Angiogenesis inhibitors: current strategies and future prospects

Angiogenesis has become an attractive target for drug therapy because of its key role in tumor growth. An extensive array of compounds is currently in preclinical development, with many now entering the clinic and/or achieving approval from the US Food and Drug Administration. Several regulatory and signaling molecules governing angiogenesis are of interest, including growth factors (eg, vascular endothelial growth factor, platelet-derived growth factor, fibroblast growth factor, and epidermal growth factor), receptor tyrosine kinases, and transcription factors such as hypoxia inducible factor, as well as molecules involved in mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K) signaling. Pharmacologic agents have been identified that target these pathways, yet for some agents (notably thalidomide), an understanding of the specific mechanisms of antitumor action has proved elusive. The following review describes key molecular mechanisms and novel therapies that are on the horizon for antiangiogenic tumor therapy.

The Role of Targeted Agents in the Treatment of Metastatic Breast Cancer

To date, blockade of growth factor receptors is the mainstay of targeted therapy in metastatic breast cancer (mBC). Monoclonal antibodies such as trastuzumab and bevacizumab represent the first generation of molecular-based therapies. Both the HER2 inhibitors and the vascular endothelial growth factor (VEGF) antagonists have shown synergism with a broad spectrum of established cytotoxins, thus being approved for first-line treatment of mBC in combination with taxanes. As a next step, tyrosine kinase inhibitors (TKIs) have been integrated into daily routine as an alternative approach for targeting HER2: The dual HER1/2 inhibitor lapatinib demonstrated activity in trastuzumab-pretreated mBC patients in combination with capecitabine. Furthermore, chemotherapy-free regimens (trastuzumab or lapatinib plus aromatase inhibitors) have been identified as additional options for hormone receptor (HR)- and HER2-positive patients. Recently published data indicate that a combination of two biologicals such as lapatinib and trastuzumab can be effective as a treatment beyond trastuzumab related progression. Multitarget TKIs have the potential to inhibit several signaling pathways involved in breast cancer-related angiogenesis. Until now, they have failed to show a clear benefit in mBC. On the other hand, poly(ADP-ribose) polymerase (PARP) inhibition, mediated by a new class of small molecules, is an interesting area of investigation. Future directions of research in HER2-positive breast cancer focus on the evaluation of novel antibodies (pertuzumab, T-DM1), and irreversible TKIs (neratinib, BIBW 2992) and inhibitors of HER2-related downstream signaling (mTOR, TORC 1/2, PI3K/Akt) and of receptor cross-talk (IGFR).

Targeted Therapy of Cancer Using Photodynamic Therapy in Combination with Multi-faceted Anti-Tumor Modalities

Photodynamic therapy (PDT) has emerged as one of the important therapeutic options in the management of cancer and other diseases. PDT involves a tumor-localized photosensitizer (PS), which when appropriately illuminated by visible light converts oxygen into cytotoxic reactive oxygen species (ROS), that attack key structural entities within the targeted cells, ultimately resulting in necrosis or apoptosis. Though PDT is a selective modality, it can be further enhanced by combining other targeted therapeutic strategies that include the use of synthetic peptides and nanoparticles for selective delivery of photosensitizers. Another potentially promising strategy is the application of targeted therapeutics that exploit a myriad of critical pathways involved in tumorigenesis and metastasis. Vascular disrupting agents that eradicate tumor vasculature during PDT and anti-angiogenic agents that targets specific molecular pathways and prevent the formation of new blood vessels are novel therapeutic approaches that have been shown to improve treatment outcome. In addition to the well-documented mechanisms of direct cell killing and damage to the tumor vasculature, PDT can also activate the body's immune response against tumors. Numerous pre-clinical studies and clinical observations have demonstrated the immuno-stimulatory capability of PDT. Herein, we aim to integrate the most important findings with regard to the combination of PDT and other novel targeted therapy approaches, detailing its potential in cancer photomedicine.

Anti-cancer, anti-diabetic and other pharmacologic and biological activities of penta-galloyl-glucose

1, 2, 3, 4, 6-penta-O-galloyl-beta-D-glucose (PGG) is a polyphenolic compound highly enriched in a number of medicinal herbals. Several in vitro and a handful of in vivo studies have shown that PGG exhibits multiple biological activities which implicate a great potential for PGG in the therapy and prevention of several major diseases including cancer and diabetes. Chemically and functionally, PGG appears to be distinct from its constituent gallic acid or tea polyphenols. For anti-cancer activity, three published in vivo preclinical cancer model studies with PGG support promising efficacy to selectively inhibit malignancy without host toxicity. Potential mechanisms include anti-angiogenesis; anti-proliferative actions through inhibition of DNA replicative synthesis, S-phase arrest, and G(1) arrest; induction of apoptosis; anti-inflammation; and anti-oxidation. Putative molecular targets include p53, Stat3, Cox-2, VEGFR1, AP-1, SP-1, Nrf-2, and MMP-9. For anti-diabetic activity, PGG and analogues appear to improve glucose uptake. However, very little is known about the absorption, pharmacokinetics, and metabolism of PGG, or its toxicity profile. The lack of a large quantity of highly pure PGG has been a bottleneck limiting in vivo validation of cancer preventive and therapeutic efficacies in clinically relevant models.

Safety and efficacy of bevacizumab with hypofractionated stereotactic irradiation for recurrent malignant gliomas

PURPOSE

Preclinical studies suggest that inhibition of vascular endothelial growth factor (VEGF) improves glioma response to radiotherapy. Bevacizumab, a monoclonal antibody against VEGF, has shown promise in recurrent gliomas, but the safety and efficacy of concurrent bevacizumab with brain irradiation has not been extensively studied. The objectives of this study were to determine the safety and activity of this combination in malignant gliomas.

METHODS AND MATERIALS

After prior treatment with standard radiation therapy patients with recurrent glioblastoma (GBM) and anaplastic gliomas (AG) received bevacizumab (10 mg/kg intravenous) every 2 weeks of 28-day cycles until tumor progression. Patients also received 30 Gy of hypofractionated stereotactic radiotherapy (HFSRT) in five fractions after the first cycle of bevacizumab.

RESULTS

Twenty-five patients (20 GBM, 5 AG; median age 56 years; median Karnofsky Performance Status 90) received a median of seven cycles of bevacizumab. One patient did not undergo HFSRT because overlap with prior radiotherapy would exceed the safe dose allowed to the optic chiasm. Three patients discontinued treatment because of Grade 3 central nervous system intratumoral hemorrhage, wound dehiscence, and bowel perforation. Other nonhematologic and hematologic toxicities were transient. No radiation necrosis was seen in these previously irradiated patients. For the GBM cohort, overall response rate was 50%, 6-month progression-free survival was 65%; median overall survival was 12.5 months, and 1-year survival was 54%.

DISCUSSION

Bevacizumab with HFSRT is safe and well tolerated. Radiographic responses, duration of disease control, and survival suggest that this regimen is active in recurrent malignant glioma.

Tumoristatic effects of endostatin in prostate cancer is dependent on androgen receptor status

BACKGROUND

Although anti-angiogenic therapy is a promising new line of therapy for prostate cancer, we recently reported that stable expression of endostatin arrested the progression of prostate cancer to poorly differentiated state and distant metastasis in TRAMP mice. However, the same therapy failed to provide any benefit when given either during or after the onset of metastatic switch. The present study determined the possible mechanisms behind the selective advantage of endostatin therapy in early-stage disease.

METHODS

Angiogenesis-related gene expression analysis was performed to identify target genes and molecular pathways involved in the therapy effects. Based on the results from in vivo studies, and recapitulation of the in vivo data in vitro using tumorigenic and non-tumorigenic human prostate cancer cells that are either androgen-sensitive or androgen-independent, analyses of possible mechanisms of the selective advantage of early treatment were performed using assays for cell proliferation, apoptosis, migration, and cell signaling. The identified mechanisms were further confirmed in vivo.

RESULTS

Results indicated that cells with high androgen receptor (AR) expression were more sensitive to endostatin treatment than androgen-independent cells with low or no AR expression. Endostatin was found to significantly downregulate the expression of growth factors, receptor tyrosine kinases, proteases, and AR both in vitro and in vivo only when the cells express high-levels of AR. Cell proliferation was not influenced by endostatin treatment but migration was significantly affected only in androgen-sensitive cells. Targeted downregulation of AR prior to endostatin treatment in androgen-sensitive cells and overexpression of AR in androgen-independent cells indicated that the effect of endostatin via AR downregulation is mediated by a non-genotropic mechanism on Ras and RhoA pathways, and independently of AR on MAPK/ERK pathway.

CONCLUSIONS

These data indicate that systemically stable endostatin expression delays the onset of metastatic switch by acting on multiple pathways involving AR.

Treatment of neovascular age-related macular degeneration: Current therapies

Choroidal neovascularization (CNV) secondary to age-related macular degeneration (AMD) is now the leading cause of blindness and severe vision loss among people over the age of 40 in the Western world. Its prevalence is certain to increase substantially as the population ages. Treatments currently available for the disease include laser photocoagulation, verteporfin photodynamic therapy, and intravitreal injections of corticosteroids and anti-angiogenic agents. Many studies have reported the benefits of each of these treatments, although none is without its risks. No intervention actually cures AMD, nor the neovascularization associated with it. However, its symptoms are treated with varying degrees of success. Some treatments stabilize or arrest the progress of the disease. Others have been shown to reverse some of the damage that has already been done. These treatments can even lead to visual improvement. This paper will review the major classes of drugs and therapies designed to treat this condition.

Higher incidence of Osteonecrosis of the Jaw (ONJ) in patients with metastatic castration resistant prostate cancer treated with anti-angiogenic agents

ONJ is an important toxicity in cancer patients receiving bisphosphonate therapy. Here we report a higher than usual incidence of ONJ, 11 of 60 (18.3%, 95% Confidence Interval, CI: 9%-28%) patients enrolled in a phase II clinical trial combining bevacizumab, docetaxel, thalidomide, and prednisone (ATTP) in chemotherapy-naive men with metastatic castration resistant prostate cancer (mCRPC). The use of bisphosphonates was allowed at study entry. Our study suggests that anti-angiogenic and chemotherapy agents can predispose to the development of ONJ in men with mCRPC on zoledronic acid. Imaging modalities, such as bone scans, may be useful in following the clinical course of patients who develop ONJ.

Phase I study of SU5416, a small molecule inhibitor of the vascular endothelial growth factor receptor (VEGFR) in refractory pediatric central nervous system tumors

SU5416 is a novel small molecule tyrosine kinase inhibitor of the VEGF receptors 1 and 2. A phase I dose escalation study stratified by concurrent use (stratum II) or absence (stratum I) of enzyme-inducing anticonvulsant drugs was undertaken to estimate the maximum-tolerated dose (MTD) and to describe the toxicity profile of SU5416 in pediatric patients with refractory brain tumors. Dose escalations were conducted independently for stratum I starting at 110 mg/m(2) while stratum II started at 48 mg/m(2). Thirty-three eligible patients were treated on stratum I (n = 23) and stratum II (n = 10). Tumor types included 23 glial tumors, 4 neural tumors, 4 ependymomas, and 2 choroid plexus carcinomas. The MTD in stratum I was initially estimated to be 110 mg/m(2). The protocol was amended to determine the MTD after excluding transient AST elevation. Re-estimation of the MTD began at the 145 mg/m(2) dose level but due to development of SU5416 being stopped by the sponsor, the trial was closed before completion. The most serious drug-related toxicities were grade 3 liver enzyme abnormalities, arthralgia, and hallucinations. The plasma pharmacokinetics of SU5416 was not significantly affected by the concurrent administration of enzyme-inducing anticonvulsant drugs. Mean values of the total body clearance, apparent volume of distribution, and terminal phase half-life of SU5416 for the 19 patients in stratum I were 26.1 +/- 12.5 l/hr/m(2), 41.9 +/- 21.4 l/m(2), and 1.11 +/- 0.41 hr, respectively. The plasma pharmacokinetics of SU5416 in children was similar to previously reported findings in adult cancer patients. Prolonged disease stabilization was observed in 4 of 16 stratum I patients.

Regulation of thrombospondin1 by extracellular proteases

The contribution of proteases to developmental, physiological and pathological processes has been well accepted. Cleavage of matrix proteins is a key requirement for cell migration and remodeling of the extracellular environment. The constant process of matrix turnover is dependent on the delicate balance between degradation and synthesis. In addition, regulated proteolysis also allows for the release and activation of growth factors and cytokines. Similarly to other extracellular matrix proteins, thrombospondins are also targets of proteolysis. While in some cases enzymatic activity is associated with degradation of the protein; in other situations, targeted and selective cleavage offers the means to release polypeptides with either alternative or enhanced function. Here, we provide a summary of the published information related to thrombospondin proteolysis within the context of how proteolysis of extracellular matrix proteins impacts diversification of protein function. We also discuss its biological relevance and potential therapeutic value of thrombospondin proteolysis with particular emphasis on angiogenesis.

A Legumain-based minigene vaccine targets the tumor stroma and suppresses breast cancer growth and angiogenesis

Tumor associated macrophages (TAMs) are well known to play a very important role in tumor angiogenesis and metastasis. The suppression of TAMs in the tumor-microenvironment (TME) provides a novel strategy to inhibit tumor growth and dissemination by remodeling the tumor's stroma. Here, we tested our hypothesis that suppression of TAMs can be achieved in syngeneic BALB/c mice with oral minigene vaccines against murine MHC class I antigen epitopes of Legumain, an asparaginyl endopeptidase and a member of the C13 family of cystine proteases which is overexpressed on TAMs in the tumor stroma. Vaccine vectors were constructed and transformed into attenuated Salmonella typhimurium (Dam ( - ) , AroA ( - )) for oral delivery. Groups of mice received either the expression vectors encoding the Legumain H-2D or 2K epitopes or the control empty vector by gavage. The efficacy of the minigene vaccines was determined by their ability to protect mice from lethal tumor cell challenges, the induction of a specific CTL response as well as IFN-gamma release, and inhibition of tumor angiogenesis. We demonstrated that the Legumain minigene vaccine provided effective protection against tumor cell challenge by inducing a specific CD8+ T-cell response against Legumain+ TAMs in our breast tumor model. The protection, induced by this T-cell response, mediated by the Legumain Kd minigene, is also responsible for lysing D2F2 breast carcinoma cells in syngeneic BALB/c mice and for suppressing tumor angiogenesis. Importantly, in a prophylactic setting, the minigene vaccine proved to be of similar anti-tumor efficacy as a vaccine encoding the entire Legumain gene. Together, our findings establish proof of concept that a Legumain minigene vaccine provides a more flexible alternative to the whole gene vaccine, which may facilitate the future design and clinical applications of such a vaccine for cancer prevention.

Gambogic acid inhibits angiogenesis and prostate tumor growth by suppressing vascular endothelial growth factor receptor 2 signaling

Gambogic acid (GA), the main active compound of Gamboge hanburyi, has been previously reported to activate apoptosis in many types of cancer cell lines by targeting transferrin receptor and modulating nuclear factor-kappaB signaling pathway. Whether GA inhibits angiogenesis, which is crucial for cancer and other human diseases, remains unknown. Here, we found that GA significantly inhibited human umbilical vascular endothelial cell (HUVEC) proliferation, migration, invasion, tube formation, and microvessel growth at nanomolar concentration. In a xenograft prostate tumor model, we found that GA effectively inhibited tumor angiogenesis and suppressed tumor growth with low side effects using metronomic chemotherapy with GA. GA was more effective in activating apoptosis and inhibiting proliferation and migration in HUVECs than in human prostate cancer cells (PC3), suggesting GA might be a potential drug candidate in cancer therapy through angioprevention with low chemotoxicity. Furthermore, we showed that GA inhibited the activations of vascular endothelial growth factor receptor 2 and its downstream protein kinases, such as c-Src, focal adhesion kinase, and AKT. Together, these data suggest that GA inhibits angiogenesis and may be a viable drug candidate in antiangiogenesis and anticancer therapies.

Antibody targeting of the insulin-like growth factor I receptor enhances the anti-tumor response of multiple myeloma to chemotherapy through inhibition of tumor proliferation and angiogenesis

Although many multiple myeloma (MM) patients initially respond to cytotoxic therapy, most eventually relapse. Novel therapeutic strategies employing a combination of chemotherapy with targeted biologics may significantly enhance the response of tumor cells to treatment. We tested a fully human anti-IGF-IR antibody (A12) against MM, and showed specific inhibition of IGF-I or serum-induced IGF-IR signaling in MM cells in vitro. The A12 as a single agent was demonstrated to exert modest to significant inhibition of tumor growth in vivo in various subcutaneous xenograft MM models. The A12 was also evaluated in a disseminated xenograft MM.1S NOD/SCID model as monotherapy or in combination with other drugs (bortezomib, melphalan) currently in clinical use. The tumor burden, as determined by luciferase bioimaging, was sharply decreased, and overall survival significantly prolonged when the therapies were combined. Immunohistochemical analysis demonstrated that the A12 treated tumors had significantly decreased vascularization compared to control tumors. Furthermore, most MM lines constitutively secreted significant quantities of VEGF, and this was enhanced following IGF-I treatment. Inhibition of IGF-IR by the A12 in vitro suppressed both constitutive and IGF-I-induced secretion of VEGF, indicating that a putative anti-angiogenic mechanism associated with the A12 treatment may contribute to its anti-tumor effect.

Successful tumor eradication was achieved by collaboration of augmented cytotoxic activity and anti-angiogenic effects following therapeutic vaccines containing helper-activating analog-loaded dendritic cells and tumor antigen DNA

We reported previously that pigeon cytochrome c-derived peptides (Pan-IA), which bind broad ranges of MHC class II molecules efficiently, activate T helper (Th) function in mice. In an experimental model, Pan-IA DNA vaccines augmented antitumor immunity in tumor antigen-immunized mice. To elicit more potent antitumor immunity and to eradicate tumors in a therapeutic setting, Pan-IA-loaded dendritic cells (DCs) were inoculated in combination with vaccines including ovalbumin (OVA) antigen DNA in tumor-bearing mice. Seventy percent of the immunized mice survived tumor-free for at least 4 months after treatment. In contrast, mice vaccinated with OVA DNA, either with or without naïve DCs, did not eliminate the tumors and died within 5 weeks. Only in mice vaccinated with OVA DNA and Pan-IA-loaded DCs were both cytotoxic and helper responses specific for OVA induced at the spleen and tumor sites as well as at the vaccination sites. Furthermore, accumulation of OVA-specific CD4(+) and CD8(+) T lymphocytes and interferon-gamma-mediated anti-angiogenesis were observed in the tumors of these mice. Thus, the combined vaccination primed both tumor-specific cytotoxicity and helper immunity resulting in augmented tumor lysis ability and anti-angiogenic effects. This is the first report to show that most established tumors were successfully eradicated by collaboration of potent antitumor immunity and anti-angiogenic effects by vaccination with tumor antigens and helper-activating analogs. This novel vaccination strategy is broadly applicable, regardless of identifying helper epitopes in target molecules, and contributes to the development of therapeutic cancer vaccines.

Endoglin (CD105) is a target for an oral DNA vaccine against breast cancer

Endoglin (CD105), a co-receptor in the TGF-beta receptor complex, is over-expressed on proliferating endothelial cells in the breast tumor neovasculature and thus offers an attractive target for anti-angiogenic therapy. Here we report the anti-angiogenic/anti-tumor effects achieved in a prophylactic setting with an oral DNA vaccine encoding murine endoglin, carried by double attenuated Salmonella typhimurium (dam-, AroA-) to a secondary lymphoid organ, i.e., Peyer's patches . We demonstrate that an endoglin vaccine elicited activation of antigen-presenting dendritic cells, coupled with immune responses mediated by CD8+ T cells against endoglin-positive target cells. Moreover, we observed suppression of angiogenesis only in mice administered with the endoglin vaccine as compared to controls. These data suggest that a CD8+ T cell-mediated immune response induced by this vaccine effectively suppressed dissemination of pulmonary metastases of D2F2 breast carcinoma cells presumably by eliminating proliferating endothelial cells in the tumor vasculature. It is anticipated that vaccine strategies such as this may contribute to future therapies for breast cancer.

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

We generated a human/mouse chimeric antibody c-SN6j of human IgG1 isotype from a murine anti-human endoglin (EDG) monoclonal antibody (mAb) SN6j that suppressed angiogenesis, tumor growth and metastasis in mice. We determined pharmacokinetics (PKs) and immunogenicity of c-SN6j in monkeys after multiple i.v. injections. A dose-escalation study was performed by administration of c-SN6j into six monkeys at the dose of 1 mg, 3 mg and 10 mg per kg body weight. In addition, both c-SN6j (3 mg/kg) and doxorubicin (0.275 mg/kg) were injected into two monkeys. c-SN6j and doxorubicin were injected twice a week for 3 weeks. We developed a unique and sensitive ELISA by sequentially targeting the common and idiotypic epitopes of c-SN6j-Fv to quantify plasma c-SN6j. Application of the ELISA showed that increasing the c-SN6j dose resulted in a proportional increase in the circulating c-SN6j after the first injection. In addition, the estimated area under the curve (AUC) for the first injection of c-SN6j is proportional to dose. We carried out detailed analyses of PKs of c-SN6j during and after the repeated injections. Our model of PKs fitted the empirical data well. Addition of doxorubicin modulated the PK parameters. We developed two ELISAs to separately determine the immune responses to the murine part and the human part of c-SN6j in monkeys. Interestingly, the murine part induced a weaker immune response than the human part. Doxorubicin potentiated the immune responses. Increasing the dose of c-SN6j increased plasma levels of c-SN6j but did not increase the immune responses to c-SN6j.

Fibrinogen E fragment selectively disrupts the vasculature and inhibits the growth of tumours in a syngeneic murine model

We recently demonstrated that a fragment of human fibrinogen, fibrinogen E fragment, inhibits the migration and differentiation of human endothelial cells in vitro. Here we show that it exerts similar effects on murine endothelial cells in vitro, and selectively disrupts tumour endothelium in vivo, causing widespread intravascular thrombosis and retarding the growth of CT26 tumours in a syngeneic murine model.

Anti-angiogenic effects of the thienopyridine SR 25989 in vitro and in vivo in a murine pulmonary metastasis model

Neovascularisation is a key step in tumour growth and establishment of distant metastases. We have recently demonstrated that the thienopyridine SR 25989 an enantiomer of the anti-aggregant clopidogrel (Plavix) lacking anti-aggregant activity, inhibits endothelial cell proliferation in vitro by increasing the expression of endogenous thrombospondin-1, a natural potent inhibitor of angiogenesis. The anti-angiogenic effect of SR 25989 was further assessed in vitro in a quantitative assay of angiogenesis comprising a fragment of rat aorta embedded in a fibrin gel and in vivo in a pulmonary metastatic model using C57BL/6 mice inoculated in the foot pad with the highly metastatic melanoma cell line B16 F10. SR 25989 induced a dose dependent inhibition of spontaneous microvessel development in vitro reaching half maximal inhibition at around less than 50 microM and caused platelet derived growth factor induced angiogenesis to regress as a function of thienopyridine concentration. In vivo, SR 25989 did not alter significantly the growth rate of the primary tumour in the foot pad and did not inhibit development of inguinal nodes which appeared after amputation. However, the number and size of lung metastases were reduced in treated animals when examined at the time of sacrifice. In addition, the few metastases over 1 mm3 did not show any neovascularisation, as confirmed by negative von Willebrand immunostaining and in contrast to intense vascularisation seen in metastases developed by control mice. These results confirm that SR 25989 possesses potent anti-angiogenic properties and is able to inhibit metastatic dissemination and growth. The lack of effect on the primary tumour and inguinal nodes illustrates the complexity of the mechanisms involved in tumoural neo-angiogenesis and points out the possibility for distinct processes leading to neovascularisation in primary tumour as opposed to metastases.

Generation of angiostatin-like fragments from plasminogen by prostate-specific antigen

Angiostatin, a potent inhibitor of angiogenesis, tumour growth and metastasis, is a biologically active fragment of plasminogen, containing the kringle domains 1-4. It is generated from plasminogen by limited proteolysis. We show that prostate-specific antigen (PSA), a serine proteinase secreted by human prostate and human prostate cancer cells, is able to convert Lys-plasminogen to biologically active angiostatin-like fragments, containing kringles 1-4, by limited proteolysis of peptide bond Glu439-Ala440 in vitro. In an in vitro morphogenesis assay, the purified angiostatin-like fragments inhibited proliferation and tubular formation of human umbilical vein endothelial cells with the same efficacy as angiostatin. This finding might help to understand growth characteristics of prostate cancer, which usually has low microvessel density and slow proliferation.

Treatment of experimental brain tumors with trombospondin-1 derived peptides: an in vivo imaging study

Antiangiogenic and antiproliferative effects of synthetic D-reverse peptides derived from the type 1 repeats of thrombospondin (TSP1) were studied in rodent C6 glioma and 9L gliosarcomas. To directly measure tumor size and vascular parameters, we employed in vivo magnetic resonance (MR) imaging and corroborated results by traditional morphometric tissue analysis. Rats bearing either C6 or 9L tumors were treated with TSP1-derived peptide (D-reverse amKRFKQDGGWSHWSPWSSac, n=13) or a control peptide (D-reverse amKRAKQAGGASHASPASSac, n=12) at 10 mg/kg, administered either intravenously or through subcutaneous miniosmotic pumps starting 10 days after tumor implantation. Eleven days later, the effect of peptide treatment was evaluated. TSP1 peptide-treated 9L tumors (50.7+/-44.2 mm3, n=7) and C6 tumors (41.3+/-34.2 mm3, n=6) were significantly smaller than tumors treated with control peptide (9L: 215.7+/-67.8 mm3, n=6; C6: 184.2+/-105.2 mm3, n=6). In contrast, the in vivo vascular volume fraction, the mean vascular area (determined by microscopy), and the microvascular density of tumors were not significantly different in any of the experimental groups. In cell culture, TSP1, and the amKRFKQDGGWSHWSPWSSac peptide showed antiproliferative effects against C6 with an IC of 45 nM for TSP1. These results indicate that TSP1-derived peptides retard brain tumor growth presumably as a result of slower de novo blood vessel formation and synergistic direct antiproliferative effects on tumor cells. We also show that in vivo MR imaging can be used to assess treatment efficacy of novel antiangiogenic drugs non-invasively, which has obvious implications for clinical trials.