The Current Landscape of Antibody-based Therapies in Solid Malignancies

Abstract

Over the past three decades, monoclonal antibodies (mAbs) have revolutionized the landscape of cancer therapy. Still, this benefit remains restricted to a small proportion of patients due to moderate response rates and resistance emergence. The field has started to embrace better mAb-based formats with advancements in molecular and protein engineering technologies. The development of a therapeutic mAb with long-lasting clinical impact demands a prodigious understanding of target antigen, effective mechanism of action, gene engineering technologies, complex interplay between tumor and host immune system, and biomarkers for prediction of clinical response. This review discusses the various approaches used by mAbs for tumor targeting and mechanisms of therapeutic resistance that is not only caused by the heterogeneity of tumor antigen, but also the resistance imposed by tumor microenvironment (TME), including inefficient delivery to the tumor, alteration of effector functions in the TME, and Fc-gamma receptor expression diversity and polymorphism. Further, this article provides a perspective on potential strategies to overcome these barriers and how diagnostic and prognostic biomarkers are being used in predicting response to mAb-based therapies. Overall, understanding these interdependent parameters can improve the current mAb-based formulations and develop novel mAb-based therapeutics for achieving durable clinical outcomes in a large subset of patients.

Keywords: Antibodies; cancer; challenges; mechanisms of action; therapy.

Figure 1

Mechanisms of action of monoclonal antibodies (mAb) in cancer therapy. Antibody-based therapeutic strategies (A-E) in solid tumors include both direct and indirect tumor cell killing. A) Antibodies acting by functional neutralization of receptor(s) on tumor cells. Antibody binding to overexpressed HER family receptors (HER1, HER2, HER3) on tumor cells, interfere with ligand binding or inhibit their homo- and hetero- dimerization with other HER family members and inhibit activation of downstream MAPK/ERK and PI3K/AKT signaling pathways that promote growth, migration, and proliferation of tumor cells. Antibodies against c- met receptor block STAT3/JNK and PI3K/AKT signaling and inhibit tumor cell transformation, survival, and proliferation. Recently developed class of agonistic antibodies to TNF superfamily death receptors DR4 and DR5 stimulate apoptosis through Bax/Bak and Caspase 9 pathway. B) Binding of antibodies to tumor vasculature receptors VEGFR1, VEGFR2, or their ligand VEGFA inhibits endothelial cell proliferation, migration, vascular permeability, and angiogenesis by interfering with PI3K/AKT, MAPK, and MEK/ERK signaling. C) Antibodies to immune checkpoint molecules, which include inhibitory (CTLA4) and co-inhibitory receptors (PD-1) present on immune cells or their ligands upregulated by tumor cells (PD-L1), reverse T cell exhaustion. Anti-PD-1 antibodies interfere with tyrosine phosphatase SHP2 recruitment and allow TCR (CD3) induced PI3K/AKT and MAPK signaling activation, cell survival, and proliferation. Anti-CTLA4 antibodies inhibit PP2A recruitment and CD3 dephosphorylation and activate PI3K/AKT, mTOR, NF-kB signaling pathways. D) Antibodies bound to tumor cells display antibody-dependent cell cytotoxic (ADCC) activity by engaging FcγR, present on the effector cells such as NK cells, neutrophils, and macrophages. This interaction induces ITAM phosphorylation and binding to tyrosine kinases ZAP-70 and SYK, which in turn activate PI3K and SOS. PIP3 generated through PI3K activation recruit BTK and PLCγ. RAS, BTK, and PLC activate downstream ERK, p38 and JNK signaling pathway along with calcium release from the endoplasmic reticulum (ER), which result in the release of cytokines and cytotoxic granules as IFNγ, perforin, and granzymes from NK cells and actin remodeling, which finally cause tumor cell apoptosis. Antibody bound tumor cells are recognized by neutrophils and macrophages and trigger oxidative burst and phagocytosis by neutrophils and macrophages, leading to lysosomal degradation of tumor cells. E) Antibody-drug conjugates (ADCs) possess specificity of mAb and cytotoxic potential of payload drug. ADCs bind to target antigen, get internalized, and undergo endocytic processing. Once in the cell, ADCs cleavage occurs, and the active cytotoxic drug is released into the cytoplasm where it inhibits microtubule polymerization and subsequently, tumor cell death. HER, human epidermal growth factor receptor; MAPK, mitogen- activated protein kinase; ERK, extracellular signal- regulated kinase; MEK, MAPK/Erk kinase 1/2; PI3K, phosphoinositide 3-kinase; SOS, son of sevenless homologue; STAT3, signal transducer and activator of transcription 3; JNK, c-JUN N- terminal kinase; DR4, death receptor 4; DR5, death receptor 5; Bax, Bcl-2-associated X protein; VEGFR1, vascular endothelial growth factor receptor-1; VEGFR2, vascular endothelial growth factor receptor-2; VEGFA, vascular endothelial growth factor A; FcγR; Fc gamma receptor; PIP3, phosphatidylinositol-3,4,5-trisphosphate; NK, natural killer; ITAM, immunoreceptor tyrosine- based activation motif; SYK, spleen tyrosine kinase; BTK, bruton's tyrosine kinase; PLCγ, phospholipase C gamma; RAS, rat sarcoma viral oncogene homolog; CTLA-4, cytotoxic T lymphocyte antigen 4; PD-1, program death-1; PD-L1, program death-1 ligand; SHP2, src homology phosphatase 2; TCR, t cell receptor; CD3, cluster of differentiation 3; mTOR, mammalian target of rapamycin; NF-kB, nuclear factor kappa-light-chain-enhancer of activated B cells.

Figure 2

Summary of factors influencing the delivery of therapeutic antibodies in solid tumors. 1) A multitude of factors, including antibody large size and affinity, antigen diversity, and altered permeability of vasculature, leading to the heterogeneous distribution of systematically administered antibodies in tumor tissues. 2) Hypoxia and pH dysregulation in the tumor microenvironment (TME) due to hypoperfusion, elevated interstitial fluid pressure (IFP) by aberrant vasculature, and dense extracellular matrix (ECM) influence the transport of therapeutic antibodies to intratumoral sites. 3) The clearance of antibodies occurs from both inside and outside the tumor. Systemic clearance of antibodies from plasma hampers the concentration gradient required for antibody diffusion into the tumor. In lower antibody doses concerning a total amount of target antigen, antigen-antibody complexes undergo endocytic consumption and degradation (also called target mediated drug disposition effect). Local endocytic clearance through antibody internalization decreases antibody penetration to the tumor site.

Figure 3

Molecular mechanisms for impaired antibody therapeutic activity in cancer. 1) Tumor-derived exosomes and Nitric Oxide (NO) secretion from tumor; 2) membrane-bound TGFβ on Treg cells and secreted IL-10 contributes to impaired ADCC activity of NK cells, characterized by downregulation of NK cell activating receptor NKG2D and CD16 (FcγR) signaling and decreased IFNγ secretion; 3) Secretion of proangiogenic cytokines from macrophages in TME, downregulate the expression of VEGFR1, VEGFR3, on vascular endothelium and resulting in resistance to anti-VEGF antibody therapies; 4) Overexpression of don't eat me a signal (CD47) on tumors and its interaction with myeloid-specific checkpoint molecule SIRPα on tumor cells, contribute to dysfunction of phagocytic activity of macrophages and neutrophils through signaling via inhibitory ITIM motif; 5) IL-10 and TGFβ produced from Myeloid-Derived Suppressor Cells (MDSCs) impair FcγR signaling and thus dampen the phagocytic activity of macrophages and neutrophils; 6) Neutrophil-mediated antibody opsonized tumor cells killing, which involves CD11b/CD18 dependent conjugate formation and trogocytosis, abolishes by CD47-SIRPα axis;7) The overexpression of membrane-bound complement regulator proteins (mCRPs) such as CD55, CD59, etc. on tumor cells interfere with the binding of immune complexes (Antigen bound antibody) to C1q component of classical complement pathway and abolish Complement- dependent Cytotoxic (CDC) activity of antibodies; 8) Dampening of antibody-induced tumor antigen-specific immune response results due to reduced IFNγ secretion from exhausted NK cells leading to insufficient DC maturation and decreased T cell activation. TGFβ, transforming growth factor β; IL-10, interleukin-10; IFNγ, interferon-gamma; SIRPα, signal regulatory protein α; ITIM, immunoreceptor tyrosine-based inhibitory motif.

Figure 4

Graphical representation of active clinical trials of mAb-based combinatorial targeting in solid tumors. We searched for active clinical trials from the clinical trial site (https://clinicaltrials.gov/) with keywords solid tumors, antibodies plus chemotherapy, cytokines, and kinase inhibitors. These combinations are categorized as cocktails of mAbs, mAb+ chemotherapy, mAb+ kinase inhibitors, mAb+ cytokines, and multiple combinations where the mixture has more than two drugs. A. Each column of the graph represents the total number of clinical trials (y-axis) for each category of combination (x-axis). Different colors in each column represent target antigens for that category. B. Pie chart shows the number of mAb-based combination therapy trials for each class of target antigen(s) in solid tumors (colored separately).