Novel super-potent conformationally constrained calcitonin (ct) analogues

Ct is a hormone of 32 aa known for its hypocalceinic effect. Little is known about the structural features of hCt bioactivity. Recently, we devised a highly potent conformationally constrained bicyclic hCt analogue with the structure (-y(lo7,2 -{Asp17, l,ys21]hCt (1). This analogue exhibited a 20x higher hypocalcemi(: effect than hCt. We hypothesized that a type I 3-turn/-sheet conformation involving residues 17-21 is important for hCt activity. To test this, the slructure of 1 was used as scaffold towards the design of new 3-turn/shoet stabilized analogues. Analogue 2, cyclolT'21-[AspLT, l)-Phe19, Lys21]hCt, designed Io (:ontain a stabilized type II turn, lacked the high hypoealcemic effect of 1. although high ,Lsheet/-turn contents were found by CD. However, analogues 3 and 4, cyclolT'2t-[Asp17, Aibis, Lys21]hCt and cyclolT'21-[Asp17, I)-LysIs, [.ys21]hCt, designed to contain a stabilized type l turn, showed similar poten(:ies to 1. Most importantly, two other superpotent analogues were i(tentified. These analogues showed activities that were either similar to or hiher than those of salmon Ct, which is the most potent known Ct. Together, our results show a strong dependency of hCt bioactivity on the nature and chirality of several residues of hCt sequence (17-21) and demonstrate the importance of a stabilized typo I 3-1urn/Mleet conformation in this region for h() bioactivilv.