Targeting KIR Opens Door to Promising Immunotherapy Combos

Although T cells have commanded most of the attention in the burgeoning immuno-oncology field, there is a growing appreciation that other immune cells have important roles in tumor surveillance and would represent attractive therapeutic targets.

As the body’s first line of defense, natural killer (NK) cells have gradually emerged as just such a target. Analogous checkpoint receptors expressed on their surface that regulate activation and inhibition of NK cell cytotoxic activity offer a particularly promising avenue for drug development.

In order to manipulate these cells, researchers have been focusing on the killer cell immunoglobulin-like receptor (KIR) pathway, one of the central receptor families involved in NK cell activation. Innate Pharma and Bristol-Myers Squibb are leading the field with their joint development of lirilumab, a monoclonal antibody that targets KIR.

Natural Born Killers

While the path to regulatory approval for this drug class is not wrinkle free, recent data suggest a potentially significant role for this treatment paradigm in rational combinations of immunotherapy and targeted therapy that capitalize on their unique mechanism of action.First discovered more than 40 years ago, the NK cells are called "natural killers" because of their ability to rapidly kill target cells without having to be primed first, for example, by exposure to an antigen, as is necessary for T cells.

These cells are on the front line of the body’s innate immune response, patrolling the body on high alert, ready to kill when they encounter target cells including virally infected and cancerous cells.

Although the speed with which the NK cells can kill is highly advantageous, it could prove very dangerous to normal cells if they were to kill indiscriminately; thus, their activation is tightly regulated. This is achieved through a delicate balance between activating and inhibitory receptors expressed on the surface of the NK cell.

The best-defined ligands of the inhibitory receptors are the major histocompatibility complex (MHC) class I proteins that take peptides from within the cell and present them on the surface to help identify a cell as “self” or “non-self.” Self-MHC class I proteins are found on the surface of all normal cells of the body and, when confronted by an NK cell, they bind to the inhibitory receptors on its surface and suppress NK-cell activity.

To become activated, the NK cell requires reversal of this inhibition, which is achieved when a cell that does not express self-MHC class I molecules is encountered (detection of missing self), as well as stimulation of the activating receptors. The latter occurs when the target cell expresses the ligands that bind these receptors; for example, the stress ligands that are expressed by cancer cells resulting from their extensive DNA damage. Combined stimulation of the activating receptors and lack of suppression of the inhibitory receptors shift the balance in favor of NK cell activation.

NK cells are derived from hematopoietic stem cells in the bone marrow and differentiate into large, granular mature NK cells that comprise between 5% and 20% of the total population of circulating lymphocytes. The granules they contain are composed of the membrane-disrupting protein perforin and proteolytic enzymes called granzymes. Once activated, the NK cell tightly associates with the target cell, forming what is known as an immunological synapse in recognition of its resemblance to a nervous system synapse. This allows the granules to be passed from the NK cell to the target cell, where they cause cell lysis and death.

NK cells can also kill target cells in several other ways, including by expressing the tumor necrosis factor (TNF) and death-inducing ligands, such as the TNF-related apoptosis-inducing ligand (TRAIL), both of which bind to death-inducing receptors on the surface of the target cell, triggering programmed cell death. NK cells also produce a variety of cytokines and chemokines that can attract other immune cells to the target cell location.

KIRs Play Dual Immune System Roles

Studies have now shown that NK cells play an important role in the antitumor immune response, but tumors also appear to have evolved mechanisms of overcoming NK cell—mediated immune surveillance. Depletion of NK cells or impaired NK cell function has been noted in various cancer types.One of the central receptor families involved in NK-cell activation is the KIRs, encoded by a group of 15 functional and 2 pseudogenes found within a section of chromosome 19 known as the leukocyte receptor complex.

When it comes to their regulation of NK-cell cytotoxicity, KIRs can be either activating or inhibitory, with their structure determining their function as well as the types of ligands with which they interact.

The potential importance of inhibitory KIRs in NK cell—mediated killing of cancer cells was first inferred more than a decade ago, when researchers observed that patients with acute myeloid leukemia (AML) who were receiving bone marrow transplants had significantly better outcomes if there was a mismatch in KIR and MHC class I ligands between donor and recipient, compared with those who received matched transplants. That is, if the recipient’s cells did not express the MHC class I ligand subtype that was recognized by the types of inhibitory KIR genes they possessed, their NK cells would not be blocked by their inhibitory KIRs binding to self-MHC molecules. Essentially, in the mismatched transplants, the recipient’s NK cells would not recognize the donor cells as self.

KIR genes are inherited independently from their MHC class I ligands and several different KIR/KIR ligand genotypes can result. Studies have suggested that this genotype may be associated with the incidence of various types of malignancies including melanoma, breast cancer, leukemia, and lymphoma, and with patient outcomes.

Limitations of KIR-Targeted Monotherapy

Since the interaction between inhibitory KIRs and their ligands keeps NK cell activity in check, they represent a novel type of immune checkpoint and an exciting opportunity for therapeutic targeting. This has not gone unappreciated by oncologists; since the reports of improved outcomes with mismatched transplants, there have been concerted efforts to reproduce these results pharmacologically by blocking inhibitory KIR function with monoclonal antibodies.To date, 2 antibodies targeting inhibitory KIRs have been developed. Lirilumab (IPH2102/BMS- 986015) is being jointly developed by Innate Pharma and Bristol-Myers Squibb and is leading the field, while Innate Pharma is also developing a second KIR-targeting drug, IPH4102. The former is a human immunoglobulin G4 monoclonal antibody directed against 3 inhibitory KIRs—KIR2DL1, KIR2DL2, and KIR2DL3—while the latter is a first-in-class anti-KIR3DL2 monoclonal antibody.

In preclinical studies, KIR-targeted antibodies were shown to block inhibitory signaling in NK cells and to enhance their production of cytokines, in addition to displaying activity against tumor cells bearing the respective ligands. These drugs also induced antibody-dependent cellular cytotoxicity (ADCC).

In phase I studies, lirilumab proved to be safe and well tolerated in patients with AML and multiple myeloma when used as a single agent, with none of the severe side effects that would be anticipated if it induced autoimmunity.

It is hypothesized that blocking inhibitory KIRs should not affect normal cells, because activating receptors also must be triggered to fully mobilize NK cells. Healthy cells do not express any of the ligands for these activating receptors but cancerous cells do; thus, KIR inhibition should only activate NK cells when a cancerous cell is encountered. This theory seems to have been borne out in the favorable safety profile of this drug class in clinical trials to date.

However, although lirilumab proved safe, thus far it has displayed limited antitumor efficacy when used as monotherapy. Most recently, a phase II trial of lirilumab in patients with smoldering multiple myeloma was closed due to lack of therapeutic benefit.

Poor antitumor efficacy has always been a possibility for NK cell—targeted therapies for several reasons, some of which are only now being revealed. NK cells are short lived and can be inactivated by tumors in several ways. Since NK cells don’t mount an effective immune response in isolation, many researchers doubt the ability of NK cell–targeted therapies to fully eliminate a tumor on their own and the development of rational combination therapy is likely to be key.

Another potential limitation of KIR blockade is that KIRs and their ligands play an important role in NK cell education, also known as licensing, the process through which immature NK cells achieve functional maturity. Although the process is still incompletely understood, essentially NK cells that express inhibitory KIRs that recognize MHC molecules found on normal tissues of the same individual are “educated” or “licensed” during their development. NK cells that don’t express inhibitory KIRs or express only KIRs that bind to non-self MHC ligands remain unlicensed.

Recent studies suggest that NK cell education is a dynamic process, maintained through constant KIR signaling. Thus, continual blockade of KIRs could impact this process and result in NK cells with reduced functionality. Ongoing clinical studies of lirilumab are investigating the effects of continuous and intermittent dosing to further tease out these effects.

Moving Forward With Combinations

Despite the disappointment, KIR-targeted monoclonal antibodies continue to be evaluated in the clinic (Table). IPH4102 is being studied as a single agent in patients with cutaneous T-cell lymphoma (CTCL), a malignancy that frequently displays expression of KIR3DL2, which is the target of this drug.

In an ongoing phase I dose-finding study, patients with relapsed/refractory CTCL are receiving IPH4102 until disease progression or unacceptable toxicity. Ten dose levels are being evaluated (0.0001 to 10 mg/kg), with the first 7 dose levels having been completed. The results from the 16 patients treated thus far were presented at the 2016 American Society for Hematology (ASH) Annual Meeting. There have been no dose-limiting toxicities and the best global response rate across all doses was 38%, with all responses ongoing at the time of analysis.

Lirilumab, meanwhile, is forging new promising therapeutic avenues as part of rational combinations of drugs. Clinical data were presented at 2016 ASH for the combination of lirilumab and the hypomethylating drug 5-azacytidine. The combination is currently being evaluated in patients with AML in a phase Ib/II trial.

Thus far, 25 patients have been treated and are evaluable for response, having received azacytidine at a dose of 75 mg/m² on days 1 through 7 and lirilumab at a dose of either 1 or 3 mg/kg on day 8. The combination is well tolerated, with no dose-limiting toxicities, and is showing promising signs of antitumor activity. The response rate is currently approximately 20% and includes 1 complete response (CR), 1 CR with insufficient count recovery, and 3 patients with hematologic improvement lasting longer than 6 months.

Preliminary results were also recently reported for an ongoing phase I/II trial of lirilumab in combination with the PD-1-targeting immune checkpoint inhibitor nivolumab in patients with advanced solid tumors. The presentation at the 2016 Society for the Immunotherapy of Cancer (SITC) Annual Meeting detailed findings from a subgroup of patients with platinum-refractory squamous cell carcinoma of the head and neck.

Among 29 patients evaluable for response, the overall response rate was 24.1%, including 3 CRs and 4 partial responses, 2 of which were near-CRs. The 6- and 12-month overall survival rates are 90% and 60%, respectively. The safety profile of the combination is similar to nivolumab monotherapy. The most common adverse events (AEs) included fatigue, pruritus, infusion-related reactions, and rash. Grade 3/4 treatment-related AEs occurred in approximately 15% of patients, of which 2.5% led to treatment discontinuation.

As our understanding of the complexities of KIR biology continues to improve, additional opportunities for clinical translation may present themselves. Growing evidence suggests that modulation of NK cell activity may form part of the mechanism of action of several other established anticancer therapies such as the proteasome inhibitor bortezomib, the immunomodulatory agent lenalidomide, and the JAK/STAT inhibitor ruxolitinib. Thus, combining NK cell—targeted therapies such as KIR blockade with these drugs could further boost this effect and enhance antitumor efficacy.

KEY RESEARCH

Bagot M, Porcu P, Ram-Wol C, et al. First-in-human, multicenter phase I study of IPH4102, first-in-class humanized anti-KIR3DL2 monoclonal antibody, in relapsed/refractory cutaneous T-cell lymphomas: preliminary safety, exploratory and clinical activity results. Presented at: American Society of Hematology 58th Annual Meeting; December 3-6, 2016; San Diego, CA. Abstract 1826.

Benson Jr DM, Caligiuri MA. Cancer immunology at the crossroads: killer immunoglobulin-like receptors and tumor immunity. Cancer Immunol Res. 2014;2(2):99-104. doi:10.1158/2326-6066.CIR-13-0219.

Carotta S. Targeting NK cells for anticancer immunotherapy: clinical and preclinical approaches. Front Immunol. 2016;7:152. doi:10.3389/ mmu.2016.00152.

Dahlberg CI, Sarhan D, Chrobok M,et al. Natural killer cell-based therapies targeting cancer: possible strategies to gain and sustain anti-tumor activity. Front Immunol. 2015;6:605. doi:10.3389/ mmu.2015.00605.

Daver N, Garcia-Manero G, Basu S, et al. Phase IB/II study of lirilumab in combination with azacytidine (AZA) in patients (pts) with relapsed acute myeloid leukemia (AML). Presented at: American Society of Hematology 58th Annual Meeting; December 3-6, 2016; San Diego, CA. Abstract 1641.

Felices M, Miller JS. Targeting KIR blockade in multiple myeloma: trouble in checkpoint paradise? Clin Cancer Res. 2016;22(21):5161- 5163. doi:10.1158/1078-0432.CCR-16-1582.

Garber K. Natural killer cells blaze into immuno-oncology. Nat Biotechnol. 2016;34(3):219-220. doi:10.1038/nbt0316-219.

Leidner R, Kang H, Haddad R, et al. Preliminary efficacy from a phase 1/2 study of the natural killer cell—targeted antibody, lirilumab, in combination with nivolumab in squamous cell carcinoma of the head and neck. Presented at: 2016 SITC Annual Meeting; November 9-13, 2016; National Harbor, MD. Abstract 456.

Morvan MG, Lanier LL. NK cells and cancer: you can teach innate cells new tricks. Nat Rev Cancer 2016;16(1):7-19. doi:10.1038/nrc.2015.5.