Antonia A. Högnäsbacka

Background: Isotopic exchange with fluorine-18 offers a conceptually simple approach to positron emission tomography (PET) tracer development, as the target compound itself serves as the labeling precursor. Results: In this study, we investigated the influence of functional groups and substituent position on the efficiency of isotopic exchange with fluorine-18. A total of 19 fluorobenzene derivatives were screened under standardized conditions, in dimethyl sulfoxide (175 °C) and dimethylformamide (150 °C) as solvents. Only a subset of compounds underwent efficient exchange; electron-withdrawing substituents strongly promoted isotopic exchange, with ortho and para isomers generally being more reactive than meta-analogues. The kinetics of the exchange in dimethyl sulfoxide were quantified for fluorobenzonitriles, nitrofluorobenzenes, and fluorobenzaldehydes. Nonlinear regression analysis indicated more rapid apparent exchange for the ortho- and para-substituted nitrile and nitro derivatives, together with higher apparent plateaus (≥84–94%), consistent with an S _N Ar-like mechanism involving stabilization of the Meisenheimer intermediate. By contrast, meta isomers were significantly less reactive, and no exchange was detected for 3-fluorobenzaldehyde. Fluorobenzaldehydes also showed lower overall apparent plateaus and signs of thermal instability in dimethyl sulfoxide. Full-scale labeling confirmed the feasibility of high molar activity, with isolated 2-[ ¹⁸ F]fluorobenzonitrile obtained at 121 ± 37 GBq/µmol. Although radiochemical yield (5.84±2.23%, decay-corrected to the addition of tetraethylammonium [ ¹⁸ F]fluoride) and radiochemical purity (88.6 ± 3.9%) remained modest, these results demonstrate that isotopic exchange can provide molar activities within the range required for PET radiopharmaceuticals. Conclusions: These findings clarify the functional group and positional influences on fluorine-19/fluorine-18 isotopic exchange, highlighting the potential of this strategy to streamline PET tracer development by circumventing the need for precursor development and extensive reaction optimization.

Background: Mutations in the epidermal growth factor receptor (EGFR) kinase domain are common in non-small cell lung cancer. Conventional tyrosine kinase inhibitors target the mutation site in the ATP binding pocket, thereby inhibiting the receptor's function. However, subsequent treatment resistance mutations in the ATP binding site are common. The EGFR allosteric inhibitor, EAI045, is proposed to have an alternative mechanism of action, disrupting receptor signaling independent of the ATP-binding site. The antibody cetuximab is hypothesized to increase the number of accessible allosteric pockets for EAI045, thus increasing the potency of the inhibitor. This work aimed to gain further knowledge on pharmacokinetics, the EGFR mutation-targeting potential, and the influence of cetuximab on the uptake by radiolabeling EAI045 with carbon-11 and tritium. Results: 2-(5-fluoro-2-hydroxyphenyl)-2-((2-iodobenzyl)amino)-N-(thiazol-2-yl)acetamide and 2-(5-fluoro-2-hydroxyphenyl)-N-(5-iodothiazol-2-yl)-2-(1-oxoisoindolin-2-yl)acetamide were synthesized as precursors for the carbon-11 and tritium labeling of EAI045, respectively. [¹¹C]EAI045 was synthesized using [¹¹C]CO in a palladium-catalyzed ring closure in a 10 ± 1% radiochemical yield (decay corrected to end of [¹¹C]CO₂ production), > 97% radiochemical purity and 26 ± 1 GBq/µmol molar activity (determined at end of synthesis) in 51 min. [³H]EAI045 was synthesized by a tritium-halogen exchange in a 0.2% radiochemical yield, 98% radiochemical purity, and 763 kBq/nmol molar activity. The ability of [¹¹C]EAI045 to differentiate between L858R/T790M mutated EGFR expressing H1975 xenografts and wild-type EGFR expressing A549 xenografts was evaluated in female nu/nu mice. The uptake was statistically significantly higher in H1975 xenografts compared to A549 xenografts (0.45 ± 0.07%ID/g vs. 0.31 ± 0.10%ID/g, P = 0.0166). The synergy in inhibition between EAI045 and cetuximab was evaluated in vivo and in vitro. While there was some indication that cetuximab influenced the uptake of [³H]EAI045 in vitro, this could not be confirmed in vivo when tumor-bearing mice were administered cetuximab (0.5 mg), 24 h prior to injection of [¹¹C]EAI045. Conclusions: EAI045 was successfully labeled with tritium and carbon-11, and the in vivo results indicated [¹¹C]EAI045 may be able to distinguish between mutated and non-mutated EGFR in non-small cell lung cancer mouse models. Cetuximab was hypothesized to increase EAI045 uptake; however, no significant effect was observed on the uptake of [¹¹C]EAI045 in vivo or [³H]EAI045 in vitro in H1975 xenografts and cells.

AZD4747 is a KRAS^G12C inhibitor recently shown to cross the non‐human primate blood‐brain barrier efficiently. In the current study, a GMP‐compliant production of [¹¹C]AZD4747 was developed to enable PET studies in human subjects. The validated procedure afforded [¹¹C]AZD4747 as an injectable solution in good radioactivity yield (1656 ± 532 MBq), excellent radiochemical purity (100%), and a molar activity of 77 ± 13 GBq/μmol at the end of the synthesis, which took 46 ± 1 min from the end of the bombardment. Quality control on the final product was performed satisfactorily and met all acceptance criteria.

Brigatinib, a tyrosine kinase inhibitor (TKI) with specificity for gene rearranged anaplastic lymphoma kinase (ALK), such as the EML4-ALK, has shown a potential to inhibit mutated epidermal growth factor receptor (EGFR). In this study, N-desmethyl brigatinib was successfully synthesized as a precursor in five steps. Radiolabeling with [¹¹C]methyl iodide produced [methylpiperazine-¹¹C]brigatinib in a 10 ± 2% radiochemical yield, 91 ± 17 GBq/μmol molar activity, and ≥95% radiochemical purity in 49 ± 4 min. [Methylpiperazine-¹¹C]brigatinib was evaluated in non-small cell lung cancer xenografted female nu/nu mice. An hour post-injection (p.i.), 87% of the total radioactivity in plasma originated from intact [methylpiperazine-¹¹C]brigatinib. Significant differences in tumor uptake were observed between the endogenously EML4-ALK mutated H2228 and the control xenograft A549. The tumor-to-blood ratio in H2228 xenografts could be reduced by pretreatment with ALK inhibitor crizotinib. Tracer uptake in EGFR Del19 mutated HCC827 and EML4-ALK fusion A549 was not significantly different from uptake in A549 xenografts.

The PET tracer [¹⁸F]F‐AraG, an arabinosyl guanine analog, has shown promise for visualizing activated T cells in multiple diseases. Herein, a practitioner's protocol is described, in which the PET tracer is prepared using minimal equipment and manual actions, making it widely accessible for preclinical applications.

Multiple small molecule PET tracers have been developed for the imaging of the epidermal growth factor receptor (EGFR). These tracers target the tyrosine kinase (TK) domain of the receptor and have been used for both quantifying EGFR expression and to differentiate between EGFR mutational statuses. However, the approaches for in vivo evaluation of these tracers are diverse and have resulted in data that are hard to compare. In this review, we analyze the historical development of the in vivo evaluation approaches, starting from the first EGFR TK PET tracer [¹¹C]PD153035 to tracers developed based on TK inhibitors used for the clinical treatment of mutated EGFR expressing non-small cell lung cancer like [¹¹C]erlotinib and [¹⁸F]afatinib. The evaluation of each tracer has been compiled to allow for a comparison between studies and ultimately between tracers. The main challenges for each group of tracers are thereafter discussed. Finally, this review addresses the challenges that need to be overcome to be able to efficiently drive EGFR PET imaging forward.