Combined effect of ACNU and 5-FU on rat glioma cells in spheroids and monolayer cultures

Isobolographic, Algebraic, and Search Methods in the Analysis of Multiagent Synergy

A combination of doses d1, d2, dn of n agents shows zero interaction when d1/D1 + d2/D2. + dn/Dn = 1, where D1,D2, Dn are the doses of the individual agents isoeffective with the combination. In synergy, the sum in this equation is less than 1, and in antagonism, it exceeds 1. This equation may be used to calculate the expected (zero interactive) effect of any combination, irrespective of the shapes of the dose-response curves of the agents and of whether they are linear or nonlinear, similar or dissimilar. For a given set of agents, finding the combination that has the maximum therapeutic (or toxic) effect may be logistically a huge problem because of the large number of variables (e.g., dose, dose interval, number of doses) that are generally involved. This problem may be tackled by (1) response surface methods, in which an equation (usually a low-order polynomial) is fitted to the observed effects of a number of different combinations, and the maximum on this response surface is found mathematically, or by (2) direct search methods, in which the response surface is explored one combination at a time without preconceived ideas about its form. For problems with many variables, direct search methods are more economic.

Cell death due to ACNU-induced DNA fragmentation: inhibition by cycloheximide

Several anticancer drugs have recently been shown to induce cell death in a manner similar to programmed cell death or apoptosis. The purpose of this study is to explore the mode of cell death caused by ACNU, a water-soluble nitrosourea. Exposure of rat glioma cell line KEG-1 to ACNU for 2 hours resulted in oligonucleosomal DNA fragmentation, creating a 'ladder' on agarose gel electrophoresis. DNA fragmentation began 18 hours after ACNU treatment, and preceded loss of membrane integrity as evaluated by the trypan blue exclusion test. The extent of DNA fragmentation increased in a dose-dependent manner, and the cell survival rate decreased reciprocally. A translational inhibitor, cycloheximide, suppressed this DNA fragmentation and enhanced cell survival rate with partial inhibition of protein synthesis. However, a transcriptional inhibitor, actinomycin D, failed to inhibit DNA fragmentation or enhance cell survival. Cycloheximide-inhibitable DNA fragmentation was also found in the KEG-1 implanted in vivo rat model following the administration of ACNU. These findings suggest that ACNU induces cell death associates with DNA fragmentation and partially with protein synthesis.

In vitro studies on drug interaction of ifosfamide and ACNU in primary and metastatic human brain tumours

Combination chemotherapy is widely employed in clinical oncology; however, there is no generally accepted model to evaluate individual tumour susceptibility to a given drug combination protocol. We therefore investigated the drug interaction of ifosfamide (4-hydroxyperoxy-ifosfamide) and ACNU in a recently developed in vitro model of paired sequential combination chemotherapy in primary and metastatic malignant brain tumours. A long-term standard [6,3-3H]-thymidine-incorporation assay, employing a liquid scintillation counting protocol, was selected to assess the drug sensitivity of human tumours. In vitro drug exposures were derived from correlating in vivo-(systemic and CNS) and in vitro-pharmacokinetic drug parameters. In combination experiments tumour cells were treated sequentially by two drugs in both sequences: drug exposures were calculated for 2 h with a 1-h drug-free interval in between. "Cut-off" concentrations (maximum in vitro exposure doses) were calculated as 1.74 microM (for primary CNS tumours: 0.58 microM) for ifosfamide and 5.4 microM (for primary CNS tumours: 1.33 microM) for ACNU. Dose/response relations were derived from isotope incorporation rates after cells had grown for approximately five population doubling times. Combination isoboles were plotted after drug doses had been transformed into "equieffective doses", enabling comparison of drug combination effects. In all three glioblastomas (with CNS exposure dose) an additive or supra-additive effect could be observed in either sequence (in one tumour a biphasic additive isobole was found for both sequences). Out of three bronchial carcinomas (small-cell type, brain metastases) in two non-identical sequences a supra-additive effect was observed in two tumours, with antagonistic effects in the third tumour. In all three malignant melanomas and in one renal carcinoma antagonistic effects were observed, whereas in a second renal carcinoma supra-additive effects were demonstrated for both sequences. We conclude that drug combination chemotherapy effects at the cellular level may be extremely heterogeneous.

Development of subrenal capsule tumours following transplantation of rat RN6 multicell spheroids

We describe here the growth rate, morphology and ultrastructure of the RN6 neurinoma spheroids in culture and after syngeneic transplantation under the renal capsule of adult BDIX rats, where sequential growth and tumour angiogenesis studies were performed. The tumour cells in solid avascular multicell spheroids (MTS) and in subrenal capsule tumours (SCT) showed similar morphology and ultrastructure. The vascularization of spheroids started 2 to 3 days after implantation under the renal capsule from proliferating venule-like vessels of the kidney parenchyma. The MTS were soon densely penetrated by hairpin loops of newly formed capillaries. Such vascularized spheroids grew rapidly. The morphology, ultrastructure and permeability characteristics of newly formed tumour vessels and their relationship to the development of tumour necrosis are described. This syngeneic model of subrenal capsule transplantation of neurogenic tumour spheroids appears to be useful for studies of the mechanisms of tumour angiogenesis, tumour necroses and the host response to malignant tumours of the nervous system.

Experimental combination chemotherapy of ACNU and 5-FU against cultured glioma model (spheroid) and subcutaneous rat glioma

The effects of combination chemotherapy of ACNU and 5-FU on cells grown exponentially as monolayers, cells in multi-cell spheroids and in s.c. transplanted tumors of rat glioma clone-6 cells were analyzed by the colony-forming assay. The cytotoxic effect of ACNU on cells in spheroids was enhanced by continuous 5-FU pretreatment for several hours, but further enhancement was obtained only if the 5-FU pretreatment lasted for more than 12 hr. The combined effect was decreased by a drug-free interval due to recovery from potentially lethal 5-FU damage, but ACNU treatment immediately after 5-FU treatment suppressed PLD recovery. Analysis with sequential trypsinization of spheroids indicated that ACNU showed more marked cytotoxicity on cells in the deeper layers in spheroids than on those in the outer layers, whereas the effect of 5-FU decreased towards the deeper cell layers. Enhancement of treatment with ACNU combined with 5-FU was evident for cells in the outer layers, but was more remarkable in the deeper layers. The combined effect of ACNU and 5-FU on s.c. tumors was similar to that in spheroids. The effect of combined treatment of ACNU with 5-FU on increase in body weight was within the additive range of both drugs acting independently, but was more than additive for the growth delay of s.c. tumors.