Identification and Optimization of EphA2-Selective Bicycles for the Delivery of Cytotoxic Payloads

Bicycles are constrained
bicyclic peptides that represent a promising
binding modality for use in targeted drug conjugates. A phage display
screen against EphA2, a receptor tyrosine kinase highly expressed
in a number of solid tumors, identified a number of Bicycle families
with low nanomolar affinity. A Bicycle toxin conjugate (BTC) was generated
by derivatization of one of these Bicycles with the potent cytotoxin
DM1 via a cleavable linker. This BTC demonstrated potent antitumor
activity in vivo but was poorly tolerated, which
was hypothesized to be the result of undesired liver uptake caused
by poor physicochemical properties. Chemical optimization of a second
Bicycle, guided by structural biology, provided a high affinity, metabolically
stable Bicycle with improved physicochemical properties. A BTC incorporating
this Bicycle also demonstrated potent antitumor activity and was very
well tolerated when compared to the initial BTC. Phage display selection
followed by chemical optimization of Bicycles can deliver potent drug
conjugates with favorable pharmaceutical properties.