Nabilone, the synthetic cannabinoid antiemetic
Nabilone is a synthetic cannabinoid (a man-made drug acting on cannabinoid receptors) held in reserve for chemotherapy-induced nausea and vomiting that standard antiemetics (drugs for sickness) have not controlled.
Remember
Nabilone is a reserve drug for chemotherapy-induced nausea and vomiting that standard antiemetics have not controlled, and its side effects, such as drowsiness and dizziness, limit its use.
Nabilone is a synthetic cannabinoid, a man-made drug that activates cannabinoid receptors, the same family of receptors that cannabis acts on. It acts on the cannabinoid receptors known as CB1 and CB2.
The licence is for chemotherapy-induced nausea and vomiting that has not responded to standard antiemetics. Licensed means the medicines regulator has approved the drug for a specific use. Nabilone is a second-step drug, a back-up rather than a first choice.
The usual drugs for chemotherapy-induced sickness are dexamethasone (a corticosteroid) plus ondansetron, which blocks 5-HT3 (serotonin) receptors. Aprepitant, which blocks NK1 receptors for substance P (a chemical messenger involved in vomiting), is added when the chemotherapy is highly emetogenic (very likely to cause vomiting). Nabilone is used after these drugs have failed, not before them.
Cannabinoid receptors are widespread in the brain, so the typical side effects of nabilone are central ones: drowsiness, dizziness and dysphoria (an unpleasant state of unease or low mood), together with postural hypotension (blood pressure falls on standing, causing light-headedness). These effects limit its use in practice.
Key facts
- Class
- a synthetic cannabinoid that activates cannabinoid receptors (CB1 and CB2).
- Licence
- chemotherapy-induced nausea and vomiting when standard antiemetics have not worked.
- Place in therapy
- second step, after the usual combination of dexamethasone plus ondansetron, with aprepitant for highly emetogenic regimens, has not worked.
- Limiting effects
- drowsiness, dizziness, dysphoria and postural hypotension.
- Receptor map
- ondansetron blocks 5-HT3 receptors, metoclopramide blocks dopamine D2 receptors and aprepitant blocks NK1 receptors, while nabilone is the one that acts on cannabinoid receptors.
- Effects of other antiemetics
- extrapyramidal effects (movement problems caused by dopamine blockade in the brain) point to metoclopramide, QT prolongation (delayed electrical recovery of the heart between beats) to ondansetron, and insomnia and hyperglycaemia (raised blood glucose) to dexamethasone.
Exam traps
- Extrapyramidal symptoms are the classic limiting effect of dopamine D2 antagonists such as metoclopramide, not of nabilone, because nabilone is not a dopamine antagonist.
- A severe rise in blood pressure is not a characteristic adverse effect of nabilone: cannabinoids do affect blood pressure, but the circulatory effect that limits its use is postural hypotension, a fall in blood pressure on standing.
- Nabilone is not a first-line way to prevent nausea when chemotherapy starts, even though it is a chemotherapy antiemetic, because the first-line approach is dexamethasone plus ondansetron, with aprepitant added for highly emetogenic regimens.
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