Les cancers colorectaux et les cancers du sein sont deux des affections malignes les plus fréquentes dans les pays industrialisés. Lorsqu’elles sont diagnostiquées à un stade précoce, ces tumeurs sont traitées efficacement par la chirurgie associée ou non à la radiothérapie. Pour des tumeurs localisées ayant certains facteurs de pronostic péjoratifs, pour les tumeurs localement avancées, ou pour les tumeurs métastasiques, une chimiothérapie est instaurée. Cependant, la chimiothérapie à elle seule ne permet pas de guérir les patients aux stades avancés de ces 2 types de cancers. C’est pourquoi d’autres alternatives comme l’immunothérapie ou des traitement plus ciblés sont en cours d’étude ou viennent récemment d’être validés.Notre travail a porté sur deux modèles d’immunothérapie dans deux formes de cancers . 1/Dans un modèle expérimental chez le rat, un traitement par un analogue de lipide A permet de guérir tous les animaux porteurs de carcinomatose péritonéale macroscopique d’origine colique. 2/ Chez la femme porteuse de carcinome mammaire localement avancé et surexprimant l’oncoprotéine HER2, un traitement préopératoire comportant du trastuzumab permet d’obtenir la disparition complète des tumeurs et des métastases axillaires chez la moitié des patientes. A partir de prélèvements tissulaires issus de ces 2 modèles, nous avons pu montrer que des analyses histologiques et surtout immunohistochimiques pouvaient permettre d’appréhender les mécanismes de régression tumorale.Dans les régressions des carcinomatoses chez le rat, le rôle du monoxyde d’azote, de l’apoptose des cellules tumorales et de l’infiltration des tumeurs par les cellules dendritiques puis des macrophages a été proposé. Dans la régression des tumeurs du sein, le rôle des cellules NK, via un mécanisme d’ADCC a été suggéré. Ces travaux ont permis de montrer que, malgré ses limites, et en particuliers l’impossibilité de démonstration mécanistique, l’immunohistochimie peut permettre de proposer des hypothèses intéressantes, qui doivent être secondairement confirmées par des expérimentations complémentaires. / In the developed countries, colorectal and breast cancers are two of the most common malignancies. When the diagnosis is made at the beginning of the disease, surgery, associated or not with radiotherapy may cure the patients. For tumors that present some pejorative prognostic factors,for locally advanced tumors or for metastasic disease, chemotherapy has to be prescribed. However, in these 2 types of cancer, chemotherapy is not able to cure patients suffering from metastasic stage and alternative treatments as immunotherapy or targeted therapies are studied or are still validated.Our work was based on 2 different models of immunotherapy in 2 types of cancer. In an experimental model of carcinomatoses of colorectal cancer in rats, Lipid A injections are able to cure all the rats, even at a macroscopic stage and in half of the women affected by locally advance HER2-overexpressing breast cancer, trastuzumab based preoperative treatment are able to obtain the total disappearance of the tumor. When we look on tumor samples obtained in these two models, we can show that histology and even more immunohistochemistry are able to propose some mechanisms of regression of the tumors. In the rat model, we can show the role of nitric oxide, apotosis, dendritic cells and macrophages and in the breast cancer regression, we show the role of NK cells and ADCC. Theses works show that immunohistochemistry, even if insufficient, may propose some interesting hypotheses that have to be confirm by other experimentations.

Clinical immunohistochemistry (IHC) techniques are not yet fully standardized. In this project, a standardization method was developed and tested for proficiency testing (PT) in external quality assurance (EQA) and quality control (QC) in clinical IHC laboratories. The breast cancer markers estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor 2 (HER2) were used as a model system. Digital image analysis (IA) was used in conjunction with new calibrated and standardized cell line microarrays (CLMA). CLMAs built from nine formalin-fixed paraffin-embedded (FFPE) breast cancer cell lines were used for both QC controls and PT samples, instead of traditionally used FFPE tissues, in the standardization of breast cancer IHC. IA was used for measurement of IHC results, and compared to evaluation by the traditional expert-assessment method. Laboratory Score: Reference Score Ratio (LSRSR) was derived from Histo-Scores (HScores) determined by IA. HScores and LSRSRs were examined statistically and evaluated as histograms and boxplots to summarize and rank participant laboratory EQA results, in comparison to a reference sample or reference laboratories in two consecutive Canada-wide EQA runs. LSRSR-derived reference ranges were highly sensitive in evaluating laboratory EQA performance in PT as well as for monitoring of controls for QC. Laboratory on-slide tissue and cell-line IHC QA controls were assessed using IA and Levey Jennings QC charts. These charts were determined to be an excellent way to observe trending in laboratory IHC staining over time, particularly when cell line controls were used. This approach also reduced the time and labor costs for PT evaluation. Overall, cell line calibration controls were functionally equivalent or better than tissue-based controls in QC and PT mainly because of cell line biological homogeneity and sample availability. This study identified an optimal design for preparation of IHC cell line controls and PT samples for breast cancer markers. Optimal, intermediate staining cell line IHC controls were identified for all three breast cancer markers. Using IA with LSRSR and cell line samples is recommended for standardization of IHC methodology. This approach advances QA for diagnostic IHC and when implemented will improve patient care

The functions of the immune system include immunosurveillance of transformed cells, i.e., the ability to eliminate these cells before they become harmful to the organism. If the transformed cells succeed to escape the immune system surveillance, an oncological disease develops. The tumour immunotherapy aims to stimulate the immune system mechanisms to fight against the tumour. Lately, there's an interest in using NK cells in the immunotherapy of tumours. These cells appertain to the innate immune system and participate in immunosurveillance. When an NK cell encounters a target cell, its activation depends on the integration of signals from the surface activating and inhibiting receptors which bind ligands on the surface of the target cell. Upon activation, NK cell exhibits a cytotoxic response against the target cell. The use of NK cells in immunotherapy includes, among others, the testing of bispecific fusion proteins which can bind a tumour surface antigen by one part and NK cell activating receptor by the other part. Thus, these fusion proteins mediate a contact between both cells and trigger the cytotoxic response. This work presents a preparation of bispecific fusion proteins which consist of an activating ligand MICA (for the receptor NKG2D) or B7H6 (for the receptor NKp30), and a nanobody...

Les rétinoïdes sont utilisés dans le traitement d’une variété de tumeurs malignes et lésions précancéreuses. Leurs effets dans des lignées cellulaires dérivées de tumeurs solides tel que le cancer du sein ont été étudiés extensivement. Cependant, les bénéfices dans le cancer du sein restent à date peu clairs. Ceci est probablement du à l’hétérogénéité des tumeurs mammaires et la réponse très variable aux effets antiprolifératifs de l’acide rétinoïque. Dans les lignées cellulaires cancéreuses mammaires, la réponse l’AR est fortement corrélée au niveau d’expression du récepteur aux estrogènes alpha (ERα), qui régule l’expression du gène qui encode le récepteur à l’acide rétinoïque alpha, RARA. Malgré cela, certaines lignées cellulaires ER-négatives, comme la lignée HER2-positive SK-BR-3, ont été décrites comme étant sensibles à l’AR. Dans le Chapter 2: de cette thèse, nous avons étudié les mécanismes de la signalisation ER-dépendante et ER-indépendante dans les cellules cancéreuses mammaires. Nous avons utilisé des lignées ER-négatives et ER-positives pour démontrer qu’une partie de la réponse à l’AR est indépendante de la signalisation par ER. Nous avons identifié plusieurs gènes cibles primaires de l’AR qui ont des effets similaires à l’AR quand ils sont surexprimés dans des cellules mammaires cancéreuses. Cette étude apporte une meilleure compréhension des mécanismes complexes qui mènent à l’arrêt de croissance induit par l’AR dans les cellules cancéreuses mammaires. Dans le Chapitre 3, nous avons regardé plus en détails la signalisation ER-indépendante par l’AR dans des cellules ayant une amplification des gènes HER2 et RARA et nous avons identifié une synergie entre l’AR et le Herceptin dans ces cellules. Nous proposons que les gènes FOXO jouent une rôle dans cette synergie. Les cellules SK BR 3, ayant une coamplification HER2/RARA, pourraient représenter une classe de tumeurs qui pourraient bénéficier d’un traitement avec des rétinoïdes, en augmentent la réponse au Herceptin et potentiellement en réduisant la résistance au Herceptin. En conclusion, les données présentées dans cette thèse aident à mieux comprendre les mécanismes menant à l’arrêt de croissance induit par l’AR dans les cellules cancéreuses mammaires et fournissent une application potentielle pour l’utilisation de l’AR dans le traitement du cancer du sein. / Retinoids are being used in the treatment of several malignancies and precancerous lesions. Their effects on cell lines derived from solid tumors, such as breast cancer, have also been described extensively. Their benefit in breast cancer, however, remains unclear. This might be because of the high levels of heterogeneity of breast tumors and the very variable response to the antiproliferative effects of retinoic acid. In mammary tumor cell lines, the response to retinoic acid is highly correlated with the expression of the estrogen receptor alpha (ERα), which regulates the expression of the retinoic acid receptor alpha gene RARA. However, some ER-negative cell lines, such as the HER2 positive SK-BR-3 cell line, have been reported to be RA-sensitive. In Chapter 2: of this thesis we have investigated the mechanisms of ER-dependent and ER-independent RA signaling in breast cancer cells. Using ER-positive and ER-negative cell lines, we show that part of the response to RA is independent of ER signaling. Several direct retinoic acid targets were identified that could mimic antiproliferative effects of retinoic acid when overexpressed in breast cancer cells. This study has provided better insight in the complex mechanisms that lead to RA-induced growth arrest in breast cancer cells. In Chapter 3: we looked further into the ER-independent RA signaling in HER2/RARA-amplified cells and identified a synergy between RA and Herceptin in these cells. We propose a role for FOXOs in mediating this synergy. HER2/RARA coamplified breast tumors might represent a subclass of tumors that could benefit from retinoid treatment, both increase antitumor effects of Herceptin, as well as in potentially reducing Herceptin resistance. In conclusion, data presented in this thesis give better insight in the mechanisms of RA induced growth arrest in breast cancer cells and provide a potential application of retinoids in a subset of breast tumors.

In the era of personalized cancer treatment, adoptive T cell therapy (ACT) shows promise for the treatment of solid cancers. However, partial or mixed responses remain common clinical outcomes due to the heterogeneity of tumours. Indeed, in many patients it is typical to see a response to ACT in one tumour nodule, while others show little or no response. Thus, defining the tumour features that distinguish those that respond to ACT from those that do not would be a significant advance, allowing clinicians to identify patients that might benefit from this treatment approach. The first chapter of this thesis provides the necessary background to understand the principals behind and components of ACT. This chapter also offers selected historical advances contributing to the current state of the field. The second chapter introduces a novel murine model of breast cancer developed to investigate the tumour-specific mechanisms associated with immune evasion in an ACT setting. The third chapter describes the in vivo characterization of mammary tumour cell lines derived from our mouse model that reliably showed complete, partial or no response to ACT. Using these cell lines, we were able to characterize in vivo tumour-specific differences in cytotoxic T cell trafficking, infiltration, activation, and proliferation associated with response to ACT. In the fourth chapter, we used bioinformatics approaches to develop a preliminary predictive gene signature associated with response to ACT in our mammary tumour model. We used this signature to predict outcome and then test a number of murine mammary tumours in vivo, with promising results, wherein 50% of tumours responded to ACT as predicted based upon gene expression. Thus, using an innovative model for breast cancer, these results suggest that there are tumour-specific features that can be used a priori to predict how a tumour will respond to adoptive T cell therapy. Importantly, these findings might facilitate the design of immunotherapy trials for human breast cancer. / Graduate

The paucity of well validated preclinical models is frequently cited as a contributing factor to the high attrition rates seen in clinical oncological trials. There remains a critical need to develop models which are accurately able to recapitulate the features of human disease. The aims of this study were to use genetically engineered mouse models (GEMMs) to explore the efficacy of novel treatment strategies in HER2 positive breast cancer and to further develop the model to facilitate the study of mechanisms underpinning drug resistance. Using the BLG--HER2KI-PTEN+/- model, we demonstrated that Src plays an important role in the early stages of tumour development. Chemopreventative treatment with dasatinib delayed tumour inititation (p= 0.046, Wilcoxon signed rank test) and prolonged overall survival (OS) (p=0.06, Wilcoxon signed rank test). Dasatinib treatment also induced squamous metaplasia in 66% of drug treated tumours. We used 2 cell lines derived from this model to further explore dasatinib’s mechanism of action and demonstrated reduced proliferation, migration and invasion following in vitro treatment. Due to the prolonged tumour latency and the low metastatic rate seen in this model, further studies were undertaken with the MMTV-NIC model. This model also allowed us to study the impact of PTEN loss on therapeutic response. We validated this model by treating a cohort of MMTV-NIC PTEN+/- mice with paclitaxel and demonstrated prolonged OS (p=0.035, Gehan Breslow Wilcoxon test). AZD8931 is an equipotent signalling inhibitor of HER2, HER3 and EGFR. We observed heterogeneity in tumour response but overall AZD8931 treatment prolonged OS in both MMTV-NIC PTEN FL/+ and MMTV-NIC PTEN+/- models. PTEN loss was associated with reduced sensitivity to AZD8931 and failure to suppress Src activity, suggesting these may be suitable predictive biomarkers of AZD8931 response. To facilitate further studies exploring resistance, we transplanted MMTV-NIC PTEN+/- fragments into syngeneic mice and generated 3 tumours with acquired resistance to AZD8931. These tumours displayed differing resistance strategies; 1 tumour continued to express HER2 whilst the remaining 2 underwent EMT and lost HER2 expression reflecting to a very limited degree some of the heterogeneity of resistance strategies seen in human disease. To further explore resistance to HER2 targeting tyrosine kinase inhibitors, we generated a panel of human cell lines with acquired resistance to AZD8931 and lapatinib. Western blotting demonstrated loss of HER2, HER3 and PTEN in all resistant lines. Acquisition of resistance was associated with a marked change in phenotype and western blotting confirmed all lines had undergone EMT. We used a combination of RPPA and mass spectrometry to further characterise the AZD8931 resistant lines and identified multiple potential novel proteins involved in the resistant phenotype, including several implicated in EMT. In conclusion, when coupled with appropriate in vitro techniques, the MMTV-NIC model is a valuable tool for selection of emerging drugs to carry forward into clinical trials of HER2 positive breast cancer.

ABD-Derived Affinity Proteins (ADAPTs) is a novel class of small engineered scaffold proteins based on albumin-binding domain (ABD) of streptococcal protein G. High affinity ADAPT  binders against various therapeutic targets can be selected.  In this thesis, we report a development of ADAPT-based radionuclide imaging agents providing high sensitivity and specificity of molecular imaging of HER2 expression in disseminated cancers. We investigated the feasibility of the use of ADAPTs as imaging agents and influence of molecular design and radiolabeling chemistry on in vivo targeting and biodistribution properties of the tracers. In Paper I we demonstrated the feasibility of the use of anti-HER2 ADAPT6 molecule as a high contrast imaging agent; In Paper II we evaluated the influence of composition of histidine-containing tag on in vivo biodistribution of ADAPT-based tracers labeled with 99mTc using 99mTc(CO)3 binding to histidine-containing tags and 111In using DOTA chelator at N-terminus; In Paper III we evaluated the influence of different aspects of N-terminus leading sequence on targeting including effect of sequence size on clearance rate and effect of the composition of the sequence on biodistribution profile; In Paper IV, we evaluated the influence of residualizing properties and positioning of the label on biodistribution and targeting; and In Paper V, we compared tumor-targeting properties of the ADAPT6 labeled at C-terminus with 99mTc using N3S chelator and 111In using DOTA chelator. In conclusion, ADAPTs constitute a very promising class of targeting probes for molecular imaging providing high contrast. Molecular design of the ADAPT proteins and chelators/linkers for labeling has an appreciable effect on their imaging properties.

HER2overexpression is associated with poor breast cancer prognosis and increased risk of metastasis. Current HER2targeted therapies include monoclonal antibody based strategies which work by reducing HER2 levels at the cell surface (trastuzumab), by preventing HER2 dimerization (pertuzumab), or via targeted delivery of a cytotoxic payload (trastuzumab emtansine). Although these therapies are successful in some cases, acquired and inherent resistance to these therapeutics remain a treatment hurdle. Oncolytic viruses (OVs) specifically target and lyse cancer cells while leaving normal cells unharmed. One such OV, VSVΔ51, replicates in interferon (IFN) defective cells, a characteristic of approximately 70% of tumours. We hypothesized that the combination of HER2 targeting therapies with VSVΔ51 could improve therapeutic efficacy. We found that HER2 overexpression was associated with increased virus sensitivity and that modulation of HER2 signaling through a subset of activating ligands and inhibitory drugs could influence infection. We further established that the HER2 monoclonal antibodies trastuzumab and pertuzumab mediate an anti-viral effect on VSVΔ51 spread. Finally, we demonstrate that conjugation of a microtubule targeting agent to trastuzumab can overcome the induced anti-viral state and enhance VSVΔ51 spread specifically in cancer cells. Overall, this work highlights the importance of HER2 signaling and activation on VSVΔ51 spread and shows that conjugation of microtubule destabilizing agents to monoclonal antibodies can enhance VSVΔ51 efficacy.

Les rétinoïdes sont utilisés dans le traitement d’une variété de tumeurs malignes et lésions précancéreuses. Leurs effets dans des lignées cellulaires dérivées de tumeurs solides tel que le cancer du sein ont été étudiés extensivement. Cependant, les bénéfices dans le cancer du sein restent à date peu clairs. Ceci est probablement du à l’hétérogénéité des tumeurs mammaires et la réponse très variable aux effets antiprolifératifs de l’acide rétinoïque. Dans les lignées cellulaires cancéreuses mammaires, la réponse l’AR est fortement corrélée au niveau d’expression du récepteur aux estrogènes alpha (ERα), qui régule l’expression du gène qui encode le récepteur à l’acide rétinoïque alpha, RARA. Malgré cela, certaines lignées cellulaires ER-négatives, comme la lignée HER2-positive SK-BR-3, ont été décrites comme étant sensibles à l’AR. Dans le Chapter 2: de cette thèse, nous avons étudié les mécanismes de la signalisation ER-dépendante et ER-indépendante dans les cellules cancéreuses mammaires. Nous avons utilisé des lignées ER-négatives et ER-positives pour démontrer qu’une partie de la réponse à l’AR est indépendante de la signalisation par ER. Nous avons identifié plusieurs gènes cibles primaires de l’AR qui ont des effets similaires à l’AR quand ils sont surexprimés dans des cellules mammaires cancéreuses. Cette étude apporte une meilleure compréhension des mécanismes complexes qui mènent à l’arrêt de croissance induit par l’AR dans les cellules cancéreuses mammaires. Dans le Chapitre 3, nous avons regardé plus en détails la signalisation ER-indépendante par l’AR dans des cellules ayant une amplification des gènes HER2 et RARA et nous avons identifié une synergie entre l’AR et le Herceptin dans ces cellules. Nous proposons que les gènes FOXO jouent une rôle dans cette synergie. Les cellules SK BR 3, ayant une coamplification HER2/RARA, pourraient représenter une classe de tumeurs qui pourraient bénéficier d’un traitement avec des rétinoïdes, en augmentent la réponse au Herceptin et potentiellement en réduisant la résistance au Herceptin. En conclusion, les données présentées dans cette thèse aident à mieux comprendre les mécanismes menant à l’arrêt de croissance induit par l’AR dans les cellules cancéreuses mammaires et fournissent une application potentielle pour l’utilisation de l’AR dans le traitement du cancer du sein. / Retinoids are being used in the treatment of several malignancies and precancerous lesions. Their effects on cell lines derived from solid tumors, such as breast cancer, have also been described extensively. Their benefit in breast cancer, however, remains unclear. This might be because of the high levels of heterogeneity of breast tumors and the very variable response to the antiproliferative effects of retinoic acid. In mammary tumor cell lines, the response to retinoic acid is highly correlated with the expression of the estrogen receptor alpha (ERα), which regulates the expression of the retinoic acid receptor alpha gene RARA. However, some ER-negative cell lines, such as the HER2 positive SK-BR-3 cell line, have been reported to be RA-sensitive. In Chapter 2: of this thesis we have investigated the mechanisms of ER-dependent and ER-independent RA signaling in breast cancer cells. Using ER-positive and ER-negative cell lines, we show that part of the response to RA is independent of ER signaling. Several direct retinoic acid targets were identified that could mimic antiproliferative effects of retinoic acid when overexpressed in breast cancer cells. This study has provided better insight in the complex mechanisms that lead to RA-induced growth arrest in breast cancer cells. In Chapter 3: we looked further into the ER-independent RA signaling in HER2/RARA-amplified cells and identified a synergy between RA and Herceptin in these cells. We propose a role for FOXOs in mediating this synergy. HER2/RARA coamplified breast tumors might represent a subclass of tumors that could benefit from retinoid treatment, both increase antitumor effects of Herceptin, as well as in potentially reducing Herceptin resistance. In conclusion, data presented in this thesis give better insight in the mechanisms of RA induced growth arrest in breast cancer cells and provide a potential application of retinoids in a subset of breast tumors.

Bröstcancer räknas idag till den vanligaste cancerformen bland kvinnor. År 2018 avled drygt 1400 personer till följd av sjukdomen, nästan alla som avled var diagnostiserade med metastaserad bröstcancer. Överlevnaden för bröstcancer är starkt kopplad till tumörstadiet vid diagnos, en tidig upptäckt är därför förenad med en bättre prognos. Human epidermal growth factor receptor 2 (HER2) är en tyrosinkinas receptor som kan hetero- eller homodimerisera med andra receptorer i samma familj, vilket är starten för en intracellulär signaleringskaskad genom olika signaleringsvägar som sedan leder till olika cellsvar såsom anti-apoptos, differentiering, proliferation och överlevnad. Denna receptor är överuttryckt i 20-30% av all bröstcancer, såkallad HER2-positiv bröstcancer. HER2-positiv bröstcancer är en aggressiv och snabbväxande bröstcancerform, förknippad med utveckling av metastaser. Vid metastaserad HER2-positiv bröstcancer ges så kallade målinriktade läkemedel, såsom antikroppar riktade mot HER2. En av dessa antikroppar är den monoklonala IgG antikroppen trastuzumab emtansin (T-DM1), vilken är konjugerad till en cytotoxisk molekyldel (DM1). Syftet med detta examensarbete var att undersöka om läkemedlet T-DM1 är effektiv som behandling i avseende på överlevnaden hos patienter med metastaserad eller avancerad HER2-positiv bröstcancer, samt att undersöka vilken relation HER2-associerade tumörbiomarkörer har för effektiviteten av läkemedlet. För att besvara frågeställningens syfte valdes fyra kliniska studier ut från den medicinska referensdatabasen PubMed. Dessa studier undersökte effektivitetsparametrarna; progression free survival (PFS), overall survival (OS) eller objective response rate (ORR) som antingen primär eller sekundär endpoint. Studiernas resultat tyder på att T-DM1 är effektiv som behandling mot HER2-positiv metastaserad bröstcancer där patienter tidigare har fått behandling i den metastaserade bröstcancerfasen då den PFS var längre för patienterna vars behandling bestod av T-DM1 jämfört mot de patienter som fick kontrollbehandling. Det är svårt att dra någon generell slutsats om hurvida T-DM1 effektivitet är kopplad till olika biomarkörer i HER2-signaleringsvägen eftersom endast tre studier har undersökts i detta syfte, dock tyder det på att ett uttryck av HER2 mRNA över median är förenat med längre PFS med T-DM1 behandling. Vidare sågs inget samband för varken PTEN-status eller PIK3CA-status i avseende på överlevnaden eftersom T-DM1 verkade vara effektiv oavsett uttryck. Dock behöver mer forskning utföras för att undersöka relationen mellan överlevnaden och PIK3CA-status, då muterad PIK3CA verkar vara förenat med en sämre prognos. Överlevnaden för patienter med HER2-positiv metastaserad cancer är låg, och sjukdomen anses inte som botbar i dagsläget. Ytterligare forskning i området behövs, samt utveckling av nya och effektivare behandlingsalternativ för att förlänga både PFS och den totala överlevnaden. / Breast cancer is considered as the most common form of cancer among women. In 2018, more than 1400 people died as a result of the disease and almost all who died were diagnosed with metastatic breast cancer. Survival is strongly linked to the stage of the tumor at the diagnosis, an earlier finding is therefore associated with better prognosis. Human epidermal growth factor receptor 2 (HER2) is a tyrosine kinase receptor, which can hetero- and homodimerize with other receptors in the same family, leading to an intracellular signalling cascade through various signalling pathways. The cascade lead to cellular responses such as anti-apoptosis, differentiation, proliferation and survival. HER2 is overexpressed in 20-30 % of all breast cancers, so-called HER2-positive breast cancer. HER2-positive breast cancer is an aggressive and fast-growing form, associated with the development of metastases. In metastatic HER2-positive breast cancer, target drugs, such as antibodies directed against HER2, are given. One of these is the monoclonal antibody trastuzumab emtansine (T-DM1), which is conjugated to a cytotoxic molecule (DM1). The purpose of this literature study was to investigate whether the drug T-DM1 is effective as a treatment to prolong the survival in patients with metastatic breast cancer, the purpose was also to investigate the relationship of HER2-related tumor biomarkers on the efficacy of the drug. To answer the purpose of the literature study, four clinical trials were selected from the medical reference database PubMed. All of the selected trials investigated one or more of the efficiency parameters; progression free survival (PFS), overall survival (OS) or objective response rate (ORR), as either primary or secondary endpoints. According to the investigated trials, the result indicate that T-DM1 is effective as a treatment for HER2-positive metastatic breast cancer, where patients previously been treated in the metastatic or advanced setting. T-DM1 showed prolonged PFS compared to control treatment. It is difficult to draw any general conclusion whether T-DM1 efficiency is linked to various biomarkers in the HER2 signaling pathway, as only three studies have been investigated for this purpose. However, it indicates that an HER2-mRNA > median is associated with a longer PFS with T-DM1 treatment. Furthermore, no relationship was seen for either PTEN- or PIK3CA-status with respect to survival, since T-DM1 appeared to be effective regardless of expression. However, more research needs to be done to investigate the relationship between survival and PIK3CA-status, as mutated PIK3CA appears to be associated with poorer prognosis. The survival rate for patients with metastatic or advanced HER2-positive breast cancer is low, and the disease is not considered as curable at present. Further research is needed, as well as the development of new and more effective treatment to extend both PFS and OS for these patients.