Mechanism of Action of Opioids and Clinical Effects

Treatment with narcotic analgesic is the core of cancer pain management. In addition, opioids are the core in anesthesia as painful afferents are induced by the surgical procedure. Although concurrent use of other approaches and interventions may be appropriate in many pain patients, and necessary in some, analgesic drugs are needed in almost every case. Drugs whose primary clinical action is the relief of pain are conventionally classified on the basis of their activity at opioid receptors as either opioid or non-opioid analgesics.

A third class, the adjuvant analgesics, is drugs with other primary indications that can be effective analgesics in specific circumstances. The major Opiate is a specific term that is used to describe drugs (natural and semi-synthetic) derived from the juice of the opium poppy (Figures II-1 and II-2). For example, morphine is an opiate but methadone (a completely synthetic drug) is not.

Opioid is a general term that includes naturally occurring, semi-synthetic, and synthetic drugs, which produce their effects by combining with opioid receptors and are competitively antagonized by naloxone. In this context the term opioid refers to opioid agonists, opioid antagonists, opioid peptides, and opioid receptors.

Narcotic is commonly used to describe morphine-like drugs and other drugs of abuse. The term is derived from the Greek narke, meaning numbness or torpor. Since this is an imprecise and pejorative term that is not useful in a pharmacological context, its use with reference to opioids is discouraged.

The source of opium is the opium poppy, Papaver somniferum, one of the few species of Papaver that produces opium (Figure II-1). Through centuries of cultivation and breeding the poppy for its opium, a species of the plant evolved that is now known as somniferum. The genus, Papaver, is the Greek word for poppy. The species, somniferum, is Latin for sleep inducing.

The psychological effects of opium may have been known to the ancient Sumerians (circa 4000 B.C.) whose symbol for the poppy was hul (joy) and gil (plant). The plant was known in Europe at least 4000 years ago, as evidenced by fossil remains of poppy seed cake and poppy pods found in the Swiss lake dwellings of the Neolithic Age. Opium was probably consumed by the ancient Egyptians and was known to the Greeks as well. References to the poppy are found in Homer's works The Iliad and The Odyssey. Hippocrates (460-357 B.C.), the Father of Medicine, recommended drinking the juice of the white poppy mixed with the seed of nettle.

Hippocrates And Opium
Figure 11-1. Fresh capsule of opium poppy plant (Papaver somniferum)

The opium poppy probably reached China about the seventh century A.D. through the efforts of Arab traders who advocated its use for medicinal purposes. In Chinese literature, however, there are earlier references to its use. The noted Chinese surgeon Hua To of the Three Kingdoms (220-264 A.D.) used opium preparations and Cannabis indica for his patients to swallow before undergoing major surgery.

The opium poppy, Papaver somniferum, is an annual plant, i.e., the plant matures one time, and does not regenerate itself. New seed must be planted each season. From a small seed, it grows, flowers, and bears fruit (a pod) only once. The entire growth cycle for most varieties of this plant takes about 120 days. The tiny seeds (like the seeds on a poppy seed roll) germinate quickly in warm air and sufficient soil moisture. In less than 6 weeks, the young plant emerges from the

Figure II-2. Cut capsule showing latex exuding from cut

soil, grows a set of four leaves, and resembles a small cabbage in appearance. The lobed, dentate (jagged-edged) leaves are bluish-green with a dull gray or blue tint.

The major legal opium production areas in the world today are in government-regulated opium farms in India, Turkey, and Tasmania (Australia). The major illegal growing areas are in Southwest Asia (Afghanistan, Pakistan, and Iran) and in the highlands of Mainland Southeast Asia (Burma, Laos, Vietnam, and Thailand) -popularly known as the Golden Triangle (Figure II-3). Opium poppy is also grown in Colombia, Mexico, and Lebanon.

Opium poppies containing small amounts of opium alkaloids were, at one time, widely grown as an ornamental plant and for seeds in the United States. The Opium Poppy Control Act of 1942 declared the possession of this plant illegal. From the cut capsule latex is exuded, which is collected and further processed in order to gain the different ingredients (Figure II-2).

Within the secreted latex collectors will find the major constituents of opium poppy, which are as follows:

1. Morphine (10%-17%), the most important alkaloid, which was discovered by the pharmacist Sertürner in a small town of Einbeck, located in Lower Saxonia in Germany in 1803. He decided to name the extract from the opium poppy morphine (Figure A) because it elicited a sedative-hypnotic and sleep inducing effect, related to the Greek god of sleep Morpheus.





I H 1





Figure A

2. Codeine (0.7%-4%), chemically a methylmorphinan (Figure B), which today is derived by methylation from the prodrug morphine.

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