Modulation of in vitro chemosensitivity by extracellular Ca++ in adriamycin sensitive and resistant P388 leukemic cells.

P388 mouse lymphocytic leukemia cells sensitive (P388/S) and resistant to adriamycin (P388/Adr), respectively, were exposed in vitro to 3 dose concentrations of adriamycin, mitoxantrone, vincristine and cisplatin in the presence and absence of extracellular Ca++ at 37 degrees C for 1 h. The absence of extracellular Ca++ enhanced the cytotoxicity of all the four drugs by 25 to 30% in P388/S cells. P388/Adr cells retained their resistance to adriamycin irrespective of the presence or absence of Ca++, however, vincristine and cisplatin to which P388/Adr cells, in normal course, show cross-resistance, exhibited a 30-40% enhancement of cytotoxicity in the absence of extracellular Ca++. Cross-resistance of P388/Adr to mitoxantrone was totally circumvented in the absence of extracellular Ca++.