Cellular Mechanisms Underlying Intertumoral Heterogeneity

Precision Medicine in Veterinary Science

Precision medicine focuses on the clinical management of the individual patient, not on population-based findings. Successes from human precision medicine inform veterinary oncology. Early evidence of success for canines shows how precision medicine can be integrated into practice. Decreasing genomic profiling costs will allow increased utilization and subsequent improvement of knowledge base from which to make better informed decisions. Utility of precision medicine in canine oncology will only increase for improved cancer characterization, enhanced therapy selection, and overall more successful management of canine cancer. As such, practitioners are called to interpret and leverage precision medicine reports for their patients.

Diversity of Intercellular Communication Modes: A Cancer Biology Perspective

From the moment a cell is on the path to malignant transformation, its interaction with other cells from the microenvironment becomes altered. The flow of molecular information is at the heart of the cellular and systemic fate in tumors, and various processes participate in conveying key molecular information from or to certain cancer cells. For instance, the loss of tight junction molecules is part of the signal sent to cancer cells so that they are no longer bound to the primary tumors and are thus free to travel and metastasize. Upon the targeting of a single cell by a therapeutic drug, gap junctions are able to communicate death information to by-standing cells. The discovery of the importance of novel modes of cell-cell communication such as different types of extracellular vesicles or tunneling nanotubes is changing the way scientists look at these processes. However, are they all actively involved in different contexts at the same time or are they recruited to fulfill specific tasks? What does the multiplicity of modes mean for the overall progression of the disease? Here, we extend an open invitation to think about the overall significance of these questions, rather than engage in an elusive attempt at a systematic repertory of the mechanisms at play.

Tissue biomarkers of immune checkpoint inhibitor therapy

Cancer immunotherapy has been rejuvenated by the growing understanding of the immune system's role in tumor activity over the past two decades. During cancer initiation and progression, tumor cells employ various mechanisms that resemble peripheral immune tolerance to evade the antitumor responses of the immune system. Immune checkpoint molecules are the major mechanism of immune resistance that are exploited by tumor cells to inhibit T-cell activation and suppress immune responses. The targeting of immune checkpoint pathways has led to substantial improvements in survival rates in a number of solid cancers. However, a lack of understanding of the heterogeneity of the tumor microenvironment (TME) has resulted in inefficient therapy responses. A greater understanding of the TME is needed to identify patients likely to respond, and those that will have resistance to immune checkpoint inhibitors (ICIs). Advancement in spatial single-cell technologies has allowed deeper insight into the phenotypic and functional diversities of cells in the TME. In this review, we provide an overview of ICI biomarkers and highlight how high-dimensional spatially resolved, single-cell approaches provide deep molecular insights into the TME and allow for the discovery of biomarkers of clinical benefit.

Monitoring the Cell Cycle of Tumor Cells in Mouse Models of Human Cancer

Cell division is obligatory to tumor growth. However, both cancer cells and noncancer cells in tumors can be found in distinct stages of the cell cycle, which may inform the growth potential of these tumors, their propensity to metastasize, and their response to therapy. Hence, it is of utmost importance to monitor the cell cycle of tumor cells. Here we discuss well-established methods and new genetic advances to track the cell cycle of tumor cells in mouse models of human cancer. We also review recent genetic studies investigating the role of the cell-cycle machinery in the growth of tumors in vivo, with a focus on the machinery regulating the G1/S transition of the cell cycle.

Regulation of Metabolic Plasticity in Cancer Stem Cells and Implications in Cancer Therapy

Cancer stem cells (CSCs), a subpopulation of tumor cells with self-renewal capacity, have been associated with tumor initiation, progression, and therapy resistance. While the bulk of tumor cells mainly use glycolysis for energy production, CSCs have gained attention for their ability to switch between glycolysis and oxidative phosphorylation, depending on their energy needs and stimuli from their microenvironment. This metabolic plasticity is mediated by signaling pathways that are also implicated in the regulation of CSC properties, such as the Wnt/β-catenin, Notch, and Hippo networks. Two other stemness-associated processes, autophagy and hypoxia, seem to play a role in the metabolic switching of CSCs as well. Importantly, accumulating evidence has linked the metabolic plasticity of CSCs to their increased resistance to treatment. In this review, we summarize the metabolic signatures of CSCs and the pathways that regulate them; we especially highlight research data that demonstrate the metabolic adaptability of these cells and their role in stemness and therapy resistance. As the development of drug resistance is a major challenge for successful cancer treatment, the potential of specific elimination of CSCs through targeting their metabolism is of great interest and it is particularly examined.

Tumoral heterogeneity in neuroblastoma

Neuroblastoma is a solid, neuroendocrine tumor with divergent clinical behavior ranging from asymptomatic to fatal. The diverse clinical presentations of neuroblastoma are directly linked to the high intra- and inter-tumoral heterogeneity it presents. This heterogeneity is strongly associated with therapeutic resistance and continuous relapses, often leading to fatal outcomes. The development of successful risk assessment and tailored treatment strategies lies in evaluating the extent of heterogeneity via the accurate genetic and epigenetic profiling of distinct cell subpopulations present in the tumor. Recent studies have focused on understanding the molecular mechanisms that drive tumoral heterogeneity in pursuing better therapeutic and diagnostic approaches. This review describes the cellular, genetic, and epigenetic aspects of neuroblastoma heterogeneity. In addition, we summarize the recent findings on three crucial factors that can lead to heterogeneity in solid tumors: the inherent diversity of the progenitor cells, the presence of cancer stem cells, and the influence of the tumor microenvironment.

Blockade of novel immune checkpoints and new therapeutic combinations to boost antitumor immunity

Immunotherapy has emerged as a promising strategy for boosting antitumoral immunity. Blockade of immune checkpoints (ICs), which regulate the activity of cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells has proven clinical benefits. Antibodies targeting CTLA-4, PD-1, and PD-L1 are IC-blockade drugs approved for the treatment of various solid and hematological malignancies. However, a large subset of patients does not respond to current anti-IC immunotherapy. An integrative understanding of tumor-immune infiltrate, and IC expression and function in immune cell populations is fundamental to the design of effective therapies. The simultaneous blockade of newly identified ICs, as well as of previously described ICs, could improve antitumor response. We review the potential for novel combinatory blockade strategies as antitumoral therapy, and their effects on immune cells expressing the targeted ICs. Preclinical evidence and clinical trials involving the blockade of the various ICs are reported. We finally discuss the rationale of IC co-blockade strategy with respect to its downstream signaling in order to improve effective antitumoral immunity and prevent an increased risk of immune-related adverse events (irAEs).

Cancer Immunotherapies: From Efficacy to Resistance Mechanisms - Not Only Checkpoint Matters

The immunotherapeutic treatment of various cancers with an increasing number of immune checkpoint inhibitors (ICIs) has profoundly improved the clinical management of advanced diseases. However, just a fraction of patients clinically responds to and benefits from the mentioned therapies; a large proportion of patients do not respond or quickly become resistant, and hyper- and pseudoprogression occur in certain patient populations. Furthermore, no effective predictive factors have been clearly screened or defined. In this review, we discuss factors underlying the elucidation of potential immunotherapeutic resistance mechanisms and the identification of predictive factors for immunotherapeutic responses. Considering the heterogeneity of tumours and the complex immune microenvironment (composition of various immune cell subtypes, disease processes, and lines of treatment), checkpoint expression levels may not be the only factors underlying immunotherapy difficulty and resistance. Researchers should consider the tumour microenvironment (TME) landscape in greater depth from the aspect of not only immune cells but also the tumour histology, molecular subtype, clonal heterogeneity and evolution as well as micro-changes in the fine structural features of the tumour area, such as myeloid cell polarization, fibroblast clusters and tertiary lymphoid structure formation. A comprehensive analysis of the immune and molecular profiles of tumour lesions is needed to determine the potential predictive value of the immune landscape on immunotherapeutic responses, and precision medicine has become more important.

Exosomes and Cancer Stem Cells in Cancer Immunity: Current Reports and Future Directions

Cancer stem cells (CSCs), which have the capacity to self-renew and differentiate into various types of cells, are notorious for their roles in tumor initiation, metastasis, and therapy resistance. Thus, underlying mechanisms for their survival provide key insights into developing effective therapeutic strategies. A more recent focus has been on exosomes that play a role in transmitting information between CSCs and non-CSCs, resulting in activating CSCs for cancer progression and modulating their surrounding microenvironment. The field of CSC-derived exosomes (CSCEXs) for different types of cancer is still under exploration. A deeper understanding and further investigation into CSCEXs’ roles in tumorigenicity and the identification of novel exosomal components are necessary for engineering exosomes for the treatment of cancer. Here, we review the features of CSCEXs, including surface markers, cargo, and biological or physiological functions. Further, reports on the immunomodulatory effects of CSCEXs are summarized, and exosome engineering for CSC-targeting is also discussed.

Stem cell quiescence: the challenging path to activation

Quiescence is a cellular state in which a cell remains out of the cell cycle but retains the capacity to divide. The unique ability of adult stem cells to maintain quiescence is crucial for life-long tissue homeostasis and regenerative capacity. Quiescence has long been viewed as an inactive state but recent studies have shown that it is in fact an actively regulated process and that adult stem cells are highly reactive to extrinsic stimuli. This has fuelled hopes of boosting the reactivation potential of adult stem cells to improve tissue function during ageing. In this Review, we provide a perspective of the quiescent state and discuss how quiescent adult stem cells transition into the cell cycle. We also discuss current challenges in the field, highlighting recent technical advances that could help overcome some of these challenges.

HLA loss of heterozygosity-mediated discordant responses to immune checkpoint blockade in squamous cell lung cancer with renal metastasis

Despite the significant success of immune checkpoint blockade therapy in advanced non-small-cell lung cancer compared with chemotherapy, efficacy varies greatly across patients, and acquired resistance frequently occurs. In particular, during immunotherapy, the dynamic changes in molecular events have not been characterized. The authors report a case of squamous cell lung carcinoma with renal metastasis, treated with pembrolizumab, in which the primary tumor and rare renal metastases showed different responses. Using whole-exome sequencing, the authors found loss of heterogeneity in HLA genes in all tumors and high levels of intratumor heterogeneity in metastases. The increased levels of HLA loss led to therapy resistance during tumor evolution. In addition to tumor mutational burden and PD-L1, HLA loss of heterozygosity and intratumor heterogeneity should be taken into consideration during immunotherapy.

Cholangiocarcinoma 2020: the next horizon in mechanisms and management

Cholangiocarcinoma (CCA) includes a cluster of highly heterogeneous biliary malignant tumours that can arise at any point of the biliary tree. Their incidence is increasing globally, currently accounting for ~15% of all primary liver cancers and ~3% of gastrointestinal malignancies. The silent presentation of these tumours combined with their highly aggressive nature and refractoriness to chemotherapy contribute to their alarming mortality, representing ~2% of all cancer-related deaths worldwide yearly. The current diagnosis of CCA by non-invasive approaches is not accurate enough, and histological confirmation is necessary. Furthermore, the high heterogeneity of CCAs at the genomic, epigenetic and molecular levels severely compromises the efficacy of the available therapies. In the past decade, increasing efforts have been made to understand the complexity of these tumours and to develop new diagnostic tools and therapies that might help to improve patient outcomes. In this expert Consensus Statement, which is endorsed by the European Network for the Study of Cholangiocarcinoma, we aim to summarize and critically discuss the latest advances in CCA, mostly focusing on classification, cells of origin, genetic and epigenetic abnormalities, molecular alterations, biomarker discovery and treatments. Furthermore, the horizon of CCA for the next decade from 2020 onwards is highlighted.

Chromosome Instability; Implications in Cancer Development, Progression, and Clinical Outcomes

Chromosome instability (CIN) refers to an ongoing rate of chromosomal changes and is a driver of genetic, cell-to-cell heterogeneity. It is an aberrant phenotype that is intimately associated with cancer development and progression. The presence, extent, and level of CIN has tremendous implications for the clinical management and outcomes of those living with cancer. Despite its relevance in cancer, there is still extensive misuse of the term CIN, and this has adversely impacted our ability to identify and characterize the molecular determinants of CIN. Though several decades of genetic research have provided insight into CIN, the molecular determinants remain largely unknown, which severely limits its clinical potential. In this review, we provide a definition of CIN, describe the two main types, and discuss how it differs from aneuploidy. We subsequently detail its impact on cancer development and progression, and describe how it influences metastatic potential with reference to cancer prognosis and outcomes. Finally, we end with a discussion of how CIN induces genetic heterogeneity to influence the use and efficacy of several precision medicine strategies, including patient and risk stratification, as well as its impact on the acquisition of drug resistance and disease recurrence.

Perturbation-Driven Entropy as a Source of Cancer Cell Heterogeneity

Intratumor heterogeneity is a key hallmark of cancer that contributes to progression and therapeutic resistance. Phenotypic heterogeneity is in part caused by Darwinian selection of subclones that arise by random (epi)genetic aberrations. In addition, cancer cells are endowed with increased cellular plasticity compared with their normal counterparts, further adding to their heterogeneous behavior. However, the molecular mechanisms underpinning cancer cell plasticity are incompletely understood. Here, I outline the hypothesis that cancer-associated perturbations collectively disrupt normal gene regulatory networks (GRNs) by increasing their entropy. Importantly, in this model both somatic driver and passenger alterations contribute to 'perturbation-driven entropy', thereby increasing phenotypic heterogeneity and evolvability. This additional layer of heterogeneity may contribute to our understanding of cancer evolution and therapeutic resistance.

Both fallopian tube and ovarian surface epithelium are cells-of-origin for high-grade serous ovarian carcinoma

The cell-of-origin of high grade serous ovarian carcinoma (HGSOC) remains controversial, with fallopian tube epithelium (FTE) and ovarian surface epithelium (OSE) both considered candidates. Here, by using genetically engineered mouse models and organoids, we assessed the tumor-forming properties of FTE and OSE harboring the same oncogenic abnormalities. Combined RB family inactivation and Tp53 mutation in Pax8 + FTE caused Serous Tubal Intraepithelial Carcinoma (STIC), which metastasized rapidly to the ovarian surface. These events were recapitulated by orthotopic injection of mutant FTE organoids. Engineering the same genetic lesions into Lgr5 + OSE or OSE-derived organoids also caused metastatic HGSOC, although with longer latency and lower penetrance. FTE- and OSE-derived tumors had distinct transcriptomes, and comparative transcriptomics and genomics suggest that human HGSOC arises from both cell types. Finally, FTE- and OSE-derived organoids exhibited differential chemosensitivity. Our results comport with a dualistic origin for HGSOC and suggest that the cell-of-origin might influence therapeutic response.

Advanced modalities of molecular imaging in precision medicine for musculoskeletal malignancies

Musculoskeletal malignancies consist of a heterogenous group of mesenchymal tumors, often with high inter- and intratumoral heterogeneity. The early and accurate diagnosis of these malignancies can have a substantial impact on optimal treatment and quality of life for these patients. Several new applications and techniques have emerged in molecular imaging, including advances in multimodality imaging, the development of novel radiotracers, and advances in image analysis with radiomics and artificial intelligence. This review highlights the recent advances in molecular imaging modalities and the role of non-invasive imaging in evaluating tumor biology in the era of precision medicine.

Prostate Stem Cells and Cancer Stem Cells

Stem/progenitor cells play central roles in processes of organogenesis and tissue maintenance, whereas cancer stem cells (CSCs) are thought to drive tumor malignancy. Here, we review recent progress in the identification and analysis of normal prostate stem/progenitor cells as well as putative CSCs in both genetically engineered mouse models as well as in human tissue. We also discuss studies that have investigated the cell type of origin for prostate cancer. In addition, we provide a critical assessment of methodologies used in stem cell analyses and outline directions for future research.

Experimental models to unravel the molecular pathogenesis, cell of origin and stem cell properties of cholangiocarcinoma

Human cholangiocarcinoma (CCA) is an aggressive tumour entity arising from the biliary tree, whose molecular pathogenesis remains largely undeciphered. Over the last decade, the advent of high-throughput and cell-based techniques has significantly increased our knowledge on the molecular mechanisms underlying this disease while, at the same time, unravelling CCA complexity. In particular, it becomes clear that CCA displays pronounced inter- and intratumoural heterogeneity, which is presumably the consequence of the interplay between distinct tissues and cells of origin, the underlying diseases, and the associated molecular alterations. To better characterize these events and to design novel and more effective therapeutic strategies, a number of CCA experimental and preclinical models have been developed and are currently generated. This review summarizes the current knowledge and understanding of these models, critically underlining their translational usefulness and limitations. Furthermore, this review aims to provide a comprehensive overview on cells of origin, cancers stem cells and their dynamic interplay within CCA tissue.

Is the Response of Tumours Dependent on the Dietary Input of Some Amino Acids or Ratios among Essential and Non-Essential Amino Acids? All That Glitters Is Not Gold

Energy production is the main task of the cancer cell metabolism because the costs of duplicating are enormous. Although energy is derived in cells by dismantling the carbon-to-carbon bonds of any macronutrient, cancer nutritional needs for energetic purposes have been studied primarily as being dependent on glycolysis. Since the end of the last century, the awareness of the dependence of cancer metabolism on amino acids not only for protein synthesis but also to match energy needs has grown. The roles of specific amino acids such as glutamine, glycine and serine have been explored in different experimental conditions and reviewed. Moreover, epidemiological evidence has revealed that some amino acids used as a supplement for therapeutic reasons, particularly the branched-chain ones, may reduce the incidence of liver cancer and a specific molecular mechanism has been proposed as functional to their protective action. By contrast and puzzling clinicians, the metabolomic signature of some pathologies connected to an increased risk of cancer, such as prolonged hyperinsulinemia in insulin-resistant patients, is identified by elevated plasma levels of the same branched-chain amino acids. Most recently, certain formulations of amino acids, deeply different from the amino acid compositions normally present in foods, have shown the power to master cancer cells epigenetically, slowing growth or driving cancer cells to apoptotic death, while being both beneficial for normal cell function and the animal’s health and lifespan. In this review, we will analyze and try to disentangle some of the many knots dealing with the complexities of amino acid biology and links to cancer metabolism.

Opportunities and Challenges in Implementation of Multiparameter Single Cell Analysis Platforms for Clinical Translation

The high-content interrogation of single cells with platforms optimized for the multiparameter characterization of cells in liquid and solid biopsy samples can enable characterization of heterogeneous populations of cells ex vivo. Doing so will advance the diagnosis, prognosis, and treatment of cancer and other diseases. However, it is important to understand the unique issues in resolving heterogeneity and variability at the single cell level before navigating the validation and regulatory requirements in order for these technologies to impact patient care. Since 2013, leading experts representing industry, academia, and government have been brought together as part of the Foundation for the National Institutes of Health (FNIH) Biomarkers Consortium to foster the potential of high-content data integration for clinical translation.

Combining Cell Type-Restricted Adenoviral Targeting with Immunostaining and Flow Cytometry to Identify Cells-of-Origin of Lung Cancer

Lung cancers display considerable intertumoral heterogeneity, leading to the classification of distinct tumor subtypes. Our understanding of the genetic aberrations that underlie tumor subtypes has been greatly enhanced by recent genomic sequencing studies and state-of-the-art gene targeting technologies, highlighting evidence that distinct lung cancer subtypes may be derived from different "cells-of-origin". Here, we describe the intra-tracheal delivery of cell type-restricted Ad5-Cre viruses into the lungs of adult mice, combined with immunohistochemical and flow cytometry strategies for the detection of lung cancer-initiating cells in vivo.

Glioblastoma Cell Malignancy and Drug Sensitivity Are Affected by the Cell of Origin

The identity of the glioblastoma (GBM) cell of origin and its contributions to disease progression and treatment response remain largely unknown. We have analyzed how the phenotypic state of the initially transformed cell affects mouse GBM development and essential GBM cell (GC) properties. We find that GBM induced in neural stem-cell-like glial fibrillary acidic protein (GFAP)-expressing cells in the subventricular zone of adult mice shows accelerated tumor development and produces more malignant GCs (mGC1_GFAP) that are less resistant to cancer drugs, compared with those originating from more differentiated nestin- (mGC2_NES) or 2,'3'-cyclic nucleotide 3'-phosphodiesterase (mGC3_CNP)-expressing cells. Transcriptome analysis of mouse GCs identified a 196 mouse cell origin (MCO) gene signature that was used to partition 61 patient-derived GC lines. Human GC lines that clustered with the mGC1_GFAP cells were also significantly more self-renewing, tumorigenic, and sensitive to cancer drugs compared with those that clustered with mouse GCs of more differentiated origin.

Homeostasis Back and Forth: An Ecoevolutionary Perspective of Cancer

The role of genetic mutations in cancer is indisputable: They are a key source of tumor heterogeneity and drive its evolution to malignancy. But, the success of these new mutant cells relies on their ability to disrupt the homeostasis that characterizes healthy tissues. Mutated clones unable to break free from intrinsic and extrinsic homeostatic controls will fail to establish a tumor. Here, we will discuss, through the lens of mathematical and computational modeling, why an evolutionary view of cancer needs to be complemented by an ecological perspective to understand why cancer cells invade and subsequently transform their environment during progression. Importantly, this ecological perspective needs to account for tissue homeostasis in the organs that tumors invade, because they perturb the normal regulatory dynamics of these tissues, often coopting them for its own gain. Furthermore, given our current lack of success in treating advanced metastatic cancers through tumor-centric therapeutic strategies, we propose that treatments that aim to restore homeostasis could become a promising venue of clinical research. This ecoevolutionary view of cancer requires mechanistic mathematical models to both integrate clinical with biological data from different scales but also to detangle the dynamic feedback between the tumor and its environment. Importantly, for these models to be useful, they need to embrace a higher degree of complexity than many mathematical modelers are traditionally comfortable with.

Hierarchical tissue organization as a general mechanism to limit the accumulation of somatic mutations

How can tissues generate large numbers of cells, yet keep the divisional load (the number of divisions along cell lineages) low in order to curtail the accumulation of somatic mutations and reduce the risk of cancer? To answer the question we consider a general model of hierarchically organized self-renewing tissues and show that the lifetime divisional load of such a tissue is independent of the details of the cell differentiation processes, and depends only on two structural and two dynamical parameters. Our results demonstrate that a strict analytical relationship exists between two seemingly disparate characteristics of self-renewing tissues: divisional load and tissue organization. Most remarkably, we find that a sufficient number of progressively slower dividing cell types can be almost as efficient in minimizing the divisional load, as non-renewing tissues. We argue that one of the main functions of tissue-specific stem cells and differentiation hierarchies is the prevention of cancer.

To limit the accumulation of somatic mutations, renewing tissues must minimize the number of times each cell divides during differentiation. Here, the authors analytically derive the lower limit of lifetime divisional load of a tissue, show that hierarchically differentiating tissues can approach this limit, and that this depends on uneven divisional rates across the hierarchy.

Aldh1 Expression and Activity Increase During Tumor Evolution in Sarcoma Cancer Stem Cell Populations

Tumors evolve from initial tumorigenic events into increasingly aggressive behaviors in a process usually driven by subpopulations of cancer stem cells (CSCs). Mesenchymal stromal/stem cells (MSCs) may act as the cell-of-origin for sarcomas, and CSCs that present MSC features have been identified in sarcomas due to their ability to grow as self-renewed floating spheres (tumorspheres). Accordingly, we previously developed sarcoma models using human MSCs transformed with relevant oncogenic events. To study the evolution/emergence of CSC subpopulations during tumor progression, we compared the tumorigenic properties of bulk adherent cultures and tumorsphere-forming subpopulations both in the sarcoma cell-of-origin models (transformed MSCs) and in their corresponding tumor xenograft-derived cells. Tumor formation assays showed that the tumorsphere cultures from xenograft-derived cells, but not from the cell-of-origin models, were enriched in CSCs, providing evidence of the emergence of bona fide CSCs subpopulations during tumor progression. Relevant CSC-related factors, such as ALDH1 and SOX2, were increasingly upregulated in CSCs during tumor progression, and importantly, the increased levels and activity of ALDH1 in these subpopulations were associated with enhanced tumorigenicity. In addition to being a CSC marker, our findings indicate that ALDH1 could also be useful for tracking the malignant potential of CSC subpopulations during sarcoma evolution.

Intravital Insights into Heterogeneity, Metastasis, and Therapy Responses

Tumor progression is driven by a series of genetic and microenvironmental changes. These events lead to heterogeneous tumors which consist of a variety of cells from which some cells may possess properties which promote survival after therapy and metastasis. Recent advances in intravital microscopy (IVM) have enabled visualization of this tumor heterogeneity over time at a single-cell resolution. We highlight here the latest IVM studies that have revealed the dynamic interactions between the tumor cells and their local microenvironment. We review the most recent data that exposes how these dynamic interactions cause an additional increase in tumor heterogeneity, resulting in multiple metastatic strategies and facilitating therapy resistance.