Next Generation of SiFAlin-Based TATE Derivatives for PET Imaging of SSTR-Positive Tumors: Influence of Molecular Design on In Vitro SSTR Binding and In Vivo Pharmacokinetics

Abstract

The Silicon-Fluoride-Acceptor (SiFA)-(18)F-labeling strategy has been shown before to enable the straightforward and efficient (18)F-labeling of complex biologically active substances such as proteins and peptides. Especially in the case of peptides, the radiolabeling proceeds kit-like in short reaction times and without the need of complex product workup. SiFA-derivatized, (18)F-labeled Tyr(3)-octreotate (TATE) derivatives demonstrated, besides strong somatostatin receptor (SSTR) binding, favorable in vivo pharmacokinetics as well as excellent tumor visualization by PET imaging. In this study, we intended to determine the influence of the underlying molecular design and used molecular scaffolds of SiFAlin-TATE derivatives on SSTR binding as well as on the in vivo pharmacokinetics of the resulting (18)F-labeled peptides. For this purpose, new SiFAlin-(Asp)n-PEG1-TATE analogs (where n = 1-4) were synthesized, efficiently radiolabeled with (18)F in a kit-like manner and obtained in radiochemical yields of 70-80%, radiochemical purities of ≥97%, and nonoptimized specific activities of 20.1-45.2 GBq/μmol within 20-25 min starting from 0.7-1.5 GBq of (18)F. In the following, the radiotracer's lipophilicities and stabilities in human serum were determined. Furthermore, the SSTR-specific binding affinities were evaluated by a competitive displacement assay on SSTR-positive AR42J cells. The obtained in vitro results support the assumption that aspartic acids are able to considerably increase the radiotracer's hydrophilicity and that their number does not affect the SSTR binding potential of the TATE derivatives. The most promising tracer (18)F-SiFAlin-Asp3-PEG1-TATE [(18)F]6 (LogD = -1.23 ± 0.03, IC50 = 20.7 ± 2.5 nM) was further evaluated in vivo in AR42J tumor-bearing nude mice via PET/CT imaging against the clinical gold standard (68)Ga-DOTATATE as well as the previously developed SiFAlin-TATE derivative [(18)F]3. The results of these evaluations showed that [(18)F]6-although showing very similar chemical and in vitro properties to [(18)F]3-exhibits not only a slowed renal clearance compared to [(18)F]3, but also a higher absolute tumor uptake compared to (68)Ga-DOTATATE, and furthermore enables excellent tumor visualization with high image resolution. These results emphasize the importance of systematic study of the influence of molecular design and applied structure elements of peptidic radiotracers, as these may considerably influence in vivo pharmacokinetics while not affecting other parameters such as radiochemistry, lipophilicity, serum stability, or receptor binding potential.

Design, Synthesis, In Vitro, and Initial In Vivo Evaluation of Heterobivalent Peptidic Ligands Targeting Both NPY(Y1)- and GRP-Receptors—An Improvement for Breast Cancer Imaging?

“…Both peptides were synthesized by standard solid phase peptide synthesis (SPPS) methods [ 13 , 21 ] by successive conjugation of the respective N α -Fmoc-amino acids after HBTU activation to the respective rink amide resin and finally modified on resin with bis -Boc-aminooxy acetic acid, giving aminooxy-PESIN ( 1 ) and [Lys 4 (aminooxy),Trp 5 ,Nle 7 ]BVD 15 ( 2 ) ( Figure 3 ). In case of PESIN, the aminooxy functionality was introduced at the N -terminal end as the peptide can be modified in this position without considerable alterations in binding affinity.…”

Molecular Imaging of Hydrolytic Enzymes Using PET and SPECT

“…Just as the development of [ 11 C]CO 2 fixation technologies 56 , 193 , 194 led to a surge in development of imaging agents for both HDAC and FAAH, other new radiosynthetic technologies are likely to drive the development of new tracers for imaging a wide array of hydrolytic enzymes. Exciting and convenient (“kit-like”) new approaches have been recently reported that could easily drive the future development of tracers for imaging hydrolytic enzymes, such as reactions with uncommon nuclei 195 including [ 18 F]trifluoroborates, 196 - 198 [ 18 F]silicon fluoride, 199 , 200 [ 18 F]sulfonyl fluoride-containing prosthetic groups, 201 and chelation of Al[ 18 F]F. 202 The major drawback to many of these techniques as they currently exist is that they require attachment of prosthetic groups or chelators to facilitate radiolabeling, which are chemical moieties that add steric bulk and can change the polarity and biodistribution of the attached drug. Another promising area of future development are new reactions for rapidly forming aryl carbon–[ 18 F]fluorine bonds.…”

Functionalizable composite nanoparticles as a dual magnetic resonance imaging/computed tomography contrast agent for medical imaging

“…Cys‐TATE (0.1 mmol) was synthesized on a solid support according to a published standard Fmoc‐based solid phase peptide synthesis approach on an Fmoc‐Thr(tBu)‐Wang resin as described elsewhere . For the chemical structure of the peptide and a schematic description of the coupling process, please see the Supporting Information S0.…”