Use and mechanism of action of AS101 in protecting bone marrow colony forming units-granulocyte-macrophage following purging with ASTA-Z 7557.

Ammonium trichloro(dioxoethylene-O,O')tellurate (AS101) has been shown previously to provide radioprotective effects when given to mice 24 h prior to irradiation and to protect mice from lethal and sublethal doses of cyclophosphamide (CTX). In this study we examined the ability of AS101 to protect mice bone marrow colony forming units-granulocyte-macrophage treated in vitro with various doses of ASTA-Z 7557, a potent derivative of cyclophosphamide. We demonstrate that prior incubation with AS101 protects colony forming units-granulocyte-macrophage from toxic effects of ASTA-Z. This protection can also be conferred by injection of mice with AS101 prior to incubation of their bone marrow in vitro with ASTA-Z. Prior incubation with AS101 was shown not to protect K562 leukemic cells or HL-60 cells from the toxic effects of ASTA-Z. We show that AS101 protection from the toxic effects of ASTA-Z in vitro and CTX in vivo can be partially ascribed to increased aldehyde dehydrogenase (ALDH) activity induced by AS101. This was shown directly by measuring cellular ALDH activity and indirectly by measuring the toxicity of ASTA-Z and CTX in the presence of cyanamide, an inhibitor of ALDH. AS101 is also demonstrated in this study to protect spleen cells from the toxic effects of 5-fluorouracil, probably through a different mechanism. These properties of AS101 make it a useful candidate for increasing the qualitative potential of bone marrow used for autologous transplantation after purging with ASTA-Z. In addition, the results suggest an increase in ALDH activity by AS101 as one of the mechanisms of protection from the toxic effects of ASTA-Z and CTX. However, the chemoprotectiveness of AS101 was found not to be restricted to cyclophosphamide, since as shown in this study, AS101 helped by other mechanisms to reconstitute the number of spleen cells after 5-fluorouracil treatment.

Mechanism of radioprotection conferred by the immunomodulator AS101.

AS101 [ammonium trichloro (dioxyethylene-0-0') tellurate] is a new synthetic compound previously described by us as having immunomodulating properties and minimal toxicity. Clinical trials are currently in progress with AS101 in AIDS and cancer patients. AS101 has recently been found to have radioprotective effects on hemopoiesis in irradiated mice when administered prior to irradiation. Since the early progenitors, spleen colony-forming units (CFU-S), are the critical cells needed for the reconstitution of the hemopoietic system, the mechanisms of action of AS101 were explored in this study by examining the compound's effect on the recovery of CFU-S, its protective effect on endogenous CFU-S and its effect on self-renewal of CFU-S. We also studied the effect of AS101 on the induction of progenitor cells into the radioresistant S-phase of the cell cycle. On days 1 and 5 after irradiation, the number of CFU-S in the bone marrow and spleen of AS101-treated mice was significantly higher than that of PBS-injected mice. Nine days after sublethal doses of irradiation, the number of endogenous spleen colonies was highest in mice given AS101 every 24 hours or every other day for 1 week prior to irradiation. AS101 administered immediately after irradiation, however, also resulted in an increase in the endogenous CFU-S. The higher number of CFU-S found in each 9-day endogenous spleen colony suggests increased self-renewal of CFU-S in AS101-treated mice. Moreover, we found that AS101 induced a higher number of progenitor cells in the S-phase of the cell cycle. These findings suggest that the radioprotection conferred by AS101 results from induction of progenitor cells in DNA synthesis (S-phase) and from the enhanced stimulation of CFU-S, not only toward proliferation but also toward CFU-S self-renewal.