Immunohistochemistry (IHC)

Patients with hormone-receptor-positive metastatic breast cancer that has stopped responding to endocrine therapy can be offered trastuzumab deruxtecan as their first chemotherapy-type treatment if the tumour shows any HER2 staining, rather than waiting until after conventional chemotherapy. Whether to use it before or after chemotherapy is now a choice, since overall survival was not shown to differ and the drug carries a risk of lung inflammation.

For hormone-receptor-positive metastatic breast cancer that has already had chemotherapy, Dato-DXd controls the disease for longer with fewer severe side effects than chemotherapy, but does not help patients live longer, so it is not a standard option here. The result is a reminder that progression-free survival is a surrogate; regulators and clinicians should wait for survival data before adopting an ADC in a setting where later therapies are effective.

Patients with newly diagnosed advanced stomach cancer should now have Claudin 18.2 tested alongside HER2, PD-L1 and mismatch repair, because roughly a third will be eligible for zolbetuximab, which adds about three months of median survival. The main practical problem is nausea and vomiting during infusions, which needs aggressive prophylaxis. How to sequence or combine it with immunotherapy in PD-L1-positive tumours is unresolved.

Patients whose breast cancer was previously called HER2-negative may now be eligible for an effective HER2-directed drug if their tumour has even low-level HER2 staining, so pathology reports must now distinguish HER2-low (1+ or 2+/ISH-negative) from HER2-zero. This applies to metastatic disease after at least one line of chemotherapy; it does not mean these patients benefit from trastuzumab or other older HER2 drugs.