Targeting the vasculature of solid tumours

Radioimmunotherapy of Human Colon Carcinoma Xenografts Using a213Bi-Labeled Domain-Deleted Humanized Monoclonal Antibody

The data presented within this paper is the first report of a humanized domain-deleted monoclonal antibody (HuCC49DeltaCH2) to be utilized in a radioimmunotherapeutic (RIT) application with 213Bi. An initial study indicated that 111In-HuCC49DeltaCH2 targets the subcutaneously implanted human colon carcinoma xenograft, LS-174T, when injected via a peritoneal route. The HuCC49DeltaCH2 was then radiolabeled with 213Bi, an alpha-emitting radionuclide with a half-life of 45.6 minutes, and evaluated for therapeutic efficacy. Dose titration studies indicated that a single dose of 500-1000 microCi, when injected by an intraperitoneal route, resulted in the growth inhibition or regression of the tumor xenograft. The radioimmunotherapeutic effect was found to be dose-dependent. Specificity of the therapeutic efficacy was confirmed in a subsequent experiment with athymic mice bearing TAG-72 negative MIP (human colorectal) xenografts. A preliminary study was also performed to assess a multiple-dose administration of 213Bi-HuCC49DeltaCH2. Doses (500 microCi) were administered at 14-day intervals after tumor implantation. A reduction in volume and/or delay in tumor growth was evident following the second and third injections of 213Bi-HuCC49DeltaCH2. As further validation of the use of 213Bi-HuCC49DeltaCH2 for RIT, a study using 131I was conducted. The overall survival of mice receiving 213Bi-HuCC49DeltaCH2 was greater than those that received 131I-HuCC49DeltaCH2.

Systematic evolution of a DNA aptamer binding to rat brain tumor microvessels. selective targeting of endothelial regulatory protein pigpen

Tumor microvessels differ in structure and metabolic function from normal vasculature, and neoangiogenesis is associated with quantitative and qualitative changes in expression of endothelial proteins. Such molecules could serve as molecular addresses differentiating the tumor vasculature from those of the normal brain. We have applied Systematic Evolution of Ligands by EXponential enrichment (SELEX) against transformed endothelial cells as a complex target to select single-stranded DNA-ligands (aptamers) that function as histological markers to detect microvessels of rat experimental glioma, a fatal brain tumor that is highly vascularized. Both the SELEX selection procedure as well as subsequent deconvolution-SELEX were analyzed by fluorescence based methods (flow cytometry and fluorescence microscopy). Of 25 aptamers analyzed, one aptamer was selected that selectively bound microvessels of rat brain glioblastoma but not the vasculature of the normal rat brain including peritumoral areas. The molecular target protein of aptamer III.1 was isolated from endothelial cells by ligand-mediated magnetic DNA affinity purification. This protein was identified by mass spectrometry as rat homologue of mouse pigpen, a not widely known endothelial protein the expression of which parallels the transition from quiescent to angiogenic phenotypes in vitro. Because neoangiogenesis, the formation of new blood vessels, is a key feature of tumor development, the presented aptamer can be used as a probe to analyze pathological angiogenesis of glioblastoma. The presented data show that pigpen is highly expressed in tumor microvessels of experimental rat brain glioblastoma and may play an important role in warranting blood supply, thus growth of brain tumors.

Active drug targeting with immunoconjugates to choroidal neovascularization

PURPOSE

Active drug targeting with monoclonal antibody to neovascular vessels may be a potential treatment for choroidal neovascularization (CNV) in age-related macular degeneration (AMD). Endoglin (CD105) is a proliferating endothelial cell marker with excellent potential for targeting. The goals of this study were to investigate the expression of CD105 in CNV membranes surgically excised from patients with AMD and CNV lesions induced by intense laser photocoagulation in a cynomolgus monkey and to evaluate the in vitro effect of immunoconjugates on endothelial cells.

METHODS

CNV membranes were surgically excised from 10 patients with AMD. Experimental CNV was induced by intense laser photocoagulation in a cynomolgus monkey. Immunolocalization of CD105 on frozen sections of CNV lesions was studied by immunohistochemical evaluation. Anti-von Willebrand's factor antibody was used as an endothelial cell marker. The cytotoxic effect of immunoconjugates of anti-CD105 monoclonal antibody and dextran binding mitomycin C on human umbilical vein endothelial cells (HUVECs) was evaluated in vitro.

RESULTS

Endothelial cells demonstrated strong immunoreactivity of CD105 in all surgically excised CNV membranes. In the monkey eye, CD105-positive cells were detected only in CNV lesions but not in normal chorioretinal tissues. Immunoconjugates with anti-CD105 monoclonal antibody showed a specific inhibitory effect on proliferating HU-VECs.

CONCLUSIONS

These results suggest that anti-CD105 monoclonal antibody-mediated drug targeting has a potential to treat CNV in AMD.

Imaging of tumour neovasculature by targeting the TGF-beta binding receptor endoglin

In vivo imaging of endothelial markers in intact tumour neovasculature would have applications in assessing the efficacy of anti-angiogenic agents in clinical trials. Although a variety of different endothelial markers have been described, few have been evaluated as imaging markers. The transforming growth factor-beta (TGF-beta) binding receptor endoglin is a proliferation-associated endothelial marker. We hypothesised that endoglin would be an ideal target for imaging since it is strongly upregulated in proliferating endothelial cells of the tumour neovasculature. We used a radiolabelled monoclonal anti-endoglin antibody and compared its neovascular binding, accumulation and in vivo behaviour to an isotype-matched control IgG(2a). Our data show that the probe binds specifically and rapidly within minutes in vivo and that correlative autoradiography and immunohistology support the in vivo imaging findings. Imaging of abundantly expressed endothelial targets circumvents delivery barriers normally associated with other tumour targeting strategies, and can potentially be used to quantitate molecular angiogenic markers.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.