Radiolabeling and biodistribution studies of the new angiogenesis tracer cyclic Asn-Gly-Arg (NGR)

Objectives The NGR (Asn-Gly-Arg) peptide motif is an aminopeptidase N (CD13) ligand, selected from a display peptide library, that targets neovasculature. NGR interacts with integrins such as αvβ3. This preliminary study endeavored to develop a new radiotracer for future angiogenesis detection in tumors, nominally radiolabeling with ^99mTc of cyclo 1-5 (NGRdYK) conjugated with MAG3 and including PEG8 in the molecule.

Methods For the labeling of MAG3-NT, sodium tartrate dissolved in labeling buffer (ammonium bicarbonate, ammonium acetate, ammonium hydroxide) was employed, along with stannous chloride solution. Reaction was induced by heating at 100°C for 20 min. Radiochemical evaluation was performed by ITLC and was confirmed by HPLC analysis. ^99mTc-MAG₃-PEG8-c(NGRYK) was injected in healthy Swiss mice (0.1mL/18.5 MBq) and biodistribution was evaluated at 5, 30, 60, 120, 240, 360 and 1440 min post-injection.

Results Radiochemical purity of the radiotracer was 99.3 +/- 0.4 %. Partition coeficient pointed towards a hydrophilic profile (log P = -3,15). About 50% of the product cleared from the blood in 30 min and 95% in 120 min post-injection (p.i.). Highest uptake was observed in kidneys (30.7 +/- 4.8 %ID/g) 5 min p.i. followed by lungs, liver and small intestine.

Conclusions No purification step was required as high purity was achieved. Biodistribution highlighted renal excretion with a blood clearance pattern quite favorable for imaging diagnosis. An ongoing protocol for angiogenesis detection in a melanoma model tends to confirm these useful properties.