GDF-15 and Its Relationship With Treatment-related ADverse Events in Breast Cancer

GRADE is trying to find out if there is a link between a hormone called GDF-15 and the side effects that people can experience when taking T-DXd. GDF-15 can be measured in the blood. GDF-15 levels in the blood will go up when the body is stressed under certain conditions, including breast cancer. There is a link between high GDF-15 levels and the nausea and vomiting experienced with "morning sickness" in pregnancy. It has also been shown that GDF-15 levels will go up with the use of other types of chemotherapy that are known to cause nausea and vomiting. Side effects such as feeling sick (nausea), vomiting and weight loss are common with T-DXd. Sometimes, these can be so severe that treatment needs to be stopped early. The investigators can't predict who will get bad side effects and who will not. If the investigators can find out if there is a link between GDF-15 and the side effects of T-DXd, they can use this information in future clinical trials.

Inclusion Criteria: 1. Participants aged ≥18 years. 2. Histologically confirmed diagnosis of metastatic/advanced unresectable HER2-positive or HER2-low breast cancer. 3. Planned to start treatment with T-DXd. 4. Life expectancy of at least 4 months. Exclusion Criteria: 1. Current active reversible causes of decreased food intake, as determined by the Investigator. 2. Receiving tube feedings or any kind of parenteral nutrition at the time of enrolment into the study. 3. Ongoing cachexia attributable to other reasons unrelated to cancer or cancer treatment as determined by the Investigator that may confound interpretation of weight loss due to T-DXd. 4. Current adherence to a calorie-restricted diet with the intention of weight loss.