2. Targeting Antigens on Lymphoid Cells
A number of different monoclonal antibodies targeting antigens on the surface of lymphocytes have been developed and studied.
Anti-CD2. Alefacept (Amevive
®, Biogen, Cambrigde, MA, USA) is a recombinant DNA dimeric fusion protein that consists of the extracellular CD2-binding portion of the human leukocyte function antigen-3 (LFA-3) linked to the Fc portion of human IgG1. By inhibiting LFA-3/CD2 interaction, alefacept interferes with lymphocyte and antigen-presenting cells (APCs) activation, causes a reduction in subsets of CD2+ T-cells (primarily memory effector subsets CD4+CD45RO+ and CD8+CD45RO+), resulting in a reduction in circulating total CD4+ and CD8+ T-lymphocyte counts, while CD2 is also expressed at low levels on the surface of natural killer (NK) cells and certain bone marrow B-cells. Alefacept has been used in two children with acute graft-versus-host disease (GvHD) and one developed aspergillus sinusitis that was successfully treated with surgery and antifungals
[7]. Of note, alefacept is no longer being marketed.
Anti-CD3. Until recently, depletion of T-lymphocytes constituted the cornerstone and end point of prophylaxis and treatment against GvHD, given that T-cells mediate immune responses towards allo-antigens. Muromonab-CD3 (Orthoclone OKT3
®, Ortho-Biotech Products, LP, Bridgewater, NJ, USA) was the first mAb ever to be approved (back in 1985) but is no longer being marketed due to decreased demand and increased infection rates, especially as regards IFDs and in particular invasive aspergillosis. Muromonab targeted CD3, a T-cell co-receptor involved in the activation of both cytotoxic CD8+ naïve T-cells and CD4+ T-helper naïve cells, explaining the detrimental impact of T-cell depletion on IFD occurrence
[4].
Anti-CD19. CD19-directed agents that have been used in pediatric blood cancers include the mAbs denintuzumab mafodotin and B43, the bispecific T-cell engagers (BiTEs) blinatumomab and DT2219ARL, and the immunotoxin combotox. The BiTE blinatumomab is the first of the above agents to be approved beyond off-label use and is indicated as monotherapy for the treatment of children aged one year or older with Philadelphia chromosome negative (Ph−) CD19+ B-precursor acute lymphoblastic leukemia (ALL) which is refractory or in relapse (r/r) after receiving at least two prior therapies or in relapse after receiving prior allogeneic hematopoietic stem cell transplantation (AHSCT)
[8]. BiTE DT2219ARL, an immunotoxin consisting of two scFv ligands targeting CD19 and CD22 linked to the first 389 amino acids of diphtheria toxin, seems to bear antineoplastic activity but the respective clinical trials involving children with hematologic cancer have not yet published results
[9].
Immunotoxin combotox is a 1:1 mixture of anti-CD19 and anti-CD22 IgG1 mAbs that are both coupled to deglycosylated ricin-A chain. Combotox has been used in a phase I study in 17 children with r/r precursor B-ALL: three out of 17 children achieved complete remission, three out of 17 children died during the course of treatment, while no IFDs have been reported
[10].
Anti-CD20. CD20-directed agents used in treatment of pediatric hematologic malignancies include: (i) first generation mAbs rituximab and
90Y-ibritumomab tiuxetan; (ii) second generation mAb ofatumumab with less immunogenicity and improved efficacy (by inducing cell lysis regardless of the level of CD20 expression and in rituximab-resistant CD20+ cells); and (iii) third generation mAb obinutuzumab, that bears an engineered fragment crystallizable region (Fc) to boost complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC)
[11]. B-cell lysis is induced by CDC, ADCC, apoptosis and sensitization to chemotherapy. Rituximab treatment leads to rapid (within three–seven days) and profound (≥90%) depletion of pre-B and mature B-cells (excluding plasma cells and B- cell precursors) along with CD3+ CD20+ cells (3–5% of T-cells) that lasts about 12 months, while serum immunoglobulin depletion lasts for five to 12 months and may require substitution in selected patients. Rituximab also leads to a reduced function of B-cells as APCs and, by increasing immature and transitional B-cells, to a dysfunction of CD4+ T-cells and abnormal cytotoxic T-cell-specific responses
[4]. Anti-CD20 blockade also affects Th17 cells, which are destined to protect mucosal barriers and contribute to pathogen clearance at mucosal surfaces
[12].
Obinutuzumab is a third generation anti-CD20 mAb that has been linked with IFDs in adults, and antifungal prophylaxis should be considered, taking into account both concomitant immunosuppressive therapy and underlying condition. Severe neutropenia has been observed in as high as 34% of CLL patients treated with obinutuzumab
[13]. Similarly, a phase III study on rituximab-refractory indolent NHL treated with obinutuzumab recorded an incidence rate of 33% of severe neutropenia, while only two cases presented with IFDs (one with PJP and one with fungal sepsis; incidence 0.5% each)
[14]. In a study of adults with previously untreated advanced-stage follicular lymphoma under anti-CD20 agents, the overall infection rates were higher in the obinutuzumab arm (77.3%) compared to the rituximab arm (70%), with this difference significantly greater during maintenance therapy (with severe infections in 19.8% and 15.6% of patients, respectively). In the same study, severe neutropenia was documented in 43.9% of subjects treated with obinutuzumab (versus 37.9% with rituximab)
[15]. In addition, a phase I study of obinutuzumab in combination with chemotherapy in CD20+ follicular NHL reported severe neutropenia as a common adverse event with 35.8% and 6.9% of affected patients during the induction and maintenance treatment phase, respectively. Severe infections were equally high in both phases of treatment with obinutuzumab (19.8% in the induction, including 1 case of PJP, versus 20.8% in the maintenance phase)
[16]. Rare opportunistic infections have been documented with obinutuzumab use. A report from two adult patients with refractory CLL (40 and 38 years old) under obinutuzumab and PJP prophylaxis (cotrimoxazole and pentamidine, respectively) suggested that IFDs are inevitable when neutropenia is prolonged and severe: the first patient developed an invasive
Candida krusei infection and PJP, while the second patient developed PJP along with bloodstream
Talaromyces marneffei infection
[17].
T. marneffei is far more frequent in patients with human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) in South East Asian countries, but is occasionally seen in those with cell-mediated immunodeficiencies involving the interleukin-12/interferon-γ (IL-12/IFN-γ) signaling pathway (e.g., in patients bearing mutations in the STAT1 gene). Among four reported cases of
T. marneffei infection in hematology patients undergoing targeted therapies (obinutuzumab, rituximab, ruxolitinib and sorafenib), only a 44-year-old man with CLL under obinutuzumab did not survive (also developing invasive
Candida glabrata infection and PJP), despite the fact that all patients received the same combination therapy with amphotericin B and voriconazole
[18]. Data of IFDs in children administered with obinutuzumab are lacking.
Third generation anti-CD20 mAbs in clinical trials are ocaratuzumab (modified via Fc mutagenesis) and ublituximab and they are both designed to augment ADCC. Severe neutropenia has been reported in 18.9% of adult patients treated with ublituximab for r/r NHL. Safety profiles of these agents are still pending
[19][20].
Anti-CD22. Off-label use of CD22-targeted agents in pediatric hematological malignancies involves the immunotoxin combotox (mentioned above with anti-CD19 agents) along with epratuzumab, inotuzumab ozogamicin and moxetumomab pasudotox. CD22 belongs to the SIGLEC (sialic acid-binding immunoglobulin-type lectins) family of I-type lectins and is a sugar binding transmembrane protein that is predominantly found on mature B-cell surfaces and on up to 90% of B-cell blasts and to a lesser extent on some immature B-cells. CD22 seems to regulate B cell receptor activation and subsequently plays a role in B-cell activation and survival, while it also serves as an adhesion molecule
[21].
With regard to epratuzumab, a study on children with r/r precursor B-ALL showed that severe infection rates were as high as 50%, while severe neutropenia was observed in only 26.3% of patients
[22]. Likewise, a study on adults with r/r ALL treated with epratuzumab recorded febrile neutropenia in 54.8% and severe infections in 41.9% of patients, half of whom eventually died
[23]. High rates of severe infections (34.7%) were also demonstrated by a study of epratuzumab use in adults with recurrent indolent NHL
[24]. Moreover, radioimmunotherapy with yttrium-90-labelled epratuzumab tetraxetan (⁹⁰Y-DOTA-epratuzumab) in adults with r/r CD22+ precursor B-ALL was associated with IFDs in 11.8% and with severe infections in 52.9% of patients
[25]. Nevertheless, long-term safety of repeated courses of epratuzumab therapy in adults with systemic lupus erythematosus reported serious infections in only 6.9%, indicating that concomitant or prior chemotherapy and underlying disease play a pivotal role in infection rates
[26]. In view of epratuzumab’s mechanism of action, which is similar to other B-cell-targeted drugs, and given that ensuing neutropenia is not excessively frequent, therapy with this agent does not seem to significantly increase the risk of infection
[27].
Inotuzumab ozogamicin (InO, Besponsa
®, Pfizer/Wyeth, Philadelphia, PA, USA) is an antibody-drug conjugate directed against CD22. In the phase 3 INO-VATE study comparing InO monotherapy with standard of care (SoC) intensive chemotherapy in adults with r/r precursor B-cell ALL, rates of treatment-related severe neutropenia were similar in both arms (36% versus 37.8%), while febrile neutropenia occurred in significantly less subjects in the InO group (26.8% versus 53.8%) and fungal pneumonia was diagnosed only in the SoC arm (2.1%)
[28]. Inotuzumab ozogamicin (Besponsa
®) SPC characterizes fungal infections as a very common adverse reaction among patients treated for relapsed or refractory precursor B-cell ALL (infection rate 48%; neutropenia 49%; febrile neutropenia 26%; fatal infections 5%)
[29]. Combination of InO with low-intensity chemotherapy in older patients with Philadelphia chromosome-negative ALL was shown to be safe and effective first-line therapy option, although it was associated with a high rate of severe infections (92.3%) and neutropenia had a median recovery duration of 16 days
[30]. Due to the incidence of prolonged neutropenia, antifungal prophylaxis should be considered during expectedly prolonged periods with low neutrophil counts. Although InO does not have any clinically relevant CYP-mediated interactions, it has been shown to increase QTc, and therefore, monitoring is recommended in case of concomitant azole administration. Antifungal prophylaxis with azoles is strongly recommended with InO, but should not be initiated until at least 24 h upon completion of InO treatment
[31].
InO is not yet approved for pediatric patients, and clinical studies are ongoing. Data on safety and efficacy in pediatric patients are scarce, but promising. A cohort of 51 children treated with InO for r/r precursor B-ALL on a compassionate use basis reported severe infections in 21.6%, severe and febrile neutropenia in 11.8% and IFDs in 3.9% of patients (one case of candidemia and one of a pulmonary fungal infection)
[32]. In another cohort including children with r/r ALL treated with InO only one IFD was recorded (2%), while severe neutropenia was estimated at 44.9%
[33]. A recent phase I study of InO in pediatric r/r ALL reported neutropenia in 56% of treated children, but low severe infection rates (8%; one with lung infection and one with sepsis after AHSCT)
[34]. A retrospective study of r/r precursor B-cell ALL in pediatric patients under compassionate use with InO and blinatumomab recorded two severe infections (6.5%) that led to death
[35]. Likewise, another retrospective study of children with r/r B-ALL treated with InO reported two events of severe infection (7.4%; one with IFD i.e., mucor-mycosis), that survived following bridging therapy with blinatumomab
[36].
Moxetumomab pasudotox (Lumoxiti
®, AstraZeneca, Cambridge, England, UK) is a recombinant anti-CD22 immunotoxin that has been recently evaluated in pediatric patients with r/r B-ALL and a respective phase I study has reported severe neutropenia in 17% and febrile neutropenia in 16% of treated patients
[37]. A respective phase II study in children with r/r B-ALL under moxetumomab pasudotox calculated a 20% rate of neutrophil count decreased and a 23.3% rate of febrile neutropenia
[38]. Of note, Lumoxiti
® is indicated for the treatment of adult patients with r/r hairy cell leukemia (HCL) who received at least two prior systemic therapies, including treatment with a purine nucleoside analog, and contains a boxed warning for capillary leak syndrome and hemolytic uremic syndrome. In a cohort of 28 adult patients with r/r HCL and moxetumomab administration, only one patient presented with severe infection (3.6%)
[39]. The extension of the previous study reported only one patient with severe neutropenia (4.8%), but none with febrile neutropenia or serious infection, corroborating that moxetumomab pasudotox spares T-cells and, due to its short half-life and its more sophisticated mechanism of action, averts prolonged B-cell depletion
[40].
Anti-CD25. Basiliximab was the first agent of this category to be approved and it is used for the prophylaxis and treatment of GvHD in the pediatric population. Inolimomab is another mAb targeting CD25 that seeks approval for steroid-resistant (SR) acute GvHD. Nevertheless, a recent phase III study on adults with GvHD under inolimomab showed no significant effect on overall survival at one year compared to usual care, while 34.7% of patients experienced at least one fungal infection
[41]. CD25 or interleukin-2 receptor alpha chain is a type I transmembrane protein present on activated T- and B-cells (in response to antigenic challenge), whereas it can also be found on thymocytes, myeloid precursors, and oligodendrocytes. Binding of IL-2 to activated T-cells is a critical signal for T-cell proliferation in allograft rejection
[42]. Due to its mechanism of action, basiliximab utilization is not expected to affect significantly IFD rates, but concomitant chemotherapy or immunosuppressive therapy and underlying disease remain the key parameters to define the respective risk.
Managing severe SR acute GvHD with basiliximab displayed favorable outcomes in a retrospective study of 34 patients (median age: 13 years old), but occurrence of IFDs was the second most fatal adverse event (26% of deceased patients)
[43]. A subsequent study with similar design showed that basiliximab was effective in treating steroid refractory acute GvHD after haplo-HSCT in 53 patients aged 8 to 52 years old, while fungal infections occurred in 20.8% of patients
[44]. Similar to the studies mentioned above, a recent study including solely pediatric patients concluded that basiliximab is an effective second-line agent, particularly when the skin is involved. Probable or documented fungal infection in this cohort regarded seven out of 100 patients
[45]. Conversely, five out of 10 children who underwent haplo-HSCT with T-cell depletion and were treated with basiliximab along with various other agents for hyperacute steroid refractory GvHD developed invasive aspergillosis and did not survive
[46]. IFDs in children with hematologic malignancies who received basiliximab are not reported in detail in the literature and precise information is lacking. One interesting case report described a 12-month-old girl who died of
Candida sepsis after receiving basiliximab for chronic GvHD after ABO-compatible liver transplantation for multi-system Langerhans cell histiocytosis (LCH)
[47]. In general, available treatment options for steroid refractory GvHD in infants and young toddlers are limited and ineffective, and anti-CD25 agents are evidently not in the spotlight of current research
[48].
Anti-CD30. Iratumumab (MDX-060), a fully humanized IgG1κ mAb that also belongs to the first-generation agents targeting CD30, is a marker of Reed-Sternberg cells in Hodgkin lymphoma (HL) and of anaplastic large cell lymphoma (ALCL), while it is also expressed in various types of other lymphomas and in embryonal carcinoma. Unfortunately, its development was suspended with no explanation given. A phase I/II study in adults receiving iratumumab for HL and ALCL showed inadequate efficacy; severe infection and pneumonia occurred in 9.7% and febrile neutropenia in 1.4% of treated patients
[49][50]. The second-generation anti-CD30 mAb XmAb2513 has an Fc region engineered to have increased binding affinity to Fcγ receptors (FcγRs) leading to improved FcγR-dependent effector cell functions. A phase I study in adults with HL showed limited efficacy and one out of 13 patients (7.7%) died of fungal pneumonia
[51].
Anti-CD33. Gemtuzumab ozogamicin
(GO) is another antibody-drug conjugate that has been recently approved for r/r CD33+ AML in pediatric patients. It targets CD33 antigen or SIGLEC-3, a transmembrane receptor expressed normally on cells of myeloid lineage, whereas it is usually found on leukemic myeloid cells, on more than 80% of AML blasts, on monocytes, granulocytes and mast cells, but not on normal precursor hematopoietic cells. Due to its cytotoxic effect on immature myeloid cells, CD33-targeted therapy leads to profound and long-lasting neutropenia and correspondingly, to increased infection incidence
[27][52]. IFDs following GO use have been under-reported in the literature but their incidence seems to range between 1.3–1.5%
[53][54].
GO has been utilized before AHSCT in r/r AML, but results were not encouraging; febrile neutropenia followed 36.4% of GO infusions and two out of 12 children died of adenovirus infection
[55]. Another pilot study of 12 children with r/r CD33+ AML, who received GO combined with busulfan and cyclophosphamide prior to AHSCT, reported severe infections in half of the patients, while one boy developed an IFD despite prophylaxis
[56]. Conversely, a single-center report of eight children with r/r AML that received fludarabine, cytarabine, and fractioned GO before AHSCT documented 11 episodes of febrile neutropenia in 13 treatment courses (84.6%) and six episodes of sepsis (46.2%), but no IFDs
[57]. GO administration for consolidation after reduced-intensity conditioning and AHSCT in a pediatric cohort with CD33+ AML noted severe neutropenia in all 14 patients (100%), but no IFDs have been documented (under prophylaxis; one death due to progressive disease and respiratory syncytial virus infection)
[58]. In the same context, all six children with r/r AML that had received reduced-intensity AHSCT before GO in a pilot study displayed severe neutropenia and four experienced severe bloodstream bacterial infections
[59]. A larger cohort of 59 children with post-consolidation GO treatment in r/r AML and after AHSCT showed that severe neutropenia followed 95% and febrile neutropenia 40% of the GO courses
[60].
Anti-CD38. Daratumumab, a novel anti-CD38 mAb approved for r/r multiple myeloma, may be an effective option in the treatment of r/r CD38+ hematological malignancies. CD38 is a glycoprotein, cyclic ADP ribose hydrolase, that exerts its function in cell adhesion, signal transduction, cytokine production and calcium signaling, and is expressed at low levels on the surface of many immune cells (CD4+, CD8+, early T- and B-cells and NK cells), plasma cells and germinal center B-cells
[27][61]. Patients with T-ALL have robust CD38 surface expression (remaining stable after multi-agent chemotherapy) and a recent preclinical study of pediatric T-ALL patient-derived xenografts (PDX) found daratumumab to be strikingly efficacious in 14 of the 15 different PDXs
[62]. In another preclinical study, treatment with daratumumab eradicated minimal residual disease (MRD) in seven of eight pediatric T-ALL PDXs
[63]. Off-label use of daratumumab in individual children has shown promising activity and limited toxicity with no indication of IFD predisposition
[64]. Clinical studies in ALL are ongoing. Anti-CD38 mAbs eliminate multiple myeloma targets by mediating CDC, ADCC, ADCP, FCγR-mediated cross-linking–induced apoptosis and nicotinamide adenine dinucleotide (NAD+) depletion
[61]. The mechanism of action in ALL has not yet been resolved. Isatuximab, another anti-CD38 mAb, is also in a phase II trial for children with r/r ALL and AML (NCT03860844).
Anti-CD52. Alemtuzumab targets the cell surface glycoprotein CD52, which is expressed in high levels on CD3+ T-cells and CD19+ B-cells, at lower levels on NK cells, monocytes and macrophages, while little or no expression is detected on neutrophils, plasma cells or bone marrow stem cells. Binding of alemtuzumab to CD52 results in ADCC and complement-mediated lysis of CD52+ cells. Alemtuzumab is a medication for the treatment of chronic lymphocytic leukemia (CLL) and multiple sclerosis in adults. Lymphopenia is typically profound after administration of alemtuzumab, reaching a nadir within a month of treatment, and lasts for three to 12 months for affected B-cells and up to three years for suppression of CD4+ and CD8+ cells
[4]. In a retrospective study of 182 patients (aged 11–79 years old) treated with alemtuzumab, the incidence of IFDs was 17%: 15 cases with invasive aspergillosis (8.2%), 10 with
Candida infection (5.5%), four with PJP (2.2%), one case of mucor-mycosis and fusariosis (0.5% each). Only an 11-year old boy treated with alemtuzumab for aplastic anemia developed invasive aspergillosis and eventually recovered. Aspergillosis resulted in a higher mortality rate than any other IFD in this cohort despite prophylaxis with fluconazole
[65]. A retrospective study of 19 pediatric patients who received alemtuzumab as a single second- or third-line treatment for acute GvHD after AHSCT reported IFDs in four out of 19 cases (21.1%)
[66]. In the same context, a cohort of 14 children administered alemtuzumab pre- and post-AHSCT and under fluconazole prophylaxis reported invasive candidiasis in two out of 14 children (14.3%)
[67]. A relevant, but older, retrospective study that utilized alemtuzumab as conditioning treatment prior to allo-HCT in nine subjects with severe aplastic anemia reported three deaths of children attributed to IFDs and mismatched grafts (33.3%; one with PJP)
[68].
Anti-CD66b.
90Y-labelled
BW 250/183 (a murine IgG1 mAb directed against carcinoembryonic antigen-related cell adhesion molecule 8 or CEACAM8 or CD66b, that is expressed on the cell surface of almost all human granulocytes and their more mature precursors) is being investigated as a form of radioimmunotherapy in children with r/r leukemia before allo-HCT, as prior dosimetry studies with indium-111 labelled anti-CD66 have shown favorable dose distributions (NCT04082286)
[69].
Anti-CD248. Ontuxizumab (MORAb-004) is a mAb directed against the C-type lectin transmembrane receptor endosialin (or tumor endothelial marker-1 TEM-1 or CD248), which is commonly found on the surface of mesenchymal cells of tumor microenvironment, including tumor endothelium, tumor-associated pericytes and activated fibroblasts, which are thought to play a key role in the development of tumor neovascular networks, cell–cell adhesion, stromal interaction and host defense. A phase I study on children with r/r solid tumors showed that ontuxizumab seems to be devoid of infectious complications in that population
[70]. A similar study on adults reported only two cases of severe infections (5.6%), but no IFDs
[71].
3. Immune Checkpoint Inhibitors
3.1. Targeting CTLA-4
Ipilimumab and tremelimumab are available cytotoxic T-lymphocyte-associated protein 4 (CTLA-4 or CD152) inhibitors, but with limited off-label use in pediatric hematologic malignancies. CTLA-4 blockade is not expected to predispose to IFDs, because it promotes T-cell priming. Interestingly, CTLA-4 gene knock-out mice seem to develop lymphoproliferative and autoimmune disorders, while clinical experience links these mAbs with various immune-related adverse effects (irAEs) including the induction of autoimmune neutropenia
[68]. Treatment of irAEs in the latter case requires corticosteroid administration, in which cases PJP prophylaxis is strongly recommended
[72]. Of note, cancer immunotherapy with check point inhibitors can cause irAEs due to loss of regulatory T-cells (Treg) homeostasis
[73]. A retrospective study of 740 patients (aged 4–93 years old) treated with immune checkpoint blockers for melanoma showed that pembrolizumab was the safest in terms of serious infection occurrence (odds ratio (OR) 0; 95% CI: 0–0.63), followed by nivolumab (OR 0.29; 95% CI: 0.03–1.68). Odds ratio for serious infection with ipilimumab was 1.05 (95% CI: 0.55–1.9), while, remarkably, the combination of ipilimumab with nivolumab was associated with a relevant risk (OR 3.26; 95% CI: 1.7–6.27). IFDs in the ipilimumab study group were 0.9%: two cases of IPA, three PJP cases, and one
Candida bloodstream infection, while nine out of 54 patients (17%) died of serious infection. No prophylaxis was given, but this study stresses the need for prophylactic measures especially in patients with irAEs that need prednisone or equivalent for at least four weeks. As expected, risk factors for serious infections in patients undergoing immune checkpoint blockade were use of corticosteroids (OR 7.71; 95% CI: 3.71–16.18) and use of infliximab (OR 4.74; 95% CI: 2.27–9.45)
[74]. No published data for IFDs in a pediatric population receiving ipilimumab were revealed, but three cases of IFDs (one with IPA and two with PJP; patient with IPA deceased) were documented in adults, sharing in common the manifestation of irAEs followed by the equivalent immunosuppressive treatment
[75][76]. IFD reports are lacking for patients under tremelimumab, but oral candidiasis seems to be a common side effect (5%)
[69].
3.2. Targeting PD-1 or PD-L1
There are ongoing clinical trials involving children with r/r lymphomas for the agents avelumab, camrelizumab, durvalumab, nivolumab and pembrolizumab, while trials on a pediatric population with atezolizumab have been terminated due to limited activity. Although response to atezolizumab was restricted, it was well-tolerated in children and young adults with previously treated solid tumors, NHL and HL in a phase I/II study (2.2% with severe neutropenia, 1.1% with febrile neutropenia, 13.3% with serial infections)
[70]. In general, PD-1/PD-L1 blockade (either as monotherapy or in combination with other anti-cancer drugs) does not seem to increase infection incidence (reporting OR 0.68; 95% CI: 0.67–0.7)
[71]. As explained above, increased risk of infection and recommendation for PJP prophylaxis refers only for patients developing irAEs that eventually require additional immunosuppression with corticosteroids and/or TNF-α targeted agents)
[72]. Risk of IFDs with TNF-α inhibitors alone or in combination with other immunosuppressive agents (like glucocorticoids) is intrinsically elevated in children
[77][78].
PD-1 (programmed cell death protein 1 or CD279) is a transmembrane protein member of the CD28 family on T-, B-, NK and myeloid cells that bears a single extracellular Ig variable domain and a cytoplasmic domain with inhibitory and switch motifs. When PD-L1 (programmed cell death 1 ligand 1 or CD274) binds to PD-1 in the presence of T-cell receptor protein complex (TCR), PD-1 delivers a co-inhibitory signal that terminates TCR/CD28 signal, and thus tumors manage to evade detection and elimination by the immune system. PD-1 activation leads to T-cell anergy and exhaustion, reduced cytotoxicity by NK cells, low cytokine production and inhibitory effects towards myeloid-derived cells like monocytes and macrophages. PD-1/PD-L1 blockade restores T-cell proliferation and the activity of anti-tumor CD8+ T-cells, enhances NK-mediated ADCC, secretes cytokines and attracts APCs
[79].
Avelumab is the first mAb of this category to be approved for pediatric patients and is indicated for subjects 12 years and older with metastatic Merkel cell carcinoma. The safety and efficacy of avelumab towards r/r lymphomas is being tested in ongoing clinical trials. Nevertheless, data on children are lacking, but IFDs have not yet been reported with avelumab
[80].
Camrelizumab has recently entered clinical trials for r/r HL in children. Reports from adult cohorts display no increase in infection rates. A single-arm study noted severe neutropenia in 2.7% of treated patients, one case of severe upper respiratory tract infection, one case of severe urinary tract infection, but no IFD
[81]. Addition of low-dose decitabine to camrelizumab in adults with r/r HL increased the incidence of severe leukocytopenia (37.3% versus zero events in the monotherapy arm), but no severe infections were documented in either group
[82]. Another single-arm study of camrelizumab plus gemcitabine, vinorelbine and pegylated liposomal doxorubicin in r/r primary mediastinal B-cell lymphoma (PMBCL) showed that severe neutropenia occurred in 18.5% of patients, while the only documented infection was a case of severe pneumonia (3.7%)
[83].
Durvalumab is a mAb that blocks the interaction between PD-L1 and PD-1 and has been studied in combination with ibrutinib in patients with r/r follicular lymphoma or diffuse large B-cell lymphoma (DLBCL). Severe neutropenia was documented in 21.3% of patients, but no severe infection or IFD was recorded
[84]. Oral candidiasis may occur (2.1%) but is mild according to Imfinzi
® SPC, while severe pneumonia is seen in 3.5%, severe upper respiratory tract infection in 0.2% and severe urinary tract infection in 4% of patients under monotherapy. Of note, febrile neutropenia is observed in 6.4% of patients receiving durvalumab in combination with chemotherapy
[85].
Treating pediatric r/r lymphoma patients with
nivolumab has displayed some initial favorable results. In a study of children with HL, 10 out of 11 children were found with PD-L1 expression in more than 50% of tumor cells
[86]. Correspondingly, a recent study on children reported PD-1 expression in 19.4% of HL cases and in 18.2% of r/r HL cases, while PD-L1 expression in more than 50% of tumor cells was documented in 67.7% of HL cases and in 72.7% of r/r HL cases, suggesting that comparable responses to PD-1/PD-L1 blockade would be expected in patients undergoing first-, second-, or third-line therapy
[87]. An open-label trial of nivolumab in children with r/r solid tumors and lymphomas reported severe neutropenia in 4.7% and febrile neutropenia in 2.4% of cases
[88]. Apart from HL, off-label nivolumab has also been administered to children with advanced malignancies with promising results, even though two out of 10 children developed severe pneumonia
[89]. Translation of next-generation molecular diagnostics into a biomarker driven treatment strategy is the aim of the INFORM (INdividualized Therapy FOr Relapsed Malignancies in Childhood) program. In this program, children and adolescents with refractory high-risk malignancies will receive nivolumab in combination with entinostat, a class I HDACi (Histone deacetylase inhibitor)
[90]. Low infection rates have been confirmed by a relevant systematic review that calculated incidence of severe infections associated with nivolumab treatment at 0.1% (five cases out of 3386 treated patients; no fatal infection; three with respiratory tract infections and two with urinary tract infections)
[91]. Reports for IFDs with nivolumab are limited to adult patients. One case report described exacerbation of IPA in a patient with lung cancer under nivolumab, while a retrospective study of lung cancer patients treated with nivolumab calculated infection incidence at 19.2% with two cases of IFDs among them (one IPA and one
Candida albicans esophagitis)
[92][93]. The literature search revealed two patients with lung cancer that developed fatal PJP while undergoing immunosuppressive therapy for nivolumab-associated immune-related pneumonitis
[94]. Another adult patient with lung cancer developed nivolumab-induced severe pancytopenia and was diagnosed with fungal pneumonia due to
Fusarium solani [95]. Of note, nivolumab has shown promising results in fighting IFDs: PD-1 blockade seems to have efficacy against neutropenic IPA and other IFDs in murine models; in addition, a woman with mucor-mycosis recovered after receiving nivolumab and IFN-γ, supporting the hypothesis that modulation of the immune system could potentially treat some forms of sepsis and IFDs
[96][97].
Pembrolizumab is the second PD-1 receptor mAb that has been approved for pediatric use and is applicable in children with refractory HL or HL that has relapsed after three or more prior lines of therapy, in children with refractory primary mediastinal B-cell lymphoma (PMBCL) or PMBCL relapsed after two or more prior lines of therapy, in children with unresectable or metastatic, microsatellite instability-high (MSI-H) or mismatch repair deficient solid tumors that have progressed following prior treatment and who have no satisfactory alternative treatment options, and in children with recurrent locally advanced or metastatic Merkel cell carcinoma
[98]. A phase I/II trial of pembrolizumab in pediatric patients with advanced melanoma or a PD-L1-positive, advanced, relapsed, or refractory solid tumor or lymphoma reported only five cases of severe neutropenia in 154 treated children (3.2%), whereas 13% of patients experienced severe infection (with one case of invasive
Candida infection) and 1.3% encountered febrile neutropenia
[99]. In a preceding study of 210 adults with r/r HL under pembrolizumab, the incidence of severe neutropenia was 2.4% and one fatal case of sepsis was noted
[100]. Another study of 127 adults with pembrolizumab for advanced rare cancers (whose tumors progressed on standard therapies) reported only one case of severe neutropenia, severe infection in 4.7% and severe lung infection in 3.1% of patients
[101]. The literature review revealed two cases of allergic bronchopulmonary aspergillosis, one case of allergic fungal rhinosinusitis and one case of IPA in adults administered with pembrolizumab
[102][103][104][105].
4. Chimeric Antigen Receptor (CAR) T-Cells
CAR T-cell recipients are at risk of infections and IFDs mainly due to prolonged leukopenia, depleted B-cells and low immunoglobulin levels. Other risk factors that predispose to severe infections are multiple previous chemotherapy treatment lines (> 3), infusion of high doses of CAR T-cells (2 × 10
7 cells/Kg) and the presence of cytokine release syndrome (CRS) or CAR-T-cell-related encephalopathy syndrome (CRES). The last two clinical entities require the administration of tocilizumab as anti-inflammatory agent (an IL-6 mAb) and dexamethasone, respectively, which increases the infection risk. IFDs occurrence has been associated with HCT and presence of CRS, while the proportion of patients that are expected to develop IFDs varies from 0–14.3% (1% with standard prophylaxis). Strict recommendation for antifungal prophylaxis applies in limited cases (prior mold infection, ≥3 weeks of neutropenia before and after CAR T-cell therapy, dexamethasone use > 0.1 mg/Kg/day for at least a week)
[106][107]. A recent position paper from Spain stated that PJP prophylaxis along with fluconazole should be standard for children under CAR T-cell therapy, while prophylaxis against filamentous fungi (posaconazole, nebulized liposomal amphotericin B or micafungin) should be added when two or more criteria are met: ≥four prior treatment lines, neutropenia prior to the infusion, CAR-T doses > 2 × 10
7 cells/Kg, previous IFD, tocilizumab and/or steroids use
[108]. Tocilizumab use alone is a risk factor for IFDs and patients should be closely monitored
[109]. Besides tisagenlecleucel that has been already approved for pediatric r/r B-ALL, another approved CAR T-cell therapy, axicabtagene ciloleucel is being developed for children. It has displayed favorable results in adults with DLBCL and demonstrated low incidence (1%) of IFDs
[110]. Interestingly, another cohort of adults treated with CD19 CAR-T cells for DLBCL displayed a cumulative incidence of IFD at one year at 4%, stressing the issue for vigilant monitoring for late (>28 days) IFDs
[111]. Brexucabtagene autoleucel (Tecartus
®, Gilead Sciences, Foster City, CA, USA) has been approved for adults with r/r mantle cell lymphoma, but has not entered trials on children.