Infrared-emitting, peptidase-resistant fluorescent ligands of the bradykinin B2 receptor: application to cytofluorometry and imaging

We have previously reported the design, pharmacological properties and imaging application of bradykinin (BK) B 2 receptor (B 2 R) ligands conjugated with fluorophores such as fluorescein derivatives at their N-terminus. To take advantage of the high penetration of infrared light into living tissues and their low autofluorescence in this region of the spectrum, additional probes conjugated with cyanine dye 7 (Cy7) were synthesized and characterized. The antagonist B-9430 (D-Arg-[Hyp 3 ,Igl 5 ,D-Igl 7 ,Oic 8 ]-BK) and the agonist B-9972 (D-Arg-[Hyp 3 ,Igl 5 ,Oic 7 ,Igl 8 ]-BK) were N-terminally extended with the infrared fluorophore Cy7, producing the peptides B-10665 and B-10666, respectively. Pharmacological studies indicated that the agonist B-10666 lost much affinity for the B 2 R vs. the parent peptide, whereas the antagonist B-10665 better retained its potency vs. B-9430 (competition of [ 3 H]BK binding to human B 2 R, contractility of the human isolated umbilical vein for which potency losses were more important in each case). Both probes stained HEK 293 cells that expressed the B 2 R-green fluorescent protein (GFP) construction in a specific manner (confocal microscopy) and with very extensive co-localization of the green and infrared fluorescence in either case. The agonist B-10666 at 100 nM promoted the endocytosis of B 2 R-GFP in live cells, but not the antagonist version at 10–25 nM. The Cy7-labeled peptides did not label cells expressing the β 2 -adrenoceptor-GFP construction. B-10665 at low nanomolar concentrations was an effective probe for the recombinant B 2 Rs in cytofluorometry and macroscopic imaging of cell wells (IVIS imaging system operated for infrared fluorescence detection). Despite a propensity for non-specific binding when used at high concentrations and limited sensitivity, Cy7-conjugated peptidase-resistant B 2 R ligands support original imaging and cytofluorometric applications.