Hege Karlsen

Radiosynthesis and Biodistribution of Cyclic RGD Peptides Conjugated with Novel [18F]Fluorinated Aldehyde-Containing Prosthetic Groups

Achieving high-yielding, robust, and reproducible chemistry is a prerequisite for the (18)F-labeling of peptides for quantitative receptor imaging using positron emission tomography (PET). In this study, we extend the toolbox of oxime chemistry to include the novel prosthetic groups [(18)F]-(2-{2-[2-(2-fluoroethoxy)ethoxy]ethoxy}ethoxy)acetaldehyde, [(18)F]5, and [(18)F]-4-(3-fluoropropoxy)benzaldehyde, [(18)F]9, in addition to the widely used 4-[(18)F]fluorobenzaldehyde, [(18)F]12. The three (18)F-aldehydes were conjugated to the same aminooxy-bearing RGD peptide and the effect of the prosthetic group on biodistribution and tumor uptake studied in mice. The peptide conjugate [(18)F]7 was found to possess superior in vivo pharmacokinetics with higher tumor to blood, tumor to liver, tumor to muscle, and tumor to lung ratios than either [(18)F]10 or [(18)F]13. The radioactivity from the [(18)F]7 conjugate excreted more extensively through the kidney route with 79%id passing through the urine and bladder at the 2 h time point compared to around 55%id for the more hydrophobic conjugates [(18)F]10 and [(18)F]13. The chemical nature of a prosthetic group can be employed to tailor the overall biodistribution profile of the radiotracer. In this example, the hydrophilic nature of the ethylene glycol containing prosthetic group [(18)F]5 clearly influences the overall excretion pattern for the RGD peptide conjugate.

A Novel Prosthetic Group for Site-Selective Labeling of Peptides for Positron Emission Tomography

Efficient methodologies for the radiolabeling of peptides with [(18)F]fluoride are a prerequisite to enabling commercialization of peptide-containing radiotracers for positron emission tomography (PET) imaging. It was the purpose of this study to investigate a novel chemoselective ligation reaction comprising conjugation of an [(18)F]-N-methylaminooxy-containing prosthetic group to a functionalized peptide. Twelve derivatives of general formula R1-CO-NH-Lys-Gly-Phe-Gly-Lys-OH were synthesized where R1 was selected from a short list of moieties anticipated to be reactive toward the N-methylaminooxy group. Conjugation reactions were initially carried out with nonradioactive precursors to assess, in a qualitative manner, their general suitability for PET chemistry with only the most promising pairings progressing to full radiochemical assessment. Best results were obtained for the ligation of O-[2-(2-[(18)F]fluoroethoxy)ethyl]-N-methyl-N-hydroxylamine 18 to the maleimidopropionyl-Lys-Gly-Phe-Gly-Lys-OH precursor 10 in acetate buffer (pH 5) after 1 h at 70 degrees C. The non-decay-corrected isolated yield was calculated to be 8.5%. The most encouraging result was observed with the combination 18 and 4-(2-nitrovinyl)benzoyl-Lys-Gly-Phe-Gly-Lys-OH, 9, where the conjugation reaction proceeded rapidly to completion at 30 degrees C after only 5 min. The corresponding non-decay-corrected radiochemical yield for the isolated (18)F-labeled product 27 was 12%. The preliminary results from this study demonstrate the considerable potential of this novel strategy for the radiolabeling of peptides.

The analogy between CO and C(CN)2. Part 2. Structural properties of N,N-dialkylaminobenzamides and the analogously substituted 2-(phenylmethylene)propanedinitriles

The barrier to rotation around the carbon amino nitrogen bond in 2-[N,N-dimethylamino(phenyl)methylene]propanedinitrile (4a), 2-[N,N-diethylamino(phenyl)methylene]propanedinitrile (4b), 2-[(phenyl)pyrrolidin-1-ylmethylene]propanedinitrile (4c), N,N-dimethylbenzamide (5a), N,N-diethylbenzamide (5b) and (phenyl)pyrrolidin-1-ylmethanone (5c) were determined using dynamic 1H NMR spectroscopy. X-Ray crystal structures of 4a, 4b, 4c and 5a were determined, and quantum chemical calculations were carried out for 4a and 5a and for the transition structures for the rotation process of these two compounds. The barriers were generally lower than for the vinyl analogues 1 and 2. Although the calculated transition structures for 4a and 5a indicate some steric strain, the reduction of the barriers, especially for series 4, indicates that steric strain is more important in the ground state, thus raising its energy relatively more than for the transition state.