Trial watch: Dendritic cell-based anticancer therapy

Abstract

The use of patient-derived dendritic cells (DCs) as a means to elicit therapeutically relevant immune responses in cancer patients has been extensively investigated throughout the past decade. In this context, DCs are generally expanded, exposed to autologous tumor cell lysates or loaded with specific tumor-associated antigens (TAAs), and then reintroduced into patients, often in combination with one or more immunostimulatory agents. As an alternative, TAAs are targeted to DCs in vivo by means of monoclonal antibodies, carbohydrate moieties or viral vectors specific for DC receptors. All these approaches have been shown to (re)activate tumor-specific immune responses in mice, often mediating robust therapeutic effects. In 2010, the first DC-based preparation (sipuleucel-T, also known as Provenge®) has been approved by the US Food and Drug Administration (FDA) for use in humans. Reflecting the central position occupied by DCs in the regulation of immunological tolerance and adaptive immunity, the interest in harnessing them for the development of novel immunotherapeutic anticancer regimens remains high. Here, we summarize recent advances in the preclinical and clinical development of DC-based anticancer therapeutics.

Keywords:

• antigen cross-presentation
• autophagy
• DC-based vaccination
• immunogenic cell death
• regulatory T cells
• Toll-like receptor agonists

Abbreviations:

• DC, dendritic cell
• FDA, Food and Drug Administration
• iDC, immature DC
• IFN, interferon
• mDC, mature DC
• MRC1, mannose receptor, C type 1
• MUC1, mucin 1
• pDC, plasmacytoid DC
• TAA, tumor-associated antigen
• TLR, Toll-like receptor
• Treg, regulatory T cell
• WT1, Wilms tumor 1

Introduction

The term “dendritic cells” (DCs) is generally employed to indicate a relatively heterogeneous and ubiquitous population of myeloid cells that (1) often, but not always, are characterized by a peculiar, tree-like morphology, and (2) tend to accumulate at sites of intense antigenic challenge (i.e., lymphoid organs, mucosal surfaces, the skin).^1-3 First discovered in 1973 by the late Nobel prize winner Ralph Steinman,^4-7 DCs have been the focus of an extraordinary intense wave of investigation over the last three decades, resulting in the increasingly precise characterization of their unique phenotypic and functional profile.^8-24 It is now clear that DCs occupy a critical position at the interface between innate and adaptive immunity. Accordingly, numeric of functional alterations in the DC network have been shown to contribute to the pathophysiology of several human diseases involving an immunological component, including a wide panel of infective, inflammatory, autoimmune, and allergic conditions.^2,25-27

Upon differentiation from common myeloid bone marrow progenitors, DCs migrate to tissues in an immature state. This implies that (1) they efficiently engulf extracellular material but secrete limited amounts of cytokines/chemokines, (2) they retain MHC Class II molecules within late endosomes, (3) they express a peculiar set of chemokine receptors; and (4) they bear low amounts of co-stimulatory molecules such as CD40, CD70, CD86, and tumor necrosis factor (ligand) superfamily, member 4 (TNFSF4, best known as OX40L) on the cell surface.^28-30 Functionally-speaking, immature DCs (iDCs) are efficient inducers of immunological tolerance, mostly owing to their ability to promote the demise of antigen-specific T-cells (a process commonly known as clonal deletion) as well as the expansion of immunosuppressive CD4⁺CD25⁺FOXP3⁺ regulatory T cells (Tregs).^31-33

In response to various cues of both microbial and endogenous origin,^1,8,28,34-36 iDCs undergo a significant functional shift as they mature.^3,37-41 The ability of mature DCs (mDCs) to take up extracellular material is limited.^3,28 Moreover, mDCs (1) bear MHC Class II molecules as well as elevated amounts of CD40, CD70, CD86, and OX40L on the cell surface; (2) express chemokine receptors that allow them to efficiently migrate to lymph nodes, such as chemokine (C-C motif) receptor 7 (CCR7); and (3) secrete increased quantities of cytokines and chemokines.^3,28 Thus, mDCs acquire a robust capacity to elicit adaptive immune responses, of both the cellular and humoral type.^42-45

Importantly, not all DCs share the same morphological, phenotypic and functional properties.^13,46-51 Thus, while some DC subsets – such as CD14⁺ dermal DCs – promote humoral immunity, others – such as epidermal Langerhans cells – preferentially stimulate CD8⁺ T-cell responses.^13,52-54 Two peculiar types of DCs are murine CD8α⁺ DCs (corresponding to human CD141⁺ DCs), which are particularly efficient at cross-presentation (i.e., at presenting on MHC Class I molecules extracellular antigens),^55-60 and plasmacytoid DCs (pDCs), which are unique in that they morphologically resemble plasma cells and in that they respond to microbial stimuli by secreting elevated levels of type I interferon (IFN).^61-63 Nonetheless, CD123⁺ pDCs activated by CpG oligodeoxynucleotides favor the expansion of CD4⁺CD25⁺FOXP3⁺ Tregs, hence mediating robust immunosuppressive effects.⁶⁴

DCs have a significant impact on oncogenesis, tumor progression and response to therapy.^35,65-68 This has been demonstrated not only in various preclinical tumor models involving alterations in the abundance or functional profile of precise DC subsets,^35,69-77 but also in a plethora of clinical studies,⁷⁸ correlating the intratumoral levels of DCs at large or specific DC subpopulations to improved^79-99 or worsened^79,100-105 disease outcome.

Throughout the past decade, consistent human and economic resources have been invested in the development of strategies to harness the robust immunomodulatory potential of DCs for the treatment of clinically relevant conditions encompassing infectious, autoimmune, asthmatic, and neoplastic disorders.^{2,12,26,79,106-112} The main DC-based anticancer interventions developed so far, each of which is associated with unique advantages and drawbacks,^79,113,114 involve: (1) untreated DCs or DCs optionally exposed to activating stimuli;^115-121 (2) DCs exposed ex vivo to preparations enriched in one or more TAAs;^122-173 (3) strategies that allow for the loading of DCs with TAAs in vivo;^9,174-186 and (4) DC-derived exosomes.^187-190 Ex vivo, the loading of DCs with TAAs can be achieved (1) by co-culturing iDCs with autologous tumor cell lysates^{122-131,191-194} or recombinant TAAs;^132-139 (2) by transfecting DCs with TAA-coding vectors, TAA-coding RNAs or bulk RNA isolated from neoplastic cells;^{140-165,195-197} and (3) by generating so-called “dendritomes," i.e., fusions between DCs and inactivated cancer cells.^{129-131,166-173,198} In vivo, TAAs can be selectively delivered to DCs if (1) fused to monoclonal antibodies, polypeptides or carbohydrate moieties that selectively target receptors expressed by DCs, such as mannose receptor, C type 1 (MRC1), CD209 (best known as DC-SIGN), and lymphocyte antigen 75 (LY75, best known as DEC-205),^{9,174-179,181,182,184,199} or glycolipids that are abundant on the DC surface, such as the glycosphingolipid globotriaosylceramide (Gb₃);^200,201 (2) encapsulated in DC-targeting immunoliposomes,^202-204 or (3) encoded in DC-targeting vectors.^205-208 Reflecting the potent tolerogenic functions of iDCs,^175,176 the efficiency of therapeutic strategies targeting DCs in vivo critically relies on the co-administration of adequate stimuli that promote DC maturation, including Toll-like receptor (TLR) agonists and immunostimulatory cytokines.^209-211 Moreover, the immune responses elicited by such approaches vary in terms of polarization and functional features (i.e., T-cell phenotype, cytotoxic activity, secretory functions, and homing properties) depending not only on the specific DC subset that is targeted, but also on the DC receptor that is harnessed to this aim.^16,212-214

Here, we summarize recent advances in the development of DC-based interventions for oncological indications, discussing the results of studies that have been released and clinical trials that have been initiated after the publication of our latest Trial Watch dealing with this topic.²¹⁵ Of note, only one cellular product involving DCs is currently approved for use in humans, sipuleucel-T (also known as Provenge®). Sipuleucel-T has been licensed by the US FDA for the treatment of asymptomatic or minimally symptomatic metastatic castration-refractory prostate cancer as early as in 2010.^216-219

Literature Update

During the last 13 mo, the results of no less than 43 clinical trials investigating the safety and efficacy of DC-based therapeutic interventions in cancer patients have been published in the peer-reviewed scientific literature (source http://www.ncbi.nlm.nih.gov/pubmed). A large fraction of these studies (24) involved autologous DCs exposed ex vivo to tumor cell lysates, TAAs or peptide thereof.^220-243 In addition, 8 of these trials were based on DCs transfected with bulk tumor cell RNA or TAA-coding RNA,^244-251 5 on autologous DCs not exposed to TAAs or TAA-coding molecules,^252-256 2 on strategies for targeting DCs in vivo,^257,258 and 1 on the adoptive transfer of allogenic DCs (in combination with donor lymphocyte infusions).²⁵⁹ Finally, the DC-based immunotherapeutic regimens employed in 2 of these studies could not be determined, owing to the lack of access to the corresponding reports.^260,261 In many instances, DCs were employed as standalone therapeutic agents, i.e., administered in the presence of standard adjuvants only.^{221-225,229,232,234,240,243,245,246,249-251} Alternatively, DC-based interventions were administered together with cytokine-induced killer (CIK) cells,^{220,233,247,252-255} adoptively transferred T lymphocytes,^{226,236,239,248,256,259} chemotherapy,^227,228,237 immunostimulatory cytokines,^230,235,242 TLR agonists,²⁵⁷ or radiation therapy.^{233,238,256,262,263} As for the main oncological indications investigating in this context, 7 trials enrolled melanoma patients,^{226,230,236,238,250,251} 4 subjects with pancreatic carcinoma,^{220,227,228,248} 4 women with breast carcinoma,^{222,255,258,260} 4 patients with various brain tumors, including glioblastoma;^{234,242,249,252} 3 ovarian carcinoma patients,^229,235 and the others a broad panel of hematological and solid neoplasms.^{221,224,225,231-233,237,239-241,243,244,246,247,253,254,256,257,259,261} Taken together, the results of these studies (the majority of which was Phase I or II trials) indicate that DC-based anticancer interventions are generally well tolerated and elicit anticancer immune responses that, at least in a fraction of patients, underpin objective clinical responses. Of note, Vassilaros and colleagues published 15-y clinical follow-up data from a Phase III study testing a variant of mucin 1 (MUC1)^264,265 targeted to DCs in vivo upon conjugation with oxidized mannan (an MRC1 ligand) vs. placebo in MUC1⁺ breast carcinoma patients.²⁵⁸ In this setting, recurrence rate was 12.5% among subjects treated with immunotherapy (mean time to recurrence: 118 mo) and 60% among patients receiving placebo only (mean time to recurrence: 65.8 mo).²⁵⁸ These data indicate that harnessing MRC1 to specifically target TAAs to DCs in vivo may constitute an efficient means to elicit therapeutically relevant immune responses. Large Phase III clinical trials are required to properly evaluate the clinical potential of this DC-based anticancer intervention.

Of note, in a recent study testing the therapeutic profile of a variant of NY-ESO-1 targeted to DEC-205 (CDX-1401), 6 of 8 patients who also received immune checkpoint inhibitors, such as the cytotoxic T lymphocyte-associated protein 4 (CTLA4)-specific, FDA-approved agent ipilimumab,^266,267 experienced objective tumor regression.²⁵⁷ In spite of the current paucity of data on combining DC-based anticancer interventions with immune checkpoint blockers,^257,268 this is expected to become an area of intense clinical investigation.

Among the numerous preclinical studies published during the past 13 mo with direct or indirect implications for DC-based anticancer immunotherapy, we found of particular interest the works of: (1) Dubrot and colleagues (University of Geneva Medical School; Geneva, Switzerland), who discovered that lymph node stromal cells are capable of taking up peptides complexed with MHC Class II molecules from DCs and present them to CD4⁺ T cells in the context of inhibitory signals, thereby promoting antigen-specific tolerance;²⁶⁹ (2) Arora and co-workers (Albert Einstein College of Medicine; Bronx, NY, US), who identified CD8α⁺DEC-205⁺ DCs as the major regulators of the innate immune response to glycolipid antigens of invariant natural killer T cells;²⁷⁰ (3) Schraml and collaborators (London Research Institute; London, UK), who proposed C-type lectin domain family 9, member A (CLEC9A, best known as DNGR1) as a phenotypic marker that allows for the precise discrimination of DCs from cells of the monocytic lineage;²⁷¹ (4) Klebanoff et al. (NCI-NIH; Bethesda, MD, US), who demonstrated the importance of retinoic acid for the homeostasis of various DC subsets;²⁷² (5) Baker and colleagues (Harvard Medical School; Boston, MA, US), who demonstrated that the neonatal Fc receptor (encoded by FCGRT) is required for DCs to take up TAAs complexed with IgGs in an optimal manner and initiate therapeutically relevant immune responses against colorectal carcinoma;²⁷³ (6) Muller and coworkers (University Hospital Basel; Basel, Switzerland), who discovered that microtubule inhibitors of the dolastatin family exert potent immunostimulatory effects as they induce DC maturation;²⁷⁴ and (7) Baghdadi and collaborators (Hokkaido University; Sapporo, Japan), who characterized a mechanism through which T-cell immunoglobulin and mucin domain containing 4 (TIMD4) promotes the autophagic degradation of dead cancer cells engulfed by DCs and macrophages, de facto inhibiting their ability to elicit a tumor-targeting immune response.²⁷⁵ These latter results add to a large amount of data demonstrating that autophagy can exert oncosuppressive functions or promote anticancer immune responses, but also favor tumor progression or inhibit immunosurveillance.^75,276-284 According to current models, the impact of autophagy on cancer can indeed vary between such extremes depending not only on the cell type mounting an autophagic response (e.g., malignant, stromal or immune cells), but also on disease stage (e.g., pre-malignant vs. established lesions) and immunological proficiency of the host.

Ongoing Trials

When this trial watch was being redacted (August 2014), official sources listed no less than 31 clinical trials initiated after August 1^st, 2013 to evaluate the safety and efficacy of DC-based anticancer interventions for oncological indications (source http://www.clinicaltrials.gov). Of these studies, 11 involve DCs pulsed ex vivo with TAAs or TAA-derived peptides (NCT01946373, NCT01944709, NCT01974661, NCT02018458, NCT02049489, NCT02061332, NCT02061423, NCT02063724, NCT02070406, NCT02111941, NCT02115126), 8 DCs exposed ex vivo to autologous tumor cell lysates (NCT01803152, NCT01946373, NCT01957956, NCT01973322, NCT02010606, NCT02146066, NCT02151448, NCT02033616), 5 DCs genetically modified to express TAAs or immunostimulatory molecules (NCT01924156, NCT01956630, NCT01983748, NCT01995708, NCT02170389), 2 sipuleucel-T (NCT01981122, NCT02159950), 2 autologous DCs expanded ex vivo but not loaded with TAAs or TAA-coding molecules (NCT01883297, NCT01926639), and 2 strategies for the targeting of DCs in vivo (NCT02122861, NCT02129075). In addition, 2 clinical trials have been launched during the last 13 mo to assess the utility of specific follow-up procedures in the context of sipuleucel-T-based immunotherapy (NCT02036918, NCT02042053) (Table 1).

Table 1. Clinical trials recently initiated to test the therapeutic profile of dendritic cell-based interventions in cancer patients

Approach	Indications	Phase	Status	TAAs	Notes	Ref.
Autologous DCs	Follicular lymphoma	II	Recruiting	n.a.	Combined with RT and rituximab	NCT01926639
	Melanoma	I	Not yet recruiting	n.a.	Combined with adoptively transferred T cells plus IL-2	NCT01883297
DCs loaded with TAAs or peptides thereof	Breast carcinoma	I	Recruiting	ERBB2	As a single agent	NCT02061423
						NCT02063724
		I/II	Recruiting	ERBB2	As a single agent	NCT02061332
				CCNB1 WT1	Combined with anakinra and/or neoadjuvant chemotherapy	NCT02018458
	Glioblastoma multiforme	I	Recruiting	CD133	As a single agent	NCT02049489
	Hepatocellular carcinoma	I	Recruiting	Various	As a single agent	NCT01974661
	Lymphoma	II	Not yet recruiting	LMP2	Combined with a TLR9 agonist	NCT02115126
	Melanoma	I/II	Terminated	MLANA NY-ESO-1	As a single agent	NCT01944709
	Ovarian carcinoma	n.a.	Recruiting	FOLR1	As a single agent	NCT02111941
	Solid tumors	I	Not yet recruiting	NY-ESO-1	Combined with NY-ESO-1-targeting PBLs	NCT02070406
DCs loaded with tumor cell lysates	Brain neoplasms	I	Recruiting	Whole TAA set	Combined with temozolomide, RT and optional bevacizumab	NCT02010606
	Glioblastoma	n.a.	Available	Whole TAA set	As a single agent	NCT02146066
	Gliosarcoma	n.a.	Recruiting	Whole TAA set	Combined with temozolomide	NCT01957956
	Melanoma	II	Recruiting	Whole TAA set	Combined with RT and IFNα-based immunotherapy	NCT01973322
	Ovarian carcinoma	II	Not yet recruiting	Whole TAA set	Combined with GM-CSF	NCT02033616
			Recruiting	Whole TAA set	Combined with standard of care chemotherapy	NCT02107378
						NCT02107937
						NCT02107950
	Peritoneal malignancies	I/II	Not yet recruiting	Whole TAA set	Combined with celecoxib, IFNα and a TLR3 agonist	NCT02151448
	Prostate carcinoma	II	Active, not recruiting	Whole TAA set	Combined with docetaxel	NCT02105675
					Combined with hormone therapy	NCT02107391
					Combined with standard RT	NCT02107430
			Recruiting	Whole TAA set	Combined with radical primary prostatectomy	NCT02107404
					Combined with docetaxel	NCT02111577
					As a single agent	NCT02137746
	Sarcoma	I	Recruiting	Whole TAA set	Combined with imiquimod and gemcitabine	NCT01803152
Genetically modified DCs	Leukemia	I/II	Recruiting	n.a.	Combined with adoptively transferred CIK cells and DLIs	NCT01956630
	Melanoma	III	Not yet recruiting	Whole TAA set (bulk RNA)	As a single agent	NCT01983748
	Multiple myeloma	I	Recruiting	MAGEA3 MAGEC1 WT1	As a single agent after autologous HSCT	NCT01995708
	Renal cell carcinoma	n.a.	Not yet recruiting	n.a.	CD40LG-expressing DCs used as a neoadjuvant single agent	NCT02170389
		I/II	Active, not recruiting	MUC1 Survivin	Combined with adoptively transferred CIK cells	NCT01924156
In vivo targeting	Melanoma	II	Recruiting	NY-ESO-1	DEC-205-targeted variant of NY-ESO-1 combined with FLT3LG and polyICLC	NCT02129075
	Solid tumors	I	Recruiting	NY-ESO-1	DC-SIGN-targeting lentiviral vector encoding NY-ESO-1, used as single agent	NCT02122861
Mixed	Melanoma	I	Recruiting	NY-ESO-1 or whole TAA set	Combined with adoptively transferred T cells plus IL-2	NCT01946373
Sipuleucel-T	Prostate carcinoma	n.a.	Recruiting	PAP	As a single agent	NCT02042053
		I	Recruiting	PAP	As a single agent	NCT02036918
		II	Recruiting	PAP	Combined with enzalutamide	NCT01981122
			Not yet recruiting	PAP	Combined with tasquinimod	NCT02159950

Abbreviations: CCNB1, cyclin B1; CD40LG, CD40 ligand; CIK, cytokine-induced killer; DC, dendritic cell; DLI, donor lymphocyte infusion; ERBB2, v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2; FLT3LG, fms-related tyrosine kinase 3 ligand; FOLR1, folate receptor 1; GM-CSF, granulocyte macrophage colony-stimulating factor; HSCT, hematopoietic stem cell transplantation; IFNα, interferon α; IL-2, interleukin-2; LMP2, latent membrane protein 2; MAGEA3, melanoma antigen family A3; MAGEC1, melanoma antigen family C1; MLANA, melan A; MUC1, mucin 1; n.a., not applicable or not available; PAP, prostate acid phosphatase; PBL, peripheral blood lymphocyte; polyICLC, polyinosinic-polycytidylic acid stabilized in poly-L-lysine and carboxymethylcellulose; RT, radiation therapy; TAA, tumor-associated antigen; TLR9, Toll-like receptor 9; WT1, Wilms tumor 1. *Based on clinical trials initiated after August 2014, 1^st (source www.clinicaltrials.gov).

In particular, (1) autologous DCs loaded ex vivo with full-length v-erb-b2 avian erythroblastic leukemia viral oncogene homolog 2 (ERBB2),^285,286 peptides thereof, or a mix of cyclin B1 (CCNB1)- and Wilms tumor 1 (WT1)-derived epitopes,^287-289 are being tested, either alone or coupled to recombinant interleukin 1 receptor antagonist (IL1RN, best known as anakinra or Kineret®) and/or neoadjuvant chemotherapy in breast carcinoma patients (NCT02018458, NCT02061332, NCT02061423, NCT02063724); (2) the safety and efficacy of DCs exposed ex vivo to purified CD133-derived peptides^290,291 are being evaluated as a standalone therapeutic intervention in subjects with glioblastoma (NCT02049489); (3) DCs co-cultured with several antigenic epitopes are being evaluated as a single agent in hepatocellular carcinoma patients (NCT01974661); (4) the clinical profile of DCs exposed ex vivo to an Epstein-Barr virus-derived TAA (latent membrane protein 2, LMP2) or peptides thereof, administered in combination with a TLR9 agonist,^{209,211,292-294} is being assessed in individuals with lymphoma (NCT02115126); (5) DCs loaded ex vivo with peptides derived from melan-A (MLANA, best known as MART1) and/or cancer/testis antigen 1B (CTAG1B, best known as NY-ESO-1)^295,296 are being tested as a single agent or as a support to adoptively transferred T cells plus high-dose interleukin-2 (IL-2)^210,297-299 in patients with melanoma or other solid tumors (NCT01944709, NCT01946373, NCT02070406); (6) DCs co-cultured with folate receptor 1 (FOLR1)-derived peptides³⁰⁰ are being investigated as a standalone therapeutic maneuver in subjects with ovarian carcinoma (NCT02111941). Of note, NCT01944709 has already been terminated owing to disease progression in several patients.

DCs exposed ex vivo to autologous tumor cell lysates are being tested (1) in patients with brain malignancies, as a standalone therapeutic intervention or combined with the vascular endothelial growth factor (VEGF)-targeting monoclonal antibody bevacizumab^301,302 (NCT02010606, NCT02146066); (2) in subjects with melanoma, as a support to adoptively transferred T cells plus high-dose IL-2, or radiotherapy²⁶² and/or IFNα-based immunotherapy^303,304 (NCT01973322, NCT01946373); (3) in patients with newly diagnosed gliosarcoma, in combination with temozolomide (an alkylating agent currently approved for the treatment of astrocytoma, glioblastoma multiforme and melanoma)^305,306 (NCT019579569); (4) in subjects with ovarian cancer, in conjunction with the immunostimulatory cytokine granulocyte macrophage colony-stimulating factor (GM-CSF)³⁰⁷ (NCT02033616) or with standard of care chemotherapy (NCT02107378; NCT02107937; NCT02107950); (5) as an adjuvant treatment of peritoneal surface malignancies, in combination with IFNα, celecoxib (an FDA-approved nonsteroidal anti-inflammatory drug specific for cyclooxygenase-2)^308,309 and rintatolimod (an experimental TLR3 agonist also known as Ampligen®)^310-312 (NCT02151448); (6) in men with prostate carcinoma, either as a standalone therapeutic intervention (NCT02137746), or combined with prostatectomy (NCT02107404), irradiation (NCT02107430), docetaxel-based chemotherapy (NCT02105675; NCT02111577) or hormone therapy (NCT02107391); and (7) in adults and children with sarcoma, in combination with gemcitabine (an immunostimulatory nucleoside analog licensed for the treatment of various carcinomas)^{67,68,313,314} and/or the FDA-approved TLR7 agonist imiquimod^315-319 (NCT01803152).

Moreover, (1) DCs transfected with bulk RNA isolated from autologous tumor cells are being tested as a standalone therapeutic intervention in uveal melanoma patients (NCT01983748); (2) DCs electroporated with RNA coding for WT1, melanoma antigen family C1 (MAGEC1) and melanoma antigen family A3 (MAGEA3)^320-323are being investigated as a consolidation therapy for multiple myeloma patients undergoing autologous stem cell transplantation (NCT01995708); (3) DCs genetically modified to express MUC1 and baculoviral IAP repeat containing 5 (BIRC5, best known as survivin)^324-326 are being evaluated as a support to the adoptive transfer of CIK cells^327-331 in subjects with renal cell carcinoma (NCT01924156); (4) DCs engineered to express the immunostimulatory molecule CD40 ligand (CD40LG)^30,332 are being assessed as a neoadjuvant intervention in patients with resectable kidney cancer (NCT02170389); and (5) DCs subjected to a not better defined genetic modification are being tested as a support to CIK cell-based immunotherapy^327-331 in leukemia patients relapsing upon allogenic stem cell transplantation (NCT01956630). Sipuleucel-T is being tested in combination with enzalutamide (an FDA-approved androgen receptor antagonist)^333,334 or tasquinimod (an experimental agent with immunomodulatory and antiangiogenic activity)^335-337 in men with castration-refractory prostate cancer (NCT01981122, NCT02159950), while autologous DCs expanded ex vivo but not loaded with TAAs or genetically modified are being investigated as a support to the adoptive transfer to restimulated tumor-infiltrating lymphocytes plus low-dose IL-2^{330,331,338,339} in melanoma patients (NCT01883297), or to radiotherapy and rituximab (and FDA-approved monoclonal antibody specific for CD20)^340,341 in subjects with follicular lymphoma (NCT01926639). As for strategies targeting DCs in vivo, (1) CDX-1401^257,342 is being tested, in combination with recombinant fms-related tyrosine kinase 3 ligand (FLT3LG)^343,344 and the experimental TLR3 ligand Hiltonol® (polyinosinic-polycytidylic acid stabilized in poly-L-lysine and carboxymethylcellulose), in melanoma patients; and (2) a lentiviral vector encoding NY-ESO-1 and targeting DC-SIGN (ID-LV305)^208,345,346 is being assessed as a standalone therapeutic intervention in patients with NY-ESO-1⁺ solid tumors (NCT02122861). Finally, 2 trials have been launched during the last 13 mo to assess the performance of positron emission tomography (PET), magnetic resonance imaging (MRI) and lymph node biopsies for predicting the immunological responses of patients with castration-refractory prostate cancer to sipuleucel-T (NCT02036918, NCT02042053).

As for the clinical trials listed in our previous Trial Watch dealing with this topic,^79,215 the following studies have changed status: NCT00722098, NCT00814892, NCT00961844, NCT01128803, NCT01334047, and NCT01783431, which have been “Terminated;" NCT01302821 and NCT01398124, which have been “Withdrawn;" NCT01216436, which has been “Suspended;" NCT00753220, NCT00795977, NCT00868114, NCT00937183, NCT00978913, NCT01082198, NCT01146262, NCT01213407, NCT01235845, NCT01530698, and NCT01567202, whose status is now “Unknown;" NCT01483274, now listed as “Not yet recruiting;" NCT01808820, NCT01876212, NCT01734304, NCT01686334, NCT01807065, NCT01804465, NCT01818986, and NCT01833208, which are now listed as “Recruiting;" NCT00601094, NCT00618891, NCT00622401, NCT00626483, NCT00639639, NCT00852007, NCT00890032, NCT00910650, NCT00970203, NCT00923351, NCT01189383, NCT01280552, NCT01347034, NCT01413295, NCT01431196, NCT01456065, NCT01487863, NCT01525017, NCT01582672, and NCT01832870, now listed as “Active, not recruiting;” NCT00612001, NCT00672542, NCT00678119, NCT00683241, NCT00715104, NCT00815607, NCT00846456, NCT00893945, NCT00913913, NCT00923910, NCT01042366, NCT01066390, NCT01171469, NCT01241682, NCT01373515, NCT01410968, NCT01574222, NCT01617629, and NCT01671592, which have been “Completed” (source http://www.clinicaltrials.gov). NCT00722098 and NCT01783431 have been terminated owing to low accrual, NCT00814892 because of funding issues, NCT00961844 due to logistic problems, and NCT01334047 since new knowledge about anticancer vaccines had to be taken into consideration. NCT01398124 has been withdrawn because the study was not to be pursued. The suspension of NCT01216436 has been dictated by a lack of funds. To the best of our knowledge, the reasons behind the termination of NCT00961844 and the withdrawal of NCT01302821, as well as the results of NCT00678119, NCT00683241, NCT00715104, NCT00815607, NCT00893945, NCT00913913, NCT01042366, NCT01171469, NCT01241682, NCT01373515, NCT01410968, NCT01574222, NCT01617629, and NCT01671592 are not available. Conversely, the results of NCT00612001,³⁴⁷ NCT00672542,³⁴⁸ NCT00846456,²⁴⁹ and NCT01066390²⁵¹ have already been disseminated (see above). NCT00923910 tested the safety and efficacy of DCs pulsed with WT1-derived peptides combined with donor lymphocytes in patients with hematological malignancies. A total of five donors and five recipients were enrolled in this study, none of whom developed severe side effects including graft-vs.-host reactions. All recipients, however, suffered from mild toxicities, including reduced platelet and white blood cell counts (in 80-100% of patients), as well as fever (in 60% of patients) (source http://www.clinicaltrials.gov).

Concluding Remarks

As summarized above, a broad panel of interventions has been developed throughout the past two decades to harness the potent immunostimulatory activity of DCs against cancer. The climax of this intense wave of translational and clinical investigation has been attained with the approval of sipuleucel-T for use in castration-refractory prostate cancer patients. Notwithstanding this achievement, the development of clinically useful DC-based immunotherapeutic regimens is hampered by several obstacles. First, most of these interventions rely on the isolation and expansion of autologous DCs that are optionally activated, loaded with TAAs and/or genetically modified, an ensemble of procedures that require good manufacturing practices (GMP)-compliant facilities and hence entail a significant cost. Second, although several TAAs have been identified and can be selectively targeted with DC-based vaccines, most of them are not bona fide tumor rejection antigens (TRAs), implying that the immune response that they elicit is often incapable of mediating complete tumor rejection. Third, only a fraction of cancer patients can be allocated to receiving peculiar types of DC-based therapy. For instance, the use of autologous DCs pulsed ex vivo with tumor cell lysates or tumor-derived RNA can only be envisioned for patients with neoplastic lesions that can be biopsied. Fourth, which specific DC subset elicits superior immune responses against malignant cells remains to be determined. Fifth, in the absence of adequate maturation signals, DCs mediate tolerogenic, rather than immunostimulatory, functions. Although this does not constitute an issue for DCs expanded ex vivo, targeting DCs in vivo may suffer from a generalized lack of potent, chemically defined, clinical grade adjuvants. Sixth, DC-based interventions are invariably employed in therapeutic, as opposed to prophylactic, settings. In these conditions, malignant cells have already established an intricate network of immunosuppressive pathways that significantly limit the ability of DCs to elicit anticancer immune responses. Finally, biomarkers that reliably predict the propensity of cancer patients to benefit from DC-based immunotherapy are missing.

Irrespective of these and other limitations, the interest of experimentalists and clinicians in DC-based anticancer interventions remains elevated, as testified by the considerable number of clinical trials launched in the past 13 mo to test this immunotherapeutic paradigm as well as by the huge amount of preclinical studies published in the same period on this topic. We surmise that the additional insights into the functional properties of specific DC subsets, the characterization of bona fide TRAs, the discovery of clinical grade adjuvants with superior immunostimulatory activity, and the identification of new predictive biomarkers will allow for the clinical implementation of novel DC-based anticancer regimens.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Funding

Authors are supported by Ligue contre le Cancer (équipe labelisée); Agence National de la Recherche (ANR); Association pour la recherche sur le cancer (ARC); Cancéropôle Ile-de-France; AXA Chair for Longevity Research; Institut National du Cancer (INCa); Fondation Bettencourt-Schueller; Fondation de France; Fondation pour la Recherche Médicale (FRM); the European Commission (ArtForce); the European Research Council (ERC); the LabEx Immuno-Oncology; the SIRIC Stratified Oncology Cell DNA Repair and Tumor Immune Elimination (SOCRATE); the SIRIC Cancer Research and Personalized Medicine (CARPEM); and the Paris Alliance of Cancer Research Institutes (PACRI).