The decapeptide ELA (ELAGIGILTV), a
Melan-A/
MART-1 antigen immunodominant
peptide analogue, is an interesting
melanoma vaccine candidate alone or in combination with other tumour
antigens. P40, the recombinant
outer membrane protein A of Klebsiella pneumoniae (kpOmpA), was recently shown to target dendritic cells and to induce
peptide-specific CTLs. Here we investigated the adjuvant role of P40 mixed or chemically conjugated to ELA. This compound is an N-terminal
glutamic acid-containing
peptide. However, it has been reported that the amino group and the gamma-carboxylic group of
glutamic acids easily condense to form pyroglutamic derivatives. Usually, to overcome this stability problem,
peptides of pharmaceutical interest were developed with a
pyroglutamic acid instead of N-terminal
glutamic acid, without loss of pharmacological properties. Unfortunately, the
pyroglutamic acid derivative (PyrELA) as well as the N-terminal acetyl capped derivative (AcELA) failed to elicit CTL activity when mixed with P40 adjuvant
protein. Despite the apparent minor modifications introduced by PyrELA and AcELA, these two derivatives have probably lower affinity than ELA for the class I Major Histocompatibility Complex. Furthermore, this stability problem is worse in the case of clinical grade ELA, produced as an
acetate salt, like most of the pharmaceutical grade
peptides. We report here that the hydrochloride shows a higher stability than the
acetate and may be suitable for use in man.