JF 1; Nalmetrene; ORF 11676;
A structural analog of Naltrexone (N285780) with opiate antagonist activity used in pharmaceutical treatment of alcoholism. Other pharmacological applications of this compound aim to reduce food cravings, drug abuse and pulmonary disease in affected individuals. Used as an opioid-induced tranquilizer on large animals in the veterinary industry. Narcotic antagonist.
|Systematic (IUPAC) name|
|Legal status||POM (UK)|
|Half-life||10.8 ± 5.2 hours|
|CAS number|| 55096-26-9
|Mol. mass||375.9 g/mol (hydrochloride)|
|Mol. Formula: C21H25NO3|
|Appearance: Off-White to Pale Yellow Solid|
|Melting Point: 182-185˚C|
|Mol. Weight: 339.43|
Nalmefene (trade name Selincro), originally known as nalmetrene, is an opioid receptor antagonist developed in the early 1970s, and used primarily in the management of alcohol dependence, and also has been investigated for the treatment of other addictions such as pathological gambling and addiction to shopping.
Nalmefene is an opiate derivative similar in both structure and activity to the opiate antagonist naltrexone. Advantages of nalmefene relative to naltrexone include longer half-life, greater oral bioavailability and no observed dose-dependent liver toxicity. As with other drugs of this type, nalmefene can precipitate acute withdrawal symptoms in patients who are dependent on opioid drugs, or more rarely when used post-operatively to counteract the effects of strong opioids used in surgery.
Nalmefene differs from naltrexone by substitution of the ketone group at the 6-position of naltrexone with a methylene group (CH2), which considerably increases binding affinity to the μ-opioid receptor. Nalmefene also has high affinity for the other opioid receptors, and is known as a “universal antagonist” for its ability to block all three.
In clinical trials using this drug, doses used for treating alcoholism were in the range of 20–80 mg per day, orally. The doses tested for treating pathological gambling were between 25–100 mg per day. In both trials, there was little difference in efficacy between the lower and higher dosage regimes, and the lower dose (20 and 25 mg, respectively) was the best tolerated, with similar therapeutic efficacy to the higher doses and less side effects. Nalmefene is thus around twice as potent as naltrexone when used for the treatment of addictions.
Intravenous doses of nalmefene at between 0.5 to 1 milligram have been shown effective at counteracting the respiratory depression produced by opiate overdose, although this is not the usual application for this drug as naloxone is less expensive.
Doses of nalmefene greater than 1.5 mg do not appear to give any greater benefit in this application. Nalmefene’s longer half-life might however make it useful for treating overdose involving longer acting opioids such as methadone, as it would require less frequent dosing and hence reduce the likelihood of renarcotization as the antagonist wears off.
Nalmefene is extensively metabolised in the liver, mainly by conjugation with glucuronic acid and also by N-dealkylation. Less than 5% of the dose is excreted unchanged. The glucuronide metabolite is entirely inactive, while the N-dealkylated metabolite has minimal pharmacological activity.
Lundbeck has licensed the drug from Biotie Therapies and performed clinical trials with nalmefene for treatment of alcohol dependence. In 2011 they submitted an application for their drug termed Selincro to the European Medicines Agency. It has not been available on the US market since at least August 2008.
- Common: drowsiness, hypertension, tachycardia, dizziness, nausea, vomiting
- Occasional: fever, hypotension, vasodilatation, chills, headache
- Rare: agitation, arrhythmia, bradycardia, confusion, hallucinations, myoclonus, itching
- Soluble in water up to 130 mg/mL, soluble in chloroform up to 0.13 mg/mL
- pKa 7.6
- Distribution half-life: 41 minutes
Nalmefene is a known opioid receptor antagonist which can inhibit pharmacological effects of both administered opioid agonists and endogenous agonists deriving from the opioid system. The clinical usefulness of nalmefene as antagonist comes from its ability to promptly (and selectively) reverse the effects of these opioid agonists, including the frequently observed depressions in the central nervous system and the respiratory system.
Nalmefene has primarily been developed as the hydrochloride salt for use in the management of alcohol dependency, where it has shown good effect in doses of 10 to 40 mg taken when the patient experiences a craving for alcohol (Karhuvaara et al, Alcohol. Clin. Exp. Res., (2007), Vol. 31 No. 7. pp 1179-1187). Additionally, nalmefene has also been investigated for the treatment of other addictions such as pathological gambling and addiction to shopping. In testing the drug in these developmental programs, nalmefene has been used, for example, in the form of parental solution (Revex™).
Nalmefene is an opiate derivative quite similar in structure to the opiate antagonist naltrexone. Advantages of nalmefene compared to naltrexone include longer half- life, greater oral bioavailability and no observed dose-dependent liver toxicity. Nalmefene differs structurally from naltrexone in that the ketone group at the 6- position of naltrexone is replaced by a methylene (CH2) group, which considerably increases binding affinity to the μ-opioid receptor. Nalmefene also has high affinity for the other opioid receptors (K and δ receptors) and is known as a “universal antagonist” as a result of its ability to block all three receptor types.
Nalmefene can be produced from naltrexone by the Wittig reaction. The Wittig reaction is a well known method within the art for the synthetic preparation of olefins (Georg Wittig, Ulrich Schόllkopf (1954). “Uber Triphenyl-phosphin- methylene ah olefinbildende Reagenzien I”. Chemische Berichte 87: 1318), and has been widely used in organic synthesis.
The procedure in the Wittig reaction can be divided into two steps. In the first step, a phosphorus ylide is prepared by treating a suitable phosphonium salt with a base. In the second step the ylide is reacted with a substrate containing a carbonyl group to give the desired alkene.
The preparation of nalmefene by the Wittig reaction has previously been disclosed by Hahn and Fishman (J. Med. Chem. 1975, 18, 259-262). In their method, naltrexone is reacted with the ylide methylene triphenylphosphorane, which is prepared by treating methyl triphenylphosphonium bromide with sodium hydride (NaH) in DMSO. An excess of about 60 equivalents of the ylide is employed in the preparation of nalmefene by this procedure.
For industrial application purposes, the method disclosed by Hahn and Fishman has the disadvantage of using a large excess of ylide, such that very large amounts phosphorus by-products have to be removed before nalmefene can be obtained in pure form. Furthermore, the NaH used to prepare the ylide is difficult to handle on an industrial scale as it is highly flammable. The use of NaH in DMSO is also well known by the skilled person to give rise to unwanted runaway reactions. The Wittig reaction procedure described by Hahn and Fishman gives nalmefene in the form of the free base. The free base is finally isolated by chromatography, which may be not ideal for industrial applications.
US 4,535,157 also describes the preparation of nalmefene by use of the Wittig reaction. In the method disclosed therein the preparation of the ylide methylene triphenylphosphorane is carried out by using tetrahydrofuran (THF) as solvent and potassium tert-butoxidc (KO-t-Bu) as base. About 3 equivalents of the ylide are employed in the described procedure.
Although the procedure disclosed in US 4,535,157 avoids the use of NaH and a large amount of ylide, the method still has some drawbacks which limit its applicability on an industrial scale. In particular, the use of THF as solvent in a Wittig reaction is disadvantageous because of the water miscibility of THF. During the aqueous work-up much of the end product (nalmefene) may be lost in the aqueous phases unless multiple re-extractions are performed with a solvent which is not miscible with water.
Furthermore, in the method described in US 4,535,157, multiple purification steps are carried out in order to remove phosphine oxide by-products of the Wittig reaction. These purification steps require huge amounts of solvents, which is both uneconomical and labor extensive requiring when running the reaction on an industrial scale. As in the case of the Wittig reaction procedure described by Hahn and Fishman (see above) the Wittig reaction procedure disclosed in US 4,535,157 also yields nalmefene as the free base, such that an additional step is required to prepare the final pharmaceutical salt form, i.e. the hydrochloride, from the isolated nalmefene base.
US 4,751,307 also describes the preparation of nalmefene by use of the Wittig reaction. Disclosed is a method wherein the synthesis is performed using anisole (methoxybenzene) as solvent and KO-t-Bu as base. About 4 equivalents of the ylide methylene triphenylphosphorane were employed in this reaction. The product was isolated by extraction in water at acidic pHs and then precipitating at basic pHs giving nalmefene as base.
Even though the isolation procedure for nalmefene as free base is simplified, it still has some disadvantages. The inventors of the present invention repeated the method disclosed in US 4,751,307 and found that the removal of phosphine oxide by-products was not efficient. These impurities co-precipitate with the nalmefene during basifϊcation, yielding a product still contaminated with phosphorus byproducts and having, as a consequence, a low chemical purity, as illustrated in example 2 herein.
There is therefore a need within the field to improve the method of producing nalmefene by the Wittig reaction. In particular, there is a need for a method that is readily applicable on a large industrial scale and which avoids the use of water- miscible solvents, such as THF, in the Wittig reaction, and permits easy isolation of nalmefene in a pure form suitable for its transformation to the final pharmaceutical salt form.
present invention the Wittig reaction may be performed by mixing a methyltriphenylphosphonium salt with 2- methyltetrahydrofuran (MTHF) and a suitable base to afford the ylide methylene triphenylphosphorane :
Methyltriphenylphosphonium salt Methylene triphenylphosphorane Yhde
The preformed ylide is subsequently reacted ‘in situ’ with naltrexone to give nalmefene and triphenylphosphine oxide (TPPO):
Naltrexone Yhde Nalmefene TPPO
Example 1 Methyltriphenylphosphonium bromide (MTPPB, 25.8 Kg) was suspended in 2- methyltetrahydrofuran (MTHF, 56 litres). Keeping the temperature in the range 20-250C, KO-t-Bu (8.8 kg) was charged in portions under inert atmosphere in one hour. The suspension turned yellow and was stirred further for two hours. An anhydrous solution of naltrexone (8.0 Kg) in MTHF (32 litres) was then added over a period of one hour at 20-250C. The suspension was maintained under stirring for a few hours to complete the reaction. The mixture was then treated with a solution of ammonium chloride (4.2 Kg) in water (30.4 litres) and then further diluted with water (30.4 litres). The phases were separated, the lower aqueous phase was discarded and the organic phase was washed twice with water (16 litres). The organic phase was concentrated to residue under vacuum and then diluted with dichloromethane (40 litres) to give a clear solution. Concentrated aqueous hydrochloric acid (HCl 37%, 2 litres) was added over one hour at 20- 250C. The suspension was stirred for at least three hours at the same temperature, and then filtered and washed with dichloromethane (8 litres) and then with acetone (16 litres). The solid was then re-suspended in dichloromethane (32 litres) at 20-250C for a few hours and then filtered and washed with dichloromethane (16 litres), affording 9.20 Kg of nalmefene hydrochloride, corresponding to 7.76 kg of nalmefene hydrochloride (99.7% pure by HPLC). Molar yield 89%.
HPLC Chromatographic conditions
Column: Zorbax Eclipse XDB C-18, 5 μm, 150 x 4.6 mm or equivalent Mobile Phase A: Acetonitrile / Buffer pH = 2.3 10 / 90
Mobile Phase B: Acetonitrile / Buffer pH = 2.3 45 / 55
Buffer: Dissolve 1.1 g of Sodium Octansulfonate in 1 L of water. Adjust the pH to 2.3 with diluted
H3PO4. Column Temperature: 35°C
Detector: UV at 230 nm
Flow: 1.2 ml/min
Injection volume: 10 μl
Time of Analysis: 55 minutes
The procedure described in US 4,751,307 was repeated, starting from 1Og of naltrexone and yielding 8.5g of nalmefene. The isolated product showed the presence of phosphine oxides by-products above 15% molar as judged by 1HNMR.
Methyltriphenylphosphonium bromide (MTPPB, 112.9g) was suspended in 2- methyltetrahydrofuran (MTHF, 245 ml). Keeping the temperature in the range 20- 25°C, KO-t-Bu (38.7 g) was charged in portions under inert atmosphere in one hour. The suspension was stirred for two hours. An anhydrous solution of naltrexone (35 g) in MTHF (144 ml) was then added over a period of one hour at 20-250C. The suspension was maintained under stirring overnight. The mixture was then treated with a solution of glacial acetic acid (17.7 g) in MTHF. Water was then added and the pH was adjusted to 9-10. The phases were separated, the lower aqueous phase was discarded and the organic phase was washed twice with water. The organic phase was concentrated to residue under vacuum and then diluted with dichloromethane (175 ml) to give a clear solution. Concentrated aqueous hydrochloric acid (HCl 37%, 10. Ig) was added over one hour at 20- 25°C. The suspension was stirred and then filtered and washed with dichloromethane and acetone. The product was dried affording 38.1g of Nalmefene HCl. Example 4
Example 3 was repeated but the Wittig reaction mixture after olefmation completeness was treated with acetone and then with an aqueous solution of ammonium chloride. After phase separation, washings, distillation and dilution with dichloromethane, the product was precipitated as hydrochloride salt using HCl 37%. The solid was filtered and dried affording 37.6 g of Nalmefene HCl.
Example 5 Preparation of Nalmefene HCl dihydrate from Nalmefene HCl Nalmefene HCl (7.67 Kg, purity 99.37%, assay 93.9%) and water (8.6 litres) were charged into a suitable reactor. The suspension was heated up to 800C until the substrate completely dissolved. Vacuum was then applied to remove organic solvents. The resulting solution was filtered through a 0.65 μm cartridge and then diluted with water (2.1 litres) that has been used to rinse the reactor and pipelines. The solution was cooled down to 500C and 7 g of Nalmefene HCl dihydrate seeding material was added. The mixture was cooled to 0-50C over one hour with vigorous stirring and then maintained under stirring for one additional hour. The solid was filtered of and washed with acetone. The wet product was dried at 25°C under vacuum to provide 5.4 Kg of Nalmefene HCl dihydrate (purity 99.89%, KF 8.3% , yield 69%).
- US patent 3814768, Jack Fishman et al, “6-METHYLENE-6-DESOXY DIHYDRO MORPHINE AND CODEINE DERIVATIVES AND PHARMACEUTICALLY ACCEPTABLE SALTS”, published 1971-11-26, issued 1974-06-04
- Barbara J. Mason, Fernando R. Salvato, Lauren D. Williams, Eva C. Ritvo, Robert B. Cutler (August 1999). “A Double-blind, Placebo-Controlled Study of Oral Nalmefene for Alcohol Dependence”. Arch Gen Psychiatry 56 (8): 719. doi:10.1001/archpsyc.56.8.719.
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