Drug Targeting Organ-Specific Strategies

246 9 Tumour Vasculature Targeting
by endothelial cells. Genetic deletion of TGF-β1 in mice leads to embryonic lethality with a
significant defect in developmental angiogenesis [84].
Endoglin is over-expressed in the vasculature of tumours and other tissues undergoing
vascular remodelling [85]. Differential upregulation of endoglin presents an opportunity to
target cytotoxic molecules to endothelial cells. Several monoclonal antibodies specific for human
endoglin have been produced [86]. Some of the monoclonal antibodies generated
against human endoglin were also found to cross react with mouse endothelial cells. In spite
of low binding, monoclonal antibodies (SN6f and SN6j) were readily internalized by mouse
endothelial cells. Using such a cross-reactive antibody, Seon et al. chemically linked ricin A
chain or deglycosylated ricin A chain (the catalytic subunit of the plant toxin ricin) [86]. Ricin
A chain is a N-glycosidase which specifically cleaves a single adenine residue from the 28-S
ribosomal RNA. Depurination irreversibly impairs the function of ribosomes and thereby inhibits
protein synthesis. Conjugates of endoglin-specific antibody and ricin A chain showed
specific cytotoxicity against endothelial cells in vitro and inhibited experimental angiogenesis
in vivo. The anti-angiogenic properties of anti-endoglin–ricin A chain conjugate were
demonstrated in a dorsal air sac model system. Most importantly, endoglin-specific immunotoxin
showed strong anti-tumour activity in a SCID mouse tumour model. In these studies,
MCF-7, a human breast cancer cell line was transplanted into mice and then treated with endoglin-specific
immunotoxin. These studies showed complete inhibition of tumour growth in
all of the treated mice without any apparent toxicity to normal tissues [87].Apart from intracellular
targeting, endoglin-specific antibodies were also successfully used to localize radionuclide
on the cell surface. Radioiodinated monoclonal antibodies (10 µCi) given to tumour-bearing
animals significantly inhibited tumour growth, indicating the clinical potential
of targeting endoglin [88].
9.3.3 Targeting Integrins to Tumour Vasculature
Interaction between cell surface-anchored integrins and extracellular matrix components
constitutes an additional pathway necessary for angiogenesis control. In fact, studies have
identified two cytokine-mediated, integrin-dependent angiogenic pathways. One of these
pathways is associated with α Vβ 3 integrin, which selectively influences FGF-2 mediated angiogenic
signals [28]. A second, non-overlapping pathway is represented by cross-talk between
α Vβ 5 integrin and PKC-dependent, VEGF- or TNFα-induced, signalling [89]. Tumour
angiogenesis can therefore be inhibited by blocking the interaction between integrins and
the RGD motif-containing extracellular matrix proteins. Furthermore, the integrins present
on tumour endothelium can serve as target epitopes via which toxic compounds can be delivered
to the endothelial cells of the tumour.
Erkki Ruoslahti and colleagues [90] developed a novel targeting strategy by using
polypeptides capable of delivering cytotoxic drugs to integrins. An in vivo selection of phage
display libraries identified peptides that specifically home to components of tumour blood
vessels (see Chapter 10 for details relating to this technology of ligand/target finding). Ruoslahti’s
research group identified two major classes of peptides, one containing the RGD
motif and the other containing an NGR motif. These polypeptides were then chemically
linked to the anti-cancer drug doxorubicin.Treatment of breast carcinoma-bearing mice with