In this review, our authors discuss the mechanism of action and clinical development of various checkpoint inhibitors in lymphoma.
Lymphoma is the most common hematologic malignancy in the United States, with an estimated 81,000 new cases of Hodgkin (HL) and non-Hodgkin lymphoma (NHL) combined and 21,000 deaths in 2015.1NHL is the sixth most common type of cancer in both males and females, accounting for 5% and 4% of new cancer cases, respectively.1Survival rates for both HL and NHL have improved significantly over the past several decades; however, outcomes continue to be poor in patients with relapsed/ refractory disease.2Recently, the introduction of novel targeted agents has ushered in a new era in management of lymphomas, especially in the relapsed/refractory setting and in frail, elderly patients with multiple comorbid conditions who are unable to tolerate cytotoxic combination chemotherapy and presents an interesting field for further research.
The immune system plays a key role in the surveillance, detection, and elimination of cancer cells. The curative role of immune therapy has long been recognized in patients with hematologic malignancies, with a prime example being allogeneic stem-cell transplantation. Immune checkpoints refer to a plethora of inhibitory pathways hardwired into the immune system that are crucial for maintaining self-tolerance. In addition, they modulate the duration and amplitude of physiological immune responses in peripheral tissues in order to minimize collateral tissue damage.3Checkpoint inhibitors act by "releasing-the-brakes" on the patient’s immune system rather than targeting the tumor directly, and have demonstrated clinically meaningful results in various solid malignancies (ie, melanoma, renal cell carcinoma, lung cancer). This therapeutic strategy emerged from the recognition that tumors can evade the host immune system by usurping immune checkpoint pathways such as the cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and programmed-death 1 (PD-1) pathways. In this review, we discuss the mechanism of action and clinical development of various checkpoint inhibitors in lymphoma.
CTLA-4, the first immune checkpoint receptor to be clinically targeted, is expressed exclusively on T cells and primarily regulates the amplitude of the early stages of T cell activation.3 CTLA-4 downregulates T cell activity and leads to a decrease in the antitumor immune response.4-6The significance of CTLA-4 in maintaining normal immunological homeostasis was confirmed by the findings that CLTA-4 deficient mice die from fatal lymphoproliferative disorders.7Blocking CTLA-4 therefore promotes the persistence and activation of intratumoral T cells.8
Primarily CTLA-4 counteracts the activity of the T-cell constimulatory receptor CD28. CD28 and CTLA-4 share identical ligands: CD80 (also known as B7-1) and CD86 (also known as B7-2). CD28 does not affect T-cell activation unless the T cell receptor (TCR) is first engaged by a cognate antigen. Once antigen recognition occurs, CD28 signaling strongly amplifies TCR signaling to activate T cells.3Since CTLA-4 has a much higher overall affinity for both ligands, its expression on the surface of T cells dampens the activation of T cells by outcompeting CD28 in binding CD80 and CD86, as well as actively delivering inhibitory signals to the T cells.4-6CTLA-4 also confers "signaling-independent" Tcell inhibition through the sequestration of CD80 and CD86 from CD28 engagement, as well as active removal of CD80 and CD86 from the antigen-presenting cell (APC) surface.9
Programmed-death (PD-1, CD 279) is a member of the B7 receptor family, expressed by activated T cells, activated B cells, natural killer cells (NK), and myeloid cells. PD-1 expression is induced when T cells become activated.3It plays a major role in regulation of immune responses by interacting with 2 ligands, programmed death ligand 1 (PD-L1) (B7-H1 or CD274) and programmed death ligand 2 (PD-L2) (B7-DC or CD273).10PD-L1 is expressed by B and T cells and macrophages, mediating a generalized anti-inflammatory effect; whereas PD-L2 is expressed by APC10 and regulates T-cell priming.11 These ligands are upregulated by the inflammatory environment and inhibit function of PD-1-bearing lymphocytes. When engaged by one of its ligands, PD-1 inhibits kinases that are involved in T-cell activation through the phosphatase SHP2. In a healthy host, PD-1/PD-L1 signaling regulates effector T-cell responses and protects bystander tissues from immune-mediated damage. This pathway is harnessed by many tumors to evade immune surveillance and forms a major resistance mechanism within the tumor microenvironment.
Mice deficient in PD-1 have an immune phenotype distinct from mice deficient in CTLA-4, as CTLA-4 is believed to primarily regulate early T-cell activation and PD-1 is believed to inhibit T-cell effector activity in the effector phase.12The inhibition of T-cell activity by PD-1 engagement appears stronger than by CTLA-4 engagement, although the phenotype of PD-1 knockout mice is less severe than that of CTLA-4 knockout.13The expression of PD-1 is also observed on other immune subsets including T-regulatory (Treg) cells, B cells, and NK cells; Treg cells heavily infiltrate many tumors and suppress effector immune responses, thus PD-1 blockage may also increase antitumor cytotoxicity through increased NK-cell killing and reduction in Treg cell number and function.114,15PD-1 binds to its ligands PD-L1 and PD-L2 and the interaction results in downstream inhibitory signals resulting in apoptosis of activated T cells.16Chronic antigen exposure, such as in chronic viral infection or cancer, can lead to persistently high PD-1 expression, which is associated with T-cell exhaustion; blockade of the PD-1/ PD-ligand pathway augmented or restored the function of viral-infection specific and tumor-specific CD4+ and CD8+ T cells in mouse and human studies.3
Lymphomas variably express PD-1 and its ligands.17PD-1 expression is common among tumor cells in chronic lymphocytic leukemia (CLL) and angioimmunoblastic T-cell lymphoma (AITL) but rare in other NHL subtypes.18,19PD-1+ T cells are frequently demonstrated in rosettes surrounding Reed-Sternberg (RS) cells in both classical Hodgkin lymphoma (cHL) and nodular lymphocyte subtypes.119,20PDL1 expression is common in tumor cells in HL and primary mediastinal B-cell lymphoma (PMBL) where it appears to be associated with amplification in chromosome 9p24.121 and aggressive virus-driven lymphomas.22A large proportion of cHL tumors have increased surface expression of PD-L1,22 strongly suggesting that HL may have a genetic dependence on the PD-1 pathway for survival. HL is characterized by the presence of few RS cells surrounded by an extensive, but ineffective immune infiltrate. Amplification of 9p24.1 is a recurrent genetic abnormality in HL and mediates PD-L1 and PD-L2 expression.21PD-1 ligand expression is also increased through the JAK2/STAT pathway, as the extended 9p24.1 amplification region also includes the JAK2 locus.21In addition, EBV infection, seen commonly in HL also increases PD-1 ligand expression.23PD-L1 is also expressed on diffuse large B-cell lymphoma (DLBCL) cells and tumor-infiltrating non-malignant cells, primarily macrophages.17,22Andorsky and colleagues studied PD-L1 expression and functional activity in cell lines and lymphoma specimens.17PD-L1 was expressed uniformly in anaplastic large cell lymphoma (ALCL) cell lines, but rarely in B-cell NHL. PDL-1 expression in B-cell lymphoma was observed in activated B-cell (ABC) DLBCL. Anti-PD-L1 blocking antibody boosted proliferation and IFN-γ secretion by allogeneic T cells responding to ALCL and DLBCL cells.17
The prognostic implication of PD-1 and its ligand expression on tumor cells in lymphoma is conflicting. In 2 studies, a high proportion of follicular lymphoma (FL) cells expressing PD-1 correlated with a favorable overall survival (OS).20,21In other reports, the prognostic impact of PD-1 expression was negligible or adverse.22 PD-1 is expressed on tumor-infiltrating lymphocytes (TILs) in DLBCL, and the presence of a large number of PD-1+ TILs is associated with favorable OS in patients with DLBCL.23,24 In a prospective cohort of DLBCL patients treated with rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R+CHOP), elevated soluble PD-L1 was adversely prognostic for OS.24In a recent study describing the clinicopathological impact of PD-L1+ DLBCL, Kiyasu et al defined a new sub-entity termed microenvironmental PD-L1+ (mPD-L11) DLBCL (ie, PD-L1DLBCL in which PD-L1+ nonmalignant cells are abundant in the tumor microenvironment).25 The prevalence rates of PD-L1+ and mPD-L1+ DLBCL were 11% and 15.3%, respectively. Both PD-L1+ and mPD-L1+ DLBCL were significantly associated with non-germinal center B-cell (GCB) type and Epstein-Barr virus positivity. Patients with PD-L1+ DLBCL had inferior OS compared with patients with PD-L1– DLBCL. In contrast, there was no significant difference in OS between mPD-L1+ and mPDL1–DLBCL.25
In summary, level of expression of PD-1 and its ligands may be used to guide early development of checkpoint inhibitors in various lymphoma subtypes; however, it is not ready for primetime as either a predictor of response or prognosis.
Ipilimumab is a recombinant human IgG1 monoclonal antibody that binds to CTLA-4 and enhances T-cell activation and proliferation. It has significant clinical benefit in solid tumors such as melanoma, and it is approved as frontline therapy in metastatic melanoma.26A phase I study of ipilimumab was performed in patients with relapsed and refractory B-cell NHL.27Eighteen patients [14 follicular lymphoma (FL), 3 DLBCL, 1 mantle cell lymphoma (MCL)] were treated with ipilimumab at 2 dose levels with an 11% (2/18) overall response rate (ORR). One DLBCL patient achieved a complete response (CR) and 1 FL patient achieved a partial response (PR). Despite low rates of response, the responses were durable for both patients lasting 31 months for the DLBCL patient and 19 months for the FL patient. Ipilimumab was well tolerated, with common adverse events being diarrhea, headache, abdominal pain, anorexia, fatigue, neutropenia, and thrombocytopenia. Correlative studies confirmed that T-cell proliferation to recall antigens was increased in a significant proportion of patients, suggesting that the immune response was activated and enhanced.
Ipilimumab was also studied in the post-allogeneic stem cell transplant (alloSCT) setting in hematological malignancies.28Twenty-nine patients with hematological malignancies that were recurrent or progressive after alloSCT received a single infusion of ipilimumab; responses were predominantly seen in patients with lymphoma. There were 2 CR in patients with HL and a PR in a patient with MCL. Interestingly, ipilimumab did not induce or exacerbate clinical graft versus host disease or graft rejection in this study. There is currently a phase I trial investigating the combination of ipilimumab and rituximab in patients with relapsed or refractory B-cell lymphoma (NCT01729806).
Nivolumab is a fully human IgG4 monoclonal antibody that inhibits PD-1 activity by binding to the PD-1 receptor and blocking its interaction with its ligands PD-L1 and PD-L2. It has shown promising results in the treatment of solid tumors and is currently approved by the FDA for treatment of advanced/metastatic melanoma and metastatic non small-cell lung cancer. A phase I study of nivolumab in patients with relapsed/refractory hematologic malignancies (NCT01592370) including classical HL, B-NHL, T-NHL, and multiple myeloma is currently underway. Results for the 23 patients with HL were published earlier this year.29
The HL study consisted of a high risk population with a median of 4 to 5 lines of prior systemic therapy with 78% of patients relapsing post brentuximab vedotin and 78% of patients relapsing post-autologous stem cell transplant (ASCT). Patients received nivolumab 3 mg/kg every 2 weeks until CR, tumor progression, or intolerable toxicity. ORR was 87% (20/23) with 17% CR and 70% PR rate. The remaining 3 patients had stable disease. The rate of progression-free survival (PFS) at 24 weeks was 86%. Many of the responses appeared durable with some patients in continued remission for over a year, though the follow-up time of this study was short. Nivolumab was well tolerated in the study population. Drug-related adverse events of any grade and of grade 3 occurred in 78% and 22% of patients, respectively. No grade 4 or 5 events were reported. The most common adverse events were rash (22%) and decreased platelet count (17%). Drug-related grade 3 events reported in 5 patients consisted of myelodysplastic syndrome, pancreatitis, pneumonitis, stomatitis, increased lipase levels, decreased lymphocyte count, and leucopenia.
Nivolumab was granted FDA "Breakthrough Therapy" designation for HL. A phase II study of nivolumab in patients with classical HL who have relapsed post-ASCT has recently completed enrollment (NCT02181738). There is considerable interest in introducing nivolumab in the frontline setting in HL; however, to our knowledge, no such clinical trials have been initiated.
Results for the remaining lymphoid malignancies have been reported in an abstract form.30Twenty-nine patients with B-NHL, 2 patients with PMBL, and 23 patients with T-NHL were enrolled. These patients were heavily pretreated with more than two-thirds of patients receiving ≥ 3 prior therapies. The most common drug-related serious adverse event was pneumonitis (7%). The ORR for patients with B-NHL was 28% (7% CR) with an ORR of 36% in patients with DLBCL, and 40% in patients with FL. The ORR was 17% in patients with T-cell NHL (no CR) with an ORR of 40% in the 5 patients with peripheral T-cell lymphoma. These results have led to 2 additional phase II clinical trials investigating the efficacy of nivolumab in relapsed or refractory FL (NCT02038946) and relapsed or refractory DLBCL (NCT02038933), which are ongoing.
Pembrolizumab is a humanized IgG4 anti-PD-1 monoclonal antibody that also blocks binding of PD-1 to its ligands PD-L1 and PD-L2. Since IgG4 cannot engage the Fc receptor and cause antibody-dependent cellular cytotoxicity (ADCC) of PD-1+ cells, enhancement of antitumor immune response is the primary mechanism of action.
Pembrolizumab was approved by the FDA for treatment of advanced/metastatic melanoma and nonsmall-cell lung cancer.
The KEYNOTE-013 trial is a phase Ib study investigating the safety and efficacy in patients with relapsed or refractory hematologic malignancies (NCT01953692). Results of the classical HL cohort of this study were presented at the American Society of Hematology (ASH) 2014 conference in an abstract form.31Twenty-nine patients with relapsed/ refractory HL received pembrolizumab 10 mg/kg every 2 weeks. More than half the patients had received at least 5 prior lines of therapy; all had received brentuximab vedotin and two-thirds had undergone ASCT. ORR was 66% (all PRs) in the whole cohort, 75% in patients relapsing post-ASCT, and 44% in patients who did not receive ASCT. Median duration of response was not reached. The most common drug-related adverse events were grade 12 respiratory events (20%) and thyroid disorders (20%). Grade 3 or higher treatment-related adverse events were rare and included axillary pain, hypoxia, joint swelling, and pneumonitis in 1 patient each. There were no grade 4 events or treatment-related deaths. The trial is still in progress with final results including data from the B-NHL arm still pending. A phase II study of pembrolizumab in relapsed/refractory classical HL is currently recruiting (KEYNOTE-087, NCT02453594).
Pembrolizumab is currently under intensive investigation in a variety of settings in lymphoma, as summarized inTABLE 1. Studies are also investigating the efficacy of pembrolizumab as consolidation therapy post ASCT in DLBCL and HL (NCT02362997), in combination with rituximab in relapsed FL (NCT02446457), and in combination with chemotherapy for advanced lymphoma (NCT02408042).
Pidilizumab is an anti-PD-1 humanized IgG1 monoclonal antibody that blocks PD-1 activity, leading to stimulation of NK-cell activity and extended effector/memory T-cell survival. These changes are associated with the enhancement of antitumor immune responses and the generation of tumorspecific memory cells.32
The first in-human study was a phase I trial conducted by Berger et al. Seventeen patients with advanced hematologic malignancies (CLL-3, DLBCL-2, HL-1, FL-1) were treated with a single dose of pidilizumab at 5 dose levels ranging from 0.2 to 6 mg/kg.32The treatment was well tolerated and no dose-limiting toxicity was observed. Six patients (35%, including 1 FL, 2 CLL and 1 HL) had clinical benefit, with 1 CR in a patient with FL that lasted over a year. Based on this encouraging efficacy signal, pidilizumab has been moved into phase II trials in lymphoid malignancies.
Armand et al conducted an international phase II study of pidilizumab in patients with DLBCL or PMBL after ASCT.33The rationale for selecting this population was the expression of PD-L1 in a subset of patients, low tumor burden post-ASCT, and remodeling of the immune system in the post-transplant setting. There was a preponderance of NK cells, monocytes, and CD45RO+ memory/effector cells among the circulating lymphocytes post ASCT, which comprise pidilizumab target populations and portend a favorable prognosis. Sixty-six patients were treated with 3 doses of pidilizumab 1.5 mg/kg beginning 1 to 3 months after ASCT. The 16-month PFS was 72%. Of note, the 18-month PFS was 70% among the 24 patients who had a positive positron emission tomography scan after pre-ASCT salvage therapy. These results compared favorably to the 52% PFS in an otherwise similarly high-risk historical control population. Among 35 patients with measurable disease post-ASCT, CR and ORR were 34% and 51%, respectively. The safety profile was favorable with no apparent autoimmune toxicity, treatment-related deaths, or infusion reactions. The main non-hematologic adverse events were fatigue, upper respiratory tract infection, diarrhea, cough, and hyperglycemia. Grade 3-4 neutropenia and thrombocytopenia occurred in 14 (20%) and 6 (8%) patients, respectively. Treatment with pidilizumab was associated with an increase in PD-L1-bearing activated helper T cells and PD-1 ligand bearing monocytes, suggesting an on-target in vivo effect of pidilizumab.
Westin et al performed a single center phase II study of pidilizumab with rituximab in patients with relapsed follicular lymphoma.34Thirty-two patients with rituximabsensitive FL relapsing after 1 to 4 previous therapies were treated with 4 doses of pidilizumab 3 mg/kg every 4 weeks, with an option to receive up to an additional 8 doses for a total of 12 infusions. Rituximab was given 375 mg/m2 weekly for 4 doses commencing 17 days after the first dose of pidilizumab. Median follow-up was 15.4 months. In the 29 response-evaluable patients, ORR was 66% with 52% CR and median PFS of 18.8 months. The combination was well tolerated with no grade 3-4 adverse events. The most common grade 1 events were anemia and fatigue, with respiratory infection being the most common grade 2 adverse event. Responders expressed higher levels of PD-L1 on peripheral blood T cells and monocytes at baseline compared to non-responders. The in vivo on-target effect of pidilizumab was demonstrated by increased expression of activation-associated genes by T cells and NK cells in day 14 samples compared to baseline.
A phase II study of pidilizumab in patients with stage III-IV DLBCL following first remission is currently recruiting participants (NCT02530125).
In summary, anti-PD-1 antibodies have shown variable response in lymphomas ranging from an impressive 87% ORR in HL to 17% in T-cell lymphomas (for nivolumab). This underlines the inherent immunologic heterogeneity of lymphomas with the immunogenic tumors such as cHL responding much more briskly than other lymphoma subtypes. The safety profiles of nivolumab, pembrolizumab, and pidilizumab have been favorable. Most patients experienced mild drug-related adverse events with no treatmentrelated deaths. The frequencies of immune-related adverse events (irAEs) commonly noted with anti-PD-1 therapy were lower compared to previously reported trials in solid tumors [nivolumab29: rash-22%, grade 3/4 none; diarrhea-13%, grade 3/4 – none; hypothyroidism – 9%, grade 3/4 – none; pancreatitis grade 3/4 – 4%; pembrolizumab 31: grade 1-2 respiratory events – 20%; grade 1-2 thyroid disorders – 20%, grade 3/4 irAEs – none; pidilizumab 33,34: serious irAEs – none], however, since most lymphoma trials are phase I, data are immature, with most results being available in only abstract form, and the patient numbers are inadequate to comment on this issue conclusively.
Several agents targeting the PD-1 axis are currently under investigation; although many have shown promising results in solid malignancies, their clinical development in lymphomas is in preliminary stages.
Multiple additional immune checkpoints are promising targets for therapeutic blockade based on preclinical experiments, and inhibitors for many of these are under active development:
As our knowledge of immune checkpoints expands, a plethora of agents are entering clinical investigation and it is likely that at least a few will reach the clinic over the next few years (See ).
Immunotherapy is revolutionizing the practice of oncology, but we have just scratched the surface of this vastly complicated biological machinery. Much needs to be done in order to deepen our understanding of the immune system so that more efficacious agents with a better toxicity profile can be developed.
Treatment with immune checkpoint inhibitors has promising clinical activity in lymphoma, particularly in patients with HL. However, responses are not uniform across various histologies of lymphoma, and further research is needed to understand the differences in the biology of both the "drug" and the "disease." With a plethora of ongoing, planned, and proposed trials, clinical testing is already outpacing our understanding of the underlying checkpoint biology. Careful scrutiny of generated data and continued collaboration between bench and bedside will maximize the therapeutic potential of this revolutionary strategy.