A modality is the kind of thing your drug is
When we say modality, we mean the broad physical category a medicine belongs to — not what disease it treats, but what it is made of and how it works at the molecular level. A pill of aspirin is a small molecule: a tidy organic compound, usually under 500 daltons, that you can often swallow. An antibody is a biologic: a large protein grown in living cells. These are different worlds of chemistry, manufacturing, and delivery, even when they hit the same target.
For most of pharmaceutical history, small molecules were the only practical option. They are cheap to make, stable on a shelf, and small enough to cross cell membranes and reach targets inside the cell. That last point matters: the great majority of human proteins live inside cells, where a large protein drug cannot easily go.
Why one tool is never enough
Not every target is druggable by a small molecule. Some targets are flat protein surfaces with no deep pocket to bind — a classic problem for protein–protein interactions. Some require exquisite selectivity between near-identical family members. And some problems aren't about blocking a protein at all, but about turning a gene up or down. Each gap invites a different modality.
- Target on the cell surface or in the blood? A monoclonal antibody can bind it with high precision — think of antibodies as the modality for the outside of cells.
- Want to silence a gene before its protein is even made? An oligonucleotide such as a siRNA works on the RNA message, upstream of the protein.
- Need to supply a missing protein? An mRNA therapeutic can instruct the body to make it for a while.
- Want to destroy an 'undruggable' intracellular protein rather than block it? A targeted protein degrader like a PROTAC may reach where a classic inhibitor cannot.