The overall aim of this study is to identify and characterize anti-NaV1.5 autoantibodies in patients with metastatic breast and colorectal cancer. These tumors are characterized by the presence of a specific target structure (called nNaV1.5) against which antibodies are produced. These antibodies may cross-react with a similar structure (called NaV1.5) that is found in the heart. This could affect channel function and increase the risk of arrhythmias.

Inclusion Criteria:

• Age ≥ 18 years
• Patients with a confirmed diagnosis of breast or colorectal cancer, either metastatic or non-metastatic
• Patients already receiving first-line therapy with a confirmed diagnosis of breast or colorectal cancer, either metastatic or non-metastatic
• Signed informed consent

Exclusion Criteria:

• Clinically significant cardiovascular diseases (atrial fibrillation, congestive heart failure, cardiomyopathies, or inherited arrhythmic syndromes)
• Known autoimmune diseases or immunodeficiencies
• Use of high-dose immunosuppressive drugs or other experimental treatments that could interfere with the assessment of study endpoints

De-identified individual participant data including demographic characteristics, clinical variables relevant to the study, and laboratory results related to the presence or absence of anti-SCN5A autoantibodies at baseline, 6-month, and 12-month follow-up assessments.