Review Articles in 2021

Circular RNAs (circRNAs) are a novel class of primarily non-coding RNAs with increasingly recognized roles in cancer development and progression through diverse mechanisms of action. Herein, the authors review the current understanding of circRNA biogenesis, regulation, physiological functions and pathophysiological roles in cancer. They also discuss the clinical potential of circRNAs as biomarkers, therapeutic agents and drug targets in oncology as well as research controversies, technical issues and biological knowledge gaps that need to be addressed before this promise can be realized.

Patients receiving cytotoxic therapies for cancer have an increased risk of cognitive and functional decline that is usually associated with ageing. In this Review, the authors describe how cancer therapies can enhance physiological ageing processes and highlight the potential for interventions that could potentially ameliorate these long-term adverse events in patients receiving active treatment for cancer and in cancer survivors.

Targeted therapies have improved the outcomes of many patients with cancer, although many more lack targetable alterations or do not derive clinical benefit for other reasons. Radiotherapy can also provide benefit to many patients, although radioresistance often limits the effectiveness of this intervention. Here, the authors describe the potential for radiotherapy to promote non-oncogene dependence on targetable signalling pathways, thus extending the benefits of both targeted therapy and radiotherapy to greater numbers of patients.

Immunotherapy is revolutionizing the treatment of many cancers and hepatocellular carcinoma (HCC) is no exception. This Review describes the heterogeneous immune microenvironments of HCC as well as their links with the various aetiologies underlying this malignancy and with response or resistance to immunotherapies. In addition, the authors provide an overview of the current landscape of clinical trials evaluating immunotherapies across all stages of HCC.

In the past few years, advances in omics technologies have led to a better understanding of the heterogeneity of triple-negative breast cancers (TNBCs) and their microenvironment, supporting a view of this breast cancer subtype as an ecosystem that encompasses the intrinsic and extrinsic features of cancer cells. The authors of this Review describe the current and upcoming therapeutic landscape of TNBC and discuss how an integrated view of the TNBC ecosystem can provide improved opportunities for tailoring treatment.

A high tumour burden has long been associated with inferior outcomes on traditional cancer therapies and emerging evidence suggests that tumour burden is particularly relevant for patients receiving immune-checkpoint inhibitors. Here, the authors summarize the available clinical and preclinical evidence for the role of tumour burden in determining the outcomes of patients receiving immune-checkpoint inhibitors and highlight areas that are likely to be of future research interest in this emerging area.

Immune-checkpoint inhibitors have dramatically improved the outcomes in patients with advanced-stage cancers, although the majority of patients will not respond to these agents. Here, the authors describe the potential of targeting emerging immunomodulatory pathways, with a focus on alternative immune checkpoints and tumour metabolism as approaches that might enable further improvements in the outcomes of patients with cancer, either as monotherapies or in combination with existing agents.

EGFR exon 19 deletions and exon 21 mutations, and HER2 amplification and/or overexpression, are predictive of response to matched molecularly targeted therapies that have greatly improved patient outcomes. However, insertion mutations in exon 20 of either EGFR or HER2 generally do not confer sensitivity to these therapies. In this Review, the authors discuss the prevalence of EGFR and HER2 exon 20 insertions across cancers, their biology and detection, and associated responses to current molecularly targeted therapies and immunotherapies. In addition, they focus on new therapeutic strategies that are being developed to target tumours driven by these non-classic EGFR and HER2 alterations.

The MYC proto-oncogenes are among the most commonly activated proteins in human cancer, yet the clinical efficacy of MYC-targeted agents remains to be demonstrated. The authors of this Review describe how activation of the MYC pathway affects cancer cells as well as the tumour microenvironment and propose strategies for the therapeutic targeting of MYC-driven cancers.

Fluorescence-guided surgery (FGS) using tumour-targeted imaging agents has emerged over the past decade as a method of intraoperative cancer detection; however, the clinical implementation of tumour-targeted FGS remains in the early stages. The authors of this Review discuss how target selection, imaging agents and detection systems could enable real-time intraoperative visualization to benefit patients with cancer.

Cancer-associated fibroblasts (CAFs) are inherently linked with cancers and have long been considered attractive therapeutic targets. However, the existence of several CAF subpopulations with substantial phenotypic and functional heterogeneity and plasticity is increasingly recognized. This Review provides an overview of the heterogeneity of CAFs and its implications with regard to the tumour-promoting and tumour-restraining roles of these cells as well as their clinical relevance in terms of prognostic value and therapeutic potential. The authors also provide insights and perspectives on future research and clinical studies involving CAFs.

Hypoxia is a common feature of tumours, contributes to many of the hallmarks of cancer and influences responses to anticancer therapies. Thus, strategies to eliminate and/or exploit tumour hypoxia have long been explored, although with limited success to date. Herein, the authors describe new insights into hypoxia biology, discuss the implications of these advances for novel hypoxia-directed therapeutic strategies, and review the progress made with longstanding methods for targeting hypoxic tumours.

Poly(ADP-ribose) polymerase (PARP) inhibitors are approved for patients with several forms of cancer, predominantly those harbouring loss-of-function BRCA1/2 mutations or other homologous recombination defects. Nonetheless, most patients receiving PARP inhibitors will ultimately develop resistance to PARP inhibitors, resulting in disease progression. In this Review, the authors describe the mechanisms of resistance to PARP inhibitors and discuss the potential treatment strategies that might overcome these effects.

The blood–brain barrier regulates the movement of various substances between the blood and the brain and therefore has a crucial role in ensuring normal brain function. In both primary brain tumours and brain metastases, the blood–brain barrier is modified to the blood–tumour barrier (BTB), resulting in altered permeability; however, the BTB continues to restrict the penetration of many therapeutic agents into intracranial tumours. Here, Patricia Steeg describes the current knowledge of BTB structure and function and discusses how this knowledge can be translated into improvements in cancer therapy and patient outcomes.

Chimeric antigen receptor (CAR) T cell therapies are generating substantial excitement and have been approved for the treatment of various haematological malignancies. All approved CARs consist of an extracellular antigen-binding domain linked to an intracellular region containing a costimulatory domain and a T cell activation domain. A key question is whether the CD28-derived and 4-1BB-derived costimulatory domains used in current commercial CAR T cell products are associated with different cellular and clinical effects. Herein, Cappell and Kochenderfer provide an overview of CD28 and 4-1BB costimulatory pathways and compare the outcomes observed in preclinical and clinical studies with CARs incorporating either costimulatory domain.

Nasopharyngeal carcinoma (NPC) is an Epstein–Barr virus (EBV)-associated malignancy endemic to southern China, southeast Asia and north Africa. The authors of this Review present a comprehensive overview of advances from the past three decades on the pathogenic role of EBV, and the genomic, epigenomic and immune landscape of NPC, which have led to the development of new biomarkers, therapeutic targets and improved treatment approaches for patients with NPC.

Immune-checkpoint inhibitors (ICIs) are now standard-of-care therapies for patients with advanced-stage non-small-cell lung cancer (NSCLC) without a targetable driver alteration. Various ICIs or combination regimens have been approved in this setting, relative to chemotherapy, although no prospective data are available comparing the various ICI-based approaches. Here, the authors provide guidance on selecting the optimal ICI-based therapy and highlight several future research directions that will probably further improve the outcomes of patients with advanced-stage NSCLC.

Hotspot genetic alterations that confer the enzymes isocitrate dehydrogenase (IDH) 1 and 2 with neomorphic activity to produce the oncometabolite D-2-hydroxyglutarate are common in several cancer types, including acute myeloid leukaemia, cholangiocarcinoma, chondrosarcoma and glioma. Herein, Pirozzi and Yan describe the current understanding of the biological, pathogenetic and prognostic implications of IDH mutations in these cancers. They also review the available preclinical and clinical data on the various therapeutic strategies that are being pursued for IDH-mutant cancers and discuss whether treatment approaches will converge or be context dependent.

Despite being the most common primary bone cancer in children and young adults, osteosarcoma is a rare cancer, a fact that has complicated efforts to improve patient outcomes. Moreover, the molecular biology of disease is highly heterogeneous and most of the recurrent genetic alterations occur in tumour-suppressor genes that are challenging therapeutic targets. Herein, Gill and Gorlick discuss the new biological discoveries, technologies, and therapeutic agents and approaches that, through collaborative efforts, are poised to generate advances in the treatment of osteosarcoma after more than four decades of stagnation.

Limited penetration into tumour tissue can restrict the activity of systemically delivered cancer immunotherapies, whereas exposure of various non-malignant tissues to high levels of such agents can lead to problematic toxicities. Intratumoural administration and/or biotechnology strategies for selective targeting of tumour tissues have the potential to circumvent these issues and thereby increase the therapeutic index. Herein, the authors review the historical origins and current landscape of intratumoural and tumour tissue-targeted immunotherapies.