^99mTc-labeling of peptidomimetic antagonist to selectively target α_vβ₃ receptor-positive tumor: Comparison of PDA and EEDA as co-ligands

Objectives: The aim of this research was to synthesize radiolabeled peptidomimetic integrin α_vβ₃ antagonist with ^99mTc for rapid targeting of integrin α_vβ₃ receptors in tumor to produce a high tumor to background ratio. Methods: The amino terminus of 4-[2-(3,4,5,6-tetra-hydropyrimidin-2-ylamino)-ethyloxy]benzoyl-2-(S)-[N-(3-amino--neopenta-1-carbamyl)]-aminoethylsulfonyl-amino-β-alanine hydrochloride (IAC) was conjugated with N-hydroxysuccinimide ester of HYNIC and labeled with ^99mTc using tricine with either 1,5-pyridinedicarboxylic acid (PDA) or ethylenediamine-N,N′-diacetic acid (EDDA) as the co-ligand. The products, ^99mTcEDDA2/HYNIC-IAC (P1) and ^99mTc PDA (tricin)/HYNIC-IAC (P2) were subjected to in vitro serum stability, receptor-binding, biodistribution and imaging studies. Results: P1 and P2 were synthesized with an overall yield of >80%. P1 was slightly more stable than P2 when incubated in serum at 37 oC for 18 hrs (84 vs 77% intact). The In vitro receptor-binding of P1 was higher than that of P2 (78.02 ±13.48 vs 51.05 ± 14.05%) when incubated with α_vβ₃ at a molar excess (0.8 μM). This receptor binding was completely blocked by a molar excess of an unlabeled peptidomimetic antagonist. Their differences shown in serum stability and the receptor-binding appeared to be related to their biological behaviors in tumor uptake and retention; the 1 h tumor uptakes of P1 and P2 were 3.17±0.52 and 2.13±0.17 % ID/g, respectively. P1 was retained in the tumor longer than P2. P1 was excreted primarily through the renal system whereas P2 complex was excreted equally via both renal and hepatobiliary systems. Thus, P1 was retained in the whole-body with 27.25 ± 3.67% ID at 4 h whereas 54.04 ± 3.57% ID of P2 remained in the whole-body at 4 h. This higher whole-body retention of P2 appeared to be resulted from a higher amount of radioactivity retained in liver and intestine. These findings were supported by imaging studies showing higher tumor-toabdominal contrast for P1 than for P2 at 3 h postinjection. Conclusions: P1 showed good tumor targeting properties with a rapid tumor uptake, prolonged tumor retention and fast whole-body clearance kinetics. These findings warrant further investigation of the HYNIC method of ^99mTc labeling of other peptidomimetic antagonists using EDDA as a coligand.