Immunostimulation by a partially modified retro-inverso-tuftsin analogue containing Thr super(1) psi (NHCO)(R,S)Lys super(2) modification

The tuftsin retro-inverso analogue H-Thr psi (NHCO)(R,S)Lys-Pro-Arg-OH was synthesized through a novel procedure for the high-yield incorporation of isolated retro-inverso bonds into peptide chains and the use of the new Meldrum's acid derivative (CH sub(3)) sub(2)C(OCO) sub(2)CH-(CH sub(2)) sub(4)NHCOCF sub(3) followed by its efficient coupling in solution to trimethylsilylated H-D-Thr(t-Bu)NH sub(2). Closely related peptide impurities were eliminated both from the crude final peptide and the fully protected tetrapeptide amide precursor via ion-exchange and reversed-phase displacement chromatography, respectively. The tuftsin retro-inverso analogue proved to be completely resistant to enzymatic degradation in vitro, either against isolated aminopeptidases or human plasma proteolytic enzymes. When administered either orally or intravenously, it was significantly more active than normal tuftsin in increasing the number of specific antibody secreting cells in spleen of mice immunized with sheep erythrocytes. Furthermore, the analogue exerted an enhanced stimulatory effect on the cytotoxic activity of splenocytes against YAC-1 tumor cells.