Efficient ¹¹C‑Carbonylation of Isolated Aryl Palladium Complexes for PET: Application to Challenging Radiopharmaceutical Synthesis

We describe the successful implementation of palladium-aryl oxidative addition complexes as stoichiometric reagents in carbonylation reactions with ¹¹CO to produce structurally challenging, pharmaceutically relevant compounds. This method enables the first ¹¹C-carbonyl labeling of an approved PET tracer, [¹¹C]­raclopride, for the dopamine D2/D3 receptor by carbonylation with excellent radiochemical purity and yield. Two other molecules, [¹¹C]­olaparib and [¹¹C]­JNJ 31020028, were efficiently labeled in this manner. The technique distinguishes itself from existing methods by the markedly improved purity profiles of the tracer molecules produced and provides access to complex structures in synthetically useful yields, hereby offering a viable alternative to other ¹¹C-labeling strategies

Biodistribution in male BALB/c nu/nu mice bearing PC-3 xenografts.

<p><b>A</b>. Biodistribution of [¹⁸F]AlF-NOTA-P2-RM26 in male BALB/c nu/nu mice bearing PC-3 xenografts at different time points (total injected mass of 45 pmol). <b>B</b>. Tumor to normal organ ratios after injection of 45 pmol of [¹⁸F]AlF-NOTA-P2-RM26 in male BALB/c nu/nu mice bearing PC-3 xenografts. The data are presented as the mean percentage of the injected dose per gram of tissue (%ID/g ± SD, n=4). </p

UV-radio-HPLC and SDS-PAGE analysis.

<p><b>A</b>. UV-radio-HPLC analyses of [¹⁸F]AlF-NOTA-P2-RM26. Upper panel: UV profile with signals corresponding to the original conjugate, NOTA-P2-RM26 (6.5 min), and the product, [¹⁸F]AlF-NOTA-P2-RM26 (6.8 min). The recording wavelength was 220 nm. The analyte was spiked with the original conjugate in order to confirm the identity. Lower panel: Radioactivity profile with the principle signal (>98%) corresponding to the product, [¹⁸F]AlF-NOTA-P2-RM26. <b>B</b>. SDS-PAGE analysis of [¹⁸F]AlF-NOTA-P2-RM26 after a 1 h incubation in murine serum at 37°C (1). Radiolabeled sample (2). Na¹⁸F-sodium fluoride was used as low molecular weight radioactivity marker on the same gel.</p

PET image.

<p>Imaging of GRPR expression in PC-3 xenografts in a BALB/c nu/nu male mouse. The animals was injected with 45 pmol of [¹⁸F]AlF-NOTA-P2-RM26 (~2 MBq). After 3 h, the animal was euthanized, the urinary bladder was dissected and the PET/CT images were acquired.</p

<i>In vitro</i> binding specificity, inhibition efficiency and cellular processing.

<p><b>A</b>. <i>In </i><i>vitro</i> binding specificity of [¹⁸F]AlF-NOTA-P2-RM26 to GRPRs was tested on PC-3 cells. Blocked dishes were pretreated with a 1000-fold excess of non-labeled peptide 5 min prior to the addition of 1 nM labeled compound. The cell-associated radioactivity is presented as a percentage of the total added radioactivity. <b>B</b>. Inhibition of ¹²⁵I-Tyr⁴-BBN binding to PC3 cells with ^natGa-NOTA-P2-RM26 or Al^natF-NOTA-P2-RM26. <b>C</b>. Cell-associated (internalized, membrane and total) radioactivity as a function of time after continuous incubation of PC-3 cells with [¹⁸F]AlF-NOTA-P2-RM26. Data are mean values ± SD of 3 culture dishes. Not all error bars are visible due to the small standard deviations.</p

Structural formula of NOTA-PEG₂-[D-Phe⁶,Sta¹³,Leu¹⁴]bombesin[6-14] (NOTA-P2-RM26).

<p>Structural formula of NOTA-PEG₂-[D-Phe⁶,Sta¹³,Leu¹⁴]bombesin[<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081932#B6" target="_blank">6</a>-<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0081932#B14" target="_blank">14</a>] (NOTA-P2-RM26).</p

<i>In vivo</i> binding specificity in male Balb/c nu/nu mice bearing PC-3 xenografts.

<p>Biodistribution of [¹⁸F]AlF-NOTA-P2-RM26 in male Balb/c nu/nu mice bearing PC-3 xenografts 1 h p.i. The total injected mass of radiolabeled conjugate was 45 pmol, and the animals in the blocked group were co-injected with 20 nmol of the non-labeled peptide. The data are presented as the mean percentage of the injected dose per gram of tissue (%ID/g ± SD, n=4). The red asterisks denote significant differences between the groups injected with 45 pmol (non-blocked) and 20 nmol (blocked) (<i>p</i><0.05).</p

¹¹C and ¹⁸F Radiolabeling of Tetra- and Pentathiophenes as PET-Ligands for Amyloid Protein Aggregates

Three oligothiophenes were evaluated as PET ligands for the study of local and systemic amyloidosis <i>ex vivo</i> using tissue from patients with amyloid deposits and <i>in vivo</i> using healthy animals and PET-CT. The <i>ex vivo</i> binding studies revealed that all three labeled compounds bound specifically to human amyloid deposits. Specific binding was found in the heart, kidney, liver, and spleen. To verify the specificity of the oligothiophenes toward amyloid deposits, tissue sections with amyloid pathology were stained using the fluorescence exhibited by the compounds and evaluated with multiphoton microscopy. Furthermore, a <i>in vivo</i> monkey PET-CT study showed very low uptake in the brain, pancreas, and heart of the healthy animal indicating low nonspecific binding to healthy tissue. The biological evaluations indicated that this is a promising group of compounds for the visualization of systemic and localized amyloidosis