Analgesic specific agents

The following section describes the most important and widely used opioid analgesics, along with features peculiar to specific agents. Data about doses approximately equivalent to 10 mg of intramuscular morphine, oral versus parenteral efficacy, duration of analgesia, and intrinsic activity (maximum efficacy) are presented in Table 31-2. 

A review of the role of the PGs in inflammation and a discussion of the various sensitivities of different tissue synthetases to NAA, underscores the fact that only the most preliminary aspects of this fascinating subject have been uncovered. 3 It has been speculated that the inhibition of PG synthetase of brain,but not of several peripheral tissues, by acetominophen is responsible for its analgesic effect and lack of antiinflammatory activity, and serves to suggest new approaches in the search for more specific agents. (See Chap. I7 for additional discussion),... 

At equipotent doses, the analgesic and anti-inflammatory activity of all NSAIDs and aspirin are similar. The selection of a specific NSAID should be based on tolerability, previous response, and cost. Some patients respond to one NSAID better than to another. If an insufficient response is achieved with one NSAID, another agent from the same or a different chemical class should be tried. Pain relief occurs rapidly (within hours), but antiinflammatory benefits are not realized until after 2 to 3 weeks of continuous therapy. This period is the minimal duration that should be considered an adequate NSAID trial. 

Few studies have explored the efficacy of opioids specifically for OA. The APS recommends against the use of codeine and propoxyphene for OA because of the high incidence of adverse effects and limited analgesic effectiveness. Oxycodone is the most extensively studied of the agents recommended for OA. However, other narcotic analgesics such as morphine, hydromorphone, methadone, and transdermal fentanyl are also effective. 

Epibatidine s antinociceptive effect can be antagonized by pretreatment with the centrally active nAChR antagonist mecamylamine, but not with the peripheral antagonist hexamethonium, so the activation of central nAChRs is presumed to be essential for nicotinic analgesics (Sullivan et al., 1994). The high toxicity of epibatidine has been attributed to its lack of selectivity for specific neuronal nAChR subtypes and has precluded its development as a therapeutic agent. 

The demonstration of the existence of strictly defined SARs. which is perhaps Ihe most important criterion of drug action at a specific receptor site, has made possible the most important pharmacologic discoveries. For example, the analgesic actions of morphine and related agents, which are indicative of specific receptors, led to die discovery of endogenous opiate peptides, i.e., the leucine and methionine enkephalins and endorphins. 

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