8Cl-cAMP cytotoxicity in both steroid sensitive and insensitive multiple myeloma cell lines is mediated by 8Cl-adenosine.

We have examined the cytotoxic effects of cyclic adenosine-3', 5'-monophosphate (cAMP) derivatives on multiple myeloma cells lines and determined that the 8-Chloro substituted derivative (8Cl-cAMP) is one of the most potent. We report here that 8Cl-cAMP is cytotoxic to both steroid sensitive and insensitive myeloma cells with a half maximal concentration of approximately 3 micromol/L. 8Cl-cAMP toxicity in myeloma cells is dependent on phosphodiesterase activity in the serum of cell culture medium. A metabolite of 8Cl-cAMP, 8-Chloro-adenosine (8Cl-AD), kills myeloma cells as effectively as 8Cl-cAMP. Adenosine deaminase (ADA) converts 8Cl-AD into 8Cl-inosine and abrogates the cytotoxic effects of 8Cl-cAMP, 8Cl-AMP, and 8Cl-AD, as does 5-(p-Nitrobenzyl)-6-Thio-Inosine (NBTI), an inhibitor of nucleoside uptake. These data suggest that 8Cl-cAMP must be converted to 8Cl-AD and that 8Cl-AD is the compound that enters the cell. Contrary to glucocorticoid-mediated cell death in myeloma cells, the pathway of 8Cl-AD-mediated cell death appears to be independent of interleukin-6 (IL-6) actions. Although the exact mode of action for this agent is currently unknown, its ability to kill steroid sensitive and insensitive multiple myeloma cells in an IL-6 independent fashion may offer exciting new therapeutic options.