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      Peripheral Opioid Receptor Antagonists for Opioid-Induced Constipation: A Primer on Pharmacokinetic Variabilities with a Focus on Drug Interactions

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          Opioid analgesics remain a treatment option for refractory acute and chronic pain, despite their potential risk for abuse and adverse events (AEs). Opioids are associated with several common AEs, but the most bothersome is opioid-induced constipation (OIC). OIC is often overlooked but has the potential to affect patient quality of life, increase associated symptom burden, and impede long-term opioid compliance. The peripherally acting µ-receptor antagonists (PAMORAs) are a class of drugs that include methylnaltrexone, naloxegol, and naldemedine. Collectively, each is approved for the treatment of OIC. PAMORAs work peripherally in the gastrointestinal tract, without impacting the central analgesic effects of opioids. However, each has unique pharmacokinetic properties that may be impacted by coadministered drugs or food. This review focuses on important metabolic and pharmacokinetic principals that are pertinent to drug interactions involving µ-opioid receptor antagonists prescribed for OIC. It highlights subtle differences among the PAMORAs that may have clinical significance. For example, unlike naloxegol or naldemedine, methylnaltrexone is not a substrate for CYP3A4 or p-glycoprotein; therefore, its plasma concentration is not altered when coadministered with concomitant medications that are CYP3A4 or p-glycoprotein inducers or inhibitors. With a better understanding of pharmacokinetic nuances of each PAMORA, clinicians will be better equipped to identify potential safety and efficacy considerations that may arise when PAMORAs are coadministered with other medications.

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          Most cited references 48

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          Drugs behave as substrates, inhibitors and inducers of human cytochrome P450 3A4.

           Feng Zhou (2008)
          Human cytochrome P450 (CYP) 3A4 is the most abundant hepatic and intestinal phase I enzyme that metabolizes approximately 50% marketed drugs. The crystal structure of bound and unbound CYP3A4 has been recently constructed, and a small active site and a peripheral binding site are identified. A recent study indicates that CYP3A4 undergoes dramatic conformational changes upon binding to ketoconazole or erythromycin with a differential but substantial (>80%) increase in the active site volume, providing a structural basis for ligand promiscuity of CYP3A4. A number of important drugs have been identified as substrates, inducers and/or inhibitors of CYP3A4. The ability of drugs to act as inducers, inhibitors, or substrates for CYP3A is predictive of whether concurrent administration of these compounds with a known CYP3A substrate might lead to altered drug disposition, efficacy or toxicity. The substrates of CYP3A4 considerably overlap with those of P-glycoprotein (P-gp). To date, the identified clinically important CYP3A4 inhibitors mainly include macrolide antibiotics (e.g., clarithromycin, and erythromycin), anti-HIV agents (e.g., ritonavir and delavirdine), antidepressants (e.g. fluoxetine and fluvoxamine), calcium channel blockers (e.g. verapamil and diltiazem), steroids and their modulators (e.g., gestodene and mifepristone), and several herbal and dietary components. Many of these drugs are also mechanism-based inhibitors of CYP3A4, which involves formation of reactive metabolites, binding to CYP3A4 and irreversible enzyme inactivation. A small number of drugs such as rifampin, phenytoin and ritonavir are identified as inducers of CYP3A4. The orphan nuclear receptor, pregnane X receptor (PXR), have been found to play a critical role in the induction of CYP3A4. The inhibition or induction of CYP3A4 by drugs often causes unfavorable and long-lasting drug-drug interactions and probably fatal toxicity, depending on many factors associated with the enzyme, drugs and the patients. The study of interactions of newly synthesized compounds with CYP3A4 has been incorporated into drug development and detection of possible CYP3A4 inhibitors and inducers during the early stages of drug development is critical in preventing potential drug-drug interactions and side effects. Clinicians are encouraged to have a sound knowledge on drugs that behave as substrates, inhibitors or inducers of CYP3A4, and take proper cautions and close monitoring for potential drug interactions when using drugs that are CYP3A4 inhibitors or inducers.
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            Role of P-glycoprotein in pharmacokinetics: clinical implications.

            P-glycoprotein, the most extensively studied ATP-binding cassette (ABC) transporter, functions as a biological barrier by extruding toxins and xenobiotics out of cells. In vitro and in vivo studies have demonstrated that P-glycoprotein plays a significant role in drug absorption and disposition. Because of its localisation, P-glycoprotein appears to have a greater impact on limiting cellular uptake of drugs from blood circulation into brain and from intestinal lumen into epithelial cells than on enhancing the excretion of drugs out of hepatocytes and renal tubules into the adjacent luminal space. However, the relative contribution of intestinal P-glycoprotein to overall drug absorption is unlikely to be quantitatively important unless a very small oral dose is given, or the dissolution and diffusion rates of the drug are very slow. This is because P-glycoprotein transport activity becomes saturated by high concentrations of drug in the intestinal lumen. Because of its importance in pharmacokinetics, P-glycoprotein transport screening has been incorporated into the drug discovery process, aided by the availability of transgenic mdr knockout mice and in vitro cell systems. When applying in vitro and in vivo screening models to study P-glycoprotein function, there are two fundamental questions: (i) can in vitro data be accurately extrapolated to the in vivo situation; and (ii) can animal data be directly scaled up to humans? Current information from our laboratory suggests that in vivo P-glycoprotein activity for a given drug can be extrapolated reasonably well from in vitro data. On the other hand, there are significant species differences in P-glycoprotein transport activity between humans and animals, and the species differences appear to be substrate-dependent. Inhibition and induction of P-glycoprotein have been reported as the causes of drug-drug interactions. The potential risk of P-glycoprotein-mediated drug interactions may be greatly underestimated if only plasma concentration is monitored. From animal studies, it is clear that P-glycoprotein inhibition always has a much greater impact on tissue distribution, particularly with regard to the brain, than on plasma concentrations. Therefore, the potential risk of P-glycoprotein-mediated drug interactions should be assessed carefully. Because of overlapping substrate specificity between cytochrome P450 (CYP) 3A4 and P-glycoprotein, and because of similarities in P-glycoprotein and CYP3A4 inhibitors and inducers, many drug interactions involve both P-glycoprotein and CYP3A4. Unless the relative contribution of P-glycoprotein and CYP3A4 to drug interactions can be quantitatively estimated, care should be taken when exploring the underlying mechanism of such interactions.
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              Methylnaltrexone for opioid-induced constipation in advanced illness.

              Constipation is a distressing side effect of opioid treatment. As a quaternary amine, methylnaltrexone, a mu-opioid-receptor antagonist, has restricted ability to cross the blood-brain barrier. We investigated the safety and efficacy of subcutaneous methylnaltrexone for treating opioid-induced constipation in patients with advanced illness. A total of 133 patients who had received opioids for 2 or more weeks and who had received stable doses of opioids and laxatives for 3 or more days without relief of opioid-induced constipation were randomly assigned to receive subcutaneous methylnaltrexone (at a dose of 0.15 mg per kilogram of body weight) or placebo every other day for 2 weeks. Coprimary outcomes were laxation (defecation) within 4 hours after the first dose of the study drug and laxation within 4 hours after two or more of the first four doses. Patients who completed this phase were eligible to enter a 3-month, open-label extension trial. In the methylnaltrexone group, 48% of patients had laxation within 4 hours after the first study dose, as compared with 15% in the placebo group, and 52% had laxation without the use of a rescue laxative within 4 hours after two or more of the first four doses, as compared with 8% in the placebo group (P<0.001 for both comparisons). The response rate remained consistent throughout the extension trial. The median time to laxation was significantly shorter in the methylnaltrexone group than in the placebo group. Evidence of withdrawal mediated by central nervous system opioid receptors or changes in pain scores was not observed. Abdominal pain and flatulence were the most common adverse events. Subcutaneous methylnaltrexone rapidly induced laxation in patients with advanced illness and opioid-induced constipation. Treatment did not appear to affect central analgesia or precipitate opioid withdrawal. (Clinical Trials.gov number, NCT00402038 [ClinicalTrials.gov].). Copyright 2008 Massachusetts Medical Society.

                Author and article information

                J Pain Res
                J Pain Res
                Journal of Pain Research
                25 February 2020
                : 13
                : 447-456
                [1 ]Department of Anesthesiology, Rutgers New Jersey Medical School , Newark, NJ, USA
                [2 ]Department of Anesthesiology, Englewood Hospital and Medical Center , Englewood, NJ, USA
                [3 ]Albany College of Pharmacy and Health Sciences , Albany, NY, USA
                [4 ]Western New England University College of Pharmacy , Springfield, MA, USA
                [5 ]Remitigate, LLC , Delmar, NY, USA
                [6 ]Stratton Veterans Affairs Medical Center , Albany, NY, USA
                Author notes
                Correspondence: Jeffrey Gudin Department of Anesthesiology, Englewood Hospital and Medical Center , 350 Engle Street, Englewood, NJ07631, USA Email jeff@paindr.com
                © 2020 Gudin and Fudin.

                This work is published and licensed by Dove Medical Press Limited. The full terms of this license are available at https://www.dovepress.com/terms.php and incorporate the Creative Commons Attribution – Non Commercial (unported, v3.0) License ( http://creativecommons.org/licenses/by-nc/3.0/). By accessing the work you hereby accept the Terms. Non-commercial uses of the work are permitted without any further permission from Dove Medical Press Limited, provided the work is properly attributed. For permission for commercial use of this work, please see paragraphs 4.2 and 5 of our Terms ( https://www.dovepress.com/terms.php).

                Page count
                Figures: 2, Tables: 3, References: 59, Pages: 10


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