INHIBITION OF ENRICHED STEM-CELLS IN-VIVO AND IN-VITRO BY THE HEMOREGULATORY PEPTIDE SK-AND-F108636

Replacement of the labile sulfhydryl group (-SH) of the hemoregulatory peptide monomer pyroGluGluAspCysLys (HP5b) with an isosteric methylen e group yields a chemically stable compound, SK&F108636. In this study , we describe the effects of SK&F108636 on highly enriched Lin(-)Sca1( +) hematopoietic stem cells, SK&F108636 significantly reduced the frac tion of cycling progenitor cells, granulocyte macrophage colony-formin g cells (GM-CFC), in vitro and in vivo, There was no effect on GM-CFC or Mix-CFC colony formation, SK&F108636 significantly inhibited prolif eration of high proliferative potential (HPP)-CFC in semisolid agar cu ltures stimulated by stem cell factor + interleukin 3 (IL-3) + IL-1, b ut had no effect in cultures stimulated with M-CSF + IL-3 + IL-1. SK&F 108636 was shown to act directly on the stem cells since SK&F108636 in hibited proliferation of Lin(-)Sca1(+) cells in single cell assays, Ad ministration of SK&F108636 to lethally irradiated mice transplanted wi th 2000 Lin(-)Sca1(+) cells significantly inhibited proliferation/diff erentiation of cells developing into colony forming units-spleen (CFU- S) (preCFU-S) and the reconstitution of HPP-CFC and GM-CFC, There was no effect of SK&F108636 on CFU-S colony formation or mature cell regen eration in bone marrow, spleen and blood, Hence, the hemoregulatory pe ptide monomer SK&F108636 is a potent primitive stem cell inhibitor in vivo and in vitro, Inhibition of stem cell proliferation by small spec ific inhibitors may protect hematopoiesis from myelotoxic side effects during chemotherapy treatment.