Chapter 24 Opioid Analgesics

http://thepointeedition.lww.com/pt/re/9780781768795/bookContentPan...DIVISIONA[3]/DIVISIONB[1]/CHAPTER[7]&highlightTo=&printPreview=yes Chapter 24 Opioid Analgesics David S. Fries Introduction Agents that decrease pain are referred to as analgesics, or analgetics. Although analgetic is grammatically correct, common use has made analgesic preferable to analgetic for the description of the pain-killing drugs. Pain relieving agents also are called antinociceptives. A number of classes of drugs are used to relieve pain. The nonsteroidal anti-inflammatory agents have primarily a peripheral site of action, are useful for mild to moderate pain, and often have an anti- inflammatory effect associated with their pain-killing action. Local anesthetics inhibit pain transmission by inhibition of voltage-regulated sodium channels. These agents often are highly toxic when used in concentrations sufficient to relieve chronic or acute pain in ambulatory patients. Dissociative anesthetics (ketamine), and other compounds that act as inhibitors of N-methyl-D-aspartate (NMDA)–activated glutamate receptors in the brain, are effective antinociceptive agents when used alone or in combination with opioids. Compounds, such as the antiseizure drug pregabulin, which inhibits voltage regulated Ca2+ ion channels, are useful in treating neuropathic pain. Most central nervous system (CNS) depressants (e.g., ethanol, barbiturates, and antipsychotics) will cause a decrease in pain perception. Inhibitors of serotonin and norepinephrine reuptake (i.e., antidepressant drugs) are useful either alone and in combination with opioids in treating certain cases of chronic pain. Current research into the antinociceptive effects of centrally acting α-adrenergic-, cannabinoid-, and nicotinic-receptor agonists may yield clinically useful analgesics working by nonopioid mechanisms. Research in one or more of the above areas may lead to new drugs, but at present, severe acute or chronic pain generally is treated most effectively with opioid agents. Historically, opioid analgesics have been called narcotic analgesics. Narcotic analgesic literally means that the agents cause sleep or loss of consciousness (narcosis) in conjunction with their analgesic effect. The term “narcotic” has become associated with the addictive properties of opioids and other CNS depressants. Because the great therapeutic value of the opioids is their ability to induce analgesia without causing narcosis, and because not all opioids are addicting, the term “narcotic analgesic” is misleading and will not be used further in this chapter. History The juice (opium in Greek) or latex from the unripe seed pods of the poppy Papaver somniferum is among the oldest recorded medications used by humans. The writings of Theophrastus around 200 BC describe the use of opium in medicine; however, evidence suggests that opium was used in the Sumerian culture as early as 3500 BC. The initial use of opium was as a tonic, or it was smoked. The pharmacist Surtürner first isolated an alkaloid from opium in 1803. He named the alkaloid morphine, after Morpheus, the Greek god of dreams. Codeine, thebaine, and papaverine are other medically important alkaloids that were later isolated from the latex of opium poppies. http://thepointeedition.lww.com/pt/re/9780781768795/bo.../DIVISIONB[1]/CHAPTER[7]&highlightTo=&printPreview=yes (1 of 58)11-09-2009 13:25:35 http://thepointeedition.lww.com/pt/re/9780781768795/bookContentPan...DIVISIONA[3]/DIVISIONB[1]/CHAPTER[7]&highlightTo=&printPreview=yes Morphine was among the first compounds to undergo structure modification. Ethylmorphine (the 3-ethyl ether of morphine) was introduced as a medicine in 1898. Diacetylmorphine (heroin), which may be considered to be the first synthetic pro-drug, was synthesized in 1874 and marketed as a nonaddicting analgesic, antidiarrheal, and antitussive agent in 1898. P.653 Clinical Significance Opioid agonists and partial agonist/antagonists generally act on δ, µ, and κ receptors. All of these receptors have subtypes that provide varying degrees of analgesia, euphoria or dysphoria, central nervous system depression, and perhaps, the potential for tolerance. By modifying their structures, properties can be changed to develop agents that require more or less hepatic metabolism and, thus, affect the duration of action and the bioavailability. Other changes in the chemical structures can yield agents with much higher affinity for analgesic receptors, which corresponds to more potency on a milligram-to-milligram basis. Other alterations of the chemical structures can lead to improved profiles regarding respiratory depression, emesis, tolerance, and allergenicity. By altering the affinities for some receptors more than others, the addictive properties also may be manipulated. Through an understanding of the relationship of chemical structures to biological activity, the clinician can improve the selection of drug to the specific patient. Jill T. Johnson, Pharm.D., BCPS Associate Professor Department of Pharmacy Practice College of Pharmacy University of Arkansas for Medical Sciences Opiate/Opioid The use of the terms “opiate” and “opioid” requires clarification. Until the 1980s, the term “opiate” was used extensively to describe any natural or synthetic agent that was derived from morphine. One could say an opiate was any compound that was structurally related to morphine. In the mid-1970s, the discovery of peptides in the brain with pharmacological actions similar to morphine prompted a change in nomenclature. The peptides were not easily related to morphine structurally, yet their actions were like those produced by morphine. At this time, the term “opioid,” meaning opium- or morphine-like in terms of pharmacological action, was introduced. The broad group of opium alkaloids, synthetic derivatives related to the opium alkaloids, and the many naturally occurring and synthetic peptides with morphine-like pharmacological effects are called opioids. In addition to having pharmacological effects similar to morphine, a compound must be antagonized by an opioid antagonist, such as naloxone, to be classed as an opioid. The neuronal- located proteins to which opioid agents bind and initiate biological responses are called opioid receptors. http://thepointeedition.lww.com/pt/re/9780781768795/bo.../DIVISIONB[1]/CHAPTER[7]&highlightTo=&printPreview=yes (2 of 58)11-09-2009 13:25:35 http://thepointeedition.lww.com/pt/re/9780781768795/bookContentPan...DIVISIONA[3]/DIVISIONB[1]/CHAPTER[7]&highlightTo=&printPreview=yes Endogenous Opioid Peptides and Their Physiological Functions Scientists had postulated for some time, based on structure–activity relationships (SARs), that opioids bind to specific receptor sites to cause their actions. It also was reasoned that morphine and the synthetic opioid derivatives are not the natural ligands for the opioid receptors and that some analgesic substance must exist within the brain. Techniques to prove these two points were not developed until the mid-1970s. Hughes et al. (1) used the electrically stimulated contractions of guinea pig ileum and the mouse vas deferens, which are very sensitive to inhibition by opioids, as bioassays to follow the purification of compounds with morphine-like activity from mammalian brain tissue. These researchers were able to isolate and determine the structures of two pentapeptides, Tyr-Gly-Gly-Phe-Met (Met-enkephalin) and Tyr- Gly-Gly-Phe-Leu (Leu-enkephalin), that caused the opioid activity. The compounds were named enkephalins after the Greek word Kaphale, which translates as “from the head.” At about the same time as Hughes and coworkers were making their discoveries, three other laboratories, using a different assay technique, were able to identify endogenous opioids and opioid receptors in the brain (2,3,4). These scientists used radiolabeled opioid compounds (radioligands), with high specific activity, to bind to opioid receptors in brain homogenates (5). They demonstrated saturable binding (i.e., the tissue contains a finite number of binding sites that can all be occupied) of the radioligands and that the bound radioligands could be displaced stereoselectively by nonradiolabeled opioids. Discovery of the enkephalins was soon followed by the identification of other endogenous opioid peptides, including β- endorphin (6), the dynorphins (7), and the endomorphins (8). The opioid peptides isolated from mammalian tissue are known collectively as endorphins, a word that is derived from a combination of endogenous and morphine. The opioid alkaloids and all of the synthetic opioid derivatives are exogenous opioids. Interestingly, the isolation of morphine and codeine in small amounts has been reported from mammalian brain (9). The functional significance of endogenous morphine remains unknown. Opioid Peptides The endogenous opioid peptides are synthesized as part of the structures of large precursor proteins (10). There is a different precursor protein for each of the major types of opioid peptides (Fig. 24.1). Proopiomelanocortin is the precursor for β-endorphin. Proenkephalin A is the precursor for Met- and Leu- enkephalin. Proenkephalin B (prodynorphin) is the precursor for dynorphin and P.654 α-neoendorphin. The pronociceptin protein has been identified and contains only one copy of the active peptide, whereas the precursor protein for the endomorphins remains to be identified. All of the pro-opioid proteins are synthesized in the cell nucleus and transported to the terminals of the nerve cells from

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