Chemokine receptor type 2, a major recruiter of circulating monocytes that subsequently develop into a protumoral type of macrophage within the tumor microenvironment, has emerged as a promising therapeutic target.
The tumor microenvironment (TME) comprises a diverse array of immune cells recruited into the supportive niche that surrounds the tumor through interactions between chemokines and their cell surface receptors. Chemokine receptor type 2 (CCR2), a major recruiter of circulating monocytes that subsequently develop into a protumoral type of macrophage within the TME, has emerged as a promising therapeutic target.
Efforts to bring CCR2 antagonists to the clinic have been abandoned by several pharmaceutical companies in recent years, but a more nuanced understanding of the role of the chemokine network in the initiation and progression of tumors is fostering development of several drugs, including a dual CCR2/chemokine receptor type 5 (CCR5) antagonist.
The CCR2 signaling axis may also offer a way to overcome challenges for an established therapeutic strategy. Chimeric antigen receptor (CAR) T cells genetically engineered to express CCR2 could penetrate more readily into the TME, which has been a barrier to the antitumor efficacy of CAR T cells in solid tumors until now.The cells of the immune system secrete a range of chemical messengers known as cytokines that facilitate communication with one another and the host. Chemokines are a subfamily of cytokines that specialize in mediating migration; they function as chemoattractants, providing directional cues for immune cells to recruit them to sites of inflammation where immune cells are needed for tissue defense and repair. There are several subgroups of chemokines, defined by the position of conserved cysteine amino acids within their structure; in the CC chemokines, there are 2 adjacent cysteines near the beginning of the protein.
Homeostatic chemokines, which maintain the status quo in many tissues, are expressed continuously. In the event of infection, injury, or tissue damage, inflammatory chemokines can also be expressed that help to address the problem.
Chemokines exert their effects by binding to transmembrane, G protein—coupled chemokine receptors on the surface of target cells. Responding cells migrate along a chemokine gradient toward areas with high local concentrations.1-3CCR2 functions predominantly as a receptor for chemokine ligand 2 (CCL2), an inflammatory CC chemokine, also known as monocyte chemoattractant protein-1, reflecting its major role in the regulation of monocyte migration to sites of pathologic inflammation.
Alternative splicing of the gene produces 2 isoforms, 2A and 2B, that are expressed in different tissues by different cells and can induce varied biological responses. The majority of CCR2 is expressed by monocytes and natural killer cells, although it is also found on other cell types.
Induced by a range of stimuli, including platelet- derived growth factor, CCL2 is expressed by different cell types, including endothelial cells, fibroblasts, and smooth muscle cells. Monocytes are also major producers of CCL2, establishing a positive feedback loop to recruit more monocytes to the site of inflammation (Figure).4-8
Monocytes are components of the mononuclear phagocyte system, a class of specialized cells that occur throughout the body and share a phagocytic function, primarily in the orchestration of the innate immune response. They are produced in the bone marrow from hematopoietic stem cells, then enter the blood stream and the spleen. They patrol the body and penetrate the tissues, ready to be mobilized in response to injury or inflammation.
The many intricacies of monocyte function in the immune system are still being teased apart—both inflammatory and no-inflammatory monocytes have been described—but their best understood and their likely primary function is to serve as the precursors for 2 other types of white blood cells: macrophages and dendritic cells (DCs). As monocytes enter the tissue, they differentiate into subtypes of macrophages and DCs, with a range of functions, depending upon local environmental cues.
Macrophages, for example, have a phenotypic continuum, from the classically activated, proinflammatory, proimmunity M1 type to the alternatively activated, anti-inflammatory, immunosuppressive M2 type at the 2 extremes. The latter are required at the later stages of the inflammatory response to limit damage to normal cells and allow time for healing and remodeling.9Tumors don’t exist in isolation; they are surrounded by host cells and tissues, with which they can interact. Tumor cells can use those interactions to manipulate their local microenvironment, creating a supportive niche. The TME fundamentally influences tumor biology and acts as a barrier to effective anticancer therapy.
A defining feature of the TME is the infiltration of immune cells, which enter the stroma in their capacity as immunosurveillers for the antitumor immune response. Monocytes and macrophages have been universally observed across tumor types, although to varying degrees. In some tumors, they constitute the majority, while in others they are less prevalent.10
In part, the TME creates a protumoral niche by providing the local environmental cues that shape the nature of the immune infiltration to suppress the antitumor immune response.10 There is evidence that CCR2 is involved in the recruitment of inflammatory monocytes into the TME, which then differentiate into tumor-associated macrophages (TAMs) that are strongly polarized toward an M2-like phenotype.11
The precise role of CCR2 signaling in tumor pathogenesis is yet to be clearly determined, but CCL2 overexpression and high numbers of CCR2-positive inflammatory monocytes have been noted in a range of tumor types, including hepatocellular carcinoma and pancreatic, breast, glioma, colorectal, lung, and bladder cancers. Additionally, elevated levels of CCL2 have been associated with advanced disease and poor prognosis in several studies.12-16 CCL2 expression levels have been shown to rise even higher after anticancer therapy.17
Study results have demonstrated that CCR2- positive inflammatory monocytes may also play a role in promoting tumor progression through their effects on metastasis and angiogenesis.7,18-20 The expression of CCR2 on breast cancer cells has been noted, but its function in this capacity remains unclear.6
A large body of research demonstrates that blocking TAM accumulation in tumors or changing their polarization to a proinflammatory, antitumor one can potentiate anticancer therapy. CCR2 offers a promising therapeutic target in this respect.11In the preclinical setting, small-molecule CCR2 antagonists appeared highly effective in a number of tumor types.12,16,17,21 Several pharmaceutical companies have taken on the challenge of developing CCR2 antagonists in the clinic, but to date, only early-phase data have been published. Several drugs have been abandoned, with the companies citing business reasons for the decision. Pfizer was developing PF-04133609, which showed promise in patients with pancreatic adenocarcinoma. In 2016, findings from an open-label, nonrandomized phase IB study published in Lancet Oncology demonstrated that combining chemotherapy with CCR2 inhibition led to a much better tumor response than chemotherapy alone. Patients were treated with FOLFIRINOX (leucovorin [folinic acid], fluorouracil, irinotecan, and oxaliplatin) alone (n = 8) or FOLFIRINOX plus PF-04133609 at a dose of 500 mg twice daily (n = 33). None of the patients treated with chemotherapy alone achieved an objective response, while the response rate was 49% among those also treated with the CCR2 inhibitor.22 A subsequent study of PF-04133609 in combination with gemcitabine and nab-paclitaxel (Abraxane) as first-line therapy for patients with metastatic pancreatic cancer was terminated for business reasons.23
Takeda also halted studies of plozalizumab (MLN1202; TAK-202), a monoclonal antibody directed against CCR2, in the treatment of melanoma and solid tumors, according to information posted on ClinicalTrials.gov.
Other CCR2 inhibitors are being studied in clinical trials (Table). The study of PF-04133609 served as a precedent for a clinical trial of ChemoCentryx’s CCX872 in combination with FOLFIRINOX in patients with pancreatic cancer. The study completed enrollment last year and preliminary results were reported at the 2018 American Society of Clinical Oncology Annual Meeting.
Fifty patients received FOLFIRINOX once every 2 weeks for a maximum of 12 cycles in combination with CCX872 at a dose of 150 mg once or twice daily for 12 weeks. Patients with at least stable disease by the end of the 12-week treatment period could continue treatment until disease progression. The all-subject overall survival (OS) rate at 18 months was 29%, and patients experienced a better OS outcome if they had a lower peripheral blood monocyte count at baseline (hazard ratio, 1.169; P =. 0071).24
Bristol-Myers Squibb is also developing a CCR2 inhibitor, BMS-813160, that targets CCR5 in addition to CCR2. The theory behind this dual inhibitor is that CCR2 inhibition will block the migration of TAMs and monocytic myeloid-derived suppressor cells (MDSCs), while CCR5 inhibition will block the migration of a different subtype of MDSCs in addition to regulatory T cells. MDSCs and regulatory T cells also mediate immunosuppressive effects, and it’s hoped the combined effect of CCR2 and CCR5 inhibition will prove even more potent.25The therapeutic potential of CCR2 is being explored from another vantage point. Adoptively transferring T cells that have been manipulated to target a cancer antigen, using CAR technology, is an exciting state-of-the-art therapeutic technique that is producing groundbreaking results in some patients with hematologic malignancies. In the treatment of solid tumors, however, this technique has proved much less effective. Among the challenges encountered in this setting is the difficulty for CAR T cells to migrate to and adequately penetrate the TME.
Genetically manipulating the CAR T cells to express a chemokine receptor that matches the chemokine expression profile of the tumor could help to overcome this challenge, allowing the CAR T cells to home in more effectively on the tumor and penetrate the TME.
CCR2-expressing CAR T cells are currently being investigated in tumors that secrete high levels of CCL2. Preclinical study results have demonstrated the potential of this approach in neuroblastoma and mesothelioma models.26,27