Summary We have previously developed a daunorubicin resistant subline of Ehrlich ascites carcinoma (EA/DR) for studies on the reversal of daunorubicin resistance. The mean survival of untreated BALB/c mice bearing drug sensitive parental tumour (EA/DS) is 18.4+0.6 days, mice bearing EA/DS treated with five daily doses of 0.3mgkg-' daunorubicin greater than 60 days, and mice bearing EA/DR treated with the same daunorubicin regimen, 21.1 + 1.4 days. We now report complete reversal of daunorubicin resistance in EA/DR by cyclosporin A (CsA). The in vitro daunorubicin IC50, defined as that concentration of daunorubicin required to inhibit 50% of DNA synthesis, in EA/DR was 6.7+1.15pgml-1 compared to 2.8+0.72 pgml-I in EA/DS. This value was reduced to 2.8+0.52 and 2.1 +0.opgml-Idaunorubicin by 3.3 and 13.2 pgml-1 CsA respectively, P<0.05. The MST of groups of host mice bearing EA/DR either untreated, treated with five daily doses of 0.3mgkg-' daunorubicin, treated with 80mgkg-' CsA in five divided daily doses or treated with combined daunorubicin-CsA were 19.0+1.0, 21.1+1.4, 24.0+2.6 and >60 days respectively. The mean survival of groups of host mice bearing EA/DR treated with 5mgkg-1 or 10mgkg-1 CsA simultaneously with daunorubicin for five days was also greater than 60 days. These differences are highly significant.The development of resistance to chemotherapeutic drugs by neoplastic cells is a major obstacle to the cure of many malignancies. Recent reports indicate that it is possible to reverse resistance to vincristine and to daunorubicin in murine tumours in vivo by use of verapamil hydrochloride, the calcium channel blocking agent (Tsuruo et al., 1981;Slater et al., 1982; Xkovsgaard et al., 1984). The use of verapamil in patients with malignancy has, however, been limited by high concentration requirements (Kessel & Wilberding, 1985a). We now describe complete reversal of daunorubicin resistance in daunorubicin resistant Ehrlich ascites carcinoma by cyclosporin A used in doses previously employed in humans.Materials and methods Tumour lines and treatment regimens Ehrlich ascites carcinoma (EA) was maintained as an ascitic tumour in BALB/c mice. A daunorubicinresistant subline was developed by sequential transfer of EA cells to subsequent generations of host mice with continuous daunorubicin treatment as previously described (Slater et al., 1982). For current studies, the daunorubicin treatment regimen consists of 0.3mgkg-' daunorubicin i.p. daily for five doses, starting 24h after the inoculation of 0.2ml, i.p. of undiluted malignant ascites harvested from preterminal animals. CsA (Sandimmune I.V., Sandoz LTD) is given i.p. either alone or simultaneously with daunorubicin at the doses indicated.
Daunorubicin inhibition of [3H]-thymidine incorporationNucleotide incorporation studies were performed by the following method as previously described (Slater et al., 1982). Cells were counted on a haemocytometer using dye exclusion, washed and resuspended in RPMI 1640 at a concentration of 2.5 x 106 ml-1. Cell aliquots...