Abstract

     5-10% of all breast cancers are associated with mutations in hereditary breast cancer genes several of which are involved in the DNA damage response.

     BRCA1 and BRCA2 mutation is not associated with poor prognosis in breast cancers, but infact better prognosis when treated with chemotherapy in comparison to similar non BRCA mutated breast cancers.

    Treatment with the small molecule PARP inhibitor olaparib after chemotherapy improves overall survival in patients with early breast cancer and germline BRCA1 or BRCA2 mutations.

     Platinum based chemotherapy is highly active in those with germline BRCA1 or BRCA2 mutations but cross resistance between platinum agents and PARP inhibitors can be mediated by somatic “reversion mutations” in BRCA1 or BRCA2 and is a clinical challenge.

     Understanding the underlying mechanisms of overlapping or distinctive resistance is vital to therapy development in this evolving area of translational medicine.

Introduction

     Approximately 5% to 10% of the 2.3 million breast cancer cases diagnosed annually are associated with a mutation in a known hereditary breast cancer predisposition gene such as BRCA1 or BRCA2.1,2 The integration of genomics into the standard diagnostic pathways for breast cancer patients and the availability of targeted treatment approaches for those with hereditary breast cancer predisposition gene mutations means that the management of these patients is now distinct to that a decade ago. In this review, we discuss the recent advances made in systemic treatments for hereditary breast cancer and highlight future challenges that must be addressed for improvements in clinical outcomes to be achieved in this distinct subgroup.

Hereditary Breast Cancer Genes

     Many of the germline mutations associated with hereditary breast cancer occur in “caretaker” tumor suppressor genes (genes whose normal function is to maintain the integrity of the genome and whose dysfunction leads to genome instability) and include BRCA1, BRCA2, PALB2, ATM, CHEK2, and p53.3 For example, germline deleterious mutations in BRCA1 (gBRCA1m) or BRCA2 (gBRCA2m), which play key roles in DNA repair by homologous recombination (HR), are associated with an increased lifetime risk of developing breast, ovarian, prostate and pancreatic cancer.2,4,5 gBRCA1m carriers have a cumulative lifetime breast cancer risk of 46% to 60% and when diagnosed with breast cancer typically have the basal-like, triple negative (B-L, TNBC), subtype of the disease. gBRCA2m carriers have a cumulative lifetime breast cancer risk of 43% to 55% and tend to develop estrogen receptor-positive (ER+), luminal B subtype, breast cancers (significantly more so than for gBRCA1m carriers).6, 7, 8, 9 gBRCA1/2m breast cancers in those who have a strong family history are usually detected at a younger age than sporadic breast cancers.

Disclosure

     E.H.-J. is a clinical PhD fellow funded by Cancer Research UK and AstraZeneca. A.T. is a consultant for AstraZeneca, Merck KGaA, Artios, Pfizer, Vertex, GE Healthcare, Inbiomotion, MD Anderson Cancer Centre; has received grant/research support from AstraZeneca, Myriad, Medivation, and Merck KGaA; and is a stockholder in Inbiomotion. Stands to gain from the use of PARP inhibitors as part of the ICR’s “rewards to inventors” scheme. C.J.L. makes the following disclosures: receives and/or has received research funding from: AstraZeneca, Merck KGaA, Artios. Received consultancy, SAB membership or honoraria payments from: Syncona, Sun Pharma, Gerson Lehrman Group, Merck KGaA, Vertex, AstraZeneca, Tango, 3rd Rock, Ono Pharma, Artios, Abingworth, Tesselate, Dark Blue Therapeutics. Has stock in: Tango, Ovibio, Enedra Tx., Hysplex, Tesselate. C.J.L. is also a named inventor on patents describing the use of DNA repair inhibitors and stands to gain from their development and use as part of the ICR “Rewards to Inventors” scheme.

1- Introduction

2- Breast cancer

3- Drug resistance

4- miRNA biogenesis

5- miR-21 in BC

6- Expression of miR-21 in cancer tissues and cells

7- Role of miR-21 as a mediator in drug resistance in BC

8- Upstream pathway of miR-21

9- Potential targets of miR-21

10- Conclusion and future perspectives

References

Abstract

Breast cancer (BC) is the most common cancer among women that is responsible for the most of the cancer-related death in worldwide. Drug resistance is remaining as a significant clinical obstacle to treat BC patients effectively. Therefore, to help overcome this problem, it is necessary to understand the mechanisms of drug resistance. microRNAs classify as highly conserved non-coding RNAs (~22 nucleotides) and interact with mRNAs-coding genes for direct post-transcriptional repression. It has been reported that miR-21 is overexpressed and also acts as oncomiR in many human malignancies by targeting of several tumor suppressor genes–associated with apoptosis, proliferation and metastasis. Specifically, it has been reported that miR-21 is responsible for the drug resistance and its overexpression is related to the development of Multi Drug Resistance (MDR) in breast cancer. In this review, we discussed about the role of miR-21 on the drug resistance of breast cancer.

Introduction

Breast cancer (BC) is responsible for the death of approximately 500,000 women per year in the world. Also, more than one million new case of BC is diagnosed across the world per year. The occurrence of BC accounts for 7-8% of the entire number of malignant tumors. Chemotherapy is known as a principal strategy for cancer treatment; however, its application is limited due to drug resistance. Resistance to chemotherapeutics is mainly divided into two extensive categories: (i) intrinsic and (ii) acquired. Intrinsic resistance shows the presence of resistance-mediating factors in the tumor before starting of treatment process that makes chemotherapy ineffective. Acquired drug resistance, however, will be developed during of the treatment [1,2]. Drug resistance brings serious clinical obstacles to the prosperous treatment of BC patients. To overcome these problems, a well understanding of the drug resistance mechanisms is definitely required. The molecular mechanism of chemotherapeutic resistance in BC cells is completely complicated, and comprises multiple processes, including epigenetic changes, gene mutation, gene amplification and microRNA expression. miRNAs are short non-coding RNAs (~22 nucleotides) that could be found in all eukaryotic cells. They have a significant role in the progression of cancer by binding to the 3′ untranslated region (UTR) of the target genes, leading to the target mRNA degradation and inhibition of translation.

1- Introduction

2- Materials and methods

3- Results

4- Discussion

References

Abstract

Drug resistance is a big problem in cancer treatment and one of the most prominent mechanisms underlain is overexpression of ATP-binding cassette (ABC) transporters, particularly ABCB1, ABCC1 and ABCG2. Inhibition of ABC transporters is an important approach to overcome drug resistance. The inositol-requiring enzyme 1α (IRE1α), an arm of unfolded protein response (UPR), splices XBP1 mRNA to generate an active transcription factor XBP1s. UPR is implicated in drug resistance. However, the underlying mechanism is unclear. We found that the anticancer drugs such as 5-fluorouracil (5-FU) activated the IRE1α-XBP1 pathway to induce the expression of ABCB1, ABCC1 and ABCG2 in colon cancer cells. Inhibition of IRE1α RNase activity with small molecule 4μ8c suppressed the drug-induced expression of these ABC transporters and sensitized 5-FU-resistant colon cancer cells to drug treatment. In vivo xenograft assay indicates that administration of 4μ8C substantially enhanced the efficacy of 5-FU chemotherapy on 5-FU-resistant colon cancer cells. These results suggest that IRE1α-targeting might be a strategy to cope with drug resistance of colon cancer.

Introduction

Cancers have the ability to develop resistance to chemotherapy, and the increasing prevalence of these drug resistant cancers necessitates further research and treatment development. Multidrug resistance is a phenomenon by which, after exposure to a chemotherapeutic agent, cancer cells develop resistance, and simultaneous cross-resistance, to a wide range of functionally and structurally unrelated chemotherapeutic drugs (1, 2). Intrinsic or acquired multidrug resistance is one of the main reasons for chemotherapy failure, leading to the recurrence of malignant tumors and ultimately, patient relapse or death (3). A variety of mechanisms have been attributed to multidrug resistance, such as enhanced drug efflux, increased DNA damage repair, reduced apoptosis, elevated autophagy, and/or altered drug metabolism (4-6). Previous studies have confirmed that multidrug resistance both in cancer cell lines and human tumor tissues is most often associated with the overexpression of the ATP-binding cassette (ABC) transporters (4, 7, 8). These ABC transporters are efflux pumps that transport various structurally unrelated and potentially dangerous substances out of the cells. They hydrolyze ATP for energy and actively pump drug components out of cells, leading to drug resistance. In these transporters, ABCB1 (also known as MDR1 or P-gp), ABCC1 (MRP1) and ABCG2 (BCRP1) play an important role in drug resistance in vivo (4, 7). Developing inhibitors of ABC transporters is an important approach to overcoming drug resistance. Extensive efforts have been made to develop inhibitors targeting ABC transporters, particularly ABCB1. However, no such agents have been developed successfully as was initially hoped.

Overcoming Target-Dependent Resistance

Overcoming Pathway-Dependent Resistance

Concluding Remarks

References

Coadministration of Allosteric and Orthosteric Drugs: A Novel Strategy to Tackle Drug Resistance

As a detrimental problem in modern biomedicine, the inevitable emergence of drug resistance poses a great challenge to pharmaceutics and threatens global health [1–7]. It is a sophisticated evolving process involving multiple events on the molecular pathway, and even organismal levels. Based on the underlying mechanism, drug resistance is classified as target-dependent resistance (see Glossary), caused by changes within the original targets, and pathway-dependent resistance, resulting from alterations in the relevant collateral pathways) [4,8,9]. Different methodologies are applicable for tackling different types of resistance; for example, structure-based drug design is more suitable against target-dependent resistance, whereas the multitargeted approach of polypharmacology may be a better choice against pathway-dependent resistance [10–12]. Many excellent reviews have already discussed various approaches to combat resistance, but most of them only focus on either drug optimization or combined treatments with different types of therapies [5,12–14]. Stemming from rapid developments in structural biology and protein allostery research [15–20], allosteric drugs can resensitize resistant targets and selectively target resistance-related signaling pathways, thereby restoring the efficacy of orthosteric drugs. Moreover, compared with monotherapy, double- or even multiple-drugging strategies can cover a broader therapeutic spectrum, achieve synergistic effects, and contribute to better clinical outcomes. Thus, combination therapy with allosteric and orthosteric drugs provides an unprecedented opportunity to overcome the notorious problem of drug resistance and boost the development of next-generation pharmacological agents. With the goal of forwarding this notion, this review first outlines typical examples of combined treatments to combat clinically relevant resistance, reviewing their modes of action and future perspectives in detail. Then, it discusses the current state of this combinatorial strategy, presenting the existent obstacles as well as possible solutions. This review highlights a promising tactic for overcoming the long-standing conundrum of drug resistance and will hopefully be instructive for future drug discovery and development.

1- Introduction

2- Mechanisms and conventional methods towards monitoring of drug resistance

3- Electrochemical and Electrochemiluminescent (ECL) sensing

4- Electrical sensing

5- Optical sensing

6- The role of nanomaterials for the screening of drug resistance

7- Electrochemical nanosensor for the monitoring of drug resistance

8- Optical nanosensors for the monitoring of drug resistance

9- Electrochemiluminescence nanosensors for the monitoring of drug resistance

10- Photoelectrochemical (PEC) Nanosensors for the monitoring of drug resistance

11- Conclusion

References

Abstract

Drug resistance is worldwide health care crisis which decrease drug efficacy and developing toxicities. Effective resistance detection techniques could alleviate treatment cost and mortality associated with this crisis. In this review, the conventional and modern analysis methods for monitoring of drug resistance are presented. Also, various types of emerging rapid and sensitive techniques including electrochemical, electrical, optical and nano-based methods for the screening of drug resistance were discussed. Applications of various methods for the sensitive and rapid detection of drug resistance are investigated. The review outlines existing key issues in the determination which must be overcome before any of these techniques becomes a feasible method for the rapid detection of drug resistance. In this review, the roles of nanomaterials on development of novel methods for the monitoring of drug resistance were presented. Also, limitations and challenges of conventional and modern methods were discussed.

Introduction

Drug resistance as an emerging health concern reducing the efficiency and potency of a drug to produce effective treatment of pathogenic microorganisms or cancer and decrease overall patient survival [1]. During the cancer invasion and metastasis, large number of failures in the chemotherapy is related with the resistance of tumor cells to anticancer agents [2]. Consequently, resistance to drugs led to many other problems including toxicity and huge costs for development of new drugs [3]. In contemporary clinical practice, resistance to anticancer drugs can only be detected after long treatment periods [3]. Various mechanisms including reduced influx or enhanced efflux of drug, drug detoxification, glutathione conjugation, disrupt in DNA repair introduced for resistance against some anticancer agents like cisplatin [4]. It is important to point out that resistant to anticancer agents already exists before of chemotherapy in about half of cancer cases [3]. Thus, diagnostic tests for drug resistance in individual prior to commencing treatment with chemotherapy agents are required to predict the efficacy of chemotherapy and avoid any toxicity in normal tissues [3]. Several in vitro test analysis methods have been developed to assessment of resistance or sensitivity [5-7].

1- Introduction

2- Noncoding RNAs and sirtuins function in cancer drug resistance

3- Future prospective and conclusion

References

Abstract

Cancer is a rising and major health issue around the world. The acquisition of resistance to chemotherapeutic drugs is a great obstacle for the effective treatment of nearly all cancers. Drug resistance is regulated by multiple factors and mechanisms including genetic mutations, abnormal expression of some cellular transporters such as multidrug resistance (MDR) transporters, changes in apoptotic pathways, cancer stem cells, tumor microenvironment, and noncoding RNAs (ncRNAs). Evidence clearly indicates a key role for sirtuins in several characteristics of cancer drug resistance. Recent studies demonstrated the crucial impact of some ncRNAs on sirtuins expression leading to modulation of chemotherapy resistance in cancers. In this review, we will focus on the current findings about the impacts of ncRNAs on the sirtuins pathway and their role in drug resistance of cancer.

Introduction

Cancer is considered a complex disease associated with some genetic mutations, deletions, epigenetic alterations and chromosomal translocations that are involved in cancer initiation, promotion, metastasis and drug resistance (Bach and Lee, 2018). Chemotherapy is one of the most widely used therapies for the treatment of cancer and improves the lifespan of patients. However, prolonged utilization of chemotherapeutic drugs. may lead to drug resistance which is a major issue in cancer treatment (Szakacs et al., 2006). Based on statistical reports, over than 90% of deaths in patients with different types of cancer are associated with chemotherapeutic drug resistance (Li et al., 2008; Longley and Johnston, 2005). Cancer cells apply many different mechanisms to impede drug treatment, including the genetic mutations, cell cycle alterations, apoptosis induction, drug metabolism, efflux alterations and DNA methylation (Balch et al., 2004; Gillet and Gottesman, 2010; Xia and Hui, 2014). NcRNAs have been reported to play an essential role in determining drug sensitivity or restoring drug sensitivity in resistant cells in many cancers. (Kapranov et al., 2010). NcRNAs are RNA molecules that do not code any protein. However, they exert an important impact on the expression of more than 60% of human genes. They are classified into two main groups including the most studied microRNAs (miRNAs) and the long non-coding RNAs (lncRNA) (Kapranov et al., 2010). MiRNAs are single-stranded RNAs, 19-23bp in length and account for approximately 30% of gene expression regulation. MiRNAs can bind to the 3′ untranslated regions (UTRs) of their target mRNAs and regulate gene expression. The most important function of miRNAs is the suppression of gene expression (Omidkhoda et al., 2019; Szulwach et al., 2010). lncRNAs are transcripts with 200 nt to ~100 kb in length. They do not have any significant open reading frames. lncRNAs are poly-adenylated and located within the cell nucleus or cytosol. They can regulate the expression of genes by cis-acting or trans-acting regulation. Cis-acting lncRNAs affect the expression of neighboring genes by acting at the site of transcription, while trans-acting lncRNAs affect the expression of genes by acting away from the site of synthesis (Barangi et al., 2019; Batista and Chang, 2013).

Head and neck cancer (HNC), which includes epithelial malignancies of the upper aerodigestive tract (oral cavity, oropharynx, pharynx, hypopharynx, larynx, and thyroid), are slowly but consistently increasing, while the overall survival rate remains unsatisfactory. Because of the multifunctional anatomical intricacies of the head and neck, disease progression and therapy-related side effects often severely affect the patient’s appearance and self-image, as well as their ability to breathe, speak, and swallow. Patients with HNC require a multidisciplinary approach involving surgery, radiation therapy, and chemotherapeutics. Chemotherapy is an important part of the comprehensive treatment of tumors, especially advanced HNC, but drug resistance is the main cause of poor clinical efficacy. The most important determinant of this phenomenon is still largely unknown. Recent studies have shown that non-coding RNAs have a crucial role in HNC drug resistance. In addition, they can serve as biomarkers in the diagnosis, treatment, and prognosis of HNCs. In this review, we summarize the relationship between non-coding RNAs and drug resistance of HNC, and discuss their potential clinical application in overcoming HNC chemoresistance.

Background

Head and neck cancer (HNC) is a major global health problem. More than 890,000 new cases of HNC and approximately 450,000 HNC-related deaths occur every year [1,2]. The majority of cases are HN squamous cell carcinoma (HNSCC) [3], involving the stratified epithelium of the oral cavity, pharynx, and larynx, with an overall 5-year survival rate of only 40%–50% [3]. Most HNC is closely related to tobacco and alcohol use [1–3]. Patients with HNC might have symptoms including hoarseness, dysphagia, pain, neck mass, and ulcers, which require a multidisciplinary approach involving surgery, radiation therapy, and chemotherapy [4]. Although the treatment is effective for early and locally advanced disease, the prognosis of recurrent and metastatic disease remains poor [5]. Clinically, more than 65% of patients are diagnosed with advanced disease because of the lack of early diagnosis [6]. However, both radiotherapy and surgery reduce patients’ quality of life at this stage [2]. Radiotherapy usually produces obvious side effects, while surgery can significantly affect the normal physiological structure and function of patients [2,7]. Additionally, targeted therapy for HNC has been incorporated into clinical treatment guidelines, but its clinical response and long-term effect are limited, and high costs are associated with this treatment [8]. In recent years, chemical treatment of advanced HNC has been increasingly recognized [9]. In early phase II clinical trials in patients with advanced HNSCC, significant single-agent response rates were up to 30% [5,10].

1- Introduction

2- Drug resistance in parasites

3- ATP-binding cassette (ABC) transporters in parasites

4- Conclusion

References

Abstract

Nowadays, drug resistance in parasites is considered to be one of the foremost concerns in health and disease management. It is interconnected worldwide and undermines the health of millions of people, threatening to grow worse. Unfortunately, it does not receive serious attention from every corner of society. Consequently, drug resistance in parasites is gradually complicating and challenging the treatment of parasitic diseases. In this context, we have dedicated ourselves to review the incidence of drug resistance in the protozoan parasites Plasmodium, Leishmania, Trypanosoma, Entamoeba and Toxoplasma gondii. Moreover, understanding the role of ATP-binding cassette (ABC) transporters in drug resistance is essential in the control of parasitic diseases. Therefore, we also focused on the involvement of ABC transporters in drug resistance, which will be a superior approach to find ways for better regulation of diseases caused by parasitic infections.

Drug resistance in parasites

Due to lack of effective vaccines, chemotherapy remains the mainstay of defence against parasites responsible for a wide spectrum of diseases in humans. Unfortunately, resistance has developed in the respective parasites for the majority of clinically used drugs against these diseases. Drug resistance mechanisms in parasites are very inconsistent in terms of the nature and life cycle of parasites [7]. In general, mechanisms of drug resistance in pathogenic parasites depend on a variety of mechanisms consisting of augmented drug efflux, reduced drug uptake, mutation events in targeted enzymes, metabolic upregulation, and deficiency of the target sites for antiparasitic drugs, etc. Apart from these issues, various genetic mechanisms including gene deletions, gene mutations and, most importantly, chimerisation of genes also play a crucial role in drug resistance mechanisms [8]. Incidentally, the formation of chimeric genes in drug resistance is not yet well established in all pathogenic parasites; to the best of our knowledge from a literature search, it is greatly studied in Trypanosoma compared with other parasites. However, in some parasites resistance appears through many other unknown mechanisms and resistance to most antiparasitic drugs in the majority of parasites is now widespread. The following sections of this review article discuss the drug resistance of the protozoan parasites Plasmodium, Leishmania, Trypanosoma, Entamoeba and T. gondii.

Most patients with advanced cancer die because their cancer exhibits or develops resistance to available therapies. This challenge remains pervasive despite many remarkable advances in cancer biology and treatment and has motivated renewed efforts to understand and combat cancer drug resistance. For example, addressing cancer drug resistance was one of the key recommendations from the Blue Ribbon Panel that advised the Beau Biden Cancer Moonshot initiative (Jacks et al., 2016). Studies of cancer drug resistance yield several types of insights. First, defining mechanisms of resistance to a specific anti-cancer regimen may identify future therapeutic strategies against such resistance. Such knowledge might guide salvage treatment after resistance has already occurred or inform new up-front strategies that prevent its emergence. For example, identification of MEK reactivation as a mechanism of resistance to RAF inhibitor monotherapy in BRAF-mutant melanoma (Johannessen et al., 2010; Nazarian et al., 2010), or of EGFRT790M as a mechanism of resistance to first-generation EGFR inhibitors in EGFR-mutant lung cancer (Kobayashi et al., 2005; Pao et al., 2005), led to the development of new therapies with clinical efficacy against these resistance mechanisms (Mok et al., 2017; Robert et al., 2015). Systematic characterization of drug resistance across tumor types and therapeutic categories may also enable new insights into cancer biology—regarding, for example, networks that differentiate the malignant state from normal, the cell-intrinsic and extrinsic factors regulating them, and how these pathways evolve to promote tumor survival during therapy. Investigating resistance to mitogen-activated protein kinase (MAPK) pathway inhibition revealed new aspects of feedback regulation within this pathway (Lito et al., 2014; Pratilas et al., 2009). Likewise, studies of EGFR inhibitor resistance in lung cancer highlighted the importance of intratumoral heterogeneity (Piotrowska et al., 2015). Despite the potential benefits of such knowledge, systematic efforts to define the full spectrum of resistance mechanisms for any given therapeutic remain the exception rather than the rule in cancer research. In fact, the skeptic might even question whether studies of resistance in general offer the most efficient or incisive means to discover either novel therapeutic targets or disease-relevant biology. However, a synthesis of emerging genomic and functional portraits of cancer drug resistance begins to reveal overarching frameworks that both offer new insights into resistant states and suggest possible therapeutic approaches to overcome them.

Extracellular vesicles (EVs) including exosomes, microvesicles, oncosomes, and microparticles have been associated with communicating anti-cancer drug-resistance. The in vitro, pre-clinical in vivo and patients’ data linking EVs to drug-resistance (and the specific drugs involved) in breast cancer, prostate cancer, lung cancer, ovarian cancer, haematological malignancies, colorectal cancer, gastric cancer, pancreatic cancer, glioblastoma, neuroblastoma, melanoma, kidney cancer and osteosarcoma. Details of the mechanisms by which the resistance seems to be occurring (e.g. EVs transferring drug-efflux pumps from drug-resistant cancer cells, EVs binding monoclonal antibodies in the peripheral circulation and so reducing their bioavailability, EVs from tumour microenvironment cells, etc.) are outlined, as are efforts to try to block such resistance. Research to date strongly supports EVs as playing a key role in drug-resistance. Further studies including tailored clinical studies are now warranted to determine how best to prevent this occurring, in the interest of patients and also for economic benefit. Furthermore, efforts to exploit safe (non-cancer origin) EVs as anti-cancer drug delivery vehicles that may achieve efficacy with more limited side-effects than free drug, deserve further investigation.

Introduction

Approximately 30 years ago, exosomes were described as involved in reticulocyte maturation by transporting transferrin receptor out of the cell [35]. Building on this knowledge, over recent years increasing evidence indicates that substantial cargos of information are released from cells via lipid bilayer-enclosed vesicles typically termed exosomes and microvesicles. These vesicles are proposed to be tailor-made specialised mini-maps of their cell of origin; are transported in the bloodstream and other body fluids; and much evidence indicates them to be involved in cell-to-cell communication. Exosomes and microvesicles, collectively termed extracellular vesicles (EVs), are often defined and sub-grouped based on size and cellular origin (exosomes ~30 nm–120 nm, endosomal origin; microvesicles/ectosomes > 120- 1000 nm, from the cell membrane). It should be noted that some reports use additional or alternative terms including, but not limited to, ectosomes, microparticles, oncosomes and prostasomes; all of which are EVs, as, indeed, are apoptotic bodies. However, once outside the cell and released into the environment (for example, the bloodstream) we cannot be certain if the EVs originated from the cells’ endosomal region or directly from the cell membrane. Furthermore, EV size distinctions are not absolute i.e. there is no known reason why vesicles budding from the cell membrane cannot be < 120 nm. In diseases such as cancer, regardless of the size and origin of EVs released, arguably the problems that EVs contribute to when released are of much importance to understand. Evidence from pre-clinical and clinical specimens’ studies, by ourselves and others, strongly associate EVs with transmitting anti-cancer drug-resistance from cell-to-cell in multiple cancer types. This phenomenon, known as multi-drug resistance or multiple-drug resistance (MDR) and initially described > 30 years ago [8], is manifest as cancer cells being resistant to anti-cancer drugs that are structurally and mechanistically unrelated. MDR may present as innate/primary or it may be acquired. Innate MDR means that, from the outset, the cancer cells are already equipped to be resistant to the anti-cancer drug being used. Acquired resistance occurs when cancer cells initially respond to treatment, but develop resistance mechanisms overtime. Advancing on early work where the mdr1 gene (which encodes the ABC transporter, ABCB1, also known as P-glycoprotein (P-gp) and now well established as causally involved in MDR) was cloned [72], many research studies and reviews (exemplified by [17,26,28,40,43,45,56–58,73,79,84]; but too many to detail here) have been published on this topic. MDR is a substantial concern in cancer management. Our understanding is that the first study showing transmission of drug-resistance by EVs, was a study performed by our group in prostate cancer [18], Numerous important studies by other research groups have been reported.

Angiogenesis research in the past two decades has contributed significantly towards understanding the molecular pathophysiology of cancer progression and inspired target-oriented research and pharma industry for the development of novel anti-angiogenic agents. Currently, over eleven drugs targeting angiogenesis have been approved by the FDA for the treatment of various malignancies. Of the registered anti-angiogenic clinical trials until the end of 2017 (ClinicalTrials.gov), over 47% were completed, 10% were terminated, 3% withdrawn, over 0.5% were suspended and only 4 trials have culminated in FDA approval for marketing. On the one hand, the clinical benefits of anti-angiogenic drugs prompted the development of novel anti-angiogenic agents. On the other hand, however, a plethora of recent studies demonstrated the emergence of tumor drug resistance towards currently used anti-angiogenic therapeutics. Series of preclinical and clinical studies have highlighted the enigma of drug resistance with functional bypass pathways, and identified compensatory or alternative angiogenic mechanisms assuring tumor growth in the midst of an antiangiogenic stress environment. In the present review the classical literature of such redundant angiogenic pathways in concert with the key angiogenic factors and specialized cells involved in anti-angiogenic escape mechanisms is described. A strategic discourse regarding increasing tumor drug resistance and future modalities for anti-angiogenic therapy is also discussed in view of recent advances.

Introduction 

Vasculogenesis is the fundamental de novo process required for the development of blood vessels during embryonic development, wherein the angioblasts (endothelial progenitor cells) differentiate into endothelial cells in response to local cues and finally assemble to form the initial vascular plexus. Angiogenesis is the extension of vasculogenesis, which involves the development of new capillaries on pre-existing vessels. A vast body of literature has accumulated which describes the dynamics and complexities of new vessel formation (Chappell et al., 2016). From a physiological perspective, angiogenesis appears to be indispensable for tissue homeostasis, as the growth and development of an organism requires a well-developed network of blood vessels to ensure a continuous supply of oxygen and nutrients. Nevertheless, important physiological processes such as embryogenesis, organogenesis, wound healing, tissue repair, etc., cannot be achieved, unless there is a well-developed, mature orchestration of vascular network. Dissolution of vascular basal membrane, degradation of extracellular matrix (ECM), increase in vascular permeability, migration, invasion and proliferation of endothelial cells (ECs) and tube formation are the hallmarks of the angiogenic process. In brief, the process of neovascularization relies on a complex network and cross talks of proangiogenic factors, stromal cell interactions and remodeling of ECM (Gacche and Meshram, 2014). Under normal physiological conditions, the process of angiogenesis is regulated by maintaining a balance between activators and inhibitors of angiogenesis. The vessels developed through physiological angiogenesis are normal, stable and rarely proliferate under physiological conditions. However, in pathological conditions like cancer, the balance is more in favor of proangiogenic factors; as a result, there is excessive remodeling of vasculature. In contrast, the complex, abnormal, leaky and torturous vessels of tumors is the hallmark of pathological angiogenesis (Goel et al., 2011).

A growing trend in medicine, especially with regard to medical oncology, is targeted therapy, which is often combined with other cancer treatments in patients with advanced disease [1–3]. While patients respond fairly well initially, in most cancers sustained treatment typically results in the failure of response to treatment (typically referred to as drug resistance) with a poorer prognosis. Furthermore, challenges and complexities regarding toxicity when combining these therapies and how to personalize medicine – selecting the right patient population that can benefit from therapies and not develop drug resistance – remain a concern.

Understanding How Cancer Cells Become Refractory to Drug Treatment

Although today we have a much greater understanding of cancer biology and genetics, one of the main reasons for our failure to overcome so-called drug resistance in cancer may relate to the difficulty of how we perceive the phenomenon (see Box 1 for the various mechanisms by which organisms can evade drug treatment; for the sake of simplicity here, we refer to it as drug resistance). Is the phenomenon driven solely by genetic mechanisms (i.e., is it irreversible and deterministic)? Alternatively, are other, non-genetic mechanisms, such as stochastic phenotypic switching or epigenetic factors that promote intrinsic diversity and phenotypic plasticity, involved? If so, can this information help in guiding treatment decisions that are currently based solely on genetic biomarkers? Unfortunately, it appears that the phenomenon is much more nuanced [4]. The water is muddied further by seemingly conflicting reports in the literature ([5] see also the examples discussed below) and the erroneous assumption that drug resistance, tolerance, and persistence are synonymous or equivalent, albeit inadvertently (Box 1). Here we consider two recent studies as a case in point; while one emphasizes the genetic underpinning of drug resistance, the other points to a non-genetic mechanism in the same cancer type. Using these two cases as recent examples, we stress the need to recognize the genetic/non-genetic duality of drug resistance in cancer and discuss how ecological and evolutionary principles may help to reconcile the duality and may even offer new strategies for treatment and prevention o drug resistance. However, the reader is referred to several excellent reviews for an in-depth discussion [6–8] and recent reports underscoring non-genetic mechanisms underlying drug resistance in cancer [9].