The human growth hormone antagonist B2036 does not interact with the prolactin receptor

The human growth hormone (hGH) antagonist B2036 combines a single amino aci d substitution impairing receptor binding sire 2 (G120K) with eight additio nal amino acid substitutions that improve binding site 1 affinity. This hGH antagonist is being tested for treating pathologies linked to excess hGH l evels. B2036-PEG is a polyethylene glycol (PEG) conjugated form of B2036 th at has an increased half-life due to reduced renal clearance, It is current ly in phase III trials for acromegaly. Human GH is also able to bind to the receptor of prolactin (PRLR). Since activation of PRLR can promote an arra y of pathological states (reproduction disorders, breast cancer), the abili ty of B2036-PEG to interact with the PRLR had to be determined. In this stu dy, we compared four hGH antagonists (G120K, G120K-PEG, B2036 and B2036-PEG ) in three bioassays : proliferation of rat Nb2 cells, binding to the human PRLR and activation of human PRLR-mediated signaling in a cell line stably expressing this receptor and a luciferase reporter gene. Agonistic and ant agonistic properties were characterized. Our data show that B2036-PEG does not bind, activate or antagonize PRLRs, either from rat or human origin. Th ese observations further demonstrate that the eight amino acid substitution s within binding site 1 provide binding specificity directed towards the hu man GH receptor.