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A 45 year-old woman complaining of breast pain was diagnosed with having HER2-positive breast cancer. The patient underwent lumpectomy followed by a period of chemotherapy including adjuvant treatment. Over the course of their treatment, the patient developed a urinary tract infection that required antimicrobial therapy, which initially consisted of a trimethoprim/sulfametoxazole combination drug. However, the patient during the course of her therapy suffered multiple recurrent infections and was, consequently, enrolled in a clinical trial to test the efficacy of a new mannoside anti-infective agent.
Question: Discuss the mechanisms underpinning current therapeutic strategies for treating HER2-positive breast cancer. In your discussion, you should include details about managing therapeutic resistance and adverse side effects such as cardiotoxicity.