In March 2019, AstraZeneca agreed to pay Daiichi Sankyo $1.35 billion before a single additional patient was dosed.
That was the upfront number. The full collaboration to jointly develop and commercialize a single experimental drug was worth up to $6.9 billion — among the largest sums ever committed to one molecule that had not yet been approved anywhere. Daiichi Sankyo, a Japanese company that most readers outside the industry had never heard of, kept exclusive rights in its home market and handed AstraZeneca the rest of the world. The drug at the center of the deal had a development code, DS-8201, and a name almost nobody could pronounce yet: trastuzumab deruxtecan. It would later ship as Enhertu.
It is worth sitting with what AstraZeneca was actually buying for that money, because it explains the whole shape of this arc. They were not buying a chemical. A small-molecule drug — the kind you can draw on a napkin, the kind a chemist optimizes one compound at a time (CS-GLEEVEC-02) — is a single thing. Enhertu is three things, engineered to behave as one. It is an antibody, a linker, and a payload, fused into a single molecule and delivered from a single vial. One name. One infusion. Three variables.
And that is the first thing to understand about an antibody–drug conjugate, before any chemistry: you do not discover one. There is no screen you run, no compound you find at the bottom of a plate, that hands you an ADC. You build it, the way you build anything with parts that fight each other — by choosing each part for a job, and then spending years on the seams where they meet. Each of the three components is its own discipline, with its own people, its own failure mode, and its own version of the Stage-4 question every drug program eventually has to answer: of all the molecules we could make, which one do we advance? (SP-04) Enhertu is that question asked three times, and a fourth time about how the answers fit together.
Take the three programs in turn — the antibody, the payload, the linker — and then the single design choice that made all three worth it, the one the rest of this arc turns on (CS-ENHERTU-02).
