Development of Hypoxia
Enhanced <sup>111</sup>In-Labeled
Bombesin Conjugates: Design, Synthesis, and In Vitro Evaluation in
PC-3 Human Prostate Cancer
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Abstract
The gastrin-releasing peptide receptor (BB2r) has shown
great promise
for tumor targeting due to the increase of the receptor expression
in a variety of human cancers including prostate, breast, small-cell
lung, and pancreatic cancer. From clinical investigations, prostate
cancer has been shown to be among the most hypoxic of the cancers
investigated. Many solid tumors contain regions of hypoxia due to
poor organization and efficiency of the vasculature. However, hypoxia
is typically not present in normal tissue. Nitroimidazoles, a thoroughly
investigated class of hypoxia selective drugs, have been shown to
be highly retained in hypoxic tissues. The purpose of this study is
to determine if the incorporation of hypoxia trapping moieties into
the structural paradigm of BB2r-targeted peptides will increase the
retention time of the agents in prostate cancer tumors. The present
work involves the design, syntheses, purification, and in vitro investigation
of hypoxia enhanced <sup>111</sup>In-BB2r-targeted radioconjugates.
A total of four BB2r-targeted conjugates (<b>1</b>–<b>4</b>) were synthesized and coupled with increasing numbers of
2-nitroimidazoles, a hypoxia trapping moiety. Conjugates were radiolabeled
with <sup>111</sup>In and purified by HPLC prior to in vitro studies.
Receptor saturation assays under both normoxic and hypoxic conditions
showed that the BB2r receptor expression on the PC-3 human prostate
cancer cell line was not significantly affected by oxygen levels.
Competitive binding assays revealed that incorporation of 2-nitroimidazoles
had a detrimental effect to BB2r binding when adequate spacer groups,
between the hypoxia trapping agent and the pharmacophore, were not
employed. All of the 2-nitroimidazole containing BB2r-targeted agents
exhibited significantly higher longitudinal retention in PC-3 cells
under hypoxic conditions compared to the analogous normoxic studies.
Protein association analysis revealed a 3-fold increase in binding
of a 2-nitroimidazole containing BB2r-targeted agent under hypoxic
relative to normoxic conditions. The positive nature of these results
indicate that further exploration into the potential of hypoxia selective
trapping agents for BB2r-targeted agents, as well as other targeted
compounds, is warranted