[I-125] PD151242 - A SELECTIVE RADIOLIGAND FOR HUMAN ET(A) RECEPTORS

Our aim was to synthesize a new endothelin ETA selective radioligand, [I-125]-PD151242 and characterize the compound in human vascular tissu e. Binding of [I-125]-PD151242 to sections of human aorta was time-dep endent and reached equilibrium after 120 min at 23 degrees C with an a ssociation rate constant of 1.26+/-0.17 x 10(8) M(-1) min(-1) (n = 3 i ndividuals+/-s.e.mean). The binding was reversible at 23 degrees C wit h an observed dissociation rate constant of 0.0025+/-0.0006 min(-1) (n = 3). Saturation binding assays using [I-125]-PD151242 revealed a sin gle population of high affinity ET receptors (n=3) in aorta (K-D = 0.7 6+/-0.17 nM; B-max = 5.98+/-1.56 fmol mg(-1) protein), pulmonary (K-D = 1.75+/-0.20 nM; B-max = 12.78+/-1.39 fmol mg(-1) protein) and corona ry arteries (K-D = 0.51+/-0.07 nM; B-max = 44.9+/-1.67 fmol mg(-1) pro tein). ET(A) selective ligands competed for [I-125]-PD151242 binding i n aorta with nanomolar affinity (BQ123, K-D = 0.41+/-0.26 nM; FR139317 , K-D = 0.55+/-0.11 nM) whereas the ET(B) selective compound, BQ3020, competed with micromolar affinity (K-D = 1.36+/-0.25 mu M). In isolate d coronary arteries, PD151242 was a functional antagonist and caused a significant, parallel rightward shift of the ET-1 dose-response curve with a pA(2) value of 5.92 (n = 5) and a slope of unity. The high aff inity and selectivity of [I-125]-PD151242 for ET(A) receptors will fac ilitate the characterization of this sub-type in human tissues.