| DrugBank Description: |
Methadone is a potent synthetic analgesic that works as a full µ-opioid receptor (MOR) agonist and N-methyl-d-aspartate (NMDA) receptor antagonist. As a full MOR agonist, methadone mimics the natural effects of the body's opioids, endorphins, and enkephalins through the release of neurotransmitters involved in pain transmission. It also has a number of unique characteristics that have led to its increased use in the last two decades; in particular, methadone has a lower risk of neuropsychiatric toxicity compared to other opioids (due to a lack of active metabolites), minimal accumulation in renal failure, good bioavailability, low cost, and a long duration of action.
Due to its unique mechanism of action, methadone is particularly useful for the management of hard to treat pain syndromes such as neuropathic pain and cancer pain requiring higher and more frequent doses of shorter-acting opioids. Compared with , the gold standard reference opioid, methadone also acts as an agonist of κ- and σ-opioid receptors, as an antagonist of the N-methyl-D-aspartate (NMDA) receptor, and as an inhibitor of serotonin and norepinephrine uptake. Specifically by inhibiting the NMDA receptor, methadone dampens a major excitatory pain pathway within the central nervous system. Compared to other opioids, methadone's effects on NMDA inhibition may explain it's improved analgesic efficacy and reduced opioid tolerance.
Methadone shares similar effects and risks of other opioids such as , , , and . However, it also has a unique pharmacokinetic profile. Compared with short-acting and even extended-release formulations of , methadone displays a comparatively longer duration of action and half-life. These effects make methadone a good option for the treatment of severe pain and addiction as fewer doses are needed to maintain analgesia and prevent opioid withdrawal symptoms. However, methadone also has an unpredictable half-life with interindividual variability, which leads to an unpredictable risk of respiratory depression and overdose when initiating or titrating therapy.
Overall, methadone's pharmacological actions result in analgesia, suppression of opioid withdrawal symptoms, sedation, miosis, sweating, hypotension, bradycardia, nausea and vomiting (via binding within the chemoreceptor trigger zone), and constipation. At higher doses, methadone use can result in respiratory depression, overdose, and death.
Treatment of opioid addiction with methadone, , or slow-release oral (SROM) is termed Opioid Agonist Treatment (OAT) or Opioid Substitution Therapy (OST). The intention of substitution of illicit opioids with the long-acting opioids used in OAT is to prevent withdrawal symptoms for 24-36 hours following dosing to ultimately reduce cravings and drug-seeking behaviours. Use of OAT is also intended to lead to social stabilization by reducing crime rates, incarceration, use of illicit opioids such as or , and ultimately marginalization. Illegally purchased opioids present many other harms in addition to overdose as they can be injected and may be laced with other substances that increase the risk of harm or overdose. Provision of OAT is often combined with education about harm reduction including use of clean needles and injection supplies in an effort to reduce the risks associated with injection drug use such as contraction of HIV and Hepatitis C and other complications including skin infections, abscesses, or endocarditis.
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| DrugBank Pharmacology: |
Overall, methadone's pharmacological actions result in analgesia, suppression of opioid withdrawal symptoms, sedation, miosis (through binding to receptors in the pupillary muscles), sweating, hypotension, bradycardia, nausea and vomiting (via binding within the chemoreceptor trigger zone), and constipation. Like many basic drugs, methadone also enters mast cells and releases histamine by a non-immunological mechanism leading to flushing, pruritus, and urticaria, which can commonly be misattributed to an allergic reaction.
Compared to other opioids, methadone has fewer active metabolites and therefore a lower risk of neuropsychiatric toxicity. This means that higher doses needed to manage severe pain or addiction are less likely to result in delirium, hyperalgesia, or seizures.
Similar to morphine, both methadone isomers are 5-HT(3) receptor antagonists, although l-methadone produces greater inhibition than d-methadone.
Methadone's effects are reversible by naloxone with a pA2 value similar to its antagonism of morphine.
**Dependence and Tolerance**
As with other opioids, tolerance and physical dependence may develop upon repeated administration of methadone and there is a potential for development of psychological dependence. Physical dependence and tolerance reflect the neuroadaptation of the opioid receptors to chronic exposure to an opioid and are separate and distinct from abuse and addiction. Tolerance, as well as physical dependence, may develop upon repeated administration of opioids, and are not by themselves evidence of an addictive disorder or abuse.
Patients on prolonged therapy should be tapered gradually from the drug if it is no longer required for pain control. Withdrawal symptoms may occur following abrupt discontinuation of therapy or upon administration of an opioid antagonist. Some of the symptoms that may be associated with abrupt withdrawal of an opioid analgesic include body aches, diarrhea, gooseflesh, loss of appetite, nausea, nervousness or restlessness, anxiety, runny nose, sneezing, tremors or shivering, stomach cramps, tachycardia, trouble with sleeping, unusual increase in sweating, palpitations, unexplained fever, weakness and yawning.
**Cardiac Conduction Effects**
Laboratory studies, both in vivo and in vitro, have demonstrated that methadone inhibits cardiac potassium channels and prolongs the QT interval. Cases of QT interval prolongation and serious arrhythmia (torsades de pointes) have been observed during treatment with methadone. These cases appear to be more commonly associated with, but not limited to, higher dose treatment (> 200 mg/day). Methadone should be administered with particular caution to patients already at risk for development of prolonged QT interval (e.g., cardiac hypertrophy, concomitant diuretic use, hypokalemia, hypomagnesemia). Careful monitoring is recommended when using methadone in patients with a history of cardiac conduction disease, those taking medications affecting cardiac conduction, and in other cases where history or physical exam suggest an increased risk of dysrhythmia.
**Respiratory Depression and Overdose**
Serious, life-threatening, or fatal respiratory depression may occur with use of methadone. Patients should be
monitored for respiratory depression, especially during initiation of methadone or following a dose increase.
Respiratory depression is of particular concern in elderly or debilitated patients as well as in those suffering from conditions accompanied by hypoxia or hypercapnia when even moderate therapeutic doses may dangerously decrease pulmonary ventilation. Methadone should be administered with extreme caution to patients with conditions accompanied by hypoxia, hypercapnia, or decreased respiratory reserve such as: asthma, chronic obstructive pulmonary disease or cor pulmonale, severe obesity, sleep apnea syndrome, myxedema, kyphoscoliosis, and CNS depression or coma. In these patients, even usual therapeutic doses of methadone may decrease respiratory drive while simultaneously increasing airway resistance to the point of apnea. Alternative, non-opioid analgesics should be considered, and methadone should be employed only under careful medical supervision at the lowest effective dose.
Infants exposed in-utero or through breast milk are at risk of life-threatening respiratory depression upon delivery or when nursed.
Methadone's peak respiratory depressant effects typically occur later, and persist longer than its peak analgesic effects, in the short-term use setting. These characteristics can contribute to cases of iatrogenic overdose, particularly during treatment initiation and dose titration.
**Head Injury and Increased Intracranial Pressure**
The respiratory depressant effects of opioids and their capacity to elevate cerebrospinal fluid pressure may be markedly exaggerated in the presence of head injury, other intracranial lesions or a pre-existing increase in intracranial pressure. Furthermore, opioids produce effects which may obscure the clinical course of patients with head injuries. In such patients, methadone must be used with caution, and only if it is deemed essential.
**Incomplete Cross-tolerance between Methadone and other Opioids**
Patients tolerant to other opioids may be incompletely tolerant to methadone. Incomplete cross-tolerance is of particular concern for patients tolerant to other µ-opioid agonists who are being converted to methadone, thus making the determination of dosing during opioid conversion complex. Deaths have been reported during conversion from chronic, high-dose treatment with other opioid agonists. A high degree of “opioid tolerance” does not eliminate the possibility of methadone overdose, iatrogenic or otherwise.
Crosstolerance between morphine and methadone has been demonstrated, as steady-state plasma methadone concentrations required for effectiveness (C50%) were higher in abstinent rats previously dosed with morphine, as compared to controls.
**Misuse, Abuse, and Diversion of Opioids**
Methadone is a mu-agonist opioid with an abuse liability similar to morphine. Methadone, like morphine and other opioids used for analgesia, has the potential for being abused and is subject to criminal diversion.
Methadone can be abused in a manner similar to other opioid agonists, legal or illicit. This should be considered when dispensing Methadone in situations where the clinician is concerned about an increased risk of misuse, abuse, or diversion.
**Hypotensive Effect**
The administration of methadone may result in severe hypotension in patients whose ability to maintain normal blood pressure is compromised (e.g., severe volume depletion).
**Gastrointestinal Effects**
Methadone and other morphine-like opioids have been shown to decrease bowel motility and cause constipation. This primarily occurs through agonism of opioid receptors in the gut wall. Methadone may obscure the diagnosis or clinical course of patients with acute abdominal conditions.
**Sexual Function/Reproduction**
Reproductive function in human males may be decreased by methadone treatment. Reductions in ejaculate volume and seminal vesicle and prostate secretions have been reported in methadone-treated individuals. In addition, reductions in serum testosterone levels and sperm motility, and abnormalities in sperm morphology have been reported. Long-term use of opioids may be associated with decreased sex hormone levels and symptoms such as low libido, erectile dysfunction, or infertility.
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