The antibody at the heart of Enhertu was never invented for Enhertu. It has the same amino-acid sequence — the same exact chain of building blocks — as trastuzumab, the antibody sold for twenty years as Herceptin. Kadcyla, a different drug from a different company era, reuses that same antibody yet again. Three products, one targeting protein, discovered once and borrowed twice. That is the first surprise of how an antibody-drug conjugate is found, and it points at the larger one: the thing a program is hunting for is not a molecule at all. It is an assembly.
An antibody-drug conjugate — an ADC — is three parts fastened together: an antibody that finds the cancer cell, a small-molecule poison that does the killing, and a chemical linker that holds the two together until the poison has arrived. The target-identification stage settled what the antibody should aim at, an address on the surface of the right cells. The next stage in any drug program is hit discovery — the step where you go from knowing what you want to attack to holding an actual molecule that does something against it. For most drugs that is one search for one compound. For an ADC it is three searches at once, run three entirely different ways, and the thing you are trying to produce is the assembled conjugate — the antibody, payload, and linker working as a single object.
