Single step 18F-labeling of dimeric cycloRGD for functional PET imaging of tumors in mice

Arylboronates afford rapid aqueous (18)F-labeling via the creation of a highly polar (18)F-aryltrifluoroborate anion ((18)F-ArBF3(-)).

Radiosynthesis of an (18)F-ArBF3(-) can be successfully applied to a clinically relevant peptide. To test this hypothesis, we labeled dimeric-cylcoRGD, [c(RGDfK)]2E because a) it is molecularly complex and provides a challenging substrate to test the application of this technique, and b) [c(RGDfK)]2E has already been labeled via several (18)F-labeling methods which provide for a preliminary comparison.

To validate this labeling method in the context of a complex and clinically relevant tracer to show tumor-specific uptake ex vivo with representative PET images in vivo.

An arylborimidine was conjugated to [c(RGDfK)]2E to give the precursor [c(RGDfK)]2E-ArB(dan), which was aliquoted and stored at -20 °C. Aliquots of 10 or 25 nmol, containing only micrograms of precursor, were labeled using relatively low levels of (18)F-activity. Following purification eight mice (pre-blocked/unblocked) with U87M xenograft tumors were injected with [c(RGDfK)]2E-(18)F-ArBF3(-) (n = 4) for ex vivo tissue dissection. Two sets of mice (pre-blocked/unblocked) were also imaged with PET-CT (n = 2).

The [c(RGDfK)]2E-ArB(dan) is converted within 15 min to [c(RGDfK)]2E-(18)F-ArBF3(-) in isolated radiochemical yields of ~10% (n = 3) at a minimum effective specific activity of 0.3 Ci/μmol. Biodistribution shows rapid clearance to the bladder via the kidney resulting in high tumor-to-blood and tumor-to-muscle ratios of >9 and >6 respectively while pre-blocking with [c(RGDfK)]2E showed high tumor specificity. PET imaging showed good contrast between tumor and non-target tissues confirming the biodistribution data.

An arylborimidine-RGD peptide is rapidly (18)F-labeled in one step, in good yield, at useful specific activity. Biodistribution studies with blocking controls show tumor specificity, which is corroborated by PET images. Advances in Knowledge and Implications for patient Care: Despite many antecedent examples of labeled RGD tracers, this work is the first to show direct aqueous labeling of bisRGD with an (18)F-ArBF3(-). Labeling occurs in near record rapidity (45 min) at useful effective specific activities and competitive yields for high contrast tumor specific images. As bisRGD has been imaged in humans with several prosthetics, this work suggests potential clinical applications of tracers appended with an (18)F-ArBF3(-). More generally, the ability to label a molecularly complex tracer suggests that this method could be useful to label many other peptides. Furthermore, these results portend the development of kits that use only microgram quantities of lyophilized precursor for on demand labeling. The ability to perform one-step aqueous labeling in under an hour to provide tracers with high T:NT ratios has important implications for developing radiotracers for use in fundamental research and in preclinical tracer studies.