Effectiveness of low-dose melphalan treatment against a nonimmunogenic primary induced mouse plasmacytoma

The fourth i.p. passage of the plasmacytoma "PR" induced by repeated i.p. injections was used for testing chemotherapy with melphalan. The development of "PR" ascitic tumours was slower and the survival time of inoculated mice was longer than that of MOPC-315 inoculated mice. Moreover, the myeloma protein secreted by the "PR" tumour cells, differed from MOPC-315 secreted myeloma protein in its electrophoretic mobility (fast gamma-2) and its characteristics as IgG immunoglobulin. Chemotherapy by a single injection of melphalan 7.5 mg/kg was effective in preventing the development of both MOPC-315 ascitic tumours and "PR" ascitic tumours. Mice cured from MOPC-315 tumours, however, developed antitumour response as shown by resistance to challenge with a high tumourigenic dose and by development of cytotoxic response in vitro against MOPC-315 tumour cells by spleen cells taken from the cured mice. On the other hand, mice cured from "PR" tumour by melphalan were highly susceptible to challenge and their spleen cells were not able to develop a cytotoxic response in vitro against target "PR" tumour cells.