Growth Kinetics of Minimal Residual Disease in the Brown Norway Rat Acute Myelocytic Leukemia

Abstract

The effect of a single i.p. dose (100 mg/kg) of cyclophosphamide (CY) on the progress of the Brown Norway rat acute myelocytic leukemia (BNML) is an almost instantaneous 5–6 decades reduction of the neoplastic cell population in the animal. The treatment was given on day 13 after the i.v. transfer of 10⁷ BNML cells. The bone marrow then contained about 10⁹ leukemic cells. The BNML population size in the bone marrow was determined at various time points using a clonogenic leukemic stemcell assay (LCFU-S). Weighted least squares fitting of various assumed growth curves revealed that “minimal residual disease” (MRD) progresses somewhat slower than the untreated disease (population doubling time .97 vs .78 d at intermediate total cell numbers, i.e., 10⁵–10⁸). This may be due to more effective kill of fast cycling cells by CY or to CY (perhaps in addition to immunological factors) enhancing the overall cell loss factor. In contrast, results from dose-survival bioassays, giving more information on MRD in the femur shortly after therapy, reproducibly showed a 7 decades reduction in BNML, followed by very fast regrowth (doubling time .48 d ≈ cell cycle time) indicating a growth fraction of one and a zero cell loss factor. The limited value of the latter method is discussed. Experiments using flow cytometry and a monoclonal antibody that recognizes BNML cells confirmed the LCFU-S data.