An antibody carrying chemotherapy added five months in a 468 person trial

One of the clearest survival results in advanced breast cancer this decade came from a randomised trial of 468 people, published in the New England Journal of Medicine in 2021. The same treatment, tested later in a different cancer, failed. Both results belong in the same account, and reading them together is what the published record actually supports [1][3].

What the treatment is

Sacituzumab govitecan is an antibody drug conjugate. An antibody is a protein the immune system uses to recognise one specific target; here it recognises Trop-2, a protein carried on the surface of most breast cancer cells. Chemically bonded to that antibody is SN-38, a chemotherapy drug that damages an enzyme cancer cells need in order to copy their DNA. The antibody is the address and the chemotherapy is the parcel [1].

The idea is to deliver a drug that would be too harsh to give freely by attaching it to something that finds the tumour first. Whether the delivery is precise enough to matter is the question every trial of this design has to answer.

The first trial

The ASCENT trial enrolled adults whose breast cancer had returned or stopped responding, in the form called triple negative, meaning the tumour carries none of the three receptors that most breast cancer treatments are aimed at. All had already received taxane chemotherapy. Participants were assigned by chance to the antibody treatment or to a single chemotherapy drug of the treating doctor's choosing, which is what these patients would otherwise have been given [1].

Among the 468 participants without cancer that had spread to the brain, median time before the disease grew again was 5.6 months on the antibody treatment against 1.7 months on chemotherapy. Median survival was 12.1 months against 6.7 months. Tumours shrank measurably in 35 percent of the antibody group against 5 percent of the chemotherapy group. All three comparisons had a P value below 0.001, meaning a gap this large would turn up by chance less than once in a thousand times if the treatments were equal [1].

Severe side effects were more common. Low white cell counts of grade 3 or worse occurred in 51 percent of the antibody group against 33 percent on chemotherapy, and severe diarrhoea in 10 percent against under 1 percent. Three people in each group died of side effects; none of those deaths were attributed to the antibody treatment [1].

The second trial, and a smaller gap

A second randomised trial, TROPiCS-02, tested the same treatment in 543 adults with a more common form of breast cancer, one driven by hormone receptors and not by the HER2 protein, after several previous treatments had been used. Its final survival analysis, published in The Lancet in 2023, reported median survival of 14.4 months against 11.2 months, a difference of 3.2 months with a P value of 0.020 [2].

That trial was open label, meaning everyone knew which treatment they were receiving. Its measures of quality of life and fatigue also moved in favour of the antibody treatment. One person died of septic shock caused by an infection during a period of low white cell counts, and that death was judged to be related to the treatment [2].

The trial that missed

TROPiCS-04, published in Annals of Oncology in 2025, took the same treatment into advanced urothelial cancer, which arises in the bladder and urinary tract, in 711 people whose disease had already progressed through platinum chemotherapy and immunotherapy. The main measurement was survival, and the trial did not meet it: median survival was 10.3 months against 9.0 months, with a P value of 0.087 [3].

Tumours did shrink more often, in 23 percent against 14 percent, so the drug was doing something. What went the other way was safety. Severe side effects occurred in 67 percent of the antibody group against 35 percent on chemotherapy, and fatal side effects in 7 percent against 2 percent. Sixteen of the 25 deaths from side effects in the antibody group were infections during periods of low white cell counts, most of them early in treatment in people who already carried several risk factors for exactly that. The authors write that these early complications may have affected the result [3].

What the three together support

That a surface protein is present on a tumour is not by itself a reason to expect benefit. The same antibody, the same chemotherapy parcel and the same target produced a large survival gap in one cancer, a smaller one in another, and none in a third. Where a treatment sits in the sequence of therapies, how sick the patients already are and how well the side effects can be managed all moved the answer [1][2][3].

The first trial was funded by Immunomedics, the company that developed the drug. The later breast cancer trial was funded by Gilead Sciences. Both papers state this, which is how it should be, and it is the reason the failed trial belongs in any honest summary alongside the two that succeeded [1][2].

What would settle it

Trials that test the treatment earlier in the course of disease, comparisons against the newer options rather than against older chemotherapy, and reporting that separates what the drug does from what happens when a person is already too frail to absorb its side effects. The five month survival gap in the first trial is real and was measured carefully. It describes one group of patients, in one cancer, at one point in their treatment [1][3].