The therapeutic index of
cytokines in
cancer therapy can be increased by targeting strategies based on
protein engineering with
peptides containing the CNGRC (NGR) motif, a
ligand that recognizes CD13-positive
tumor vessels. We show here that the targeting domain of recombinant CNGRC-
cytokine fusion
proteins, such as NGR-TNF (a CNGRC-
tumor necrosis factor-α (TNF) conjugate used in clinical studies) and NGR-
EMAP-II, undergoes various post-translational modification and degradation reactions that lead to the formation of markedly heterogeneous products. These modifications include N-terminal
cysteine acetylation or the formation of various
asparagine degradation products, the latter owing to intramolecular interactions of the
cysteine α-amino group with
asparagine and/or its
succinimide derivative. Blocking the
cysteine α-amino group with a
serine (SCNGRC) reduced both post-translational and degradation reactions. Furthermore, the
serine residue reduced the
asparagine deamidation rate to
isoaspartate (another degradation product) and improved the affinity of NGR for CD13. Accordingly, genetic engineering of NGR-TNF with the N-terminal
serine produced a more stable and homogeneous
drug (called S-NGR-TNF) with improved antitumor activity in
tumor-bearing mice, either when used alone or in combination with
chemotherapy. In conclusion, the targeting domain of NGR-
cytokine conjugates can undergo various untoward modification and degradation reactions, which can be markedly reduced by fusing a
serine to the N-terminus. The SCNGRC
peptide may represent a
ligand for
cytokine delivery to
tumors more robust than conventional CNGRC. The S-NGR-TNF conjugate (more stable, homogeneous, and active than NGR-TNF) could be rapidly developed for clinical trials.