Naltrexone

INDICATIONS AND USAGE Treatment with naltrexone for extended-release injectable suspension should be part of a comprehensive management program that includes psychosocial support. Naltrexone for extended-release injectable suspension contains naltrexone, an opioid antagonist, and is indicated for the treatment of alcohol dependence in patients who are able to abstain from alcohol in an outpatient setting prior to initiation of treatment with naltrexone for extended-release injectable suspension. Patients should not be actively drinking at the time of initial naltrexone for extended-release injectable suspension administration (1.1) . Naltrexone for extended-release injectable suspension is indicated for the prevention of relapse to opioid dependence, following opioid detoxification (1.2) . Naltrexone for extended-release injectable suspension should be part of a comprehensive management program that includes psychosocial support (1) . 1.1 Alcohol Dependence Naltrexone for extended-release injectable suspension is indicated for the treatment of alcohol dependence in patients who are able to abstain from alcohol in an outpatient setting prior to initiation of treatment with naltrexone for extended-release injectable suspension. Patients should not be actively drinking at the time of initial naltrexone for extended-release injectable suspension administration. 1.2 Opioid Dependence Naltrexone for extended-release injectable suspension is indicated for the prevention of relapse to opioid dependence, following opioid detoxification.

DOSAGE AND ADMINISTRATION Naltrexone for extended-release injectable suspension must be prepared and administered by a healthcare provider (2.1 , 2.6) . Prior to initiating naltrexone for extended-release injectable suspension, an opioid-free duration of a minimum of 7–10 days is recommended for patients, to avoid precipitation of opioid withdrawal that may be severe enough to require hospitalization (2.1) . The recommended dose of naltrexone for extended-release injectable suspension is 380 mg delivered intramuscularly (deep) as a gluteal injection, every 4 weeks or once a month, alternating buttocks for each subsequent injection, using the carton components provided (2.1) . Naltrexone for extended-release injectable suspension must ONLY be administered as a deep intramuscular gluteal injection ( 2.1 ) . See Full Prescribing Information for complete Directions for Use (2.6) . 2.1 Important Dosage and Administration Information Naltrexone for extended-release injectable suspension must be prepared and administered by a healthcare provider. Naltrexone for extended-release injectable suspension must ONLY be administered as a deep intramuscular gluteal injection. Parenteral products should be visually inspected for particulate matter and discoloration prior to administration whenever solution and container permit. A properly mixed suspension will be milky white, will not contain clumps, and will move freely down the wall of the vial [see Dosage and Administration ( 2.6 ) ] . Prior to initiating naltrexone for extended-release injectable suspension, an opioid-free duration of a minimum of 7–10 days is recommended for patients, to avoid precipitation of opioid withdrawal that may be severe enough to require hospitalization [see Warnings and Precautions ( 5.3 ) ]. The recommended dose of naltrexone for extended-release injectable suspension is 380 mg delivered intramuscularly (deep) as a gluteal injection every 4 weeks or once a month, alternating buttocks for each subsequent injection, using the carton components provided [see How Supplied/Storage and Handling ( 16 ) ] . The needles provided in the carton are customized needles. Naltrexone for extended-release injectable suspension must not be injected using any other needle. The needle lengths (either 1 1/2 or 2 inches) may not be adequate in every patient because of body habitus. Body habitus should be assessed prior to each injection for each patient to assure that needle length is adequate for intramuscular administration. For patients with a larger amount of subcutaneous tissue overlying the gluteal muscle, the administering healthcare provider may utilize the supplied 2-inch needle with needle protection device to help ensure that the injectate reaches the intramuscular mass. For very lean patients, the 1 1/2-inch needle may be appropriate to prevent the needle contacting the periosteum. Either needle may be used for patients with average body habitus. Healthcare providers should ensure that the naltrexone for extended-release injectable suspension injection is given correctly, and should consider alternate treatment for those patients whose body habitus precludes an intramuscular gluteal injection with one of the provided needles. If a patient misses a dose, he/she should be instructed to receive the next dose as soon as possible. Pretreatment with oral naltrexone is not required before using naltrexone for extended-release injectable suspension. 2.2 Patient Access to an Opioid Reversal Agent for the Emergency Treatment of Opioid Overdose At the initial naltrexone for extended-release injectable suspension injection and with each subsequent injection, inform patients and caregivers about opioid overdose reversal agents (e.g., naloxone, nalmefene) and discuss the importance of having access to an opioid overdose reversal agent. Because patients being treated for opioid use disorder have the potential for relapse, putting them at risk for opioid overdose after naltrexone for extended-release injectable suspension treatment is discontinued, at the end of the naltrexone for extended-release injectable suspension dosing interval (i.e., near the end of the month that naltrexone for extended-release injectable suspension was administered), or after a dose of naltrexone for extended-release injectable suspension is missed, strongly consider recommending or prescribing an overdose reversal agent for the emergency treatment of opioid overdose [see Warnings and Precautions ( 5.1 ) ] . Discuss the options for obtaining an opioid overdose reversal agent (e.g., prescription, over-the-counter, or as part of a community-based program [see Warning and Precautions (5.1) ] . There are important differences among the opioid overdose reversal agents, such as route of administration, product strength, approved patient age range, and pharmacokinetics. Be familiar with these differences, as outlined in the approved labeling for those products, prior to recommending or prescribing such an agent. 2.3 Reinitiation of Treatment in Patients Previously Discontinued There are no data to specifically address reinitiation of treatment. Patients reinitiating treatment with naltrexone for extended-release injectable suspension should be opioid-free at the time of dose administration [see Indications and Usage ( 1 ) , Contraindications ( 4 ) , and Warnings and Precautions ( 5.3 ) ] . 2.4 Switching From Oral Naltrexone There are no systematically collected data that specifically address the switch from oral naltrexone to naltrexone for extended-release injectable suspension. 2.5 Switching from Buprenorphine, Buprenorphine/Naloxone, or Methadone There are no systematically collected data that specifically address the switch from buprenorphine or methadone to naltrexone for extended-release injectable suspension; however, review of postmarketing case reports have indicated that some patients may experience severe manifestations of precipitated withdrawal when being switched from opioid agonist therapy to opioid antagonist therapy [see Warnings and Precautions ( 5.3 ) ] . Patients transitioning from buprenorphine or methadone may be vulnerable to precipitation of withdrawal symptoms for as long as 2 weeks. Healthcare providers should be prepared to manage withdrawal symptomatically with non-opioid medications. 2.6 Directions for Use Naltrexone for extended-release injectable suspension must be prepared and administered by a healthcare provider. Naltrexone for extended-release injectable suspension must ONLY be administered as a deep intramuscular gluteal injection. To ensure proper dosing, it is important that you follow the preparation and administration instructions outlined in this document. Naltrexone for extended-release injectable suspension must be suspended only in the diluent supplied in the carton and must be administered only with one of the administration needles supplied in the carton. The microspheres, diluent, preparation needle, and an administration needle with needle protection device are required for preparation and administration. Two thin-walled 1 1/2-inch needles with needle protection device and two 2-inch thin-walled needles with needle protection device have been provided to accommodate varying patient body habitus. For patients with a larger amount of subcutaneous tissue overlying the gluteal muscle, the administering healthcare provider may utilize the supplied 2-inch needle with needle protection device to help ensure that the injectate reaches the intramuscular mass. For very lean patients, the 1 1/2-inch needle may be appropriate to prevent the needle contacting the periosteum. Either needle may be used for patients with average body habitus. A spare administration needle of each size is provided in case of clogging [see How Supplied/Storage and Handling ( 16 ) ] . Do not substitute any other components for the components of the carton. Prior to preparation, allow drug to reach room temperature (approximat

WARNINGS Vulnerability to Opioid Overdose After opioid detoxification, patients are likely to have reduced tolerance to opioids. As the blockade of exogenous opioids provided by naltrexone hydrochloride wanes and eventually dissipates completely, patients who have been treated with naltrexone hydrochloride may respond to lower doses of opioids than previously used, just as they would shortly after completing detoxification. This could result in potentially life-threatening opioid intoxication (respiratory compromise or arrest, circulatory collapse, etc.) if the patient uses previously tolerated doses of opioids. Cases of opioid overdose with fatal outcomes have been reported in patients after discontinuing treatment. Patients should be alerted that they may be more sensitive to opioids, even at lower doses, after naltrexone hydrochloride treatment is discontinued. It is important that patients inform family members, and the people closest to the patient of this increased sensitivity ot opioids and the risk of overdose (see PRECAUTIONS, Information for Patients ). There is also the possibility that a patient who is treated with naltrexone hydrochloride could overcome the opioid blockade effect of naltrexone hydrochloride. Although naltrexone hydrochloride is a potent antagonist, the blockade produced by naltrexone hydrochloride is surmountable. The plasma concentration of exogenous opioids attained immediately following their acute administration may be sufficient to overcome the competitive receptor blockade. This poses a potential risk to individuals who attempt, on their own, to overcome the blockade by administering large amounts of exogenous opioids. Any attempt by a patient to overcome the antagonism by taking opioids is especially dangerous and may lead to life-threatening opioid intoxication or fatal overdose. Patients should be told of the serious consequences of trying to overcome the opioid blockade (see PRECAUTIONS, Information for Patients ). Patient Access to an Opioid Overdose Reversal Agent for the Emergency Treatment of Opioid Overdose When initiating and renewing treatment with naltrexone hydrochloride tablets, USP, inform patients and caregivers about opioid overdose reversal agents (e.g., naloxone, nalmefene) and discuss the importance of having access to an opioid overdose reversal agent. Because of the risks for opioid overdose described above, strongly consider recommending or prescribing an opioid overdose reversal agent for the emergency treatment of an opioid overdose, both when initiating and renewing treatment with naltrexone hydrochloride tablets, USP. Discuss the options for obtaining an opioid overdose reversal agent (e.g., prescription, over-the-counter (some products), or as part of a community-based program). There are important differences among the opioid overdose reversal agents, such as route of administration, product strength, approved patient age range, and pharmacokinetics. Be familiar with these differences, as outlined in the approved labeling for those products, prior to recommending or prescribing such an agent. Educate patients and caregivers on how to recognize the signs and symptoms of an opioid overdose and how to use an opioid overdose reversal agent for the emergency treatment of opioid overdose. Emphasize the importance of calling 911 or getting emergency medical help in all cases of known or suspected opioid overdose, even if an opioid overdose reversal agent is administered. Precipitated Opioid Withdrawal The symptoms of spontaneous opioid withdrawal (which are associated with the discontinuation of opioid in a dependent individual) are uncomfortable, but they are not generally believed to be severe or necessitate hospitalization. However, when withdrawal is precipitated abruptly by the administration of an opioid antagonist to an opioid-dependent patient, the resulting withdrawal syndrome can be severe enough to require hospitalization. Symptoms of withdrawal have usually appeared within five minutes of ingestion of naltrexone hydrochloride and have lasted for up to 48 hours. Mental status changes including confusion, somnolence and visual hallucinations have occurred. Significant fluid losses from vomiting and diarrhea have required intravenous fluid administration. Review of postmarketing cases of precipitated opioid withdrawal in association with naltrexone treatment has identified cases with symptoms of withdrawal severe enough to require hospital admission, and in some cases, management in the intensive care unit. To prevent occurrence of precipitated withdrawal in patients dependent on opioids, or exacerbation of a pre-existing subclinical withdrawal syndrome, opioid-dependent patients, including those being treated for alcohol dependence, should be opioid-free (including tramadol) before starting naltrexone hydrochloride treatment. An opioid-free interval of a minimum of 7 to 10 days is recommended for patients previously dependent on short-acting opioids. Patients transitioning from buprenorphine or methadone may be vulnerable to precipitation of withdrawal symptoms for as long as two weeks. If a more rapid transition from agonist to antagonist therapy is deemed necessary and appropriate by the healthcare provider, monitor the patient closely in an appropriate medical setting where precipitated withdrawal can be managed. In every case, healthcare providers should always be prepared to manage withdrawal symptomatically with non-opioid medications because there is no completely reliable method for determining whether a patient has had an adequate opioid-free period. A naloxone challenge test may be helpful; however, a few case reports have indicated that patients may experience precipitated withdrawal despite having a negative urine toxicology screen or tolerating a naloxone challenge test (usually in the setting of transitioning from buprenorphine treatment). Patients should be made aware of the risks associated with precipitated withdrawal and encouraged to give an accurate account of last opioid use. Patients treated for alcohol dependence with naltrexone hydrochloride should also be assessed for underlying opioid dependence and for any recent use of opioids prior to initiation of treatment with naltrexone hydrochloride. Precipitated opioid withdrawal has been observed in alcohol-dependent patients in circumstances where the prescriber had been unaware of the additional use of opioids or co-dependence on opioids. Hepatotoxicity Cases of hepatitis and clinically significant liver dysfunction were observed in association with naltrexone hydrochloride exposure during the clinical development program and in the postmarketing period. Transient, asymptomatic hepatic transaminase elevations were also observed in the clinical trials and postmarketing period. When patients presented with elevated transaminases, there were often other potential causative or contributory etiologies identified, including pre-existing alcoholic liver disease, hepatitis B and/or C infection, and concomitant usage of other potentially hepatotoxic drugs. Although clinically significant liver dysfunction is not typically recognized as a manifestation of opioid withdrawal, opioid withdrawal that is precipitated abruptly may lead to systemic sequelae, including acute liver injury. Patients should be warned of the risk of hepatic injury and advised to seek medical attention if they experience symptoms of acute hepatitis. Use of naltrexone hydrochloride should be discontinued in the event of symptoms and/or signs of acute hepatitis. Depression and Suicidality Depression, suicide, attempted suicide and suicidal ideation have been reported in the postmarketing experience with naltrexone hydrochloride used in the treatment of opioid dependence. No causal relationship has been demonstrated. In the literature, endogenous opioids have been theorized to contribute to a variety of conditions. Alcohol- and opioid-dependent patients

CONTRAINDICATIONS Naltrexone for extended-release injectable suspension is contraindicated in: Patients receiving opioid analgesics [see Warnings and Precautions ( 5.3 ) ] . Patients with current physiologic opioid dependence [see Warnings and Precautions ( 5.3 ) ] . Patients in acute opioid withdrawal [see Warnings and Precautions ( 5.3 ) ] . Any individual who has failed the naloxone challenge test or has a positive urine screen for opioids [see Warnings and Precautions ( 5.3 ) ] . Patients who have previously exhibited hypersensitivity to naltrexone, PLG, carboxymethylcellulose, or any other components of the diluent [see Warnings and Precautions ( 5.8 ) ]. Naltrexone for extended-release injectable suspension is contraindicated in: Patients receiving opioid analgesics (4) . Patients with current physiologic opioid dependence (4) . Patients in acute opioid withdrawal (4) . Any individual who has failed the naloxone challenge test or has a positive urine screen for opioids (4) . Patients who have previously exhibited hypersensitivity to naltrexone, polylactide-co-glycolide (PLG), carboxymethylcellulose, or any other components of the diluent (4) .

Drug Interactions Studies to evaluate possible interactions between naltrexone hydrochloride and drugs other than opiates have not been performed. Consequently, caution is advised if the concomitant administration of naltrexone hydrochloride and other drugs is required. The safety and efficacy of concomitant use of naltrexone hydrochloride and disulfiram is unknown, and the concomitant use of two potentially hepatotoxic medications is not ordinarily recommended unless the probable benefits outweigh the known risks. Lethargy and somnolence have been reported following doses of naltrexone hydrochloride and thioridazine. Patients taking naltrexone hydrochloride may not benefit from opioid containing medicines, such as cough and cold preparations, antidiarrheal preparations, and opioid analgesics. In an emergency situation when opioid analgesia must be administered to a patient receiving naltrexone hydrochloride, the amount of opioid required may be greater than usual, and the resulting respiratory depression may be deeper and more prolonged (see PRECAUTIONS ).

ADVERSE REACTIONS During two randomized, double-blind placebo-controlled 12-week trials to evaluate the efficacy of naltrexone hydrochloride as an adjunctive treatment of alcohol dependence, most patients tolerated naltrexone hydrochloride well. In these studies, a total of 93 patients received naltrexone hydrochloride at a dose of 50 mg once daily. Five of these patients discontinued naltrexone hydrochloride because of nausea. No serious adverse events were reported during these two trials. While extensive clinical studies evaluating the use of naltrexone hydrochloride in detoxified, formerly opioid-dependent individuals failed to identify any single, serious untoward risk of naltrexone hydrochloride use, placebo-controlled studies employing up to five fold higher doses of naltrexone hydrochloride (up to 300 mg per day) than that recommended for use in opiate receptor blockade have shown that naltrexone hydrochloride causes hepatocellular injury in a substantial proportion of patients exposed at higher doses (see WARNINGS and PRECAUTIONS, Laboratory Tests ). Aside from this finding, and the risk of precipitated opioid withdrawal, available evidence does not incriminate naltrexone hydrochloride, used at any dose, as a cause of any other serious adverse reaction for the patient who is "opioid-free." It is critical to recognize that naltrexone hydrochloride can precipitate or exacerbate abstinence signs and symptoms in any individual who is not completely free of exogenous opioids. Patients with addictive disorders, especially opioid addiction, are at risk for multiple numerous adverse events and abnormal laboratory findings, including liver function abnormalities . Data from both controlled and observational studies suggest that these abnormalities, other than the dose-related hepatotoxicity described above, are not related to the use of naltrexone hydrochloride. Among opioid-free individuals, naltrexone hydrochloride administration at the recommended dose has not been associated with a predictable profile of serious adverse or untoward events. However, as mentioned above, among individuals using opioids, naltrexone hydrochloride may cause serious withdrawal reactions (see CONTRAINDICATIONS , WARNINGS , DOSAGE AND ADMINISTRATION ). Reported Adverse Events Naltrexone hydrochloride has not been shown to cause significant increases in complaints in placebo-controlled trials in patients known to be free of opioids for more than 7 to 10 days. Studies in alcoholic populations and in volunteers in clinical pharmacology studies have suggested that a small fraction of patients may experience an opioid withdrawal-like symptom complex consisting of tearfulness, mild nausea, abdominal cramps, restlessness, bone or joint pain, myalgia, and nasal symptoms. This may represent the unmasking of occult opioid use, or it may represent symptoms attributable to naltrexone. A number of alternative dosing patterns have been recommended to try to reduce the frequency of these complaints. Alcoholism In an open label safety study with approximately 570 individuals with alcoholism receiving naltrexone hydrochloride, the following new-onset adverse reactions occurred in 2% or more of the patients: nausea (10%), headache (7%), dizziness (4%), nervousness (4%), fatigue (4%), insomnia (3%), vomiting (3%), anxiety (2%) and somnolence (2%). Depression, suicidal ideation, and suicidal attempts have been reported in all groups when comparing naltrexone, placebo, or controls undergoing treatment for alcoholism. RATE RANGES OF NEW ONSET EVENTS Naltrexone Placebo Depression 0 to 15% 0 to 17% Suicide Attempt/Ideation 0 to 1% 0 to 3% Although no causal relationship with naltrexone hydrochloride is suspected, physicians should be aware that treatment with naltrexone does not reduce the risk of suicide in these patients (see PRECAUTIONS ). Opioid Addiction The following adverse reactions have been reported both at baseline and during the naltrexone hydrochloride clinical trials in opioid addiction at an incidence rate of more than 10%: Difficulty sleeping, anxiety, nervousness, abdominal pain/cramps, nausea and/or vomiting, low energy, joint and muscle pain, and headache. The incidence was less than 10% for: Loss of appetite, diarrhea, constipation, increased thirst, increased energy, feeling down, irritability, dizziness, skin rash, delayed ejaculation, decreased potency, and chills. The following events occurred in less than 1% of subjects: Respiratory Nasal congestion, itching, rhinorrhea, sneezing, sore throat, excess mucus or phlegm, sinus trouble, heavy breathing, hoarseness, cough, shortness of breath. Cardiovascular Nose bleeds, phlebitis, edema, increased blood pressure, non-specific ECG changes, palpitations, tachycardia. Gastrointestinal Excessive gas, hemorrhoids, diarrhea, ulcer. Musculoskeletal Painful shoulders, legs or knees; tremors, twitching. Genitourinary Increased frequency of, or discomfort during, urination; increased or decreased sexual interest. Dermatologic Oily skin, pruritus, acne, athlete's foot, cold sores, alopecia. Psychiatric Depression, paranoia, fatigue, restlessness, confusion, disorientation, hallucinations, nightmares, bad dreams. Special senses Eyes blurred, burning, light sensitive, swollen, aching, strained; ears–"clogged," aching, tinnitus. General Increased appetite, weight loss, weight gain, yawning, somnolence, fever, dry mouth, head "pounding," inguinal pain, swollen glands, "side" pains, cold feet, "hot spells." Post-marketing Experience Data collected from post-marketing use of naltrexone hydrochloride show that most events usually occur early in the course of drug therapy and are transient. It is not always possible to distinguish these occurrences from those signs and symptoms that may result from a withdrawal syndrome. Events that have been reported include anorexia, asthenia, chest pain, fatigue, headache, hot flushes, malaise, changes in blood pressure, agitation, dizziness, hyperkinesia, nausea, vomiting, tremor, abdominal pain, diarrhea, elevations in liver enzymes or bilirubin, hepatic function abnormalities or hepatitis, palpitations, myalgia, anxiety, confusion, euphoria, hallucinations, insomnia, nervousness, somnolence, abnormal thinking, dyspnea, rash, increased sweating, vision abnormalities and idiopathic thrombocytopenic purpura. In some individuals the use of opioid antagonists has been associated with a change in baseline levels of some hypothalamic, pituitary, adrenal, or gonadal hormones. The clinical significance of such changes is not fully understood. Adverse events, including withdrawal symptoms and death, have been reported with the use of naltrexone hydrochloride in ultra rapid opiate detoxification programs. The cause of death in these cases is not known (see WARNINGS ). Laboratory Tests In a placebo controlled study in which naltrexone hydrochloride was administered to obese subjects at a dose approximately five-fold that recommended for the blockade of opiate receptors (300 mg per day), 19% (5/26) of naltrexone hydrochloride recipients and 0% (0/24) of placebo-treated patients developed elevations in serum transaminases (i.e. peak ALT values ranging from 121 to 532; or 3 to 19 times their baseline values) after three to eight weeks of treatment. The patients involved were generally clinically asymptomatic, and the transaminase levels of all patients on whom follow-up was obtained returned to (or toward) baseline values in a matter of weeks. Transaminase elevations were also observed in other placebo controlled studies in which exposure to naltrexone hydrochloride at doses above the amount recommended for the treatment of alcoholism or opioid blockade consistently produced more numerous and more significant elevations of serum transaminases than did placebo. Transaminase elevations occurred in 3 of 9 patients with Alzheimer's Disease who received naltrexone hydrochloride (at doses up to 300 mg/day) for 5 to 8 weeks

CLINICAL PHARMACOLOGY Pharmacodynamic Actions Naltrexone hydrochloride is a pure opioid antagonist. It markedly attenuates or completely blocks, reversibly, the subjective effects of intravenously administered opioids. When coadministered with morphine, on a chronic basis, naltrexone hydrochloride blocks the physical dependence to morphine, heroin and other opioids. Naltrexone hydrochloride has few, if any, intrinsic actions besides its opioid blocking properties. However, it does produce some pupillary constriction, by an unknown mechanism. The administration of naltrexone hydrochloride is not associated with the development of tolerance or dependence. In subjects physically dependent on opioids, naltrexone hydrochloride will precipitate withdrawal symptomatology. Clinical studies indicate that 50 mg of naltrexone hydrochloride will block the pharmacologic effects of 25 mg of intravenously administered heroin for periods as long as 24 hours. Other data suggest that doubling the dose of naltrexone hydrochloride provides blockade for 48 hours, and tripling the dose of naltrexone hydrochloride provides blockade for about 72 hours. Naltrexone hydrochloride blocks the effects of opioids by competitive binding (i.e., analogous to competitive inhibition of enzymes) at opioid receptors. This makes the blockade produced potentially surmountable, but overcoming full naltrexone blockade by administration of very high doses of opiates has resulted in excessive symptoms of histamine release in experimental subjects. The mechanism of action of naltrexone hydrochloride in alcoholism is not understood; however, involvement of the endogenous opioid system is suggested by preclinical data. Naltrexone hydrochloride, an opioid receptor antagonist, competitively binds to such receptors and may block the effects of endogenous opioids. Opioid antagonists have been shown to reduce alcohol consumption by animals, and naltrexone hydrochloride has been shown to reduce alcohol consumption in clinical studies. Naltrexone hydrochloride is not aversive therapy and does not cause a disulfiram-like reaction either as a result of opiate use or ethanol ingestion. Pharmacokinetics Naltrexone hydrochloride is a pure opioid receptor antagonist. Although well absorbed orally, naltrexone is subject to significant first pass metabolism with oral bioavailability estimates ranging from 5 to 40%. The activity of naltrexone is believed to be due to both parent and the 6-β-naltrexol metabolite. Both parent drug and metabolites are excreted primarily by the kidney (53% to 79% of the dose), however, urinary excretion of unchanged naltrexone accounts for less than 2% of an oral dose and fecal excretion is a minor elimination pathway. The mean elimination half-life (T 1/2 ) values for naltrexone and 6-β-naltrexol are 4 hours and 13 hours, respectively. Naltrexone and 6-β-naltrexol are dose proportional in terms of AUC and C max over the range of 50 to 200 mg and do not accumulate after 100 mg daily doses. Absorption Following oral administration, naltrexone undergoes rapid and nearly complete absorption with approximately 96% of the dose absorbed from the gastrointestinal tract. Peak plasma levels of both naltrexone and 6-β-naltrexol occur within one hour of dosing. Distribution The volume of distribution for naltrexone following intravenous administration is estimated to be 1350 liters. In vitro tests with human plasma show naltrexone to be 21% bound to plasma proteins over the therapeutic dose range. Metabolism The systemic clearance (after intravenous administration) of naltrexone is ~3.5 L/min, which exceeds liver blood flow (~1.2 L/min). This suggests both that naltrexone is a highly extracted drug (>98% metabolized) and that extrahepatic sites of drug metabolism exist. The major metabolite of naltrexone is 6-β-naltrexol. Two other minor metabolites are 2-hydroxy-3-methoxy-6-β-naltrexol and 2-hydroxy-3-methyl-naltrexone. Naltrexone and its metabolites are also conjugated to form additional metabolic products. Elimination The renal clearance for naltrexone ranges from 30 to 127 mL/min and suggests that renal elimination is primarily by glomerular filtration. In comparison, the renal clearance for 6-β-naltrexol ranges from 230 to 369 mL/min, suggesting an additional renal tubular secretory mechanism. The urinary excretion of unchanged naltrexone accounts for less than 2% of an oral dose; urinary excretion of unchanged and conjugated 6-β-naltrexol accounts for 43% of an oral dose. The pharmacokinetic profile of naltrexone suggests that naltrexone and its metabolites may undergo enterohepatic recycling. Hepatic and Renal Impairment Naltrexone appears to have extra-hepatic sites of drug metabolism and its major metabolite undergoes active tubular secretion (see Metabolism above). Adequate studies of naltrexone in patients with severe hepatic or renal impairment have not been conducted (see PRECAUTIONS: Special Risk Patients ). Clinical Trials Alcoholism: The efficacy of naltrexone hydrochloride as an aid to the treatment of alcoholism was tested in placebo-controlled, outpatient, double blind trials. These studies used a dose of naltrexone hydrochloride tablets 50 mg once daily for 12 weeks as an adjunct to social and psychotherapeutic methods when given under conditions that enhanced patient compliance. Patients with psychosis, dementia, and secondary psychiatric diagnoses were excluded from these studies. In one of these studies, 104 alcohol-dependent patients were randomized to receive either naltrexone hydrochloride tablets 50 mg once daily or placebo. In this study, naltrexone hydrochloride proved superior to placebo in measures of drinking including abstention rates (51% vs. 23%), number of drinking days, and relapse (31% vs. 60%). In a second study with 82 alcohol-dependent patients, the group of patients receiving naltrexone hydrochloride were shown to have lower relapse rates (21% vs. 41%), less alcohol craving, and fewer drinking days compared with patients who received placebo, but these results depended on the specific analysis used. The clinical use of naltrexone hydrochloride as adjunctive pharmacotherapy for the treatment of alcoholism was also evaluated in a multicenter safety study. This study of 865 individuals with alcoholism included patients with comorbid psychiatric conditions, concomitant medications, polysubstance abuse and HIV disease. Results of this study demonstrated that the side effect profile of naltrexone hydrochloride appears to be similar in both alcoholic and opioid dependent populations, and that serious side effects are uncommon. In the clinical studies, treatment with naltrexone hydrochloride supported abstinence, prevented relapse and decreased alcohol consumption. In the uncontrolled study, the patterns of abstinence and relapse were similar to those observed in the controlled studies. Naltrexone hydrochloride was not uniformly helpful to all patients, and the expected effect of the drug is a modest improvement in the outcome of conventional treatment. Treatment of Opioid Addiction Naltrexone hydrochloride has been shown to produce complete blockade of the euphoric effects of opioids in both volunteer and addict populations. When administered by means that enforce compliance, it will produce an effective opioid blockade, but has not been shown to affect the use of cocaine or other non-opioid drugs of abuse. There are no data that demonstrate an unequivocally beneficial effect of naltrexone hydrochloride on rates of recidivism among detoxified, formerly opioid-dependent individuals who self-administer the drug. The failure of the drug in this setting appears to be due to poor medication compliance. The drug is reported to be of greatest use in good prognosis opioid addicts who take the drug as part of a comprehensive occupational rehabilitative program, behavioral contract, or other compliance-enhancing protocol. Naltrexone hydrochloride, unlike methadone or