Variation of specific activities of ⁶⁸Ga-aquibeprin and ⁶⁸Ga-avebetrin enables selective PET imaging of different expression levels of integrins α₅β₁ and α_vβ₃

⁶⁸Ga-aquibeprin and ⁶⁸Ga-avebetrin are tracers for selective in vivo mapping of integrins α₅β₁ and α_vβ₃, respectively, by PET. Because both tracers exhibit high affinity to their respective targets, the aim of this study was to investigate the influence of the specific activity of preparations of both tracers on in vivo imaging results. Methods: Fully automated ⁶⁸Ga labeling of 0.3 nmol of aquibeprin or avebetrin was done using buffered eluate fractions (600-800 MBq, pH 2) of an SnO₂-based generator, affording the radiopharmaceuticals with specific activities greater than 1,000 MBq/nmol. Lower values ranging from 150 to 0.4 MBq/nmol were adjusted by addition of inactive compound (∼0.15-50 nmol) to the injected activity (∼20 MBq for PET, 5-7 MBq for biodistribution). For in vivo experiments, 6- to 12-wk-old female severe combined immunodeficiency mice bearing M21 xenografts (human melanoma, expressing both integrins α₅β₁ and α_vβ₃) were used. The expression density of integrin β₃ was determined by immunohistochemistry on paraffin slices. Results: For mass doses (specific activities) of less than 20 pmol (>1,000 MBq/nmol) and 1 nmol (20 MBq/nmol) per mouse, respectively, uptake of ⁶⁸Ga-aquibeprin and ⁶⁸Ga-avebetrin in M21 tumors dropped from 5.3 and 3.5 to 3.0 and 2.4 percentage injected dose per gram (%ID/g), respectively. When less than 20 pmol was applied, high uptake of ⁶⁸Gaaquibeprin in the eyes (4.5 %ID/g) or ⁶⁸Ga-avebetrin in adrenals (25.9 %ID/g), respectively, were found, which was reduced by 90% and 65% (0.44 and 6.2 %ID/g, respectively), for doses of 1 nmol. The highest tumor-to-tissue ratios were observed both in ex vivo biodistribution and PET for comparably large doses, for example, 6 nmol (0.65 mg/kg) ⁶⁸Ga-aquibeprin per mouse (3.5 MBq/nmol). Conclusion: Presumably because of their high affinities, ⁶⁸Ga-aquibeprin and ⁶⁸Ga-avebetrin allow for selective addressing of target sites with different integrin expression levels by virtue of adjusting specific activity, which can be exploited for visualization of low-level target expression or optimization of tumor-to-background contrast.