macrocycle

A macrocycle is a molecule built around one large ring, typically twelve atoms or more in the loop. Where most small-molecule drugs are compact and chain-like, a macrocycle is a big flexible hoop that can drape itself over a target and wrap around features that a small rigid molecule could never grasp.

That large ring gives two useful properties. First, the ring pre-organizes the molecule into a shape, so it pays less of an entropic penalty when binding and can engage large, flat targets such as protein-protein interfaces. Second, by folding so that polar groups tuck inward, some macrocycles hide their water-loving parts and slip across cell membranes despite being bigger than usual rules allow.

Macrocycles occupy a 'beyond rule of five' space between classic small molecules and biologics, capturing some of the reach of peptides while sometimes keeping oral availability. Nature makes many of them, including drugs like cyclosporine and erythromycin. The downside is that they are harder and costlier to synthesize, and their flexibility makes their properties tricky to predict and optimize.