discodermolide and ixabepilone

Because induction of drug efflux transporters is one of the major underlying mechanisms of drug resistance in cancer chemotherapy, and human pregnane X receptor (hPXR) is one of the principal "xenobiotic" receptors whose activation induces transporter and drug-metabolizing enzyme gene transcription, it would be ideal to develop chemotherapy drugs that do not activate hPXR. This report describes studies undertaken to explore the characteristics of hPXR stimulation and mechanisms of drug-receptor interactions in vitro with new anti-tubulin drugs.. In vitro transient transcription, glutathione S-transferase pull-down assays, and mammalian one-hybrid and two-hybrid systems were used to explore drug-receptor interactions. Loss of righting reflex was used to assess effects of drugs on PXR activity in vivo.. The current study showed that paclitaxel, discodermolide, and an analogue of epothilone B, BMS-247550, induced CYP3A4 protein expression in HepG2 hepatoma cells. Transient transcription assays of a luciferase reporter in the presence and absence of a GAL4-steroid and xenobiotic receptor (SXR) plasmid in HepG2 cells showed that these drugs activate hPXR. This was not true for the inactive analogue of paclitaxel, baccatin III, or for an analogue of epothilone A, analogue 5, none of which stabilizes microtubules. To determine the mechanisms by which paclitaxel, discodermolide, and BMS-247550 activate hPXR, a mammalian two-hybrid assay was done using VP16SRC-1 (coactivator) and GAL4-SXR. SRC-1 preferentially augmented the effects of these drugs on hPXR. Expression of SMRT (corepressor) but not NCoR suppressed the drug-induced activation of SXR by approximately 50%, indicating a selectivity in corepressor interaction with hPXR. These drugs resulted in shortened duration of loss of righting reflex in vivo, indicating drug-induced activation of PXR in mice.. These findings suggest that activation of hPXR with selective displacement of corepressors is an important mechanism by which microtubule-stabilizing drugs induce drug-metabolizing enzymes both in vitro and in vivo.

Topics: Alkanes; Animals; Antineoplastic Agents; Antineoplastic Agents, Phytogenic; Aryl Hydrocarbon Hydroxylases; Blotting, Northern; Carbamates; Cytochrome P-450 CYP3A; Cytochrome P-450 Enzyme System; DNA-Binding Proteins; Epothilones; Glutathione Transferase; Histone Acetyltransferases; Humans; Immunoblotting; Lactones; Mice; Mice, Inbred C57BL; Nuclear Proteins; Nuclear Receptor Co-Repressor 1; Nuclear Receptor Co-Repressor 2; Nuclear Receptor Coactivator 1; Oxidoreductases, N-Demethylating; Paclitaxel; Plasmids; Pregnane X Receptor; Pyrones; Receptors, Cytoplasmic and Nuclear; Receptors, Steroid; Reflex, Abnormal; Repressor Proteins; Transcription Factors; Transcription, Genetic; Transcriptional Activation; Two-Hybrid System Techniques; Xenobiotics