| dc.description.abstract |
<p>Antivascular targeting is a promising strategy for tumor therapy. This strategy
overcomes many of the transport barriers and has shown efficacy in many preclinical
models, but targeting epitopes on tumor vasculature can also promote accumulation
in healthy tissues. We used Elastin-like Polypeptide (ELP) to form block copolymers
(BCs) consisting of two separate ELP blocks seamlessly fused at the genetic level.
ELPBCs self-assemble into spherical micelles at a critical micelle temperature (CMT),
allowing external control over monovalent unimer and multivalent micelle forms. We
hypothesized that thermal self-assembly could trigger specific binding of ligand-ELPBC
to target receptors via the multivalency effect as a method to spatially restrict
high-avidity interactions. We termed this approach Dynamic Affinity Modulation (DAM).
The objectives of this study were to design, identify, and evaluate protein-based
drug carriers that specifically bind to target receptors through static or dynamic
multivalent ligand presentation.</p>
<p>ELPBCs were modified to include a low-affinity GRGDS or GNGRG ligand and a unique
conjugation site for hydrophobic compounds. This addition did not disrupt micelle
self-assembly and facilitated thermally-controlled multivalency. The ability of ligand-ELPBC
to specifically interact with isolated AvB3 or CD13 was tested using an in vitro binding
assay incorporating an engineered cell line. RGD-ELPBC promoted specific receptor
binding in response to multivalent presentation but NGR-ELPBC did not. Enhanced binding
with multivalent presentation was also observed only with constructs exhibiting CMT
< body temperature. This study establishes proof-of-principle of DAM, but ELPBC requires
thermal optimization for use with applied hyperthermia. Static affinity targeting
of fluorescent ligand-ELPBC was then analyzed in vivo using intravital microscopy
(IM), immunohistochemistry (IHC), and custom image processing algorithms. IM showed
increased accumulation of NGR-ELPBC in tumor tissue relative to normal tissue while
RGD-ELPBC and non-ligand ELPBC did not, and IHC verified these observations. This
study shows (1) multivalent NGR presentation is suitable for static multivalent targeting
of tumors and tumor vasculature, (2) multivalent RGD presentation may be suitable
for DAM with thermal optimization, and (3) ELPBC micelles may selectively target proteins
at the tumor margin.</p>
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