Current Pharmaceutical Design - Volume 10, Issue 7, 2004

In this issue of Current Pharmaceutical Design, five manuscripts delve deep into the history and present day studies of morphine and related opioids, which have been the cornerstone of pain treatment since ancient history. Sabatowski [1] presents a history of pain treatment, starting from the early day opium use in Sumeria and Egypt, mandrake in the middle ages, and passing via Serturner, who in 1805 described for the first time morphine in its pure form as the active, analgesic principle of opium, finally arriving to the pain theories from the 19 th and 20 th century. The first total synthesis of morphine, the finding of the opioid antagonists, the discovery of the synthetic analgesics Fentanyl* , and its analogs by Paul Janssen during the 1950s, the discovery of the enkephalins and endorphins, the body's endogenous analgesics, and the characterization of three opioid receptors during the seventies were other milestones in the history of pain treatment. In the sixties and seventies, Bentley studied the orvinols, a class of rigid morphinans with extraordinary potency.They resulted from the Diels-Alder adducts of thebaine with ethyl acrylate or methyl vinyl ketone. Because of their rigidity, they formed the basis of a description of the features of the opioid receptor, which, as we know now, was in fact the μ-opioid receptor. Lewis and Husbands [2] took up the study of the compounds again, and using modern chemistry and pharmacology, arrived at a new model for the κ-opioid receptor. Nonpeptide ligands for the third opioid receptor, the d receptor, are discussed by Calderon and Coop [3]. The discovery of SNC-80, the first of a new class of selective, nonpeptide d-agonists, initiated a whole new direction in the study of opioids. Although the actions of morphine and its analogs have been mainly directed at the central nervous system, DeHaven-Hudkins and Dolle [4] make a case for a novel class of opioids, which are restricted to the peripheral nervous system, therewith avoiding some of the centrally mediated side effects of the classical opioids. Finally, James Frost [5] gives a different view of opioid receptor in a literal sense. He describes the application of positron emission tomography (PET) in the visualization of opioid receptors, and its application in the study of pain phenomena. Lastly, I would like to express my sincere thanks to all collaborators to this issue of Current Pharmaceutical Design for their efforts in describing some new vistas of pain treatment and their patience.

Pain is a common experience of mankind. Pain theories and the management of pain have been modified throughout the history of mankind. This article gives a brief review on pain, pain believes and pain management from early magico-demonic and magico-religeous ideas and procedures to more empiric-scientific models; from ancient times and primitive cultures to the 20 th century. Due to new anatomical, physiological and biochemical insights, modern pain theories developed in the 19 th and 20 th century. Modern analgesics were synthesized and new invasive procedures were approved having a major impact on pain management strategies. However, older traditional beliefs and attitudes have not been replaced completely and have survived to some degree in modern patients.

The thevinols and orvinols derived from thebaine via the thebaine-methylvinyl ketone adduct (thevinone) were thoroughly investigated in the 1960's and 1970's by the Reckitt group. From this work a number of important opioids emerged. Buprenorphine is a m partial agonist, κ / δ-antagonist that is now used primarily in the treatment of heroin abuse and dependence though it was initially launched as an analgesic for the treatment of moderate to severe pain. Etorphine and dihydroetorphine are very potent m agonists that have found application in vetinary and human medicine respectively. Diprenorphine is primarily a m antagonist though it also has some κ-partial agonist effects. It has high affinity for all types of opioid receptors and as a 'universal' opioid ligand has been much in demand as a pharmacological tool. It has also been converted into a [ 11 C] version for use in Positron Emission Tomography (PET) studies of brain function related to the opioid receptor system. More recent medicinal chemistry investigations have been concerned with gaining a greater understanding of buprenorphine's unique opioid profile. This has involved the synthesis and evaluation of a number of series of buprenorphine analogues in which the C20 t-butyl group has been constrained in a ring system. These studies have suggested that the methyls in the t-butyl group inhibit the conformational changes in the k-receptor required for generation of an agonist response. Introduction of a 7α-cinnamoylaminomethyl group in place of the orvinol tertiary alcohol function leads to selective irreversible μ antagonism.

The discovery of the selective delta (δ) opioid agonists SNC 80 and BW373U86, which possess a diarylmethylpiperazine structure unique among opioids, was a major advance in the field of δ-opioid ligands. Much research has been performed to uncover the structure-activity relationships (SAR) of this class of ligands and also to compare the diarylmethylpiperazines with the traditional morphinan-based d opioids. This review focuses on the development of the SAR of this unique series of ligands, and discusses questions which remain unanswered.

Mediation of antinociception via opioid receptors located in the periphery is a viable strategy to produce analgesia without the occurrence of side effects associated with stimulation of opioid receptors located in the central nervous system. Peripheral opioid receptors are particularly important in inflammatory pain states and in the responses to pruritogenic stimuli, and have been implicated in the transmission of visceral pain. Medicinal chemistry approaches to achieve peripheralization of opioid agonists have started with a centrally acting opioid agonist as a template, and introduced features of lipophilicity, hydrophilicity, or combined lipophilicity and hydrophilicity to achieve amphiphilicity. Quaternarization of centrally acting opioid agonists or identification of compounds that serve as substrates for the mdr transporter to achieve transport out of the brain has also been employed. The in vivo assays used to identify peripherally selective compounds have measured a variety of behavioral and pharmacokinetic endpoints, with varying degrees of predictability. This review focuses on a discussion of these methods, as well as a review of those compounds where sufficient data exist to support a claim of peripheralization in vivo.

The endogenous opioid system plays a central role in pain. Recent advances have permitted imaging of opioid receptors by PET in human subjects while experiencing pain and detection of changes in receptor occupancy. The ability to perform these types of studies is dependent on the development of opioid tracer ligands labeled with positron emitting isotopes. This article follows the development and radiochemistry of opioid tracer molecules through their use in human subjects and subsequent application to the study of pain. The role of mu, delta and kappa opioid receptors in pain is reviewed. Occupancy changes in mu receptors have been observed with PET in human subjects subjected to experimental pain paradigms. The implication of this approach to the study of pain and pain syndromes, and possible clinical applications, is also addressed

Maneuvering single gene expression is not only an optimal way to study gene function but also an ambitious goal, which will lead to the treatment of a variety of human diseases whose main pathogenetic event is a genetic alteration. The recent efforts focusing on the genome project have led to array based, high throughput, gene expression analysis techniques that allow the study of complex molecular networks. Combining these powerful new technologies with modulation of gene expressions is making it possible to unravel complex molecular networks or, vice versa, to find new gene products responsible for pathological conditions on which exogenous modulation can be productive. Efficient and specific modulation of gene expression can be obtained either by producing transgenic or gene knockout organisms or cells (gene targeting), or by treating organisms or cells with short synthetic nucleic acid segments in antisense orientation with respect to the targeted mRNAs (mRNA targeting by antisense strategy). While genome manipulation is a time consuming and expensive approach, requiring invasive intervention, the “antisense strategy” is characterized by high flexibility resulting from safeness, specificity, reversibility, modulability, and low cost. The rationale of the antisense strategy is that, once one gene sequence is known, its expression can be silenced by application of synthetic single-strand nucleic acid segments (oligonucleotides) whose sequence is in antisense orientation compared to the targeted mRNA. Recently, this “informational” strategy has been boosted by the discovery of the RNA interference: a natural mechanism by which cells are thought to fight detrimental exogenous viruses and endogenous transposons. Despite promising futures, antisense-based therapeutics are far from being an established reality. This review analyses the recent improvements in antisense-based gene expression modulation, focuses on the treatment of diseases in the light of the past, and provides our personal findings on this topic.

The tragic failure of gene therapy resulted in rolling back the research of gene-based medicine. Because of the poor delivery of gene-based medicines, such as antisense oligonucleotides, ribozyme, triplex, or gene both in vitro and in vivo, further development of gene-based medicines as therapeutic agents have stagnated. Although the delivery system plays a critical role in the overall efficacy of oligonucleotides, inappropriate target selection, improper evaluation methods and misinterpretation of results often caused the pessimistic view. Still, the decoding of the whole human genome has rekindled the enthusiastic development of delivery tools for gene-based medicine. We would like to focus on the newly developed delivery systems mainly for antisense oligonucleotides in this article. There are two ways to improve delivery efficacy of antisense oligonucleotides: One is the chemical modification of the antisense oligonucleotide backbone. The other way is by means of delivery vehicles, such as cationic liposomes, synthetic polymers, or non-viral vectors. We will review the current status of delivery vehicles both in vitro and in vivo. Delivery efficiency depends on the oligonucleotides' chemistry, length, size, net charge, cell / tissue type and administration route. It is difficult to deduce a common rule that affects delivery efficiency. Some cells like keratinocytes rapidly internalize oligonucleotides without a delivery system, which is contrary to common belief. Although we cannot extensively cover all reports, we will summarize several experiments with delivery system in vitro and in vivo. We will then address the possible factors promoting the efficient delivery of oligonucleotides.

Antisense oligonucleotides (ODN) technology is one of the most promising therapeutic strategies to prevent the progress of diseases through inhibiting the specific gene expression. They are well established to serve as molecular tools for several biologic applications, from the study of single gene function up to complex target validations. From the theoretical simple action, sequence-specific inhibition of mRNA functions after complex formation and presumably enzymatic degradation of the target mRNA, they obviously carry a high therapeutic potential to treat human diseases. In addition to the potential for the treatment, antisense ODN may be applicable for investigations of the mechanism and stereochemistry of biochemical reactions, mapping of nucleic acid protein interactions, and diagnostic applications. However, the design of antisense ODN, is very difficult because many factors affecting their activity and stability must be considered. Especially, the modifications of ODN are very critical and many researchers are trying to establish ODN which have resistance to nucleolytic degradation, high affinity to complementary nucleic acid, high selectivity in binding with complementary nucleic acid, the ability to activate ribonuclease H that selectively degrades the RNA strand of ODN-RNA complex, cell permeability, and favorable pharmacokinetic and pharmacodynamic attributes. In this review we would like to introduce some modifications of ODN design and examples of our applications of antisense ODN in cardiovascular disease in animal models.

A current goal in molecular medicine is the development of new strategies for the selective inhibition of cancer-critical genes. Triplex-forming oligonucleotides and peptide nucleic acids bind to the double helix of DNA in a sequence-specific manner and with great affinity. Because of these properties, these molecules have been proposed as anti-gene therapeutic drugs. This review summarizes recent results on the use of oligonucleotides and peptide nucleic acids to downregulate gene expression in cultured cells. The data are discussed from the perspective of the recent literature on new molecular strategies with potential therapeutic applications.