sign in sign up
<<
Home , Chlornaltrexamine

PostDoc Journal Journal of Postdoctoral Research Vol. 1, No. 4, April 2013 www.postdocjournal.com

Opioid receptors in drug design – the past, present and future Ajay S. Yekkirala1, 2 1F. M Kirby Neurobiology Center, Children’s Hospital Boston, 2Department of Neurobiology, Harvard Medical School, Boston MA 02115 USA Email: [email protected]

Abstract Pain is a common phenomenon that is expressed due to external tissue injury or innate physiological dysfunction. While everyone has experienced pain in some form or the other, over 40% of all Americans visited the clinic due to chronic pain each year according to the American Pain Society. In spite of certain deleterious side effects, such as, respiratory depression, tolerance, and addictive potential, have remained the drugs of choice for the treatment of pain. This review provides a brief snapshot of the past, and ongoing, research in the field, to generate potent without the debilitating side effects.

Opioid receptors and their ligands – a brief meperidine, could also elicit potent analgesic history effects was a departure that revitalized the It has been known for over five millennia field4. In time, other analgesics such as that the alkaloids obtained from the juice of , , morphinans and poppy seeds proffered analgesic and benzomorphans were synthesized. However, the euphoric properties1. The various ligands in the ideal opioid that would produce potent analgesia opium alkaloid cocktail were termed ‘’, of without deleterious side effects remained which is the major component2. elusive. Decades of concerted research have expanded By the early 1950s, structure-activity the list of compounds that have similar relationship (SAR) studies with the prevailing pharmacological effects, and the opioid ligands suggested that ligand structure, armamentarium as a whole is designated as size and shape were all important for analgesic ‘opioid’ ligands1,2. Even with the advent of activity. This led Beckett and Casy to first several classes of analgesic drugs, opioids propose a unique that followed agonists such as, morphine and are the the lock and key mechanism to interact with analgesics of choice to treat pain in the clinic. opioids5. To simplify their hypothesis, they The major problems with managing pain proposed that all opioids adopt a morphine-like using morphine are respiratory depression, structure within the receptor, which allowed for tolerance, constipation, and physical the similarity of activity for a diverse group of dependence associated with its use. In 1929, it molecules. However, there were several was proposed that making structural anomalies that did not fit such a structurally rigid modifications to the morphine scaffold would receptor1,4. result in a molecule devoid of the side effects In an effort to address the while retaining its more salutary attributes3,4. inconsistencies in the Beckett and Casy model, Unfortunately, none of the synthetic molecules Portoghese suggested an alternate hypothesis in showed any reduction in addictive potential. This 19656. Again using SAR, he showed that parallel was a considerable blow to the opioid field as changes of the N-substituent of rigid scaffolds the prevailing thought at the time was that it (morphine, morphinan or benzomorphan) would be impossible for a non-morphine scaffold produced similar effects on the analgesic to have potent analgesic properties. Hence, the activity. This suggested that the rigid parent discovery that the rather simple piperidine, structures were all interacting with the active 10 Journal of Postdoctoral Research 2013: 9-23

Figure 1. Selective ligands for mu opioid receptors site in a simi1lar fashion. However, for non-rigid that the mouse vas deferens showed greater scaffolds (methadone, meperidine, etc) similar affinity for than morphine led to the modification did not produce parallel changes in suggestion of the delta (δ) opioid receptor18. Two activity indicating the rigid and non-rigid studies by Cuatrecasas and colleagues showed molecules were binding differently to the opioid the existence of the -preferring delta receptor. This was hence called the bimodal site in the brain, and they attempted to describe binding model of opioids. A prescient opioid receptor distribution in various brain interpretation by Portoghese was that instead of regions19,20. However, to perform such detailed different modes of interaction with the same analyses it was paramount to develop ligands active site, the data also indicated the possible selective for the various suggested opioid existence of multiple opioid receptors6. receptors types (Fig 1-3). Around the same time, Goldstein and colleagues also used structural determinants in Selective opioid ligands opioids to propose the existence of a unique The development of high specific opioid receptor7. As the radioligand binding activity tritiated ligands in the 1970s and 1980s assays gained prevalence8, three independent opened up the field for receptor characterization laboratories simultaneously described the first and localization studies. After the discovery of opioid receptor sites in rat brain membranes9-11. enkephalins, synthetic efforts were centered on Shortly thereafter, Hughes and coworkers12 making substitutions that would render stability isolated the first endogenous -like factors, towards hydrolysis by 21. These methionine enkephalin (Met-enkephalin) and efforts led to the synthesis of the enkephalin leucine enkephalin (Leu-enkephalin). The analogs, DADLE and DSLET, that were shown to discovery of the biological aspects of the opioid be delta-selective peptides18-21 (Fig 2). However, system was well underway. they still had affinity for mu receptors. The To add to the initial suggestion by synthesis of conformationally restricted bis- Portoghese6 that the diversity in opioid ligand pencillamine enkephalins afforded the highly structure would require multiple opioid receptor delta-selective analog DPDPE22. sites, the experiments by Martin and The isolation of the endogenous linear coworkers13-16 in dogs led to suggestion of mu heptapeptides from frog skin extracts led to the (µ), kappa (κ) and sigma (σ) opioid receptors. discovery of deltorphins23,24. The first ligand that The activity of sigma receptors is not - was isolated had the sequence: Tyr-D-Met-Phe- reversible and these receptors are no longer His-Leu-Met-Asp-NH2. Two more peptides were considered as opioid receptors17. An observation later reported that had D-Alanine as the second Ajay S. Yekkirala 11

Figure 2. Selective ligands for delta opioid receptors kappa, when compared with mu or delta residue, with either aspartate or glutamate receptors, making it the most selective kappa residue in position 4, and were named [D-Ala2]- ligand. This study also showed that I (DELT-I) and II (DELT-II), respectively. was less selective and could bind to all the three These linear peptides showed the highest affinity opioid receptors. Thus, in early studies, for delta receptors compared to any of the morphine 27,28 and [D-Ala2-MePhe4-Glyol5] ligands at the time. Indeed, the two [D-Ala2] enkephalin (DAMGO; 21,29) have been described had ~200-fold greater affinity for and used widely as selective ligands for mu delta receptors than DPDPE. receptors, DADLE, D-Pen5D-Pen5enkephalin For kappa receptors (Fig 3), (DPDPE; 22) and deltorphin-II (DELT-II) 30,31 for ethylketocyclazocine (EKC, Fig 1.5) stood as the delta receptors, and EKC, U69593 26,32, and prototypic ligand throughout the 1970s. bremazocine 32 for kappa receptors. However, Von Voightlander and colleagues used binding and in vivo behavioral procedures to Early attempts to tackle morphine’s adverse show that the benzeneacetamide, U50,488, is a effects in the clinic: Mixed agonist-antagonist kappa ligand with greater selectivity25. They ligands showed that U50,488 displaced [3H]EKC and the The development of tolerance, physical displacement was not blocked by high doses of dependence, and respiratory depression are the . In addition, U50,488 did not major side effects associated with morphine produce cross-tolerance to morphine tolerance pharmacotherapy. The design of improved suggesting that U50,488 was mediating its ligands with reduced deleterious effects and effects via a non-mu opioid receptor. pA2 studies efforts to elucidate mechanisms that lead to with naloxone further implicated the tolerance and dependence remain a major focus involvement of kappa receptors in the activity of of opioid research. was an early U50,488 and bremazocine. that was used in the clinic for At the time, there still wasn’t a highly . However, Lasagna and selective tritiated ligand available for kappa coworkers were surprised to observe that receptors. The introduction of [3H]U69593, an nalorphine could produce potent analgesic analog of U50,488, provided the opportunity to activity by itself33. Since nalorphine was shown determine the distribution and expression levels to reverse morphine-induced withdrawal, it was of kappa receptors in various tissues26. U69395 deemed that a mixture of kappa agonist opioid was shown to be 484-fold more selective at with mu antagonist properties would be a preferred opioid analgesic. The hypothesis was

12 Journal of Postdoctoral Research 2013: 9-23 that µ antagonism would ensure that the ligand would mitigate mu opioid side effects like respiratory depression, tolerance, and dependence, while the kappa agonism would confer the antinociceptive ability2. With this premise a number of kappa agonist/mu antagonist ligands were developed. , a benzomorphan-derived analgesic, was one of the first drugs in this class that was initially considered to have the right attributes and was used extensively in the clinic33,34. Depending on the route of administration, pentazocine was considered to be one-half to one-fifth as potent as morphine35- 38. Pentazocine could attenuate abstinence syndrome in patients dependent on low-dose morphine (30 mg/kg), but not at higher doses of morphine. Indeed, pentazocine has been shown to precipitate withdrawal-like syndrome in morphine-addicted patients which may be due to its mu antagonistic activity39. Pentazocine has also been shown to precipitate physical dependence, and naloxone administration in Figure 3. Selective ligands for kappa opioid these patients can produce moderate receptors withdrawal symptoms39,40. In addition, pentazocine can also produce dysphoric and is an opioid ligand of the morphinan psychotomimetic effects that limited its use39,41- class of compounds and has been used 43. It was also contraindicated in patients with extensively in the clinic. Butorphanol produces coronary problems44. pharmacological effects that are also very much is a like pentazocine. By the intramuscular route of analogue that shows effects that are very much administration, butorphanol has been found to like pentazocine. However, nalbuphine has been be >15-fold more potent than pentazocine and 5-8 times more potent than morphine47-51. shown to produce substantially fewer 45 Similar difference in potency was also observed psychotomimetic effects . In postoperative pain when both morphine and butorphanol were management, nalbuphine was equipotent to administered by the intravenous route52. morphine, but at least 3 to 5-fold more potent Butorphanol was reported to be fast acting with than pentazocine. Physical dependence was observed only with chronic administration of limited side effects. Indeed, psychotomimetic ~200 mg of nalbuphine per day. Interestingly, effects were only observed in patients who had a nalbuphine did not produce any abstinence history of using drugs which suggested that these effects may represent mild syndrome in morphine-dependent patients, 53 unlike naloxone46. withdrawal syndrome . In spite of the largely favorable pharmacological profile (limited tolerance and abuse potential when compared with morphine) of the above analgesics, the psychotomimetic and dysphoric effects made them eventually unpopular in the clinic. Indeed, even today, the Ajay S. Yekkirala 13 issue of limiting the abuse potential of opioids A major step in the quest to develop remains the central focus of most opioid selective antagonists for kappa receptors investigators. occurred with the discovery of TENA, a bivalent Opioid antagonists ligand containing two -derived The introduction of naloxone as pharmacophores tethered by a spacer of 10 a potent narcotic antagonist remains a atoms64. Portoghese and colleagues reported watershed moment in the history of opioid that shorter spacers promoted greater selectivity research54,55. It is a potent and non-selective for kappa receptors, while increased spacer opioid antagonist that has been used so often length promoted mu selectivity65. With this in that those ligands whose effects are not mind, bivalent ligands with short spacers were naloxone-reversible are not considered to act via developed. Pyrrole ring was chosen as a spacer opioid receptors. It has been most useful in the to conformationally restrict the orientation of clinic as an antidote for narcotic overdose and the molecule in an effort to facilitate better has helped save many lives. Even though binding to the kappa receptors. This led to the naltrexone was more potent than naloxone, the development of (norBNI, Fig lack of intrinsic activity has helped naloxone 3) that possessed the highest affinity and remain as the prototypic opioid antagonist56 (Fig selectivity for kappa receptors. Indeed, even 1). with the synthesis of several other kappa The elucidation of effects of the selective antagonists, to date norBNI has different opioid receptors was greatly facilitated remained the standard kappa antagonist in by the synthesis of selective opioid antagonists. opioid research66,67. In the early 1980s, Portoghese and coworkers A few years before the synthesis of described the synthesis, pharmacological norBNI, the “message-address concept” was properties, and mechanism of action of β- proposed by Schwyzer as a way to reconcile funaltrexamine (β-FNA) as an affinity label for peptide ligand selectivity for receptors68. Based mu opioid receptors57-60. β-FNA produced long- on specific sequence identifiers in opioid acting and irreversible antagonism of mu peptides, this model was shown to be applicable agonists in the guinea pig ileum (GPI) to non-peptide opioid ligands as well69,70. For preparation, mouse vas deferens (MVD), and in instance, it was suggested that adding Phe-Leu 71 vivo59,60. Indeed, β-FNA produced long-acting to mu ligands conferred delta selectivity . This antagonism of morphine, even 120 hrs after i.c.v. was used as the design strategy to convert or s.c. administration in mice. Interestingly, β- naltrexone into a non-peptide delta antagonist. FNA produced short-acting agonism that was It was hypothesized that the benzene ring in 4 reversible and was attributed to activity at kappa Phe of Leu-enkephalin will need to be receptors58. incorporated into the morphinan core of In search of a reversible antagonist for naltrexone via a rigid spacer. While a pyrrole ring mu opioid receptors, Hruby and colleagues was chosen as the spacer for synthetic ease, it developed D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen- led to the development of the prototypic delta 72 Thr-NH2 (CTOP) and D-Phe-Cys-Tyr-D-Trp-Arg- antagonist, (NTI, Fig 2) . NTI was Thr-Pen-Thr-NH2 (CTAP), cyclic derivatives of 240-fold more selective for delta receptors than somatostatin, as highly selective mu ligands61,62 naltrexone, giving credence to the incorporation (Fig 1). They attributed the mu selectivity by of the delta address-mimic into a non-selective determining the ability of CTOP in displacing ligand. Using a furan ring instead of the pyrrole 73 [3H]naloxone and [3H]DPDPE. The results showed led to the synthesis of (NTB, Fig 2) that CTOP displaced [3H]naloxone ~ 4,829-fold which is less potent, but with greater affinity greater than [3H]DPDPE. In 1994, CTOP was than NTI. shown to be highly selective for the cloned mu opioid receptor63. Genetic receptors vs pharmacological receptors

14 Journal of Postdoctoral Research 2013: 9-23

For years investigators had been piecing the DALCE and BNTX selectively inhibited DPDPE, together the complex physiological effects of but not DELT-II. On the other hand NTB opioid receptors with the help of ligands. selectively antagonized DELT-II, but did not However, given that most ligands are not specific antagonize DPDPE. This led to the postulation of for a single receptor, it was near impossible to two distinct delta subtypes: δ1 which is selective determine the receptor effects in isolation. The for DPDPE and BNTX, and δ2 which is selective advent of molecular biology changed this for DELT-II and NTB. scenario when two groups independently cloned Genetic manipulation of delta receptors the delta opioid receptor74,75. Within a couple of revealed tissue specific effects of ligands years other investigators cloned the kappa76 and targeting delta opioid receptors. Using antisense mu opioid77,78 receptors and genetically mapped oligos that were administered intrathecally (i.t.), the gene sequences to specific chromosomal Pasternak and colleagues showed that the spinal 79-81 locations . activity of both DPDPE (δ1) and DELT-II (δ2) was The studies showed that opioid completely abolished86. However, Bilsky and receptors are class A members of the G protein coworkers showed that the antisense molecules coupled-receptor (GPCR) superfamily. The inhibited the antinociceptive activity of DELT-II general GPCR structure consists of seven (δ2), but not DPDPE (δ1), when administered transmembrane (7TM) domains linked by three supraspinally87. In both studies the effects of alternating intracellular and extracellular loops. DAMGO (µ) or U69,593 (κ) were unaffected by There is high amino acid homology (~60%) within the delta-selective oligos. the opioid receptor family that constitutes a Since molecular cloning only identified a group of four receptor types: MOP (mu), DOP single gene for the delta receptor, it was 82 (delta), KOP (kappa) and ORL-1 () . speculated that different splice variants could Though ORL-1 was originally placed within the lead to the expression of the δ1 and δ2 receptor opioid family due to sequence homology, it is subtypes. There are three exons contained in the not known to interact with any of the non- delta receptor gene, which prompted Pasternak selective opioid ligands and produces and coworkers to design oligonucleotides for all downstream effects that are unlike the other the three exons to gain a comprehensive three opioid receptors. We have, therefore, understanding of delta receptor expression88. In focused our research efforts on the mu, kappa all, five oligonucleotides were designed. All the and delta opioid receptors. oligos inhibited the antinociception of DPDPE Most of the homology between the and DELT-II when administered i.t. However, opioid receptors occurs in the TM domains, while the antinociception mediated by DELT-II 17 intracellular loops and in the C-terminus . was abolished by i.c.v. administration of all the However, it was surprising that all the cloning oligos, only oligos targeting exon 3 attenuated studies pointed to just three different gene the i.c.v. activity of DPDPE. This led to the products when the literature at the time suggestion that δ1 and δ2 are distinct receptors suggested the existence of three mu (µ1, µ2, µ3), that are expressed due to splice variants that rd two delta (δ1 and δ2), and three kappa (κ1, κ2 and contain either all three exons (δ2), or only the 3 κ3) receptor subtypes based on distinctive exon (δ1). While the study had tremendous pharmacological effects. implications, no such splice variants have been isolated to date. Opioid receptor subtypes One of the criticisms of antisense oligo In the early 1990s, several reports methods is that the level of knockdown is rarely emerged that showed that the delta opioid homogenous across all the cells in the tissue of agonists, DPDPE and deltorphin II (DELT-II), were interest. This can lead to challenges in inhibited to different extents by delta interpretation, as some tissues may be more 83-85 antagonists . For instance, it was shown that permeable to both the oligos and ligands than Ajay S. Yekkirala 15 other tissues. To get around this problem, words, if the ligands are not binding to any knockout mouse models were developed that receptor in the brain, where is the supraspinal allowed for the entire genes to be abolished. In activity coming from? an ineteresting study, Pintar and coworkers Several studies also suggested developed a delta knockout (DORKO) mouse pharmacological subtypes for kappa opioid model and studied the antinociceptive activity of receptors90-93. Such an idea stemmed from the the two subtype selective ligands, DPDPE and fact that benzenomorphan ligands like DELT-II89. In these knockout animals, neither bremazocine consistently showed greater [3H]DPDPE or [3H]DELT-II showed any binding in amount of binding than arylacetamide ligands90. the brain. In addition, the spinal activity of both The receptors that arylacetamides bound were ligands was attenuated. In contrast to the considered κ1, while the residual binding for antisense studies, the supraspinal bremazocine that was observed in the presence antinociceptive activity of both DPDPE and DELT- of excess δ, µ and κ1 selective ligands was II remained intact. The authors summarized that considered to be κ2. However, genetic knockout the lack of binding indicated that both δ1 and δ2 studies suggested that bremazocine is a non- were expressed by the same δ receptor gene, selective opioid ligand that binds all three opioid but the supraspinal activity of these ligands was receptors. In a sequential knockout study by mediated by delta-like receptor that is different Simonin and colleagues90 in mu, delta, double from the cloned delta receptor. mu/delta, and triple mu/kappa/delta knockout animals, [3H]bremazocine binding was shown to represent 68% from mu, 27% from delta and only 14.5% from kappa receptor genes. Indeed all of the labeling due to [3H]bremazocine was abolished in the triple knockout mice. Thus, the putative κ2 receptor, for which bremazocine has been considered to be the prototype agonist, may just stem from the simultaneous occupancy of all the three receptors. While there have also been suggestions for the existence of multiple mu opioid receptor subtypes94, as in the case of delta and kappa receptors, there is little genetic evidence to support it. Though numerous mu opioid splice Figure 4. An illustration for the concept of variants have been isolated, further studies are receptor oligomerization. When two similar needed to elucidate their importance in receptors form a complex, it is termed a modulating physiological and pharmacological homomer while dissimilar receptors effects. oligomerize to form heteromers.

But this interpretation has several Oligomerization of opioid receptors – a game inconsistencies. The authors suggest that the changer supraspinal activity of these ligands is mediated There is an additional by a delta-like receptor89. However, they also consideration that had been consistently gaining showed that there was no binding of the ground to explain the evidence for tritiated ligands in the brain. If there was a pharmacological receptor subtypes. Even in the different receptor that was mediating the early 1980s, independent studies by Portoghese antinocieption, the authors should have still and Rothman groups suggested the possibility of observed residual binding in the brain. In other adjacent and interacting opioid receptors that

16 Journal of Postdoctoral Research 2013: 9-23 are organized as complexes65,95. For instance, receptor oligomerization is the functional and Portoghese and coworkers showed that bivalent physiological outcome of the resultant ‘meta’- ligands containing two pharmacophores receptor. An obvious progression of this idea is separated by variable length spacer, can occupy the possibility that ligands targeting such proximal receptor active sites64,65 in the Guinea oligomers may have novel pharmacological pig ileum (GPI) and mouse vas deferens (MVD). effects that can lead to tantalizing therapeutic Studies by Vaught et al., suggested that mu and discoveries. Again, the work of the Portoghese delta receptors interact in the spinal cord and group has provided the first examples of ligands promote distinct downstream effects96. In targeting opioid receptor heteromers and has addition, Rothman and colleagues showed that been reviewed previously105. mu and delta ligands antagonized each other Delta opioid receptors have been shown non-competitively at low doses, suggesting a to modulate some of the side effects produced level of allosteric coupling between mu and delta by mu opioid agonists. For instance, the opioid receptors97-99. These provocative results, administration of the delta antagonist led to further experimentation and the concept naltrindole (NTI)106, delta receptor antisense of oligomerization of opioid receptors gained knockdown 107 and delta receptor knockout 108 traction. have shown attenuated tolerance and dependence due to the treatment of morphine. Experimental evidence for opioid oligomers Given the fact that mu and delta opioid To better appreciate the concept receptors oligomerize to form heteromers100,103, of receptor oligomerization, it would be helpful Portoghese and colleagues synthesized a series to visualize the simplest minimal functional unit, of MDAN bivalent ligands that contain mu a dimer. When two similar receptors form a agonist and delta antagonist pharmacophores complex, we have a homogenous complex or a tethered via spacers of varying length109. While ‘homomer’, and the complexation of two the ligands with shorter spacers produced different receptors is termed a ‘heteromer’97 (Fig tolerance and physical dependence, the 21-atom 4). In the case of opioid receptors, the groups of spacer bivalent ligand was reported to produce Devi and George were instrumental in potent antinociception devoid of tolerance, elucidating the various opioid homomers and physical dependence, or place preference,109,110. heteromers. Devi and colleagues isolated This study suggested that MDAN-21 was bridging homomers of delta98, kappa99, mu opioid the active sites of adjacent mu and delta receptors100,101 and later, the kappa-delta protomers within a mu-delta heteromer leading heteromer102, one of the first GPCR heteromers to the lack of deleterious side effects. to have been isolated. The mu-delta heteromer An effort to develop ligands for kappa- was subsequently identified by the research delta heteromers led to the identification of 6’- groups of both Devi and George100,103. Finally, GNTI111 that selectively activates kappa-delta Sadee and coworkers utilized bioluminescence heteromers in HEK-293 cells and produces resonance energy transfer (BRET) to show that antinociception in mice only when administered all the opioid receptors can associate to form intrathecally (i.t.), but not homomers and heteromers104, thus providing intracerebroventricularly (i.c.v.). These data independent evidence using a different suggested the tissue-specific expression of experimental technique. kappa-delta heteromers in the spinal cord of mice. The study where Wessendorf and Ligands targeting opioid receptor heteromers – coworkers showed that kappa and delta the future? receptors are extensively colocalized in the When we take note of the fact that most rodent spinal cord112 provides independent GPCRs can exist as monomeric units and be support for the possibility of such tissue-specifc functionally active, the most intriguing aspect of expression. Ajay S. Yekkirala 17

Recently, we reported on the discovery Fig 4. An illustration for the concept of receptor of N-Naphthoyl-β-naltrexamine (NNTA), a ligand oligomerization. When two similar receptors that selectively activates mu-kappa opioid form a complex, it is termed a homomer while receptor heteromers with binding affinities in dissimilar receptors oligomerize to form the sub-picomolar range113. NNTA produced heteromers. antinociception intracerebroventricularly (i.c.v.), but was ~100-fold more potent in the spinal References cord. Significantly, NNTA did not produce any physical dependence or place preference in mice suggesting that targeting mu-kappa heteromers 1. Fries, D.S. Analgesics. in Principles of may produce analgesics devoid of those side Medicinal Chemsitry (eds. Foye, W.O., effects. Lemke, T.L. & Williams, D.A.) 247-269 (William & Wilkins, Pennsylvania, 1995). 2. Gutenstein, H.a.A., H. Opioid analgesics. Conclusions in Goodman and Gilman's In spite of concerted efforts to develop Pharmacological basis of therapeutics new analgesics, opioids still remain the drugs of (The McGraw Hill companies, Inc., USA, choice in the clinic. However, the deleterious 2006). side effects such as tolerance, physical 3. Small, L.F., Eddy, N.B., Mosettig, E. & dependence, and respiratory depression lead to Himmelsbach, C.K. Studies on Drug complications during therapy. As discussed in the Addiction. in Public Health Reports review, historically there have been several (Washington DC, 1929). attempts to reduce these side effects with little 4. Willette, R.E. Analgesic agents. in Wilson to no success. The discovery that opioid and Gisvold's textbook of organic receptors, and other GPCRs, can form higher medicinal and pharmaceutical chemistry order complexes suggests novel permutations of (ed. Doerge, R.F.) 607-656 (J. B. molecular and physiological effects that can be Lippincott Company, Philadelphia, PA, exploited using selective ligands. Indeed, the 1982). discovery that ligands targeting mu-delta and 5. Beckett, A.H., Casy, A.F. & Kirk, G. Alpha- mu-kappa receptors can produce potent and beta- type compounds. J antinociception without tolerance, dependence Med Pharm Chem 1, 37-58 (1959). or place preference show considerable promise 6. Portoghese, P.S. A new concept on the towards developing therapeutics that will mode of interaction of narcotic revolutionize pain treatments. Only time will tell analgesics with receptors. J Med Chem 8, if it is the desired mountain peak, or the 609-16 (1965). proverbial abysmal cliff at the end of this path. 7. Goldstein, A., Aaronow, L. & Kalman, S.M. Priniciples of Drug Action: The Basis Acknowledgements of Pharmacology, (Harper and Row, New I sincerely thank Dr. Philip S Portoghese for the York, 1968). discussions and clarifications as I was compiling 8. Goldstein, A., Lowney, L.I. & Pal, B.K. this review. I also thank Dr. Sonia Das for her Stereospecific and nonspecific help in looking over the manuscript for errors. interactions of the morphine congener in subcellular fractions of Figure Legends mouse brain. Proc Natl Acad Sci U S A 68, Fig 1. Selective ligands for mu opioid receptors 1742-7 (1971). Fig 2. Selective ligands for delta opioid receptors 9. Pert, C.B., Pasternak, G. & Snyder, S.H. Fig 3. Selective ligands for kappa opioid Opiate agonists and antagonists receptors discriminated by receptor binding in

18 Journal of Postdoctoral Research 2013: 9-23

brain. Science (New York, N.Y.) 182, in the brain and differential binding of 1359-1361 (1973). opiates and opioid peptides. Mol 10. Simon, E.J., Hiller, J.M. & Edelman, I. Pharmacol 16, 91-104 (1979). Stereospecific binding of the potent 20. Chang, K.J. & Cuatrecasas, P. Multiple narcotic analgesic (3H) to rat- opiate receptors. Enkephalins and brain homogenate. Proc Natl Acad Sci U morphine bind to receptors of different S A 70, 1947-9 (1973). specificity. J Biol Chem 254, 2610-8 11. Terenius, L. Stereospecific interaction (1979). between narcotic analgesics and a 21. Handa, B.K. et al. Analogues of beta- synaptic plasm a membrane fraction of LPH61-64 possessing selective agonist rat cerebral cortex. Acta Pharmacol activity at mu-opiate receptors. Toxicol (Copenh) 32, 317-20 (1973). European journal of pharmacology 70, 12. Hughes, J. et al. Identification of two 531-540 (1981). related pentapeptides from the brain 22. Mosberg, H.I. et al. Bis-penicillamine with potent opiate agonist activity. enkephalins possess highly improved Nature 258, 577-80 (1975). specificity toward delta opioid receptors. 13. Martin, W.R., Eades, C.G., Thompson, Proceedings of the National Academy of J.A., Huppler, R.E. & Gilbert, P.E. The Sciences of the United States of America effects of morphine- and nalorphine- like 80, 5871-5874 (1983). drugs in the nondependent and 23. Kreil, G. et al. Deltorphin, a novel morphine-dependent chronic spinal dog. amphibian skin peptide with high J Pharmacol Exp Ther 197, 517-32 selectivity and affinity for delta opioid (1976). receptors. Eur J Pharmacol 162, 123-8 14. Gilbert, P.E. & Martin, W.R. The effects (1989). of morphine and nalorphine-like drugs in 24. Erspamer, V. et al. Deltorphins: a family the nondependent, morphine- of naturally occurring peptides with high dependent and -dependent affinity and selectivity for delta opioid chronic spinal dog. J Pharmacol Exp Ther binding sites. Proc Natl Acad Sci U S A 86, 198, 66-82 (1976). 5188-92 (1989). 15. Gilbert, P.E. & Martin, W.R. Sigma effects 25. Vonvoigtlander, P.F., Lahti, R.A. & of nalorphine in the chronic spinal dog. Ludens, J.H. U-50,488: a selective and Drug Alcohol Depend 1, 373-6 (1976). structurally novel non-Mu (kappa) opioid 16. Martin, W., Bell, J., Gilbert, P., Sloan, J. & agonist. J Pharmacol Exp Ther 224, 7-12 Thompson, J. The effects of naltrexone in (1983). the chronic spinal dog and acute spinal 26. Lahti, R.A., Mickelson, M.M., McCall, cat; possible interaction with naturally- J.M. & Von Voigtlander, P.F. 3H]U-69593 occurring morphine-like agonists. NIDA a highly selective ligand for the opioid Res Monogr, 27-30 (1976). kappa receptor. European journal of 17. Akil, H. et al. Endogenous opioids: pharmacology 109, 281-284 (1985). overview and current issues. Drug 27. Su, T.P., Clements, T.H. & Gorodetzky, Alcohol Depend 51, 127-40 (1998). C.W. Multiple opiate receptors in 18. Lord, J.A., Waterfield, A.A., Hughes, J. & guinea-pig ileum. Life Sciences 28, 2519- Kosterlitz, H.W. Endogenous opioid 2528 (1981). peptides: multiple agonists and 28. Tyers, M.B. A classification of opiate receptors. Nature 267, 495-9 (1977). receptors that mediate antinociception 19. Chang, K.J., Cooper, B.R., Hazum, E. & in animals. British journal of Cuatrecasas, P. Multiple opiate pharmacology 69, 503-512 (1980). receptors: different regional distribution Ajay S. Yekkirala 19

29. Zhao, G.M., Qian, X., Schiller, P.W. & Analgesics. Pharmacol Rev 16, 47-83 Szeto, H.H. Comparison of [Dmt1]DALDA (1964). and DAMGO in binding and G protein 39. Jasinski, D.R., Martin, W.R. & Hoeldtke, activation at mu, delta, and kappa opioid R.D. Effects of short- and long-term receptors. The Journal of pharmacology administration of pentazocine in man. and experimental therapeutics 307, 947- Clin. parmac. Ther. 11, 385-403 (1970). 954 (2003). 40. Houde, R.W. The use and misuse of 30. Qi, J.A., Mosberg, H.I. & Porreca, F. in the treatment of chronic Antinociceptive effects of [D- pain. in Advances in Neurology, Vol. 4 Ala2]deltorphin II, a highly selective (ed. Bonica, J.J.) 527-536 (Raven Press, delta agonist in vivo. Life Sci 47, PL43-7 New York, 1974). (1990). 41. Hamilton, R.C., Dundee, J.W., Clarke, 31. Kreil, G. et al. Deltorphin, a novel R.S., Loan, W.B. & Morrison, J.D. Studies amphibian skin peptide with high of drugs given before anaesthesia. 13. selectivity and affinity for delta opioid Pentazocine and other opiate receptors. European journal of antagonists. Br J Anaesth 39, 647-56 pharmacology 162, 123-128 (1989). (1967). 32. Leighton, G.E., Rodriguez, R.E., Hill, R.G. 42. Dundee, J.W. A Clinical Trial of a Mixture & Hughes, J. kappa-Opioid agonists of Levorphanol and as an produce antinociception after i.v. and Oral Analegesic. Br J Anaesth 36, 486-93 i.c.v. but not intrathecal administration (1964). in the rat. British journal of 43. Beaver, W.T., Wallenstein, S.L., Houde, pharmacology 93, 553-560 (1988). R.W. & Rogers, A. A comparison of the 33. Houde, R.W. Analgesic effectiveness of analgesic effects of pentazocine and the narcotic agonist-antagonists. Br J Clin morphine in patients with cancer. Clin Pharmacol 7 Suppl 3, 297S-308S (1979). Pharmacol Ther 7, 740-51 (1966). 34. Seitner, P.G., Martin, B.C., Cochin, J. & 44. Alderman, E.L., Barry, W.H., Graham, Harris, L. Survey of Analgesic Drug A.F. & Harrison, D.C. Hemodynamic Prescribing Patterns. (Am. Med. Assoc. effects of morphine and pentazocine Center for Health Services Research & differ in cardiac patients. N Engl J Med Development, Washington DC, 1975). 287, 623-7 (1972). 35. Keats, A.S. & Telford, J. Studies of 45. Blumberg, H. & Dayton, H.B. Naloxone, Analgesic Drugs. Viii. A Narcotic naltrexone, and related Antagonist Analgesic without noroxymorphones. Adv Biochem Psychotomimetic Effects. J Pharmacol Psychopharmacol 8, 33-43 (1973). Exp Ther 143, 157-64 (1964). 46. Jasinski, D.R. & Mansky, P.A. Evaluation 36. Stoelting, V.K. Analgesic action of of nalbuphine for abuse potential. Clin pentazocine compared with morphine in Pharmacol Ther 13, 78-90 (1972). postoperative pain. Anesth Analg 44, 47. Dobkin, A.B., Eamkaow, S. & Caruso, F.S. 769-72 (1965). Butorphanol and pentazocine in patients 37. Gordon, R.A. & Moran, J.H. Studies of with severe postoperative pain. Clin pentazocine (WIN-20228). I. Evaluation Pharmacol Ther 18, 547-53 (1975). as an analgesic in post-operative 48. Gilbert, M.S., Hanover, R.M., Moylan, patients. Can Anaesth Soc J 12, 331-6 D.S. & Caruso, F.S. Intramuscular (1965). butorphanol and meperidine in 38. Lasagna, L. The Clinical Evaluation of postoperative pain. Clin Pharmacol Ther Morphine and Its Substitutes as 20, 359-64 (1976).

20 Journal of Postdoctoral Research 2013: 9-23

49. Gilbert, M.S., Forman, R.S., Moylan, D.S. antagonistic and reversible agonistic & Caruso, F.S. Butorphanol: a double- properties of the fumaramate methyl blind comparison with pentazocine in ester derivative of naltrexone. European post-operative patients with moderate journal of pharmacology 70, 445-451 to severe pain. J Int Med Res 4, 255-64 (1981). (1976). 59. Ward, S.J., Portoghese, P.S. & Takemori, 50. Andrews, I.C. Butorphanol and A.E. Pharmacological profiles of beta- pentazocine: A double-blind funaltrexamine (beta-FNA) and beta- intramuscular comparison in patients chlornaltrexamine (beta-CNA) on the with moderate to severe postoperative mouse vas deferens preparation. pain. Curr. ther. Res. 22, 697-706 (1977). European journal of pharmacology 80, 51. Tavakoli, M., Corssen, G. & Caruso, F.S. 377-384 (1982). Butorphanol and morphine: a double- 60. Ward, S.J., Portoghese, P.S. & Takemori, blind comparison of their parenteral A.E. Pharmacological characterization in analgesic activity. Anesth Analg 55, 394- vivo of the novel opiate, beta- 401 (1976). funaltrexamine. The Journal of 52. Delpizzo, A. Butorphanol, a new pharmacology and experimental intravenous analgesic: double-blind therapeutics 220, 494-498 (1982). comparison with morphine sulfate in 61. Gulya, K. et al. H-D-Phe-Cys-Tyr-D-Trp- postoperative patients with moderate or Orn-Thr-Pen-Thr-NH2: a potent and severe pain. Curr Ther Res Clin Exp 20, selective antagonist opioid receptors. 221-32 (1976). NIDA Res Monogr 75, 209-12 (1986). 53. Houde, R.W., Wallenstein, S.L. & Rogers, 62. Gulya, K., Pelton, J.T., Hruby, V.J. & A. Clinical assessment of narcotic Yamamura, H.I. Cyclic somatostatin agonist-antagonist analgesics. in octapeptide analogues with high affinity Advances in Pain Research and Therapy, and selectivity toward mu opioid Vol. 1 (eds. Bonica, J.J. & Albe-Fessard, receptors. Life Sci 38, 2221-9 (1986). D.) 647-751 (Raven Press., New York, 63. Raynor, K. et al. Pharmacological 1976). characterization of the cloned kappa-, 54. Lewenstein, M.J. & Fishman, J. Morphine delta-, and mu-opioid receptors. Mol Derivative. (ed. Office, U.S.P.) (USA, Pharmacol 45, 330-4 (1994). 1961). 64. Portoghese, P.S. & Takemori, A.E. TENA, 55. Foldes, F.F., Lunn, J.N., Moore, J. & a selective kappa opioid receptor Brown, I.M. N-Allylnoroxy-morphone: a antagonist. Life Sciences 36, 801-805 new potent narcotic antagonist. Am J (1985). Med Sci 245, 23-30 (1963). 65. Erez, M., Takemori, A.E. & Portoghese, 56. Foldes, F.F. The Human Pharmacology P.S. Narcotic antagonistic potency of and Clinical Use of Narcotic Antagonists. bivalent ligands which contain beta- Med Clin North Am 48, 421-43 (1964). naltrexamine. Evidence for bridging 57. Portoghese, P.S., Larson, D.L., Sayre, between proximal recognition sites. L.M., Fries, D.S. & Takemori, A.E. A novel Journal of medicinal chemistry 25, 847- opioid receptor site directed alkylating 849 (1982). agent with irreversible narcotic 66. Portoghese, P.S., Lipkowski, A.W. & antagonistic and reversible agonistic Takemori, A.E. and nor- activities. Journal of medicinal chemistry binaltorphimine, potent and selective 23, 233-234 (1980). kappa-opioid receptor antagonists. Life 58. Takemori, A.E., Larson, D.L. & Sciences 40, 1287-1292 (1987). Portoghese, P.S. The irreversible narcotic Ajay S. Yekkirala 21

67. Takemori, A.E., Ho, B.Y., Naeseth, J.S. & Proc Natl Acad Sci U S A 89, 12048-52 Portoghese, P.S. Nor-binaltorphimine, a (1992). highly selective kappa-opioid antagonist 76. Meng, F. et al. Cloning and in analgesic and receptor binding assays. pharmacological characterization of a rat The Journal of pharmacology and kappa opioid receptor. Proceedings of experimental therapeutics 246, 255-258 the National Academy of Sciences of the (1988). United States of America 90, 9954-9958 68. Schwyzer, R. ACTH: a short introductory (1993). review. Ann N Y Acad Sci 297, 3-26 77. Thompson, R.C., Mansour, A., Akil, H. & (1977). Watson, S.J. Cloning and 69. Portoghese, P.S. Bivalent ligands and the pharmacological characterization of a rat message-address concept in the design mu opioid receptor. Neuron 11, 903-913 of selective opioid receptor antagonists. (1993). Trends in pharmacological sciences 10, 78. Wang, J.B. et al. mu opiate receptor: 230-235 (1989). cDNA cloning and expression. Proc Natl 70. Takemori, A.E. & Portoghese, P.S. Acad Sci U S A 90, 10230-4 (1993). Selective naltrexone-derived opioid 79. Wang, J.B. et al. Human mu opiate receptor antagonists. Annual Review of receptor. cDNA and genomic clones, Pharmacology and Toxicology 32, 239- pharmacologic characterization and 269 (1992). chromosomal assignment. FEBS Lett 338, 71. Lipkowski, A.W., Tam, S.W. & 217-22 (1994). Portoghese, P.S. Peptides as receptor 80. Yasuda, K., Espinosa, R., 3rd, Takeda, J., selectivity modulators of opiate Le Beau, M.M. & Bell, G.I. Localization of pharmacophores. Journal of medicinal the kappa opioid receptor gene to chemistry 29, 1222-1225 (1986). human chromosome band 8q11.2. 72. Portoghese, P.S., Sultana, M. & Genomics 19, 596-7 (1994). Takemori, A.E. Naltrindole, a highly 81. Befort, K., Mattei, M.G., Roeckel, N. & selective and potent non-peptide delta Kieffer, B. Chromosomal localization of opioid receptor antagonist. European the delta opioid receptor gene to human journal of pharmacology 146, 185-186 1p34.3-p36.1 and mouse 4D bands by in (1988). situ hybridization. Genomics 20, 143-5 73. Portoghese, P.S., Nagase, H., (1994). MaloneyHuss, K.E., Lin, C.E. & Takemori, 82. Cox, B.M. & al., e. Opioid receptors. A.E. Role of spacer and address International Union of Basic and Clinical components in peptidomimetic delta Pharmacology opioid receptor antagonists related to database (IUPHAR-DB). naltrindole. Journal of medicinal 83. Sofuoglu, M., Portoghese, P.S. & chemistry 34, 1715-1720 (1991). Takemori, A.E. Differential antagonism of 74. Evans, C.J., Keith, D.E., Jr., Morrison, H., delta opioid agonists by naltrindole and Magendzo, K. & Edwards, R.H. Cloning of its benzofuran analog (NTB) in mice: a delta opioid receptor by functional evidence for delta opioid receptor expression. Science (New York, N.Y.) 258, subtypes. The Journal of pharmacology 1952-1955 (1992). and experimental therapeutics 257, 676- 75. Kieffer, B.L., Befort, K., Gaveriaux-Ruff, C. 680 (1991). & Hirth, C.G. The delta-opioid receptor: 84. Sofuoglu, M., Portoghese, P.S. & isolation of a cDNA by expression cloning Takemori, A.E. delta-Opioid receptor and pharmacological characterization. binding in mouse brain: evidence for heterogeneous binding sites. European

22 Journal of Postdoctoral Research 2013: 9-23

journal of pharmacology 216, 273-277 93. Magnan, J. & Tiberi, M. Selectivity profile (1992). of nor-binaltorphimine and ICI-197,067 85. Sofuoglu, M., Portoghese, P.S. & in guinea pig brain. Prog Clin Biol Res Takemori, A.E. 7-Benzylidenenaltrexone 328, 73-6 (1990). (BNTX): a selective delta 1 opioid 94. Pasternak, G.W. Insights into mu opioid receptor antagonist in the mouse spinal pharmacology the role of mu opioid cord. Life Sciences 52, 769-775 (1993). receptor subtypes. Life Sci 68, 2213-9 86. Standifer, K.M., Chien, C.C., Wahlestedt, (2001). C., Brown, G.P. & Pasternak, G.W. 95. Rothman, R.B. & Xu, H. Mu-delta Selective loss of delta opioid analgesia interactions in vitro and in vivo. in The and binding by antisense Delta Receptor (eds. Chang, K., Porreca, oligodeoxynucleotides to a delta opioid F. & Woods, J.H.) 373-382 (Mercel receptor. Neuron 12, 805-10 (1994). Dekker, Inc, 2004). 87. Bilsky, E.J. et al. Selective inhibition of 96. Vaught, J.L., Rothman, R.B. & Westfall, [D-Ala2, Glu4]deltorphin antinociception T.C. Mu and delta receptors: their role in by supraspinal, but not spinal, analgesia in the differential effects of administration of an antisense opioid peptides on analgesia. Life Sci 30, oligodeoxynucleotide to an opioid delta 1443-55 (1982). receptor. Life Sci 55, PL37-43 (1994). 97. Ferre, S. et al. Building a new conceptual 88. Rossi, G.C., Su, W., Leventhal, L., Su, H. & framework for receptor heteromers. Nat Pasternak, G.W. Antisense mapping Chem Biol 5, 131-4 (2009). DOR-1 in mice: further support for delta 98. Cvejic, S. & Devi, L.A. Dimerization of the receptor subtypes. Brain Res 753, 176-9 delta opioid receptor: implication for a (1997). role in receptor internalization. J Biol 89. Zhu, Y. et al. Retention of supraspinal Chem 272, 26959-64 (1997). delta-like analgesia and loss of morphine 99. Jordan, B.A., Cvejic, S. & Devi, L.A. tolerance in delta opioid receptor Opioids and their complicated receptor knockout mice. Neuron 24, 243-252 complexes. Neuropsychopharmacology (1999). 23, S5-S18 (2000). 90. Simonin, F. et al. Analysis of 100. George, S.R. et al. Oligomerization of [3H]bremazocine binding in single and mu- and delta-opioid receptors. combinatorial opioid receptor knockout Generation of novel functional mice. Eur J Pharmacol 414, 189-95 properties. The Journal of biological (2001). chemistry 275, 26128-26135 (2000). 91. Tiberi, M. & Magnan, J. Demonstration 101. Li-Wei, C. et al. Homodimerization of of the heterogeneity of the kappa-opioid human mu-opioid receptor receptors in guinea-pig cerebellum using overexpressed in Sf9 insect cells. Protein selective and nonselective drugs. Eur J Pept Lett 9, 145-52 (2002). Pharmacol 188, 379-89 (1990). 102. Jordan, B.A. & Devi, L.A. G-protein- 92. Tiberi, M. & Magnan, J. Quantitative coupled receptor heterodimerization analysis of multiple kappa-opioid modulates receptor function. Nature receptors by selective and nonselective 399, 697-700 (1999). ligand binding in guinea pig spinal cord: 103. Gomes, I. et al. Heterodimerization of resolution of high and low affinity states mu and delta opioid receptors: A role in of the kappa 2 receptors by a opiate synergy. The Journal of computerized model-fitting technique. neuroscience : the official journal of the Mol Pharmacol 37, 694-703 (1990). Society for Neuroscience 20, RC110 (2000). Ajay S. Yekkirala 23

104. Wang, D., Sun, X., Bohn, L.M. & Sadee, 109. Daniels, D.J. et al. Opioid-induced W. Opioid receptor homo- and tolerance and dependence in mice is heterodimerization in living cells by modulated by the distance between quantitative bioluminescence resonance pharmacophores in a bivalent ligand energy transfer. Mol Pharmacol 67, series. Proceedings of the National 2173-84 (2005). Academy of Sciences of the United States 105. Yekkirala, A.S. Two to tango: GPCR of America 102, 19208-19213 (2005). oligomers and GPCR-TRP channel 110. Lenard, N.R., Daniels, D.J., Portoghese, interactions in nociception. Life Sci 92, P.S. & Roerig, S.C. Absence of 438-45. conditioned place preference or 106. Abdelhamid, E.E., Sultana, M., reinstatement with bivalent ligands Portoghese, P.S. & Takemori, A.E. containing mu-opioid receptor agonist Selective blockage of delta opioid and delta-opioid receptor antagonist receptors prevents the development of pharmacophores. European journal of morphine tolerance and dependence in pharmacology 566, 75-82 (2007). mice. The Journal of pharmacology and 111. Waldhoer, M. et al. A heterodimer- experimental therapeutics 258, 299-303 selective agonist shows in vivo relevance (1991). of G protein-coupled receptor dimers. 107. Sanchez-Blazquez, P., Garcia-Espana, A. Proceedings of the National Academy of & Garzon, J. Antisense Sciences of the United States of America oligodeoxynucleotides to opioid mu and 102, 9050-9055 (2005). delta receptors reduced morphine 112. Wessendorf, M.W. & Dooyema, J. dependence in mice: role of delta-2 Coexistence of kappa- and delta-opioid opioid receptors. J Pharmacol Exp Ther receptors in rat spinal cord axons. 280, 1423-31 (1997). Section Title: Mammalian Hormones 108. Nitsche, J.F. et al. Genetic dissociation of 298, 151-154 (2001). opiate tolerance and physical 113. Yekkirala, A.S. et al. N-naphthoyl-beta- dependence in delta-opioid receptor-1 naltrexamine (NNTA), a highly selective and preproenkephalin knock-out mice. and potent activator of mu/kappa-opioid The Journal of neuroscience : the official heteromers. Proc Natl Acad Sci U S A journal of the Society for Neuroscience 108, 5098-103 (2011). 22, 10906-10913 (2002).