In vivo evaluation of two tissue transglutaminase PET tracers in an orthotopic tumour xenograft model

BACKGROUND: The protein cross-linking enzyme tissue transglutaminase (TG2; EC 2.3.2.13) is associated with the pathogenesis of various diseases, including cancer. Recently, the synthesis and initial evaluation of two high-potential radiolabelled irreversible TG2 inhibitors were reported by us. In the present study, these two compounds were evaluated further in a breast cancer (MDA-MB-231) tumour xenograft model for imaging active tissue transglutaminase in vivo. RESULTS: The metabolic stability of [(11)C]1 and [(18)F]2 in SCID mice was comparable to the previously reported stability in Wistar rats. Quantitative real-time polymerase chain reaction analysis on MDA-MB-231 cells and isolated tumours showed a high level of TG2 expression with very low expression of other transglutaminases. PET imaging showed low tumour uptake of [(11)C]1 (approx. 0.5 percentage of the injected dose per gram (%ID/g) at 40–60 min p.i.) and with relatively fast washout. Tumour uptake for [(18)F]2 was steadily increasing over time (approx. 1.7 %ID/g at 40–60 min p.i.). Pretreatment of the animals with the TG2 inhibitor ERW1041E resulted in lower tumour activity concentrations, and this inhibitory effect was enhanced using unlabelled 2. CONCLUSIONS: Whereas the TG2 targeting potential of [(11)C]1 in this model seems inadequate, targeting of TG2 using [(18)F]2 was achieved. As such, [(18)F]2 could be used in future studies to clarify the role of active tissue transglutaminase in disease.