Acute T cell leukaemia in the rat : antitumour effect of cyclosporin A alone and in combination with α-difluoromethylornithine
This study had three objectives: (i) to characterise an acute T cell leukaemia (Roser) in male PVG rats with respect to immunophenotype and clinical presentation and assess its suitability as a model of human acute lymphoblatic leukaemia (ALL); (ii) to examine the effect of the anti-T cell agent cyclosporin A (CsA on growth of the Roser leukaemia (RL) and (iii) to investigate the influence of CsA administered together with the polyamine antimetabolite α-difluoromethylornithine (DFMO) on a) tumour growth, host survival and polyamine levels in RL-bearing rats and b) growth of the EL4 (T cell) lymphoma in C57BL/6. Animals received 20.10^3 viable tumour cells intramuscularly (day 0) and leukamic cells were recovered on day 17 for lymphoid cell separation, monoclonal antibody staining and flow cytometric analysis. CsA (12.5mg/kg or 25mg/kg) or drug vehicle was administered to rats by gavage from day 0 or day 14, by which latter time leukaemic blasts usually appeared in the circulation; mice inoculated with the EL4 lymphoma received CsA 75mg/kg subcutaneously. DFMO 2-3% (w/v) was given in the drinking water from day 0. Animals were killed, either on day 17 or on reaching the terminal phase of disease, for analysis of malignant lymphoblasts and polyamine levels in blood and organs. The leukaemic blasts were shown to have a mature T-helper phenotype, ie W3/25^+ (CD4), Ox44^+ (MHC Class I) and also to express the thymocyte antigen 0x7 (Thy 1.1), corresponding to a population comprising 5&37 of normal medullary thymocytes. Growth characteristics were consistent and bore some similarity to human acute leukaemia. Administration of CsA 25mg/kg from day 0 significantly delayed the appearance of leukaemic cells in the circulation, whereas a dose of 12.5mg/kg was without effect. Leukaemic infiltration of spleen and liver was reduced in animals killed on day 17, but no effect was discernible in lymph nodes or kidneys. In CsA-treated leukaemic animals, there was greater impairment of renal function than in untreated tumour-bearing controls. As with CsA, DFMO, administered from the time of tumour injection, markedly reduced numbers of circulating lymphoblastoids. Host survival was also prolonged in DFMO-treated rats; CsA, however, had no effect on survival. CsA and DFMO combination further reduced blood-borne malignant blast cells, but had no additional effects on either host survival or tumour growth within organs, compared to that achieved with DFMO alone. At the humane terminal end point, organs were extensively infiltrated by malignant cells in all treatment groups. Neither CsA nor DFMO administered from the time of tumour cell injection, nor both drugs in combination affected peritoneal growth of the EL4 lymphoma, although survival was significantly prolonged with DFMO. As anticipated, DFMO inhibited polyamine synthesis in vivo with a pattern of polyamine depletion consistent with ornithine decarboxylase inhibition. CsA treatment, however, did not reduce polyamine synthesis, but produced a rise in putrescine, spermidine and total polyamine levels. It is unlikely that the striking end-tumour effect on circulating RL blasts, observed with CsA and DFMO administered in combination, is due to an additive effect on inhibition of polyamine biosynthesis. By reducing polyamine synthesis however, DFMO may enhance the vulnerability of the malignant T cells, susceptible to another, as yet unexplained, inhibitory action of CsA on cell proliferation.