Gene Ther Biol Vol 13, 82-90, 2009
Delivery of opioid analgesics to the brain: the role of blood-brain barrier
Sebastiano Mercadante1*, Edoardo Arcuri2
1Pain Relief and Palliative Care Unit, La Maddalena Cancer Center, Palermo
2Pain therapy and Intensive Care, National Cancer Institute, Rome
*Correspondence: Dr Sebastiano Mercadante, MD, Pain Relief and Palliative Care Unit, La Maddalena Cancer Center, Via San Lorenzo 312, 90146 Palermo, Italy, Tel: 39 0916806521; Fax: 39 0916806110; e-mail: email@example.com
Key words: Opioids, brain-blood-barrier, and pain
Received: 26 May 2009; Revised: 1 June 2009;
Accepted: 26 May 2009; electronically published: June 2009
The Blood-brain-barrier (BBB) has been found to have multiple functions rather than being a simple anatomic structure. The BBB provides a stable milieu, essential for the complex integrative functions of the CNS, and has also a protective action against potentially damaging neuroactive substances and other neurotransmitters with highly variable changes in systemic extracellular fluids, blocking their entry or actively transforming by hydrolyzing and conjugating enzymes, and removing them from brain via active efflux transporters. A large number of transporters are present in the BBB, most of them being designed to carry polar substances into the brain that would otherwise have minimal access to the CNS. All the observations regarding the unpredictability of traditional factors influencing the drug passage through the BBB may be partially explained by recent investigations regarding membrane-bound drug transporters capable of actively pumping a variety of drugs out of the CNS. The most characterized transporter system is represented by P-glycoprotein (P-gp), a saturable transporting system systemically expressed widely in several tissues, impeding the delivery of therapeutic agents. Many drugs, including opioids, which are potentially effective at their site of action, may have a decreased effect due to a failure to deliver them in sufficient amount to the CNS. Opioid transport is often the result of the concerted action of efflux and uptake pumps located in the BBB. Polymorphisms affecting drug transporters are of interest for opioid analgesia, as altered function or expression of transporters might cause differences in the extracellular brain concentrations of opioids. Given such an important role of P-gp in the drug disposition process, it is not surprising to see increasing interest on the role of polymorphisms in this transporter as a potential determinant of interindividual variability in opioid response.
Opioids analgesics produce analgesia at peripheral and central sites. However, the principal target of action remains the central nervous system (CNS). The presence of the blood brain barrier (BBB) presents a challenge for effective delivery of analgesics to the CNS. In recent years, it has become evident that a complex group of interacting cells constituting the BBB have other properties, influencing solute transportation and metabolic activity, other than offering an anatomical barrier to the free passage of drugs (Begley, 2004a). Many drugs, including opioids, which are potentially effective at their site of action, may have a decreased effect due to a failure to deliver them in sufficient amount to the CNS. The aims of this review were multiple. Thus, to provide a comprehensive reading, before describing how opioids may be delivered to brain, the characteristics of the BBB will be described, as well as some specific conditions most frequently encountered in clinical daily practice when using opioids, like cancer or chronic pain conditions, able to modify BBB function.
II. Anatomy and physiology of BBB
The function of the BBB is twofold. The BBB has either an anatomic and neuroprotective function. A stable milieu is essential for CNS activity to perform its complex integrative activity. BBB enables the creation of a stable compartment of intracerebral extracellular fluid, distinct from the systemic extracellular fluid. The somatic extracellular fluid contains many substances at variable concentrations that cannot be tolerated in the CNS, which operates in an extremely stable background. The BBB has also a protective action against potentially damaging neuroactive substances and other neurotrasmitters with highly variable changes in systemic extracellular fluids, blocking their entry or actively transforming by hydrolyzing and conjugating enzymes, and removing them from brain via active efflux transporters. For example, amino acids present in high plasma concentrations are potent excitatory neurotransmitters in CNS. A typical example of metabolic function of the BBB is represented by the model of successful treatment of ParkinsonÕs disease. L-Dopa crosses the BBB via a neutral amino acid transporter and is then converted by dopa-decarboxylase to biologically active dopamine (Neuwelt, 2004). Furthermore, there is an enzymatic barrier at the cerebral endothelia, capable of metabolizing drugs and nutrients, and include gamma-glutamyl transpeptidase, alkaline phosphatase and decarboxylases. Finally, drug efflux transporters, such as P-glycoprotein, are mainly present on the luminal membrane surface (see below).
The BBB is a thin, membraneous structure formed by the cerebral endothelial cells of brain and spinal cord capillaries, astrocytes, basement membrane, pericytes, and neurones in physical proximity to the endothelium. Endothelial cells are connected by tight junctions produced by the interaction of transmembrane proteins, which interact to impede free diffusion for polar solutes. Despite the capability of endocytosis is lower in comparison with peripheral endothelial cells, this mechanism is significant for transportation across BBB of macromolecules, such as peptides and proteins.
Endothelial cells express transport proteins in the two interfaces, in both sides (bidirectional) or one side (unidirectional). Endothelial cells, adjacent pericytes, and the end-feet of astroglia induce the tight junction formation, the expression of transport proteins, and differentiation of cerebral endothelium, and contribute to the proteins of the surrounding extracellular matrix, which in turn influence the differentiation of the cells constituting the BBB, also named neurovascular unit. The interaction of these junctional proteins blocks free diffusion for polar solutes from blood. These transmembraneous proteins include occludin, claudins, junctional adhesion molecules, and membrane-associated guanylate kinase-like proteins (Persidsky et al, 2006).
Some substances can be transported into the brain, and others out. As tight junctions seal off the brain to polar solutes, endothelial cells are required to maintain high level of expression of transport proteins for essential polar metabolites to facilitate their efflux (Begley 2004b). The endothelial cells forming BBB also exhibit a polarized expression of transport proteins, so that the transport of some solutes can be facilitated in either direction depending on the direction of gradient concentration. Finally, perivascular macrophages and microglia are part of the BBB and contribute differentiating and modulatory signals (Zenker et al, 2002).
A similar barrier, the blood-cerebrospinal fluid barrier (B-CSF-B) is present in the epithelium of the choroid plexus and circumventricular organs, although these structures result to be more permeable. The total surface area of circumventricular organs is smaller than that of the BBB and results to be less efficient than expected to delivery substances, other than for several pharmacokinetic considerations (Bickel et al, 2001). Choroid plexus produces CSF and regulates the movement of solutes in a bidirectional way, while circumventricular organs are fenestrated to allow a restricted volume of extracellular fluid to rapidly equilibrate with plasma, the composition resembling a plasma ultrafiltrate. Dendritic processes and receptors on neurons within this area can interact with blood-borne solutes and their nervous activity can be modulated. The cells forming the avascular arachnoid membrane enveloping the whole CNS also possess tight junctions, that effectively seal the paracellular diffusional pathway between these cells (Begley, 2004a). CSF drains proteins and other metabolites from the interstitial fluid. The volume of CSF that leaves the CNS via pseudolymphatic pathways may nearly equal the volume of CSF that leaves via arachnoid granulations. Because of these blood-CNS barriers and the lack of a true lymphatic vessels, the CNS is often considered immunologically privileged, although a slow influx of white cells has been noted in some circumstances and may subject to changes in disease states (see below) (Neuwelt, 2004).
III. Passage of drugs through the BBB
Physicochemical characteristics of drugs are determinant for passing BBB, the molecular weight being between 150 and 500 Da and a log octanol-water partition coefficient between 0.5 and 6. Only the uncharged fraction of drugs partially ionized determines the diffusion gradient for a passive diffusion across BBB.
The ability of drugs to cross the BBB has long been recognized to be predicted by molecular weight, degree of ionization, protein binding, and lipid solubility. Some small solutes with high lipophilicity are observed to poorly penetrate the BBB and show lower permeability into the brain than expected (Hamabe et al, 2006). In contrast, some substances may have an even larger CNS entry, not commensurate with that predicted on the basis of their lipid solubility penetration. These molecules, which are mainly essential metabolites for the brain, are facilitated by a carrier-mediated or energy-dependent concentrative mechanism entry dependent on specific proteins inserted in the membranes of the capillary endothelial cells. Transport is often asymmetric. Passive paracellular transport of hydrophilic compounds is restricted by tight junctions between endothelial cells of the BBB (Fromm, 2004).
A transport lipidic vector interacts with a cell surface receptor initiating transcytosis. After tissue enzymes cleaves the peptide from its vector, the free substance will be then available for interaction with its specific receptor in brain cells (Bickel et al, 2001) or converted into an active metabolite at their site of action (Bodor and Buchwald, 2003). Other than lipid solubility, favouring CNS penetration, stable plasma concentration associated with long half-life helps maintaining the diffusion gradient over an extended period.
Other than direct instillation of chemotherapeutic agents, spinal therapies are increasingly used for chronic pain, both malignant and non-malignant. In general, there is a positive correlation between the degree of water solubility and both the spread of analgesia and adverse effects. Highly water-soluble opioids like morphine administered intrathecally exhibit a greater degree of rostral spread in comparison with lipophilic opioids like fentanyl, offering better supraspinal analgesia and more extensive coverage (Cohen and Dragovich, 2007).
Transmucosal route is a viable and interesting possibility for the delivery of some drugs to the brain. In fact olfactory neurons, surrounded by arachnoid membrane, containing CSF, terminates penetrating the olfactory mucosa (Dale et al, 2002). Alternately, nasally administered drugs are taken up by the olfactory nerves and transported by retrograde axonal cytoplasmic flow back into CSF (Begley, 2004a). Thus, CSF has direct communication with olfactorial mucosa via perineuronal space, and drugs may enter the CSF without restrictions usually linked to the BBB.
IV. BBB transporters
A large number of transporters are present in the BBB, most of them being designed to carry polar substances into the brain that would otherwise have minimal access to the CNS. The large neutral aminoacid carrier is the transport system that appears to accept the widest variety of substrates (Begley et ao, 2004a). All the observations regarding the unpredictability of traditional factors influencing the drug passage through the BBB may be partially explained by recent investigations regarding membrane-bound drug transporters capable of actively pumping a variety of drugs out of the CNS. Most of the BBB transporters are from the superfamily of ATP-binding cassette (ABC) proteins, which mediate cellular extrusion of substances with diverse structures. The most characterized of the ABC transporters is the ABCB1 (MDR1, P-glycoprotein) (P-gp). P-gp is a saturable transporting system systemically expressed widely in several tissues, having a protective role and conferring drug-resistance by impeding the delivery of therapeutic agents. Together with xenobiotic-metabolizing enzymes, P-gp expression is believed to be an important protective mechanism against potentially toxic xenobiotics. P-gp limits drug entry as a result of its expression in enterocytes, hepatocytes, proximal tube cells in the kidneys, and at BBB level. Thus, P-gp is an important component of the BBB that limits accumulation of many compounds in brain via active flux across the luminal membrane of capillary endothelium. P-gp is also expressed in the spinal cord and choroids plexus (Dagenais et al, 2004). A reduction of this functional protein could lead to highly increased brain distribution of P-gp substrates (Taylor, 2002). P-gp-knockout experiments have contributed significantly to understanding of the importance of P-gp in limiting drug transfer to the brain. Limited penetration through the BBB could be an obstacle to adequate drug therapy, and a state of P-gp over expression may be one mechanism underlying pharmaco-resistance.
P-gp interacts with a diverse set of hydrophilic and amphiphatic compounds as well as lipophilic compounds and confers resistance to tumor cells by extruding cytotoxic natural product hydrophobic drugs using the energy of ATP hydrolysis. Other complex interactions are known to occur with modulators of the multidrug resistance phenotype (Sauna and Ambudkar, 2001). P-gp acts as an energy-dependent efflux pump, which transports a wide array of structurally divergent compounds, including chemotherapeutic agents, calcium channel blockers, antiarrythmics, immunosuppressants, HIV preotease-inhibitors, and many opioid analgesics, from the intracellular to the extracellular compartment (Ambudkar et al, 1999). As the system is saturable, various compounds can compete with each other, including many drugs that currently in clinical use, such as quinidine, verapamil, and ketokonazole. Some substances, like probenecid, cyclosporine and verapamil, may inhibit the activity of these transporters and could improve delivery across BBB (Neuwelt, 2004). P-gp inhibitors reverse drug resistance in cancer patients and improve treatment outcome (Gottesman, 2002), although P-gp modulators might increase toxicity to the CNS. Antisense approach showed a down regulation of P-gp, which in turn, reduced the efflux of the compounds from the brain into circulation (King et al, 2001).
V. Factors impairing BBB integrity in cancer
Several diseases may produce relevant changes in BBB, and transport of some specific substances can be impaired (Neuwelt 2004). The efficacy of BBB may decline with age, although it is not clearly demonstrated (Preston, 2001). The importance of P-gp was first recognized in cancer cells, where it is responsible for the development of resistance to cytotoxic agents. Brain tumors may affect BBB function due to changes in microvessels morphology with some discontinuation and development of fenestration (Schlageter et al, 1999). The degree of BBB integrity is variable in the different areas of the tumor, and these different permeabilities result in sharply reduced concentrations of chemotherapeutic agents (Siegal and Zylber-Katz, 2002). Moreover BBB integrity often recovers when the bulk of a tumor decrease with treatment. Finally, some glial tumors demonstrated increased levels of P-gp (Neuwelt, 2004).
In damaged brain tissue after traumatic injury, an increased BBB permeability to morphine has been reported in the penumbra zone surrounding a focal mass lesion and in apparently normal brain regions in patients with general brain swelling (Ederoth et al, 2003). Extravasation into the brain through the BBB may be influenced by weak inflammatory stimuli such as pain, or stronger stimuli such as infection and other immunological factors.
The BBB compromise and cell extravasation with modulation by cytokines and chemokines, are key events in the changes of BBB permeability. Corticosteroids and other therapeutic agents decrease the passage of immune cells across the BBB (Gasperini et al, 1998). Peripheral inflammatory hyperalgesia has been found to produce an up-regulation P-gp in BBB endothelial cells, restricting the passage of morphine across the BBB (Seelback et al, 2007). This information suggests that pathophysiological states, such as inflammation and infection can impact drug availability (McRae et al, 2003).
VI. Pain and BBB
Pain is a complex phenomenon involving endocrine and immunological responses. The immune response is characterized by the release of inflammatory mediators in response to a stimulus and is typified by increased vascular permeability, edema formation and leukocyte migration. Other kinins, prostaglandins, matrix metalloproteinases, cellular adhesion molecules, excitatory peptides and aminoacids, numerous cytochines, both excitatory and inhibitory, are involved in the immune response when microglia, neurons, astroglia, perivascular cells are activated. Levels of cytochine expression depend on the brain region. Chemokines recruit leukocytes to the site of inflammation playing inflammatory and homeostatic roles. Cytochines have also been shown to activate hypothalamic-pituitary-adrenal (HPA) axis producing a stress cascade response down to corticosteroids, which in turn modulate the expression of cytochines, chemiotaxis, and production of inflammatory mediators (Wolka et al, 2003). Inflammatory mediators have also been shown to alter receptor-mediated endocytic reuptake at the BBB. Cytochines appear to play a role in BBB disruption, increasing endothelial permeability due to loss of the tight junction proteins. Adhesions of leukocytes induced by chemokines, produce similar damage. Mast cell activation in response to hormonal stress also influences BBB permeability. One consequence of BBB disruption has been shown to be increased drug delivery to the brain. This altered delivery may be detrimental, resulting in unexpected toxicity, or beneficial, resulting in improved therapeutic outcome.
On the other hand, the ability of the brain to secrete active peptides into circulation may be a mechanism providing neurochemical links to peripheral sites. Evidence suggests that BBB may be unidirectional, and able to secrete a wide range of structurally diverse compounds from the brain to circulation. Some cytokines, for example, do not pass into the brain to an appreciable degree in normal condition when given intravenously. Yet, they are rapidly excreted from the brain to the blood (King et al, 2001).
VII. Opioid passage through BBB
It is well known that the doses of opioids needed for pain relief clinically vary between individuals. The response to opioids depends on several factors, including variable bioavailability, differences in pain mechanism, differences in pharmacodynamics at the μ-receptor, in drug metabolism, and genetics (Mercadante and Portenoy, 2001). Recent studies revealed that inherited differences in drug-metabolizing enzymes, opioid receptors, and drug-transporters, might affect the effectiveness of opioid drugs in individual patients (Klepstad et al, 2005). Although there are many factors that can influence the pharmacokinetics-pharmacodynamics of a drug, transport of opioids across the BBB could be an important step in permitting opioids to exert a centrally mediated analgesic effect. Many opioids have been identified as P-gp substrates with variable degrees (Graff and Pollack, 2004, Skorpen et al, 2008, Waldel et al, 2002, Kalvas et al, 2007).
Both efflux and uptake carrier systems have been implicated in the transport of opioid drugs. Transporter expression at the BBB has the potential to significantly influence the clinical efficacy and safety of opioids, whose major site of action lies within the CNS. The two major families of drug transporters of relevance to opioid pharmacokinetics are the ABC superfamily of efflux transporters, and the solute carrier (SLC) superfamily of influx transporters. P-gp expression at the BBB is of particular importance to opioids, regulating the access to their site of action and consequently affecting efficacy. Opioid analgesics given systematically have limited distribution into the brain because of their interaction with P-gp, with the brain uptake and the antinociceptive effects of opioids being dependent on P-gp function. Opioids activate P-gp ATPase, producing energy requisite for P-gp transport. The relative dependence of opioids of P-gp is an important determinant for opioid action. Among opioid analgesics there are some differences in substrate specificity for P-gp. Morphine and fentanyl have been found to activate P-gp ATPase in the brain capillary endothelial cell membranes and showed higher analgesic potencies in P-gp-deficient mice suggesting that their analgesic effects are considerably dependent on P-gp expression. In contrast meperidine did not show ant-activating effect of P-gp ATPase (Hamabe et al, 2006).
Studies have indicated that the presence of P-gp reduces both the magnitude and duration of analgesia produced by morphine, methadone, and fentanyl, whereas the analgesic efficacy of M6G and meperidine is less affected (Thompson et, 2002), suggesting that some opioids may be weak substrates for P-gp. However, in another experiment assessing the initial bran uptake, increased brain accumulation was demonstrated only for morphine. It can be expected that the effect under steady-state conditions would be more significant also for other opioids (Dagenais et al, 2004). Factors interacting with P-gp functioning may alter the response to opioids. Drugs that are not themselves substrates for P-gp but may inhibit P-gp are a potential source of important drug interactions, allowing the increased penetration into CNS of concomitant drugs. The inhibition of P-gp results in substantial alteration of drug tissue availability of concomitantly administered drugs, with unexpected more clinical effects, but also producing a potential to cause adverse effects of those with a narrow safety margin.
Loperamide, which has a prevalent peripheral effect, is a high affinity P-gp substrate that is effectively pumped out of the CNS that pharmacologically effective concentrations are not achieved in the brain, despite being a potent opioid (Wandel et al, 2002).
Morphine is an effective analgesic, which requires high plasma concentrations to penetrate to enter in small amount the BBB, due to its hydrophilicity. Data regarding modulation of P-gp modulation are contrasting. The active efflux of morphine across the BBB has been demonstrated, also producing a delay of the clinical effects (Mantione et al, 2005). But an acute inhibition of P-gp did not affect pharmacokinetics of morphine in volunteers (Drewe et al, 2000). Individual differences in morphine analgesia were negatively correlated with relative cortical P-gp expression levels and basal P-gp ATP-ase activity (Hamabe et al, 2005). This effect was not observed with high doses of morphine, possibly due to the saturation of morphine-P-gp coupling, leading to the increment of non-specific permeation across the BBB. Of interest, although morphine injection may increase P-gp ATP-ase activity, this returns to the basal level after elimination of substrate drug, suggesting that this mechanism is not relevant for developing antinociceptive tolerance (Al-Shavi et al, 2003). However, other studies supported that down regulation of P-gp enhanced both the potency and duration of action of systemic morphine, and blocked the development of tolerance (King et al, 2001). Induction of P-gp may be one mechanism involved in the development of morphine tolerance. Chronic morphine exposure appeared to induce P-gp in rat brain, enhancing the morphine efflux from the brain (Aquilante et al, 2000). Down-regulation of P-gp enhanced both the potency and duration of action of systemic morphine, and blocked the development of tolerance (King et al, 2001). On chronic administration the up-regulated P-gp would be expected to result in lower brain concentration of morphine and thereby exacerbating tolerance to the central analgesic effects (Mercier et al, 2007). It has also been suggested that P-gp transport system may provide a mechanism of communication between the CNS and the periphery throughout the secretion of peripherally acting peptides and hormones. P-gp active transport of morphine out of the brain into circulation could be crucial in providing a synergistic interaction between peripheral and central opioid systems (King et al, 2001). In MDR-1 knockout mice there was a two-fold increased net brain uptake of morphine, lending support to the notion that P-gp may affect the brain morphine disposition (Schinkel et al, 1996). Down regulating P-gp expression with antisense reduced the brain to blood transport of morphine and other opioids resulting in significantly enhanced systemic morphine analgesia. However, analgesic analgesia of centrally administered morphine decreased, suggesting that supraspinal analgesia depends on a combination of central and peripheral mechanisms activated by morphine transported from the brain to the blood. Similar effects were produced disrupting the MDR1 gene (King et al, 2001).
M6G is a metabolite of morphine that appears to have a greater analgesic potency than morphine. M6G has a higher potency when administered by intracerebroventricular route and produce a longer antinociceptive effect than morphine. It is highly hydrophilic and has BBB permeability 57 times lower than morphine. However the brain uptake rate is only 32 times lower suggesting that an active transport mechanism might exist (Mantione et al, 2005). Although efflux transporters act on M6G at the BBB, the probenecid-sensitive transporters seem to be not involved in the brain efflux, as the ratio was unaltered when probenecid was co-administered (Tumblad et al, 2005), and possibly is trapped by other transporters (Bourasset et al, 2006).
Chemical lipidization may increase brain penetration of morphine. A lipophilic drug with limited activity, able to penetrate the BBB, can be converted in an active substance within CNS, and get more polar to be effectively retained in the CNS. For example, in codeine, the hydroxyl groups of morphine are substituted, the molecule increases lipid solubility and increases brain uptake. Codeine is rapidly transported into the brain and quickly reached distribution equilibrium with the same unbound concentrations in blood and brain. Therefore the influx is identical to efflux, which suggests that only passive process participate in codeine BBB transport (Xie and ammarlund-Udenaes, 2005). Substituting acetyl groups to form dyacethylmorphine (heroin), it is possible to substantially increase CNS penetration, providing an excellent example of chemical manipulation. However, dyacethylmorphine is also rapidly metabolized back to the parent drug, and this form, after the typical flash clinically observed, prevalently interacts with opioid receptors. Morphine maintains CNS levels, as it cannot easily back diffuse across the BBB.
Drugs with high octanol-water partition ratios, like fentanyl, tend to have high tissue/blood partition ratios. This is evidence for both inward and outward vectors of transport relative to the microvascular lumen. At the BBB, P-gp actively pumps a variety of lipophilic drugs away from the brain tissue, thus reducing the tissue/blood partition ratio that would exist by passive diffusion alone. The active p-gp-mediated extrusion of fentanyl, however, is overshadowed by an active inward transport process, mediated by an unidentified transporter. Moreover studies also showed that fentanyl may act as P-gp inhibitor (Hentorn et al, 1999).
Despite a lower affinity to opioid receptor, oxycodone and morphine have a similar potency. This observation is explained by the finding that oxycodone is actively influxed at the BBB, in comparison with morphine and its metabolites, and codeine. It has been shown that the influx clearance is 3-fold greater than the efflux clearance, resulting in a higher concentration for oxycodone than could be anticipated from plasma concentrations (Bolstrom et al, 2006). The concentration of unbound oxycodone in brain ISF was six times higher than that of morphine for the same unbound plasma concentrations, meaning that the rate of transport of oxycodone is very high compared with morphine (Bolstrom et al, 2008). This is probably due to transport proteins with saturable mechanisms (Bolstrom et al, 2006). These transporters are energy-dependent, proton-coupled antiporter (Okura et al, 2008).
Methadone is characterized by the large variability in response, which has been related to the complex pharmacodynamics and pharmacokinetics, or activity of P-gp. Methadone has been shown to be a substrate of P-gp, suggesting a possible role of this efflux protein in methadoneÕs individual variability. P-gp in BBB greatly limits the brain entry of (R) and (S) methadone to their central nervous system acting sites (Wang et al, 2004). The polymorphic expression of P-gp in BBB may represent a source of variation for the access and effects of methadone in the brain. Oral methadone potency was increased three fold in patients treated with a P-gp-inhibitor, valspodar (Rodriguez et al, 2004). Methadone dosage requirement has been reported to be influenced by ABCB1 haplotypes in opioid dependent subjects in methadone maintenance treatment (Persidsky et al, 2006). This finding was not confirmed in a similar group of patients (Coller et al, 2006), possibly for the different dose used, which saturated P-gp (Crettol et al, xxxx).
P-gp was found to mediate brain to blood efflux transport of buprenorphine across the BBB, at least in part. The use of inhibitors, such as cyclosporine A, quinidine, and verapamil, enhanced uptake of buprenorphine by 1.5-fold (Suzuki et al, 2007).
The potential exists for drug interactions to result during chronic opioid therapy because of P-gp inhibition for other drugs. P-gp substrates may competitively inhibit P-gp, resulting in an increased uptake of opioids into brain. For example, cyclosporine and verapamil are both inhibitors and substrates for P-gp, digoxin, loperamide, vincristine, and dexamathasone are substrates for P-gp, whereas quinidine and ketoconazole are inhibitors but not substrates for P-gp (Waldel et al, 2002). While P-gp-inducers such as rifampicin and St JohnÕs wort cause a substantial increase in withdrawal symptoms (Coller et al, 2006), P-gp-inhibitors verapamil, quinidine, and probenecid have been shown to reduce the efflux clearance of morphine (King et al, 2001, Tumblad et al, 2005). Valspodar, an analogue of cyclosporine D, has been developed for its high potency to reverse the resistance to chemotherapy of cancer cells by inhibiting P-gp. However, in humans, where direct measurements of changes in brain disposition of morphine and M6G were not possible, and relied on peripheral blood measurements, the coadministration of valspodar with morphine did not significantly affect the pharmacokinetic and pharmacodynamic profile of morphine (Drewe et al, 2000). While most inhibitors are not particularly potent at clinically concentrations, some drugs, such as corticosteroids may reach concentrations that are high enough to affect the distribution and clearance of opioids, such as morphine, methadone, and fentanyl (Thompson et al, 2000).
P-gp-inhibition as a way of increasing the concentrations of drugs in the brain and thus increasing the central effects was tested in studies using loperamide. Loperamide, an opioid presumed to not producing central effects, elicits potent centrally mediated opioid-like effects and evidences increased brain accumulation in P-gp-deficient mice (Xie and Hammarlund-Udenaes, 1998). When administered with quinidine, a known relatively selective P-gp-inhibitor, loperamide produced respiratory depression, independently from the plasma concentration of the drug (Begley, 2004b). Moreover, in P-gp knockout mice, doses of loperamide that are normally without effect in wild mice were lethal (Schinkel et al, 1996).
VIII. P-gp pharmacogenomics and opioids
Many studies have increased our knowledge and understanding of how pharmacogenetics can influence the opioid response and contribute to interpatient variability. However, the importance of pharmacogenomics at present is at the level of explaining variability in drug response and toxicity, and not necessarily its immediate translation into clinical practice (Skorpen et al, 2008). Drug transport is often the result of the concerted action of efflux and uptake pumps located both in the basolateral and apical membranes of epithelial cells. Polymorphisms affecting drug transporters are of interest for opioid analgesia, as altered function or expression of transporters might cause differences in the extracellular brain concentrations of opioids. Although there are many transporters that theoretically could be involved in transport of opioids, most studies have focused on P-gp.
P-gp functions as a transmembrane efflux pump that translocates its substrates from its intracellular domain to its extracellular domain. Given such an important role of P-gp in the drug disposition process, it is not surprising to see increasing focus on the role of polymorphisms in this transporter as a potential determinant of interindividual variability in pharmacological response. Differences in opioid potencies may also be due to structure-activity relationship (Mercier et al, 2007).
The ABCB1 gene encoding P-gp is highly polymorphic, with more than 50 single nucleotide polymorphisms, and possesses the potential to affect the expression and function of the transporter. Genetic or chemical disruption of P-gp has been shown to enhance the antinociceptive and toxic effects of some opioids, although the extent of this phenomenon has yet to be better understood. Genetic polymorphisms of P-gp may affect opioid pharmacokinetics by changing the levels of P-gp expression. Many polymorphisms have been identified in the ABCB1 gene and several have been associated with differences in protein expression and function (Skorpen et al, 2008).
Animal and human studies have produced conflicting results regarding the functional consequences of polymorphisms in terms of opioid activity, although there is some evidence that the brain distribution of morphine, which is transported by these systems with less efficiency than loperamide, may be affected by 3435 genotype. This polymorphism did not influence the pharmacodynamic effects of methadone (Lotsch et al, 2006). No association between morphine dose requirements and polymorphisms 2677G/A or 3435C>T was found (Coulbault et al, 2006). However, highly significant association between variability of pain relief and genotypes of the 3435C>T polymorphism was recently found (Campa et al, 2008). The effect of haplotypes, meaning significant linkage disequilibrium across ABCB1 gene, is more likely to predict P-gp expression and function. For example, employing ABCB1 haplotypes, subjects for the wild-type haplotypes required significant higher dose of methadone than heterozygous or non-carriers, and carriers of AGCTT haplotype required lower doses compared with non-carriers (Somogyi et al, 2007). Similarly, the ABCB1 GG2677/CC3435 diplotype has been found to predict morphine adverse effects than either of the two analyzed separately (Coulbault et al, 2006).
Genotypes susceptible to fentanyl were associated with early decrease of respiration rate as compared with resistant genotypes (Park et al, 2006). Adverse effects were not correlated to 3435C>T genotype in morphine-treated cancer patients (Campa et al, 2008).
The hypothetical effects of two polymorphic genes, one involved in pharmacokinetics (ABCB1, MDR1, encoding P-gp), and another involved in pharmacodynamics (OPRM1, encoding for μ-opioid receptor), have been evaluated. Pain relief variability was associated with both polymorphism, the single-nucleotide polymorphism C3435T of ABCB1 and A80G of OPRM1, and combining the extreme genotypes of both genes, the association between patient polymorphism and pain relief improved (Campa et al, 2008). The commonest adverse effects that limit dose titration are drowsiness and confusion. The occurrence of these central adverse effects is the main reason to switch from one opioid to another. In a study of markers of the need of opioid switching in cancer patients with pain who poorly responded to morphine, genetic variation in MDR-1 has been found to be independently associated with moderate-severe drowsiness and confusion or hallucinations, with G2677T/A showing the strongest association. Of interest, no differences were observed in serum morphine levels between genotypes, reflecting the poor correlation between serum and CNS levels f morphine (Ross et al, 2008).
The discrepancies among the results of the various studies of ABCB1 genetic polymorphisms or haplotypes on the function of P-gp variants include small study population, lack of standardization for the determination of P-gp expression, interethnic differences, and unknown roles of other transporters (Fromm, 2004).
Of the multidrug resistance-associated proteins in the ABCC family, ABCC1, ABCC2, and ABCC3 are the most likely to be involved in opioid transport and a number of glucuronide conjugates (Somogyi et al, 2007). Several polymorphisms have been identified, but functional consequences to the transport of opioids have not been determined yet.
The organic anion transporting popypeptides (SLCO) family consists of different isoforms. SLCO1A2 is predominantly expressed in the brain capillary endothelial cells, differently from SLCO1B3, which is almost expressed in the liver. The identification of functionally relevant genetic polymorphisms warrants further investigation (Somogyi et al, 2007).
Different models and experimental conditions have been used, including inhibition of binding or transport of another substrate, use of non-specific inhibitors, antinociceptive response without concurrent measurement of brain accumulation, making it difficult to assess quantitatively the interaction of opioids with P-gp. Moreover, other factors, such as plasma protein binding, affinity for brain tissue, egress component, other peripheral sites, for example intestinal, liver, and kidney P-gp, different metabolic genotypes, pharmacological interactions, receptor polymorphisms, may also minimize the influence of BBB P-gp functioning, which should be considered as a part of the complex framework determining the final response to opioids in individuals.
The polymorphisms that affect the response to opioids are likely to be complex and multigenetic, with a number of alleles each making a modest contributes to the overall phenotype. In addition, environmental and behavioural factors combine with pharmacokinetic and pharmacodynamic factors. This information may help in the development of an effective strategy for customization of opioid therapy.
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