Role of central versus peripheral opioid receptors in analgesia induced by repeated administration of opioid antagonists

Discovery of EST73502, a Dual μ-Opioid Receptor Agonist and σ1 Receptor Antagonist Clinical Candidate for the Treatment of Pain

The synthesis and pharmacological activity of a new series of 4-alkyl-1-oxa-4,9-diazaspiro[5.5]undecane derivatives as potent dual ligands for the σ1 receptor (σ1R) and the μ-opioid receptor (MOR) are reported. A lead optimization program over the initial 4-aryl analogues provided 4-alkyl derivatives with the desired functionality and good selectivity and ADME profiles. Compound 14u (EST73502) showed MOR agonism and σ1R antagonism and a potent analgesic activity, comparable to the MOR agonist oxycodone in animal models of acute and chronic pain after single and repeated administration. Contrary to oxycodone, 14u produces analgesic activity with reduced opioid-induced relevant adverse events, like intestinal transit inhibition and naloxone-precipitated behavioral signs of opiate withdrawal. These results provide evidence that dual MOR agonism and σ1R antagonism may be a useful strategy for obtaining potent and safer analgesics and were the basis for the selection of 14u as a clinical candidate for the treatment of pain.

Management of opioid-induced constipation in cancer patients: focus on methylnaltrexone

Opioids are one of the most widely used therapies for the palliative treatment of cancer pain; however, despite their proven analgesic efficacy, they are associated with several adverse effects. Associated with psychological distress and multiple concomitant clinical concerns, constipation is the most commonly occurring adverse effect of chronic opioid therapy in cancer patients. Whilst prophylaxis remains the first-line management option, methylnaltrexone is a recommended treatment option for opioid-related constipation if administration of laxatives is ineffective. Due to its inability to cross the blood-brain barrier, methylnaltrexone exerts a peripheral inhibition of opioid-related effects without influencing the opioid-induced central effects; as a result, the analgesic effect of opioids is unaffected. Moreover, multiple clinical trials, albeit not always conducted specifically in cancer patients, have demonstrated that up to 4 months' treatment with either intravenous or subcutaneous methylnaltrexone provides effective relief from opioid-related constipation and is well tolerated. Preliminary evidence indicates that the addition of methylnaltrexone to standard care for opioid-related constipation may also be advantageous from a pharmacoeconomic perspective. In addition, preliminary data suggest that methylnaltrexone could be associated with some further clinical benefits other than the treatment of opioid-related constipation, such as the improvement of gastric emptying, the relief of nausea/vomiting, and the reduction of the risk of regurgitation and pulmonary aspiration. This narrative review examines the most recent evidence and evaluates the current role of methylnaltrexone in the management of opioid-related constipation, and its potential efficacy in cancer patients. The pharmacokinetics, pharmacodynamics, efficacy and tolerability of methylnaltrexone are discussed.

Methylnaltrexone

Methylnaltrexone is a selective mu-opioid receptor antagonist that has restricted ability to cross the blood-brain barrier, thus enabling reversal of opioid-induced peripheral effects, such as constipation, without affecting the central effects, such as pain relief. Treatment with subcutaneous methylnaltrexone 0.15-0.30 mg/kg, relative to placebo, significantly increased the rescue-free laxation response rate within 4 hours of the first dose (primary endpoint) in adult patients with opioid-induced constipation and advanced illness in two randomized, double-blind, placebo-controlled, multicentre, phase III studies; one was a single-dose study (n = 154), the other a multiple-dose study (n = 133). In the multiple-dose study, rescue-free laxation response rates within 4 hours after at least two of the first four doses (coprimary endpoint) were also significantly higher in methylnaltrexone recipients than in placebo recipients. Moreover, median time to laxation after the first dose was significantly shorter in methylnaltrexone recipients than in placebo recipients in both studies. Methylnaltrexone was not associated with any significant changes in pain scores or central opioid withdrawal in these studies. Methylnaltrexone was generally well tolerated in clinical trials; most adverse events were of mild to moderate severity.

Methylnaltrexone in the treatment of opioid-induced constipation

Constipation is a significant problem related to opioid medications used to manage pain. This review attempts to outline the latest findings related to the therapeutic usefulness of a μ opioid receptor antagonist, methylnaltrexone in the treatment of opioid-induced constipation. The review highlights methylnaltrexone bromide (Relistor™; Progenics/Wyeth) a quaternary derivative of naltrexone, which was recently approved in the United States, Europe and Canada. The Food and Drug Administration in the United States approved a subcutaneous injection for the treatment of opioid bowel dysfunction in patients with advanced illness who are receiving palliative care and when laxative therapy has been insufficient. Methylnaltrexone is a peripherally restricted, μ opioid receptor antagonist that accelerates oral-cecal transit in patients with opioid-induced constipation without reversing the analgesic effects of morphine or inducing symptoms of opioid withdrawal. An analysis of the mechanism of action and the potential benefits of using methylnaltrexone is based on data from published basic research and recent clinical studies.

Methylnaltrexone mechanisms of action and effects on opioid bowel dysfunction and other opioid adverse effects

OBJECTIVE

To review the mechanisms of action of methylnaltrexone and its effects on opioid bowel dysfunction, as well as its effects on other opioid-induced adverse effects (ADEs), and its potential roles in clinical practice.

DATA SOURCES

A literature search using the MEDLINE and Cochrane Collaboration databases for articles published between 1966 and March 2007 was performed. Additional data sources were obtained from manual searches of recent journal articles, book chapters, and monographs. An updated literature search showed no additional publications.

STUDY SELECTION AND DATA EXTRACTION

Abstracts and original preclinical and clinical research reports published in the English language were identified for review. Review articles, commentaries, and news reports of this compound were excluded. Literature related to opioids, opioid receptors, opioid antagonists, methylnaltrexone, opioid-induced bowel dysfunction, constipation, nausea, and vomiting was evaluated and selected based on consideration of the support shown for the proof of concept, mechanistic findings, and timeliness. Fifty-eight original articles from preclinical studies and clinical trials using methylnaltrexone were identified. Pharmacologic action, benefits, and ADEs of methylnaltrexone were reviewed, with a focus on its effects on bowel dysfunction after opioids. Emphases were placed on its receptor binding activities and therapeutically relevant sites of action (peripheral vs central), in which peripheral opioid receptors in the body contribute to physiological and drug-induced effects.

DATA SYNTHESIS

Morphine and related opioids are associated with a number of limiting ADEs, including opioid-induced bowel dysfunction. Methylnaltrexone, a quaternary derivative of naltrexone, blocks peripheral effects of opioids while sparing central analgesic effects. It is currently under late-stage clinical investigation for the treatment of opioid-induced constipation in patients with advanced illness. Reported results showed the drug to be generally well-tolerated. The rapid reversal of constipation is very encouraging. Hastening postoperative discharge may also be possible.

CONCLUSIONS

Methylnaltrexone has the potential to prevent or treat opioid-induced peripherally mediated ADEs on bowel dysfunction without interfering with central analgesia. The study of methylnaltrexone leads to a greater understanding of the mechanisms of action of opioid pharmacology.

Methylnaltrexone, a novel peripheral opioid receptor antagonist for the treatment of opioid side effects

Methylnaltrexone is an investigational peripheral opioid receptor antagonist, a quaternary derivative of naltrexone. Methylnaltrexone has greater polarity and lower lipid solubility, thus it does not cross the blood-brain barrier in humans. Methylnaltrexone offers the therapeutic potential to block or reverse the undesired side effects of opioids that are mediated by receptors located in the periphery (e.g., in the gastrointestinal tract), without affecting analgesia or precipitating the opioid withdrawal symptoms that are predominantly mediated by receptors in the CNS. This article reviews preclinical studies and clinical opioid bowel dysfunction trial data, and briefly discusses other potential roles of this compound in clinical practice.

Effects of acute selective 5-HT1, 5-HT2, 5-HT3 receptor and alpha 2 adrenoceptor blockade on naloxone-induced antinociception

Several studies have demonstrated a paradoxical form of antinociception induced by the repeated administration of opioid antagonists accompanied by exposure to a painful stimulus. The underlying mechanism of this naloxone-induced antinociception (NIA) is still unknown, but the results of several studies suggest that it is a non-opioid response. This study was designed to investigate serotonergic and noradrenergic involvement in NIA. Rats were treated daily with systemic injections of 5 mg/kg naloxone, followed by a 45-s hot plate test of nociception (temperature = 51.5 +/- 0.5 degree C). After rats reached plateau levels of NIA, they received a test trial in which they were treated with various doses of different selective 5-HT or alpha 2 adrenoceptor antagonists in addition to naloxone before the hot plate test. Rats treated with 0.16, 0.32 and 0.63 mg/kg pirenperone or 2.5 mg/kg ritanserin showed significant reductions in paw lick latency with respect to rats treated with vehicle. In addition, high doses of yohimbine (7.5-10 mg/kg) also effectively reversed NIA. In contrast, NIA was not affected by acute blockade of 5-HT1 or 5-HT3 receptors by methiothepin or MDL 72222, respectively, or by the alpha 2 adrenoceptor blocker idazoxan. None of the 5-HT or alpha 2 adrenoceptor antagonists had any effect on the paw lick latencies of saline-treated rats. A possible role of 5-HT2 receptors in the antinociception induced by opioid receptor blockade is discussed.

Naloxone-induced hypoalgesia: evidence from the formalin test

Two experiments examined the hypoalgesic effects that accrue from pairing exposure to a heat stressor with the opioid antagonist naloxone. Experiment 1 confirmed that rats given separate exposures to a heated floor and to naloxone are hypoalgesic when then tested on that floor. Further, the results confirmed that pairing the initial exposure to the heat stressor with naloxone resulted in a more profound hypoalgesic response. Experiment 2 provided new evidence for the hypoalgesic effects of separate exposures to the heat stressor and to naloxone, and for naloxone's enhancement of acquisition of the hypoalgesia, in rats tested for responsiveness to formalin. These results, therefore, demonstrate generalisability of the hypoalgesic effects across assays for acute and chronic pain.

Narcotic receptor blockade and its effect on the analgesic response to placebo and ibuprofen after oral surgery

The purpose of this study was to evaluate the contribution of endogenous opiates to the analgesic response after treatment with placebo, codeine, and ibuprofen after oral surgery. Eighty-one patients undergoing complicated dental extractions were pretreated with either a placebo or the narcotic antagonist naltrexone 50 mg, 30 minutes before surgery. After surgery, patients self administered one of three possible postsurgical medications, which included placebo, codeine 60 mg, and ibuprofen 400 mg, when their pain reached a moderate or severe intensity. The study was double-blind with the three postsurgical treatments being randomly allocated within each presurgical treatment block. Pain intensity, pain relief, pain half gone, and overall evaluations were assessed for up to 6 hours. Ibuprofen was significantly more efficacious (p < .05) than codeine or placebo for most analgesic measures. The administration of naltrexone before surgery reduced the analgesic response to both placebo and codeine. Pretreatment with naltrexone did not diminish the peak analgesic response to ibuprofen, but surprisingly prolonged (p < .05) the duration of its action. The results suggest that a blockade of endogenous opiates by naltrexone diminished the placebo response, but that naltrexone may prolong ibuprofen analgesia by some unknown mechanism.

Effects of naltrindole and nor-binaltorphimine treatment on antinociception induced by sub-acute selective mu opioid receptor blockade

When administered repeatedly, in conjunction with hot plate testing, naloxone and naltrexone have the paradoxical effect of producing antinociception in rats and mice. Recently, we have found that the sub-acute selective blockade of mu opioid receptors leads to the development of antinociception and an augmentation of kappa receptor-mediated antinociception. In this study, acute delta/kappa antagonist treatment produced a significant decrease in paw lick latency in rats displaying antinociception induced by sub-acute mu blockade, however, the response level of these animals was still significantly above the baseline. In addition, rats receiving sub-acute combined mu and delta antagonist treatment took longer to develop an antinociceptive response than those treated with a mu antagonist alone. Sub-acute selective blockade of kappa or delta opioid receptors had no overall effect on paw lick latency during the course of 5 days of hot plate testing. The results indicate that delta receptor activity may play a role in the antinociception induced by sub-acute mu blockade. However, while delta antagonist treatment effected the expression, it did not completely attenuate the antinociception induced by sub-acute mu blockade suggesting that there is still a significant non-opioid component to this analgesic response. The results of a final experiment, in which acute delta antagonist treatment had no effect on antinociception induced by repeated systemic injections of naloxone, supported this hypothesis.