An active area of prostate cancer research is in the synthesis of radiolabeled peptides for in vivo tumor imaging or therapy. This method is possible since specific receptors are expressed in high concentration on certain tumor tissue. One receptor that is expressed in high concentration on prostate cancer tissue is the gastrin-releasing peptide (GRP). The amphibian peptide, bombesin (BBN), has high affinity and specificity to the GRP receptors; therefore, when BBN is radiolabeled with an appropriate radionuclide, non-invasive prostate tumor images and therapy can be obtained. The aim of this research was to produce kinetically inert bifunctional chelate complexes that would effectively contain the radionuclide Rhenium-188 (188Re), Technetium-99m (99mTc), Copper-64 (64Cu) or Bismuth-213 ( 213Bi).
My first aim was to produce kinetically inert rhenium Schiff base complexes, for the potential in developing 188Re thearapeutic agents. Rhenium complexes with tetradentate Schiff base ligands have previously yielded both rearranged ReV complexes and reduced ReIII complexes on reaction with tertiary phosphine ligands. To further understand this chemistry, the rigid diiminediphenol (N2O2) Schiff base-ligand sal2phen (N,Ñ-O-phenylenebis(salicylaldimine)) was reacted with (n-Bu4N)[ReVOCl4] to yield trans-[ReVOCl(sal2phen)]·CHCl 3 (1). On reaction with triphenylphosphine (PPh3), a rearranged ReV product cis-[ReO(PPh 3)(sal2phen*)]PF6·CHCl2 ( 2) was isolated, in which one of the imines was reduced to the amine during the reaction; a reduced ReIII product trans -[ReCl(PPh3)(sal2phen)]·CH2Cl 2 (3), and reduced ReIII product trans-[Re(PPh3)2sal2phen]) +, was isolated. Reaction of sal2phen with [ReIII Cl3(PPh3)2(MeCN)] resulted in the isolation of [ReCl2(PPh3)(sal2phen)]·2 CH2Cl2 (4). All compounds were characterized by 1H NMR, 31P NMR, FT-IR, and ESI-MS spectra. Complexes 1, 2, 3, and 4 were also characterized by single crystal X-ray crystallography.
My second aim was to produce kinetically inert technetium tricarbonyl bifunctional chelate complexes for the potential in developing molecular imaging agents. Here, I report a synthetic approach toward design of a new tridentate amine ligand for the organometallic aqua-ion [99mTc(H 2O)3(CO)3]+. The new chelating ligand framework, 2-(N,Ñ-Bis(tert-butoxycarbonyl)diethylenetriamine) acetic acid (DTMA), was synthesized from a diethylenetriamine precursor and fully characterized by mass spectrometry and nuclear magnetic resonance spectroscopy (1H and 13C). DTMA was conjugated to H 2N-(X)-BBN(7-14)NH2, where X = β-Ala, GGG, GSG and SSS, by means of solid phase peptide synthesis. [DTMA-(X)-BBN(7-14)NH2] conjugates were purified by RP-HPLC and characterized by ESI-MS. The new conjugates were radiolabeled with [99mTc(H2O)3(CO) 3]+ produced via Isolink® radiolabeling kits to produce [99mTc(CO)3-DTMA-(X)-BBN(7-14)NH 2]. Radiolabeled conjugates were purified by reversed-phase high performance chromatography. Effective receptor binding behavior was evaluated in vitro and in vivo. [99mTc(CO) 3-DTMA-(X)-BBN(7-14)NH2] conjugates displayed very high affinity for the gastrin releasing peptide receptor in vitro and in vivo.
My third aim was to produce kinetically inert copper bifunctional chelate complexes for the potential in developing molecular imaging and therapy agents. Here, I report the development of novel [NO2A-X-BBN(7-14)NH2] conjugates, where NO2A = 1,4,7-triazacyclononane-1,4-diacetic acid and X = PABA, β-Ala, 6-Ahx or 9-Anc. [NO2A-(X)-BBN(7-14)NH2] conjugates were purified by RP-HPLC and characterized by ESI-MS. The novel conjugates were radiolabeled with [64CuCl2] to produce [64Cu-NO2A-(X)-BBN(7-14)NH 2]. The NO2A chelator effectively stabilized Cu(II) under in vivo conditions. [64Cu-NO2A-(X)-BBN(7-14)NH2 ] conjugates showed affinity and specificity towards GRPr-positive tissue and had efficient localization and clearance properties. High-resolution microPET/microCT images were obtained for the [64Cu-NO2A-(X)-BBN(7-14)NH 2] conjugates (where X = PABA, 6-Ahx, 8-Aoc, and 9-Anc) in PC-3 xenografted tumors in a SCID mouse model.
My fourth aim was to produce kinetically inert bismuth bifunctional chelate complexes for the potential in developing therapy agents. Here, I report the development of novel [CHX-Á´-(8-Aoc)-BBN(7-14)NH2] conjugates, where CHX-Á´ = (N-[( R)-2-amino-3-(p-aminophenyl)propyl]-trans -(S,S)-cyclohexane-1,2-diamine-N,N,Ñ,Ñ´,Ñ´´-pentaacetic acid). Conjugates were purified by RP-HPLC and characterized by ESI-MS. The conjugate was radiolabeled with [205BiI3], to produce [205Bi-CHX-Á´-(8-Aoc)-BBN(7-14)NH2]. The pharmacokinetics of the [205Bi-CHX-Á´-(8-Aoc)-BBN(7-14)NH 2] conjugate was investigated in nude mice at 0.5, 1, 4, 8 and 24 h post-tail vein injection. Unfortunately, poor receptor-specific localization was observed.