Enprofylline

Enprofylline is hands down the most interesting xanthine most people have never heard of; it's essentially theophylline with the annoying parts filed off. It relaxes airway smooth muscle harder than theophylline gram for gram, yet it pretty much skips the adenosine blockade, so you don't get the jittery CNS buzz, the diuresis, the gastric burn or the seizure risk that come with caffeine-family drugs.

In head-to-head asthma crossovers it matched or beat theophylline for bronchodilation at much lower plasma levels, and unlike theophylline it left coronary blood flow alone. It works low and clean; the main real-world nuisance is early headache and a short half-life that wants a sustained-release format. It never made it to broad market, so treat it as an investigational xanthine rather than a shelf staple; still, as a research tool for teasing apart what adenosine actually does, it's a genuinely elegant molecule.

Enprofylline is an investigational xanthine that was never broadly marketed; it is not risk-free. The most consistently reported side effects in human asthma trials were headache (more frequent than with theophylline, especially early in treatment) and mild nausea. It is a smooth-muscle-active drug with a short elimination half-life (about 1 hour in children), so it was studied largely as a sustained-release tablet. It is eliminated mostly unchanged in urine, which limits metabolic drug interactions relative to theophylline. Long-term safety data are limited because it did not achieve widespread clinical use.