Original article: Preclinical antitumour activity and animal toxicology studies of rhizoxin, a novel tubulin-interacting agent

Summary: Rhizoxin is a 16-membered antifungal macro-cyclic lactone isolated from the plant pathogenic fungus Rhizopus chinensis. The compound binds to tubulin, preventing microtubule formation, and inhibiting mitosis. It possesses antitumour activity in vivo against various preclinical murine models, both leukaemias and solid tumours model, as well as in vincristine- and doxorubicin-resistant leukaemia lines. In the present study, cytotoxic activity was observed in human tumour cell lines in vitro at very low concentrations (±10-¹⁰M) particularly against melanoma, colon, renal, non-small cell and small cell lung cancer. In vivo antitumour activity was demonstrated in murine P388 and L1210 murine leukaemias, solid tumour models B16 melanoma and M5076 sarcoma, and in 5 out of 9 human solid tumour xenografts: LOX melanoma, MX-1 breast cancer, non-small cell lung cancer A549, and small cell lung cancers LXFS 605 and LXFS 650. The absence of cross-resistance to vinca alkaloids was confirmed in vivo against the vincristine-resistant P388 leukaemia subline and the vincristine-resistant human small cell lung cancer LXFS 650. In addition, the antitumour activity of rhizoxin was improved by prolonged or repeated drug administration indicating a schedule dependency. In animal toxicology studies, transient changes in erythrocyte and leukocyte numbers, local phlebitis, diarrhea, and sper-matogenic arrest were observed. The LD₁₀value of rhizoxin after a single intravenous injection was 2.8 mg/kg (8.4 mg/ m²). One-tenth of the mouse equivalent LD₁₀ (0.84 mg/m²), the starting dose for clinical phase I studies, was considered to be safe in rats. The antitumour activity of rhizoxin, its unique interactions with tubulin and the absence of non-manageable toxic effects in the animal toxicological studies have led to rhizoxin's selection for clinical trials. A phase 1clinical trial has been completed showing leukopenia, muco-sitis and diarrhea to be the dose-limiting toxicities. In some cases phlebitis was observed. These toxicities were predicted from the animal toxicological studies. In addition, rhizoxin caused minor responses in three heavily pretreated patients with recurrent breast cancer. Phase II clinical trials will start soon within the framework of the EORTC and CRC.