cancers Review Trastuzumab Mechanism of Action; 20 Years of Research to Unravel a Dilemma Hamid Maadi 1, Mohammad Hasan Soheilifar 2, Won-Shik Choi 1, Abdolvahab Moshtaghian 3,4 and Zhixiang Wang 5,* 1 Department of Oncology, Cross Cancer Institute, University of Alberta, Edmonton, AB T6G 1Z2, Canada; [email protected] (H.M.); [email protected] (W.-S.C.) 2 Department of Medical Laser, Medical Laser Research Center, Yara Institute, ACECR, Tehran 1315795613, Iran; [email protected] 3 Department of Molecular and Cell Biology, Faculty of Basic Sciences, University of Mazandaran, Babolsar 4741695447, Iran; [email protected] 4 Deputy of Research and Technology, Semnan University of Medical Sciences, Semnan 3514799442, Iran 5 Department of Medical Genetics and Signal, Transduction Research Group, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, AB T6G 2H7, Canada * Correspondence: [email protected] Simple Summary: Overexpression of HER2 receptors have been identified in various types of cancer including breast cancer and ovarian cancer. HER2 overexpression is generally associated with poor clinical outcomes in patients with HER2-positve tumors. Trastuzumab, an antibody specifically targeting HER2 receptors, showed promising clinical benefits for patients with HER2-positive tumors. Studies show that trastuzumab suppresses HER2 receptors’ oncogenic functions in HER2-postive tumors. Moreover, trastuzumab has been shown to provoke immune responses against the HER2- amplified tumors. Citation: Maadi, H.; Soheilifar, M.H.; Choi, W.-S.; Moshtaghian, A.; Wang, Z. Trastuzumab Mechanism of Abstract: Trastuzumab as a first HER2-targeted therapy for the treatment of HER2-positive breast Action; 20 Years of Research to cancer patients was introduced in 1998. Although trastuzumab has opened a new avenue to treat Unravel a Dilemma. Cancers 2021, 13, patients with HER2-positive breast cancer and other types of cancer, some patients are not responsive 3540. https://doi.org/10.3390/ or become resistant to this treatment. So far, several mechanisms have been suggested for the mode cancers13143540 of action of trastuzumab; however, the findings regarding these mechanisms are controversial. In this review, we aimed to provide a detailed insight into the various mechanisms of action of trastuzumab. Academic Editors: Jonas Cicenas and Kylie Gorringe Keywords: trastuzumab; breast cancer; HER2; PI3K/AKT pathway; MAPK pathway Received: 6 May 2021 Accepted: 12 July 2021 Published: 15 July 2021 1. Introduction HER2 overexpression in approximately 20–30% of breast cancer (BC) patients has Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in motivated researchers for years to find a decent drug to specifically target HER2. A ® published maps and institutional affil- humanized monoclonal antibody named trastuzumab (Herceptin ), which targets the iations. extracellular domain of HER2, was developed. Since the approval of trastuzumab, many investigations have been carried out to unravel the trastuzumab mechanism of action and find an appropriate combination therapy to enhance the treatment efficacy. In this review, we aimed to shine the light on the various mechanisms through which trastuzumab exerts its anti-tumoral effects. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 2. From Characterization of HER2 to Developing Trastuzumab This article is an open access article distributed under the terms and In 1984, Schechter et al. isolated a gene from ethylnitrosourea-induced rat neuro/gliob- conditions of the Creative Commons lastomas and confirmed the ability of this gene to transform the NIH3T3 cells [1]. This Attribution (CC BY) license (https:// gene, which is named ‘neu’, was later shown to encode a protein with 185 kDa molecular creativecommons.org/licenses/by/ weight [1]. One year later, Coussens et al. identified and characterized a gene located at 4.0/). the human q21 region of chromosome 17 which was homologous to the rat neu gene [2]. Cancers 2021, 13, 3540. https://doi.org/10.3390/cancers13143540 https://www.mdpi.com/journal/cancers Cancers 2021, 13, x FOR PEER REVIEW 2 of 18 Cancers 2021, 13, 3540 2 of 17 gene, which is named ‘neu’, was later shown to encode a protein with 185 kDa molecular weight [1]. One year later, Coussens et al. identified and characterized a gene located at Duethe human to the q21 significant region of similarities chromosome in 17 the which protein was sequence homologous between to the rat the neu EGFR gene (HER1) [2]. andDue the to the product significant of the similaritiesneu gene, in it the was protein named sequence HER2 and between suggested the EGFR to play (HER1) a role and as a kinasethe product receptor of the at neu the gene, cell surface. it was named In 1986, HER2 Drebin and etsuggested al. demonstrated to play a role the as tumorigenic a kinase potentialreceptor at of the NIH3T3 cell surface. cells, In when 1986, transfected Drebin et al with. demonstrated the HER2 thegene tumorigenicin vivo [3]. potential They also showedof NIH3T3 that cells, treating when the transfected mice with with an antibodythe HER2 againstgene in vivo the neu [3].gene They productalso showed (anti-HER2 that antibody)treating the significantly mice with an suppresses antibody tumoragainst growth. the neu Thegene oncogenicproduct (anti function-HER2 ofantibody) HER2 was alsosignificantly confirmed suppresses by another tumor research growth. group The aboutoncogenic a year function later [of4]. HER2 In 1987, was Slamon also con- et al. foundfirmed that by another the HER2 research gene is group amplified about in a year about later 30% [4] of. In BC 1987, tumors, Slamon and et this al. found amplification that isthe significantly HER2 gene correlatedis amplified with in about poor 30% prognosis of BC tumors in BC patients, and this [ 5amplification]. In 1989, Hudziak is signifi- et al. cantly correlated with poor prognosis in BC patients [5]. In 1989, Hudziak et al. isolated a isolated a monoclonal antibody named “4D5”, which has a high specificity toward HER2, monoclonal antibody named “4D5”, which has a high specificity toward HER2, from mice from mice exposed to this receptor and showed its antiproliferative effects against SKBR3 exposed to this receptor and showed its antiproliferative effects against SKBR3 HER2- HER2-positive BC cells [6]. In 1992, Carter et al. humanized the 4D5 monoclonal antibody positive BC cells [6]. In 1992, Carter et al. humanized the 4D5 monoclonal antibody which which made it suitable for further clinical evaluation [7]. This antibody, which was named made it suitable for further clinical evaluation [7]. This antibody, which was named “Trastuzumab”, showed significant clinical benefits toward patients with HER2-positive “Trastuzumab”, showed significant clinical benefits toward patients with HER2-positive BC and increased the efficacy of conventional chemotherapies [8]. Finally, trastuzumab BC and increased the efficacy of conventional chemotherapies [8]. Finally, trastuzumab waswas approvedapproved in in the the USA USA and and Europe Europe in 1998 in 1998 and and2000, 2000,respectively respectively [9,10]. The [9,10 key]. Thefind- key findingsings related related to trastuzumab to trastuzumab history history are summarized are summarized in Figure in Figure 1. After1. Afterdemonstrating demonstrating the theclinical clinical benefits benefits of trastuzumab, of trastuzumab, many many studies studies were were carried carried out outin an in attempt an attempt to unravel to unravel thethe mechanisms mechanisms ofof action of of trastuzumab. trastuzumab. In In this this review, review, we we discuss discuss different different mechanisms mechanisms ofof action action proposedproposed for tra trastuzumab.stuzumab. FigureFigure 1. 1.Timeline Timeline of of trastuzumab trastuzumab history, history, fromfrom HER2HER2 identification identification to to demonstrating demonstrating the the clinical clinical benefits benefits of trastuzumab. of trastuzumab. 3.3. HER2 HER2 SignalingSignaling TheThe ErbB receptor receptor tyrosine tyrosine kinase kinase 2 (ErbB2 2 (ErbB2 or HER2) or HER2) belongs belongs to the ErbB to the (HER) ErbB fam- (HER) familyily of ofreceptor receptor tyrosi tyrosinene kinases kinases along along with with three three other other members members,, including including epithelial epithelial growthgrowth factorfactor receptorreceptor (EGFR), ErbB3 ErbB3 (HER3), (HER3), and and ErbB4 ErbB4 (HER4) (HER4) [11] [11. Generally,]. Generally, these these receptorsreceptors consist of of three three main main domains: domains: (1) the (1) extracellular the extracellular domain domain that contains that contains the four the foursubdomains subdomains that mediate that mediate HER receptor HER receptor ligand-dependent ligand-dependent or -independent or -independent dimerization, dimer- ization,(2) transmembrane(2) transmembrane domain domainthat connects that connectsextracellular extracellular and cytoplasmic and cytoplasmic domains, and domains, (3) andthe (3)C-terminal the C-terminal cytoplasmic cytoplasmic domain that domain contains that containstyrosine kinase tyrosine and kinase regulatory and regulatorysubdo- subdomainsmains [12]. Upon [12]. Uponthe binding the binding of HER of-specific HER-specific ligands ligands to HER to receptors, HER receptors, these receptors these recep- torsform form homo homo-- or heterodimers. or heterodimers. Subsequently, Subsequently, certain certain tyrosine tyrosine residues residues in cytoplasmic