Specific toxicity of 2-chlorodeoxyadenosine toward resting and proliferating human lymphocytes

2-Chlorodeoxyadenosine (CdA), an adenosine-deaminase-resistant purine deoxynucleoside, is markedly toxic toward human T-lymphoblastoid cell lines in vitro and is an effective agent against L1210 leukemia in vivo. The present studies have examined the toxicity, and in some cases, metabolism, of CdA in (1) multiple established human cell lines of varying phenotype, (2) leukemia and lymphoma cells taken directly from patients, (3) normal bone marrow cells, and (4) normal peripheral blood lymphocytes. Nanomolar concentrations of CdA blocked the proliferation of lymphoblastoid cell lines with a high ratio of deoxycytidine kinase to deoxynucleotidase. The drug had virtually no effect on the growth of cell lines derived from solid tissues. The CdA inhibited the spontaneous uptake of tritiated thymidine by many T and non-T, non-B acute lymphoblastic leukemia cell specimens at concentrations less than or equal to 5 nM. The same concentrations did not impair either thymidine uptake or granulocyte-monocyte colony formation by normal bone marrow cells. In common with deoxyadenosine, but unlike several other agents affecting purine and purine metabolism, CdA was lethal to resting normal T lymphocytes and to slowly dividing malignant T cells. In both resting and proliferating lymphocytes, the CdA was phosphorylated by deoxycytidine kinase and entered a rapidly turning over nucleotide pool. Dividing lymphocytes also incorporated abundant CdA into DNA. The selective toxicity of CdA toward both dividing and resting lymphocytes may render the drug useful as an immunosuppressive or antileukemic agent.