The application of certain cytostatic drugs causes the recruitment of pluripotent hemopoietic stem cells (CFU-S) into active proliferation. Further application of the drug(s) may then lead to severe and long lasting disturbances of hemopoiesis. We investigated if the hemoregulatory peptide pGlu-Glu-Asp-Cys-Lys (HP5b) could be used to inhibit stem cell recruitment and consequently to protect mice against the toxicity of repeated high doses of 1-β-d-arabinofuranosylcytosine (ara-C). CFU-S recruitment (induced by injecting a single dose of 900 mg/kg ara-C) was prevented by either treating the bone marrow of these mice in vitro with 1 × 10-7m/liter HP5b, or by injecting 0.6 µg HP5b (10-9 mol, 30 µg/kg) at -2, +2, and +6 h relative to the ara-C injection. Multiple high dose ara-C applications (4 × 900 mg/kg at 0, 7, 24, and 30 h) lead to proliferative activation of CFU-S and resulted in the death of 90% of the mice within 7–9 days. Reconstitution of the hemopoietic system by a bone marrow transplant given after ara-C application decreased the mortality to about 45%, indicating the nonhematological component of ara-C toxicity. A single injection of HP5b (30 µg/kg at 26 h, when few CFU-S were found in S phase) decreased the mortality to 59%, not significantly different from the transplanted group. Inactive peptides given instead of HP5b had no protective effect. HP5b did not change the ara-C sensitivity of transformed cell lines (HL-60, Raji, Friend), even not in such cases (myeloid cell lines) where it had a direct inhibitory effect on the cells (e.g., HL-60). These results suggest that HP5b may be used as a myeloprotector in cancer chemotherapy by keeping hemopoietic stem cells out of cycle during the most hazardous treatment phase. Its lack of species specificity, its low toxicity, its high selectivity for hemopoiesis, the small size, as well as the availability through standard synthetic techniques may be of advantage for its clinical use.

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