In drug discovery, what is a hit?

• A. A compound that passes screening consisting of testing many compounds in assays relevant to the disease in question.
• B. A compound with proven efficacy in humans.
• C. A compound already approved.
• D. A compound with perfect pharmacokinetics.

Answer: (A)

In drug discovery, a hit is an initial active compound identified from screening a large library against a disease-relevant assay. Through high-throughput screening or similar tests, compounds that meet a predefined activity threshold and show reproducible, dose-dependent effects are labeled hits. They are not yet proven in humans and aren’t optimized for pharmacokinetics or safety; they’re starting points that go on to secondary validation and medicinal chemistry to become leads and, later, drug candidates. The other statements describe later-stage outcomes (efficacy in humans, regulatory approval) or ideal properties that aren’t what defines a hit.