Trastuzumab-emtansine (T-DM1/Kadcyla®; Genentech/Roche) is an antibody-drug conjugate (ADC) which incorporating the human epidermal growth factor receptor 2 (HER2)–targeted antitumor properties of trastuzumab, a humanized immunoglobulin G1 (IgG1) monoclonal antibody that targets the HER2 extracellular domain (ECD), with the cytotoxic activity of the microtubule polymerization inhibitor agent DM1. In this ADC, trastuzumab and DM1 are covalently bound via a stable thioether linker.
Approximately 25% of all breast cancers are classified as HER2+. In these cancers the HER2 protein, as measured by immunohistochemistry, is overexpressed. Activation of HER2 triggers an intracellular signaling cascade which results in cell proliferation, survival, invasion, and angiogenesis. HER2-positive breast cancers have an aggressive phenotype. With the advent of HER2-targeted therapy, the natural history of HER2-positive breast cancer has been dramatically improved 
Clinical trials have shown that T-DM1 significantly prolongs progression-free and overall survival with less toxicity than lapatinib (Tykerb®; GSK) plus capecitabine (Xeloda®; Genentech/Roche) in patients with HER2-positive advanced breast cancer previously treated with trastuzumab and a taxane.
Dose limiting toxicity
Thrombocytopenia was the dose-limiting toxicity in the phase I study with T-DM1. Researchers observed transient grade 4 thrombocytopenia with a T-DM1 regimen of 4.8 mg/kg q3w but not with T-DM1 3.6 mg/kg q3w. 
Furthermore, grade ≥3 thrombocytopenia occurred in up to 13% of patients receiving T-DM1 in phase III studies.
In a recent study, Hirdesh Uppal, Estelle Doudement, Kaushiki Mahapatra, and their colleagues at Genentech investigated the mechanism of T-DM1-induced thrombocytopenia. The results were published in ￼the November 4, 2014 OnlineFirst edition of Clinical Cancer Research. 
In this study, the effect of T-DM1 on platelet function was measured by aggregometry and by flow cytometry to detect markers of activation. The effect of T-DM1 on differentiation and maturation of megakaryocytes (MKs) from human hematopoietic stem cells was then assessed by flow cytometry and microscopy. Binding, uptake, and catabolism of T-DM1 in MKs, were assessed by various techniques including fluorescence microscopy, scintigraphy to detect T-[H3]-DM1 and 125I-T-DM1, and mass spectrometry.
The scientists assessed the role of FcγRIIa using blocking antibodies and mutant constructs of trastuzumab that do not bind FcγR.
T-DM1 had no direct effect on platelet activation and aggregation, but it did markedly inhibit MK differentiation via a cytotoxic effect. Inhibition occurred with DM1-containing antibody-drug conjugates, but not with trastuzumab demonstrating a role for DM1.
MKs internalized these antibody-drug conjugates in a HER2-independent, FcγRIIa-dependent manner, resulting in intracellular release of DM1. Binding and internalization of T-DM1 diminished as megakaryocytes matured.
The researchers noted that prolonged exposure of mature megakaryocytes to T-DM1 resulted in a disrupted cytoskeletal structure.
Based on these outcomes, the researchers concluded that their data confirms and supports the hypothesis that T-DM1-induced thrombocytopenia is mediated in large part by DM1-induced impairment of megakaryocytes differentiation, with a less pronounced effect on mature megakaryocytes.
The researchers further hypothesize that other antibody-drug conjugates with an IgG1 backbone conjugated to a tubulin inhibitor via a maleimidomethyl]cyclohexane-1-carboxylate (MCC), a nonreducible thioether linker may, potentially, also result in a clinical risk for dose-limiting thrombocytopenia.