Effect of P-glycoprotein modulation on the clinical pharmacokinetics and adverse effects of morphine

Refinement of safety and efficacy of anti-cancer chemotherapeutics by tailoring their site-specific intracellular bioavailability through transporter modulation

Low intracellular bioavailability, off-site toxicities, and multi drug resistance (MDR) are the major constraints involved in cancer chemotherapy. Many anticancer molecules fail to become a good lead in drug discovery because of their poor site-specific bioavailability. Concentration of a molecule at target sites is largely varied because of the wavering expression of transporters. Recent anticancer drug discovery strategies are paying high attention to enhance target site bioavailability by modulating drug transporters. The level of genetic expression of transporters is an important determinant to understand their ability to facilitate drug transport across the cellular membrane. Solid carrier (SLC) transporters are the major influx transporters involved in the transportation of most anti-cancer drugs. In contrast, ATP-binding cassette (ABC) superfamily is the most studied class of efflux transporters concerning cancer and is significantly involved in efflux of chemotherapeutics resulting in MDR. Balancing SLC and ABC transporters is essential to avoid therapeutic failure and minimize MDR in chemotherapy. Unfortunately, comprehensive literature on the possible approaches of tailoring site-specific bioavailability of anticancer drugs through transporter modulation is not available till date. This review critically discussed the role of different specific transporter proteins in deciding the intracellular bioavailability of anticancer molecules. Different strategies for reversal of MDR in chemotherapy by incorporation of chemosensitizers have been proposed in this review. Targeted strategies for administration of the chemotherapeutics to the intracellular site of action through clinically relevant transporters employing newer nanotechnology-based formulation platforms have been explained. The discussion embedded in this review is timely considering the current need of addressing the ambiguity observed in pharmacokinetic and clinical outcomes of the chemotherapeutics in anti-cancer treatment regimens.

Pharmacological data science perspective on fatal incidents of morphine treatment

Morphine prescribed for analgesia has caused drug-related deaths at an estimated incidence of 0.3% to 4%. Morphine has pharmacological properties that make it particularly difficult to assess the causality of morphine administration with a patient's death, such as its slow transfer between plasma and central nervous sites of action and the existence of the active metabolite morphine-6-glucuronide with opioid agonistic effects, Furthermore, there is no well-defined toxic dose or plasma/blood concentration for morphine. Dosing is often adjusted for adequate pain relief. Here, we summarize reported deaths associated with morphine therapy, including associated morphine exposure and modulating patient factors such as pharmacogenetics, concomitant medications, or comorbidities. In addition, we systematically analyzed published numerical information on the stability of concentrations of morphine and its relevant metabolites in biological samples collected postmortem. A medicolegal case is presented in which the causality of morphine administration with death was in dispute and pharmacokinetic modeling was applied to infer the administered dose. The results of this analytical review suggest that (i) inference from postmortem blood concentrations to the morphine dose administered has low validity and (ii) causality between a patient's death and the morphine dose administered remains a highly context-dependent and collaborative assessment among experts from different medical specialties.

Pharmacogenetics in the Treatment and Supportive Care of Patients With Cancer

For patients undergoing treatment for cancer-induced pain, the identification and evaluation of pharmacogenetic variability may improve outcomes. Metabolism of opioids and other analgesic agents is influenced by patient-specific variables and drug–drug interactions, which often pose clinical challenges. Consultation with a clinical pharmacist or pharmacogenetics specialist is recommended to aid in the interpretation and evaluation of pharmacogenetic test results. The current NCCN Guidelines for Adult Cancer Pain provide pharmacogenetic considerations and recommendations for the treatment and supportive care of this population.

Pharmacokinetic Drug Interaction Study of Sorafenib and Morphine in Rats

A combination of the tyrosine kinase inhibitor—sorafenib—and the opioid analgesic—morphine—can be found in the treatment of cancer patients. Since both are substrates of P-glycoprotein (P-gp), and sorafenib is also an inhibitor of P-gp, their co-administration may affect their pharmacokinetics, and thus the safety and efficacy of cancer therapy. Therefore, the aim of this study was to evaluate the potential pharmacokinetic drug–drug interactions between sorafenib and morphine using an animal model. The rats were divided into three groups that Received: sorafenib and morphine (I_SOR+MF), sorafenib (II_SOR), and morphine (III_MF). Morphine caused a significant increase in maximum plasma concentrations (C_max) and the area under the plasma concentration–time curves (AUC_0–t, and AUC_0–∞) of sorafenib by 108.3 (p = 0.003), 55.9 (p = 0.0115), and 62.7% (p = 0.0115), respectively. Also, the C_max and AUC_0–t of its active metabolite—sorafenib N-oxide—was significantly increased in the presence of morphine (p = 0.0022 and p = 0.0268, respectively). Sorafenib, in turn, caused a significant increase in the C_max of morphine (by 0.5-fold, p = 0.0018). Moreover, in the presence of sorafenib the C_max, AUC_0–t, and AUC_0–∞ of the morphine metabolite M3G increased by 112.62 (p < 0.0001), 46.82 (p = 0.0124), and 46.78% (p = 0.0121), respectively. Observed changes in sorafenib and morphine may be of clinical significance. The increased exposure to both drugs may improve the response to therapy in cancer patients, but on the other hand, increase the risk of adverse effects.

Voriconazole greatly increases the exposure to oral buprenorphine

PURPOSE

Buprenorphine has low oral bioavailability. Regardless of sublingual administration, a notable part of buprenorphine is exposed to extensive first-pass metabolism by the cytochrome P450 (CYP) 3A4. As drug interaction studies with buprenorphine are limited, we wanted to investigate the effect of voriconazole, a strong CYP3A4 inhibitor, on the pharmacokinetics and pharmacodynamics of oral buprenorphine.

METHODS

Twelve healthy volunteers were given either placebo or voriconazole (orally, 400 mg twice on day 1 and 200 mg twice on days 2-5) for 5 days in a randomized, cross-over study. On day 5, they ingested 0.2 mg (3.6 mg during placebo phase) oral buprenorphine. We measured plasma and urine concentrations of buprenorphine and norbuprenorphine and monitored their pharmacological effects. Pharmacokinetic parameters were normalized for a buprenorphine dose of 1.0 mg.

RESULTS

Voriconazole greatly increased the mean area under the plasma concentration-time curve (AUC_0-18) of buprenorphine (4.3-fold, P < 0.001), its peak concentration (C_max) (3.9-fold), half-life (P < 0.05), and excretion into urine (A_e; P < 0.001). Voriconazole also markedly enhanced the C_max (P < 0.001), AUC_0-18 (P < 0.001), and A_e (P < 0.05) of unconjugated norbuprenorphine but decreased its renal clearance (P < 0.001). Mild dizziness and nausea occurred during both study phases.

CONCLUSIONS

Voriconazole greatly increases exposure to oral buprenorphine, mainly by inhibiting intestinal and liver CYP3A4. Effect on some transporters may explain elevated norbuprenorphine concentrations. Although oral buprenorphine is not commonly used, this interaction may become relevant in patients receiving sublingual buprenorphine together with voriconazole or other CYP3A4 or transporter inhibitors.

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.

Opioids Resistance in Chronic Pain Management

Abstract: Chronic pain management represents a serious healthcare problem worldwide. Chronic pain affects approximately 20% of the adult European population and is more frequent in women and older people. Unfortunately, its management in the community remains generally unsatisfactory and rarely under the control of currently available analgesics. Opioids have been used as analgesics for a long history and are among the most used drugs; however, while there is no debate over their short term use for pain management, limited evidence supports their efficacy of long-term treatment for chronic non-cancer pain. Therapy with opioids is hampered by inter-individual variability and serious side effects and some opioids often result ineffective in the treatment of chronic pain and their use is controversial. Accordingly, for a better control of chronic pain a deeper knowledge of the molecular mechanisms underlying resistance to opiates is mandatory.

Morphine-induced hyperalgesia involves mu opioid receptors and the metabolite morphine-3-glucuronide

Opiates are potent analgesics but their clinical use is limited by side effects including analgesic tolerance and opioid-induced hyperalgesia (OIH). The Opiates produce analgesia and other adverse effects through activation of the mu opioid receptor (MOR) encoded by the Oprm1 gene. However, MOR and morphine metabolism involvement in OIH have been little explored. Hence, we examined MOR contribution to OIH by comparing morphine-induced hyperalgesia in wild type (WT) and MOR knockout (KO) mice. We found that repeated morphine administration led to analgesic tolerance and hyperalgesia in WT mice but not in MOR KO mice. The absence of OIH in MOR KO mice was found in both sexes, in two KO global mutant lines, and for mechanical, heat and cold pain modalities. In addition, the morphine metabolite morphine-3beta-D-glucuronide (M3G) elicited hyperalgesia in WT but not in MOR KO animals, as well as in both MOR flox and MOR-Nav1.8 sensory neuron conditional KO mice. M3G displayed significant binding to MOR and G-protein activation when using membranes from MOR-transfected cells or WT mice but not from MOR KO mice. Collectively our results show that MOR is involved in hyperalgesia induced by chronic morphine and its metabolite M3G.

ABCC3 and OCT1 genotypes influence pharmacokinetics of morphine in children

AIM

Large interindividual variability in morphine pharmacokinetics could contribute to variability in morphine analgesia and adverse events.

METHODS

Influence of weight, genetic polymorphisms, race and sex on morphine clearance and metabolite formation from 220 children undergoing outpatient adenotonsillectomy was studied. A nonlinear mixed effects model was developed in NONMEM to describe morphine and morphine glucuronide pharmacokinetics.

RESULTS

Children with ABCC3 -211C>T polymorphism C/C genotype had significantly higher levels of morphine-6-glucuronide and morphine-3-glucuronide formation (∼40%) than C/T+T/T genotypes (p < 0.05). In this extended cohort similar to our earlier report, OCT1 homozygous genotypes (n = 13, OCT1*2-*5/*2-*5) had lower morphine clearance (14%; p = 0.06), and in addition complementing lower metabolite formation (∼39%) was observed. ABCB1 3435C>T TT genotype children had lower levels of morphine-3-glucuronide formation though no effect was observed on morphine and morphine-6-glucuronide pharmacokinetics.

CONCLUSION

Our data suggest that besides bodyweight, OCT1 and ABCC3 genotypes play a significant role in the pharmacokinetics of intravenous morphine and its metabolites in children.

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.

Cyclosporine-inhibitable blood-brain barrier drug transport influences clinical morphine pharmacodynamics

BACKGROUND

The blood-brain barrier is richly populated by active influx and efflux transporters influencing brain drug concentrations. Morphine, a drug with delayed clinical onset, is a substrate for the efflux transporter P-glycoprotein in vitro and in animals. This investigation tested whether morphine is a transporter substrate in humans.

METHODS

Fourteen healthy volunteers received morphine (0.1 mg/kg, 1-h IV infusion) in a crossover study without (control) or with the infusion of validated P-glycoprotein inhibitor cyclosporine (5 mg/kg, 2-h infusion). Plasma and urine morphine and morphine glucuronide metabolite concentrations were measured by mass spectrometry. Morphine effects were measured by miosis and analgesia.

RESULTS

Cyclosporine minimally altered morphine disposition, increasing the area under the plasma morphine concentration versus time curve to 100 ± 21 versus 85 ± 24 ng/ml·h (P < 0.05) without changing maximum plasma concentration. Cyclosporine enhanced (3.2 ± 0.9 vs. 2.5 ± 1.0 mm peak) and prolonged miosis, and increased the area under the miosis-time curve (18 ± 9 vs. 11 ± 5 mm·h), plasma effect-site transfer rate constant (k(e0), median 0.27 vs. 0.17 h(-1)), and maximum calculated effect-site morphine concentration (11.5 ± 3.7 vs. 7.6 ± 2.9 ng/ml; all P < 0.05). Analgesia testing was confounded by cyclosporine-related pain.

CONCLUSIONS

Morphine is a transporter substrate at the human blood-brain barrier. Results suggest a role for P-glycoprotein or other efflux transporters in brain morphine access, although the magnitude of the effect is small, and unlikely to be a major determinant of morphine clinical effects. Efflux may explain some variability in clinical morphine effects.

The role of multidrug resistance-associated protein in the blood-brain barrier and opioid analgesia

The blood-brain barrier protects the brain from circulating compounds and drugs. The ATP-binding cassette (ABC) transporter P-glycoprotein (Pgp) is involved with the barrier, both preventing the influx of agent from the blood into the brain and facilitating the efflux of compounds from the brain into the blood, raising the possibility of a similar role for other transporters. Multidrug resistance-associated protein (MRP), a 190 kDa protein, similar to Pgp is also ABC transporter that has been implicated in the blood-brain barrier. The current study explores its role in opioid action. Immunohistochemically, it is localized in the choroid plexus in rats and can be selectively downregulated by antisense treatment at both the level of mRNA, as shown by RT-PCR, and protein, as demonstrated immunohistochemically. Behaviorally, downregulation of MRP significantly enhances the analgesic potency of systemic morphine in MRP knockout mice and in antisense-treated rats by lowering the blood-brain barrier. Following intracerebroventricular administration, a number of compounds, including some opioids, are rapidly secreted from the brain into the blood where they contribute to the overall analgesic effects by activating peripheral systems. MRP plays a role in this efflux. Downregulating MRP expression leads to a corresponding decrease in the transport and a diminished analgesic response from opioids administered intracerebroventricularly. Thus, the transporter protein MRP plays a role in maintaining the blood-brain barrier and modulates the activity of opioids.

Plasma morphine and metabolite concentrations are associated with clinical effects of morphine in cancer patients

CONTEXT

Morphine is the opioid of choice for cancer-related pain, but for many patients the benefits of morphine are outweighed by its side effect profile. Morphine is metabolized to morphine-3-glucuronide and morphine-6-glucuronide; however, little is known about the contribution of these metabolites to analgesia and morphine-related side effects.

OBJECTIVES

We investigated the association between plasma morphine and metabolite concentrations and the clinical effects of morphine in cancer patients.

METHODS

A prospective study was performed in cancer patients taking oral morphine for moderate-to-severe cancer pain. Subjects who responded well to morphine (responders) and subjects who failed to respond to morphine because of lack of analgesia and/or the presence of intolerable side effects (nonresponders/switchers) were recruited. Pain and toxicity scores were recorded and blood samples were analyzed for plasma morphine, morphine-3-glucuronide, and morphine-6-glucuronide concentrations.

RESULTS

Results showed that 1) morphine responders have higher plasma morphine and metabolite concentrations compared with nonresponders, 2) lower pain scores are associated with higher plasma morphine and metabolite concentrations, 3) central side effects are associated with a higher metabolite:plasma morphine ratio, and 4) myoclonus is associated with extremely high concentrations of plasma morphine and metabolites.

CONCLUSION

This study has shown that plasma morphine and metabolite concentrations are associated with the clinical effects of morphine therapy. These results are important because they demonstrate the relevance of measuring plasma metabolite concentrations in clinical trials and the potential for metabolite data to deepen our understanding of factors that influence an individual's response 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.

Pilot study examining the frequency of several gene polymorphisms involved in morphine pharmacodynamics and pharmacokinetics in a morbidly obese population

Morbidly obese patients are at significantly elevated risk of postsurgery complications and merit closer monitoring by health care professionals after bariatric surgery. It is now recognized that genetic factors influence individual patient's response to drug used in anesthesia and analgesia. Among the many drug administered by anesthetists, we focused in this pilot study on morphine, since morphine patient-controlled anesthesia in obese patients undergoing gastric bypass surgery is frequently prescribed. We examined the allelic frequency of three polymorphisms involved in morphine pharmacodynamics and pharmacokinetics in patients with body mass index (BMI) >40. One hundred and nine morbidly obese patients (BMI = 49.1 ± 7.7 kg/m²) were genotyped for three polymorphisms c.A118G of mu opioid receptor (OPRM1), c.C3435T of the P-glycoprotein gene (ABCB1), and p.Val158Met of catechol-O-methyltransferase gene (COMT). Allelic frequencies were 118G-0.22, C3435-0.55, and 158Met-0.5 in our whole population and 0.23, 0.5, and 0.47 in Caucasian population. Allelic frequencies did not differ according to gender. Mean BMI did no differ according to the allelic variant. OPRM1118G allele was more frequent in our population than in most previously described European populations. Since the concept of "personalized medicine" promises to individualize therapeutics and optimize medical treatment in term of efficacy and safety, especially when prescribing drugs with a narrow therapeutic index such as morphine, further clinical studies examining the clinical consequences of the OPRM1 c.A118G polymorphism in patients undergoing gastric bypass surgery are needed.

Prediction of codeine toxicity in infants and their mothers using a novel combination of maternal genetic markers

Substantial variation exists in response to standard doses of codeine ranging from poor analgesia to life-threatening central nervous system (CNS) depression. We aimed to discover the genetic markers predictive of codeine toxicity by evaluating the associations between polymorphisms in cytochrome P450 2D6 (CYP2D6), UDP-glucuronosyltransferase 2B7 (UGT2B7), P-glycoprotein (ABCB1), mu-opioid receptor (OPRM1), and catechol O-methyltransferase (COMT) genes, which are involved in the codeine pathway, and the symptoms of CNS depression in 111 breastfeeding mothers using codeine and their infants. A genetic model combining the maternal risk genotypes in CYP2D6 and ABCB1 was significantly associated with the adverse outcomes in infants (odds ratio (OR) 2.68; 95% confidence interval (CI) 1.61-4.48; P(trend) = 0.0002) and their mothers (OR 2.74; 95% CI 1.55-4.84; P(trend) = 0.0005). A novel combination of the genetic and clinical factors predicted 87% of the infant and maternal CNS depression cases with a sensitivity of 80% and a specificity of 87%. Genetic markers can be used to improve the outcome of codeine therapy and are also probably important for other opioids sharing common biotransformation pathways.

Pharmacokinetics of PSC 833 (valspodar) in its Cremophor EL formulation in rat

Valspodar is a P-glycoprotein inhibitor widely used in preclinical and clinical studies for overcoming multidrug resistance. Despite this, the pharmacokinetics of valspodar in rat, a commonly used animal model, have not been reported. Here, we report on the pharmacokinetics of valspodar in Sprague-Dawley rats following intravenous and oral administration of its Cremophor EL formulation, which has been used for humans in clinical trials. After intravenous doses, valspodar displayed properties of slow clearance and a large volume of distribution. Its plasma unbound fraction was around 15% in the Cremophor EL formulation used in the study. After 10 mg kg(-1) orally it was rapidly absorbed with an average maximal plasma concentration of 1.48 mg l(-1) within approximately 2 h. The mean bioavailability of valspodar was 42.8%. In rat, valspodar showed properties of low hepatic extraction and wide distribution, similar to that of its structural analogue cyclosporine A.

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.

Pharmacology of morphine in obese patients: clinical implications

Morphine is an analgesic drug used to treat acute and chronic pain. Obesity is frequently associated with pain of various origins (e.g. arthritis, fibromyalgia, cancer), which increases the need for analgesic drugs. Obesity changes drug pharmacokinetics, and for certain drugs, specific modalities of prescription have been proposed for obese patients. However, scant data are available regarding the pharmacokinetics and pharmacodynamics of morphine in obesity. Prescription of morphine depends on pain relief but the occurrence of respiratory adverse effects correlates with obesity, and is not currently taken into account. Variations in the volume of distribution, elimination half-life and oral clearance of morphine, as well as recent advances in the respective roles of drug-metabolizing enzymes, catechol-O-methyltransferase and the mu opioid receptor in morphine pharmacokinetics and pharmacodynamics, may contribute to differences between obese and non-obese patients. In addition, drug-drug interactions may alter the disposition of morphine and its glucuronide metabolites, which may either increase the risk of adverse effects or reduce drug efficacy.

Utilizing nature as a source of new probes for opioid pharmacology

Background: Traditional and current opioid pharmacology is fundamentally based on interactions between opioid receptors and compounds isolated from natural sources. Adverse effects associated with opioids have led to the search for compounds with diminished side effects. Discussion: Recent discoveries of non-nitrogenous and structurally diverse alkaloids as novel opioid ligands have led to renewed interest in the development of novel chemotypes for opioid receptors. Conclusion: The strong history of natural products as opioid receptor ligands suggests that nature is one of the most promising for the identification of novel opioids. This review highlights the vast potential of investigating natural products as novel probes of opioid receptors.

Alterations of chemotherapeutic pharmacokinetic profiles by drug-drug interactions

BACKGROUND

Drug interactions in oncology are common place and largely ignored as we tolerate high thresholds of 'toxic' drug responses in these patients. However, in the era of 'targeted' or seemingly 'less toxic' therapy, these interactions are more commonly flagged and contribute significantly towards poor 'quality of life' and medical fatalities.

OBJECTIVE

This review and opinion article focuses on alteration of chemotherapeutic pharmacokinetic profiles by drug interactions in the setting of polypharmacy. The assumption is that the drugs, with changes in their pharmacokinetics, will contribute towards changes in their pharmacodynamics.

METHODS

The examples cited for such drug-drug interactions are culled from published literature with an emphasis on those interactions that have been well characterized at the molecular level.

RESULTS

Although very few drug interaction studies have been performed on approved oncology based drugs, it is clear that drugs whose pharmacokinetics profiles are closely related to their pharmacodynamics will indeed result in clinically important drug interactions. Some newer mechanisms are described that involve interactions at the level of gene transcription, whereby, drug metabolism is significantly altered. However, for any given drug interaction, there does not seem to be a comprehensive model describing interactions.

CONCLUSIONS

Mechanisms based drug interactions are plentiful in oncology; however, there is an absolute lack of a comprehensive model that would predict drug-drug interactions.

Itraconazole, a potent inhibitor of P-glycoprotein, moderately increases plasma concentrations of oral morphine

BACKGROUND

Individual variation in opioid response is considerable, partly due to pharmacokinetic factors. Transporter proteins are becoming increasingly interesting also in the pharmacokinetics of opioids. The efflux transporter P-glycoprotein can affect gastrointestinal absorption and tissue distribution, particularly brain access of many opioids. The aim of this study was to evaluate whether itraconazole, which is a potent inhibitor of P-glycoprotein and CYP3A4, would change the pharmacokinetics or the pharmacodynamics of oral morphine.

METHODS

Twelve healthy male volunteers ingested, in a randomized crossover study, once daily 200 mg itraconazole or placebo for 4 days. On day 4, 1 h after the last pre-treatment dose, the subjects ingested 0.3 mg/kg morphine. Blood samples for the determination of plasma morphine, morphine-3-glucuronide (M3G), morphine-6-glucuronide (M6G) and itraconazole concentrations were drawn up to 48 h after morphine ingestion. Pharmacodynamic effects were evaluated using a questionnaire, visual analogue scales, a reaction time test, the Digit Symbol Substitution Test and the Critical Flicker Fusion Test.

RESULTS

Itraconazole increased the mean area under the plasma concentration-time curve [AUC (0-9)] of morphine by 29% (P=0.002), its AUC (0-48) by 22% (P=0.013) and its peak plasma concentration by 28% (P=0.035). Itraconazole did not significantly affect the pharmacokinetic variables of M3G or M6G or the pharmacodynamic effects of morphine.

CONCLUSIONS

Itraconazole moderately increases plasma concentrations of oral morphine, probably by enhancing its absorption by inhibiting intestinal wall P-glycoprotein. A possible improvement of morphine penetration to the brain could not be observed.

Pharmacogenetic insights into codeine analgesia: implications to pediatric codeine use

Codeine has been used medicinally since the 1800s as an analgesic and antitussive agent. Although very few studies have methodically examined the safety of codeine use in the pediatric age group, it is nonetheless commonly prescribed to children and breastfeeding mothers. Empirical evidence over the last century has suggested variability in the efficacy of codeine, and recent genomic advancements have shed important light on the mechanisms leading to such variability. Aside from evaluating the role of genetic variability in drug-metabolizing enzymes, receptors and transporters, the development of the blood–brain-barrier and the ontogeny of drug-metabolizing enzymes must also be considered in newborns and young children.

Effect of grapefruit juice, naringin, naringenin, and bergamottin on the intestinal carrier-mediated transport of talinolol in rats

The effect of two varieties of grapefruit juice (white and ruby red) and its selected components (naringin, naringenin, and bergamottin) was investigated on the activity of the P-glycoprotein (P-gp) in male Sprague-Dawley rats. Talinolol, a nonmetabolized P-gp substrate, was used as a marker compound. The white grapefruit juice (GFJ) had a minor effect on talinolol pharmacokinetics, but the ruby red GFJ reduced the C max and the AUC (0-infinity) by 60% and 50% of the control, respectively. However, among the GFJ constituents tested, bergamottin (0.22 mg/kg) was the most potent component augmenting the C max and the AUC (0-infinity) of talinolol by 2.4- and 1.8-fold, respectively, if compared to the control group. The flavonoids naringenin (0.7 mg/kg) and naringin (2.4 and 9.4 mg/kg) had a similar effect increasing the talinolol C max and AUC (0-infinity) by 1.5- to 1.8-fold, respectively. In conclusion, the effect of GFJ on P-gp activity seems to depend on the variety, the concentration of compounds in the juice, and the composition of different ingredients.

Opioids and efflux transporters. Part 2: P-glycoprotein substrate activity of 3- and 6-substituted morphine analogs

Continuing our studies investigating opioids with reduced P-glycoprotein (P-gp) substrate activity, a series of known 3- and 6-hydroxy, -methoxy, and -desoxymorphine analogs was synthesized and analyzed for P-gp substrate activity and opioid binding affinity. 6-Desoxymorphine ( 7) showed high affinity for opioid receptors and did not induce P-gp-mediated ATP hydrolysis. Additionally, 7 demonstrated morphine-like antinociceptive potency in mice, indicating this compound as an ideal lead to further evaluate the role of P-gp in opioid analgesic tolerance development.

Pharmacogenetics of Opioids

Opioids are used for acute and chronic pain and dependency. They have a narrow therapeutic index and large interpatient variability in response. Genetic factors regulating their pharmacokinetics (metabolizing enzymes, transporters) and pharmacodynamics (receptors and signal transduction elements) are contributors to such variability. The polymorphic CYP2D6 regulates the O-demethylation of codeine and other weak opioids to more potent metabolites with poor metabolizers having reduced antinociception in some cases. Some opioids are P-glycoprotein substrates, whereas, ABCB1 genotypes inconsistently influence opioid pharmacodynamics and dosage requirements. Single-nucleotide polymorphisms in the mu opioid receptor gene are associated with increasing morphine, but not methadone dosage requirements and altered efficacy of mu opioid agonists and antagonists. As knowledge regarding the interplay between genes affecting opioid pharmacokinetics including cerebral kinetics and pharmacodynamics increases, our understanding of the role of pharmacogenomics in mediating interpatient variability in efficacy and side effects to this important class of drugs will be better informed. Opioid drugs as a group have withstood the test of time in their ability to attenuate acute and chronic pain. Since the isolation of morphine in the early 1800s by Friedrich Sertürner, a large number of opioid drugs beginning with modification of the 4,5-epoxymorphinan ring structure were developed in order to improve their therapeutic margin, including reducing dependence and tolerance, ultimately without success.

Genetic variants of the P-glycoprotein gene Abcb1b modulate opioid-induced hyperalgesia, tolerance and dependence

OBJECTIVE

Opioid-induced hyperalgesia (OIH) is a state of paradoxically increased nociceptive sensitivity seen in both humans and rodents following the resolution of the acute opioid antinociceptive effects or during periods of chronic opioid administration. Using the power of genetic analysis, we hoped to discover novel mechanisms modulating this trait.

BASIC METHODS

The degree of opioid-induced hyperalgesia displayed in response to a thermal stimulus applied to the hind paw was measured in 16 strains of inbred mice after 4 days of morphine administration. The degree of thermal sensitization was then used in a recently developed in silico haplotypic mapping algorithm along with a haplotypic map constructed from a database containing 209,000 single nucleotide polymorphisms.

MAIN RESULTS

Analysis of the data resulted in the identification of several haplotype blocks strongly associated with the thermal opioid-induced hyperalgesia trait. The most strongly associated block was located within the Abcb1b P-glycoprotein drug transporter gene. Experiments using the P-glycoprotein inhibitor cyclosporine A and P-glycoprotein null mutant mice supported the hypothesis that a functional association exists between P-glycoprotein transporters and opioid-induced hyperalgesia. The observation of a correlation between morphine brain concentrations and the development of opioid-induced hyperalgesia was consistent with this hypothesis as well. In addition, P-glycoprotein gene deletion and pharmacological inhibition altered morphine ED50, tolerance and physical dependence.

CONCLUSIONS

We conclude that the use of haplotypic mapping to identify novel mechanisms controlling complex traits is a viable approach. Variants of the Abcb1b gene may explain some portion of the interstrain differences in OIH and perhaps other consequences of chronic opioid administration.

Individualizing analgesic prescription Part I: pharmacogenetics of opioid analgesics

The current use of analgesics is based on the empiric administration of a given drug with clinical monitoring for efficacy and toxicity. However, individual responses to drugs are influenced by a combination of pharmacokinetic and pharmacodynamic processes, and each of these components, in addition to pain perception and processing, seem to be regulated by genetic factors. Whereas polymorphic drug-metabolizing enzymes and drug transporters may affect the pharmacokinetics of drugs, polymorphic drug targets and disease-related pathways may influence the pharmacodynamic action of drugs. After usual dose, drug toxicity, as well as inefficacy, can be observed depending on the polymorphism, the analgesic considered and the presence or absence of active metabolites. Thus, cytochrome P450 (CYP)2D6 polymorphism influences codeine and tramadol analgesic effects, CYP2C9 has an impact on the disposition of some nonsteroidal anti-inflammatory drugs, and opioid receptor polymorphism (118A>G) may reduce morphine potency. Moreover, drug interaction mimics genetic deficiency and contributes to the variability in response to analgesics. This two-part review summarizes the available data on the pharmacokinetic-pharmacodynamic consequences of known polymorphisms of drug-metabolizing enzymes (CYP and uridine diphosphate glucuronosyltransferase), drug transporters (multidrug resistance proteins, multidrug resistance-associated proteins, organic anion-transporting polypeptides, and serotonin transporters), relevant drug targets (such as µ-opioid receptor, serotonin receptor and cyclooxygenases) and other nonopioid biological systems, on currently prescribed central and peripheral analgesics.

Transport is not rate-limiting in morphine glucuronidation in the single-pass perfused rat liver preparation

Binding, transport, and metabolism are factors that influence morphine (M) removal in the rat liver. For M and the morphine 3beta-glucuronide metabolite (M3G), modest binding existed with 4% bovine serum albumin (unbound fractions of 0.89 +/- 0.07 and 0.98 +/- 0.09, respectively), and there was partitioning of M into red blood cells. Transport studies of M (<750 microM) showed similar, concentration-independent uptake clearances (CLs) of 1.5 ml min(-1) g(-1) among zonal and homogeneous, isolated rat hepatocytes. Transport of M3G, ascertained in multiple indicator dilution studies at various steady-state M3G concentrations (10-262 microM), uncovered a low and concentration-independent influx clearance (<10% of flow rate). The outflow dilution curve of [(3)H]M3G was superimposable onto that of [(14)C]sucrose, the extracellular reference, displaying similarity in transit times (23.5 and 22.2 s), negligible biliary excretion, and almost complete dose recovery from perfusate. In contrast, M3G occurred abundantly in both perfusate and bile in single-pass perfusion studies of the precursor, M, and revealed a biliary clearance of formed M3G that was 12.3-fold that of preformed M3G, suggesting a sinusoidal, diffusional barrier for M3G. With increasing concentrations of M (9-474 microM), clearance decreased, and metabolism and biliary excretion displayed concentration-dependent kinetics. Fitting of the data to a physiologically based liver model revealed that M removal mechanisms were saturable, with a K(m,met) of 52.2 microM and V(max,met) of 58.8 nmol min(-1) g(-1) for metabolism, and a K(m,ex) of 41.2 microM and V(max,ex) of 8.1 nmol min(-1) g(-1) for excretion. Sinusoidal transport was not rate-limiting for M removal.

Cyclosporine A (CsA) affects the pharmacodynamics and pharmacokinetics of the atypical antipsychotic amisulpride probably via inhibition of P-glycoprotein (P-gp)

The importance of P-glycoprotein (P-gp) in the pharmacokinetics of amisulpride and the effects of a P-gp inhibitor cyclosporine A (CsA) was investigated both, in vitro and in vivo. In vitro and in vivo results indicated amisulpride as a substrate of P-gp. Amisulpride was not metabolized by rat liver microsomes. Open field behavior showed time dependent abolishment in locomotion by amisulpride (50 mg kg(-1)). Co-administration of CsA (50 mg kg(-1)) resulted in a higher and significantly longer antipsychotic effect (24 h after drug administration). Accordingly, the area under concentration-time curve in serum and brain was higher in CsA co-treated rats (13.5 vs. 29.8 micromol h l(-1) for serum and 2.16 vs 2.98 micromol h l(-1) for brain tissue) while renal clearance was not affected. These results pointed to a pharmacokinetic drug interaction between CsA and amisulpride most likely caused by inhibition of P-gp.

Polymorphisme génétique et interactions médicamenteuses : leur importance dans le traitement de la douleur

OBJECTIVES

To evaluate the impact of certain genetic polymorphisms on variable responses to analgesics

SOURCES

Systematic review, by means of a structured computerized search in the Medline database (1966-2004). Articles in English and French were selected. References in relevant articles were also retrieved.

MAIN FINDINGS

Most analgesics are metabolized by CYP isoenzymes subject to genetic polymorphism. NSAIDs are metabolized by CYP2C9; opioids described as "weak" (codeine, tramadol), anti-depressants and dextromethorphan are metabolized by CYP2D6 and some "potent" opioids (buprenorphine, methadone or fentanyl) by CYP3A4/5. After the usual doses have been administered, drug toxicity or, on the contrary, therapeutic ineffectiveness may occur, depending on polymorphism and the substance. Drug interactions mimicking genetic defects because of the existence of CYP inhibitors and inducers, also contribute to the variable response to analgesics. Some opioids are substrates of P-gp, a transmembrane transporter also subject to genetic polymorphism. However, P-gp could only play a minor modulating role in man on the central effects of morphine, methadone and fentanyl.

CONCLUSION

In the near future, pharmacogenetics should enable us to optimize therapeutics by individualizing our approach to analgesic drugs and making numerous analgesics safer and more effective. The clinical usefulness of these individualized approaches will have to be demonstrated by appropriate pharmacoeconomic studies and analyses.

Genetic variability and clinical efficacy of morphine

The individual variability of opioid pharmacology suggests that the patients' genetic disposition influences the response to opioids. Given the complexity of morphine pharmacology, variability may be caused by several genes. We review data which shows that variability in genes coding the enzyme metabolizing morphine (UGT2B7 gene), mu-opioid receptors (OPRM gene) and blood-brain barrier (BBB) transport of morphine by multidrug resistance transporters (MDR1 gene) influences the clinical efficacy of morphine. Furthermore, variability in an enzyme degrading catecholamines (COMT gene) alters the efficacy of morphine demonstrating that genetic variability in non-opioid systems may indirectly influence the clinical efficacy from morphine. Thus, results obtained so far strongly argue that opioid efficacy is partly related to inborn properties caused by genetic variability.

A Fluorometric Screening Assay for Drug Efflux Transporter Activity in the Blood-Brain Barrier

PURPOSE

To examine the capability of a fluorometric assay to identify and characterize the drug efflux interactions of a broad spectrum of drug agents in an in vitro model of the blood-brain barrier (BBB).

METHODS

Various concentrations of drug agent (1, 10, and 100 microM) were evaluated for their effect on the cellular accumulation of the P-glycoprotein (P-gp) probe R123 (3.2 microM), and the mixed P-gp and multidrug resistance-associated protein (MRP) probe, BCECF (1 microM), in bovine brain microvessel endothelial cell (BBMEC) monolayers. Drugs demonstrating a significant effect were further quantitated using an expanded concentration range and a nonlinear regression curve fit to determine the potency (IC50) and efficacy (Imax) of the drug for P-gp and/or MRP.

RESULTS

Several of the 36 therapeutic agents examined showed drug efflux transporter interactions in BBMEC monlayers. Melphalan and risperidone significantly enhanced the accumulation of R123 over control (1.47- and 1.82-fold, respectively) with resulting IC50s of 1.4 and 14.6 microM, respectively. Chlorambucil and valproic acid significantly enhanced the accumulation of BCECF compared to control monolayers (2.02- and 4.01-fold, respectively) with resulting IC50s of 146.1 and 768.5 microM, respectively.

CONCLUSIONS

The current study demonstrates the feasibility of a fluorometric assay consisting of R123 and BCECF in assessing the drug efflux interactions of a variety of drugs in the BBB.

The impact of P-glycoprotein on the disposition of drugs targeted for indications of the central nervous system: evaluation using the MDR1A/1B knockout mouse model

Thirty-two structurally diverse drugs used for the treatment of various conditions of the central nervous system (CNS), along with two active metabolites, and eight non-CNS drugs were measured in brain, plasma, and cerebrospinal fluid in the P-glycoprotein (P-gp) knockout mouse model after subcutaneous administration, and the data were compared with corresponding data obtained in wild-type mice. Total brain-to-plasma (B/P) ratios for the CNS agents ranged from 0.060 to 24. Of the 34 CNS-active agents, only 7 demonstrated B/P area under the plasma concentration curve ratios between P-gp knockout and wild-type mice that did not differ significantly from unity. Most of the remaining drugs demonstrated 1.1- to 2.6-fold greater B/P ratios in P-gp knockout mice versus wild-type mice. Three, risperidone, its active metabolite 9-hydroxyrisperidone, and metoclopramide, showed marked differences in B/P ratios between knockout and wild-type mice (6.6- to 17-fold). Differences in B/P ratios and cerebrospinal fluid/plasma ratios between wild-type and knockout animals were correlated. Through the use of this model, it appears that most CNS-active agents demonstrate at least some P-gp-mediated transport that can affect brain concentrations. However, the impact for the majority of agents is probably minor. The example of risperidone illustrates that even good P-gp substrates can still be clinically useful CNS-active agents. However, for such agents, unbound plasma concentrations may need to be greater than values projected using receptor affinity data to achieve adequate receptor occupancy for effect.

Genetic Predictors of the Clinical Response to Opioid Analgesics

This review uses a candidate gene approach to identify possible pharmacogenetic modulators of opioid therapy, and discusses these modulators together with demonstrated genetic causes for the variability in clinical effects of opioids. Genetically caused inactivity of cytochrome P450 (CYP) 2D6 renders codeine ineffective (lack of morphine formation), slightly decreases the efficacy of tramadol (lack of formation of the active O-desmethyl-tramadol) and slightly decreases the clearance of methadone. MDR1 mutations often demonstrate pharmacogenetic consequences, and since opioids are among the P-glycoprotein substrates, opioid pharmacology may be affected by MDR1 mutations. The single nucleotide polymorphism A118G of the mu opioid receptor gene has been associated with decreased potency of morphine and morphine-6-glucuronide, and with decreased analgesic effects and higher alfentanil dose demands in carriers of the mutated G118 allele. Genetic causes may also trigger or modify drug interactions, which in turn can alter the clinical response to opioid therapy. For example, by inhibiting CYP2D6, paroxetine increases the steady-state plasma concentrations of (R)-methadone in extensive but not in poor metabolisers of debrisoquine/sparteine. So far, the clinical consequences of the pharmacogenetics of opioids are limited to codeine, which should not be administered to poor metabolisers of debrisoquine/sparteine. Genetically precipitated drug interactions might render a standard opioid dose toxic and should, therefore, be taken into consideration. Mutations affecting opioid receptors and pain perception/processing are of interest for the study of opioid actions, but with modern practice of on-demand administration of opioids their utility may be limited to explaining why some patients need higher opioid doses; however, the adverse effects profile may be modified by these mutations. Nonetheless, at a limited level, pharmacogenetics can be expected to facilitate individualised opioid therapy.

Peroxisome proliferator-activated receptor alpha induces hepatic expression of the human bile acid glucuronidating UDP-glucuronosyltransferase 2B4 enzyme

Glucuronidation, a major metabolic pathway for a large variety of endobiotics and xenobiotics, is catalyzed by enzymes belonging to the UDP-glucuronosyltransferase (UGT) family. Among UGT enzymes, UGT2B4 conjugates a large variety of endogenous and exogenous molecules and is considered to be the major bile acid conjugating UGT enzyme in human liver. In the present study, we identify UGT2B4 as a novel target gene of the nuclear receptor peroxisome proliferator-activated receptor alpha (PPAR alpha), which mediates the hypolipidemic action of fibrates. Incubation of human hepatocytes or hepatoblastoma HepG2 and Huh7 cells with synthetic PPAR alpha agonists, fenofibric acid, or Wy 14643 resulted in an increase of UGT2B4 mRNA levels. Furthermore, treatment of HepG2 cells with Wy 14643 induced the glucuronidation of hyodeoxycholic acid, a specific bile acid UGT2B4 substrate. Analysis of UGT2B mRNA and protein levels in PPAR alpha wild type and null mice revealed that PPAR alpha regulates both basal and fibrate-induced expression of these enzymes in rodents also. Finally, a PPAR response element was identified in the UGT2B4 promoter by site-directed mutagenesis and electromobility shift assays. These results demonstrate that PPAR alpha agonists may control the catabolism of cytotoxic bile acids and reinforce recent data indicating that PPAR alpha, which has been largely implicated in the control of lipid and cholesterol metabolism, is also an important modulator of the metabolism of endobiotics and xenobiotics in human hepatocytes.

The conduct of in vitro and in vivo drug-drug interaction studies: a Pharmaceutical Research and Manufacturers of America (PhRMA) perspective

Current regulatory guidances do not address specific study designs for in vitro and in vivo drug-drug interaction studies. There is a common desire by regulatory authorities and by industry sponsors to harmonize approaches, to allow for a better assessment of the significance of findings across different studies and drugs. There is also a growing consensus for the standardization of cytochrome P450 (P450) probe substrates, inhibitors and inducers and for the development of classification systems to improve the communication of risk to health care providers and to patients. While existing guidances cover mainly P450-mediated drug interactions, the importance of other mechanisms, such as transporters, has been recognized more recently, and should also be addressed. This article was prepared by the Pharmaceutical Research and Manufacturers of America (PhRMA) Drug Metabolism and Clinical Pharmacology Technical Working Groups and represents the current industry position. The intent is to define a minimal best practice for in vitro and in vivo pharmacokinetic drug-drug interaction studies targeted to development (not discovery support) and to define a data package that can be expected by regulatory agencies in compound registration dossiers.

abcb1ab P-glycoprotein is involved in the uptake of citalopram and trimipramine into the brain of mice

The phenomenon of a heterogeneous response to the same drug in different patients is well-known. An important reason is that, even at equal concentrations, the bioavailability of a drug depends on the interaction of the drug with the blood-brain barrier (BBB). In part, this is due to the drug-transporting P-glycoprotein (P-gp), a product of the multiple drug resistance gene (ABCB1), which can transport drugs against a concentration gradient across the BBB back into the plasma and thereby reduce the bioavailability in the brain. In the present study, we have examined the uptake of the antidepressants citalopram and trimipramine into the brain of abcb1ab knockout mice compared with controls. One hour after s.c. injection of the drugs, concentrations of the two drugs and of the metabolite d-trimipramine in brain, spleen, kidney, liver and plasma were measured with HPLC. Significantly higher brain concentrations in knockout mice, showing that these drugs are substrates of P-gp and that the presence of P-gp reduces the effective bioavailability of these substances in the brain. The results of our study contradict an earlier report that citalopram is not actively transported from endothelial cells. These results were derived from an in vitro study, showing that due to the complexity of the BBB-drug interaction, it is difficult to transfer results from in vitro studies to the in vivo situation. We hypothesize that inter-individual differences in the activity of the ABCB1 gene can account in part for the great variation in clinical response to antidepressants in psychiatric patients, even at comparable plasma levels.

Pharmacokinetic drug interactions of morphine, codeine, and their derivatives: theory and clinical reality, part I

Pharmacokinetic drug-drug interactions with morphine, hydromorphone, and oxymorphone are reviewed in this column. Morphine is a naturally occurring opiate that is metabolized chiefly through glucuronidation by uridine diphosphate glucuronosyl transferase (UGT) enzymes in the liver. These enzymes produce an active analgesic metabolite and a potentially toxic metabolite. In vivo drug-drug interaction studies with morphine are few, but they do suggest that inhibition or induction of UGT enzymes could alter morphine and its metabolite levels. These interactions could change analgesic efficacy. Hydromorphone and oxymorphone, close synthetic derivatives of morphine, are also metabolized primarily by UGT enzymes. Hydromorphone may have a toxic metabolite similar to morphine. In vivo drug-drug interaction studies with hydromorphone and oxymorphone have not been done, so it is difficult to make conclusions with these drugs.

Phenytoin pharmacokinetics and clinical effects in African children following fosphenytoin and chloramphenicol coadministration

AIMS

Some children with malaria and convulsions also have concurrent bacterial meningitis. Chloramphenicol is used to treat the latter whereas phenytoin is used for convulsions. Since chloramphenicol inhibits the metabolism of phenytoin in vivo, we studied the effects of chloramphenicol on phenytoin pharmacokinetics in children with malaria.

METHODS

Multiple intravenous (i.v.) doses of chloramphenicol succinate (CAP) (25 mg kg-1 6 hourly for 72 h) and a single intramuscular (i.m.) seizure prophylactic dose of fosphenytoin (18 mg kg-1 phenytoin sodium equivalents) were concomitantly administered to 15 African children with malaria. Control children (n = 13) with malaria received a similar dose of fosphenytoin and multiple i.v. doses (25 mg kg-1 8 hourly for 72 h) of cefotaxime (CEF). Blood pressure, heart rate, respiratory rate, oxygen saturation, level of consciousness and convulsion episodes were monitored. Cerebrospinal fluid (CSF) and plasma phenytoin concentrations were determined.

RESULTS

The area under the plasma unbound phenytoin concentration-time curve (AUC(0, infinity ); means (CAP, CEF): 58.5, 47.6 micro g ml-1 h; 95% CI for difference between means: -35.0, 11.4), the peak unbound phenytoin concentrations (Cmax; medians: 1.12, 1.29 micro g ml-1; 95% CI: -0.5, 0.04), the times to Cmax (tmax; medians: 4.0, 4.0 h; 95% CI: -2.0, 3.7), the CSF:plasma phenytoin ratios (means: 0.21, 0.22; 95% CI: -0.8, 0.10), the fraction of phenytoin unbound (means: 0.06, 0.09; 95% CI: -0.01, 0.07) and the cardiovascular parameters were not significantly different between CAP and CEF groups. However, mean terminal elimination half-life (t1/2,z) was significantly longer (23.7, 15.5 h; 95% CI: 1.71, 14.98) in the CAP group compared with the CEF group. Seventy per cent of the children had no convulsions during the study period.

CONCLUSIONS

Concomitant administration of chloramphenicol and a single i.m. dose of fosphenytoin alters the t1/2,z but not the other pharmacokinetic parameters or clinical effects of phenytoin in African children with severe malaria. Moreover, a single i.m. dose of fosphenytoin provides anticonvulsant prophylaxis in the majority of the children over 72 h. However, a larger study would be needed to investigate the effect of concomitant administration of multiple doses of the two drugs in this population of patients.

Increased CNS uptake and enhanced antinociception of morphine-6-glucuronide in rats after inhibition of P-glycoprotein

Morphine-6-glucuronide (M6G) is a substrate of P-glycoprotein (P-gp), which forms an outward transporter at the blood-brain barrier. Inhibition of P-gp may therefore be expected to cause increased CNS uptake of M6G. We directly assessed the spinal concentrations of M6G and its antinociceptive effects in rats following pharmacological inhibition of P-gp. Spinal cord tissue concentrations of M6G were assessed by microdialysis with probes transversally implanted through the dorsal horns of the spinal cord at level L4. Ten rats received M6G intravenously (0.018 mg/kg loading dose plus 0.00115 mg/kg/min for an 8-h infusion), five of them together with PSC833 to inhibit P-gp (32-h infusion, starting 24 h before the addition of M6G). Antinociceptive effects were explored by means of formalin tests. After having obtained evidence for enhanced CNS uptake and antinociception of M6G in the presence of PSC833, additional behavioural experiments were performed in another 32 rats to assess the dose dependency of the antinociceptive effects of M6G either with or without PSC833 in comparison with both PSC833 alone and placebo. Inhibition of P-gp increased the M6G concentrations in the spinal cord approximately three-fold whereas the plasma concentrations were increased only by a factor of 1.4, which resulted in a more than doubled spinal cord/plasma concentration ratio (from 0.08 +/- 0.03 for M6G alone to 0.17 +/- 0.08 for M6G plus PSC833). Antinociceptive effects of M6G were significantly enhanced by inhibition of P-gp. Inhibition of P-gp alters the transport of M6G across the blood-brain barrier, resulting in enhanced spinal cord uptake and enhanced antinociception.

Drug interactions with patient-controlled analgesia

Patient-controlled analgesia (PCA) has become standard procedure in the clinical treatment of pain. Its widespread use in patients with all kinds of diseases opens a variety of possible interactions between analgesics used for PCA and other drugs that might be administered concomitantly to the patient. Many of these drug interactions are of little clinical importance. However, some drug interactions have been reported to result in serious clinical problems. Drug interactions can either predominantly affect the pharmacokinetics or pharmacodynamics of the drug. Most important pharmacokinetic drug interactions occur at the level of drug metabolism or protein binding. Acceleration of methadone metabolism caused by cytochrome P450 (CYP) 3A4 induction by antiretroviral drugs or rifampicin (rifampin) has caused methadone withdrawal symptoms. Lack of morphine formation from codeine as a result of CYP2D6 inhibition by quinidine results in an almost complete loss of the analgesic effects of codeine. Alterations of methadone protein binding caused by an inhibition of alpha1-acid glycoprotein synthesis by alkylating substances are another possibility for predominantly pharmacokinetically based drug interactions during PCA. Furthermore, inhibition of P-glycoprotein by anticancer drugs could result in altered transmembrane transport of morphine, methadone or fentanyl, although this has not been shown to be of clinical relevance. Synergistic effects of systemically administered opioids with spinally or topically delivered opioids or anaesthetics have been reported frequently. The same is true for the opioid-sparing effects of coadministered non-opioid analgesics. Antidepressants, anticonvulsants or alpha2-adrenoreceptor agonists have also been shown to exert additive analgesic effects when administered together with an opioid. Inconsistent findings, however, are reported regarding the treatment of patients with opioid-induced nausea and sedation, since coadministration of antiemetics either increased or decreased the respective adverse effects or revealed additional unwanted drug effects.

Morphine-6-glucuronide: an analgesic of the future?

Morphine-6-beta-glucuronide (M6G) is an opioid agonist that plays a role in the clinical effects of morphine. Although M6G probably crosses the blood-brain barrier with difficulty, during long term morphine administration it may reach sufficiently high CNS concentrations to exert clinically relevant opioid effects. As a consequence of its almost exclusive renal elimination, M6G may accumulate in the body of patients with impaired renal function and cause severe opioid adverse effects with insidious onset and long persistence. Its profile of receptor affinities, however, gives reason to speculate that M6G may exhibit analgesic effects while causing fewer adverse effects than morphine. This is supported by reports of the good tolerability of intrathecal and intravenous injections of M6G in humans with intact renal function. M6G may thus be contemplated as an analgesic for short term postoperative analgesia, especially for intrathecal analgesic therapy. In addition, its possibly higher potency than morphine makes M6G a candidate opioid for local or peripheral analgesic therapy. However, current knowledge is too incomplete to finally judge the clinical usefulness of M6G. The next topics for clinical research on M6G should include: (i) a comparison of the potencies of M6G and morphine to cause wanted and unwanted clinical effects; (ii) development of a predictive population pharmacokinetic-pharmacodynamic model of M6G with calculation of the transfer half-life between plasma and effect site; and (iii) identification of cofactors influencing the action of M6G that can serve as predictors for the clinical outcome of morphine/M6G therapy in an individual including the pharmacogenetics of M6G.

Recent advances in opioid pharmacology

Despite their many and sometimes life-threatening side-effects, opioids in general and morphine in particular are valuable and potent painkillers. This article describes recent developments in sex-related differences in opioid (morphine) pharmacodynamics, morphine metabolites, the nociceptin/orphanin FQ receptor system, acute opioid tolerance and opioid-induced side-effects, such as opioid-induced respiratory depression and itch, and P-glycoprotein modulation of opioid effect.