Who this is for
This page is for people with HER2-positive metastatic breast cancer who have previously received trastuzumab and a taxane, and are being told that treatment options have changed in recent years.
If you have been told that a newer drug has “replaced the old standard,” it can feel confusing. Many patients wonder why something that worked for years is suddenly no longer first choice. The reason is not fashion or marketing. It is data — and the data in this case was unusually strong.
What changed in HER2-positive metastatic breast cancer
For many years, T-DM1 (trastuzumab emtansine) was the standard second-line treatment for HER2-positive metastatic breast cancer.
It was effective.
It was widely accepted.
It was supported by strong evidence.
Then DESTINY-Breast03 was published.
This was a large, international, phase III clinical trial comparing trastuzumab deruxtecan (T-DXd) against T-DM1 in patients previously treated with trastuzumab and a taxane.
The results were decisive.
Key results from DESTINY-Breast03
• Progression-free survival hazard ratio: 0.28
• 12-month progression-free survival: approximately 75% with T-DXd vs 34% with T-DM1
• Overall survival hazard ratio (updated analysis): approximately 0.64
What this means
A hazard ratio of 0.28 for progression-free survival represents a 72% relative reduction in the risk of disease progression compared with the prior standard.
This magnitude of benefit is uncommon in metastatic breast cancer trials.
The separation of the survival curves was early and sustained.
That is why practice changed quickly. It was an important moment in the history of oncology.
Why this drug is different
Trastuzumab deruxtecan is an antibody-drug conjugate.
It combines:
• A HER2-targeted antibody
• A potent chemotherapy payload
• A linker that allows targeted delivery into cancer cells
Importantly, it carries a higher drug-to-antibody ratio than earlier agents, allowing more chemotherapy molecules to be delivered directly to HER2-expressing cells.
This mechanism likely explains the improved outcomes observed in the trial.
Side effects and important considerations
Like all effective cancer treatments, T-DXd has risks.
The most important safety consideration is interstitial lung disease (ILD), which was observed in a minority of patients and requires careful monitoring.
Other side effects include nausea, fatigue and bone marrow suppression.
In experienced centres in Melbourne and across Australia, structured monitoring protocols are used to identify and manage these risks early.
The key point is that improved efficacy must always be balanced with careful safety oversight.
Why this matters in Melbourne and Australia
Following DESTINY-Breast03, trastuzumab deruxtecan became the preferred second-line therapy in HER2-positive metastatic breast cancer internationally.
In Australia, access pathways and PBS listing evolved following regulatory approval.
This reflects how rapidly high-quality data can shift clinical standards.
When results are this clear, practice adapts.
International patients, including Sri Lanka
Patients from Sri Lanka and other countries sometimes seek clarification regarding access to trastuzumab deruxtecan and how it compares with older HER2-directed therapies.
While access may differ depending on healthcare systems, the clinical data underpinning its use is global and robust.
A structured review of prior HER2 therapies and sequencing remains essential.
Why this trial stands out
Oncology rarely sees hazard ratios as strong as 0.28 in the metastatic setting.
That is why DESTINY-Breast03 is widely regarded as practice-changing.
It did not introduce incremental improvement.
It redefined the standard.
This is also an example of how antibody-drug conjugates are reshaping cancer therapy more broadly.
Key Takeaway
• DESTINY-Breast03 showed a 72% relative reduction in progression risk compared with the prior standard (hazard ratio 0.28).
• Trastuzumab deruxtecan rapidly became the preferred second-line therapy for HER2-positive metastatic breast cancer and on track to be the first line agent.
• Treatment decisions should balance strong efficacy data with careful safety monitoring.