Amplified pulmonary metastases of a rat rhabdomyosarcoma in response to nitrosourea treatment.

Repeated observations in our laboratory show that the chloroethylnitrosourea of cysteamine ( CNCC ) induces slowed tumor growth rate and decreased lymph node metastasis in rats bearing a rhabdomyosarcoma but concomitantly enhances metastatic dissemination in the lung. Tumors obtained by sc graft of tumor cells, in syngeneic rats, gave a reproducible pattern of metastases at nodal and pulmonary sites after a 60-80-day period. CNCC was administered orally at a dose of 50 mg/kg once a week for 5 weeks beginning at the time of tumor appearance. Forty-five of 46 CNCC -treated rats had lung metastases with 95 (+/- 9.7) nodules; in the control group 29 of 41 rats had lung metastases with 7 (+/- 1.5) nodules. This amplifying effect was found after treatment with two other nitrosoureas (chlorozotocin and hydroxyethylchloroethylnitrosourea ) but not with cyclophosphamide and methotrexate. Lung metastatic amplification was also observed after treatment of the 13762 mammary adenocarcinoma in Fischer rats and treatment of nickel-induced soft tissue tumor. Several hypotheses have been proposed. The dissociated effect of nitrosourea on local tumor, lymph nodes, and pulmonary metastases does not support the concept of systemic immunosuppression as the main mechanism of this phenomenon, but a decrease of local immunological defenses exerted by NK cells, for example, could be possible. Alternatively, a direct effect of the drug on lung tissue, especially lesions of endothelial tissue, could be responsible for the observed effect. Nitrosourea treatment of rats after surgical excision of the tumor, as adjuvant chemotherapy, was responsible for an amplification effect in association with local recurrences. From this fact we hypothesized that nitrosourea treatment could modify the equilibrium of cell subpopulations in the tumor by selecting highly metastatic drug-resistant variants. Although the mechanism of the amplifying effect of nitrosoureas has not been elucidated, our study shows a possible risk in the use of these drugs for inductive or adjuvant chemotherapy.