Ovarian reticular cell sarcoma of the mouse (M5076) made resistant to cyclophosphamide.

Resistance of mouse M5076 (M5) ovarian reticular cell sarcoma to cyclophosphamide (CTX) was obtained in vivo by repeated drug treatment followed by transplantation of the regrowing tumor. After 16 passages, we obtained an M5 subline resistant to CTX (M5-CTX-16R). Median survival times were approximately 29 and 39 days for M5 and M5-CTX-16R, respectively. Survival of M5-bearing mice given a single i.p. dose of 200 or 300 mg/kg was 160 and 168% of controls, respectively, whereas in M5-CTX-16R it ws 103 and 123%, respectively. The resistance was not reversible after 14 additional passages with no further CTX treatment. M5 and M5-CTX-16R appear similar in histological features, pattern of metastasis formation, and DNA content, as assessed by flow cytometry (hypotetraploid). Metastases of M5-CTX-16R were also resistant to CTX. Flow cytometry studies 12 and 24 hr after CTX treatment revealed a block in S and G2-M phases in both tumors. After 48 hr and at subsequent times, no cytokinetic pertubation was evident in M5-CTX-16R, whereas in M5 marked accumulation of cells in G2-M was observed at 48, 72, 96, and 120 hr. Cross-resistance was found between CTX, L-phenylalanine mustard, chlorambucil, and hexamethylmelamine. M5-CTX-16R was sensitive, but less so than M5, to cis-platinum, 1,3-bis(2-chloroethyl)-1-nitrosourea, and imidazole-4-carboxamide,5-(3,3-dimethyl-1-triazene). Adriamycin was equally active on M5 and M5-CTX-16R, while 4'-demethylepipodophyllotoxin-9-(4,6-O-ethylidine-beta-D-glucopyranoside) was inactive. This model appears to be suitable for studies on the mechanism of resistance to CTX and alkylating agents and for screening new, non-cross-resistant drugs.