Mechanisms of resistance to trastuzumab emtansine (T-DM1) in HER2-positive breast cancer

Abstract

The HER2-targeted antibody-drug conjugate trastuzumab emtansine (T-DM1) is approved for the treatment of metastatic, HER2-positive breast cancer after prior trastuzumab and taxane therapy, and has also demonstrated efficacy in the adjuvant setting in incomplete responders to neoadjuvant therapy. Despite its objective activity, intrinsic and acquired resistance to T-DM1 remains a major clinical challenge. T-DM1 mediates its activity in a number of ways, encompassing HER2 signalling blockade, Fc-mediated immune response and payload-mediated microtubule poisoning. Resistance mechanisms relating to each of these features have been demonstrated, and we outline the findings of these studies in this review. In our overview of the substantial literature on T-DM1 activity and resistance, we conclude that the T-DM1 resistance mechanisms most strongly supported by the experimental data relate to dysfunctional intracellular metabolism of the construct and subversion of DM1-mediated cell killing. Loss of dependence on signalling initiated by HER2-HER2 homodimers is not substantiated as a resistance mechanism by clinical or experimental studies, and the impact of EGFR expression and tumour immunological status requires further investigation. These findings are instructive with respect to strategies that might overcome T-DM1 resistance, including the use of second-generation anti-HER2 antibody-drug conjugates that deploy alternative linker-payload chemistries.

Conflict of interest statement

F.W.H. is an employee and shareholder of Johnson & Johnson, Inc., received honoraria from Threshold Pharmaceuticals, Inc., received research funding from Roche and received consulting fees from Merck KGaA. M.J.P.-G. received honoraria and consulted for Genentech/Roche. C.S. is named on patents on gene expression and methylation signatures, participated in advisory boards, and/or spoke at meetings, and/or was a recipient of travel support for participation in medical meetings for/from (in alphabetical order): Amgen, Astellas, AstraZeneca, Bayer, Celgene, Nanostring Technologies, Novartis, Pfizer, Puma Biotechnology, Roche, and Seattle Genetics. Other authors declare no conflict of interest.

Figures

Fig. 1. Structure of T-DM1.

T-DM1 is comprised of the monoclonal antibody trastuzumab conjugated via a non-cleavable MCC thioether linker to 3–3.6 moieties of the potent tubulin polymerisation inhibitor mertansine (DM1).

Fig. 2. Mechanisms of action of T-DM1.

T-DM1 exerts anti-tumour activity via at least three distinct mechanisms. As for trastuzumab, engagement of HER2 receptors by T-DM1 inhibits downstream signalling pathways (via RAS–mitogen-activated protein kinase [MAPK] and phosphatidylinositol 3-kinase [PI3K]–AKT–mammalian target of rapamycin [mTOR]) and ectodomain shedding while also eliciting immune effector cell function (e.g. antibody-dependent cellular cytotoxicity) mediated via Fc receptors. T-DM1–HER2 complexes are also internalised via receptor-mediated endocytosis, after which endocytic vesicles mature through the endosomal pathway for ultimate delivery to lysosomes. Trastuzumab is proteolytically degraded in lysosomes, liberating lysine-MCC-DM1 for active transport into the cytoplasm, where it inhibits tubulin polymerisation resulting in failure of the mitotic spindle and ultimate mitotic catastrophe.

Fig. 3. T-DM1 resistance arising from loss of trastuzumab-mediated activity.

A reduction in HER2 expression can impair T-DM1 binding and internalisation, preventing the intracellular release of DM1. Shedding of the extracellular domain of HER2 to generate the truncated form, p95HER2, also can prevent T-DM1 binding and DM1 intracellular release. Mutations in PIK3CA and loss of function of PTEN can lead to constitutive phosphatidylinositol 3-kinase (PI3K)–AKT–mammalian target of rapamycin (mTOR) signalling despite T-DM1-mediated inhibition of HER2. Heterodimerisation of HER2 with HER3 or epidermal growth factor receptor (EGFR) can induce PI3K and mitogen-activated protein kinase (MAPK) signalling in the presence of T-DM1. Finally, immunosuppression can limit the antibody-dependent cell-mediated cytotoxicity (ADCC) associated with T-DM1.

Fig. 4. T-DM1 resistance arising from dysfunctional intracellular trafficking and metabolism.

HER2–T-DM1 complex internalisation might be reduced by enhanced recycling of HER2–T-DM1 complexes back to the plasma membrane, thereby promoting the efflux of T-DM1. Altered expression of certain endocytic and cytoskeletal proteins could impair normal transit of HER2–T-DM1 complexes through the endosomal maturation pathway. Altered lysosomal pH regulation resulting in decreased acidity of lysosomal vesicles can reduce catabolism of HER2–T-DM1 to lysine-MCC-DM1 and prevent the release of the active compound. Reduced expression of lysosomal transporter proteins, such as SLC46A3, might also impair the release of lysine-MCC-DM1 into the cytoplasm.

Fig. 5. T-DM1 resistance arising from impairment of DM1-mediated cytotoxicity.

Increased expression of drug efflux transporters for which DM1 is a substrate might promote the efflux of lysine-MCC-DM1 from cells. Alternatively, cells might escape from DM1-mediated mitotic catastrophe through reduced induction of cyclin B1 or increased expression of polo-like kinase 1 (PLK1), allowing cells to complete mitosis and avoid apoptosis despite having an abnormal mitotic spindle.