Biodistribution of radiolabeled, monoclonal antibody (MoAb)-conjugated peptidomimetic antagonists to access status of tumor angiogenesis via selective targeting of α_vβ₃ receptors

Abstract

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Objectives: The aim was to synthesize I-125 and In-111 labeled, antibody-conjugated peptidomimetic antagonists and evaluate their targeting of integrin α_vβ₃ receptors on tumor-induced angiogenesis

Methods: A MoAb-antagonist conjugate, T101-(A-S-S[PEO]₃S-S-A-IA)n with n=10 IA was synthesized by the conjugation of S-acetylthioaceto groups to T101, an irrelevant MoAb to our tumor model and to IA, a peptidomimetic integrin α_vβ₃ antagonist, 4-[2-(3,4,5,6-tetrahydropyrimidine-2-ylamino)-ethyloxy]benzoyl-2-(S)-amino-ethylsulfonyl-amino-β-alanine (IA); generation of -SH groups by deacylation of the acetyl-S protecting group, and addition of the SH groups to a homobifunctional 1,8-bis-maleimidotriethyleneglycol (BM-[PEO]₃). The final product was labeled with either I-125 using the Iodogen method or conjugated with 2-(p-isothio-cyanatobenzyl)cyclohexyl-DTPA and labeled with In-111, and purified by size exclusion HPLC. The In-111 and I-125 products that bound specifically to α_vβ₃ receptor at a molar excess in vitro (70% bound to α_vβ₃ at 0.4 μM) were used for biodistribution in nude mice (n = 5) bearing receptor-positive M21 human melanoma xenografts. Under this binding condition, In-111 labeled IA monomer did not bind to the receptor. The mice received i.v. the In-111 product (2 μCi//0.2 μg) with or without coinjection of 240 μg IA or the I-125 product (2 μCi//0.5 μg) in 0.2 mL PBS containing 1% BSA.

Results: The tumor uptake was similar between the I-125 product and the In-111 product for a 44 hr period. In contrast, the blood radioactivity was lower, but liver and other organ uptakes were much higher for the In-111 label than for the I-125. This was reflected in the whole-body retention with 12% ID of the I-125 label retained at 44 hr vs 64% ID for the In-111 label. The coinjection of the In-111 label with cold IA (240 μg) drastically decreased the liver uptake at 21 hr (P=0.01) whereas substantially increased the In-111 in the blood (P=0.08) compared to those of the In-111 without the cold IA.

Conclusions: The rapid blood clearance, a short peak tumor uptake time, and a low peak tumor uptake value with prolonged tumor retention of this macromolecule appears to indicate that it primarily bound to α_vβ₃ receptors on angiogenic vessels, but not on tumor. It warrants further study to confirm the receptor targeting on angiogenic vessels and the effect of the cold dose.

Biodistribution of radiolabled MoAb-IA (%ID/g mean ± S.D.)