Patent Publication Number: US-2007105940-A1

Title: Method for treating pain

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to the treatment or prevention of pain or nociception.  
      2. Related Art  
      Pain is a sensory experience distinct from sensations of touch, pressure, heat and cold. It is often described by sufferers by such terms as bright, dull, aching, pricking, cutting or burning and is generally considered to include both the original sensation and the reaction to that sensation. This range of sensations, as well as the variation in perception of pain by different individuals, renders a precise definition of pain difficult, however, many individuals suffer with severe and continuous pain.  
      Pain that is caused by damage to neural structures is often manifest as a neural supersensitivity or hyperalgesia and is termed “neuropathic” pain. Pain can also be “caused” by the stimulation of nociceptive receptors and transmitted over intact neural pathways, such pain is termed “nociceptive” pain.  
      The level of stimulation at which pain becomes noted is referred to as the “pain threshold.” Analgesics are pharmaceutical agents which relieve pain by raising the pain threshold without a loss of consciousness. After administration of an analgesic drug a stimulus of greater intensity or longer duration is required before pain is experienced. In an individual suffering from hyperalgesia an analgesic drug may have an anti-hyperalgesic effect. In contrast to analgesics, agents such as local anaesthetics block transmission in peripheral nerve fibers thereby blocking awareness of pain. General anaesthetics, on the other hand, reduce the awareness of pain by producing a loss of consciousness.  
     BRIEF SUMMARY OF INVENTION  
      It has now been discovered that certain compounds which exhibit the properties of blocking transient receptor potential vanilloid type 1 (TRPV1) channels and activating cannabinoid CB 1  receptors have a utility for the amelioration of pain and particularly for the amelioration of neuropathic pain.  
      Therefore, in one aspect, the method of the present invention utilizes a compound, N-arachidonoyl-serotonin (AA-5-HT) and its congeners according to formula I  
                 
 
 wherein n is an integer of from 1 to 10, m is an integer of from 1 to 4 and the total carbon atoms in the alkenyl amide chain is from about 11 to about 20 to ameliorate or treat pain. 
 
      The compounds of this invention may be prepared by reacting serotonin or an analogue or homologue thereof with a carboxylic acid to form the corresponding amide of said carboxylic acid and serotonin (or analogue or homologue thereof). This reaction may be carried out at conditions known in the art for preparing amides of fatty acids e.g., which fatty acids have similar reaction properties as the above carboxylic acids.  
      In another aspect, the invention provides a method for the treatment of pain using a compound in accord with formula I, the method comprising administering a pain-ameliorating effective amount of the compound.  
      In another embodiment, the method comprises administration of a pain-ameliorating effective amount of a compound according to formula I in the form of a pharmaceutical composition comprising a compound according to formula I as an active ingredient together with one or more pharmaceutically-acceptable additives.  
      In a further embodiment, the method comprises binding a compound according to formula I to the TRPV1 channel of a warm-blooded animal, such as a human being, so as to beneficially inhibit the activity of said channel to activation by capsaicin, for example.  
      In a further embodiment, the method comprises binding a compound according to formula I to the fatty acid amide hydrolase of a warm-blooded animal, such as a human being so as to beneficially enhance endocannabinoid levels and increase the activity of CB1 receptors.  
      Yet other aspects of the invention are pharmaceutical compositions which contain the compound in accord with formula I and the use of the compound in accord with formula I for the preparation of medicaments and pharmaceutical compositions. 
    
    
     DESCRIPTION OF THE DRAWING FIGURES  
      FIG. 1A is a plot showing the IC 50  of AA-5-HT against capsaicin.  
      FIG. 1B is a plot showing that AA-5-HT behaves as a non-competitive antagonist.  
      FIG. 2A shows that AA-5-HT inhibits nociceptive response to formalin injected into a rat paw.  
      FIG. 2B shows that the effect of AA-5-HT is reversed by AM 251 and occluded by capsazepine.  
      FIG. 3 shows the change in mechanical nociceptive response as a result of administration of URB-597 and AA-5-HT.  
      FIG. 4 shows the change in thermal nociceptive response as a result of administration of URB-597 and AA-5-HT. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Genetic or pharmacological targeting of fatty acid amide hydrolase (FAAH), one of the enzymes catalysing endocannabinoid degradation, was shown to result in analgesic and anti-hyperalgesic actions that are due to the “indirect” activation (via enhancement of endocannabinoid levels) of cannabinoid CB 1  receptors. Additionally, genetic or pharmacological targeting of transient receptor potential vanilloid type 1 (TRPV1) channels was found to abolish thermal and inflammatory analgesia. We describe a class of “hybrid” FAAH inhibitors/TRPV1 antagonists with high efficacy against inflammatory hyperalgesia. These “hybrid” FAAH inhibitors have the general formula I:  
                 
 
 wherein n is an integer of from 1 to 10, m is an integer of from 1 to 4 and the total carbon atoms in the alkenyl amide chain is from about 11 to about 20 to ameliorate or treat pain; 
 
      N-arachidonoyl-serotonin (AA-5-HT) and its congeners with general chemical structure shown here, inhibit FAAH as mixed inhibitors and in a range of concentrations between 1.5 and 15 μM depending of the animal species and type of enzyme preparation. We now show that these compounds also interact, by blocking their activation by capsaicin, with TRPV1 channels, whose gating plays a permissive role in the development of hyperalgesia, e.g. following formalin injection into the paw of laboratory animals (see FIG. 1A, B). For AA-5-HT the IC 50  against capsaicin (100 nM) was calculated to be 130 nM (FIG. 1A), similar to that previously reported for the well known TRPV1 antagonist capsazepine (60 nM). The compound behaved as a non-competitive antagonist (slope=0.5) (FIG. 1B). Similar results were obtained with other congeners, provided that at least one double bond was present in the acyl chain.  
      When injected directly into the periaqueductal grey (PAG) of rats, AA-5-HT (4 μg/rat) potently inhibited both phases of the nociceptive response to formalin injected into the rat paw (FIG. 2A) and concomitantly elevated anandamide levels in this area of the brainstem, when these were measured 20 min following injection. The anti-hyperalgesic effect was counteracted by the CB 1  receptor antagonist, AM251 (nmol/rat) and was occluded by the TRPV1 antagonist, capsazepine (6 nmol/rat). Thus, while not wishing to be bound by theory, it is believed that the compounds of formula I ameliorate pain by the dual mechanism of action of AA-5-HT via both “indirect” activation of CB 1  and antagonism of TRPV 1 . The compound acted at the supraspinal level by blocking the inhibitory effect of formalin on the OFF cells of the rostral ventromedial medulla, which receive synapses with cells from the PAG. Also this effect of AA-5-HT was reversed by AM251 and occluded by capsazepine (FIG. 2B). When injected into the paw, AA-5-HT (1 mg/rat, s.c.) selectively blocked the 2 nd , inflammatory phase of the nocifensive response to formalin, again in a way counteracted by AM251 and occluded by capsazepine, thus suggesting also a peripheral mode of action.  
      AA-5-HT and its congeners are novel agents against anti-inflammatory pain, acting by enhancing endocannabinoid levels (via FAAH inhibition) and at the same time by antagonizing TRPV1.  
      The advantage of having in one molecule a FAAH inhibitor and a TRPV1 antagonist comes from the several experimental observations suggesting that FAAH inhibitors (i.e. “indirect” agonists of cannabinoid and fatty acid amide receptors) as well as direct cannabinoid receptor agonists (both CB1 and CB2) are very promising against inflammatory and neuropathic pain, and so are compounds that block TRPV1 receptors. However, different populations of neurons/cells and different mechanisms are involved in CB1/CB2- and TRPV1-mediated anti-inflammatory and anti-hyperalgesic/anti-allodynic effects. Therefore, if for example following nerve injury, only one of these different populations is destroyed, a compound only acting on that population will be ineffective, whereas a compound with “hybrid” activity will always be more effective. On the other hand if different nociceptive mechanisms cause pain, a drug targeting more of these mechanisms will be more efficacious than a drug specific for only one of them.  
      To use the compound of the invention or a pharmaceutically-acceptable salt thereof for the therapeutic treatment, which may include prophylactic treatment, of pain in mammals, which may be humans, the compound can be formulated in accordance with standard pharmaceutical practice as a pharmaceutical composition.  
      Suitable pharmaceutical compositions that contain the compounds of the invention may be administered in conventional ways, for example by oral, topical, parenteral, buccal, nasal, vaginal or rectal administration or by inhalation. For these purposes a compound of the invention may be formulated by means known in the art into the form of, for example, tablets, capsules, aqueous or oily solutions, suspensions, emulsions, creams, ointments, gels, nasal sprays, suppositories, finely divided powders or aerosols for inhalation, and for parenteral use (including intravenous, intramuscular or infusion) sterile aqueous or oily solutions or suspensions or sterile emulsions. A preferred route of administration is orally by tablet or capsule.  
      In addition to a compound of the present invention a pharmaceutical composition of this invention may also contain one or more other pharmacologically-active agents, or such pharmaceutical composition may be simultaneously or sequentially co-administered with one or more other pharmacologically-active agents.  
      Pharmaceutical compositions of this invention will normally be administered so that a pain-ameliorating effective daily dose is received by the subject. The daily dose may be given in divided doses as necessary, the precise amount of the compound received and the route of administration depending on the weight, age and sex of the patient being treated and on the particular disease condition being treated according to principles known in the art. A preferred dosage regime is once daily.  
      A further embodiment of the invention provides a pharmaceutical composition which contains a compound of the invention as defined herein or a pharmaceutically-acceptable salt thereof, in association with a pharmaceutically-acceptable additive such as an excipient or carrier.  
      A yet further embodiment of the invention provide the use of the compound of the invention, or a pharmaceutically-acceptable salt thereof, in the manufacture of a medicament useful for blocking the TRPV1 channel in a warm-blooded animal such as a human being.  
      Still another embodiment of the invention provides a method of binding the compound of the invention to the TRPV1 channel of a warm-blooded animal, such as a human being, in need of treatment for pain, which method comprises administering to said animal an effective amount of a compound of formula I or a pharmaceutically-acceptable salt thereof.  
      A yet further embodiment of the invention provide the use of the compound of the invention, or a pharmaceutically-acceptable salt thereof, in the manufacture of a medicament useful for indirectly activating the cannabinoid CB 1  receptor in a warm-blooded animal such as a human being.  
      Still another embodiment of the invention provides a method of binding the compound of the invention to the FAAH of a warm-blooded animal, such as a human being, in need of treatment for pain, which method comprises administering to said animal an effective amount of a compound of formula I or a pharmaceutically-acceptable salt thereof.  
      The invention is further illustrated by the following examples which are illustrative of specific modes of practicing the invention and are not intended as limiting the scope of the appended claims.  
     EXAMPLE 1  
      Surgery and Treatments  
      Neuropathic pain was induced by chronic constrictory injury (CCI) of the sciatic nerve. Rats were anaesthetized with sodium pentobarbital (60 mg/kg i.p.), the right sciatic nerve was exposed and four ligatures were loosely tied around the nerve just proximal to the trifurcation. Control rats underwent a sham surgery with exposure of the sciatic nerve without ligature. URB-597 (3 mg/kg i.p.), AA-5-HT (5 mg/kg i.p.) and respective vehicle (20% DMSO in 0.9% NaCl) were administered to sham and CCI groups (n=5) of rats for 7 days, starting the day after surgery (day 1).  
      Nociceptive Behaviour  
      Changes in thermal and mechanical nociceptive responses have been evaluated using a Plantar Test Apparatus and Dynamic Plantar Anaesthesiometer (Ugo Basile, Varese, Italy). Thermal paw withdrawal latency and mechanical paw withdrawal threshold (PWL and PWT) were measured (in sec and grams, respectively) for each group of rats every 30 min for 3 h.  
     CONCLUSIONS  
      AA-5-HT was as efficacious as URB-597, a known inhibitor of FAAH, at reducing thermal nociceptive responses. Regarding mechanical allodynia, which is the true sign of chronic pain in this model, URB-597 was ineffective whereas AA-5-HT totally abolished it. This strengthens the hypothesis that this compound, which in vitro is 100-fold less potent than URB-597 at inhibiting FAAH, also acts by additional mechanisms.  
      These results are reported in FIGS. 3 and 4.  
      The foregoing description details specific methods and compositions that can be employed to practice the present invention, and represents the best mode contemplated. Thus, however detailed the foregoing may appear in text, it should not be construed as limiting the overall scope hereof; rather, the ambit of the present invention is to be governed only by the lawful construction of the appended claims.