Classification of Opioids

There are multiple systems used to classify opioids. They may be categorized according to receptor affinity or by their intrinsic activity at that receptor site. According to the latter, they are classified as agonist, partial agonists, agonist/antagonist, or antagonist (Table 7.2). When classified according to receptor affinity, they may be classified as either weak or strong opioids. Based on derivation, opioids may be grouped as natural (morphine and codeine), semisynthetic, or synthetic (Table 7.3). Chemical classes of opioids include phenanthrenes, benzomorphans, phenylpiperidines, and diphenylheptanes.

Opium serves as the main source for production of the two naturally occurring alkaloid opioid chemical classes. One alkaloid group, consisting of morphine, codeine, and thebaine, contains the three-ringed phenanthrene nucleus. The other group consists of benzylisoquino-line alkaloids, papaverine, and noscapine, which lack analgesic activity. The semisynthetic opioids are produced by altering the alkaloid ring structure of naturally occurring opioids. Thebaine serves as precursor for the potent opioid oxycodone and the antagonist naloxone, while morphine serves as the building block for heroin. Synthetic opioids belong to one of the four chemical classes listed above. These chemical classes result after progressively reducing rings from the original five-ring structure of morphine. Phenanthrenes consist of a four-ring nucleus while phenylpiperidines contain only a two-ring nucleus (Gustein and Akil 2006).

Table 7.2 Classification of opioids: intrinsic receptor activity.

Class

Definition

Example

Agonist

A drug which causes maximal stimulation of the opioid

Morphine, fentanyl,

receptor when bound

sufentanil, remifentanil

Antagonists

A drug which fails to cause any stimulation of the

Naloxone

receptor when bound

Partial agonists

A drug that, when bound to the receptor, stimulates the

Buprenorphine

receptor below maximal intensity

Mixed agonists/

A drug which acts simultaneously on several receptor

Nalbuphine, butorphanol

antagonists

subtypes. Drug acts as agonist on one or more

subtypes and as antagonist at one or more subtypes

Modified from Wall (1994).

Modified from Wall (1994).

Table 7.3 Classification of opioids by derivation.

Naturally occurring

Semisynthetic

Synthetic

Phenanthrene

Morphine

Morphinans

Morphine

Diacetylmorphine

Levorphanol

Codeine

Dihydromorphinone

Nalbuphine

Thebaine

Dihydrohydroxymorphinone

Benzylisoquinoline

Thebaine derivatives

Phenylheptylamines

Papaverine

Buprenorphine

Methadone

Noscapine

Oxycodone

l-Alpha-acetylmethadol (LAAM)

Propoxyphene

Phenylpiperidine

Alfentanil

Alphaprodine

Fentanyl

Ketobemidone

Meperidine

Remifentanil

Sufentanil

Modified from Uppington (2005).

Modified from Uppington (2005).

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