Synthesis and NMR Spectroscopy of Tripeptide Derived Biomolecules for Site Specific Radiopharmaceuticals

Abstract

<p>This thesis describes a bifunctional approach to the development of new, site-specific radioimaging agents. The approach taken involved covalently linking a chelate, which binds strongly to ⁹⁹ᵐTc (the most commonly used radionuclide in diagnostic medicine), to a drug, usually through a spacer chain. The biological molecule, which has a well defined receptor site in the body, is then expected to guide the labelled chelate to that receptor. The initial stages of the work entailed improving the synthesis of the diamidodithiol (DADT) chelate. This chelant is of particular interest because its technetium complex is known to be stable in vivo. Tripeptide analogues of the DADT chelate were then synthesised by the use of standard peptide coupling chemistry. The peptides were of the type mercaptoacetic acid-X-L-cysteine, where X can be any amino acid. Several variations on X (Gly, Phe, His, Ile, Ser, Met, Tyr) were synthesised to provide a series of chelates with varying solubility, coordination chemistry and sites of derivatization. The rhenium complexes of two peptides, Tr-S-Mer-L-His-S-Bn-L-Cys-OMe and Tr-S-Mer-L-Ile-S-Bn-L-Cys-OMe were prepared and characterized by NMR and the former by X-ray crystallography and NMR spectrocscopy. The reaction of rhenium with the histidine containing peptide resulted in the formation of two diastereomers while the rhenium complex of the second chelnt formed only one isomer because of the steric hindrance from the isoleucine side chain. The second phase of this work involved coupling the chelants to biomolecules. The DADT chelant was coupled to tamoxifen, a drug used in the treatment of hormone-dependent breast cancer. A total synthesis was required to conjugate the chelant to an appropriate site on tamoxifen. The desired chelate-tamoxifen species was prepared in 18 steps in 7% overall yield. The 4-hydroxy analogue of tamoxifen, with a linker arm for conjugation to a chelate was also prepared. Further development towards cancer imaging agents was accomplished by conjugating one of the tripeptide chelants, Tr-S-Mer-L-Ser-S-Bn-L-Cys, to the alkylating agen chlorambucil. The product of the 10 step synthesis (30% overll yield) was characterized by several high resolution NMR techniques.</p>