In vitro cross-resistance to nucleoside analogues and inhibitors of topoisomerase 1 and 2 in acute myeloid leukemia.

Only about one third of all patients with acute myeloid leukemia (AML) will be cured by common chemotherapy regimens. Susceptibility towards chemotherapy either of the leukemic bulk or the leukemic stem cell is considered the major determining parameter for long-term outcome. The purpose of the present study was to investigate whether chemoresistance was correlated between different antileukemic drugs or not. We determined the lethal concentration of chemotherapy necessary to reduce viability of cells to 50% compared to untreated control (LC50) as a surrogate marker of chemotherapy susceptibility of six established chemotherapeutic agents [cytarabine (median 0.83 microg/ml), daunorubicine (0.09 microg/ml), idarubicine (0.03 microg/ml), mitoxantrone (0.05 microg/ml), etoposide (4.81 microg/ml), and topotecan (0.14 microg/ml)] in an overall number of 147 samples from consecutive patients with AML by WST-1 assay in vitro. We found that susceptibility to chemotherapy was significantly correlated between all six agents (all p values < 0.01). A homogenous response of the blast populations was significantly correlated to high chemoresistance. These data indicate that cross-resistance in AML against antileukemic drugs exists between agents with different modes of action and seems not to be mediated by drug-specific resistance mechanisms but rather by more generalized death-defying features of the affected cells (e.g., inhibited apoptosis).

Modeling the pharmacodynamics of highly schedule-dependent agents: exemplified by cytarabine-based regimens in acute myeloid leukemia.

BACKGROUND: Many agents in antineoplastic chemotherapy are highly schedule dependent. Therefore, variables such as total dose and also the area under the curve (AUC) that are schedule insensitive are generally insufficient to adequately represent treatment strength. PURPOSE: To establish a descriptor of treatment strength that takes into account the differential contribution of plasma concentrations (C) and exposure times (T) towards the cytotoxic effect and to investigate whether such a pharmacodynamically weighed descriptor is better correlated to the clinical effect than conventional variables. PATIENTS AND METHODS: The paradigm "C(N) x T = constant" (for an isoeffect) incorporates a weighing factor N (concentration coefficient) into the conventional description of the AUC that quantitates the differential contribution of C and T towards the cytotoxic effect. N was to be numerically derived from a multitude of in vitro isoeffect analyses of the major agents in acute myeloid leukemia (AML) therapy from patient samples (n = 57). RESULTS: For cytarabine, N was 0.45, numerically expressing the substantially higher relevance of T versus C for its cytotoxic effect. In a meta-analysis of 49 study arms involving >10,000 patients, neither total dose, dose intensity, nor AUC was correlated to the clinical effect. However, when AUC was pharmacodynamically weighed (N-weighed AUC, N-AUC = C(0.45) x T), this new descriptor was highly significantly correlated to the clinical effect (P < 0.001). CONCLUSION: The N-AUC concept is able to characterize schedule-dependent agents and is the only descriptor of cytarabine treatment strength actually correlated to the clinical effect in AML.

Functional analysis of apoptosis induction in acute myeloid leukaemia-relevance of karyotype and clinical treatment response.

Deficiencies or structural defects of the apoptotic machinery have been postulated as a potential mechanism for a broad resistance of acute myeloid leukaemia (AML) blasts towards cytotoxic therapy comprising chemotherapeutic agents with diverse pharmacodynamic principles but also cell-mediated cytotoxicity of the graft-versus-leukaemia effect, for example, in the setting of allogeneic transplantation. This hypothesis was systematically tested by functionally analysing the early, intermediate and late events of the apoptotic process in primary AML (n = 31) blasts following activation of the intrinsic and extrinsic pathway of apoptosis (etoposide and cytarabine as DNA damaging agents, FAS-ligand as an activator of the death receptor pathway). Activation of the extrinsic pathway by FAS-ligand did not induce apoptosis in primary AML, instead the proapoptotic signal was shown to 'fade', even in the early phase of the apoptotic sequence. However, activation of the intrinsic pathway induced severe cytotoxicity in all samples that showed the characteristic features of typical apoptosis, with a prominent apoptotic volume decrease (blebbing) in the early phase, significant increases in caspase 3 activity (intermediate or effector phase) and breakdown of cellular energy production in the late phase of apoptosis. These characteristics did not differ between prognostically favourable versus unfavourable AML karyotypes or between clinically responding versus refractory AML--indicating that a functional apoptotic apparatus is present even in the unfavourable AML subgroups. Our data indicate that the mechanism for a broad clinical resistance is not a dysfunctional apparatus per se but rather the consequence of anti-apoptotic regulation impeding otherwise functional apoptotic machinery.