Antiangiogenic chimeric anti-endoglin (CD105) antibody: pharmacokinetics and immunogenicity in nonhuman primates and effects of doxorubicin

Efficacy and Safety of TRC105 Plus Pazopanib vs Pazopanib Alone for Treatment of Patients With Advanced Angiosarcoma: A Randomized Clinical Trial

Question

Does the combination of pazopanib plus carotuximab improve progression-free survival compared with pazopanib alone in patients with advanced angiosarcoma?

Findings

This phase 3 randomized clinical trial of 123 patients found no significant difference in median progression-free survival between patients receiving pazopanib plus carotuximab compared with pazopanib alone.

Meaning

The study’s findings indicate that the combination of pazopanib plus carotuximab is not superior to pazopanib alone in treating patients with advanced angiosarcoma.

Importance

Angiosarcoma is a rare sarcoma subtype with a poor outcome. Carotuximab plus pazopanib produced a median progression-free survival (PFS) of 7.8 months in pazopanib-naive patients with chemotherapy-refractory angiosarcoma in a phase 1/2 trial.

Objective

To determine whether carotuximab plus pazopanib improves PFS compared with pazopanib alone in patients with advanced angiosarcoma.

Design, Setting, and Participants

The TAPPAS Trial: An Adaptive Enrichment Phase 3 Trial of TRC105 and Pazopanib vs Pazopanib Alone in Patients With Advanced Angiosarcoma was a multinational, multicenter, open-label, parallel-group, phase 3 randomized clinical trial of 123 patients 18 years or older with advanced angiosarcoma that was conducted between February 16, 2017, and April 12, 2019, at 31 sites in the US and the European Union. Patients were randomized 1:1 to receive pazopanib alone or carotuximab plus pazopanib. The trial incorporated an adaptive enrichment design. Inclusion criteria were no more than 2 prior lines of systemic therapy and an Eastern Cooperative Oncology Group performance status of 0 or 1. The efficacy analysis used the intent-to-treat population; the safety analysis included all patients who received a dose of either study drug.

Exposures

Oral pazopanib, 800 mg/d, or intravenous carotuximab, 10 mg/kg, administered weekly, plus oral pazopanib, 800 mg/d, with dose modification allowed per patient tolerance or until disease progression.

Main Outcomes and Measures

The primary end point was PFS, assessed by blinded independent radiographic and cutaneous photographic review per Response Evaluation Criteria in Solid Tumors (RECIST) guidelines, version 1.1. Secondary end points included the objective response rate and overall survival. An interim analysis to determine the final sample size was conducted after enrollment of 123 patients. PFS in the group receiving pazopanib alone was compared with PFS in the group receiving carotuximab plus pazopanib using the log rank test.

Results

Of 114 patients with evaluable data (53 in the pazopanib arm and 61 in the carotuximab plus pazopanib arm), 69 (61%) were female and the median age was 68 years (range, 24-82 years); 57 (50%) had cutaneous disease and 32 (28%) had had no prior treatment. The primary end point (PFS) was not reached (hazard ratio [HR], 0.98; 95% CI, 0.52-1.84; P = .95), with a median of 4.3 months (95% CI, 2.9 months to not reached) for pazopanib and 4.2 months (95% CI, 2.8-8.3 months) for the combination arm. The most common all-grade adverse events in the single-agent pazopanib arm vs the combination arm were fatigue (29 patients [55%] vs 37 [61%]), headache (12 patients [23%] vs 39 [64%]), diarrhea (27 patients [51%] vs 35 [57%]), nausea (26 patients [49%] vs 29 [48%]), vomiting (12 patients [23%] vs 23 [38%]), anemia (5 patients [9%] vs 27 [44%]), epistaxis (2 patients [4%] vs 34 [56%]), and hypertension (29 patients [55%] vs 22 [36%]).

Conclusions and Relevance

In this phase 3 randomized clinical trial, carotuximab plus pazopanib did not improve PFS compared with pazopanib alone in patients with advanced angiosarcoma.

Trial Registration

ClinicalTrials.gov Identifier: NCT02979899

Endoglin Targeting: Lessons Learned and Questions That Remain

Approximately 30 years ago, endoglin was identified as a transforming growth factor (TGF)-β coreceptor with a crucial role in developmental biology and tumor angiogenesis. Its selectively high expression on tumor vessels and its correlation with poor survival in cancer patients led to the exploration of endoglin as a therapeutic target for cancer. The endoglin neutralizing antibody TRC105 (Carotuximab^®, Tracon Pharmaceuticals (San Diego, CA, USA) was subsequently tested in a wide variety of preclinical cancer models before being tested in phase I-III clinical studies in cancer patients as both a monotherapy and in combination with other chemotherapeutic and anti-angiogenic therapies. The combined data of these studies have revealed new insights into the role of endoglin in angiogenesis and its expression and functional role on other cells in the tumor microenvironment. In this review, we will summarize the preclinical work, clinical trials and biomarker studies of TRC105 and explore what these studies have enabled us to learn and what questions remain unanswered.

An open-label phase Ib dose-escalation study of TRC105 (anti-endoglin antibody) with bevacizumab in patients with advanced cancer

PURPOSE

Endoglin, an endothelial cell membrane receptor expressed on angiogenic tumor vessels, is essential for angiogenesis and upregulated in the setting of VEGF inhibition. TRC105 is an anti-endoglin IgG1 monoclonal antibody that potentiates VEGF inhibitors in preclinical models. This study assessed safety, pharmacokinetics, and antitumor activity of TRC105 in combination with bevacizumab.

EXPERIMENTAL DESIGN

Patients (n = 38) with advanced solid tumors, Eastern Cooperative Group performance status 0-1, and normal organ function were treated with escalating doses of TRC105 plus bevacizumab until disease progression or unacceptable toxicity using a standard 3 + 3 phase I design.

RESULTS

TRC105 and bevacizumab were well tolerated at their recommended single-agent doses (10 mg/kg) when the initial dose of TRC105 was delayed by one week and divided over 2 days to limit the frequency of headache. The concurrent administration of bevacizumab and TRC105 did not otherwise potentiate known toxicities of TRC105 or bevacizumab. Hypertension and proteinuria were observed, though not at rates expected for single-agent bevacizumab. Several patients who had previously progressed on bevacizumab or VEGF receptor tyrosine kinase inhibitor (VEGFR TKI) treatment experienced reductions in tumor volume, including two partial responses by RECIST, and 6 remained without progression for longer periods than during their prior VEGF inhibitor therapy.

CONCLUSIONS

TRC105 was well tolerated with bevacizumab and clinical activity was observed in a VEGF inhibitor-refractory population. Ongoing clinical trials are testing TRC105 in combination with bevacizumab in glioblastoma and with VEGFR TKIs in renal cell carcinoma, hepatocellular carcinoma, and soft tissue sarcoma.

Facilitation of endoglin-targeting cancer therapy by development/utilization of a novel genetically engineered mouse model expressing humanized endoglin (CD105)

Endoglin (ENG) is a TGF-β coreceptor and essential for vascular development and angiogenesis. A chimeric antihuman ENG (hENG) monoclonal antibody (mAb) c-SN6j (also known as TRC105) shows promising safety and clinical efficacy features in multiple clinical trials of patients with various advanced solid tumors. Here we developed a novel genetically engineered mouse model to optimize the ENG-targeting clinical trials. We designed a new targeting vector that contains exons 4-8 of hENG gene to generate novel genetically engineered mice (GEMs) expressing functional human/mouse chimeric (humanized) ENG with desired epitopes. Genotyping of the generated mice confirmed that we generated the desired GEMs. Immunohistochemical analysis demonstrated that humanized ENG protein of the GEMs expresses epitopes defined by 7 of our 8 anti-hENG mAbs tested. Surprisingly the homozygous GEMs develop normally and are healthy. Established breast and colon tumors as well as metastasis and tumor microvessels in the GEMs were effectively suppressed by systemic administration of anti-hENG mAbs. Additionally, test result indicates that synergistic potentiation of antitumor efficacy can be induced by simultaneous targeting of two distinct epitopes by anti-hENG mAbs. Sorafenib and capecitabine also showed antitumor efficacy in the GEMs. The presented novel GEMs are the first GEMs that express the targetable humanized ENG. Test results indicate utility of the GEMs for the clinically relevant studies. Additionally, we generated GEMs expressing a different humanized ENG containing exons 5-6 of hENG gene, and the homozygous GEMs develop normally and are healthy.

Modulation of circulating protein biomarkers following TRC105 (anti-endoglin antibody) treatment in patients with advanced cancer

TRC105 is an endoglin-targeting drug that possesses anti-angiogenic and antitumor potential. Analysis of the initial phase I trial of TRC105 demonstrated good tolerability and efficacy in cancer patients. In this report, we analyzed multiple circulating biomarkers at baseline, cycle 2 day 1 (C2D1), and end of study (EOS) for each patient. The baseline level and the fold change from baseline to both C2D1 and EOS for each marker were statistically analyzed. At C2D1, seven markers were significantly downregulated (angiopoietin-2 [Ang-2], insulin-like growth factor-binding protein-3 [IGFBP-3], plasminogen activator inhibitor-1 [PAI-1] total, platelet-derived growth factor [PDGF]-AA, PDGF-BB, thrombospondin-1 [TSP-1], and vascular endothelial growth factor [VEGF]-D). Meanwhile, seven markers were upregulated by C2D1 (E-Cadherin, soluble Endoglin [sEnd], E-Selectin, interleukin-6 [IL-6], osteopontin [OPN], TSP-2, and von Willebrand factor [vWF]). At EOS, seven markers were upregulated including Ang-2, C-reactive protein (CRP), intercellular adhesion molecule-1 (ICAM-1), IGFBP-1, IL-6, TSP-2, and vascular cell adhesion molecule-1 (VCAM-1). A statistical trend was also seen for increases of VEGF-A and placenta growth factor (PlGF) at EOS. Throughout treatment, sEnd levels significantly increased, an observation that was recapitulated in cultured endothelial cells. This is the first report of plasma-based biomarkers in patients receiving TRC105. TRC105 treatment by C2D1 was associated with decreases in several angiogenic factors, including Ang-2, PDGF isoforms, and VEGF isoforms, offering insight into the mechanisms underlying TRC105's anti-angiogenic, antitumor function. Increases in sEnd were the most significant of all observed biomarker changes and may reflect direct drug effects. Additionally, biomarker changes in response to TRC105 are distinct from those seen in patients treated with VEGF-targeting drugs, suggesting the possible utility of combining these two classes of angiogenesis inhibitors in patients.

In vitro and in vivo effects of an anti-mouse endoglin (CD105)-immunotoxin on the early stages of mouse B16MEL4A5 melanoma tumours

TGF-beta superfamily co-receptors are emerging as targets for cancer therapy, acting both directly on cells and indirectly on the tumour neovasculature. Endoglin (CD105), an accessory component of the TGF-beta receptor complex, is expressed in certain melanoma cell lines and the endothelial cells of tumour neovessels. Targeting endoglin with immunotoxins is an attractive approach for actively suppressing the blood supply to tumours. Here, we report evidence indicating that endoglin is expressed in mouse melanoma B16MEL4A5 and mouse fibroblast L929 cell lines. We prepared an immunotoxin to target endoglin by coupling the rat anti-mouse MJ7/18 (IgG2a) monoclonal antibody (mAb) to the non-toxic type 2 ribosome-inactivating protein nigrin b (Ngb) with N-succinimidyl 3-(2-pyridyldithio)-propionate (SPDP) as a linker with a molar nigrin b at a MJ7/18 stoichiometry of 2:1. The MJ7-Ngb immunotoxin generated killed both cell lines, with IC50 values of 4.2 × 10(-9) M for B16MEL4A5 and 7.7 × 10(-11) M for L929 cells. For in vivo assays of the immunotoxin, B16MEL4A5 cells were injected subcutaneously into the right flanks of 6-week-old C57BL/6 J mice. When the animals developed palpable solid tumours, they were subjected to treatment with the immunotoxin. While treatment with either MJ7/18 mAb or Ngb did not affect tumour development, treatment with the immunotoxin completely and steadily blocked tumour growth up to 7 days, after which some tumours re-grew. Thus, vascular-targeting therapy with this anti-vascular immunotoxin could promote the destruction of newly created tumour vessels at early stages of B16MEL4A5 tumour development and readily accessible CD105+ B16MEL4A5 melanoma cells.

A phase I first-in-human study of TRC105 (Anti-Endoglin Antibody) in patients with advanced cancer

PURPOSE

TRC105 is a chimeric IgG1 monoclonal antibody that binds CD105 (endoglin). This first-in-human, phase I, open-label study assessed safety, pharmacokinetics, and antitumor activity of TRC105 in patients with advanced refractory solid tumors.

PATIENTS AND METHODS

Patients received escalating doses of intravenous TRC105 until disease progression or unacceptable toxicity using a standard 3 + 3 phase I design.

RESULTS

Fifty patients were treated with escalating doses of TRC105. The maximum tolerated dose (MTD) was exceeded at 15 mg/kg every week because of dose-limiting hypoproliferative anemia. TRC105 exposure increased with increasing dose, and continuous serum concentrations that saturate CD105 receptors were maintained at 10 mg/kg weekly (the MTD) and 15 mg/kg every 2 weeks. Common adverse events including anemia, telangiectasias, and infusion reactions reflected the mechanism of action of the drug. Antibodies to TRC105 were not detected in patients treated with TRC105 from Chinese hamster ovary cells being used in ongoing phase Ib and phase II studies. Stable disease or better was achieved in 21 of 45 evaluable patients (47%), including two ongoing responses at 48 and 18 months.

CONCLUSION

TRC105 was tolerated at 10 mg/kg every week and 15 mg/kg every 2 weeks, with a safety profile that was distinct from that of VEGF inhibitors. Evidence of clinical activity was seen in a refractory patient population. Ongoing clinical trials are testing TRC105 in combination with chemotherapy and VEGF inhibitors and as a single agent in prostate, ovarian, bladder, breast, and hepatocellular cancer.

Stem cell pathways contribute to clinical chemoresistance in ovarian cancer

PURPOSE

Within heterogeneous tumors, subpopulations often labeled cancer stem cells (CSC) have been identified that have enhanced tumorigenicity and chemoresistance in ex vivo models. However, whether these populations are more capable of surviving chemotherapy in de novo tumors is unknown.

EXPERIMENTAL DESIGN

We examined 45 matched primary/recurrent tumor pairs of high-grade ovarian adenocarcinomas for expression of CSC markers ALDH1A1, CD44, and CD133 using immunohistochemistry. Tumors collected immediately after completion of primary therapy were then laser capture microdissected and subjected to a quantitative PCR array examining stem cell biology pathways (Hedgehog, Notch, TGF-β, and Wnt). Select genes of interest were validated as important targets using siRNA-mediated downregulation.

RESULTS

Primary samples were composed of low densities of ALDH1A1, CD44, and CD133. Tumors collected immediately after primary therapy were more densely composed of each marker, whereas samples collected at first recurrence, before initiating secondary therapy, were composed of similar percentages of each marker as their primary tumor. In tumors collected from recurrent platinum-resistant patients, only CD133 was significantly increased. Of stem cell pathway members examined, 14% were significantly overexpressed in recurrent compared with matched primary tumors. Knockdown of genes of interest, including endoglin/CD105 and the hedgehog mediators Gli1 and Gli2, led to decreased ovarian cancer cell viability, with Gli2 showing a novel contribution to cisplatin resistance.

CONCLUSIONS

These data indicate that ovarian tumors are enriched with CSCs and stem cell pathway mediators, especially at the completion of primary therapy. This suggests that stem cell subpopulations contribute to tumor chemoresistance and ultimately recurrent disease.

Endoglin-targeted cancer therapy

Vascular-targeting antiangiogenic therapy (VTAT) of cancer can be advantageous over conventional tumor cell targeted cancer therapy if an appropriate target is found. Our hypothesis is that endoglin (ENG; CD105) is an excellent target in VTAT. ENG is selectively expressed on vascular and lymphatic endothelium in tumors. This allows us to target both tumor-associated vasculature and lymphatic vessels to suppress tumor growth and metastasis. ENG is essential for angiogenesis/vascular development and a co-receptor of TGF-β. Our studies of selected anti-ENG monoclonal antibodies (mAbs) in several animal models and in vitro studies support our hypothesis. These mAbs and/or their immunoconjugates (immunotoxins and radioimmunoconjugates) induced regression of preformed tumors as well as inhibited formation of new tumors. In addition, they suppressed metastasis. Several mechanisms were involved in the suppressive activity of the naked (unconjugated) anti-ENG mAbs. These include direct growth suppression of proliferating endothelial cells, induction of apoptosis, ADCC (antibody-dependent cell-mediated cytotoxicity) and induction of T cell immunity. To facilitate clinical application, we generated a human/mouse chimeric anti-ENG mAb termed c-SN6j and performed studies of pharmacokinetics, toxicology and immunogenicity of c-SN6j in nonhuman primates. No significant toxicity was detected by several criteria and minimal immune response to the murine part of c-SN6j was detected after multiple i.v. injections. The results support our hypothesis that c-SN6j can be safely administered in cancer patients. This hypothesis is supported by the ongoing phase 1 clinical trial of c-SN6j (also known as TRC105) in patients with advanced or metastatic solid cancer in collaboration with Tracon Pharma and several oncologists (NCT00582985).

Targeting tumor vasculature with novel Listeria-based vaccines directed against CD105

The FDA approval of bevacizumab (Avastin®, Genentech/Roche), a monoclonal antibody raised against human VEGF-A, as second-line therapy for colon and lung carcinoma validated the approach of targeting human tumors with angiogenesis inhibitors. While the VEGF/VEGFR pathway is a viable target for anti-angiogenesis tumor therapy, additional targets involved in tumor neovascularization have been identified. One promising target present specifically on tumor vasculature is endoglin (CD105), a member of the TGF-β receptor complex expressed on vascular endothelium and believed to play a role in angiogenesis. Monoclonal antibody therapy and preventive vaccination against CD105 has met with some success in controlling tumor growth. This report describes the in vivo proof-of-concept studies for two novel therapeutic vaccines, Lm-LLO-CD105A and Lm-LLO-CD105B, directed against CD105 as a strategy to target neovascularization of established tumors. Listeria-based vaccines directed against CD105 lead to therapeutic responses against primary and metastatic tumors in the 4T1-Luc and NT-2 mouse models of breast cancer. In a mouse model for autochthonous Her-2/neu-driven breast cancer, Lm-LLO-CD105A vaccination prevented tumor incidence in 20% of mice by week 58 after birth while all control mice developed tumors by week 40. In comparison with previous Listeria-based vaccines targeting tumor vasculature, Lm-LLO-CD105A and Lm-LLO-CD105B demonstrated equivalent or superior efficacy against two transplantable mouse models of breast cancer. Support is provided for epitope spreading to endogenous tumor antigens and reduction in tumor vascularity after vaccination with Listeria-based CD105 vaccines. Reported here, these CD105 therapeutic vaccines are highly effective in stimulating anti-angiogenesis and anti-tumor immune responses leading to therapeutic efficacy against primary and metastatic breast cancer.

CD105 is a marker of tumour vasculature and a potential target for the treatment of head and neck squamous cell carcinoma

BACKGROUND

The importance of angiogenesis in solid tumour growth is well recognised. Tumour angiogenesis is considered the result of an imbalance between pro- and anti-angiogenic factors produced by both the malignancy and normal cells. Endoglin (CD105) is a proliferation-associated, hypoxia-inducible glycoprotein that seems to be clinically superior to other pan-endothelial markers in the selective evaluation of tumour angiogenesis. Several studies have revealed CD105 up-regulation in a wide range of tumour endothelia. Since 2002, endothelial CD105 expression has also been retrospectively investigated in patients with head and neck squamous cell carcinoma (HNSCC).

METHODS

An exhaustive literature review was performed to investigate available evidence on CD105 expression and its biological role and therapeutic potential in HNSCC.

RESULTS

The available evidence supports the hypothesis that CD105 expression in HNSCC may be a valuable parameter for pinpointing patients at greater risk of recurrent malignancy and with a worse prognosis. A high CD105 expression in HNSCC was associated with metastatic lymph nodes in most of the studies.

CONCLUSIONS

Prospective studies are mandatory to confirm that CD105 expression is a significant prognostic hallmark in HNSCC. The results of prospective studies could be relevant for the adoption of stricter follow-up protocols and/or alternative therapeutic regimens for patients with a high CD105 expression in HNSCC. Great interest is currently being focused on vascular targeting for therapeutic purposes. Preclinical studies on appropriate animal models resembling HNSCC to investigate the effects of inhibiting CD105 may show the efficacy of combined treatment strategies associating angiogenic-targeted with conventional therapies for HNSCC.

Overexpression of endoglin (CD105) is associated with recurrence in radically resected gastric cancer

The aim of this study was to examine the expression of endoglin (CD105) in resected gastric cancer and to evaluate the relationship between microvessel density (MVD) and tumor recurrence after surgery. One hundred and thirty-two patients who had undergone curative surgery for primary gastric cancer were immunohistochemically stained for CD105, and MVD was assessed based on the number of CD105-positive vessels. MMP-7 expression was also investigated to determine the relationship with MVD. High MVD was significantly correlated with worse relapse-free survival in all cases and stage I-II cases by log-rank test (p=0.0005 and p=0.0154, respectively). Furthermore, in stage I-II cases, high MVD was the only independent predictor for relapse-free survival by multivariate analysis of the Cox proportional hazard model [p=0.028; hazard ratio (HR)=4.582; 95% confidence interval (CI) 1.184-17.737]. Regarding the specific patterns of recurrence, high MVD was independently related to locoregional recurrence [p=0.011; odds ratio (OR)=15.208; 95% CI 1.886-122.662] and hematogenous recurrence (p=0.002; OR=5.718; 95% CI 1.875-17.442) by multivariate logistic analysis. However, the expression of MMP-7 or CD105 combined with MVD was associated with relapse-free survival, although these prognostic impacts were not maintained in multivariate analysis. In conclusion, MVD recognized by CD105 may be useful as a predictor for the recurrence of resected gastric cancer and may have a specific association with the development of locoregional and hematogenous recurrence.

Tgf-Beta superfamily receptors-targets for antiangiogenic therapy?

The TGF-beta pathway controls a broad range of cellular behavior including cell proliferation, differentiation, and apoptosis of various cell types including tumor cells, endothelial cells, immune cells, and fibroblasts. Besides TGF-beta's direct effects on tumor growth and its involvement in neoangiogenesis have received recent attention. Germline mutations in TGF-beta receptors or coreceptors causing Hereditary Hemorrhagic Teleangiectasia and the Loeys-Dietz syndrome underline the involvement of TGF-beta in vessel formation and maturation. Several therapeutic approaches are evaluated at present targeting the TGF-beta pathway including utilization of antisense oligonucleotides against TGF-beta itself or antibodies or small molecule inhibitors of TGF-beta receptors. Some of these therapeutic agents have already entered the clinical arena including an antibody against the endothelium specific TGF-beta class I receptor ALK-1 targeting tumor vasculature. In conclusion, therapeutic manipulation of the TGF-beta pathway opens great opportunities in future cancer therapy.

Targeting cancer vasculature via endoglin/CD105: a novel antibody-based diagnostic and therapeutic strategy in solid tumours

Endoglin/CD105 is well acknowledged as being the most reliable marker of proliferation of endothelial cells, and it is overexpressed on tumour neovasculature. Our current knowledge of its structure, physiological role, and tissue distribution suggests that targeting of endoglin/CD105 is a novel and powerful diagnostic and therapeutic strategy in human malignancies, through the imaging of tumour-associated angiogenesis and the inhibition of endothelial cell functions related to tumour angiogenesis. Among biotherapeutic agents, monoclonal antibodies have shown a major impact on the clinical course of human malignancies of different histotypes. Along this line, the potential efficacy of anti-endoglin/CD105 antibodies and their derivatives for clinical purposes in cancer is supported by a large body of available pre-clinical in vitro and in vivo data. In this review, the main findings supporting the translation of antibody-based endoglin/CD105 targeting from pre-clinical studies to clinical applications in human cancer are summarized and discussed.

Anti-endoglin monoclonal antibodies are effective for suppressing metastasis and the primary tumors by targeting tumor vasculature

Anti-metastatic activity of an antitumor agent is exceedingly important because metastasis is the primary cause of death for most solid cancer patients. In this report, we show that 3 anti-endoglin (ENG) monoclonal antibodies (mAbs) SN6a, SN6j and SN6k which define individually distinct epitopes of ENG of tumor vasculature are capable of suppressing tumor metastases in the multiple metastasis models. The metastasis models were generated by i.v., s.c. (into flank) or mammary gland fat pad injection of 4T1 murine mammary carcinoma cells and splenic injection of two types of colon26 murine colorectal carcinoma cells. Individual mAbs were injected i.v. via the tail vein of mice. SN6a and SN6j effectively suppressed the formation of metastatic colonies of 4T1 in the lung in all of the three 4T1 metastatic models. In addition, these mAbs were effective for suppressing the primary tumors of 4T1 in the skin and mammary fat pad. These mAbs effectively suppressed microvessel density and angiogenesis in tumors as measured by the Matrigel plug assay in mice. No significant side effects of the administered mAbs were detected. Furthermore, SN6a and SN6j extended survival of the tumor-bearing mice. SN6j, SN6k and their immunoconjugates with deglycosylated ricin A-chain were all effective for suppressing hepatic metastasis of colon26. The findings in the present study are clinically relevant in view of the ongoing clinical trial of a humanized (chimerized) form of SN6j.

Murine endoglin-specific single-chain Fv fragments for the analysis of vascular targeting strategies in mice

Endoglin has been identified as a promising cell surface antigen for vascular targeting approaches in cancer therapy, e.g. employing antibody molecules as targeting moieties. However, in vivo analysis of such strategies in mouse models requires antibodies recognizing endoglin on mouse endothelial cells. Here we describe the isolation of single-chain Fv fragments (scFvs) from phage display libraries, which bind to the extracellular region of mouse endoglin. One of these clones, scFv mE12, showed strong (K(d)=11 nM) and selective binding to purified endoglin and also to the endoglin-expressing mouse endothelioma cell line eEnd.2. This antibody recognized a linear epitope located in the N-terminal region (aa 27-361) of endoglin. Cell binding was further increased by generating a bivalent scFv-Fc fusion protein composed of scFv mE12 and the human gamma1 Fc part. Moreover, scFv mE12 was endowed with an additional cysteine residue in the linker region and applied for the generation of anti-endoglin scFv immunoliposomes capable of selectively binding to endoglin-expressing cells. Thus, anti-mouse endoglin scFv mE12 should be useful to analyze vascular targeting strategies in mice.

Endoglin (CD105): a marker of tumor vasculature and potential target for therapy

Endoglin (CD105) is an accessory protein of the transforming growth factor-beta receptor system expressed on vascular endothelial cells. Mutation of the endoglin gene is associated with hereditary hemorrhagic telangiectasias, or Osler-Weber-Rendu syndrome, and has been studied extensively in the context of this disease. The expression of endoglin is elevated on the endothelial cells of healing wounds, developing embryos, inflammatory tissues, and solid tumors. Endoglin is a marker of activated endothelium, and its vascular expression is limited to proliferating cells. Recent studies identified endoglin expression in several solid tumor types, with the level of expression correlating with various clinicopathologic factors including decreased survival and presence of metastases. Attempts to target endoglin and the cells that express this protein in tumor-bearing mice have yielded promising results.

Anti-cancer therapies targeting the tumor stroma

For anti-tumor therapy different strategies have been employed, e.g., radiotherapy, chemotherapy, or immunotherapy. Notably, these approaches do not only address the tumor cells themselves, but also the tumor stroma cells, e.g., endothelial cells, fibroblasts, and macrophages. This is of advantage, since these cells actively contribute to the proliferative and invasive behavior of the tumor cells via secretion of growth factors, angiogenic factors, cytokines, and proteolytic enzymes. In addition, tumor stroma cells take part in immune evasion mechanisms of cancer. Thus, approaches targeting the tumor stroma attract increasing attention as anti-cancer therapy. Several molecules including growth factors (e.g., VEGF, CTGF), growth factor receptors (CD105, VEGFRs), adhesion molecules (alphavbeta3 integrin), and enzymes (CAIX, FAPalpha, MMPs, PSMA, uPA) are induced or upregulated in the tumor microenvironment which are otherwise characterized by a restricted expression pattern in differentiated tissues. Consequently, these molecules can be targeted by inhibitors as well as by active and passive immunotherapy to treat cancer. Here we discuss the results of these approaches tested in preclinical models and clinical trials.

Endoglin expression is associated with poor oncologic outcome in oral and oropharyngeal carcinoma

CONCLUSIONS

The present preliminary results suggest that endoglin (CD105)-assessed micro-vessel density (MVD) in primary oral and oropharyngeal squamous cell carcinomas (SCCs) may identify patients at risk of disease recurrence or poor oncological outcome after treatment.

OBJECTIVES

MVD is an independent prognostic indicator in several human malignancies. Endoglin antibodies have shown a greater specificity for tumour vasculature in comparison with pan-endothelial markers. The present explorative study evaluated endoglin expression and its prognostic role in oral and oropharyngeal SCCs.

PATIENTS AND METHODS

The study considered 13 consecutive cases of oral and oropharyngeal SCC with lymph node metastases (pN+) and 13 consecutive pN0 cases. CD105-assessed MVD was calculated at 400x magnification.

RESULTS

The mean MVDs were 3.6 and 3.1 in pN+ and pN0 groups, respectively (p>0.05). The mean CD105-assessed MVDs were 4.7 in the group with locoregional recurrence and 2.9 in the group without locoregional recurrence or post-treatment diagnosis of distant metastasis (p=0.01). The mean CD105-assessed MVD in primary oral and oropharyngeal SCCs with poor oncological outcome (recurrence of disease or occurrence of distant metastasis) was 4.3. The mean MVD in primary oral and oropharyngeal SCCs with good outcome was 2.9. Statistical analysis showed a significant difference between CD105-assessed MVD in poor and good outcome groups (p=0.02).

Tumor stroma-associated antigens for anti-cancer immunotherapy

Immunotherapy has been widely investigated for its potential use in cancer therapy and it becomes more and more apparent that the selection of target antigens is essential for its efficacy. Indeed, limited clinical efficacy is partly due to immune evasion mechanisms of neoplastic cells, e.g. downregulation of expression or presentation of the respective antigens. Consequently, antigens contributing to tumor cell survival seem to be more suitable therapeutic targets. However, even such antigens may be subject to immune evasion due to impaired processing and cell surface expression. Since development and progression of tumors is not only dependent on cancer cells themselves but also on the active contribution of the stromal cells, e.g. by secreting growth supporting factors, enzymes degrading the extracellular matrix or angiogenic factors, the tumor stroma may also serve as a target for immune intervention. To this end several antigens have been identified which are induced or upregulated on the tumor stroma. Tumor stroma-associated antigens are characterized by an otherwise restricted expression pattern, particularly with respect to differentiated tissues, and they have been successfully targeted by passive and active immunotherapy in preclinical models. Moreover, some of these strategies have already been translated into clinical trials.