Opioid Analgesics—Morphine Type

Sources of opioids. Morphine is an opium alkaloid (p.4). Besides morphine, opium contains alkaloids devoid of analgesic activity, e.g., the spasmolytic papaverine. All semisynthetic derivatives (hydromorphone) and fully synthetic derivatives (pentazocine, pethidine = meperidine, l-methadone, and fentanyl) that possess the analgesic effect of morphine are collectively referred to as opiates and opioids, respectively. The high analgesic effectiveness of xenobiotic opioids derives from their af nity for receptors normally acted upon by endogenous opioids (enkephalins, β-endorphin, dynorphins; A). Opioid receptors occur on nerve cells. They are found in various brain regions and the spinalmedulla,aswellasinintramuralnerve plexuses that regulate the motility of the alimentary and urogenital tracts. There are several types of opioid receptors, designated µ, δ, κ, which mediate the various opioid effects, and all belong to the superfamily of G-protein coupled receptors (p.66).

Endogenous opioids are peptidesthat are cleaved from the precursors, proenkephalin, pro-opiomelanocortin, and prodynorphin. All contain the amino acid sequence of the pentapeptides [Met]- or [Leu]-enkephalin (A). The effects of the opioids can be abolishedbyantagonists(e.g., naloxone; A), with the exception of buprenorphine.

Mode of action of opioids. Most neurons react to opioids with a hyperpolarization, reflecting an increase in K+ conductance. Ca2+ influx into nerve terminals during excitation is decreased, leading to a decreased release of transmittersand decreased synaptic activity (A). Depending on the cell population affected,thissynapticinhibition translates into a depressant or excitant effect (B).

Effects of opioids (B). The analgesic effect results from actions at the level of the spinal cord (inhibition of nociceptive impulse transmission) and the brain (disinhibition

of the descending antinociceptive system, attenuation of impulse spread, and inhibition of pain perception). Attention and ability to concentrate are impaired. There is a mood change, the direction of which depends on the initial condition. Aside from the relief associated with the abatement of strong pain, there is a feeling of detachment (floating sensation) and a sense of wellbeing (euphoria), particularly after intravenous injection and, hence, rapid build-up of drug levels in the brain. The desire to reexperience this state by renewed administration of drug may become overpowering: development of psychological dependence.

The attempt to quit repeated use of the drug results in withdrawal signs of both physical (e.g., cardiovascular disturbances) andpsychological(e.g., restlessness, anxiety, depression) nature.

Opioids meet the criteria of drugs producing dependence, that is, a psychological and physiologicalneedtocontinueuseofthedrug aswellasthecompulsiontoincreasethedose.

For these reasons, prescription of opioids is subject to special rules (Controlled Substances Act, USA; Narcotic Control Act, Canada; etc). Certain opioid analgesics such as codeine and tramadol may be prescribed in the usual manner, because of their lesser potential for abuse and development of dependence. Differences among opioids in ef - cacy and liability to cause dependence probably reflect differing patterns of af nity and intrinsic activity at individual receptor subtypes, as well as genetic variation of opioid receptors. A given substance does not necessarily act as an agonist or antagonist at each subtype;instead,itmaybehave asanagonist at one subtype and as a partial agonist/antagonist at another, or even as a pure antagonist (p.212). In particular, the addictive potential is determined by kinetic properties; only with rapid drug entry into the brain can the euphoriant “rush” be experienced. All high-potency opioids pose the risk of respiratory depression (paralysis of the respiratorycenter) after overdosage. Themaximally

possible extent of respiratory depression is thought to be less for partial agonists/antagonists (pentazocine, nalbuphine).

The cough-suppressant (antitussive) effect produced by inhibition of the cough reflex is independent of the effects on nociception or respiration (antitussives: codeine, noscapine).

Stimulation of chemoreceptors in the area postrema (p.342) results in vomiting, particularly after first-time administration or in the ambulant patient. The emetic effect disappears with repeated use because a direct inhibition of the emetic center then predominates.

Opioids elicit pupillary narrowing (miosis) by stimulating the parasympathetic portion (Edinger–Westphal nucleus) of the oculomotor nucleus.

The peripheral effects concern the motility and tonus of gastrointestinal smooth muscle; segmentation is enhanced but propulsive peristalsis is inhibited. The tonus of sphincter muscles is markedly raised (spastic constipation). The antidiarrhetic effect is used therapeutically (loperamide, p.180). Gastric emptying is delayed (pyloric spasm) and drainage of bile and pancreatic juice is impeded because the sphincter of Oddi contracts. Likewise, bladder function is affected; specifically bladder emptying is impaired owing to an increased tone of the vesicular sphincter.

Kinetics of opioids. The endogenous opioids (e.g., metenkephalin, leuenkephalin, β-en- dorphin) cannot be used therapeutically because, owing to their peptide nature, they are rapidly degraded or excluded from passage through the blood–brain barrier, thus preventing access to their sites of action even after parenteral administration (A). Morphine can be given orally or parenterally, as well as epidurally or intrathecally in the spinal cord. Fentanyl is of such potency and high tissue penetrability as to permit application in the form of a patch (transdermal

delivery system) (A). In opiate abuse, drug (“stuff,” “junk,” “smack,” “jazz,” “China white”; mostly heroin = diacetylmorphine) is injected intravenously (“mainlining”) to achieve the fastest possible rate of rise in brain concentration. Evidently, psychic effects (“kick,” “buzz,” “rush”) are especially intense with this route of delivery. The user may also resort to other more unusual routes: opium is smoked; heroin can be taken as snuff (B).

Metabolism (C). Like other opioids bearing a free hydroxyl group, morphine is conjugated to glucuronic acid and excreted renally. Glucuronidation of the OH group at position 6, unlike that at position 3, does not affect af nity for the receptors.

Tolerance. With repeated administration of opioids, their CNS effects undergo habituation (adaptation). In the course of therapy, progressively larger doses are thus needed to achieve the same degree ofpain relief. The peripheral effects are less altered by the development of tolerance, so that persistent constipation during prolonged use may force a discontinuation of analgesic therapy however urgently needed. Frequently laxatives must be prescribed.

Morphine antagonists and partial agonists.

The effectsofopioidscan be abolished bythe antagonists naloxone or naltrexone (A), irrespective of the receptor type involved. Given by itself, neither has any effect in normal subjects; however, in opioid-dependent subjects, both precipitate acute withdrawal signs. Because of its rapid presystemic elimination, naloxone is only suitable for parenteral use. Naltrexone is metabolically more stable and can be given orally. Naloxone is effective as antidote in the treatment of opioid-induced respiratory paralysis. Since it is more rapidly eliminated than most opioids, repeated doses may be needed. Naltrexone may be used as adjunct in withdrawal therapy.

Racemate of tramadol

Drug concentration in plasma

Analgesia

Methadone t1/2 = 55 h high dose

Methadone low dose