Aminopeptidase resistant Arg-Gly-Asp analogs are stable in plasma and inhibit platelet aggregation

Tetrapeptides containing the sequence Arg-Gly-Asp (RGD) antagonize fibrinogen binding to its platelet receptor (gp IIb/IIIa, integrin α_11bβ₃) and inhibit platelet aggregation in vitro. The peptides RGDS and RGDY(Me)-NH₂ were rapidly degraded when incubated in human, rat, and dog plasma. HPLC analysis indicated that amino acids were sequentially removed from the peptide N-terminus, and this degradation was prevented by the aminopeptidase inhibitor bestatin. Analogs of RGDY(Me)-NH₂ with an acetylated or deleted α-amino group were prepared. Both analogs were stable when incubated in plasma, blocked ¹²⁵I-fibrinogen binding to activated platelets (IC₅₀= 10–30μm ) and inhibited ADP induced platelet aggregation (IC₅₀= 10–30μm ). This study concludes that aminopeptidase rapidly degrades RGD peptides in plasma, an important issue for in vivo testing of RGD peptides and analogs. RGD analogs intrinsically stabilized against aminopeptidase are stable in plasma and are important tools for antithrombotic studies involving antagonism of gp IIb/IIIa.