Potent somatostatin undecapeptide agonists selective for somatostatin receptor 1 (sst1)

A family of analogues of des-AA(1,2,5)-[DTrp(8)/D2Nal(8)]-SRIF that contain a 4-(N-isopropyl)-aminomethylphenylalanine (IAmp) at position 9 was identified that has high affinity and selectivity for human somatostatin receptor subtype 1 (sst1). The binding affinities of des-AA(1,2,5)-[DTrp(8),IAmp(9)]-SRIF (c[H-Cys-Lys-Phe-Phe-DTrp-IAmp-Thr-Phe-Thr-Ser-Cys-OH], CH-275) (7), des-AA(1,5)-[Tyr(2),DTrp(8),IAmp(9)]-SRIF (CH-288) (16), des-AA(1,2,5)-[Tyr(7),DTrp(8),IAmp(9)]-SRIF (23), and des-AA(1,2,5)-[DTrp(8),IAmp(9),Tyr(11)]-SRIF (25) are about (1)/(7), (1)/(4), (1)/(125), and (1)/(4) that of SRIF-28 (1) to sst1, respectively, about (1)/(65), (1)/(130), <(1)/(1000), and <(1)/(150) that of 1 to sst3, respectively, and about or less than (1)/(1000) that of 1 to the other three human SRIF receptor subtypes. A substitution of DTrp(8) by D2Nal(8) in 7 to yield des-AA(1,2,5)-[D2Nal(8),IAmp(9)]-SRIF (13) and in 16 to yield des-AA(1,5)-[Tyr(2),D2Nal(8),IAmp(9)]-SRIF (17) was intended to increase chemical stability, selectivity, and affinity and resulted in two analogues that were less potent or equipotent with similar selectivity, respectively. Carbamoylation of the N-terminus as in des-AA(1,2,5)-[DTrp(8),IAmp(9),Tyr(11)]-Cbm-SRIF (27) increased affinity slightly as well as improved selectivity. Monoiodination of 25 to yield 26 and of 27 to yield 28 resulted in an additional 4-fold increase in affinity at sst1. Desamination of the N-terminus of 17 to yield 18, on the other hand, resulted in significant loss of affinity. Attempts at reducing the size of the ring with maintenance of selectivity failed in that des-AA(1,4,5,13)-[Tyr(2),DTrp(8),IAmp(9)]-SRIF (33) and des-AA(1,4,5,6,12,13)-[Tyr(2),DTrp(8),IAmp(9)]-SRIF (34) progressively lost affinity for all receptors. Both des-AA(1,2,5)-[DTrp(8),IAmp(9),Tyr(11)]-Cbm-SRIF (27) and des-AA(1,2,5)-[DCys(3),DTrp(8),IAmp(9),Tyr(11)]-Cbm-SRIF (29) show agonistic activity in a cAMP assay; therefore, the structural basis for the agonist property of this family of analogues is not contingent upon the chirality of the Cys residue at position 3 as shown to be the case in 18-membered ring SRIF octapeptides. None of the high affinity structures described here showed receptor antagonism. We have prepared the radiolabeled des-AA(1,2,5)-[DTrp(8),IAmp(9),(125)ITyr(11)]-SRIF ((125)I-25) and des-AA(1,2,5)-[DTrp(8),IAmp(9), (125)ITyr(11)]-Cbm-SRIF ((125)I-27), used them as in vitro tracers, and found them to be superior to des-AA(1,5)-[(125)ITyr(2),DTrp(8),IAmp(9)]-SRIF ((125)I-16) for the detection of sst1 tumors in receptor autoradiography studies.