Resistance to the microtubule inhibitor podophyllotoxin: Selection and partial characterization of mutants in CHO cells

Stable mutants two- to fourfold more resistant to the microtubule inhibitor podophyllotoxin (PodRI)have been selected in a single step in Chinese hamster ovary (CHO) cells. Mutants showing higher levels of resistance to the drug can be obtained in a second-step selection using the PodRImutants (PodRIIclass).Prior treatment of cells with the mutagen ethylmethane sulfonate markedly enhanced the frequency of both PodRIand PodRIImutants in culture. The spontaneous rate of mutation of PodRIas determined by fluctuation analysis was found to be 1−2×10−5 and 4× 10−7mutations/cell/generation at 20 and 30 ng/ml podophyllotoxin, respectively. Somatic cell hybrids between PodRI × PodRII and PodRII × PodSshowed intermediate levels of resistance to the drug, indicating that the PodRphenotype behaves codominantly under these conditions. Cross-resistance studies with various PodRI and PodRIImutants show that they are not cross-resistant to other microtubule inhibitors, e.g., colchicine, colcemid, griseofulvin, vinblastin, that have been examined, or to a compound such as VP16-213, which is a podophyllotoxin derivative which lacks microtubule inhibitory activity. These results indicate that the lesion in PodRmutants is highly specific and probably does not involve a permeability alteration.