- Prior exposure to any aromatase inhibitor (aminoglutethimide, formestane anastrozole, letrozole or exemestane) for more than 28 days. Patients that have already started on aromatase inhibitors (AIs) will be eligible for the protocol if they meet all other eligibility requirements and receive loading dose of trastuzumab not more than 28 days after starting AI therapy. Patients who initially received anastrozole or exemestane will be switched to letrozole.