Which medication is primarily designed to treat chronic migraines rather than act acutely?

Propranolol is primarily designed to treat chronic migraines as a preventive medication rather than as an acute treatment. It is a non-selective beta-blocker that is effective in reducing the frequency of migraine attacks and is often used as a first-line treatment option for migraine prophylaxis. The mechanism behind its efficacy is not fully understood but is believed to involve modulation of vascular tone and stabilization of neuronal excitability.

The other medications listed are more suited to acute treatment or have a different primary indication. Sumatriptan, for instance, is a triptan specifically formulated to relieve migraine symptoms once an attack has started, making it an effective acute treatment rather than a preventative measure. Desipramine, an antidepressant, can be used off-label for migraine prophylaxis, but it isn't considered a first-line treatment specifically for migraines. Ibuprofen, while it can relieve pain during a migraine, also serves an acute role and is not intended for chronic preventive treatment. Thus, propranolol is the most appropriate choice for chronic migraine management in this context.