Erenumab

Erenumab is a human monoclonal antibody against the calcitonin gene-related peptide receptor, given once monthly by subcutaneous injection for prevention of migraine. It is used when other preventive drugs have failed or were not tolerated. Constipation and hypertension are its notable risks.

Mechanism. Binds and blocks the calcitonin gene-related peptide receptor, reducing trigeminal pain signalling. It is the only antibody in its class that targets the receptor rather than the peptide.

Pharmacokinetics. Half-life is about 28 days. Clearance is target-mediated and saturable. It is not metabolised by cytochrome P450 enzymes, so interactions are few.

Specialist use. Preventive treatment of migraine in adults with at least 4 migraine days each month, usually after other preventive drugs have failed.

• Subcutaneous erenumab: 70 mg once monthly. Some people need 140 mg once monthly, given as two 70 mg injections or one 140 mg injection.

• Review benefit after 3 months, and stop if there is no response by 3 to 6 months.

• After 6 to 12 months of good control, consider a planned break from treatment.

Kidney, liver, weight and age.

• Kidney and liver: no adjustment.

• Older adults: limited data over age 65.

• Children: not licensed under 18 years.

Contraindications. Hypersensitivity. Some presentations have a needle cap that contains dry natural rubber (latex), so use caution with latex allergy.

Serious adverse effects. Severe constipation with complications (obstruction, ileus, perforation), new or worsening hypertension, hypersensitivity including angioedema and anaphylaxis, and Raynaud phenomenon.

Common adverse effects. Injection site reactions, constipation, muscle spasm and cramps, and alopecia.

Interactions. No clinically important interactions are described. Use caution with other constipating drugs (opioids, anticholinergics, calcium channel blockers).

Common prescribing errors. Not asking about constipation. Missing hypertension. Continuing without a benefit review.

Special groups.

• Pregnancy: avoid unless clearly needed, because the antibody crosses the placenta in later pregnancy.

• Breastfeeding: no data; use with caution.

• Cardiovascular disease: limited data; use caution in people with Raynaud phenomenon or vascular disease.

• Regional: very costly and seldom available, and cold storage is needed.

Formulations. Prefilled autoinjector or pen 70 mg and 140 mg in 1 mL. Inject subcutaneously into the abdomen, thigh or upper arm, rotating sites. Store at 2 to 8 degrees Celsius, protected from light, and allow the injection to reach room temperature for 30 minutes before use. It may be kept at room temperature for up to 7 days.

Monitoring.

• Baseline blood pressure and bowel habit, then blood pressure periodically.

• Headache diary of monthly migraine days. Review at 3 months and about every 6 to 12 months.

Stopping. No taper is needed. Effects wear off over several months.

Referral and escalation. Start under specialist care. Refer urgently for severe constipation with abdominal pain or vomiting, or for angioedema.