and low HER2 expression in the randomized phase 3 DESTINY-Breast04 study, according to 32-month follow-up data.
The overall safety profile of the HER2-directed antibody drug conjugate was also comparable to that observed at the primary analysis in 2022, and longer exposure did not appear to increase toxicity, Shanu Modi, MD, reported on behalf of the DESTINY-Breast04 investigators at the European Society of Medical Oncology (ESMO) Congress 2023.
“These results continue to support the use of T-DXd as the new standard of care after one line of chemotherapy in patients with HER2-low metastatic breast cancer,” said Dr. Modi, a breast oncologist and attending physician at Memorial Sloan Kettering Cancer Center, New York.
DESTINY-Breast04 enrolled 557 patients 2:1 to receive 5.4 mg/kg of T-DXd every 3 weeks or physicians’ choice of capecitabine, eribulin, gemcitabine, paclitaxel, or nab-paclitaxel, and established HER2-low mBC as “a new targetable patient population with T-DXd as a new standard of care,” she explained.
Median overall survival (mOS) with a median of 18.4 months of follow-up at the Jan. 11, 2022, primary data cut-off was 23.4 months in the T-DXd arm versus 16.8 months in the TPC arm and 23.9 versus 17.5 months, respectively, in the hormone receptor–positive (HR+) cohort (hazard ratio, 0.64 for both groups). At the preplanned extended follow-up with data cut-off on March 1, 2023, the mOS was 22.9 versus 16.8 months for T-DXd versus TPC, and 23.9 versus 17.6 months for the HR+ cohort, respectively (HR, 0.69 for both).
Median progression-free survival (PFS) by investigator assessment was 8.8 versus 4.2 months for the full cohort, and 9.6 versus 4.2 months for the HR+ cohort (HR, 0.36 and 0.37, respectively). PFS was consistent with the results from the primary analysis.
The benefits in the HR+ patients were consistent across all patient subgroups, Dr. Modi noted.
“I do think it’s interesting to point out that at the landmark 2-year point, all patients on standard chemotherapy discontinued study treatment, whereas 15% on T-DXd remain [on treatment] without any evidence of disease progression, Dr. Modi added
An exploratory analysis in the hormone receptor–negative (HR–) cohort showed mOS of 18.2 versus 8.3 months at the primary analysis (HR, 0.48), and a “clinically meaningful and numerical advantage for T-DXd” persisted at the planned follow-up (mOS, 17.1 vs. 8.3; HR, 0.58), she said.
PFS in the HR- cohort was 8.5 versus 2.9 months at the primary analysis, and 6.3 versus 2.9 months at the update (HR, 0.46 and 0.29, respectively).
An assessment of post-study therapies received by patients showed that those therapies did not account for the significant survival advantage conferred by T-DXd, Dr. Modi said.
She noted, however, that while no new safety signals were observed at follow-up, lung toxicity remains a “toxicity of special interest,” having occurred in 12.1% of cases at the time of the primary analysis.
Most cases were grade 1 or 2, and no new cases were observed at follow-up, but one patient with lung toxicity and an initial grade 3 event experienced clinical deterioration and later died from lung toxicity, which underscores the importance of remaining vigilant and intervening promptly in all cases of lung toxicity, Dr. Modi stressed.
Invited discussant Giampaolo Bianchini, MD, reiterated that T-DXd is an effective treatment option and said, “we must accurately identify patients and avoid improperly denying this important therapeutic option.”
Although HER2-low disease is not a unique biological disease entity, it is a “practical and pragmatic definition used to select patients with ‘some degree’ of HER2 protein expression adopting a test and a scoring system already implemented in the routine clinical practice,” said Dr. Bianchini, head of the breast cancer group and head of clinical translational and immunotherapy research at IRCCS Ospedale, San Raffaele, Milan.
However, the current definition may be inadequate, he said, explaining that the ongoing DESTINY-Breast06 study “will challenge the current definition of what we consider HER2-low definition,” potentially extending the T-DXd indication to HER2 ultra-low.
Furthermore, current HER2 testing was designed to discriminate cases with high abundant protein – not for the low HER2 dynamic range, which leads to technical inaccuracy.
Given these considerations, he suggested considering a new biopsy, if feasible, in patients with an immunohistochemistry (IHC) score of 0 in all tumor biopsies, and having a revision performed by the pathologist.
In patients with an IHC score of 1 or greater only in one biopsy, there is no need to confirm the HER2-low status, he said.
DESTINY-Breast04 is funded by Daiichi Sankyo Inc. and AstraZeneca. Dr. Modi reported relationships with Daiichi Sankyo, Genentech, AstraZeneca, Seagen, and MacroGenics. Dr. Bianchini reported relationships with AstraZeneca, Daiichi Sankyo, Gilead, MSD, Seagen, Roche, Sanofi, Lilly, EISAI, Novartis, Pfizer, Stemline, Exact Science, and Agendia.