Synthesis, Characterization, and Labeling with 99mTc/188Re of Peptide Conjugates Containing a Dithia-bisphosphine Chelating Agent

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Abstract
Radiolabeling of small receptor-avid peptides at specific predetermined chelation sites with radioactive metals has been an effective approach for production of target-specific radiopharmaceuticals for diagnosis and therapy of diseases. Among various electron-donating groups found on chelator frameworks, phosphines are unique because they display versatile coordination chemistry with a wide range of transition metals. We have recently reported the utility of a dithia-bis(hydroxymethyl)phosphine-based (P2S2) bifunctional chelating agent (BFCA) containing air-stable primary phosphine groups to form 99mTc-labeled receptor-avid peptides by the preconjugation approach. Here we report a novel strategy for labeling small peptides with both 99mTc and 188Re using the P2S2-COOH (6,8-bis[3-(bis(hydroxymethyl)phosphanyl)propylsulfanyl]octanoic acid) BFCA by a postconjugation radiolabeling approach. The first step in this approach involves the coupling of the corresponding (PH2)2S2-COOH intermediate to the N-terminus of the peptide(s). Formylation of P−H bonds with aqueous formaldehyde in the presence of HCl in ethanol affords the corresponding (hydroxymethyl)phosphine−P2S2−peptide conjugates in the form of an oxidatively stable phosphonium salt. The P2S2−peptide conjugates are generated (where the PH2 groups are converted to P(CH2OH)2 groups) by treatment of the P2S2−peptide phosphonium salt(s) with 1 M sodium bicarbonate solution at pH 8.5. Complexation of BFCA conjugates with 99mTc is achieved by direct reduction with Sn(II) tartarate to yield the 99mTc−P2S2−peptide conjugate in near quantitative yields. Complexation of the BFCA conjugates with 188Re is achieved by transchelation with 188Re citrate in yields of ≥90%. In this study, (PH2)2S2-COOH BFCA was conjugated to model peptides. The glycineglycine ethyl ester (GlyGlyOEt)−(PH2)2S2-COOH BFCA conjugate was converted to the hydroxymethylene phosphine form and complexed with 99mTc to produce the 99mTcO2−P2S2−GlyGlyOEt conjugate 8 in RCPs of ≥95%. This singular 99mTc product is stable over 24 h in aqueous solution as confirmed by HPLC. Identical retention times of the 99mTcO2−P2S2−GlyGlyOEt complex and its cold rhenium analogue (ReO2−P2S2−GlyGlyOEt) on HPLC indicates similarity in structures at the macroscopic and the tracer levels. The utility of this postconjugation strategy was further demonstrated by synthesizing a P2S2−d-Lys6-LHRH conjugate and producing its corresponding 99mTc complex in RCPs of ≥88%. Finally, the P2S2−5-Ava-BBN[7−14]NH2 bombesin (BBN) analogue was synthesized, the PH2 groups converted to P(CH2OH)2 groups and subsequently labeled with 188Re to yield a 188Re-labeled bombesin analogue with a RCP of ≥90%. The biological integrity of this conjugate was demonstrated in both in vitro and in vivo. The results of this investigation demonstrate that the (PH2)2S2-COOH BFCA can be conveniently used as a precursor for labeling small receptor-avid peptides with diagnostic (99mTc) and therapeutic (188Re) radionuclides via the postconjugation approach in high yields.