Synergistic killing of human leukaemic lymphoblasts by glucocorticoids and cytosine arabinoside

Abstract

Summary Previous work has shown that the lethal effects of glucocorticoids on the human lymphoblastoid cell line, CEM-C7, are antagonized by the simultaneous presence of 1-,B-D-arabinofuranosylcytosine (Ara-C). A possible cell cycle mechanism prompted further studies using flow microfluorimetry. We now report that (1) Ara-C (10-100nM) blocks cells in S-phase and (2) the block is reversible after the drug is removed. A second treatment protocol, in which glucocorticoid is added to cells recovering from the effects of 24h exposure to Ara-C, results in a clear synergism between the 2 drugs. This synergism is observed over a range of concentrations (5-100nM), but is most significant at low doses, where inhibition of cell growth by Ara-C occurs but cell killing is minimal. Prior treatment with Ara-C increases the number of cells killed in the presence of steroid during the period 12-24h after removal of the S-phase block. Combinations of Ara-C and steroid can thus be either synergistic or antagonistic, depending on the drug scheduling.

Synchronisation by hydroxyurea does not affect the sensitivity of CEM-C7 lymphoblasts to glucocorticoids

“…The difference between SAB and SA.SB did not vary significantly from zero when the drugs were combined according to protocol 1, while the only measureable interactions for protocol 2 was a very small antagonistic effect manifested at low HU concentrations. These results are clearly different from the antagonism (protocol 1) and synergism (protocol 2) observed with prednisolone and Ara-C (Gledhill & Norman, 1981;Gledhill et al, 1983). The results presented above show that HU, like Ara-C, inhibited DNA synthesis in CEM-C7 cells, resulting in accumulation of cells in S-phase of the cell cycle (Figure 1).…”

“…Measurement of cellular DNA content by flow microfluorimetry (FMF) confirmed that Ara-C blocked CEM-C7 cells in S-phase, and that the block was reversible (Gledhill et al, 1983). A second treatment protocol (protocol 2) was devised in which the cells were exposed to steroid after prior treatment with Ara-C alone.…”

“…The glucocorticoid-sensitive human lymphoblastoid cell line CEM-C7 is a suitable model system for studying glucocorticoid-induced cell killing in vitro, and for investigating possible interactions between glucocorticoids and other antileukaemic drugs Gledhill et al, 1983).…”

“…This procedure, based on the method of Crissman & Tobey (1974), has been described previously (Gledhill et al, 1983). Cell aliquots (106 cells) were fixed in 70% ethanol and stored at 40C prior to analysis.…”

“…This period of time was marked by an increase in the number of cells in mitosis and an abrupt increase in the total cell count. CEM-C7 cells were treated with prednisolone and HU using the two protocols developed in previous investigations (Norman et al, 1978;Gledhill et al, 1983). In protocol 1 cells were first treated for 24h with prednisolone (10-6M) alone, and then for a further 24h with prednisolone plus HU.…”