Patent Abstract:
this invention concerns inhibitors of the tolerance and dependence induced by opiate analgesics . excitatory amino acids control the action of the nmda receptor through allosteric binding sites . early studies showed that channel blockers and allosteric site antagonists of the nmda receptor inhibited the development of tolerance to and dependence on opiate analgesics , but also produced their own undesirable side effects . partial agonists of the nmda allosteric sites tend to normalize nmda receptor function and do not produce strong undesirable side effects . it has been discovered that partial agonists inhibit the development of tolerance to opiate analgesics , and it is taught that partial agonists also inhibit development of dependence on opiate analgesics . acpc , 1 - aminocyclopropanecarboxylic acid , in particular , is an exemplary partial agonist at the glysine site useful to treat opiate addiction . compounds containing an opiate analgesic and a partial agonist , produce analgesia without inducing tolerance or dependence .

Detailed Description:
this detailed description is presented in three parts . first , several terms frequently used in this description are defined , some of them because their meanings are not yet standardized in the literature . second , a brief description of the materials and methods which were used to measure the effects of partial agonists on opiate induced tolerance . third , so that those skilled in the art may more fully understand and use the invention , scientific examples of the use of the partial agonists are given . the following terms are defined so that their use in this application is unambiguous : the term &# 34 ; agonist &# 34 ; as used herein means any compound which increases the flow of ca ++ through the nmda receptor -- a channel opener . the term &# 34 ; antagonist &# 34 ; as used herein means any compound which reduces the flow of ca ++ through the nmda receptor -- a channel closer . the term &# 34 ; ligand &# 34 ; as used herein means any compound which binds to a site on the nmda receptor . 1 ) any compound which is derived from opium including , but not limited to thebaine , morphine , diacetylmorphine , hydromorphone , oxycodone , codeine , and oxymorphone ; 2 ) any synthetic opioid acting at the same mu receptor sites as the compounds of subsection ( 1 ) hereof including , but not limited to meperidine , pentazocine , methadone , fentanyl , and butorphanol ; and 3 ) any endogenous or synthetic opioid peptides acting at the mu or delta receptors . the term &# 34 ; nmda receptor &# 34 ; as used herein means a postsynaptic receptor which is stimulated , at a minimum , by the excitatory amino acids glutamate and glycine . it is a ligand - gated receptor with a strychnine insensitive glycine site . the term &# 34 ; partial agonist &# 34 ; as used herein means a compound which modulates an allosteric site on the nmda receptor so as to increase or decrease the flux of ca ++ through the ligand - gated channel depending on the presence or absence of the principal site ligand . in the absence of the principal site ligand , a partial agonist increases the flow of ca ++ through the ligand - gated channel but at a lower flux than achieved by the principal site ligand -- a partial agonist may thus partially open the nmda receptor channel . in the presence of the principal site ligand , a partial agonist decreases the flow of ca ++ through the ligand - gated channel below the flux normally achieved by the principal site ligand -- a partial agonist may thus partially close the nmda receptor channel . the term &# 34 ; lower alkyl &# 34 ; as used herein means an alkyl radical having 1 - 9 carbon atoms , which may be straight or branched , including , for example , methyl , ethyl , propyl , isopropyl , butyl , isobutyl , tertbutyl , amyl , isoamyl , hexyl , heptyl , octyl , nonyl , or the like . the term &# 34 ; halogen &# 34 ; as used herein refers to fluorine , chlorine , bromine , and iodine atoms . the term &# 34 ; aryl &# 34 ; as used herein means an organic radical derived from an aromatic hydrocarbon , e . g ., phenyl from benzene . the term &# 34 ; pharmaceutically acceptable salt &# 34 ; as used herein includes acid addition salts , ammonium salts , hydrates , alcolates and other salts of formulas i and ii compounds disclosed herein which are physiologically compatible in warm blooded animals , the addition salts may be formed by either strong or weak acids . representative of strong acids are hydrochloric , sulfuric and phosphoric . representative of weak acids are fumaric , maleic , succinic , oxalic , citric , tartaric , cyclohexaminic and the like . experimental animals : male cd - 1 mice ( 25 - 35 g . ; charles river breeding laboratories , wilmington , mass .) were maintained on a 12 hr . light : 12 hr . dark cycle with purina rodent chow and water available ad libitum . mice were housed in groups of 5 until testing . experimental drugs : morphine sulfate was obtained from mallinckrodt ( st . louis , mo .) and u50 , 488h { trans - 3 , 4 - dichloro - n - methyl - n -[ 2 -( 1 - pyrrolindinyl ) cyclohexyl ]- benzeneacetamide , methane sulfonate hydrate } was supplied by upjohn pharmaceutics ( kalamazoo , mich .). naloxone benzoylhydrazone ( nalbzoh ) was synthesized as described by luke et al . ( luke , hahn , price , and pasternadk , 1988 ). acpc was obtained from senn chemicals , zurich , switzerland . the delta peptide [ d - pen2 , d - pen5 ] enkephalin ( dpdpe ) was obtained from the research technology branch of nida . naibzoh was dissolved in 30 % etoh , which did not produce analgesia in control mice , and doses are stated as the free base . all other drugs were dissolved in saline or water . experimental methods : single daily doses of all analgesics except for dpdpe along with a fixed dose of acpc were administered subcutaneously to the mice for a period of five to ten days . dpdpe was administered intrathecally on a daily basis . a dose ( mg / kg body weight ) of analgesic was chosen which would produce analgesia in approximately 50 % to 60 % of the mice . this value was chosen since it avoided the possibility of using too high a dose which would provide excess analgesia and at the same time would provided a sufficient dose so that the effects of tolerance and the inhibition of tolerance could be studied . analgesia in the mice was determined using the radiant heat tail - flick technique ( paul , levinson , howard , pick , hahn , and pasternak , 1990 ; paul , pick , tive , and pasternak , 1991 ). in this analgesic assay , the latency to withdraw the tail from a focussed light stimulus was measured electronically using a photocell . baseline latencies ( 2 . 0 - 3 . 0 sec .) were determined before experimental treatments for all animals as the mean of two trials . post - treatment tail - flick latencies were determined 30 minutes after the analgesic administration . the stimulus was removed after a maximal latency of 10 sec . to minimize tissue damage . analgesia was defined quantally as a doubling or greater of the baseline values for each mouse . all analgesic values presented represent at least 10 mice . to avoid testing animals on more than 6 days in order to minimize tissue damage , all animals were tested on day 1 after which several groups ( n = 10 each ) were used for the extended duration studies . dose - response curves were determined using three doses of morphine , with each dose including ten mice . the curves were analyzed using a modification of the bliss - 20 computer program . this program maximizes the log - likelihood function to fit a parallel set of gaussian normal sigmoid curves to the dose response data ( umans , and inturrisi , 1981 ). single - dose comparisons were analyzed using the fisher exact test . as indicated above , the prior art teaches that antagonists of the nmda receptor , i . e ., compounds which reduce the ca ++ ion flow , are required to inhibit the development of tolerance and dependence . in addition , inhibiting nitric oxide synthase , which is biochemically equivalent to reducing ca ++ ion flux through the nmda receptor with an antagonist , also inhibits dependence . the discovery of this invention that partial agonists have a similar effect is unexpected and surprising . applicant believes that the prior art has too narrowly concentrated on the individual separate agonistic and antagonistic effects of the partial agonists and that partial agonists should be viewed as regulators of nmda receptor function which always tend to normalize the receptor function . the fact that the use of partial agonists does not produce the neurotoxic side effects seen with competitive or non - competitive ( channel blocker ) antagonists of the nmda receptor supports the view that the partial agonists normalize nmda function . by utilizing the partial agonists , as described herein , it is possible to treat tolerance and dependence induced by opiate analgesics without producing unwanted side effects such as schizophrenia - like symptoms , loss of normal nmda receptor - mediated synaptic plasticity ( which can possibly affect learning and memory ), amnesia , confusional states , and muscle relaxation . this unexpected beneficial result may provide , at long last , a solution to several vexing problems involving the use of opiates including the management of chronic pain in severely ill patients and alleviation of the pain of withdrawal both in legitimate and illegitimate drug users . several partial agonists of the nmda receptor are known which work at either the glycine site or the polyamine associated site such as d - cycloserine and the con - g derivatives disclosed in the maccecchini patent application cited earlier , and all are considered within the scope of the present invention . other suitable compounds useful in this invention include partial agonists of the glycine site having the following formulas i and ii below : ## str1 ## wherein a is -- nh 2 , -- nhr 1 or -- nr 1 r 2 ; r 1 , r 2 , and r 3 , same or different , are selected from lower alkyl and lower alkyl substituted by halogen , hydroxyl , alkoxy , oxo , mercapto , aryl or amino ; or a pharmaceutically acceptable salt thereof . ## str2 ## wherein a 1 is -- nh 2 , -- nhr 1 or -- nr 1 r 2 , r 1 , r 2 and r 3 , same or different , are lower alkyl , or a pharmaceutically acceptable salt thereof . the partial agonists of formulas i & amp ; ii are exemplified by 1 - aminocyclopropanecarboxylic acid ( acpc ). while the experimental results presented below were obtained with acpc , this is in no way to be construed as limiting applicant &# 39 ; s general disclosure herein of a method and compounds for inhibiting tolerance and dependence induced by opiate analgesics . applicant considers within the scope of this disclosure relating to inhibition of opiate induced tolerance and dependence , all partial agonists of the nmda receptor including , but not limited to , those presently identified for the glycine and polyamine associated sites . further , it is believed that the herein disclosed use for partial agonists will encourage further discovery efforts to identify natural or synthetic compounds having potentially even broader ranges of partial agonist activity which may be used to alleviate the suffering produced by tolerance and dependence on opiate analgesics . the inhibition of the development of tolerance to the opiate morphine and the opioids dpdpe , u50 , 488h , and nalbzoh by the partial agonist acpc was studied in mice by administering single daily constant doses of each analgesic along with a fixed dose of acpc for a period of five to ten days . the extent of analgesia was measured using the tail - flick assay . fig2 shows the effects of acpc on the development of tolerance to morphine . the solid oval curve of fig2 demonstrates that the animals developed typical analgesic tolerance to the morphine . the dose of morphine which was sufficient to produce analgesia in 60 % of the animals on day 1 produced 2 no analgesia by day 5 . this curve is typical of the phenomenon of tolerance as also seen in humans . however , the administration of acpc along with the morphine inhibited the development of tolerance to morphine as can be seen in the open and solid triangle curves of fig2 . acpc significantly retarded the development of tolerance to morphine . at five days , the analgesic response of the acpc treated animals was not much different than the response of the animals at day 1 but was significantly different from the animals receiving no acpc who exhibited no analgesic effect of the morphine . further , even after ten days acpc inhibited the development of tolerance . lowering the dose of acpc to 50 mg / kg yielded virtually identical results demonstrating that acpc effectively inhibits tolerance at very low dose levels . animal studies carried out on monkeys have demonstrated that acute administration of acpc does not produce noticeable side effects until 2 . 0 g / kg of body weight is given . further , acute administration to monkeys of up to 8 g / kg acpc is not lethal . these levels represent extraordinarily high doses ; much greater than needed for therapies . in other animal studies , an injection of 300 mg / kg has been found to yield a concentration of 500 μm in brain tissue . further , in vitro studies on rodent derived nmda receptors have demonstrated the effect of acpc at concentrations of 1 - 5 μm . acpc may also be administered orally . the effects seen in fig2 were not due to any analgesic effect produced by acpc itself and acpc does not potentiate the effect of morphine . table 1 below demonstrates the effect of acpc on morphine analgesia . table 1______________________________________effect of acpc on morphine analgesiaacpc treatment morphine ed . sub . 50 ( mg / kg ) ______________________________________none 4 . 4 ( 3 . 6 , 5 . 3 ) acute 4 . 6 ( 3 . 2 , 6 . 5 ) chronic 3 . 9 ( 2 . 4 , 5 . 7 ) mice were given acpc ( 150 mg / kg ) either immediatelybefore performing a morphine dose - response curve ( acute ) or daily for 5 days before determining the morphine ed . sub . 50 ( chronic ). ed . sub . 50 values were determined using at least threegroups of mice , each comprised of 10 subjects . the valuesin parentheses are the 95 % confidence limits for thedetermination . ______________________________________ acutely , acpc ( 150 mg / kg ) did not change morphine &# 39 ; s ed 50 . chronic administration of acpc followed by analgesic testing of morphine also did not significantly change morphine &# 39 ; s analgesic potency . these results are very similar to those seen with nmda antagonists including mk - 801 . fig3 shows the effect of acpc ( 150 mg / kg ) on the development of tolerance to dpdpe , a synthetic peptide enkephalin analog , which acts at a delta receptor instead of at a mu receptor as does morphine . after five days without acpc , as shown by the solid oval curve , the test animals developed a tolerance to dpdpe . the difference between the 10 % analgesic value indicated on the graph and 0 % is not considered statistically significant in this measure . daily administration of acpc at 150 mg / kg , as shown by the open triangle curve , inhibited the development of tolerance in a manner similar to the inhibition of morphine tolerance . acpc virtually eliminated the tolerance seen at five days . thus , acpc inhibits the development of tolerance to a delta receptor ligand as well as to a mu receptor ligand . fig4 shows the effect of acpc at 150 mg / kg on the development of tolerance to u50 , 488h , a kappa 1 analgesic . essentially , acpc does not prevent the development of tolerance to the kappa 1 analgesic . fig5 shows the effect of acpc at 150 mg / kg on the development of tolerance to nalbzoh , a kappa 3 analgesic . as with the kappa 1 analgesic , acpc does not prevent the development of tolerance to the kappa3 analgesics . these results parallel those seen with the kappa 1 and kappa 3 analgesics and the nitric oxide synthase inhibitor n g - nitro - l - arginine ( kolesnikov , pick , ciszewska , and pasternak , 1993 ). from the above it can be seen that a partial agonist of the glycine site on the nmda receptor inhibits the development of tolerance to morphine in a manner similar to that seen with the channel blocker mk - 801 and the nitric oxide synthase inhibitor n g - nitro - l - arginine . since mk - 801 and ha - 966 , a competitive antagonist for the glycine site , also inhibit the development of dependence , it is part of the teaching of the present invention that partial agonists of allosteric sites on the nmda receptor also inhibit the development of dependence . accordingly , this invention encompasses the use of partial agonists to inhibit the development of dependence . it is also reasonable to expect that compositions comprising several partial agonists , having activities either at the same site or at different sites , may be particularly effective in inhibiting development of tolerance and dependence , and such simultaneous use of multiple partial agonists is also within the scope of this invention . the partial agonists of formula i and ii of this invention may be formulated into sterile pharmaceutical compositions for injection by combination with appropriate pharmaceutically acceptable carriers or diluents or may be formulated into preparations in liquid or solid forms for other routes of administration . administration of the compounds of formula i & amp ; ii can be by any technique which delivers the compounds into the bloodstream including oral administration , and by intravenous , intramuscular , or subcutaneous injection . the following methods and excipients are , therefore , merely exemplary and are in no way to be construed as limiting the present inventive methods or compositions . in pharmaceutical dosage forms , partial agonists of formulas i & amp ; ii encompassed by the present invention may be used in the form of their pharmaceutically acceptable salts , and may also be used alone or in appropriate association , as well as in combination with other pharmaceutically active compounds such as dependence - inducing and tolerance - inducing compounds . the compounds of formulas i & amp ; ii may be formulated into preparations for injection by dissolving , suspending or emulsifying them in an aqueous or non - aqueous solvent , such as vegetable oil , synthetic aliphatic acid glycerides , esters of higher aliphatic acids or propylene glycol ; and , if desired , with conventional additives such as solubilizers , isotonic agents , suspending agents , emulsifying agents , stabilizers and preservatives . alternatively , if an oral dosage is desired containing the partial agonists of formulas i & amp ; ii this invention alone or the partial agonists in combination with tolerance - inducing and dependence - inducing compounds , commonly used and pharmaceutically acceptable tabletting excipients , such as lactose , microcrystalline cellulose , corn starch stearic acid , or the like , may be used , if desired , to prepare such dosage forms . the amount of the partial agonist compounds of the present invention to be used may vary according to the degree of tolerance or dependence exhibited and may be adjusted for increases or decreases in the dosage of the tolerance - inducing or dependence - inducing compounds administered as well as the period of time during which such tolerance - inducing or dependence - inducing compounds are administered . nonetheless , when the compounds of formula i and ii are injected , a suitable dosage is thought to be about 0 . 1 to 20 mg / kg body weight , and preferably 2 to 10 mg / kg body weight . the most preferred dosages is , of course , that amount sufficient to inhibit tolerance or dependence in the subject . the partial agonists of this invention may be formulated into unit dosage forms , wherein the term &# 34 ; unit dosage &# 34 ; refers to physically discrete units suitable as unitary dosages for human subjects , each unit containing a predetermined quantity of a partial agonist either alone or in conjunction with a dependence - inducing or toleranceinducing compound , calculated in an amount sufficient with a pharmaceutically acceptable diluent , carrier , or vehicle . the specification for such unit dosage forms depends on the particular partial agonist or partial agonist and tolerance - inducing or dependence - inducing compound employed and the effect to be achieved , and the pharmacodynamics associated with each compound in the treated subject . any necessary adjustments in dose can be readily made to meet the needs of a given subject by the skilled practitioner . although the invention has been described with reference to acpc as representative of the compounds of formulas i & amp ; ii , which in turn are representative of all partial agonists of the nmda receptor , this should not be construed as a limitation on the invention . rather , various other partial agonists , equivalents , modifications , and other derivatives as would be obvious to one skilled in the art upon learning of the invention and without departing from the spirit and scope of the invention are intended to be included within the scope of the disclosure and of the following claims . 1 . chemistry news : frog venom cocktail yields a one - handed painkiller . 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