Morphine antinociception is enhanced in mdr1a gene-deficient mice

Essential role of P-glycoprotein in the mechanism of action of oliceridine

Mu opioid receptor (MOR) agonists comprise the most effective analgesics, but their therapeutic utility is limited by adverse effects. One approach for limiting such effects has been to develop "biased" MOR agonists that show preference for activating G protein over β-Arrestin signaling. However, the notion of biased agonism has been challenged by recent studies. Oliceridine (Olinvyk®, TRV-130, OLC) is a selective MOR agonist approved by the FDA in 2020 for pain management in controlled clinical settings. Oliceridine purportedly demonstrates diminished adverse effects compared to morphine or other MOR agonists, a profile attributed to its biased agonism. However, recent studies suggest that oliceridine does not display biased agonism but instead weak intrinsic efficacy for G protein and β-Arrestin activation. Nevertheless, these insights have been derived from in vitro studies. To better understand oliceridine's in vivo efficacy profile, we performed a comprehensive assessment of its in vitro and in vivo pharmacology using both cultured cells and rodents. In vitro, oliceridine displayed high MOR affinity and weak intrinsic efficacy. In vivo, oliceridine showed impaired brain penetrance and rapid clearance, effects we attributed to its interaction with the P-glycoprotein (P-gp) efflux transporter. Moreover, we found that P-gp was essential for oliceridine's in vivo efficacy and adverse effect profiles. Taken together with prior studies, our results suggest that oliceridine's in vivo efficacy and adverse effect profiles are not attributed solely to its weak intrinsic efficacy or biased agonism but, to a large extent, its interaction with P-gp as well.

Morphine Accumulates in the Retina Following Chronic Systemic Administration

Opioid transport into the central nervous system is crucial for the analgesic efficacy of opioid drugs. Thus, the pharmacokinetics of opioid analgesics such as morphine have been extensively studied in systemic circulation and the brain. While opioid metabolites are routinely detected in the vitreous fluid of the eye during postmortem toxicological analyses, the pharmacokinetics of morphine within the retina of the eye remains largely unexplored. In this study, we measured morphine in mouse retina following systemic exposure. We showed that morphine deposits and persists in the retina long after levels have dropped in the serum. Moreover, we found that morphine concentrations (ng/mg tissue) in the retina exceeded brain morphine concentrations at all time points tested. Perhaps most intriguingly, these data indicate that following chronic systemic exposure, morphine accumulates in the retina, but not in the brain or serum. These results suggest that morphine can accumulate in the retina following chronic use, which could contribute to the deleterious effects of chronic opioid use on both image-forming and non-image-forming visual functions.

Morphine promotes migration and lung metastasis of mouse melanoma cells

BACKGROUND

Morphine is an analgesic agent used for cancer pain management. There have been recent concerns that the immunosuppressant properties of morphine can also promote cancer metastasis. Morphine is an agonist for toll like receptor 4 (TLR4) that has a dual role in cancer development. The promotor or inhibitor role of morphine in cancer progression remains controversial. We investigated the effects of morphine on migration and metastasis of melanoma cells through TLR4 activation.

METHODS

Mouse melanoma cells (B16F10) were treated with only morphine (0, 0.1, 1, and 10 μM) or in combination with a TLR4 inhibitor (morphine10 μM +CLI-095 1μM) for either 12 or 24 hours. Migration of cells was analyzed by transwell migration assays. Twenty C57BL/6 male mice were inoculated with B16F10 cells via the left ventricle of the heart and then randomly divided into two groups (n = 10 each) that received either morphine (10 mg.kg^-1, sub-q) or PBS injection for 21 days (control group). Animals were euthanized and their lungs removed for evaluation of metastatic nodules.

RESULTS

Morphine (0.1, 1, and 10 μM) increased cell migration after 12 hours (p < 0.001) and after 24 hours of treatment with morphine (10 μM) (p < 0.001). Treatment with CLI-095 suppressed migration compared to cells treated with morphine alone (p < 0.001). Metastatic nodules in the morphine-treated group (64 nodules) were significantly higher than in the control group (40 nodules) (p < 0.05).

CONCLUSION

Morphine increases the migration and metastasis of mouse melanoma cells by activating TLR4.

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.

Impact of P-Glycoprotein-Mediated Active Efflux on Drug Distribution into Lumbar Cerebrospinal Fluid in Nonhuman Primates

Estimation of unbound drug concentration in the brain (C_u,brain) is an essential part of central nervous system (CNS) drug development. As a surrogate for C_u,brain in humans and nonhuman primates, drug concentration in cerebrospinal fluid (C_CSF) collected by lumbar puncture is often used; however, the predictability of C_u,brain by lumbar C_CSF is unclear, particularly for substrates of the active efflux transporter P-glycoprotein (P-gp). Here, we measured lumbar C_CSF in cynomolgus monkey after single intravenous administration of 10 test compounds with varying P-gp transport activities. The in vivo lumbar cerebrospinal fluid (CSF)-to-plasma unbound drug concentration ratios (K_{p,uu,lumbar CSF}) of nonsubstrates or weak substrates of P-gp were in the range 0.885-1.34, whereas those of good substrates of P-gp were in the range 0.195-0.458 and were strongly negatively correlated with in vitro P-gp transport activity. Moreover, concomitant treatment with a P-gp inhibitor, zosuquidar, increased the K_{p,uu,lumbar CSF} values of the good P-gp substrates, indicating that P-gp-mediated active efflux contributed to the low K_{p,uu,lumbar CSF} values of these compounds. Compared with the drug concentrations in the cisternal CSF and interstitial fluid (ISF) that we previously determined in cynomolgus monkeys, the lumbar C_CSF were more than triple for two and all of the good P-gp substrates examined, respectively. Although lumbar C_CSF may overestimate cisternal CSF and ISF concentrations of good P-gp substrates, lumbar C_CSF allowed discrimination of good P-gp substrates from the weak and nonsubstrates and can be used to estimate the impact of P-gp-mediated active efflux on drug CNS penetration. SIGNIFICANCE STATEMENT: This is the first study to systematically evaluate the penetration of various P-glycoprotein (P-gp) substrates into lumbar cerebrospinal fluid (CSF) in nonhuman primates. Lumbar CSF may contain >3-fold higher concentrations of good P-gp substrates than interstitial fluid (ISF) and cisternal CSF but was able to discriminate the good substrates from the weak or nonsubstrates. Because lumbar CSF is more accessible than ISF and cisternal CSF in nonhuman primates, these findings will help increase our understanding of drug central nervous system penetration at the nonclinical stage.

Nonclinical safety assessment of PF614: A novel TAAP prodrug of oxycodone for chronic pain indication

PF614, a novel trypsin activated abuse protection (TAAP) prodrug of oxycodone, is being studied as chronic pain analgesic with extended release and abuse resistant properties. A series of nonclinical safety studies were conducted to support PF614 introduction to clinical trials. Ames assays (PF614 and its metabolites), comet assay (PF614 ≤ 50 mg/kg/day oral gavage in rats) and micronucleus assay (PF614 ≤ 175 mg/kg/day oral gavage in rats) were negative. hERG assay IC₅₀ for PF614 was ≥300 μM. PF614 (0.1 and 10 μM) showed a low permeability in Caco-2 cells (≤1.17 x 10-6 cm/s) and was not a P-gp or BCRP substrate or inhibitor. The mean percent unbound PF614 among all concentrations in plasma ranged from 91.2 to 98.4, 79.4 to 100, and 52.9-79.9% in rat, dog, and human, respectively. Also, PF614 was metabolically stable in rat, dog, and human hepatocytes with no metabolites identified. Safety pharmacology study in dog indicated moderately lower heart rate at ≥ 2 mg/kg oral gavage doses. Toxicity studies of PF614 in rat and dog with daily oral doses of 25 and 18 mg/kg, respectively, for 14 Days were well tolerated with favorable safety profile supporting its further clinical evaluation.

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.

Pharmacokinetics of Morphine in Rats with Adjuvant-induced Arthritis

We investigated the in vivo dynamics and analgesic effect of morphine using an adjuvant-induced arthritis (AA) rat as a model of chronic inflammation. Morphine generally binds to μ-opioid receptors in the brain to exert its effects. After several minutes, it is metabolized by glucuronidation via a UDP-glucuronosyltransferase (UGT). Here, we showed that in AA rats, UGT activity in liver microsomes was reduced. Morphine-free serum fractions in AA rats were also decreased (control, 84.9%; AA, 63.9%) and the expression of ATP-binding cassette, sub-family B (MDR/TAP), member 1 (ABCB1), which plays a crucial role in morphine bile excretion, decreased to 23.0% that of the control group. However, we observed no significant difference between the AA and control groups regarding blood concentrations of morphine and morphine-3-glucuronide. In contrast, the analgesic effect of morphine increased 4-fold in AA rats. Our results showed that the pharmacokinetics of morphine is not changed, but the pharmacodynamics of morphine is enhanced in chronic inflammation.

Morphine enhances renal cell carcinoma aggressiveness through promotes survivin level

Background: Morphine is an opioid analgesic drug often used for pain relief in cancer patients. However, there is growing evidence that morphine may modulate tumor growth, progression and metastasis. Unfortunately, the results obtained by these studies are still contradictory.

Methods: In this study, we investigated the effect of morphine in human clear cell renal cell carcinoma 786-O, RLC-310 cells and whether morphine affects on tumor growth in human clear cell renal cell carcinoma 786-O, RLC-310 cells. The cell proliferation was determined by MTT assay, cell proliferation, migration and invasion assays. Immunofluorescence staining and Q-PCR was used to determine the Survivin expression.

Results: It was shown that morphine enhances proliferation of 786-O, RLC-310 cells, whereas morphine promoted the growth and aggressive phenotype of 786-O and RLC-310 cells in vitro though Survivin-dependent signaling.

Conclusions: Our data showed that morphine promotes RCC growth and increases RCC progression via over-expression of Survivin.

Morphine, a potential antagonist of cisplatin cytotoxicity, inhibits cisplatin-induced apoptosis and suppression of tumor growth in nasopharyngeal carcinoma xenografts

Morphine is an opioid analgesic drug often used for pain relief in cancer patients. However, there is growing evidence that morphine may modulate tumor growth, progression and metastasis. In this study, we evaluated whether morphine modulates cisplatin-induced apoptosis in human nasopharyngeal carcinoma CNE-2 cells and whether morphine affects the antitumor activity of cisplatin on tumor growth in human nasopharyngeal carcinoma CNE-2 xenografts in nude mice. We showed that a pretreatment with morphine (1 μg/ml) inhibited the sensitivity of CNE-2 cells to cisplatin by inhibiting cisplatin-induced CNE-2 cell apoptosis, decreasing caspase-3 activity and increasing the Bcl-2/Bax ratio. However, a high dose of morphine (1000 μg/ml) had the opposite effect. We also showed that at a low dose, morphine enhances chemoresistance in an in vivo nasopharyngeal carcinoma (NPC) model by inhibiting cisplatin-induced apoptosis and decreasing neovascularization. Taken together, our results indicate that a low dose of morphine may lead to chemoresistance of cisplatin in NPC models in vitro and in vivo by inhibiting cisplatin-induced apoptosis and decreasing neovascularization.

The ontogeny of P-glycoprotein in the developing human blood-brain barrier: implication for opioid toxicity in neonates

BACKGROUND

Neonates have been shown to have a heightened sensitivity to the central depressive effects of opioids compared to older infants and adults. The limited development of P-glycoprotein (P-gp) may limit the ability of the neonate to efflux morphine from the brain back to the systemic circulation. The objective of the study was to determine the ontogeny of P-gp in the human brain.

METHODS

Postmortem cortex samples from gestational age (GA) 20-26 wk, GA 36-40 wk, postnatal age (PNA) 0-3 mo, PNA 3-6 mo, and adults were immunostained for P-gp.

RESULTS

The intensity of P-gp staining in adults was significantly higher compared to at GA 20-26 wk (P < 0.05), GA 36-40 wk (P < 0.05), and PNA 0-3 mo (P < 0.05). P-gp intensity at GA 20-26 wk (P < 0.05), GA 36-40 wk (P < 0.05), and PNA 0-3 mo (P < 0.05) was significantly lower compared to at PNA 3-6 mo.

CONCLUSION

P-gp expression in the brain is limited at birth, increases with postnatal maturation, and reaches adult levels at ~3-6 mo of age. Given the immaturity of blood-brain barrier (BBB) P-gp after birth, morphine may concentrate in the brain. This provides mechanistic support to life threatening opioid toxicity seen with maternal codeine use during breastfeeding.

P-glycoprotein in the developing human brain: a review of the effects of ontogeny on the safety of opioids in neonates

The human blood brain barrier is responsible for maintaining brain homeostasis and protecting against potentially toxic substances. The ATP-binding cassette drug efflux protein, P-glycoprotein (P-gp) is a key player in actively extruding a wide range of xenobiotics such as opioids from the brain. Because the blood brain barrier is structurally and functionally immature in neonates, opioids may have a greater penetration to the central nervous system. This may influence the efficacy and safety of opioids in the newborn. Understanding the extent of P-gp's expression in the brain in the embryo, fetus, and newborn will facilitate rational opioid use during pregnancy and the neonatal period. This review aims to summarize the current evidence that associates the ontogeny of P-gp and the susceptibility to opioid-induced adverse respiratory effects in neonates. To date, evidence suggests that the expression of P-gp in the human brain is low at birth, contributing to increased susceptibility.

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.

Morphine Promotes Tumor Angiogenesis and Increases Breast Cancer Progression

Morphine is considered a highly potent analgesic agent used to relieve suffering of patients with cancer. Several in vitro and in vivo studies showed that morphine also modulates angiogenesis and regulates tumour cell growth. Unfortunately, the results obtained by these studies are still contradictory. In order to better dissect the role of morphine in cancer cell growth and angiogenesis we performed in vitro studies on ER-negative human breast carcinoma cells, MDA.MB231 and in vivo studies on heterotopic mouse model of human triple negative breast cancer, TNBC. We demonstrated that morphine in vitro enhanced the proliferation and inhibited the apoptosis of MDA.MB231 cells. In vivo studies performed on xenograft mouse model of TNBC revealed that tumours of mice treated with morphine were larger than those observed in other groups. Moreover, morphine was able to enhance the neoangiogenesis. Our data showed that morphine at clinical relevant doses promotes angiogenesis and increases breast cancer progression.

Liposomal conjugates for drug delivery to the central nervous system

Treatments of central nervous system (CNS) diseases often fail due to the blood–brain barrier. Circumvention of this obstacle is crucial for any systemic treatment of such diseases to be effective. One approach to transfer drugs into the brain is the use of colloidal carrier systems—amongst others, liposomes. A prerequisite for successful drug delivery by colloidal carriers to the brain is the modification of their surface, making them invisible to the reticuloendothelial system (RES) and to target them to specific surface epitopes at the blood–brain barrier. This study characterizes liposomes conjugated with cationized bovine serum albumin (cBSA) as transport vectors in vitro in porcine brain capillary endothelial cells (PBCEC) and in vivo in rats using fluorescently labelled liposomes. Experiments with PBCEC showed that sterically stabilized (PEGylated) liposomes without protein as well as liposomes conjugated to native bovine serum albumin (BSA) were not taken up. In contrast, cBSA-liposomes were taken up and appeared to be concentrated in intracellular vesicles. Uptake occurred in a concentration and time dependent manner. Free BSA and free cBSA inhibited uptake. After intravenous application of cBSA-liposomes, confocal fluorescence microscopy of brain cryosections from male Wistar rats showed fluorescence associated with liposomes in brain capillary surrounding tissue after 3, 6 and 24 h, for liposomes with a diameter between 120 and 150 nm, suggesting successful brain delivery of cationized-albumin coupled liposomes.

[Effect of repeated oral treatment with etoposide on the expression of intestinal P-glycoprotein and oral morphine analgesia]

Currently, the World Health Organization recommends oral administration of opioid analgesics for patients with cancer to treat cancer-related pain from the initial stage of treatment. Furthermore, many anticancer drugs have been newly-developed and approved as oral form. Because of this trend, the chances of drug-drug interactions between anticancer drugs and opioid analgesics during absorption process from the intestine are likely to increase. To investigate these possible drug-drug interactions, we have focused on intestinal P-glycoprotein (P-gp) which regulates the absorption of various substrate drugs administered orally. Previously, we have found that repeated oral treatment with etoposide (ETP), an anticancer drug, attenuates analgesia of oral morphine, a substrate drug for P-gp, by increasing the expression and activity of intestinal P-gp. However, the mechanism by which ETP treatment increases the intestinal P-gp expression and decreases oral morphine analgesia remains unclear. RhoA, a small G-protein, and ROCK, an effector of RhoA, pathway has been attracted attention with regard to their involvement in the regulatory mechanism of the expression and activity of P-gp. Interestingly, this pathway is activated in response to various signaling induced by some anticancer drugs. Furthermore, it has been reported that ezrin/radixin/moesin (ERM) play a key role in the plasma membrane localization of P-gp, and that RhoA/ROCK pathway regulates the activation process of ERM. This review article introduces the result of our previous research as well as recent findings on the involvement of ERM via activation of RhoA/ROCK in the increased expression of intestinal P-gp and decreased oral morphine analgesia induced by repeated oral treatment with ETP.

Impact of P-glycoprotein at the blood-brain barrier on the uptake of heroin and its main metabolites: behavioral effects and consequences on the transcriptional responses and reinforcing properties

RATIONALE

Transport across the BBB is a determinant of the rate and extent of drug distribution in the brain. Heroin exerts its effects through its principal metabolites 6-monoacetyl-morphine (6-MAM) and morphine. Morphine is a known substrate of P-glycoprotein (P-gp) at the blood-brain-barrier (BBB) however, little is known about the interaction of heroin and 6-MAM with P-gp.

OBJECTIVE

The objective of this paper is to study the role of the P-gp-mediated efflux at the BBB in the behavioral and molecular effects of heroin and morphine.

METHODS

The transport rates of heroin and its main metabolites, at the BBB, were measured in mice by in situ brain perfusion. We then examined the effect of inhibition of P-gp on the acute nociception, locomotor activity, and gene expression modulations induced by heroin and morphine. The effect of P-gp inhibition during the acquisition of morphine-induced place preference was also studied.

RESULTS

Inhibition of P-gp significantly increased the uptake of morphine but not that of heroin nor 6-MAM. Inhibition of P-gp significantly increased morphine-induced acute analgesia and locomotor activity but did not affect the behavioral effects of heroin; in addition, acute transcriptional responses to morphine were selectively modulated in the nucleus accumbens. Increasing morphine uptake by the brain significantly increased its reinforcing properties in the place preference paradigm.

CONCLUSIONS

The present study demonstrated that acute inhibition of P-gp not only modulates morphine-induced behavioral effects but also its transcriptional effects and reinforcing properties. This suggests that, in the case of morphine, transport across the BBB is critical for the development of dependence.

Involvement of moesin in the development of morphine analgesic tolerance through P-glycoprotein at the blood-brain barrier

Altered expression of P-glycoprotein (P-gp), a drug efflux transporter expressed by brain capillary endothelial cells (BCECs), may contribute to the development of opioid analgesic tolerance, as demonstrated by cumulative evidence from research. However, the detailed mechanism by which chronic morphine treatment increases P-gp expression remains unexplained. Ezrin/radixin/moesin (ERM) are scaffold proteins that are known to regulate the plasma membrane localization of some drug transporters such as P-gp in peripheral tissues, although a few reports suggest its role in the central nervous system as well. In this study, we investigated the involvement of ERM in the development of morphine analgesic tolerance through altered P-gp expression in BCECs. Repeated treatment with morphine (10 mg/kg/day, s.c. for 5 days) decreased its analgesic effect in the tail-flick test and increased P-gp protein expression in BCECs, as determined by Western blotting. Furthermore, moesin protein expression increased in the same fraction whereas that of ezrin decreased; no change was observed in the radixin expression. Furthermore, immunoprecipitation and immunofluorescence assays revealed interaction between moesin and P-gp molecules, along with co-localization, in BCECs. In conclusion, an increase in moesin expression may contribute to the increased expression of P-gp in BCECs, leading to the development of morphine analgesic tolerance.

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.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

The role of morphine in animal models of human cancer: does morphine promote or inhibit the tumor growth?

Morphine, a highly potent analgesic agent, is widely used to relieve pain and suffering of patients with cancer. Additionally, it has been reported that morphine is important in the regulation of cancerous tissue. Morphine relieves pain by acting directly on the central nervous system, although its activities on peripheral tissues are responsible for many adverse side effects. For these reasons, it is very important also to understand the role of morphine in cancer treatment. The published literature reporting the effect of morphine on tumor growth presents some discrepancies, with reports suggesting that morphine may either promote or inhibit the tumor growth. It has been also demonstrated that morphine modulates angiogenesis which is important for primary tumour growth, invasiveness, and the development of metastasis. This review will focus on the latest findings on the role of morphine in the regulation of cancer cell growth and angiogenesis.

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.

Induction of P-glycoprotein and Bcrp at the rat blood-brain barrier following a subchronic morphine treatment is mediated through NMDA/COX-2 activation

Subchronic morphine treatment induces P-glycoprotein (P-gp) up-regulation at the blood-brain barrier. This study investigates the rate and extent to which P-gp and breast cancer-resistance protein (Bcrp) increase at the rat blood-brain barrier following subchronic morphine treatment. Rats were given increasing doses of morphine (10-40 mg/kg) or saline i.p. twice daily for 5 days. The brain cortex large vessels and microvessels were then mechanical isolated 6, 9, 12, 24, and 36 h after the last injection. The gene and protein expression of P-gp and Bcrp in morphine-treated and control rats were compared by qRT-PCR and western blotting. The levels of Mdr1a and Bcrp mRNAs were not significantly modified 6 h post morphine, but the Mdr1a mRNA increased 1.4-fold and Bcrp mRNA 2.4-fold at 24 h. P-gp and Bcrp protein expression in brain microvessels was unchanged 6 h post morphine and increased 1.5-fold at 24 h. This effect was more pronounced in large vessels than in microvessels. However, extracellular morphine concentrations of 0.01-10 μM did not modify the expressions of the MDR1 and BCRP genes in hCMEC/D3 human endothelial brain cells in vitro. MK-801 (NMDA antagonist) and meloxicam (cyclo-oxygenase-2 inhibitor) given after morphine treatment completely blocked P-gp and Bcrp up-regulation. Interestingly, misoprostol and iloprost, two well-known agonists of prostaglandin E2 receptors induced both MDR1 and BCRP mRNA levels in hCMEC/D3. Thus, morphine does not directly stimulate P-gp and Bcrp expression by the brain endothelium, but glutamate released during morphine withdrawal may do so by activating the NMDA/cyclo-oxygenase-2 cascade.

P-glycoprotein is a major determinant of norbuprenorphine brain exposure and antinociception

Norbuprenorphine is a major metabolite of buprenorphine and potent agonist of μ, δ, and κ opioid receptors. Compared with buprenorphine, norbuprenorphine causes minimal antinociception but greater respiratory depression. It is unknown whether the limited antinociception is caused by low efficacy or limited brain exposure. Norbuprenorphine is an in vitro substrate of the efflux transporter P-glycoprotein (Mdr1), but the role of P-glycoprotein in norbuprenorphine transport in vivo is unknown. This investigation tested the hypothesis that limited norbuprenorphine antinociception results from P-glycoprotein-mediated efflux and limited brain access. Human P-glycoprotein-mediated transport in vitro of buprenorphine, norbuprenorphine, and their respective glucuronide conjugates was assessed by using transfected cells. P-glycoprotein-mediated norbuprenorphine transport and consequences in vivo were assessed by using mdr1a(+/+) and mdr1a(-/-) mice. Antinociception was determined by hot-water tail-flick assay, and respiratory effects were determined by unrestrained whole-body plethysmography. Brain and plasma norbuprenorphine and norbuprenorphine-3-glucuronide were quantified by mass spectrometry. In vitro, the net P-glycoprotein-mediated efflux ratio for norbuprenorphine was nine, indicating significant efflux. In contrast, the efflux ratio for buprenorphine and the two glucuronide conjugates was unity, indicating absent transport. The norbuprenorphine brain/plasma concentration ratio was significantly greater in mdr1a(-/-) than mdr1a(+/+) mice. The magnitude and duration of norbuprenorphine antinociception were significantly increased in mdr1a(-/-) compared with mdr1a(+/+) mice, whereas the reduction in respiratory rate was similar. Results show that norbuprenorphine is an in vitro and in vivo substrate of P-glycoprotein. P-glycoprotein-mediated efflux influences brain access and antinociceptive, but not the respiratory, effects of norbuprenorphine.

[Altered intestinal P-glycoprotein expression levels affect pharmacodynamics under diabetic condition]

P-glycoprotein (P-gp), one of the important drug-efflux pumps, is known to affect pharmacokinetics and pharmacodynamics of P-gp substrate drugs. We have previously reported that intestinal P-gp expression levels are transiently decreased in streptozotocin (STZ)-induced type 1 diabetic mouse model. Herein, we examined the analgesic effects of orally administered morphine and its pharmacokinetic properties under diabetic conditions, specifically focusing on the involvement of intestinal P-gp in a type 1 diabetic mouse model. Type 1 diabetes was induced in male ddY mice by an i.p. injection of STZ (230 mg/kg). We assessed the oral morphine analgesia using the tail-flick test. Serum and brain morphine content were determined on a HPLC-ECD system. Intestinal P-gp expression levels were significantly decreased on day 9 after STZ administration. On the other hands, oral morphine analgesia, and serum and brain morphine content were significantly increased on day 9 after STZ administration. The decrease in the intestinal P-gp expression levels were suppressed by aminoguanidine, a specific iNOS inhibitor. Interestingly, the increase in the analgesic effect of morphine, as well as serum and brain morphine content, was suppressed by aminoguanidine. Conversely, there was no change in the analgesic effect obtained with subcutaneous morphine in STZ-treated mice. In conclusions, our findings suggest that the oral morphine analgesia is dependent on intestinal P-gp expression, and that may be one of the problems against obtaining stable pharmacological effects of morphine in diabetic patients.

The role of morphine in regulation of cancer cell growth

Morphine is considered the "gold standard" for relieving pain and is currently one of the most effective drugs available clinically for the management of severe pain associated with cancer. In addition to its use in the treatment of pain, morphine appears to be important in the regulation of neoplastic tissue. Although morphine acts directly on the central nervous system to relieve pain, its activities on peripheral tissues are responsible for many of the secondary complications. Therefore, understanding the impact, other than pain control, of morphine on cancer treatment is extremely important. The effect of morphine on tumor growth is still contradictory, as both growth-promoting and growth-inhibiting effects have been observed. Accumulating evidence suggests that morphine can affect proliferation and migration of tumor cells as well as angiogenesis. Various signaling pathways have been suggested to be involved in these extra-analgesic effects of morphine. Suppression of immune system by morphine is an additional complication. This review provides an update on the influence of morphine on the growth and migration potential of tumor cells.

Synthesis and in vivo brain distribution of carbon-11-labeled δ-opioid receptor agonists

Three new radiolabeled compounds, [(11)C]SNC80 ((+)-4-[(αR)-α-{(2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl}-3-[(11)C]methoxybenzyl-N,N-diethylbenzamide), N,N-diethyl-4-[3-methoxyphenyl-1-[(11)C]methylpiperidin-4-ylidenemethyl)benzamide and N,N-diethyl-4-[(1-[(11)C]methylpiperidin-4-ylidene)phenylmethyl]benzamide, were prepared as potential in vivo radiotracers for the δ-opioid receptor. Each compound was synthesized by alkylation of the appropriate desmethyl compounds using [(11)C]methyl triflate. In vivo biodistribution studies in mice showed very low initial brain uptake of all three compounds and no regional specific binding for [(11)C]SNC80. A monkey positron emission tomography study of [(11)C]SNC80 confirmed low brain permeability and uniform regional distribution of this class of opioid agonists in a higher species. Opioid receptor ligands of this structural class are thus unlikely to succeed as in vivo radiotracers, likely due to efficient exclusion from the brain by the P-glycoprotein efflux transporter.

Effects of repeated morphine treatment on the antinociceptive effects, intestinal absorption, and efflux from intestinal epithelial cells of morphine

The present study was conducted to investigate the effects of repeated treatment with morphine on the drug's antinociceptive effects, intestinal absorption, and transepithelial transport. The antinociceptive effects of morphine in rats were markedly decreased after repeated oral administration of the drug for 5 d, indicating the development of tolerance. In the morphine-tolerant rats, intestinal absorption of morphine was determined using the in situ loop method. Absorption of morphine from the jejunum was significantly decreased after repeated administration. The permeability of human intestinal epithelial Caco-2 cells was increased in the efflux direction after repeated treatment. The repeated administration of morphine also reduced the cellular accumulation and efflux of P-glycoprotein substrates ([(3)H]vincristine and rhodamine123) from Caco-2 cells, suggesting that it enhances P-glycoprotein-mediated efflux in Caco-2 cells. These results suggest that repeated use enhances the efflux of morphine in the epithelial cells of the small intestine, subsequently decreasing its intestinal absorption.

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.

Effects of quinidine on antinociception and pharmacokinetics of morphine in rats

Aim

The aim of this study was to investigate the effect of quinidine, a P-glycoprotein inhibitor, on the pharmacokinetics and pharmacodynamics of morphine in rats.

Methods

Rats were given morphine (30 mg/kg p.o. or 30 mg/kg over 10 min i.v.) 30 min after pretreatment with quinidine (30 mg/kg p.o.). Antinociceptive effects were determined using the tail immersion test. Concentrations of morphine in plasma and brain were also determined.

Key findings

The antinociception of morphine was significantly enhanced by oral administration of quinidine, with a 3.1-fold greater area under the effect–time curve than that in vehicle-treated rats. Morphine concentrations in plasma and brain were significantly increased by quinidine. The area under the plasma concentration–time curve after oral or intravenous administration of morphine was increased 5.2- and 1.7-fold, respectively, in quinidine-pretreated rats compared with vehicle-pretreated rats. Quinidine caused a 40% decrease in the total clearance of morphine and increased the concentration of morphine in the brain, although the brain-to-plasma concentration ratio was not changed.

Conclusions

Oral administration of quinidine increases the absorption of morphine from the gastrointestinal tract and subsequently enhances the concentration in the brain and its antinociceptive effect. Enhanced intestinal absorption of morphine may be due largely to inhibition of intestinal P-glycoprotein by quinidine.

Pharmacokinetic and pharmacodynamic implications of P-glycoprotein modulation

Modulation of P-glycoprotein (Pgp)-mediated transport has significant pharmacokinetic implications for Pgp substrates. Pharmacokinetic alterations may be at the systemic (blood concentrations), regional (organ or tissue concentrations), or local (intracellular concentrations) level. Regardless of the particular location of Pgp modulation, changes in substrate pharmacokinetics will have the potential to alter the magnitude and duration of pharmacologic effect (pharmacodynamics). It is important to understand each of the aspects of Pgp modulation for a given Pgp substrate in order to predict the degree to which Pgp modulation may affect that substrate, to minimize untoward effects associated with that modulation, or to exploit that modulation for specific therapeutic advantage.

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.

The influence of opioids on urokinase plasminogen activator on protein and mRNA level in MCF-7 breast cancer cell line

Urokinase plasminogen activator plays a key role in tumor-associated processes, increasing cancer cell invasion and metastasis, and is therefore used as a marker in cancer prognosis. In this study, we have determined the effect of mu-opioid receptor agonists and antagonists on the urokinase plasminogen activator secretion in MCF-7 cell line. It was shown that mu-opioid receptor agonists, such as morphine and endomorphins, greatly stimulate urokinase plasminogen activator secretion, while naloxone and MOR-selective antagonists elicit the opposite effect. The same tendency was observed also on the urokinase plasminogen activator mRNA level. However, neither agonists nor antagonists had any effect on proliferation of MCF-7 cells. The findings reported in this study may be useful in designing further experiments aimed at elucidating the role of the opioid system in cancer cells.

Evaluation of the P-glycoprotein (Abcb1) affinity status of a series of morphine analogs: comparative study with meperidine analogs to identify opioids with minimal P-glycoprotein interactions

One of the major shortcomings of many commonly used opioids is the fact that they are P-gp substrates, which represents a major obstacle towards effective pain management. P-gp can affect opioids' oral absorption, CNS accumulation, systemic clearance, antinociceptive activity, and tolerance development to their analgesic effects. Moreover, P-gp can be the locus of drug-drug interactions between opioids and other concomitantly administered drugs that are P-gp substrates/inhibitors. The objective of this study was to identify opioids that are non-P-gp substrates to overcome some of the mentioned shortcomings. We evaluated the P-gp affinity status (substrate, non-substrate, or inhibitor) of a series of morphine analogs (10 opioid agonist and 2 opioid antagonists) and compared them to previously reported meperidine analogs. The fold stimulation of the morphine analogs ranged from 1.01 to 1.54 while for the meperidine analogs the fold stimulation ranged from 1.10 to 3.66. From each series (morphine and meperidine analogs) we selected potential candidate opioids that are non-P-gp substrates and conducted in vivo assessments of their antinociceptive effects using P-gp knockout and P-gp competent mice. 6-Desoxymorphine, meperidine and N-phenylbutyl normeperidine did not significantly (p>0.05) stimulate the basal P-gp ATPase activity, where, the fold stimulations of the basal P-gp ATPase activity were 1.01+/-0.11, 1.51+/-0.29 and 1.10+/-0.23, respectively. Evaluation of the influence of P-gp ablation on their antinociceptive effects indicated that P-gp did not significantly (p>0.05) affect their antinociceptive effects. Among the evaluated opioids in vivo, 6-desoxymorphine showed high potency and induced no apparent toxicity upon low- and high-dose administration. 6-Desoxymorphine is therefore an ideal lead compound to create a library of opioids that have negligible P-gp affinity for better management of pain.

Aripiprazole brain concentration is altered in P-glycoprotein deficient mice

P-glycoprotein (P-gp) is a transporter that mediates the tissue disposition of numerous drugs. To evaluate the role of P-glycoprotein (P-gp) in aripiprazole tissue distribution and penetration across the blood-brain barrier, mice deficient in the P-gp gene (Abcb1a/b-/-) were dosed intraperitoneally with 2 microg/g mouse of the antipsychotic drug aripiprazole. Wildtype FVB mice were administered the same dose as transgenic animals. At one, two, and three hours after dosing, blood and tissue samples were collected and assayed for aripiprazole concentration by HPLC. Deficiency of P-gp did not result in significantly altered plasma drug concentrations but had dramatic effects on drug concentrations in brain tissue. At 1, 2, and 3 h after dosing, aripiprazole brain concentrations in the Abcb1a/b-/- mice were 4.6-, 4.1- and 3.0-fold higher, respectively (P<0.01), compared with the wildtype mice. Increases in drug concentration were also observed in testes and muscle in Abcb1a/b -/- mice. All other tissues including gut, lung, heart, kidney, liver, and spleen did not show significant differences between the two groups. These data provide evidence that aripiprazole is a transportable substrate of P-gp. Thus, factors influencing P-gp activity within the blood brain barrier in humans may have implications for the therapeutic effects and tolerability of aripiprazole.

Enhancement by grapefruit juice of morphine antinociception

The aim of this study was to investigate the effect of grapefruit juice intake on the antinociception of morphine in rats. The antinociception of morphine (30 mg/kg, per os (p.o.)) was significantly enhanced by the oral administration of grapefruit juice (2 ml/rat). Further, the effect of grapefruit juice was examined in morphine-tolerant rats. The repeated administration of morphine (100 mg/kg p.o.) for 5 d caused a marked decrease in the antinociception, indicating the development of morphine-tolerance. In the morphine-tolerant rats, oral administration of grapefruit juice potentiated significantly the antinociceptive effect of morphine. To examine the pharmacokinetics of morphine after the repeated treatment with morphine for 5 d, microdialysis probes were implanted into the jugular vein and spinal intrathecal space in rats. The morphine concentrations in the blood and intrathecal cerebrospinal fluid (CSF) were gradually decreased by the repeated treatment with morphine. The grapefruit juice treatment significantly increased the blood concentration of morphine in morphine-tolerant rats. These results suggest that oral administration of grapefruit juice enhances the morphine antinociception by increasing the intestinal absorption of this agent.

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

The involvement of P-glycoprotein (P-gp) in buprenorphine (BNP) transport at the blood-brain barrier (BBB) in rats was investigated in vivo by means of both the brain uptake index technique and the brain efflux index technique. P-gp inhibitors, such as cyclosporin A, quinidine and verapamil, enhanced the apparent brain uptake of [3H]BNP by 1.5-fold. The increment of the BNP uptake by the brain suggests the involvement of a P-gp efflux mechanism of BNP transport at the BBB. [3H]BNP was eliminated with an apparent elimination half-life of 27.5 min after microinjection into the parietal cortex area 2 regions of the rat brain. The apparent efflux clearance of [3H]BNP across the BBB was 0.154 ml/min/g brain, which was calculated from the elimination rate constant (2.52 x 10- 2 min- 1) and the distribution volume in the brain (6.11 ml/g brain). The efflux transport of [3H]BNP was inhibited by range from 32 to 64% in the presence of P-gp inhibitors. The present results suggest that BNP is transported from the brain across the BBB via a P-gp-mediated efflux transport system, at least in part.

Effect of chronic exposure to morphine on the rat blood-brain barrier: focus on the P-glycoprotein

Morphine may affect the properties of the blood-brain barrier (BBB) by modifying the expression of certain BBB markers. We have determined the effect of chronic morphine treatment on the expression and function of some BBB markers in the rat. The mRNAs of 19 selected genes encoding caveolins, endothelial transporters, receptors and tight junctions proteins in the total RNA of isolated cortex microvessels were assayed by quantitative RT-PCR (qRT-PCR). The expression of genes Mdr1a, Mrp1, Bcrp, Glut-1 and Occludin, was slightly increased, while that of Flk-1 was decreased in microvessels from morphine-treated rats. The expression of the Mrd1a and Mdr1b genes encoding the P-glycoprotein (P-gp) also increased in the whole hippocampus and cortex of morphine-treated rats. The Mdr1a gene induction (1.38-fold) observed by qRT-PCR was also confirmed using in situ hybridization technique (1.40-fold). Immunoblotting revealed an increase in P-gp expression in the hippocampus (1.8-fold) and cortex (1.36-fold) of morphine-treated rats, but no effect in isolated microvessels. In contrast, morphine treatment increased by 1.48-fold the expression of P-gp in a large vessel-enriched fraction. The integrity of the BBB, measured by in situ brain perfusion of [(14)C]-sucrose, and the activity of P-gp at the BBB, measured with the P-gp substrate [(3)H]-colchicine, were not modified by morphine. Immunohistofluorescence experiments revealed that P-gp expression is restricted to large vessels and microvessels in control rats and that morphine treatment did not induce the expression of P-gp in the brain parenchyma (astrocytes or neurons). Taken together, our results showed that chronic morphine treatment does not significantly alter BBB integrity or P-gp activity. The impact of morphine-mediated P-gp induction observed in large vessels remains to be determined in terms of brain disposition of drugs that are P-gp substrates.

Opioid-induced regulation of µ-opioid receptor gene expression in the MCF-7 breast cancer cell line

The aim of the study was to investigate the presence of opioid receptor types in human breast adenocarcinoma MCF-7 cells and to characterize the changes in MOR expression induced by opioid agonist and antagonist treatment. We have shown that all three types of opioid receptors, but predominantly MOR, are expressed in MCF-7 cells. Selective MOR agonists, morphine, endomorphin-1, and endomorphin-2 downregulated MOR mRNA levels in a concentration- and time-dependent manner, but the effect produced by endomorphins was much stronger. Downregulation was blocked by the opioid antagonist naloxone. Naloxone alone produced a slight increase in MOR gene expression. Immunoblotting with antiserum against MOR-1 confirmed these results at the protein level. The results of our study indicate that, in MCF-7 cells, MOR gene expression is downregulated by opioid agonists and upregulated by opioid antagonists. We propose that the opioid-induced regulation of MOR mRNA expression is mediated by reduced binding of the transcription factors NFκB and AP-1 to the promoter region on the MOR gene.

Oxycodone induces overexpression of P-glycoprotein (ABCB1) and affects paclitaxel's tissue distribution in Sprague Dawley rats

Previous studies suggest that P-glycoprotein (P-gp) modulates the PK/PD of many compounds including opioid agonists and chemotherapeutic agents. The objective of this study was to assess the P-gp affinity status of oxycodone, the P-gp expression, and the paclitaxel's tissue distribution in oxycodone-treated rats. P-gp ATPase assay, Caco-2 transepithelial permeability studies, and mdr1a/b (-/-) mice were used to assess the P-gp affinity status of oxycodone. P-gp expression was determined by Western blot analysis while [(14)C] paclitaxel's distributions in the liver, kidney, brain, and plasma tissues were determined by liquid scintillation counter. Oxycodone stimulated the P-gp ATPase activity in a concentration-dependant manner. The Caco-2 secretory transport of oxycodone was reduced from 3.64 x 10(-5) to 1.96 x 10(-5) cm/s (p < 0.05) upon preincubation with the P-gp inhibitor, verapamil. The brain levels of oxycodone in mdr1a/b (+/+) were not detectable (<15 ng/mL) while in mdr1a/b (-/-) the average levels were 115 +/- 39 ng/mL. The P-gp protein levels were increased by 1.3-4.0 folds while paclitaxel's tissue distributions were decreased by 38-90% (p < 0.05) in oxycodone-treated rats. These findings display that oxycodone is a P-gp substrate, induces overexpression of P-gp, and affects paclitaxel's tissue distribution in a manner that may influence its chemotherapeutic activity.

Pharmacokinetics and pharmacodynamics of alfentanil in P-glycoprotein-competent and P-glycoprotein-deficient mice: P-glycoprotein efflux alters alfentanil brain disposition and antinociception

Previous studies have indicated that P-glycoprotein (P-gp) attenuates the central nervous system penetration and central activity of some opioids. The impact of P-gp-mediated efflux on the disposition and efficacy of the synthetic opioid alfentanil currently is unknown. In this study, P-gp-competent [mdr1a(+/+)] and P-gp-deficient [mdr1a(-/-)] mice were used to investigate the impact of P-gp-mediated efflux on the systemic pharmacokinetics, brain disposition, and central activity of alfentanil. Equipotent doses of alfentanil were administered to mdr1a(+/+) and mdr1a(-/-) mice (0.2 and 0.067 mg/kg, respectively), and the time course of brain and serum concentrations as well as antinociception were determined. A pharmacokinetic-pharmacodynamic (PK-PD) model was fit to the data and used to assess the impact of P-gp on parameters associated with alfentanil disposition and action. The mdr1a(+/+) mice were less sensitive to alfentanil than mdr1a(-/-) mice, requiring a 3-fold higher dose to produce similar antinociception. PK-PD modeling revealed no differences in alfentanil systemic pharmacokinetics between P-gp expressers and nonexpressers. However, the steady-state brain-to-serum concentration ratio (K(p,brain,ss)) was approximately 3-fold lower in mdr1a(+/+) mice compared with mdr1a(-/-) mice (0.19 +/- 0.01 versus 0.54 +/- 0.04, respectively). Consistent with the approximately 3-fold lower K(p,brain,ss), the antinociception versus serum concentration relationship in mdr1a(+/+) mice was shifted approximately 3-fold rightward compared with mdr1a(-/-) mice. However, there was no difference in the antinociception versus brain concentration relationship, or in the brain tissue EC(50) (11 +/- 1.8 versus 9.2 +/- 1.7 ng/g), between mdr1a(+/+) and mdr1a(-/-) mice. These results indicate that alfentanil is an in vivo P-gp substrate and are consistent with the hypothesis that P-gp-mediated efflux attenuates antinociception by reducing alfentanil K(p,brain,ss).