Two anti-angiogenic TKI-PET tracers, [C-11]axitinib and [C-11]nintedanib: Radiosynthesis, in vivo metabolism and initial biodistribution studies in rodents

Introduction
Tyrosine kinase inhibitors (TKIs) are very attractive targeted drugs, although a large portion of patients remains unresponsive. PET imaging with EGFR targeting TKIs ([11C]erlotinib and [18F]afatinib) showed promise in identifying treatment sensitive tumors. The aim of this study was to synthesize two anti-angiogenic TKI tracers, [11C]axitinib and [11C]nintedanib, and to evaluate their potential for PET.

Methods
Following successful tracer synthesis, biodistribution studies in VU-SCC-OE and FaDu xenograft bearing mice were performed. Furthermore, tracer stability studies in mice were performed employing (radio-)HPLC and LC–MS/MS techniques. For [11C]nintedanib an LC–MS/MS method was developed to detect the primary carboxylic acid metabolite, resulting from methylester cleavage, in plasma and tumors, because this metabolite is postulated to be important for nintedanib efficacy. LC–MS/MS was also explored to assess the metabolic fate of [11C]axitinib in vivo, since axitinib has an isomerizable double bond.

Results
[11C]axitinib and [11C]nintedanib were successfully synthesized with 10.5 ± 2.6% and 25.6 ± 3.3% radiochemical yield (corrected for decay), respectively. Biodistribution studies only demonstrated tumor uptake of [11C]nintedanib in FaDu xenografts of 1.66 ± 0.02% ID/g at 60 min p.i. In vivo stability analysis of [11C]axitinib at 45 min p.i. revealed the formation of predominantly non-polar metabolites (36.6 ± 6.8% vs 47.1 ± 8.4% of parent tracer and 16.3 ± 2.1% of polar metabolites), while for [11C]nintedanib mostly polar metabolites were found (70.9 ± 4.1 vs 26.7 ± 3.9% of parent tracer and only 2.4 ± 1.6 of a non-polar metabolites). No isomerization of [11C]axtinib was observed in vivo; however, a sulfoxide metabolite could be detected using LC–MS/MS. For [11C]nintedanib, LC–MS/MS revealed formation of the reported primary carboxylic acid metabolite when in vitro plasma incubations were performed, with large differences in plasmas from different species. In vivo metabolite analysis, however, did not demonstrate the presence of the carboxylic acid in plasma or tumor tissue.

Conclusions
Reliable syntheses of [11C]axitinib and [11C]nintedanib were successfully developed. Tumor uptake was observed for [11C]nintedanib, albeit modest. The metabolic profiles of the tracers suggest that rapid metabolism is partly responsible for the modest tumor targeting observed.