretion of ATFX glucuronide. Significant decrease in the steady state biliary clearance of ATFX and its glucuronide was observed in CCl(4)-EHI(24h) rats. 5. These results indicate that multiple transporters are possibly involved in the biliary excretion of ATFX.

Cobalamin potentiates vinblastine cytotoxicity through downregulation of mdr-1 gene expression in HepG2 cells

BACKGROUND

P-glycoprotein (Pgp), produced by multidrug resistance-1 gene (mdr-1), is a main mechanism developed by cancer cells to guard against anti-cancer drugs. Alterations of DNA methylation of the mdr-1 gene promoter are known to be linked to mdr-1 gene expression and are probably related to intracellular S-adenosyl-methionine. We here used HepG2 cells to determine the role of the methionine cycle (through the use of the Methionine-Synthase (MS) cofactor, cobalamin) on mdr-1 gene expression.

METHODS

Semiquantitative RT-PCR of mdr-1 gene, cellular retention of rhodamine-123, and vinblastine cytotoxicity were carried out on cells cultivated with and without cobalamin. Methylation status of the mdr-1 gene promoter was determined by methylation-specific PCR.

RESULTS

Addition of cobalamin to the cells led to an increase in MS activity, to a significant decrease in mdr-1 gene expression which is correlated to an increase in retention of the Pgp substrate Rhodamine 123. Furthermore, cobalamin potentiated cell sensitivity to vinblastine to the same range as that of the Pgp blocker verapamil and prevented methotrexate-induced up-regulation of mdr-1 gene expression. However, no modification in methylation of the mdr-1 gene promoter was observed.

CONCLUSION

Cobalamin downregulates mdr-1 gene expression, as well as Pgp expression and function, and significantly increases cytotoxicity of vinblastine. The identification of this novel way of diminishing cellular resistance to the chemotherapeutic agent vinblastine holds promises of leading to better treatments for cancer patients.