A novel colony-stimulating factor 1 (CSF1) translocation involving human endogenous retroviral element in a tenosynovial giant cell tumor.

Tenosynovial giant cell tumors (TSGCTs) are rare tumors arising in tendons or the synoviae of joints and bursae. The localized type is benign while the diffuse type shows expansive growth leading to greater morbidity and is therefore considered locally aggressive. Typical recurrent chromosomal aberrations are found in the majority of TSCGT and the CSF1 gene is frequently involved. In this article, we describe a newly identified gene fusion mediated by an inversion in a case of diffuse TSGCT. Multicolor-fluorescence in situ hybridization (FISH) molecular karyotyping identified a pericentric inversion of chromosome 1 in 7 out of 17 analyzed cells 46,XX,inv(1)(p13.3q24.3) [7]/46,XX [10], and with interphase FISH the involvement the CSF1 locus was detected. After performing transcriptome sequencing analysis for fusion detection, only one out of five fusion gene algorithms detected a fusion involving the CSF1 gene product. The resulting chimera fuses a sequence from a human endogenous retrovirus (HERV) gene to CSF1 Exon 6 on chromosome 1, abrogating the regulatory element of the 3' untranslated region of the CSF1 gene. This new translocation involving Exon 6 of the CSF1 gene fused to 1q24.1, supports the hypothesis that a mutated CSF1 protein is likely to play a vital role in the pathogenesis of TSGCT. The role of the HERV partner identified as a translocation partner, however, remains unclear. Our data add to the complexity of involved translocation partners in TSGCT and point to the potential difficulty of identifying fusion partners in tumor diagnostics using transcriptome sequencing when HERV or other repeat elements are involved.

Role of colony-stimulating factor 1 in the neoplastic process of tenosynovial giant cell tumor.

Tenosynovial giant cell tumors (TGCTs) are rare, locally aggressive, mesenchymal neoplasms, most often arising from the synovium of joints, bursae, or tendon sheaths. Surgical resection is the first-line treatment, but recurrence is common, with resulting impairments in patients' mobility and quality of life. Developing and optimizing the role of systemic pharmacologic therapies in TGCT management requires an understanding of the underlying disease mechanisms. The colony-stimulating factor 1 receptor (CSF1R) has emerged as having an important role in the neoplastic processes underlying TGCT. Lesions appear to contain CSF1-expressing neoplastic cells derived from the synovial lining surrounded by non-neoplastic macrophages that express the CSF1R, with lesion growth stimulated by both autocrine effects causing proliferation of the neoplastic cells themselves and by paracrine effects resulting in recruitment of CSF1 R-bearing macrophages. Other signaling pathways with evidence for involvement in TGCT pathogenesis include programmed death ligand-1, matrix metalloproteinases, and the Casitas B-cell lymphoma family of ubiquitin ligases. While growing understanding of the pathways leading to TGCT has resulted in the development of both regulatory approved and investigational therapies, more detail on underlying disease mechanisms still needs to be elucidated in order to improve the choice of individualized therapies and to enhance treatment outcomes.

Interactions in CSF1-Driven Tenosynovial Giant Cell Tumors.

PURPOSE: A major component of cells in tenosynovial giant cell tumor (TGCT) consists of bystander macrophages responding to CSF1 that is overproduced by a small number of neoplastic cells with a chromosomal translocation involving the CSF1 gene. An autocrine loop was postulated where the neoplastic cells would be stimulated through CSF1R expressed on their surface. Here, we use single-cell RNA sequencing (scRNA-seq) to investigate cellular interactions in TGCT. EXPERIMENTAL DESIGN: A total of 18,788 single cells from three TGCT and two giant cell tumor of bone (GCTB) samples underwent scRNA-seq. The three TGCTs were additionally analyzed using long-read RNA sequencing. Immunofluorescence and IHC for a range of markers were used to validate and extend the scRNA-seq findings. RESULTS: Two recurrent neoplastic cell populations were identified in TGCT that are highly similar to nonneoplastic synoviocytes. We identified GFPT2 as a marker that highlights the neoplastic cells in TCGT. We show that the neoplastic cells themselves do not express CSF1R. We identified overlapping MAB features between the giant cells in TGCT and GCTB. CONCLUSIONS: The neoplastic cells in TGCT are highly similar to nonneoplastic synoviocytes. The lack of CSF1R on the neoplastic cells indicates they may be unaffected by current therapies. High expression of GFPT2 in the neoplastic cells is associated with activation of the YAP1/TAZ pathway. In addition, we identified expression of the platelet-derived growth factor receptor in the neoplastic cells. These findings suggest two additional pathways to target in this tumor.

Tenosynovial giant cell tumors (TGCT): molecular biology, drug targets and non-surgical pharmacological approaches.

INTRODUCTION: Tenosynovial giant cell tumor (TGCT) is a mono-articular, benign or locally aggressive and often debilitating neoplasm. Systemic therapies are becoming part of the multimodal armamentarium when surgery alone will not confer improvements. Since TGCT is characterized by colony-stimulating factor-1 (CSF1) rearrangements, the most studied molecular pathway is the CSF1 and CSF1 receptor (CSF1R) axis. Inhibiting CSF1-CSF1R interaction often yields considerable radiological and clinical responses; however, adverse events may cause treatment discontinuation because of an unfavorable risk-benefit ratio in benign disease. Only Pexidartinib is approved by the US FDA; however, the European Medicines Agency has not approved it due to a uncertain risk-benefit ratio. Thus, there is a need for safer and effective therapies. AREAS COVERED: Light is shed on disease mechanisms and potential drug targets. The safety and efficacy of different systemic therapies are evaluated. EXPERT OPINION: The CSF1-CSF1R axis is the principal drug target; however, the effect of CSF1R inhibition on angiogenesis and the role of macrophages, which are essential in the postoperative course, needs further elucidation. Systemic therapies have a promising role in treating mainly diffuse-type, TGCT patients who are not expected to clinically improve from surgery. Future drug development should focus on targeting neoplastic TGCT cells.

Results from Phase I Extension Study Assessing Pexidartinib Treatment in Six Cohorts with Solid Tumors including TGCT, and Abnormal CSF1 Transcripts in TGCT.

PURPOSE: To assess the response to pexidartinib treatment in six cohorts of adult patients with advanced, incurable solid tumors associated with colony-stimulating factor 1 receptor (CSF1R) and/or KIT proto-oncogene receptor tyrosine kinase activity. PATIENTS AND METHODS: From this two-part phase I, multicenter study, pexidartinib, a small-molecule tyrosine kinase inhibitor that targets CSF1R, KIT, and FMS-like tyrosine kinase 3 (FLT3), was evaluated in six adult patient cohorts (part 2, extension) with advanced solid tumors associated with dysregulated CSF1R. Adverse events, pharmacokinetics, and tumor responses were assessed for all patients; patients with tenosynovial giant cell tumor (TGCT) were also evaluated for tumor volume score (TVS) and patient-reported outcomes (PRO). CSF1 transcripts and gene expression were explored in TGCT biopsies. RESULTS: Ninety-one patients were treated: TGCT patients (n = 39) had a median treatment duration of 511 days, while other solid tumor patients (n = 52) had a median treatment duration of 56 days. TGCT patients had response rates of 62% (RECIST 1.1) and 56% (TVS) for the full analysis set. PRO assessments for pain showed improvement in patient symptoms, and 76% (19/25) of TGCT tissue biopsy specimens showed evidence of abnormal CSF1 transcripts. Pexidartinib treatment of TGCT resulted in tumor regression and symptomatic benefit in most patients. Pexidartinib toxicity was manageable over the entire study. CONCLUSIONS: These results offer insight into outcome patterns in cancers whose biology suggests use of a CSF1R inhibitor. Pexidartinib results in tumor regression in TGCT patients, providing prolonged control with an acceptable safety profile.

A New Simple and Practical Clinical Classification for Tenosynovial Giant Cell Tumors of the Knee.

OBJECTIVE: To propose a simple and practical clinical classification for tenosynovial giant cell tumor (TGCT) of the knee. METHODS: A retrospective study was conducted to verify the value and significance of this clinical classification. TGCT growth patterns, knee joint capsule, and bone erosion were applied to establish this novel clinical classification. Seventy-eight patients who underwent surgery for TGCT from 2008 to 2016 were identified. This novel clinical classification was retrospectively applied to patients' existing classification, and patients with different TGCT types were statistically compared to verify the significance of the clinical classification. RESULTS: The clinical classification included three types and four subtypes. Type 1: localized TGCT, Subtype 1a: localized intra-articular TGCT, Subtype 1b: localized extra-articular TGCT. Type 2: diffuse TGCT, Subtype 2a: diffuse intra-articular TGCT with bone normal, Subtype 2b: diffuse intra-articular TGCT with bone destruction. Type 3: diffuse TGCT across the knee joint capsule. The mean follow-up time for the 78 patients was 59.6 months. Twenty-one patients were in Subtype 1a, four were Subtype 1b, 38 were Subtype 2a, seven were Subtype 2b, and eight were Type 3. Oncological results and surgical complications differed significantly (P = 0.000, P = 0.000). The mean Musculoskeletal Tumor Society functional scores differed significantly at 27.8 for Type 1 patients, 22.9 for Type 2 patients, and 17.0 for Type 3 patients (P = 0.000). CONCLUSIONS: This clinical classification can be easily used to evaluate TGCT of all knees prior to surgery or other treatments and can help determine surgical options.

Neoplastic synovial lining cells that coexpress podoplanin and CD90 overproduce CSF-1, driving tenosynovial giant cell tumor.

Tenosynovial giant cell tumor (TCGT) is a rare neoplasm affecting the synovium of joints, bursae, and tendon sheaths. The overproduction of colony-stimulating factor-1 (CSF-1) by a minority of the tumor population works in a paracrine fashion to drive tumor growth. Pathology of the reactive, monocytic component has been well elucidated, whereas the populations of neoplastic cells and all the sources of CSF-1 overproduction are incompletely characterized. Podoplanin (PDPN), or gp38, is a cell surface glycoprotein that is expressed on fibroblast-like synovial cells and upregulated in rheumatoid arthritis and many cancers; it governs cell mobility, epithelial-mesenchymal transition, and other functions and is associated with lymphangiogenesis and poor prognosis in many solid tumors, which underscores its local and possible systemic effects. We found higher PDPN expression in TGCT than in internal controls of patients' healthy synovium. Flow cytometry partitioned PDPNhigh cells into PDPNhigh CD90+ and PDPNhigh CD14+ populations. Quantitative real-time polymerase chain reaction analysis of the PDPNhigh CD90+ cells revealed that CSF-1 expression was 10-fold higher than in PDPNhigh CD14+ cells. Therefore, we conclude that the lining fibroblast-like synovial cells, which express PDPNhigh CD90+ , are responsible for the overproduction of CSF-1 and for driving tumor growth.

Prospects of Treating Tenosynovial Giant Cell Tumor through Pexidartinib: A Review.

BACKGROUND: Tenosynovial giant cell tumor refers to a group of rarely occurring tumors that are formed in the joints, which are characterized by pain, swelling, and limitation of movement of the joint. Surgery is the main treatment strategy, but the tumor is likely to recur, especially in pigmented villonodular synovitis, which is the diffuse-type giant cell tumor. Pexidartinib was approved in August 2019 by the Food and Drug Administration (FDA) with a brand name TURALIO as the first systemic approved therapy for patients having Tenosynovial Giant Cell Tumors (TGCT). OBJECTIVE: In this review, different aspects pertaining to pexidartinib have been summarized, including the pathophysiology of TGCT, chemistry, pharmacokinetics and pharmacodynamics of pexidartinib. Special attention is given to various reported clinical trials of pexidartinib. METHODS: A comprehensive literature search was conducted in the relevant databases to identify studies published in this field during recent years. CONCLUSION: Pexidartinib acts by inhibiting the Colony-Stimulating Factor (CSF1)/CSF1 receptor pathway, which leads to the inhibition of the cell lines proliferation and promotes the autophosphorylation process of the ligand-induced CSF1 receptor. Pexidartinib emerged as a potential drug candidate for the treatment of TGCT.

PK/PD Mediated Dose Optimization of Emactuzumab, a CSF1R Inhibitor, in Patients With Advanced Solid Tumors and Diffuse-Type Tenosynovial Giant Cell Tumor.

Targeted biological therapies may achieve maximal therapeutic efficacy at doses below the maximum tolerated dose (MTD); therefore, the search for the MTD in clinical studies may not be ideal for these agents. Emactuzumab is an investigational monoclonal antibody that binds to and inhibits the activation of the cell surface colony-stimulating factor-1 receptor. Here, we show how modeling target-mediated drug disposition coupled with pharmacodynamic end points was used to optimize the dose of emactuzumab without defining an MTD. The model could be used to recommend doses across different disease indications. The approach recommended an optimal biological dose of emactuzumab for dosing every 2 weeks (q2w) ≥ 900 mg, approximately three-fold lower than the highest dose tested clinically. The model predicted that emactuzumab doses ≥ 900 mg q2w would achieve target saturation in excess of 90% over the entire dosing cycle. Subsequently, a dose of 1,000 mg q2w was used in the extension phase of a phase I study of emactuzumab in patients with advanced solid tumors or diffuse-type tenosynovial giant cell tumor. Clinical data from this study were consistent with model predictions. The model was also used to predict the optimum dose of emactuzumab for use with dosing every 3 weeks, enabling dosing flexibility with respect to comedications. In summary, this work demonstrates the value of quantitative clinical pharmacology approaches to dose selection in oncology as opposed to traditional MTD methods.

Pexidartinib, a Novel Small Molecule CSF-1R Inhibitor in Use for Tenosynovial Giant Cell Tumor: A Systematic Review of Pre-Clinical and Clinical Development.

Tenosynovial giant cell tumor (TGCT) is a rare benign tumor that involves the synovium, bursa, and tendon sheath, resulting in reduced mobility of the affected joint or limb. The current standard of care for TGCT is surgical resection. However, some patients have tumor recurrence, present with unresectable tumors, or have tumors that are in locations where resection could result in amputations or significant debility. Therefore, the development of systemic agents with activity against TGCT to expand treatment options is a highly unmet medical need. Pathologically, TGCT is characterized by the overexpression of colony-stimulating factor 1 (CSF-1), which leads to the recruitment of colony-stimulating factor-1 receptor (CSF-1R) expressing macrophages that make up the primary cell type within these giant cell tumors. The binding of CSF-1 and CSF-1R controls cell survival and proliferation of monocytes and the switch from a monocytic to macrophage phenotype contributing to the growth and inflammation within these tumors. Therefore, molecules that target CSF-1/CSF-1R have emerged as potential systemic agents for the treatment of TGCT. Given the role of macrophages in regulating tumorigenesis, CSF1/CSF1R-targeting agents have emerged as attractive therapeutic targets for solid tumors. Pexidartinib is an orally bioavailable and potent inhibitor of CSF-1R which is one of the most clinically used agents. In this review, we discuss the biology of TGCT and review the pre-clinical and clinical development of pexidartinib which ultimately led to the FDA approval of this agent for the treatment of TGCT as well as ongoing clinical studies utilizing pexidartinib in the setting of cancer.

Detection of CSF1 rearrangements deleting the 3' UTR in tenosynovial giant cell tumors.

Tenosynovial giant cell tumors (TGCTs) are characterized by rearrangements of CSF1, thought to drive overexpression of macrophage colony-stimulating factor (CSF1), thereby promoting tumor growth and recruitment of non-neoplastic mononuclear and multinucleated inflammatory cells. While fusions to collagen promoters have been described, the mechanism of CSF1 overexpression has been unclear in a majority of cases. Two cohorts of TGCT were investigated for CSF1 rearrangements using fluorescence in situ hybridization (FISH) and either RNA-seq or DNA-seq with Sanger validation. The study comprised 39 patients, including 13 localized TGCT, 21 diffuse TGCT, and five of unspecified type. CSF1 rearrangements were identified by FISH in 30 cases: 13 translocations, 17 3' deletions. Sequencing confirmed CSF1 breakpoints in 28 cases; in all 28 the breakpoint was found to be downstream of exon 5, replacing or deleting a long 3' UTR containing known miRNA and AU-rich element negative regulatory sequences. We also confirmed the presence of CBL exon 8-9 mutations in six of 21 cases. In conclusion, TGCT in our large cohort were characterized by variable alterations, all of which led to truncation of the 3' end of CSF1, instead of the COL6A3-CSF1 fusions previously reported in some TGCTs. The diversity of fusion partners but consistent integrity of CSF1 functional domains encoded by exons 1-5 support a hypothesis that CSF1 overexpression results from transcription of a truncated form of CSF1 lacking 3' negative regulatory sequences. The presence of CBL mutations affecting the linker and RING finger domain suggests an alternative mechanism for increased CSF1/CSF1R signaling in some cases.

Pexidartinib: First Approval.

Pexidartinib (TURALIO™) is an orally administered small molecule tyrosine kinase inhibitor with selective activity against the colony-stimulating factor 1 (CSF1) receptor, KIT proto-oncogene receptor tyrosine kinase (KIT) and FMS-like tyrosine kinase 3 harboring an internal tandem duplication mutation (FLT3-ITD). In August 2019, the US FDA approved pexidartinib capsules for the treatment of adult patients with symptomatic tenosynovial giant cell tumor (TGCT) associated with severe morbidity or functional limitations and not amenable to improvement with surgery. This approval was based on positive results from the phase III ENLIVEN trial. Pexidartinib is being investigated in various malignancies as monotherapy or combination therapy. This article summarizes the milestones in the development of pexidartinib leading to its first approval for TGCT.

Randomized placebo-controlled double-blind phase II study of zaltoprofen for patients with diffuse-type and unresectable localized tenosynovial giant cell tumors: a study protocol.

BACKGROUND: A tenosynovial giant cell tumor (TGCT) is a locally aggressive benign neoplasm arising from intra- or extra-articular tissue. Diffuse TGCT (D-TGCT) most commonly develops in the knee, followed by the hip, ankle, elbow, and shoulder. Surgical removal is the only effective treatment option for the patients. However, a local recurrence rate as high as 47% has been reported. Recently, we revealed that zaltoprofen, a nonsteroidal anti-inflammatory drug possessing the ability to activate peroxisome proliferator-activated receptor gamma (PPARγ), can inhibit the proliferation of TGCT stromal cells via PPARγ. PPARγ is a ligand-activated transcription factor that belongs to the nuclear hormone receptor superfamily. It plays an important role in the differentiation of adipocytes from precursor cells and exhibits antitumorigenic effects on certain malignancies. Therefore, we are conducting this investigator-initiated clinical trial to evaluate whether zaltoprofen is safe and effective for patients with D-TGCT or unresectable localized TGCT (L-TGCT). METHODS: This study is a randomized, placebo-controlled, double-blind, multicenter trial to evaluate the safety and efficacy of zaltoprofen for patients with D-TGCT or L-TGCT. For the treatment group, zaltoprofen 480 mg/day will be administered for 48 weeks; the placebo group will receive similar dosages without zaltoprofen. Twenty participants in each group are needed in this trial (40 participants total). The primary outcome is the progression-free rate at 48 weeks after treatment administration. "Progression" is defined as any serious events (1. Repetitive joint swelling due to hemorrhage, 2. Joint range of motion limitation, 3. Invasion of adjacent cartilage or bone, 4. Severe joint space narrowing, 5. Increase in tumor size) requiring surgical interventions. We hypothesize that the zaltoprofen group will have a higher progression-free rate compared to that of the placebo group at 48 weeks. DISCUSSION: This is the first study to evaluate the efficacy of zaltoprofen in patients with D-TGCT or unresectable L-TGCT. We believe that the results of this trial will validate a novel treatment option, zaltoprofen, to stabilize disease progression for TGCT patients. TRIAL REGISTRATION: University Hospital Medical Information Network (UMIN) Clinical Trials Registry ( UMIN000025901 ) registered on 4/01/2017.

Does CSF1 overexpression or rearrangement influence biological behaviour in tenosynovial giant cell tumours of the knee?

AIMS: Localised- and diffuse-type tenosynovial giant cell tumours (TGCT) are regarded as different clinical and radiological TGCT types. However, genetically and histopathologically they seem indistinguishable. We aimed to correlate CSF1 expression and CSF1 rearrangement with the biological behaviour of different TGCT-types with clinical outcome (recurrence). METHODS AND RESULTS: Along a continuum of extremes, therapy-naive knee TGCT patients with >3-year follow-up, mean age 43 (range = 6-71) years and 56% females were selected. Nine localised (two recurrences), 16 diffuse-type (nine recurrences) and four synovitis as control were included. Rearrangement of the CSF1 locus was evaluated with split-apart fluorescence in-situ hybridisation (FISH) probes. Regions were selected to score after identifying CSF1-expressing regions, using mRNA ISH with the help of digital correlative microscopy. CSF1 rearrangement was considered positive in samples containing >2 split signals/100 nuclei. Irrespective of TGCT-subtype, all cases showed CSF1 expression and in 76% CSF1 rearrangement was detected. Quantification of CSF1-expressing cells was not informative, due to the extensive intratumour heterogeneity. Of the four synovitis cases, two also showed CSF1 expression without CSF1 rearrangement. No correlation between CSF1 expression or rearrangement with clinical subtype and local recurrence was detected. Both localised and diffuse TGCT cases showed a scattered distribution in the tissue of CSF1-expressing cells. CONCLUSION: In diagnosing TGCT, CSF1 mRNA-ISH, in combination with CSF1 split-apart FISH using digital correlative microscopy, is an auxiliary diagnostic tool to identify rarely occurring neoplastic cells. This combined approach allowed us to detect CSF1 rearrangement in 76% of the TGCT cases. Neither CSF1 expression nor presence of CSF1 rearrangement could be associated with the difference in biological behaviour of TGCT.

Tumefactive foreign body giant cell reaction following high-pressure paint injection injury: A case report and review of literature.

High-pressure paint injection injury is an uncommon but well-described injury. The histologic features of long-term paint injection injury with retained material are less recognized. A 46-year-old male presented clinically as "recurrent giant cell tumor of tendon sheath." The right index finger demonstrated fusiform enlargement by a pigmented mass with diffuse infiltration into the soft tissue of the hand. Histologically the tumor showed multiple giant cells in a fibrotic stroma extending into the dermis. There were multiple types of foreign material including diffuse brown black pigment, weakly optically polarizing foreign material and white inclusions with a "train track" appearance. The cells were positive for CD68 and negative for S100 antigen. Further investigation revealed that the patient had a history of high-pressure paint injection injury to his digit 6 years prior. Foreign material injected under high pressure into tissues may result in a pseudo-neoplastic foreign body granulomatous reaction that can mimic giant cell tumor of tendon sheath. Our case demonstrates that this reaction can be florid and can have slow growth over years. A high index of suspicion, a good clinical history and careful examination can distinguish these 2 entities.

Abundant lubricin expression suggests a link between synoviocytes, synovial tumors, and myxomas.

Progenitor cell differentiation into fibroblast-like synoviocytes (FLSs) and their ensuing phenotypic changes are incompletely explored. Synovial lining is composed of intimal macrophages and FLSs. FLSs have epithelioid morphology and directionally secrete components of synovial fluid, including lubricin. We stained human tissues and tumors using two anti-lubricin antibodies. Lubricin was found in FLSs in synovium and in tenosynovial giant cell tumors (TSGCTs) and not in the associated monocyte/macrophage cells, which were identified by double immunostaining for CD163. In TSGCTs, giant cells, known to form by fusion of mononuclear cells, were negative for both lubricin and CD163. Occasional mononuclear cells with the same phenotype were also seen, suggesting that the precursors of the giant cells are derived from the minor CD163-negative monocyte subset. Lubricin was also detected in intramuscular myxomas, in early myxoid changes of ganglion cysts, and in one of five low-grade myxofibrosarcomas, but not in other fibroconnective tissues, epithelial tissues, or other tumors tested. This suggests that lubricin expression may typify adaptive and neoplastic changes along a pathway toward FLSs. Further support for this concept comes from ganglion cysts and juxta-articular myxoma tumors, which show a spectrum of myxoid, cystic and synovial differentiation, and in which moderate lubricin staining of myxoid stroma was seen.