Pharmacokinetics
The conversion of codeine to morphine occurs in the liver and is catalysed by the cytochrome P450 enzyme CYP2D6. CYP3A4 produces norcodeine and UGT2B7 conjugates codeine, norcodeine, and morphine to the corresponding 3- and 6- glucuronides. Approximately 6–10% of the Caucasians, 2% of Asians, and 1% of Arabs are "poor metabolizers"; they have little CYP2D6, and codeine is less effective for analgesia in these patients (Rossi, 2004). Srinivasan, Wielbo and Tebbett speculate that codeine-6-glucuronide is responsible for a large percentage of the analgesia of codeine, and, thus, these patients should experience some analgesia. Many of the adverse effects will still be experienced in poor metabolizers. Conversely, 0.5-2% of the population are "extensive metabolizers"; multiple copies of the gene for 2D6 produce high levels of CYP2D6 and will metabolize drugs through that pathway more quickly than others.
Some medications are CYP2D6 inhibitors and reduce or even completely block the conversion of codeine to morphine. The most well-known of these are two of the selective serotonin reuptake inhibitors, paroxetine (Paxil) and fluoxetine (Prozac) as well as the antihistamine diphenhydramine and the antidepressant, bupropion (Wellbutrin, also known as Zyban). Other drugs, such as rifampicin and dexamethasone, induce CYP450 isozymes and thus increase the conversion rate.
Since codeine is a prodrug, metabolism differences have the opposite effect. Thus an extensive metabolizer may have adverse effects from a rapid buildup of codeine metabolites while a poor metabolizer may get little or no pain relief. CYP2D6 is dysfunctional in 7% of white and black Americans, resulting in reduced metabolism of codeine. Other individuals may have two or more copies of the CYP2D6 gene, resulting in rapid metabolism of the target drug. CYP2D6 metabolizes and activates codeine into morphine, which then undergoes glucuronidation. Life-threatening intoxication, including respiratory depression requiring intubation, can develop over a matter of days in patients who have multiple functional alleles of CYP2D6, resulting in ultra-rapid metabolism of opioids such as codeine into morphine.
The active metabolites of codeine, notably morphine, exert their effects by binding to and activating the μ-opioid receptor.
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