New rhenium complexes with ciprofloxacin as useful models for understanding the properties of [^99mTc]-ciprofloxacin radiopharmaceutical

Rhenium complexes with the antibiotic ciprofloxacin have been prepared to be studied as models of technetium radiopharmaceuticals. With this aim, the new rhenium complexes 1 {[ReO(Cpf)2]Cl}, 2 {[ReO(CpfH) 2]Cl3} and 3 {fac-[Re(CO)3(H 2O)(Cpf)]} with ciprofloxacin (CpfH = ciprofloxacin; Cpf = conjugated base of ciprofloxacin) have been synthesised and characterised by elemental analyses, IR, NMR (1H, 19F and 13C CP-MAS) spectroscopy, as well as MS measurements. All spectroscopic data are consistent with the coordination of ciprofloxacin in all these complexes through the carbonyl and the carboxylate oxygen atoms with the formation of a six member chelate ring. The study of a Tc-ciprofloxacin solution by ESI-MS reveals the presence of [TcO(Cpf)2]+ cations, which agrees with the hypothesis that complexes 1 and 2 can be seen as model rhenium complexes of this radiopharmaceutical. Antimicrobial and DNA gyrase inhibition studies performed with complexes 2 and 3 have shown a very similar behaviour between complex 2 and the free antibiotic, whereas complex 3 exhibit a lower antimicrobial activity. Based on a joint analysis of the data reported in the literature and the chemical and biological results obtained in this study, a tentative proposal to explain some aspects of the behaviour of Tc-ciprofloxacin radiopharmaceutical has been made. © 2014 Elsevier Ltd. All rights reserved.