Trial watch: Monoclonal antibodies in cancer therapy

Trial watch: DNA vaccines for cancer therapy

The foundation of modern vaccinology dates back to the 1790s, when the English physician Edward Jenner uncovered the tremendous medical potential of prophylactic vaccination. Jenner’s work ignited a wave of nationwide vaccination campaigns abating the incidence of multiple life-threatening infectious diseases and culminating with the eradication of natural smallpox virus, which was definitively certified by the WHO in 1980. The possibility of using vaccines against cancer was first proposed at the end of the 19th century by Paul Ehrlich and William Coley. However, it was not until the 1990s that such a hypothesis began to be intensively investigated, following the realization that the immune system is not completely unresponsive to tumors and that neoplastic cells express immunogenic tumor-associated antigens (TAAs). Nowadays, anticancer vaccines are rapidly moving from the bench to the bedside, and a few prophylactic and therapeutic preparations have already been approved by FDA for use in humans. In this setting, one interesting approach is constituted by DNA vaccines, i.e., TAA-encoding circularized DNA constructs, often of bacterial origin, that are delivered to patients as such or by means of specific vectors, including (but not limited to) liposomal preparations, nanoparticles, bacteria and viruses. The administration of DNA vaccines is most often performed via the intramuscular or subcutaneous route and is expected to cause (1) the endogenous synthesis of the TAA by myocytes and/or resident antigen-presenting cells; (2) the presentation of TAA-derived peptides on the cell surface, in association with MHC class I molecules; and (3) the activation of potentially therapeutic tumor-specific immune responses. In this Trial Watch, we will summarize the results of recent clinical trials that have evaluated/are evaluating DNA vaccines as therapeutic interventions against cancer.

Trial watch: Chemotherapy with immunogenic cell death inducers

It is now clear that the immune system plays a critical role not only during oncogenesis and tumor progression, but also as established neoplastic lesions respond to therapy. Selected cytotoxic chemicals can indeed elicit immunogenic cell death, a functionally peculiar type of apoptosis that stimulates tumor-specific cognate immune responses. Such immunogenic chemotherapeutics include cyclophosphamide, doxorubicin and oxaliplatin (which are approved by FDA for the treatment of various hematological and solid malignancies), mitoxantrone (which is currently employed both as an anticancer agent and against multiple sclerosis) and patupilone (a microtubular poison in clinical development). One year ago, in the second issue of OncoImmunology, we discussed the scientific rationale behind immunogenic chemotherapy and reviewed the status of recent clinical trials investigating the off-label use of cyclophosphamide, doxorubicin, oxaliplatin and mitoxantrone in cancer patients. Here, we summarize the latest developments in this area of clinical research, covering both high-impact studies that have been published during the last 13 months and clinical trials that have been initiated in the same period to assess the antineoplastic profile of immunogenic chemotherapeutics.

Drugs derived from phage display: from candidate identification to clinical practice

Phage display, one of today’s fundamental drug discovery technologies, allows identification of a broad range of biological drugs, including peptides, antibodies and other proteins, with the ability to tailor critical characteristics such as potency, specificity and cross-species binding. Further, unlike in vivo technologies, generating phage display-derived antibodies is not restricted by immunological tolerance. Although more than 20 phage display-derived antibody and peptides are currently in late-stage clinical trials or approved, there is little literature addressing the specific challenges and successes in the clinical development of phage-derived drugs. This review uses case studies, from candidate identification through clinical development, to illustrate the utility of phage display as a drug discovery tool, and offers a perspective for future developments of phage display technology.

Tumor imaging and targeting potential of an Hsp70-derived 14-mer peptide

BACKGROUND

We have previously used a unique mouse monoclonal antibody cmHsp70.1 to demonstrate the selective presence of a membrane-bound form of Hsp70 (memHsp70) on a variety of leukemia cells and on single cell suspensions derived from solid tumors of different entities, but not on non-transformed cells or cells from corresponding 'healthy' tissue. This antibody can be used to image tumors in vivo and target them for antibody-dependent cellular cytotoxicity. Tumor-specific expression of memHsp70 therefore has the potential to be exploited for theranostic purposes. Given the advantages of peptides as imaging and targeting agents, this study assessed whether a 14-mer tumor penetrating peptide (TPP; TKDNNLLGRFELSG), the sequence of which is derived from the oligomerization domain of Hsp70 which is expressed on the cell surface of tumor cells, can also be used for targeting membrane Hsp70 positive (memHsp70+) tumor cells, in vitro.

METHODOLOGY/PRINCIPAL FINDINGS

The specificity of carboxy-fluorescein (CF-) labeled TPP (TPP) to Hsp70 was proven in an Hsp70 knockout mammary tumor cell system. TPP specifically binds to different memHsp70+ mouse and human tumor cell lines and is rapidly taken up via endosomes. Two to four-fold higher levels of CF-labeled TPP were detected in MCF7 (82% memHsp70+) and MDA-MB-231 (75% memHsp70+) cells compared to T47D cells (29% memHsp70+) that exhibit a lower Hsp70 membrane positivity. After 90 min incubation, TPP co-localized with mitochondrial membranes in memHsp70+ tumors. Although there was no evidence that any given vesicle population was specifically localized, fluorophore-labeled cmHsp70.1 antibody and TPP preferentially accumulated in the proximity of the adherent surface of cultured cells. These findings suggest a potential association between membrane Hsp70 expression and cytoskeletal elements that are involved in adherence, the establishment of intercellular synapses and/or membrane reorganization.

CONCLUSIONS/SIGNIFICANCE

This study demonstrates the specific binding and rapid internalization of TPP by tumor cells with a memHsp70+ phenotype. TPP might therefore have potential for targeting and imaging the large proportion of tumors (∼50%) that express memHsp70.

The immune microenvironment: a major player in human cancers

Cancer is a major public health issue and figures among the leading causes of death in the world. Cancer development is a long process, involving the mutation, amplification or deletion of genes and chromosomal rearrangements. The transformed cells change morphologically, enlarge, become invasive and finally detach from the primary tumor to metastasize in other organs through the blood and/or lymph. During this process, the tumor cells interact with their microenvironment, which is complex and composed of stromal and immune cells that penetrate the tumor site via blood vessels and lymphoid capillaries. All subsets of immune cells can be found in tumors, but their respective density, functionality and organization vary from one type of tumor to another. Whereas inflammatory cells play a protumoral role, there is a large body of evidence of effector memory T cells controlling tumor invasion and metastasis. Thus, high densities of memory Th1/CD8 cytotoxic T cells in the primary tumors correlate with good prognosis in most tumor types. Tertiary lymphoid structures, which contain mature dendritic cells (DC) in a T cell zone, proliferating B cells and follicular DC, are found in the tumor stroma and they correlate with intratumoral Th1/CD8 T cell and B cell infiltration. Eventually, tumors undergo genetic and epigenetic modifications that allow them to escape being controlled by the immune system. This comprehensive review describes the immune contexture of human primary and metastatic tumors, how it impacts on patient outcomes and how it could be used as a predictive biomarker and guide immunotherapies.

Trial Watch: Toll-like receptor agonists in oncological indications

Toll-like receptors (TLRs) are an evolutionarily conserved group of enzymatically inactive, single membrane-spanning proteins that recognize a wide panel of exogenous and endogenous danger signals. Besides constituting a crucial component of the innate immune response to bacterial and viral pathogens, TLRs appear to play a major role in anticancer immunosurveillance. In line with this notion, several natural and synthetic TLR ligands have been intensively investigated for their ability to boost tumor-targeting immune responses elicited by a variety of immunotherapeutic and chemotherapeutic interventions. Three of these agents are currently approved by the US Food and Drug Administration (FDA) or equivalent regulatory agencies for use in cancer patients: the so-called bacillus Calmette-Guérin, monophosphoryl lipid A, and imiquimod. However, the number of clinical trials testing the therapeutic potential of both FDA-approved and experimental TLR agonists in cancer patients is stably decreasing, suggesting that drug developers and oncologists are refocusing their interest on alternative immunostimulatory agents. Here, we summarize recent findings on the use of TLR agonists in cancer patients and discuss how the clinical evaluation of FDA-approved and experimental TLR ligands has evolved since the publication of our first Trial Watch dealing with this topic.

Chloroquine and hydroxychloroquine for cancer therapy

Macroautophagy (herein referred to as autophagy) is a highly conserved mechanism for the lysosomal degradation of cytoplasmic components. Autophagy is critical for the maintenance of intracellular homeostasis, both in baseline conditions and in the context of adaptive responses to stress. In line with this notion, defects in the autophagic machinery have been etiologically associated with various human disorders including infectious, inflammatory and neoplastic conditions. Once tumors are established, however, autophagy sustains the survival of malignant cells, hence representing an appealing target for the design of novel anticancer regimens. Accordingly, inhibitors of autophagy including chloroquine and hydroxychloroquine have been shown to mediate substantial antineoplastic effects in preclinical models, especially when combined with chemo- or radiotherapeutic interventions. The pharmacological profile of chloroquine and hydroxychloroquine, however, appear to involve mechanisms other than autophagy inhibition. Here, we discuss the dual role of autophagy in oncogenesis and tumor progression, and summarize the results or design of clinical studies recently completed or initiated to evaluate the therapeutic activity of chloroquine derivatives in cancer patients.

Anti-melanoma vaccines engineered to simultaneously modulate cytokine priming and silence PD-L1 characterized using ex vivo myeloid-derived suppressor cells as a readout of therapeutic efficacy

Efficacious antitumor vaccines strongly stimulate cancer-specific effector T cells and counteract the activity of tumor-infiltrating immunosuppressive cells. We hypothesised that combining cytokine expression with silencing programmed cell death ligand 1 (PD-L1) could potentiate anticancer immune responses of lentivector vaccines. Thus, we engineered a collection of lentivectors that simultaneously co-expressed an antigen, a PD-L1-silencing shRNA, and various T cell-polarising cytokines, including interferon γ (IFNγ), transforming growth factor β (TGFβ) or interleukins (IL12, IL15, IL23, IL17A, IL6, IL10, IL4). In a syngeneic B16F0 melanoma model and using tyrosinase related protein 1 (TRP1) as a vaccine antigen, we found that simultaneous delivery of IL12 and a PD-L1-silencing shRNA was the only combination that exhibited therapeutically relevant anti-melanoma activities. Mechanistically, we found that delivery of the PD-L1 silencing construct boosted T cell numbers, inhibited in vivo tumor growth and strongly cooperated with IL12 cytokine priming and antitumor activities. Finally, we tested the capacities of our vaccines to counteract tumor-infiltrating myeloid-derived suppressor cell (MDSC) activities ex vivo. Interestingly, the lentivector co-expressing IL12 and the PD-L1 silencing shRNA was the only one that counteracted MDSC suppressive activities, potentially underlying the observed anti-melanoma therapeutic benefit. We conclude that (1) evaluation of vaccines in healthy mice has no significant predictive value for the selection of anticancer treatments; (2) B16 cells expressing xenoantigens as a tumor model are of limited value; and (3) vaccines which inhibit the suppressive effect of MDSC on T cells in our ex vivo assay show promising and relevant antitumor activities.

Trial watch: Immunostimulatory cytokines in cancer therapy

Tumor-targeting immune responses provide a significant contribution to (when they do not entirely account for) the clinical activity of diverse antineoplastic regimens, encompassing not only a large panel of immunotherapeutic strategies but also conventional cytotoxic molecules, targeted anticancer agents and irradiation. In line with this notion, several approaches have been devised to elicit novel or boost existing anticancer immune responses, including the administration of immunomodulatory cytokines. Such a relatively unspecific intervention suffices to mediate clinical effects in (at least a subset of) patients bearing particularly immunogenic tumors, like melanoma and renal cell carcinoma. More often, however, immunostimulatory cytokines are administered to boost the immunogenic potential of other agents, including (but not limited to) immune checkpoint-blocking antibodies, anticancer vaccines, oncolytic viruses and immunogenic chemotherapeutics. Here, we summarize the latest advances in the clinical development of recombinant cytokines as an immunomodulatory intervention for cancer therapy.

Trial Watch:: Oncolytic viruses for cancer therapy

Oncolytic viruses are natural or genetically modified viral species that selectively infect and kill neoplastic cells. Such an innate or exogenously conferred specificity has generated considerable interest around the possibility to employ oncolytic viruses as highly targeted agents that would mediate cancer cell-autonomous anticancer effects. Accumulating evidence, however, suggests that the therapeutic potential of oncolytic virotherapy is not a simple consequence of the cytopathic effect, but strongly relies on the induction of an endogenous immune response against transformed cells. In line with this notion, superior anticancer effects are being observed when oncolytic viruses are engineered to express (or co-administered with) immunostimulatory molecules. Although multiple studies have shown that oncolytic viruses are well tolerated by cancer patients, the full-blown therapeutic potential of oncolytic virotherapy, especially when implemented in the absence of immunostimulatory interventions, remains unclear. Here, we cover the latest advances in this active area of translational investigation, summarizing high-impact studies that have been published during the last 12 months and discussing clinical trials that have been initiated in the same period to assess the therapeutic potential of oncolytic virotherapy in oncological indications.

Monoclonal Antibodies in Cancer Therapy: Mechanisms, Successes and Limitations

Rituximab was the first chemotherapeutic monoclonal antibody (CmAb) approved for clinical use in cancer therapeutics in 1997 and has significantly improved the clinical outcomes in non-Hodgkin's lymphoma. Since then, numerous CmAbs have been developed and approved for the treatment of various haematologic and solid human cancers. In this review, the classification, efficacy and significantly reduced toxicity of CmAbs available for use in the United States of America are presented. Finally, the limitations of CmAbs and future considerations are explored.

Trial Watch: Adoptive cell transfer for anticancer immunotherapy

The expression "adoptive cell transfer" (ACT) is commonly employed to indicate an immunotherapeutic regimen involving the isolation of autologous blood-borne or tumor-infiltrating lymphocytes, their selection/expansion/activation ex vivo, and their reinfusion into the patient, most often in the context of lymphodepleting pre-conditioning and in combination with immunostimulatory treatments. Optionally, the cellular material for ACT is genetically manipulated before expansion to (1) target specific tumor-associated antigens; (2) endogenously express immunostimulatory molecules; and/or (3) persist for long periods upon reinfusion. Consistent efforts have been dedicated at the amelioration of this immunotherapeutic regimen throughout the past decade, resulting in the establishment of ever more efficient and safer ACT protocols. Accordingly, the number of clinical trials testing ACT in oncological indications does not cease to increase. In this Trial Watch, we summarize recent developments in this exciting area of research, covering both high-impact studies that have been published during the last 12 months and clinical trials that have been launched in the same period to evaluate the safety and therapeutic potential of ACT in cancer patients.

Therapeutic human monoclonal antibodies against cancer

There are over 30 monoclonal antibodies that are FDA approved for a variety of diseases ranging from malignancies to autoimmune diseases to macular degeneration. These antibodies include murine, fully humanized, and chimeric antibodies. There are a number of monoclonal antibodies used in the treatment of malignancies; in fact, three of the top five grossing antibodies (bevacizumab, trastuzumab, and rituximab) are used in oncology Scolnik (mAbs 1:179-184, 2009).

Trial Watch: Chemotherapy with immunogenic cell death inducers

Accumulating evidence suggests that the clinical efficacy of selected anticancer drugs, including conventional chemotherapeutics as well as targeted anticancer agents, originates (at least in part) from their ability to elicit a novel or reinstate a pre-existing tumor-specific immune response. One of the mechanisms whereby chemotherapy can stimulate the immune system to recognize and destroy malignant cells is commonly known as immunogenic cell death (ICD). Cancer cells succumbing to ICD are de facto converted into an anticancer vaccine and as such elicit an adaptive immune response. Several common chemotherapeutics share the ability of triggering ICD, as demonstrated in vaccination experiments relying on immunocompetent mice and syngeneic cancer cells. A large number of ongoing clinical trials involve such ICD inducers, often (but not always) as they are part of the gold standard therapeutic approach against specific neoplasms. In this Trial Watch, we summarize the latest advances on the use of cyclophosphamide, doxorubicin, epirubicin, oxaliplatin, and mitoxantrone in cancer patients, discussing high-impact studies that have been published during the last 13 months as well as clinical trials that have been initiated in the same period to assess the antineoplastic profile of these immunogenic drugs as off-label therapeutic interventions.

Trial Watch: Immunostimulatory monoclonal antibodies in cancer therapy

Immunostimulatory monoclonal antibodies (mAbs) exert antineoplastic effects by eliciting a novel or reinstating a pre-existing antitumor immune response. Most often, immunostimulatory mAbs activate T lymphocytes or natural killer (NK) cells by inhibiting immunosuppressive receptors, such as cytotoxic T lymphocyte-associated protein 4 (CTLA4) or programmed cell death 1 (PDCD1, best known as PD-1), or by engaging co-stimulatory receptors, like CD40, tumor necrosis factor receptor superfamily, member 4 (TNFRSF4, best known as OX40) or TNFRSF18 (best known as GITR). The CTLA4-targeting mAb ipilimumab has been approved by the US Food and Drug Administration for use in patients with unresectable or metastatic melanoma in 2011. The therapeutic profile of ipilimumab other CTLA4-blocking mAbs, such as tremelimumab, is currently being assessed in subjects affected by a large panel of solid neoplasms. In the last few years, promising clinical results have also been obtained with nivolumab, a PD-1-targeting mAb formerly known as BMS-936558. Accordingly, the safety and efficacy of nivolumab and other PD-1-blocking molecules are being actively investigated. Finally, various clinical trials are underway to test the therapeutic potential of OX40- and GITR-activating mAbs. Here, we summarize recent findings on the therapeutic profile of immunostimulatory mAbs and discuss clinical trials that have been launched in the last 14 months to assess the therapeutic profile of these immunotherapeutic agents.

Harnessing the immune system to provide long-term survival in patients with melanoma and other solid tumors

Accumulating data from patients treated with checkpoint inhibitors and other immunomodulatory agents indicate that harnessing the power of the immune system is integral to achieve improve long-term cancer containment rates and prolong patient survival. Due to their mechanism of action, immunotherapeutic approaches have the potential to be effective against almost every tumor type. Durable responses to immunotherapy and prolonged patient survival have indeed been documented in individuals with melanoma, as well as kidney and lung cancer. These advances call for the re-evaluation of how clinical benefit is measured in an era in which long-term tumor control and survival are achievable treatment goals.

Dendritic cell-based immunotherapy for myeloid leukemias

Acute and chronic myeloid leukemia (AML, CML) are hematologic malignancies arising from oncogene-transformed hematopoietic stem/progenitor cells known as leukemia stem cells (LSCs). LSCs are selectively resistant to various forms of therapy including irradiation or cytotoxic drugs. The introduction of tyrosine kinase inhibitors has dramatically improved disease outcome in patients with CML. For AML, however, prognosis is still quite dismal. Standard treatments have been established more than 20 years ago with only limited advances ever since. Durable remission is achieved in less than 30% of patients. Minimal residual disease (MRD), reflected by the persistence of LSCs below the detection limit by conventional methods, causes a high rate of disease relapses. Therefore, the ultimate goal in the treatment of myeloid leukemia must be the eradication of LSCs. Active immunotherapy, aiming at the generation of leukemia-specific cytotoxic T cells (CTLs), may represent a powerful approach to target LSCs in the MRD situation. To fully activate CTLs, leukemia antigens have to be successfully captured, processed, and presented by mature dendritic cells (DCs). Myeloid progenitors are a prominent source of DCs under homeostatic conditions, and it is now well established that LSCs and leukemic blasts can give rise to "malignant" DCs. These leukemia-derived DCs can express leukemia antigens and may either induce anti-leukemic T cell responses or favor tolerance to the leukemia, depending on co-stimulatory or -inhibitory molecules and cytokines. This review will concentrate on the role of DCs in myeloid leukemia immunotherapy with a special focus on their generation, application, and function and how they could be improved in order to generate highly effective and specific anti-leukemic CTL responses. In addition, we discuss how DC-based immunotherapy may be successfully integrated into current treatment strategies to promote remission and potentially cure myeloid leukemias.

Trial Watch: Peptide vaccines in cancer therapy

Throughout the past 3 decades, along with the recognition that the immune system not only influences oncogenesis and tumor progression, but also determines how established neoplastic lesions respond therapy, renovated enthusiasm has gathered around the possibility of using vaccines as anticancer agents. Such an enthusiasm quickly tempered when it became clear that anticancer vaccines would have to be devised as therapeutic, rather than prophylactic, measures, and that malignant cells often fail to elicit (or actively suppress) innate and adaptive immune responses. Nonetheless, accumulating evidence indicates that a variety of anticancer vaccines, including cell-based, DNA-based, and purified component-based preparations, are capable of circumventing the poorly immunogenic and highly immunosuppressive nature of most tumors and elicit (at least under some circumstances) therapeutically relevant immune responses. Great efforts are currently being devoted to the identification of strategies that may provide anticancer vaccines with the capacity of breaking immunological tolerance and eliciting tumor-associated antigen-specific immunity in a majority of patients. In this sense, promising results have been obtained by combining anticancer vaccines with a relatively varied panels of adjuvants, including multiple immunostimulatory cytokines, Toll-like receptor agonists as well as inhibitors of immune checkpoints. One year ago, in the December issue of OncoImmunology, we discussed the biological mechanisms that underlie the antineoplastic effects of peptide-based vaccines and presented an abundant literature demonstrating the prominent clinical potential of such an approach. Here, we review the latest developments in this exciting area of research, focusing on high-profile studies that have been published during the last 13 mo and clinical trials launched in the same period to evaluate purified peptides or full-length proteins as therapeutic anticancer agents.

Trial Watch: Lenalidomide-based immunochemotherapy

Lenalidomide is a synthetic derivative of thalidomide currently approved by the US Food and Drug Administration for use in patients affected by multiple myeloma (in combination with dexamethasone) and low or intermediate-1 risk myelodysplastic syndromes that harbor 5q cytogenetic abnormalities. For illustrative purposes, the mechanism of action of lenalidomide can be subdivided into a cancer cell-intrinsic, a stromal, and an immunological component. Indeed, lenalidomide not only exerts direct cell cycle-arresting and pro-apoptotic effects on malignant cells, but also interferes with their physical and functional interaction with the tumor microenvironment and mediates a robust, pleiotropic immunostimulatory activity. In particular, lenalidomide has been shown to stimulate the cytotoxic functions of T lymphocytes and natural killer cells, to limit the immunosuppressive impact of regulatory T cells, and to modulate the secretion of a wide range of cytokines, including tumor necrosis factor α, interferon γ as well as interleukin (IL)-6, IL-10, and IL-12. Throughout the last decade, the antineoplastic and immunostimulatory potential of lenalidomide has been investigated in patients affected by a wide variety of hematological and solid malignancies. Here, we discuss the results of these studies and review the status of clinical trials currently assessing the safety and efficacy of this potent immunomodulatory drug in oncological indications.

Trial watch: Dendritic cell-based interventions for cancer therapy

Dendritic cells (DCs) occupy a privileged position at the interface between innate and adaptive immunity, orchestrating a large panel of responses to both physiological and pathological cues. In particular, whereas the presentation of antigens by immature DCs generally results in the development of immunological tolerance, mature DCs are capable of priming robust, and hence therapeutically relevant, adaptive immune responses. In line with this notion, functional defects in the DC compartment have been shown to etiologically contribute to pathological conditions including (but perhaps not limited to) infectious diseases, allergic and autoimmune disorders, graft rejection and cancer. Thus, the possibility of harnessing the elevated immunological potential of DCs for anticancer therapy has attracted considerable interest from both researchers and clinicians over the last decade. Alongside, several methods have been developed not only to isolate DCs from cancer patients, expand them, load them with tumor-associated antigens and hence generate highly immunogenic clinical grade infusion products, but also to directly target DCs in vivo. This intense experimental effort has culminated in 2010 with the approval by the US FDA of a DC-based preparation (sipuleucel-T, Provenge®) for the treatment of asymptomatic or minimally symptomatic metastatic castration-refractory prostate cancer. As an update to the latest Trial Watch dealing with this exciting field of research (October 2012), here we summarize recent advances in DC-based anticancer regimens, covering both high-impact studies that have been published during the last 13 mo and clinical trials that have been launched in the same period to assess the antineoplastic potential of this variant of cellular immunotherapy.

Trial Watch: Anticancer radioimmunotherapy

Radiotherapy has extensively been employed as a curative or palliative intervention against cancer throughout the last century, with a varying degree of success. For a long time, the antineoplastic activity of X- and γ-rays was entirely ascribed to their capacity of damaging macromolecules, in particular DNA, and hence triggering the (apoptotic) demise of malignant cells. However, accumulating evidence indicates that (at least part of) the clinical potential of radiotherapy stems from cancer cell-extrinsic mechanisms, including the normalization of tumor vasculature as well as short- and long-range bystander effects. Local bystander effects involve either the direct transmission of lethal signals between cells connected by gap junctions or the production of diffusible cytotoxic mediators, including reactive oxygen species, nitric oxide and cytokines. Conversely, long-range bystander effects, also known as out-of-field or abscopal effects, presumably reflect the elicitation of tumor-specific adaptive immune responses. Ionizing rays have indeed been shown to promote the immunogenic demise of malignant cells, a process that relies on the spatiotemporally defined emanation of specific damage-associated molecular patterns (DAMPs). Thus, irradiation reportedly improves the clinical efficacy of other treatment modalities such as surgery (both in neo-adjuvant and adjuvant settings) or chemotherapy. Moreover, at least under some circumstances, radiotherapy may potentiate anticancer immune responses as elicited by various immunotherapeutic agents, including (but presumably not limited to) immunomodulatory monoclonal antibodies, cancer-specific vaccines, dendritic cell-based interventions and Toll-like receptor agonists. Here, we review the rationale of using radiotherapy, alone or combined with immunomodulatory agents, as a means to elicit or boost anticancer immune responses, and present recent clinical trials investigating the therapeutic potential of this approach in cancer patients.

Trial Watch: Toll-like receptor agonists for cancer therapy

Toll-like receptors (TLRs) have long been known for their ability to initiate innate immune responses upon exposure to conserved microbial components such as lipopolysaccharide (LPS) and double-stranded RNA. More recently, this family of pattern recognition receptors has been attributed a critical role in the elicitation of anticancer immune responses, raising interest in the development of immunochemotherapeutic regimens based on natural or synthetic TLR agonists. In spite of such an intense wave of preclinical and clinical investigation, only three TLR agonists are currently licensed by FDA for use in cancer patients: bacillus Calmette-Guérin (BCG), an attenuated strain of Mycobacterium bovis that operates as a mixed TLR2/TLR4 agonist; monophosphoryl lipid A (MPL), a derivative of Salmonella minnesota that functions as a potent agonist of TLR4; and imiquimod, a synthetic imidazoquinoline that activates TLR7. One year ago, in the August and September issues of OncoImmunology, we described the main biological features of TLRs and discussed the progress of clinical studies evaluating the safety and therapeutic potential of TLR agonists in cancer patients. Here, we summarize the latest developments in this exciting area of research, focusing on preclinical studies that have been published during the last 13 mo and clinical trials launched in the same period to investigate the antineoplastic activity of TLR agonists.

Trial Watch: Immunostimulatory cytokines

During the past two decades, the notion that cancer would merely constitute a cell-intrinsic disease has gradually been complemented by a model postulating that the immune system plays a relevant role during all stages of oncogenesis and tumor progression. Along with this conceptual shift, several strategies have been devised to stimulate tumor-specific immune responses, including relatively unselective approaches such as the systemic administration of adjuvants or immunomodulatory cytokines. One year ago, in the July issue of OncoImmunology, we described the main biological features of this large group of proteins and discussed the progress of ongoing clinical studies evaluating their safety and therapeutic potential in cancer patients. Here, we summarize the latest developments in this area of clinical research, focusing on high impact studies that have been published during the last 13 mo and clinical trials launched in the same period to investigate which cytokines can be employed as safe and efficient immunostimulatory interventions against cancer.

Trial Watch: Adoptive cell transfer for anticancer immunotherapy

Adoptive cell transfer (ACT) represents a prominent form of immunotherapy against malignant diseases. ACT is conceptually distinct from dendritic cell-based approaches (which de facto constitute cellular vaccines) and allogeneic transplantation (which can be employed for the therapy of hematopoietic tumors) as it involves the isolation of autologous lymphocytes exhibiting antitumor activity, their expansion/activation ex vivo and their reintroduction into the patient. Re-infusion is most often performed in the context of lymphodepleting regimens (to minimize immunosuppression by host cells) and combined with immunostimulatory interventions, such as the administration of Toll-like receptor agonists. Autologous cells that are suitable for ACT protocols can be isolated from tumor-infiltrating lymphocytes or generated by engineering their circulating counterparts for the expression of transgenic tumor-specific T-cell receptors. Importantly, lymphocytes can be genetically modified prior to re-infusion for increasing their persistence in vivo, boosting antitumor responses and minimizing side effects. Moreover, recent data indicate that exhausted antitumor T lymphocytes may be rejuvenated in vitro by exposing them to specific cytokine cocktails, a strategy that might considerably improve the clinical success of ACT. Following up the Trial Watch that we published on this topic in the third issue of OncoImmunology (May 2012), here we summarize the latest developments in ACT-related research, covering both high-impact studies that have been published during the last 13 months and clinical trials that have been initiated in the same period to assess the antineoplastic profile of this form of cellular immunotherapy.

Trial watch: Oncolytic viruses for cancer therapy

Oncolytic virotherapy is emerging as a promising approach for the treatment of several neoplasms. The term "oncolytic viruses" is generally employed to indicate naturally occurring or genetically engineered attenuated viral particles that cause the demise of malignant cells while sparing their non-transformed counterparts. From a conceptual standpoint, oncolytic viruses differ from so-called "oncotropic viruses" in that only the former are able to kill cancer cells, even though both display a preferential tropism for malignant tissues. Of note, such a specificity can originate at several different steps of the viral cycle, including the entry of virions (transductional specificity) as well as their intracellular survival and replication (post-transcriptional and transcriptional specificity). During the past two decades, a large array of replication-competent and replication-incompetent oncolytic viruses has been developed and engineered to express gene products that would specifically promote the death of infected (cancer) cells. However, contrarily to long-standing beliefs, the antineoplastic activity of oncolytic viruses is not a mere consequence of the cytopathic effect, i.e., the lethal outcome of an intense, productive viral infection, but rather involves the elicitation of an antitumor immune response. In line with this notion, oncolytic viruses genetically modified to drive the local production of immunostimulatory cytokines exert more robust therapeutic effects than their non-engineered counterparts. Moreover, the efficacy of oncolytic virotherapy is significantly improved by some extent of initial immunosuppression (facilitating viral replication and spread) followed by the administration of immunostimulatory molecules (boosting antitumor immune responses). In this Trial Watch, we will discuss the results of recent clinical trials that have evaluated/are evaluating the safety and antineoplastic potential of oncolytic virotherapy.