KATHERINE

Design, population and eligibility

Treatment and comparator

T-DM1 3.6 mg/kg IV every 3 weeks for 14 cycles versus trastuzumab 6 mg/kg IV every 3 weeks (with standard loading dose) for 14 cycles.

Primary and secondary endpoints

Primary endpoint: invasive disease-free survival (iDFS). Key secondary endpoint: overall survival; additional endpoints included distant recurrence-free survival and safety.

Key efficacy results

At median follow-up 8.4 years, iDFS HR 0.54 (95% CI 0.44–0.66). Seven-year iDFS was 80.8% with T-DM1 versus 67.1% with trastuzumab (absolute difference 13.7 percentage points). OS HR 0.66 (95% CI 0.51–0.87; P=0.003); 7-year OS 89.1% versus 84.4% (difference 4.7 points). Seven-year distant recurrence-free survival was 84.5% versus 76.2% (HR 0.60; 95% CI 0.47–0.76).

Source-reported outcome visualization

Seven-year iDFS and OS.

Kaplan–Meier landmark rates at 7 years; NNTs below are calculated from these fixed-time absolute differences, not from median survival.

Absolute effect

At 7 years, iDFS improved by 13.7 percentage points (80.8% vs 67.1%); approximate NNT 8 to prevent one iDFS event by 7 years. OS improved by 4.7 points (89.1% vs 84.4%); approximate NNT 22 for one additional survivor at 7 years. These are landmark Kaplan–Meier estimates.

Trial-specific toxicity and treatment burden

Grade ≥3 adverse events: 26.1% with T-DM1 versus 15.7% with trastuzumab. Permanent discontinuation due to adverse reactions: 18% versus 2.1%. Important T-DM1 toxicities include thrombocytopenia, elevated transaminases/bilirubin, peripheral neuropathy, fatigue and nausea; grade ≥3 thrombocytopenia was a notable event. Monitor platelets, liver tests, neuropathy and cardiac function.

Quality of life and patient-reported outcomes

Patient-reported outcomes showed a modest early deterioration in some symptoms with T-DM1, with overall health-related QoL generally maintained. The additional treatment burden is 14 cycles and greater toxicity-related discontinuation.

Subgroups and applicability

Benefit was broadly consistent across major clinical subgroups, including hormone-receptor status and residual disease characteristics. Activity was observed even in patients with HER2-negative residual tumour on retesting when the original tumour was HER2-positive; do not reclassify eligibility solely from residual specimen.

Current-practice context · reviewed 3 November 2026

Adjuvant T-DM1 for 14 cycles is standard for HER2-positive early breast cancer with residual invasive disease after appropriate neoadjuvant HER2-directed therapy. This is post-neoadjuvant escalation, not a trial of T-DM1 for patients with pCR.

Limitations and critical interpretation

Open-label trial; the OS analysis was a second interim analysis at the final iDFS follow-up. Toxicity and discontinuation are higher with T-DM1. Results do not apply to patients with pCR or HER2-negative primary disease.

Bottom line: KATHERINE is practice-defining: T-DM1 improves long-term iDFS and OS in residual HER2-positive disease after neoadjuvant therapy, with a clinically important absolute benefit and a meaningful increase in grade ≥3 toxicity and discontinuation.

Primary sources and follow-up publications

1. Final iDFS and updated OS, NEJM 2025
2. Original primary report, NEJM 2019
3. Patient-reported outcomes
4. ClinicalTrials.gov

Outcome values are sourced from the linked reports. Unreported values are not imputed. NNT/NNH is only stated when an explicit fixed-time absolute difference supports a defensible calculation; no NNT is derived from median survival.