STABILITY AND IN-VITRO METABOLISM OF THE MITOGENIC NEUROPEPTIDE ANTAGONISTS [D-ARG(1),D-PHE(5),D-TRP(7,9),LEU(11)]-SUBSTANCE-P AND [ARG(6),D-TRP(7,9),MEPHE(8)]-SUBSTANCE-P-(6-11) CHARACTERIZED BY HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

The substance P (SP) analogues [D-Arg(1), D-Phe(5), D-Trp(7,9), Leu(11)]-SP and [Arg(6), D-Trp(7,9), MePhe(8)]-SP (6-11) (antagonists D and G, respectively) are under consideration as new anticancer drugs. In this report, the stability and in vitro metabolism of both antagonists in up to seven different media (water, 1 M acetic acid, human plasma, nude mouse liver and WX 322 human SCLC xenograft homogenized in either 1 M acetic acid or phosphate buffered saline (PBS), pH 7.4) have been characterized by both isocratic and gradient elution reversed-phase HPLC. Antagonist D was stable (never >13% degradation over 24 h, at 37 degrees C) in water, 1 M acetic acid and plasma but was metabolized by PBS liver homogenates (10%, w/v) sequentially to two stable metabolites with a half life of 0.98 h at a concentration of 500 mu g ml(-1). The major pathway of degradation of antagonist G appeared to be C-terminal methionine oxidation (particularly in plasma) as well as hydrolysis, with even aqueous solutions being significantly affected at low concentrations of peptide (0.1 mu g ml(-1), half life 20.9 h at 37 degrees C). Stable metabolites of antagonist G were also detected in incubations with PBS liver homogenates (half life 1.53 h at 500 mu g ml(-1), 37 degrees C). Overall, the data presented indicate that the modifications made to SP have been relatively successful in preserving chemical and biological stability.