[en] Radioisotopes of Pb(II) have been of some interest in radioimmunotherapy and radioimmunoimaging (RII). However, the absence of a kinetically stable bifunctional chelating agent for Pb(II) has hampered its use for these applications. ²⁰³Pb (T_1/2=52.02 h) has application potential in RII, with a γ-emission that is ideal for single photon emission computerized tomography, whereas ²¹²Pb (T_1/2=10 h) is a source of highly cytotoxic α-particles via its decay to its ²¹²Bi (T_1/2=60 min) daughter. The synthesis of the novel bifunctional chelating agent 2-(4-isothiocyanotobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra-(2-carbamoyl methyl)-cyclododecane (4-NCS-Bz-TCMC) is reported herein. The Pb[TCMC]²⁺ complex was less labile to metal ion release than Pb[DOTA]^2- at pH 3.5 and below in isotopic exchange experiments. In addition to increased stability to Pb²⁺ ion release at low pH, the bifunctional TCMC ligand was found to have many other advantages over the bifunctional 1,4,7,10-tetraazacyclodocane-1,4,7,10-tetraacetic acid (DOTA) ligand. These include a shorter and more straightforward synthetic route, a more efficient conjugation reaction to a monoclonal antibody (mAb), with a higher chelate to protein ratio, a higher percent immuroreactivity, and a more efficient radiolabeling reaction of the mAb-ligand conjugate with ²⁰³Pb

[en] The studies reported herein present the first in vitro and in vivo comparison of radioimmunoconjugates (RIC) radiolabeled with ¹⁷⁷Lu using the acyclic CHX-A''-DTPA ligand and the macrocyclic ligands, C-DOTA and PA-DOTA. The in vivo studies include pharmacokinetics and biodistribution of the formed ¹⁷⁷Lu-labeled immunoconjugates in a tumor bearing murine model with engineered monoclonal antibody HuCC49ΔCH2. The in vitro analysis indicated that the CHX-A'' RIC was superior with respect to immunoreactivity, radiolabeling with ¹⁷⁷Lu, and specific activity. The in vivo pharmacokinetic data by itself indicated that the Lu(III)-PA-DOTA complex may not be as stable as Lu(III) complexes with CHX-A'' or C-DOTA. All three RICs demonstrated tumor targeting of human colon carcinoma xenografts in athymic mice. In these biodistribution studies, there appears to be no overall pattern or trend of one RIC over the other two. Based on these in vitro and in vivo studies, the CHX-A'' DTPA ligand should be considered a suitable bifunctional chelate for the radiolabeling of monoclonal antibodies with ¹⁷⁷Lu for radioimmunotherapy applications

[en] To develop targeting molecules to be used for vascular targeting of short half-lived α-emitters for radioimmunotherapy, linear peptide phage display libraries were selected in vivo for binding to IC-12 rat tracheal tumors growing in severe combined immune deficient mice. After three rounds of selection, 15 phage clones were analyzed for DNA sequence, and the deduced translation products of cDNA inserts were compared. Three consensus sequences were chosen from three separate experimental selection series and peptides of these sequences with added -gly-gly-tyr were obtained. Peptides were radiolabeled on tyrosine with ¹²⁵I and the biodistribution in tumor-bearing mice was determined. The radioiodinated peptides were stable in vitro and when injected in tumor-bearing mice ∼3.0 %ID/g accumulated in the tumor; however, much of the ¹²⁵I was found in the gastrointestinal tract and thyroid, indicative of dehalogenation of the labeled peptide. Radiolabeling peptide 2 with N-succinimidyl-3-¹²⁵I-iodobenzoate resulted in faster excretion, which in turn resulted in lower levels in tumor and other organs, especially thyroid and gastrointestinal tract. Peptide 2 was derivatized with the bifunctional isothiocyanates of cyclohexyl-B diethylenetriaminepentaacetic acid (DTPA) or CHX-A'' DTPA by direct conjugation or with a hydroxylamine derivative of 1B4M-DTPA (2-(p-[O-(carboxamylmethyl)hydroxylamine]benzyl)-6 -methyl-diethylenetriamine-N,N,N',N'',N''-pentaacetic acid ) coupled at the N-terminus. The primary molecular species in the conjugated products were shown by mass spectrometry to have one DTPA per peptide. Peptide chelate conjugates were radiolabeled with ²¹³Bi and the products tested for biodistribution in tumor-bearing mice. The data show that chelation of ²¹³Bi to peptides was accomplished by both the direct method of DTPA attachment and by the method using the linker at the N-terminus. Only small amounts of peptide accumulated at tumor sites. We conclude that phage display is a powerful tool to select peptides with restricted binding specificity; however, the peptides isolated to date do not bind with high retention to tumor sites in vivo