Pembrolizumab for Early Triple-Negative Breast Cancer

Inclusion Criteria: * Age ≥18 years * Female biological sex * Newly diagnosed, previously untreated, histologically confirmed early triple-negative breast cancer * Plan to initiate preoperative anti-PD-1 immunotherapy using pembrolizumab * Overweight or obesity, defined as a body mass index \>=25 kg/m2 * Ability to provide written informed consent * Allow the collection and storage of biospecimens and data for future use Exclusion Criteria: * Active autoimmune disease * Concomitant conditions that require the use of antibiotics (e.g., chronic sinusitis) * Digestive disease disorders (e.g., irritable bowel syndrome, Crohn's disease) * Currently pregnant, breastfeeding, or planning to become pregnant within the next 24 weeks * Recent (within 90 days) use of glucocorticoids for more than 10 consecutive days * Any other cardiovascular, pulmonary, orthopedic, neurologic, psychiatric, or other condition that, in the investigator's opinion, would preclude participation or successful compliance with the protocol * Any other situation that, in the opinion of the investigator, would negatively impact subject safety or successful compliance with the protocol