Ethyldeshydroxy-sparsomycin (EdSm) is a rlbosomal pro tein synthesis Inhibitor which synerglstically enhances the antitumor activity of cisplatin against L.1210 leukemia In vivo. Because cellular glutathione (GSH) and glutathione Stransferases (GST) are reported to interfere with the antitumor activity of cisplatin, we analyzed the effect of EdSm and cisplatin on GSH and GST activity in selected tumor cells. For this purpose we used three murine leukemia tumors with different sensitivities towards EdSm and cisplatin: L1210-WT, sensitive to both drugs, L1210-Sm, resistant to EdSm, and L1210-CDDP, resistant to cisplatin. No significant differences were detectable between these three cell lines regarding the population doubling time, the cell size, and the cellular level of protein and glutathione. Neither of the resistant L1210 subclones showed P-glycoprotein expression. Drug exposure, however, changed the Intracellular dynamics. Exposure to EdSm strongly de creased the amount of cellular protein, decreased the overall GST activity and led to GSH depletion, whereas exposure to cisplatin induced a rise in the amount of protein, in GSH, and In the total GST activity. These effects are dose-dependent and correlate well with the sensitivity of the tumor cells for EdSm or cisplatin. In addition, exposure to EdSm lowered the of GST in L121Q-WT and L1210-Sm; however, In L1210-CDDP both the Vm QX and the Km were Increased. That this was not a direct effect of EdSm on GST was shown in a cell-free system, where EdSm did not Influence the GST activity nor could It act as a substrate for GST. Our results suggest that the synergis tic combination of EdSm and cisplatin might be explained by EdSm switching off the cellular detoxification mechan ism for cisplatin, i.e. by Inhibition of de novo synthesis and subsequent depletion of GSH and GST.

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The efficacy of cisplatin (CDDP) in combination with the protein synthesis inhibitor ethyldeshydroxysparsomycin (EDSM) has been tested in two tumor models at various schedules. Mice with L1210 leukemia or B16 melanoma were treated with CDDP alone or in combination with EDSM. Against L1210 leukemia, which is sensitive to CDDP, combinations elicited increases in life-span for all treatment schedules compared to those achieved with the corresponding dose of CDDP. Moreover, the combination of EDSM with this platinum compound yielded a cure rate > 80%, compared to < 35% for single CDDP treatment. Although the B16 melanoma is rather resistant to both CDDP and EDSM, combinations of these agents against B16 melanoma showed schedule dependent efficacy and in certain schedules significant therapeutic advantage over individual drug treatment, but cures were not observed. Our results suggest that EDSM has significant synergistic capabilities in both animal tumor models, but strong therapeutic enhancement of cisplatin efficacy is only seen when the tumor is sensitive to CDDP.

The cultured murine leukaemia L1210 cell populations used in the present study were derived from L1210 cells that had been grown in vivo. Subclones resistant to sparsomycin (L1210/Sm) or cisplatin (L1210/CDDP) were also developed in vivo. The doubling times of the cultured cell populations were identical. Fractions surviving after drug treatment in vitro were determined by colony formation in soft agar. The results, based on the differential sensitivity of the cell populations to ethyldeshydroxysparsomycin (EdSm) and CDDP, indicated that after a short exposure, cultured L1210/CDDP cells were cross-resistant to EdSm. L1210/Sm cells, however, were not cross-resistant to CDDP. The results obtained in cultured cell populations were confirmed in vivo. CD2f1 mice bearing i.p. implants of 1 x 10(5) tumour cells were given EdSm or CDDP and a combination of the two agents. Drugs were given once daily every 4 days for 3 doses starting at 24 h after tumour implantation. Treatment of mice bearing L1210/wt leukaemia with combined EdSm and CDDP caused strongly synergistic antitumour activity. In animals bearing the two resistant subclones, however, combined drug treatment did not improve the antitumour activity. The corresponding median survival of mice receiving combined drug treatment was 60 days in each group containing 6 mice bearing L1210/wt, with 4-6 cures being noted; 19 days in animals harbouring L1210/Sm, with 2 cures being recorded among 6 mice; and 11 days in mice bearing L1210/CDDP, with no cure being obtained. The results of this study indicate that the synergism resulting from combined treatment with CDDP and EdSm is a function of the cellular properties of the target tumour-cell populations and is independent of host factors.

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