The search for a better analgesic

A Journey through Diastereomeric Space: The Design, Synthesis, In Vitro and In Vivo Pharmacological Activity, and Molecular Modeling of Novel Potent Diastereomeric MOR Agonists and Antagonists

Four sets of diastereomeric C9-alkenyl 5-phenylmorphans, varying in the length of the C9-alkenyl chain, were designed to examine the effect of these spatially distinct ligands on opioid receptors. Functional activity was obtained by forskolin-induced cAMP accumulation assays and several compounds were examined in the [³⁵S]GTPgS assay and in an assay for respiratory depression. In each of the four sets, similarities and differences were observed dependent on the length of their C9-alkenyl chain and, most importantly, their stereochemistry. Three MOR antagonists were found to be as or more potent than naltrexone and, unlike naltrexone, none had MOR, KOR, or DOR agonist activity. Several potent MOR full agonists were obtained, and, of particular interest partial agonists were found that exhibited less respiratory depression than that caused by morphine. The effect of stereochemistry and the length of the C9-alkenyl chain was also explored using molecular modeling. The MOR antagonists were found to interact with the inactive (4DKL) MOR crystal structures and agonists were found to interact with the active (6DDF) MOR crystal structures. The comparison of their binding modes at the mouse MOR was used to gain insight into the structural basis for their stereochemically induced pharmacological differences.

CT-guided injection of a TRPV1 agonist around dorsal root ganglia decreases pain transmission in swine

One approach to analgesia is to block pain at the site of origin or along the peripheral pathway by selectively ablating pain-transmitting neurons or nerve terminals directly. The heat/capsaicin receptor (TRPV1) expressed by nociceptive neurons is a compelling target for selective interventional analgesia because it leaves somatosensory and proprioceptive neurons intact. Resiniferatoxin (RTX), like capsaicin, is a TRPV1 agonist but has greater potency. We combine RTX-mediated inactivation with the precision of computed tomography (CT)-guided delivery to ablate peripheral pain fibers in swine. Under CT guidance, RTX was delivered unilaterally around the lumbar dorsal root ganglia (DRG), and vehicle only was administered to the contralateral side. During a 4-week observation period, animals demonstrated delayed or absent withdrawal responses to infrared laser heat stimuli delivered to sensory dermatomes corresponding to DRG receiving RTX treatment. Motor function was unimpaired as assessed by disability scoring and gait analysis. In treated DRG, TRPV1 mRNA expression was reduced, as were nociceptive neuronal perikarya in ganglia and their nerve terminals in the ipsilateral dorsal horn. CT guidance to precisely deliver RTX to sites of peripheral pain transmission in swine may be an approach that could be tailored to block an array of clinical pain conditions in patients.

A new splice of life for the μ-opioid receptor

μ-Opioid agonists mediate their analgesic effect through GPCRs that are generated via alternate splicing of the Oprm1 transcript. While the majority of μ-opioids interact with receptors comprising the canonical 7 transmembrane (7TM) domain, a recently identified class of μ-opioids appears to require a 6TM domain variant. In this issue of the JCI, Lu and colleagues provide an in vivo proof-of-concept demonstration that a 6TM isoform of the μ-opioid receptor can support functional analgesia in Oprm1-deficent animals. The 6TM isoform was pharmacologically distinct from the canonical 7TM μ-opioid receptor, and 6TM agonists had a reduced side effect profile, which confers a strong therapeutic advantage over standard opioid analgesics. The observations of Lu et al. extend the reach of opioid-receptor neurobiology and pharmacology into a new era of analgesic discovery. This advance emerges from a series of fundamental research analyses in which elements of the endogenous opioid system were frequently in the vanguard.

Chronic methadone treatment shows a better cost/benefit ratio than chronic morphine in mice

Chronic treatment of pain with opiate drugs can lead to analgesic tolerance and drug dependence. Although all opiate drugs can promote tolerance and dependence in practice, the severity of those unwanted side effects differs depending on the drug used. Although each opiate drug has its own unique set of pharmacological profiles, methadone is the only clinically used opioid drug that produces substantial receptor endocytosis at analgesic doses. Here, we examined whether moderate doses of methadone carry any benefits over chronic use of equianalgesic morphine, the prototypical opioid. Our data show that chronic administration of methadone produces significantly less analgesic tolerance than morphine. Furthermore, we found significantly reduced precipitated withdrawal symptoms after chronic methadone treatment than after chronic morphine treatment. Finally, using a novel animal model with a degrading μ-opioid receptor we showed that, although endocytosis seems to protect against tolerance development, endocytosis followed by receptor degradation produces a rapid onset of analgesic tolerance to methadone. Together, these data indicated that opioid drugs that promote receptor endocytosis and recycling, such as methadone, may be a better choice for chronic pain treatment than morphine and its derivatives that do not.

The chemical and pharmacological importance of morphine analogues

The object of this review is to summarize the efforts which resulted in the discovery of therapeutically useful morphine-like drugs. The search for new analgesics can be divided into three stages: (a) search for analgesics with high efficacy and reduced unwanted side-effects; (b) understanding of structure-activity relationships; (c) studies on the mechanism of pain perception and its alleviation by investigation of the pharmacology of opioids. An immense body of literature has been produced on the syntheses of thousands of new compounds which resulted in the development of detailed structure-activity relationships. The physical and psychologic dependence of opioid analgesics also facilitated investigators to solve the problem of the separation of strong analgesia from addiction liability. In the past decades more mixed agonist-antagonist analgesics, pure antagonists devoid of agonist action and potent opioids like the 6,14-ethenomorphinan derivatives were developed. Naloxone, Naltrexone, Buprenorphine and Pentazocine are the outstanding representatives which are introduced into clinical therapy.

Stimulating the development of mechanism-based, individualized pain therapies

Biomedical science has greatly improved our understanding of pain in recent decades, but few novel molecular entities that address fundamentally new pain mechanisms have entered the clinic, despite dramatically increased pharmaceutical investment. Indeed, virtually all new analgesics approved over the past 25 years are derivatives or reformulations of opioids or aspirin-like drugs, existing drugs given for a new indication or older drugs given by a different route of administration. Here, we discuss factors contributing to this lack of innovation in therapies for pain and advocate public-private partnerships (PPPs) to translate new knowledge into more efficacious and safer treatments.

The role of crystallography in drug design

Structure and function are intimately related. Nowhere is this more important than the area of bioactive molecules. It has been shown that the enantioselectivity of an enzyme is directly related to its chirality. X-ray crystallography is the only method for determining the "absolute" configuration of a molecule and is the most comprehensive technique available to determine the structure of any molecule at atomic resolution. Results from crystallographic studies provide unambiguous, accurate, and reliable 3-dimensional structural parameters, which are prerequisites for rational drug design and structure-based functional studies.

The ladder and the clock: cancer pain and public policy at the end of the twentieth century

The origins of the WHO Cancer Pain Relief Program (the Analgesic Ladder) and its research basis in two very different research traditions, one at Memorial Sloan-Kettering Cancer Center in New York, the other at St. Christopher's Hospice in London, are discussed. The Sloan-Kettering group emphasized precise relative differences in analgesic effects of various drugs, whereas Twycross at St. Christopher's used patient well-being as the crucial benchmark. Despite these differences, both traditions presented evidence of the safe and effective use of strong opioids for cancer pain relief, in a setting of individualized attention and close physician monitoring. The success and limitations of the Ladder as a global health policy are briefly addressed.

Debating the Controlled Substances Act

In the United States, the basis of modern drug regulation is the Controlled Substances Act (CSA) of 1970. The CSA laid out the authority of the federal government and provided a framework within which all existing and new substances could be regulated on their abuse potential, safety, and medical utility. The debates over the CSA centered on several critical issues: where to place the authority to make scheduling designations, the impact of scheduling on drug research, and defining what constituted drug "abuse" for purposes of scheduling. Passage of the CSA was aided by broad language that provided a kind of "big tent" which could accommodate diverse points of view. A retrospective assessment of the CSA shows it to have greatly expanded federal administrative authority over the nation's drug supply, much as its authors intended. Other impacts of the CSA, however, are much less certain. This article concludes by highlighting the issues and questions that should guide future retrospective research on the efficacy of drug control regimes.

A critical structural determinant of opioid receptor interaction with phenolic 5-phenylmorphans

The opioid receptor binding affinities of N-methyl- and N-phenethyl-5-phenylmorphans with a meta-hydroxy substituent [3-(2-methyl-2-azabicyclo[3.3.1]non-5-yl)-phenol (1a), and 3-(2-phenethyl-2-azabicyclo[3.3.1]non-5-yl)-phenol (1b)] were compared with the affinities of four new ligands bearing an ortho- or para-hydroxyl substituent (2-(2-methyl-2-azabicyclo[3.3.1]non-5-yl)-phenol (2a) and 2-(2-phenethyl-2-azabicyclo[3.3.1]non-5-yl)-phenol (2b), 4-(2-methyl-2-azabicyclo[3.3.1]non-5-yl)-phenol (3a), and 4-(2-phenethyl-2-azabicyclo[3.3.1]non-5-yl)-phenol (3b)) that were synthesized from 2-bromoanisole or the known 2-methyl-5-phenyl-2-azabicyclo[3.3.1]nonane (13), respectively. The data indicated that either the electronic state of the phenolic ring is critical for the ligand's interaction with an opioid receptor, or that there must be a specific distance and angle for a hydrogen bond between the phenolic moiety and an amino acid in the binding domain that cannot be altered.

Deletion of vanilloid receptor 1-expressing primary afferent neurons for pain control

Control of cancer, neuropathic, and postoperative pain is frequently inadequate or compromised by debilitating side effects. Inhibition or removal of certain nociceptive neurons, while retaining all other sensory modalities and motor function, would represent a new therapeutic approach to control severe pain. The enriched expression of transient receptor potential cation channel, subfamily V, member 1 (TRPV1; also known as the vanilloid receptor, VR1) in nociceptive neurons of the dorsal root and trigeminal ganglia allowed us to test this concept. Administration of the potent TRPV1 agonist resiniferatoxin (RTX) to neuronal perikarya induces calcium cytotoxicity by opening the TRPV1 ion channel and selectively ablates nociceptive neurons. This treatment blocks experimental inflammatory hyperalgesia and neurogenic inflammation in rats and naturally occurring cancer and debilitating arthritic pain in dogs. Sensations of touch, proprioception, and high-threshold mechanosensitive nociception, as well as locomotor function, remained intact in both species. In separate experiments directed at postoperative pain control, subcutaneous administration of RTX transiently disrupted nociceptive nerve endings, yielding reversible analgesia. In human dorsal root ganglion cultures, RTX induced a prolonged increase in intracellular calcium in vanilloid-sensitive neurons, while leaving other, adjacent neurons unaffected. The results suggest that nociceptive neuronal or nerve terminal deletion will be effective and broadly applicable as strategies for pain management.

X-ray studies on ligands

Advances in x-ray crystallographic data collection, structure solution, and refinement/validation have reduced the time required and expanded the range of samples amenable to x-ray crystallographic studies. Consequently, we can now collect complete atomic resolution data sets on physically smaller crystals and solve larger problems by direct methods beyond what could have been accomplished even five years ago. Applying these improved methods to the study of opioid ligands has enhanced our knowledge of the opioid pharmacophore. Despite considerable progress, it is still difficult to define the pharmacophoric parameters required for highly selective and potent opioid peptides. In part this is due to the conformational flexibility remaining even in conformationally constrained peptides.

Effects of nor-binaltorphimine on the development of analgesic tolerance to and physical dependence on morphine

The effects of a highly selective kappa antagonist, nor-binaltorphimine (nor-BNI), on the development of tolerance to morphine analgesia and physical dependence on morphine were examined. Pretreatment with nor-BNI (5 mg/kg s.c.) 2 h prior to injection of morphine or a selective kappa agonist, U-50,488H, significantly antagonized the analgesic effect of U-50,488H, but not morphine analgesia in mice. The development of tolerance to morphine analgesia was significantly potentiated by pretreatment of mice with nor-BNI 2 h prior to morphine treatment during chronic morphine treatment for 5 days. Additionally, the pretreatment with nor-BNI during chronic treatment with the high dose of morphine for 5 days significantly potentiated the naloxone-induced body weight loss in morphine-dependent mice and rats. These findings suggest that inactivation of the kappa opioid system may potentiate the development of tolerance to morphine analgesia in mice and may aggravate the naloxone-precipitated body weight loss in morphine-dependent mice and rats.

Effects of a selective kappa-opioid agonist, U-50,488H, on morphine dependence in rats

Rats were made dependent on morphine by mixing the drug with their only source of food. Naltrexone (0.5 mg/kg) injection precipitated a syndrome of withdrawal signs including weight loss. Pretreatment with the selective kappa agonist, U-50,488H (1.0, 30.0 or 10.0 mg/kg), generally had no effects on the signs of morphine withdrawal. In other subjects, U-50,488H was repeatedly administered (1.0, 3.0 or 10.0 mg/kg per 12 h) during the development of morphine dependence. In these subjects, the course of naltrexone-precipitated withdrawal was unchanged. These results suggest that agonist activity at kappa receptors is not sufficient to alter morphine dependence or withdrawal.

Analgesic and mood effects of heroin and morphine in cancer patients with postoperative pain

We designed a study to determine the relative analgesic potency of intramuscular heroin and morphine and to compare mood and side effects in 166 cancer patients with postoperative pain. Heroin was about twice as potent as morphine (95 per cent confidence limits, 1.6 to 2.6 times) in graded-dose, twin-crossover assays. Heroin provided an analgesic peak effect earlier than morphine (1.2 plus or minus 0.08 and 1.5 plus or minus 0.10 hours, respectively [mean plus or minus S.E.M.]). Doses with equal analgesic effects provided comparable improvements in various elements of mood, particularly feelings of peacefulness. Peak mood improvement occurred earlier after heroin than after morphine (1.2 plus or minus 0.10 and 1.8 plus or minus 0.13 hours, respectively). Both analgesia and mood improvement were less sustained after heroin at doses providing equal peak analgesic effects. The drugs shared the most common side effects, with no marked differences in their occurrence; sleepiness was the most frequent side effect after both drugs (46 per cent with each). Heroin has no apparent unique advantages or disadvantages for the relief pain in patients with cancer.

Postoperative analgesia

Postoperative analgesia both by drugs and regional techniques is reviewed. In the United Kingdom in the last 25 years or more there has been little advance on either front. Some marginal improvement in regard to drugs might be brought about by better education of both doctors and nurses and better patient contact. Extradural analgesia and intercostal block do not offer a complete solution, though a judicious increase in the use certainly of the former might be beneficial. The problem awaits a radical new approach.

Methadone maintenance and narcotic blocking drugs. Appendix

The research history of norcodeine as a narcotic blocking drug is presented. Because of the limitations found for norcodeine usage, naloxone, cyclazocine, and naltrexone have recently received attention, and the results of research with them are detailed.

Opiate Receptors and Opiate Antagonists: Progress in the “War on Drugs”

The “War on Drugs” has spawned major research efforts that have led to the discovery of the “opiate receptor” and the development of new narcotic antagonists which may be useful adjuncts in the rehabilitation of chronic opiate addicts. At the moment, these findings are rich with potential applications and potential discovery. The authors review the current state of the art.

Markers for detection of supplementation in narcotic programs--deuterium-labeled methadone

Specific deuterium labeling of methadone and use of gas chromatography-mass spectroscopy technique permits rapid and quanitative determination of the ratio of the labeled to unlabeled drug in body fluids. A trideuertiomethadone (methadone-d3) was shown to have exactly the same analgesic activity and toxicity in mice as methadone. The rates of absorption, distribution, and excretion of methadone-d3 and methadone were identical in rats. These observations suggest that methadone-d3 may be used as an in vivo marker for monitoring methadone intake of patients, and thus may improve the effectiveness of methadone treatment programs.

Opiate agonists and antagonists discriminated by receptor binding in brain

Receptor binding of opiate agonists and antagonists can be differentiated in vivo and in vitro. Administration of either rapidly elevates stereospecific [(3)H]dihydromorphine binding to mouse brain extracts by 40 to 100 percent, but antagonists are 10 to 1000 times more potent than agonists; as little as 0.02 milligram of naloxone per kilogram of body weight significantly enhances opiate receptor binding. Sodium enhances antagonist binding in vitro but decreases agonist binding, a qualitative difference that may be relevant to the divergent pharmacological properties of opiate agonists and antagonists.