Targeted technetium ( 99m Tc) and rhenium ( 188 Re) radiopharmaceuticals consist of a targeting vector that determines the specificity for the target (receptor, biochemical process) and is linked to an organic ligand that forms a complex (or chelate) with the radiometal. Our long-range goal is to design new conjugates consisting of the 99m Tc/ 188 Re chelate and the targeting vector (Figure 1) that will have exceptional stability, targeting ability and rapid elimination of unbound radioactivity. The objective of this proposal is to design, synthesize and test simple Tc/Re complexes (chelates) based on two Tc/Re """"""""cores"""""""", [M v =O] 3+ and M I (CO)3 + , M= 99m Tc, 188 Re, and complementary peptide ligand systems.
(Aims 1 and 2) The peptide ligands are chosen to provide an understanding of the factors (structure, lipophilicity, stereochemistry) that lead to 1) the formation of one major product (two, in the case of stereoisomers) upon radiolabeling, 2) the metal chelates remaining intact in in vivo and in vitro assays (high stability) and 3) elimination of the intact metal chelates in the urine via the kidneys (renal system). This understanding is key to the design of targeted 99m Tc and 188 Re radiopharmaceuticals because we hypothesize that the metal chelate has a profound effect on the stability of the radiometal and the biodistribution of the entire molecule. The chelates designed in this project can be easily conjugated to target vectors (biomolecules) by standard solid phase peptide synthesis (SPPS) techniques for further in vivo testing for specificity to the target and application as imaging or radiotherapy agents.
Aim 3, a collaborative effort with New York University School of Medicine proposes to conjugate the best chelates from Aims 1 and 2 to a 13 amino acid peptide that is the active site of the HU177 antibody. Both the peptide and the antibody bind to an antigen of collagen type IV that is exposed when collagen unravels in the vicinity of tumors as part of angiogenesis. The 99m Tc and 188 Re conjugates will be tested for in vivo tracking of the peptide and antibody and for applications in imaging and therapy.Our proposed study differs from other Tc and Re radiopharmaceutical studies because we will work with both the tracer ( 99m Tc, 188 Re, [10[ -10] -10[ -12] M]) and the macroscopic ( 99 Tc and Re [10[-2] -10[-3] M]) species. This is critical to gain a complete understanding of the structure, speciation and chemistry of the radiopharmaceutical. In this effort we employ techniques that have not been used for 99m Tc and 188 Re radiopharmaceutical studies and will provide important information on structures and chemistry. These techniques include, but are not limited to, preparative HPLC to prepare the 99 Tc macroscopic analogs, that can be related to the tracer species, and the use of Circular Dichroism and 17 O NMR Spectroscopy.
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