Synthesis and comparative in vivo evaluation of ^99mTc(CO)₃‐labeled PEGylated and non‐PEGylated cRGDfK peptide monomers

Abstract

This work aimed at studying the effect of insertion of medium PEG (PEG ) on the pharmacokinetic behavior of cRGDfK peptide in comparison with the non-PEGylated analogue. The cRGDfK peptide has thus been derivatized at ε-amino group of lysine by conjugation with N -PEG -COOH/N -CH -COOH to prepare a PEGylated and a non-PEGylated analogue of cRGDfK. A tridentate chelator was then incorporated by click chemistry conjugation of the two peptide azides for radiolabeling with [ Tc(CO) (H O) ] precursor. Comparative in vivo evaluation of the two Tc(CO) -labeled radiotracers, Tc(CO) -Pra-Tz-CH -cRGDfK 5 and Tc(CO) -Pra-Tz-PEG -cRGDfK 6, was carried out in C57BL/6 mice bearing α β -positive melanoma tumors to determine their potential toward targeting integrin α β receptors. The radiotracers exhibited excellent stability in saline as well as in serum. Maximum tumor uptake for the two radiotracers was observed at 30 min p.i. (5: 3.0 ± 0.7% ID/g; 6: 4.1 ± 0.5% ID/g). The two neutral Tc(CO) radiotracers prepared exhibited receptor-mediated uptake in melanoma tumor. The increase in the tumor uptake on introduction of PEG unit was accompanied by slower clearance from other organs which resulted in decreased target-to-background ratios. The in vivo kinetics of Tc(CO) -labeled radiotracer, Tc(CO) -Pra-Tz-CH -cRGDfK 5 with only methylene unit as the spacer, was found to be more favorable due to higher tumor/blood, tumor/liver, tumor/kidney, and tumor/lung ratios.

Design, synthesis, and comparative evaluation of 99mTc(CO)3‐labeled N‐terminal and C‐terminal modified asparagine–glycine–arginine peptide constructs

“…The azide group of NGR peptide was subjected to click reaction with propargyl glycine to introduce a tridentate chelating scaffold for radiolabeling with [ 99m Tc(CO) 3 ] + core . Briefly, the azide‐peptide Lys(N 3 )c (CNGRC)G‐CONH 2 (2 mg) dissolved in 1:1 t‐butanol/water (100 μL) was added to a solution of propargyl glycine (0.35 mg), copper sulphate (0.2 mg), and sodium ascorbate (0.6 mg) followed by incubation at 50°C for 6 hours under nitrogen environment.…”

“…The azide/alkyne‐modified peptides can subsequently be joined with metal chelators through click reaction. The peptide‐chelator constructs thus assembled can be readily radiometalated for diagnosis or therapy of various cancers …”

“…The three water molecules in [ 99m Tc(CO) 3 (H 2 O) 3 ] + core can be replaced by a tridentate chelator to coordinate with 99m Tc and form a thermodynamically and kinetically stable complex . The azide alkyne click ligation generates 1,2,3‐triazole (Tz) that provides the third coordination site, nitrogen of the triazole ring, for tridentate chelation with [ 99m Tc(CO) 3 (H 2 O) 3 ] + core …”

“…The peptide-chelator constructs thus assembled can be readily radiometalated for diagnosis or therapy of various cancers. 15,23 Radiometalation of NGR peptides has been reported with 64 Cu, 68 Ga, and 99m Tc for diagnostic purpose. [7][8][9][10][11][12] However, single-photon emission computed tomography (SPECT) radioisotope, 99m Tc has wider coverage in nuclear medicine hospitals because of lower infrastructure and financial requirement in comparison with positron emission tomography (PET) isotopes, 64 Cu and 68 Ga. 24,25 The nuclear (E γ = 140 keV, 89%) and physical (t ½ = 6 h) properties of 99m Tc are ideally suited for carrying out labeling and imaging procedures.…”

“…[28][29][30] The azide alkyne click ligation generates 1,2,3-triazole (Tz) that provides the third coordination site, nitrogen of the triazole ring, for tridentate chelation with [ 99m Tc(CO) 3 (H 2 O) 3 ] + core. 23,31 In this report, CNGRC peptide was azide-functionalized at C-and N-termini, and the two azide peptides were then subjected to click ligation with an alkyne chelator, propargylglycine to introduce a tridentate ligand scaffold suitable for radiolabeling with [ 99m Tc(CO) 3 (H 2 O) 3 ] + core. The radiochemical yield of the two 99m Tc(CO) 3 -labeled NGR peptide constructs was determined.…”