Naloxegol (NKTR-118)

Management of opioid-induced constipation

Up to 40% of patients taking opioids develop constipation. Opioid-induced constipation (OIC) may limit the adequate dosing of opioids for pain relief and reduce quality of life. Health professionals must therefore inquire about bowel function in patients receiving opioids. The management of OIC includes carefully re-evaluating the necessity, type and dose of opioids at each visit. Lifestyle modification and alteration of aggravating factors, the use of simple laxatives and, when essential, the addition of newer laxatives or opioid antagonists (naloxone, naloxegol or methylnaltrexone) can be used to treat OIC. This review discusses the recent literature regarding the management of OIC and provides a rational approach to assessing and managing constipation in individuals receiving opioids.

Opioids

The opioids are a large class of medications related in structure to the natural plant alkaloids found in opium that are derived from the resin of the opium poppy, Papaver somniferum. The natural alkaloids are also referred to as opiates and include morphine and codeine. Synthetic derivatives include heroin, fentanyl, hydromorphone, methadone, buprenorphine and others. The opioids are highly potent and effective analgesics, but most have a high potential for dependency and abuse.

Opioids act by engagement of specific cell surface receptors; the opiate receptors, which are designated ? [mu], κ [kappa] and δ [delta]. These receptors are found predominantly in the central nervous system, brain and spinal column, but are also present on vascular, cardiac, lung, gut and even peripheral blood mononuclear cells. Engagement of the opiate receptors generates a series of intracellular signals, including inhibition of adenylate cyclase, decreased opening of calcium channels, increased potassium currents and activation of protein kinase C (PKC). The major effect of these pathways is reduction in cell excitability and neurotransmission. The natural ligands for the opiate receptors are the so-called endogenous opioid peptides such as the enkephalins, endorphins and endomorphins.

The opioids have a variety of clinical effects, but are predominantly known and used for their profound pain relieving effects. Other effects that are often linked to opiate analgesia include euphoria, changes in mood, drowsiness and mental clouding. However, the distinctive feature of the analgesia induced by the opioids is the lack of loss of consciousness. The pain is often described as less intense, but still present although better tolerated. Thus, the opioids do not decrease or treat the cause of the painful stimulus, but rather decrease its perception.

Other effects of opioids include respiratory depression, decreased gastrointestinal motility, sedation, nausea, vomiting, constipation and intestinal bloating. Opioids also have direct cardiovascular effects, decreasing blood pressure, causing vasodilation and decreasing cardiac work.

Most opioids have similar effects and side effects, although pharmacokinetic differences, tissue distribution, and receptor type specificity probably account for the variation in effects of the various synthetic and semisynthetic derivatives of morphine. Morphine is considered the prototype opiate, against which other agents are measured for their analgesic effects as well as adverse side effects.

The opioids can be categorized into subclasses on the basis of their chemical structure as opium alkaloids (opiates: codeine, morphine), semisynthetic derivatives of the natural alkaloids (hydrocodone, hydromorphone, oxycodone, buprenorphine), and various classes of synthetic opioids such as the anililopiperidines (fentanyl, alfentanil, sufentanil, remifentanil), diphenylpropylamine derivatives (propoxyphene, dextropropoxyphene, methadone, diphenoxylate, loperamide), and others (pentazocine, butorphanol, nalbuphine, levorphanol, tramadol), and, the opioid antagonists (nalmefene, naloxone and naltrexone). They can also be informally classified based upon their major use such as anesthesia (fentanyl, alfentanil, remifentanil, sufentanil), severe pain (morphine, hydromorphone, levorphanol, meperidine), moderate-to-severe acute or chronic pain (transdermal or transbuccal fentanyl, codeine, oxycodone, hydrocodone, levorphanol, methadone), diarrhea (loperamide, diphenoxylate), and cough (codeine, hydrocodone). Finally, opioids can be categorized on the basis of their action as full agonists, partial agonists or mixed agonists/antagonists, and antagonists of opiate receptors.

Opioid receptor antagonists are used to reverse the effects of opioids and are invaluable in the management of opioid overdose (naloxone, naltrexone, nalmefene). Specialized opioid antagonists can be used to reverse unwanted opioid effects, such as constipation in patients with chronic pain on long-term opioids. These agents (naldemedine, naloxegol) are generally modified so as not to cross the blood brain and reverse the central nervous system effects of opiates.

Opioids are rare causes of drug induced liver disease and are not mentioned in large case series of clinically apparent liver injury caused by medications. In physiological, pain relieving doses, opioids have not been implicated in causing clinically apparent liver injury, acute liver failure, chronic hepatitis or vanishing bile duct syndrome. However, overdoses of the more potent opioids have been linked to cases of acute liver injury, usually with a precipitous onset and pattern of acute toxicity with marked elevations in serum aminotransferase levels and early onset of signs of hepatic failure. This syndrome has been best characterized after buprenorphine overdose or abuse, but likely occurs with others. It is possible that the implicated opioids are not directly toxic to the liver, but cause ischemic liver injury due to respiratory failure, cardiovascular collapse, shock and anoxia that can occur with severe opioid overdose. The clinical syndrome resembles acute hepatic necrosis and liver failure, but is rapidly reversible and rarely the primary cause of death from overdose.

A special form of liver injury linked to opioid use occurs with their fixed drug combinations with acetaminophen. These combinations are commonly used for moderate to moderately severe pain and can lead to abuse. If taken too frequently, acetaminophen doses may reach toxic levels, particularly with overuse for several days in the face of malnutrition, alcohol abuse or intercurrent illness. These other stresses can lower hepatic glutathione levels and predispose to acetaminophen hepatotoxicity. This constellation of events is referred to as inadvertent or unintended acetaminophen overdose or more colloquially as a “therapeutic misadventure”. Because of their potential for hepatotoxicity, opioid combinations in which the dose of acetaminophen is greater than 325 mg per tablet or capsule were discontinued.

References to the safety and hepatotoxic potential of the various opiate agonists are given together at the end of this overview section. References to the opioids and the opiate antagonists used to treat substance abuse are given separately with each agent (buprenorphine, methadone, nalmefene, naloxone, naltrexone). The opioids are discussed individually or as groups of agents and links to each are given below.

Full and partial agonists:

1. Alfentanil

2. Buprenorphine

3. Butorphanol

4. Codeine

5. Diphenoxylate

6. Fentanyl

7. Heroin

8. Hydrocodone

9. Hydromorphone

10. Levorphanol

11. Loperamide

12. Meperidine

13. Methadone

14. Morphine

15. Opium

16. Oxycodone

17. Oxymorphone

18. Pentazocine

19. Remifentanil

20. Sufentanil

21. Tramadol

Opiate antagonists:

1. Naldemedine

2. Nalmefene

3. Naloxegol

4. Naloxone

5. Naltrexone

Opioid-Induced Constipation in Oncological Patients: New Strategies of Management

Cancer-associated pain has traditionally been treated with opioid analgesics, often in escalating doses. Opioid-induced constipation (OIC) is a common problem associated with chronic use of opioid analgesics. Typical treatment strategies to alleviate constipation are based on dietary changes, exercise, and laxatives. However, laxatives have a nonspecific action and do not target underlying mechanisms of OIC. This article will review prevalent, clinical presentation and recommendations for the treatment of OIC. An independent literature search was carried out by the authors. We reviewed the literature for randomized controlled trials that studied the efficacy of laxatives, naloxone, and naloxegol in treating OIC. Newer strategies addressing the causal pathophysiology of OIC are needed for a more effective assessment and management of OIC. Finally, traditional recommended therapies are appraised and compared with the latest pharmacological developments. Future research should address whether naloxegol is more efficacious by its comparison directly with first-line treatments, including laxatives.

Diagnosis and management of chronic constipation in adults

Constipation is a heterogeneous, polysymptomatic, multifactorial disease. Acute or transient constipation can be due to changes in diet, travel or stress, and secondary constipation can result from drug treatment, neurological or metabolic conditions or, rarely, colon cancer. A diagnosis of primary chronic constipation is made after exclusion of secondary causes of constipation and encompasses several overlapping subtypes. Slow-transit constipation is characterized by prolonged colonic transit in the absence of pelvic floor dysfunction. This subtype of constipation can be identified using either the radio-opaque marker test or wireless motility capsule test, and is best treated with laxatives such as polyethylene glycol or newer agents such as linaclotide or lubiprostone. If unsuccessful, subspecialist referral should be considered. Dyssynergic defecation results from impaired coordination of rectoanal and pelvic floor muscles, and causes difficulty with defecation. The condition can be identified using anorectal manometry and balloon expulsion tests and is best managed with biofeedback therapy. Opioid-induced constipation is an emerging entity, and several drugs including naloxegol, methylnaltrexone and lubiprostone are approved for its treatment. In this Review, we provide an overview of the burden and pathophysiology of chronic constipation, as well as a detailed discussion of the available diagnostic tools and treatment options.

Naloxegol

No information is available on the excretion of naloxegol into breastmilk. Because of the possibility of inducing opioid withdrawal in the breastfed infant, the manufacturer recommends that breastfeeding is not recommended during treatment with naloxegol.