The previous installment ended on a loose thread, and it is worth pulling on before anything else.
The story so far was an engineering one: an antibody borrowed from Herceptin, a payload too poisonous to give on its own, and a linker built to keep that poison sealed until it reached a HER2-positive cell (CS-ENHERTU-01). Everything depended on containment. The whole point was that the warhead went where the antibody aimed it and nowhere else. But the same installment noticed a problem with that tidy picture. The conjugate is carried into a HER2-positive cell whole and the payload — a small molecule called DXd — is cut loose only once it is inside; to do its work it then has to slip across the cell’s oily membranes under its own power. And a molecule that can move through a membrane like that is, by the same chemistry, a molecule that can slip back out of the cell it was released in — and into the cells around it.
Hold that thought, because it is about to stop being a worry and start being the reason the drug exists.
Here is the puzzle in its sharpest form. Enhertu and Kadcyla carry the same antibody — trastuzumab, the same protein sequence, residue for residue, aimed at the same target, HER2. If you believed the antibody was what made the drug work — find the HER2, latch on, deliver the kill — you would expect the two drugs to reach the same tumors. They do not. Enhertu turned out to work in patients Kadcyla could not touch: patients whose tumors carry so little HER2 that, for twenty years, every HER2-directed drug had treated them as out of reach. Same antibody. Same target. Different reach.
When the antibody is held constant and the result changes, the antibody is not the answer. The answer is in the payload — and specifically in the one property the last installment flagged as a worry: that DXd can cross a membrane and DM1, the payload Kadcyla carries, largely cannot. That single difference in chemistry, established in the lab long before any of this reached a patient, is the whole of the explanation. And it has a name. The bystander effect.
