Anti-endoglin Monoclonal Antibodies Research Articles

Expression Profile of Endoglin in Different Grades of Endometrial Cancer.

Endoglin is a marker of active, proliferating endothelial cells of blood vessels. In many cancers, it is present in both peripheral vessels and vessels located inside the tumor. Endoglin is more specific and sensitive compared to other tumor angiogenesis markers. It is suggested that endoglin can be considered a reliable marker of disease outcome. The aim of the study was to assess the expression of endoglin and to determine its potential usefulness as a complementary molecular marker of endometrial cancer. The study included 60 women who underwent hysterectomy: 45 with endometrioid endometrial cancer (study group) and 15 without neoplastic changes (control group). The study group was further divided according to the degree of histological differentiation: G1, 17; G2, 15; and G3, 13. The expression of endoglin was determined immunohistochemically with mouse anti-Endoglin monoclonal antibody. The obtained reactions were evaluated using light microscopy. Analysis of endoglin expression in endothelium showed that it reached 145% of the control. In G2, we observed that the endoglin level decreased and was similar to the control, while in G3 it increased and was even higher than in G1. In cancer cells, endoglin expression increased with the grade of endometrial cancer. Endoglin can be considered a valuable complementary molecular marker, allowing to visualize the advancement of the cancer process, including endometrial cancer.

Durable remission for a woman with refractory choriocarcinoma treated with anti-endoglin monoclonal antibody and bevacizumab: A case from the New England Trophoblastic Disease Center, Brigham and Women's Hospital and Dana-Farber Cancer Institute

A 36-year-old with metastatic and refractory choriocarcinoma following single- and multi-agent chemotherapy and surgical metastectomy experienced a durable remission after receiving therapy with an anti-endoglin monoclonal antibody and bevacizumab. Treatment options and scientific advances in this disease are highlighted.

345 - Targeting Ewing Sarcoma cells and the tumor microenvironment with OMTX003 anti-endoglin monoclonal antibodies

345 - Targeting Ewing Sarcoma cells and the tumor microenvironment with OMTX003 anti-endoglin monoclonal antibodies

A phase Ib dose-escalation study of TRC105 (anti-endoglin antibody) in combination with axitinib in patients with metastatic renal cell carcinoma.

426 Background: Resistance to VEGF-targeted therapy is a major challenge in contemporary treatment of metastatic renal cell carcinoma (mRCC), and endoglin (CD105) activation may be an important mechanism leading to resistance. Endoglin is an essential angiogenic receptor expressed on proliferating tumor vessels and mRCC cancer stem cells that is upregulated following VEGF inhibition. TRC105 is an anti-endoglin monoclonal antibody that potentiates bevacizumab (Bev) and VEGF receptor tyrosine kinase inhibitors (VEGFR TKI) in preclinical models. Methods: Heavily-pretreated mRCC pts with ECOG PS 0-1, and acceptable organ function were treated with TRC105 weekly (8 mg/kg and then 10 mg/kg) in combination with axitinib (initially at 5 mg PO BID and then escalated per patient tolerance to a maximum of 10 mg PO BID). Results: Eighteen mRCC pts (median age=61.5; M:F 16:2; median number of prior therapies=3, including >1 VEGFR TKI, clear cell=13, prior axitinib allowed) were treated. TRC105 dose escalation proceeded from 8 mg/kg (n=3) to 10 mg/kg (n=15) without dose limiting toxicity. Low grade AEs characteristic of each drug were not increased in frequency or severity at the recommended phase 2 doses of the two drugs. Three pts (18%) were PR by RECIST and 8 of 17 pts (47%) exhibited >10% tumor reduction. Median PFS is not mature and is at least 5.8 months in the overall population and at least 5.9 months in ccRCC pts. The single patient who progressed on axitinib immediately prior to study entry remains progression free at month 5 with minor tumor regression. Conclusions: TRC105 at 8 and 10 mg/kg was well tolerated with axitinib in mRCC pts with signs of activity. A multicenter randomized phase II trial of axitinib +/- TRC105 is accruing at this time. Clinical trial information: NCT01806064.

Endoglin (CD105) Silencing Mediated by shRNA Under the Control of Endothelin-1 Promoter for Targeted Gene Therapy of Melanoma.

Endoglin (CD105), a transforming growth factor (TGF)-β coreceptor, and endothelin-1, a vasoconstrictor peptide, are both overexpressed in tumor endothelial and melanoma cells. Their targeting is therefore a promising therapeutic approach for melanoma tumors. The aim of our study was to construct a eukaryotic expression plasmid encoding the shRNA molecules against CD105 under the control of endothelin-1 promoter and to evaluate its therapeutic potential both in vitro in murine B16F10-luc melanoma and SVEC4-10 endothelial cells and in vivo in mice bearing highly metastatic B16F10-luc tumors. Plasmid encoding shRNA against CD105 under the control of the constitutive U6 promoter was used as a control. We demonstrated the antiproliferative and antiangiogenic effects of both plasmids in SVEC4-10 cells, as well as a moderate antitumor and pronounced antimetastatic effect in B16F10-luc tumors in vivo. Our results provide evidence that targeting melanoma with shRNA molecules against CD105 under the control of endothelin-1 promoter is a feasible and effective treatment, especially for the reduction of metastatic spread.

Abstract CT222: Differences in pharmacokinetics of TRC105 (anti-endoglin antibody) when administered as a single agent versus in combination with bevacizumab (Bev)

Abstract Background: TRC105 is an anti-endoglin chimeric monoclonal antibody that inhibits angiogenesis and tumor growth and is being studied in randomized Phase 2 trials with Bev. TRC105 is cleared through binding to endoglin expressed on proliferating endothelium when given as a single agent to cancer patients (Spencer et al, ASCO 2012). Preclinical data indicate that endoglin expression is increased in response to VEGF targeted treatment, and increased endoglin expression in response to Bev may increase the clearance of TRC105 when administered in combination to cancer patients. Methods: Patients (pts) with solid tumors (ST) or ovarian cancer (OC) received 10 mg/kg/wk TRC105 as a single agent, and additional patients with ST (primarily ovarian and colorectal) received 10 mg/kg/wk TRC105 with Bev. Peak and trough levels were assessed by ELISA in 39 patients who received TRC105 alone and compared to that of 35 patients (largely Bev refractory) who received TRC105 and Bev. Pts administered TRC105 and Bev were considered a population sub-group and treated as a covariate. A population pharmacokinetic model of TRC105 disposition was built using rich sampling from the ST trial, with sparse data from OC and TRC105+Bev included in the base model. A two-compartment model with nonlinear elimination best fit the data, utilizing Michaelis-Menten parameters for saturable clearance. Results: TRC105 peak and trough concentrations exceeded target serum concentrations of TRC105 known to saturate endoglin receptors in all pts dosed with 10 mg/kg/wk of TRC105 with and without Bev. The PK of TRC105 given with Bev had mean predicted (following 10,000 simulations) parameters of volume of distribution in the central compartment (VC), VMAX, and KM that were increased compared to population estimated parameters of TRC105 given as a single agent [VC= 44.5±2 (SE) (mL/kg) Pop mean estimate vs. 68.8±4 (mL/kg) Bev predicted; VMAX = 92.6±16 (μg/hr) vs. 297.5±40 (μg/hr) predicted, and KM= 5.91±2 (μg/mL) vs. 61.6±8.8 (μg/mL)]. All differences were significant (p<0.001). Observable data from patients administered the combination yielded PK parameters that were consistent with increased target-mediated clearance of TRC105 when given with Bev. Conclusions: Peak and trough TRC105 serum levels exceed target serum concentrations when given at 10 mg/kg/wk as a single agent or with Bev. Central compartment distribution of TRC105 increased when given with Bev, which is consistent with increased endoglin expression on proliferating endothelium following Bev treatment. The maximum rate of elimination (Vmax) also increased, consistent with increased turnover; however the intrinsic clearance ratio of Vmax/Km remained the same, suggesting no change in endoglin turnover efficiency. Future studies will assess whether PK parameters correlate with responses to the combination of TRC105 and Bev in Bev refractory patients. Citation Format: Shawn D. Spencer, Lee S. Rosen, Michael S. Gordon, Francisco Robert, Daniela Matei, Cody J. Peer, Bonne Adams, Delia Alvarez, Ben K. Seon, Charles P. Theuer, W. Douglas Figg. Differences in pharmacokinetics of TRC105 (anti-endoglin antibody) when administered as a single agent versus in combination with bevacizumab (Bev). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr CT222. doi:10.1158/1538-7445.AM2014-CT222

887P - Trc105 (Anti-Endoglin Antibody) in Combination with Bevacizumab (Bev) and As a Single Agent for Platinum Resistant Ovarian Cancer

ABSTRACT Aim: Endoglin (CD105) is an endothelial cell membrane receptor densely expressed on tumor vessels that is essential for angiogenesis and upregulated by hypoxia and VEGF inhibition. TRC105 is an anti-endoglin monoclonal antibody that potentiates BEV in preclinical models. Methods: Patients with advanced metastatic ovarian cancer, ECOG PS 0-1, and normal organ function were treated with escalating doses of IV TRC105 (3, 6, 8 or 10 mg/kg/wk) plus BEV at 15 mg/kg q3wk or 10 mg/kg q2wk (n = 11) or with 10 mg/kg/wk TRC105 as a single agent (n = 23). Patients were assessed for safety, pharmacokinetics and response. Results: TRC105 was well tolerated as a single agent in 23 patients with advanced platinum resistant ovarian cancer (median age = 63; median 2 prior therapies; range 1-5). The combination of TRC105 and BEV was well tolerated at the recommended single agent doses (10 mg/kg) when the initial dose of TRC105 was divided over two days to limit the frequency of headache. The concurrent administration of BEV and TRC105 did not otherwise potentiate known toxicities of TRC105 or BEV. Target TRC105 serum concentrations were achieved in all patients who received the combination or single agent TRC105 at 10 mg/kg. Mucocutaneous telangiectasia, a marker of TRC105 target modulation, was observed at higher rates with the combination of the two drugs (64% vs 35%; p = 0.15). Reductions in CA-125 were noted in 7 of 20 patients treated with TRC105 as a single agent and 7 of 11 treated with the combination, including 5 of 8 patients who received prior BEV or VEGF receptor tyrosine kinase inhibitor (VEGFR TKI) therapy. Radiographic reductions in tumor volume were observed in 7 of 11 patients (64%) treated with the combination including 4 of 8 patients who received prior BEV or VEGFR TKI, including two patients with PFS > 6 months; one of whom had a partial response by RECIST 1.1. Conclusions: TRC105 10 mg/kg wkly was well tolerated with and without BEV 10 mg/kg q2wk in patients with advanced platinum resistant ovarian cancer. The combination demonstrated activity in BEV and VEGFR TKI refractory patients. Disclosure: A.A. Garcia: The institution received funding for participating in the study; E.A. Alvarez: Dr. Alvarez receives compensation from TRACON Pharmaceuticals, Inc for service on the Safety Review Board Committee; D.S. Mendelson: Institution received funding to conduct the study; B.K. Seon: I am an inventor of TRC105. I do not own stock of TRACON Pharma. I am not a member on the advisory board or board of directors of TRACON Pharma. I had a corporate-sponsored research agreement with TRACON Pharma; D. Alvarez, B.J. Adams and C.P. Theuer: I am employed by TRACON Pharma and own stock in the company. M. Gordon: Institution received funding to conduct the study. All other authors have declared no conflicts of interest.

Abstract A2: Exploratory textural CT evaluation of the combination of TRC105 (anti-endoglin monoclonal antibody; MAb) and bevacizumab (BEV) indicates partial response by Choi criteria in BEV refractory advanced cancer patients (pts) and identifies candidate markers of response.

Abstract Background: Endoglin (CD105) is an endothelial cell membrane receptor, highly expressed on angiogenic tumor vessels, that is essential for angiogenesis and upregulated by hypoxia and VEGF inhibition. TRC105 is an anti-endoglin MAb that potentiates VEGF inhibitors in preclinical models. TRC105 10 mg/kg weekly was well tolerated with BEV 10 mg/kg q2wk and the combination demonstrated activity in BEV and VEGF TKI refractory pts. This study assessed radiographic responses to TRC105 + BEV using Choi criteria and tumor morphology by applying novel quantitative textural analysis (QTA: TexRad -University of Sussex, UK) in pts with durable stable disease by RECIST, to determine predictive markers of response. Methods: Contrast enhanced CT scans from 5 pts with advanced solid tumors who demonstrated stable disease by RECIST in a trial of escalating doses of TRC105 (3, 6, 8 or 10 mg/kg/wk) plus BEV were reviewed. Seventeen target lesions were selected from baseline scans and assessed for target lesion diameter, whole lesion density, and tumor volume at baseline and follow-up. QTA analysis was assessed on the same target lesions using six different filter levels at baseline and on follow-up scans. The results were correlated to anatomic tumor response using non-parametric evaluation and regression analysis. Statistical significance was defined as a two-tailed p < 0.05. Results: Scans from 5 patients (median age 56; M:F 2:3; median 4 prior regimens; 3 metastatic colorectal and 2 ovarian cancer) 4 of whom progressed following VEGF inhibitor treatment were selected and demonstrated stable disease by RECIST for at least 4 months (range: 4-14 months) of treatment with TRC105 + BEV. Four of five patients (80%) had partial responses by Choi criteria. Predictive markers of tumor response on baseline scans included 1) elevated mean pixel density (median values of responder (R) vs non responder (NR): 27.2 vs -4.3) that correlated with subsequent tumor size reduction, 2) elevated entropy (a measure of tumor heterogeneity; median R vs NR: 5.1 vs 4.7) that correlated with subsequent decrease in mean tumor volume, and 3) low kurtosis (a measure of tumor heterogeneity; median R vs NR: 0.2 vs 1.1) that correlated with subsequent reduction in lesion density (p<0.01). Mean positive pixel values (an indicator of hypoxia) on follow-up scans correlated with decreased tumor density. Conclusions: Assessment of radiographic response using Choi criteria identified VEGF inhibitor refractory patients who demonstrated partial response to the combination of TRC105 + BEV. Using novel QTA measures, markers of tumor heterogeneity and hypoxia correlated with individual lesion responses and are worthy of prospective evaluation as predictive imaging biomarkers. Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A2. Citation Format: Ron L. Korn, Michael S. Gordon, Lee S. Rosen, Francisco Robert, Daniela Matei, Jonathan W. Goldman, David S. Mendelson, E. Gabriela Chiorean, Robert Matthew Strother, Ben K. Seon, Delia Alvarez, Bonne J. Adams, Charles P. Theuer. Exploratory textural CT evaluation of the combination of TRC105 (anti-endoglin monoclonal antibody; MAb) and bevacizumab (BEV) indicates partial response by Choi criteria in BEV refractory advanced cancer patients (pts) and identifies candidate markers of response. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A2.

Abstract 378: Serum endoglin (CD105): Molecular heterogeneity and marked differences among different anti-endoglin monoclonal antibodies in the detection of metastasis and tumor progression

Abstract Endoglin (ENG, CD105) is a proliferation-associated homodimeric cell membrane antigen of endothelial cells and leukemia cells. It is strongly expressed on angiogenic vascular endothelium in tumors and also expressed on lymphatic vessels in tumors. Furthermore, ENG is a TGF-β coreceptor and essential for angiogenesis. ENG represents a more specific marker for tumor angiogenesis and/or tumor progression than the commonly used pan-endothelial markers such as CD34 and CD31. Previously we developed a double antibody sandwich radioimmunoassay (DAS-RIA) using two anti-ENG monoclonal antibodies (mAbs) SN6h and SN6a to quantify nanograms/ml levels of serum ENG and found association of serum ENG levels with tumor metastasis (Clin Cancer Res, 2001, 7: 524). The objective of the present study is to characterize molecular nature of serum ENG and to test a hypothesis that sensitivity and specificity of DAS-RIA can be substantially improved by selecting appropriated pairs of anti-ENG mAbs. We generated 12 anti-ENG mAbs and epitopes defined by these mAbs were mapped. We selected 5 pairs of anti-ENG mAbs defining different epitopes to compare their sensitivity and specificity. Anti-ENG mAb SN6h was used as the capture antibody in each pair because SN6h possesses an extremely high antigen-binding avidity (association K = 1.38 × 1011 liters/mol) and it strongly binds both native and denatured forms of ENG. The order of sensitivity among the five pairs was determined to be SN6f/SN6h>> SN6g/SN6h> SN6a/SN6h ≈ SN6j/SN6h> SN6/SN6h. SN6f/SN6h and SN6g/SN6h pairs were particularly effective for distinguishing between Metastasis-positive (Meta+) and Meta− samples. In an additional study, we developed a sensitive DAS-ELISA using SN6f/SN6h and SN6g/SN6h to facilitate easier quantification of soluble ENG. Reasons for the different capacity of different pairs of anti-ENG mAbs to distinguish between Meta+ and Meta− samples will be attributable to molecular heterogeneity of serum ENG, different epitopes defined by individual mAbs, and differences in the antigen-binding avidity among these mAbs. Western blot of serum samples from cancer patients showed highly heterogeneous patterns of serum ENG. Furthermore, certain mAbs were able to detect only a fraction of the heterogeneous ENG molecules while others were capable of detecting a series of heterogeneous ENG molecules. Test results suggest that the major ENG component in the heterogeneous ENG corresponds to a complex of TGF-β1/TGF-β3, TGF-β receptor II and ENG fragment(s). Assay results of serum ENG will be strongly influenced by the molecular heterogeneity of serum ENG, different capacity of individual anti-ENG mAbs in detection of different components of serum ENG, and differences in the heterogeneous patterns of serum ENG among different cancer patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 378.

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.

Anti‐tumor activity of an anti‐endoglin monoclonal antibody is enhanced in immunocompetent mice

In the present study, we investigated the mechanisms by which anti-endoglin (EDG; CD105) monoclonal antibodies (mAbs) suppress angiogenesis and tumor growth. Antihuman EDG mAb SN6j specifically bound to murine endothelial cells and was internalized into the cells in vitro. SN6j effectively suppressed angiogenesis in mice in the Matrigel plug assay. We found that SN6j is more effective for tumor suppression in immunocompetent mice than in SCID mice. We hypothesized that T cell immunity is important for effective antitumor efficacy of SN6j in vivo. To test this hypothesis, we investigated effects of CpG oligodeoxynucleotides (ODN) and depletion of CD4(+) T cells and/or CD8(+) T cells on antitumor efficacy of SN6j in mice. Systemic (i.v.) administration of a relatively small dose (0.6 mug/g body weight/dose) of SN6j suppressed growth of established s.c. tumors of colon-26 in BALB/c mice and improved survival of the tumor-bearing mice. Addition of CpG ODN to SN6j synergistically enhanced antitumor efficacy of SN6j. In contrast, such enhancing effects of CpG ODN were not detected in SCID mice. Antitumor efficacy of SN6j in BALB/c mice was abrogated when CD4(+) T cells and/or CD8(+) T cells were depleted; effect of CD8(+) T cell depletion was stronger. Interestingly, CD4-depletion decreased tumor growth while CD8-depletion enhanced tumor growth in the absence of SN6j. SN6j induced apoptosis in human umbilical vein endothelial cells in a dose-dependent manner which indicates an additional mechanism of antiangiogenesis by SN6j. (c) 2008 Wiley-Liss, Inc.

A Novel Class of Vascular Endothelial Growth Factor-responsive Genes That Require Forkhead Activity for Expression

Recently, we have shown that transient phosphorylation and inhibition of the pro-apoptotic transcription factor, forkhead, by vascular endothelial growth factor (VEGF) is essential for endothelial cell (EC) survival and proliferation. The goal of the present study was to determine whether forkhead (FKHR) also plays a positive role in agonist-mediated gene induction. Human coronary artery ECs were transduced with adenovirus overexpressing constitutively active phosphorylation-resistant triple mutant FKHR or transfected with small interference RNA (siRNA) against FKHR. The cells were then treated in the absence or presence of VEGF and assayed for gene expression using quantitative real-time PCR and Northern blots analyses. The data revealed a novel set of VEGF-responsive genes that require FKHR activity for optimal expression in ECs, including bone morphogenic protein 2, cbp/p300-interacting transactivator 2, decay accelerating factor (DAF), vascular cell adhesion molecule-1 (VCAM-1), manganese superoxide dismutase, endothelial-specific molecule-1, RING1 and YY1-binding protein, and matrix metalloproteinase-10. Consistent with a positive role for FKHR in mediating VEGF induction of DAF and VCAM-1 mRNA, siRNA against FKHR attenuated the effect of VEGF on complement-mediated EC lysis and monocyte adhesion, respectively. VEGF induction of the forkhead-dependent genes was down-regulated by the NF-kappaB inhibitor, constitutively active Ad-IkappaB, and in some cases by the nuclear factor of activated T-cells (NF-AT) inhibitor, cyclosporin. Together, these findings suggest that the VEGF-forkhead signaling axis plays an important functional role in ECs beyond the regulation of cell survival/apoptosis and cell cycle.

Synergy between anti‐endoglin (CD105) monoclonal antibodies and TGF‐β in suppression of growth of human endothelial cells

Endoglin (CD105) is a proliferation-associated cell membrane antigen of endothelial cells and strongly expressed in the angiogenic vasculature of solid tumors. Endoglin is essential for angiogenesis/vascular development and an ancillary transforming growth factor beta (TGF-beta) receptor. Certain anti-endoglin monoclonal antibodies (mAbs), termed SN6 series mAbs, inhibited angiogenesis, tumor growth and metastasis in mice. We investigated the mechanisms by which anti-endoglin mAbs suppress growth of proliferating endothelial cells. We found that 4 SN6 series mAbs suppressed growth of human umbilical vein endothelial cells (HUVECs) in a dose-dependent manner in the absence of any effector cells or complement. Significant differences in the growth suppression between the 4 anti-endoglin mAbs defining different epitopes were observed. These differences were not determined by antigen-binding avidities of the mAbs. Combination of TGF-beta1 and each of the 4 anti-endoglin mAbs exerted synergistic growth suppression of HUVECs. Binding of anti-endoglin mAbs to endoglin-expressing cells did not block the subsequent binding of TGF-beta1. Conversely, preincubation of HUVECs with TGF-beta1 did not change cell surface expression of endoglin. The present results suggest that direct suppression of the endothelial cell growth by SN6 series mAbs is one of the underlying mechanisms by which anti-endoglin mAbs exert antiangiogenic and tumor-suppressive activity in vivo. The results further suggest that TGF-beta1 plays an important role in the in vivo antiangiogenic efficacy of anti-endoglin mAbs by synergistically enhancing the activity of these mAbs. Further studies of the present novel findings may provide valuable information about the functional roles of endoglin and anti-endoglin mAbs in the TGF-beta-mediated cell regulation.

Endoglin (CD105): a target for anti-angiogenetic cancer therapy.

Targeting of tumor vasculature is a promising strategy for cancer treatment. Among endothelial cell markers, Endoglin, a cell membrane glycoprotein, is emerging as an attractive therapeutic target on angiogenetic blood vessels, and it currently represents a powerful marker to quantify tumor angiogenesis. In normal human tissues, Endoglin is weakly expressed on erytroid precursors, stromal cells and activated monocytes, whereas it is strongly expressed on proliferating endothelial cells. In human neoplasias of different histotype, Endoglin is mainly present on endothelial cells of both peri- and intra-tumoral blood vessels, while it is weakly expressed or absent on neoplastic cells. Endoglin is an accessory component of the receptor complex of Transforming Growth Factor (TGF)-beta, a pleiotropic cytokine that modulates angiogenesis by the regulation of different cellular functions including proliferation, differentiation and migration. Interestingly, the over-expression of Endoglin antagonizes several cellular responses to TGF-beta1, while its down-regulation potentiates cellular responses to TGF-beta1. In animal models, administration of radiolabeled anti-Endoglin monoclonal antibodies (mAb) efficiently images primary tumors, and naked or conjugated anti-Endoglin mAb suppress angiogenesis and tumor growth. In this review we will summarize the complex of experimental evidences pointing to Endoglin as a vascular target to design innovative bioimmunotherapeutic strategies in human neoplasias.

Antiangiogenic radioimmunotherapy of human solid tumors in SCID mice using (125)I-labeled anti-endoglin monoclonal antibodies.

Endoglin (CD105), which is a component of the TGF-beta receptor complex, is highly expressed at the surface of proliferating human endothelial cells such as those of tumor vessels. In the present study, we tested the antitumor efficacy of (125)I-labeled anti-endoglin monoclonal antibodies (MAbs), SN6f and SN6j, against s. c. tumors of MCF-7 human breast cancer cells in SCID mice by i.v. administration. SN6f and SN6j cross-react weakly with mouse endothelial cells, but show no significant reactivity with MCF-7 tumor cells. These MAbs are effectively internalized into the cells after binding to the cell surface antigen of endothelial cells. Four groups of SCID mice (n = 10 or 9 in each group) inoculated s.c. with 8 x 10(6) MCF-7 cells were treated with (125)I-SN6f (10 microCi), (125)I-SN6j (10 microCi), a (125)I-labeled isotype-matched control IgG (10 microCi) or PBS. The systemic therapy was performed in 2 series, i.e., on days 3, 5, 7 and days 58, 60, 62. Both (125)I-SN6f and (125)I-SN6j showed significant growth suppression of the tumors, whereas the (125)I-labeled control IgG did not show any significant antitumor efficacy. No significant toxicity or weight loss was observed in mice treated with either (125)I-SN6f or (125)I-SN6j. After 100 days of observation, autopsies revealed no significant organ damage. Our results show the possible usefulness of antiangiogenic radioimmunotherapy using (125)I-labeled anti-endoglin MAbs.

Associations between TGF-beta1 receptors in human bone marrow stromal cells.

Bone marrow stromal cells are required for sustained haemopoiesis. Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine present in the bone marrow microenvironment which regulates the expression of several cytokines, cytokine receptors and cell adhesion elements. The TGF-beta receptors type I and II, and endoglin, mediate TGF-beta1 binding to the membrane of human bone marrow stromal cells. [125I]TGF-beta1-affinity labelling experiments showed that three different anti-endoglin monoclonal antibodies co-immunoprecipitated a 68 kD TGF-beta1-labelled polypeptide together with TGF-beta1/endoglin complexes. Here, we have shown that the 68 kD receptor corresponds to the type I receptor, indicating that endoglin and the type I receptor associate on the membrane of these cells upon ligand binding. The expression of endoglin by stromal cells was found to be up-regulated by TGF-beta1, but not by IL-1beta. The association of endoglin with signalling components of the TGF-beta receptor system on the membrane of bone marrow stromal cells might modulate TGF-beta1 access to the signalling pathways, and therefore it could regulate TGF-beta1-mediated stromal cellular responses.