Enlargement and multinucleation of u937 leukemia and MCF7 breast carcinoma cells by antineoplastic agents to enhance sensitivity to low frequency ultrasound and to DNA-directed anticancer agents.

Cytochalasin B is a mycogenic toxin that preferentially damages malignant cells through multiple mechanisms. The microfilament-disrupting agent inhibits cytokinesis, producing enlarged and multinucleated neoplastic cells without enlarging or producing multinucleated normal cells. In addition, cytochalasin B has been shown to induce apoptosis and to increase the mitochondrial activity of malignant cells. In spite of these pharmacological properties potentially exploitable in cancer chemotherapy, no cytochalasin congener or derivative and indeed no microfilament-directed agent has yet been examined in the clinic. Nevertheless, it will likely be necessary to combine microfilament-directed agents with other chemotherapeutic agents, and potentially with other anti-neoplastic modalities to amplify the mechanisms by which microfilament-directed agents inflict damage. These combinations could increase the likelihood of obtaining clinically useful activities with microfilament-directed agents and decrease the often inevitable emergence of drug resistance. Therefore, this study intends to determine appropriate chemotherapeutic agents to use concurrently with cytochalasin B and with other microfilament-directed agents. Since cytochalasin B has shown in vitro efficacy against anchorage-independent growth, as well as against attached malignancies, both U937 human monocytic leukemia and MCF7 human breast carcinoma cells were evaluated. These cell lines were assessed for their sensitivity to a comprehensive array of chemotherapeutic agents that could amplify the cytoskeletal effects of microfilament-directed agents or that could themselves be potentiated by the cellular effects of such agents. In addition, clinically-approved microtubule-directed agents, as well as clinically-active anti-neoplastic agents not specifically cytoskeletal-directed, were examined for their ability to potentiate cell enlargement, one of the hallmark features of microfilament-directed agents. Conditions for inducing optimal enlargement and multinucleation of neoplastic cells with cytochalasin B were also defined. U937 and MCF7 cells have differing sensitivities to chemotherapeutic agents indicating that different regimens will likely be needed for various cell types in concomitant cytochalasin B-mediated chemotherapy. It was noted that microtubule-directed agents (paclitaxel and vincristine) would likely have a synergistic effect with cytochalasin B as they produced a substantial enlargement in viable cells at their 50% inhibitory (IC50) values. Interestingly, doxorubicin and mitomycin C also produced considerable cell enlargement, suggesting that nucleic acid-directed agents may be used to further enhance the cell-enlargement and multinucleation effects of microfilament-directed agents if appropriate sequences and concentrations can be found for the combination of agents. A subsequent publication in this series will examine the optimal combinations of chemotherapeutic agents with microfilament-directed agents in regards to drug concentrations and sequential timing. U937 cells exposed to cytochalasin B exhibited substantial cell enlargement and multinucleation that was still prevalent 8 days post-administration depending on the concentration used. Taken together, it appears that cytochalasin B has substantial synergistic potential with microtubule- and nucleic acid-directed agents.