Structure-activity relationships of linear and cyclic peptides containing the NGR tumor-homing motif

Cyclic and linear peptides containing the Asn-Gly-Arg (NGR) motif have
proven useful for delivering various anti-tumor compounds and viral
particles to tumor vessels. We have investigated the role of cyclic
constraints on the structure and tumor-homing properties of NGR peptides
using tumor necrosis factor-alpha (TNF) derivatives containing
disulfide-bridged (CNGRC-TNF) and linear (GNGRG-TNF) NGR domains.
Experiments carried out in animal models showed that both GNGRG and
CNGRC can target TNF to tumors. However, the antitumor activity of
CNGRC-TNF was > 10-fold higher than that of GNGRG-TNF. Molecular dynamic
simulation of cyclic CNGRC showed the presence of a bend geometry
involving residues Gly(3)-Arg(4). Molecular dynamic simulation of the
same peptide without disulfide constraints showed that the most
populated and thermodynamically favored configuration is characterized
by the presence of a beta-turn involving residues Gly(3)-Arg(4) and
hydrogen bonding interactions between the backbone atoms of Asn(2) and
Cys(5). These results suggest that the NGR motif has a strong propensity
to form beta-turn in linear peptides and may explain the finding that
GNGRG peptide can target TNF to tumors, albeit to a lower extent than
CNGRC. The disulfide bridge constraint is critical for stabilizing the
bent conformation and for increasing the tumor targeting efficiency.