A novel class of drugs, antibody-drug conjugates (ADCs), are now rapidly emerging as highly effective treatments for solid tumours. ADCs conjugate conventional chemotherapeutics with highly selective targeted monoclonal antibodies. Anti-HER2 therapies selectively target cancer cells expressing human epidermal growth factor receptor 2 (HER2), among them trastuzumab has been the first HER2-targeting monoclonal antibody to achieve successful results that made it the backbone of anti-HER2 therapies. Trastuzumab drug conjugates (T-DCs), use trastuzumab as a selective antibody to lead cytotoxic drugs inside cancer cells. Trastuzumab-emtansine (T-DM1) and trastuzumab-deruxtecan (T-Dxd) are the two approved T-DCs. T-Dxd along with other five T-DCs represents “second generation ADCs” that has been firstly tested in HER2 positive breast cancer (BC) and then in HER2-low BC and other cancers showing promising results thanks to extraordinary and innovative pharmacokinetic and pharmacodynamic characteristics. The evidence generated so far are establishing them as a completely new class of agents effective in solid cancer treatments but also warrants physicians against unconventional toxicity profiles. The role of T-DCs in HER2-positive BC has been largely reviewed, while in this review, we provided for the first time in literature an overview of trastuzumab drug conjugates (T-DCs) approved and/or in clinical development with a specific focus on their efficacy and safety profile in HER2-low BC and other solid tumours different from BC. We started by analysing T-DCs biological characteristics that underly the differences in T-DCs pharmacodynamics and safety profile, then presented the main evidence on the activity and efficacy of these emerging T-DCs in HER2-low BC and other HER2 overexpressing and/or mutated solid tumours and lastly, we provided an overview of the complex and still evolving scenario in which these compounds should be allocated. A specific focus on possible combination strategies with other drugs such as immunotherapy, chemotherapy and target therapy, to increase T-DCs activity and eventually overcome future upcoming resistance mechanisms, are here also critically reviewed.
