I'd say that API is the active pharmaceutical ingredient, so that is the active substance in a drug, which can administered through a pill (orally), or an injection (sub-cutaneously).
The abbreviation API refers to the "application programming interface." An application programming interface (API) is a mechanism that enables users to query resources, get data in a manner that is readable by machines, and download that data.
An agent could produce a therapeutic effect against illness after its administration via any type of formulation as well as the route of administration that agent is called as biological agent or Active Pharmaceutical Agent (API).