REGULATION BY RETINOIC ACID AND HYDROCORTISONE OF THE ANTHRACYCLINE SENSITIVITY OF BLAST CELLS OF ACUTE MYELOBLASTIC-LEUKEMIA

The experiments reported here continue the study of regulated drug sen sitivity by extending the observations to anthracyclines. Previous wor k has shown that hydrocortisone (HC) protects AML blast stem cells fro m the lethal effects of cytosine arabinoside (ara-C) while retinoic ac id (ATRA) increases ara-C sensitivity; further mechanisms of regulatio n of ara-C sensitivity might include increase or decrease in repair of sublethal damage. Anthracyline dose-response curves are characterized by an initial shoulder, followed by exponential decrease in survival with increasing dose. The shoulder portion of such curves may indicate the accumulation of sublethal damage. We used two assays to look for evidence of regulation of anthracycline sensitivity by HC or ATRA; the clonogenic assay for blast stem cells detects drug effects on this cr ucial population, but only after several days on incubation, during wh ich time repair might occur. Measurements of nicks in DNA show damage in the bulk population of cells, but these can be detected very soon a fter exposure to drug. Both methods showed the HC protected cells in t wo continuous cell lines (OCI/AML-2 and OCI/AML-5) while ATRA made the cells more sensitive. Blast cells freshly-obtained from six AML patie nts were also tested. Both assays showed HC protection and ATRA sensit ization in three populations. The clonogenic assay detected both effec ts in cells from a fourth patient; the nicked DNA assay confirmed both effects in a fifth patient, where the results of the clonogenic assay did not reach statistical significance. Neither ATRA nor HC influence d the sensitivity of blasts from a sixth patient; but these cells were highly resistant to drug. Kinetic studies showed that damage persiste d longer after treatment with anthracyclines than with ara-C. OCI/AML- 2 cells treated with HC before drug accumulated fewer cells with nicke d DNA after daunorubicin (DNR). Cells exposed to ATRA after DNR showed increased toxicity in kinetic experiments. We conclude that sensitivi ty to anthracyclines may be regulated by ligands for steroid receptors . Furthermore, since growth factors do not regulate anthracyclines' se nsitivity, different mechanisms may be operative for the action of lig ands for cell surface receptors. Finally, we suggest that retinoic aci d might be considered for inclusion in standard anthracycline/ara-C re gimens for the treatment of AML.