Cytotoxic synergism between trimetrexate and etoposide. Evidence that trimetrexate potentiates etoposide-induced protein-associated DNA strand breaks in L1210 leukemia cells through alterations in intracellular ATP concentrations.

Using an outgrowth method, combinations of trimetrexate and etoposide were synergistic against L1210 leukemia as assessed by the median-effect method. Trimetrexate was also found to stimulate etoposide-mediated protein-associated DNA strand breaks by nearly 2-fold when L1210 cells were exposed to 0.5 microM drug(s) for 2 hr. Trimetrexate had no effect on the transport of etoposide or the repair of etoposide-induced DNA strand breaks. Other drugs that interfere with de novo purine biosynthesis, including methotrexate and 5,10-dideazatetrahydrofolate, also potentiated etoposide-induced DNA strand breaks, whereas agents that specifically reduce intracellular concentrations of pyrimidines (pyrazofurin or CB-3717) had no effect. Only those protectants that restored ATP levels (adenosine or hypoxanthine) could abolish the stimulatory effect of trimetrexate. Finally, it was shown that by exposing cells to various concentrations of 2,4-dinitrophenol, there was an inverse relationship between the number of DNA strand breaks produced by etoposide and the intracellular concentrations of ATP down to about 600 microM. The results indicate that trimetrexate stimulates etoposide-induced DNA strand breaks possibly by modulating intracellular ATP levels which may contribute to the synergistic interaction between these drugs.