2',3'-DIDEOXYADENOSINE KILLING OF TDT-POSITIVE CELLS IS DUE TO A TRACE CONTAMINANT

We have previously reported that the chain-terminating nucleoside anal ogue 2',3'-dideoxyadenosine (ddA) is specifically cytotoxic for TdT-po sitive cells in the co-presence of the adenosine deaminase (ADA) inhib itor coformycin (CF). Further studies with ddA/CF revealed that cytoto xicity occurs only if ddA is supplied from the Calbiochem or Fluka com panies. ddA supplied from other commercial sources (Pharmacia, Sigma) or the NCI Pharmaceutical Resources Branch is non-cytotoxic. To explor e the basis for this difference, ddA from various sources was subjecte d to reverse-phase high-pressure liquid chromatography (HPLC) analysis . The Calbiochem and Fluka ddA had a unique HPLC peak, with a retentio n time of 12.8 min, representing a contaminant of less than 0.1% of th e bulk material applied to the C-18 HPLC column. Study of all HPLC pea ks resolved from the bulk material showed cytotoxic activity in only t he 12.8 min peak. To identify the nature of the unknown compound, we c ompared HPLC characteristics of the synthetic intermediates and byprod ucts of ddA synthesis to the peak eluting at 12.8 min. Of these, only 3'-deoxyadenosine (cordycepin) had similar HPLC characteristics. In ad dition, in the co-presence of CF, cordycepin was specifically cytotoxi c (IC50 < 0.5 mu M) for all TdT-positive cell lines tested. Cytotoxici ty was seen in TdT-negative cells only at concentrations 10-100-fold h igher. We conclude that our previous report on ddA/CF as a TdT-specifi c cytotoxic combination was due to contamination of the ddA supplied b y Calbiochem by cordycepin. ddA itself is non-cytotoxic for TdT-positi ve cells. Cordycepin in the co-presence of an ADA inhibitor may be eff ective in the treatment of TdT-positive hematological malignancies.