Oxorhenium(V) and Oxotechnetium(V) Complexes of Cysteine

Earlier attempts to obtain technetium complexes with cysteine always resulted in the formation of a product
contaminated with polymeric species. A pure product, which could be chemically characterized and adopted for
radiopharmaceutical preparation, has now been obtained by using cystine as the precursor of cysteine. This
method has been extended to prepare the corresponding rhenium chelate, isolated as the tetraphenylphosphonium
salt [Ph4P]+[{ReO(Cys)2}-{HReO(Cys)2}]·4H2O. The X-ray crystal structure of this compound revealed the
presence of both neutral and anionic chelated species. In [HReO(Cys)2], the cysteine carboxylate moiety is
unidentatedly bound to rhenium, while the carboxylic acid of the second cysteine remains as free COOH. The
coordination environment around rhenium in the anionic species [ ReO(Cys)2-] is similar, the only difference
being that the uncoordinated carboxylate moiety is present as a COO- anion. The thiolate, amine coordination
of the ligand with the metal is present in both the chelate units. The compound crystallized in an orthorhombic
system with the space group P212121, and having four formula units in each cell. The crystal data are a =
9.700(2) Å, b = 12.836(3) Å, and c = 36.228(3) Å. The rhenium chelate has been structurally correlated with
the technetium chelates through comparable spectroscopic and chromatographic data. The technetium-99m analogue
of this rhenium chelate exhibited renal tubular transport and renal retention, which makes this radiopharmaceutical
useful for evaluation of the clinical status of renal patients.