rhizoxin and Breast-Neoplasms

Microtubules, which are composed of polymerized tubulin dimers, play an important role in various cell functions. For example, they maintain cell shape, form mitotic spindles in M phase of cell cycle, and carry an axonal transport in nerve cells. Microtubules have also been an important target of cancer chemotherapy. Vinca alkaloids depolymerize microtubules, the mechanisms of which action have extensively been investigated recently. Clinical trials of vinorelbine (navelbine), a new semisynthetic vinca alkaloid, are ongoing in Japan. One of advantages of the drug is reduced risk of neurotoxicity. Estramustine may act on microtubule-associated proteins (MAPs) as well as tubulin. It shows additive or synergistic cytotoxicity preclinically when used in combination with vinblastine. This combination was active against hormone-refractory prostate cancer. Another novel drug rhizoxin, which has a similar mechanism of action to that of vinca alkaloids, is also a promising cytotoxic agent and is examined clinically in Europe. Taxanes, which include paclitaxel (Taxol) and taxotere, are interesting drugs because they promote polymerization of tubulin and stabilize microtubules. They show promising antitumor activity against breast, ovarian and lung cancers. Phase I and II trials are ongoing in Japan. Paclitaxel may also potentiate cytotoxicity of radiation. There are several mechanisms of resistance to microtubule-acting drugs. One is multidrug resistance mediated by P-glycoprotein. Other mechanisms include mutation of tubulin.

Topics: Animals; Antibiotics, Antineoplastic; Antineoplastic Agents; Breast Neoplasms; Clinical Trials, Phase I as Topic; Clinical Trials, Phase II as Topic; Female; Humans; Lactones; Lung Neoplasms; Macrolides; Microtubule-Associated Proteins; Microtubules; Ovarian Neoplasms; Paclitaxel; Tubulin; Vinca Alkaloids

Rhizoxin is a new anti-tumour agent isolated from the pathogenic fungus Rhizopus chinensis. It has shown broad activity against murine tumour models and is also active against vinca alkaloid-resistant cells. The purpose of our studies was to determine the clinical activity of this compound in patients with advanced breast cancer and melanoma. Based on the results of a phase I study, 2.0 mg m-2 was administered as intravenous infusion over 5 min every 21 days. Nineteen patients were entered into the breast cancer phase II trial and received a total of 50 courses (median 2, range 1-6). Of these, dose reductions were performed in three courses because of leucopenia or stomatitis (1.5 mg m-2, one course; 1.45 mg m-2, two courses). Twenty-six patients were entered into the melanoma trial and received a total of 70 courses (median 2, range 1-12). No dose reductions were required. All patients were eligible for toxicity. Haematological toxicity included neutropenia CTC grade 3 (29/120 courses, 24.2%) and grade 4 (11/20 courses, 9.2%). Only drug-related CTC grade 1 thrombocytopenia was observed. Non-haematological toxicity included alopecia in all patients after two courses of treatment as well as CTC grade 3/4 stomatitis and asthenia. In the breast cancer study, one patient achieved a more than 50% tumour reduction after six cycles but was progressing after 6 weeks. Another patient showed a partial remission after the first course but was taken off the study because of CTC grade 3 skin toxicity. One patient was not evaluable for response (early death). No objective remissions were observed in 15 evaluable patients. In melanoma, no objective remissions were observed. We conclude that rhizoxin can be safely administered at 2.0 mg m-2 every 3 weeks. However, it has little activity in patients with advanced breast cancer and melanoma.

Topics: Adult; Aged; Antibiotics, Antineoplastic; Breast Neoplasms; Female; Hematopoiesis; Humans; Lactones; Macrolides; Male; Melanoma; Middle Aged

Rhizoxin is a macrocyclic lactone compound that binds to tubulin and inhibits microtubule assembly. Rhizoxin demonstrated preclinical anti-tumour activity against a variety of human tumour cell lines and xenograft models. Phase I evaluation found a maximum tolerated rhizoxin dose of 2.6 mg m-2, with reversible, but dose-limiting, mucositis, leucopenia and diarrhoea. Clinical trials were then initiated by the EORTC ECSG in melanoma, breast, head and neck, and non-small-cell lung cancers with the recommended phase II rhizoxin dose of 2 mg m-2. Pharmacological studies were instituted with the phase II trials to complement the limited pharmacokinetic data available from the phase I trial. Blood samples were obtained from 69 of 103 eligible patients enrolled in phase II rhizoxin studies, and these were evaluable for pharmacokinetic analysis in 36 patients. Plasma rhizoxin concentrations were determined by high-performance liquid chromatography (HPLC), and post-distribution pharmacokinetic parameters were estimated by a one-compartment model. Rhizoxin was rapidly eliminated from plasma, with a median systemic clearance of 8.41 min-1 m-2 and an elimination half-life of 10.4 min. Rhizoxin area under the concentration-time curve (AUC) was higher in patients obtaining a partial response or stable disease than in those with progressive disease (median 314 vs 222 ng ml-1 min; P = 0.03). As predicted from previous studies, haematological and gastrointestinal toxicity was observed, but could not be shown to be related to rhizoxin AUC. This study demonstrated the rapid and variable elimination of rhizoxin from the systemic circulation. The presence of pharmacodynamic relationships and the low level of systemic toxicity suggest that future trials of rhizoxin with alternative dosage or treatment schedules are warranted.

Topics: Antibiotics, Antineoplastic; Area Under Curve; Breast Neoplasms; Carcinoma, Non-Small-Cell Lung; Female; Head and Neck Neoplasms; Humans; Injections, Intravenous; Lactones; Lung Neoplasms; Macrolides; Male; Melanoma; Treatment Outcome

The recent success of antibody-drug conjugates (ADCs) in the treatment of cancer has led to a revived interest in microtubule-destabilizing agents. Here, we determined the high-resolution crystal structure of the complex between tubulin and maytansine, which is part of an ADC that is approved by the US Food and Drug Administration (FDA) for the treatment of advanced breast cancer. We found that the drug binds to a site on β-tubulin that is distinct from the vinca domain and that blocks the formation of longitudinal tubulin interactions in microtubules. We also solved crystal structures of tubulin in complex with both a variant of rhizoxin and the phase 1 drug PM060184. Consistent with biochemical and mutagenesis data, we found that the two compounds bound to the same site as maytansine and that the structures revealed a common pharmacophore for the three ligands. Our results delineate a distinct molecular mechanism of action for the inhibition of microtubule assembly by clinically relevant agents. They further provide a structural basis for the rational design of potent microtubule-destabilizing agents, thus opening opportunities for the development of next-generation ADCs for the treatment of cancer.

Topics: Animals; Antibiotics, Antineoplastic; Antineoplastic Agents, Phytogenic; Binding Sites; Breast Neoplasms; Cattle; Clinical Trials, Phase I as Topic; Crystallography, X-Ray; Female; Humans; Macrolides; Maytansine; Microtubules; Polyketides; Pyrones; Tubulin; Tubulin Modulators

We describe a cell-based assay for antimitotic compounds that is suitable for drug discovery and for quantitative determination of antimitotic activity. In the assay, cells arrested in mitosis as a result of exposure to antimitotic agents in pure form or in crude natural extracts are detected by ELISA using the monoclonal antibody TG-3. The assay was used to screen >24,000 extracts of marine microorganisms and invertebrates and terrestrial plants and to guide the purification of active compounds from 5 of 119 positive extracts. A new rhizoxin analogue was found in a Pseudomonas species, six new eleutherobin analogues were identified from the octocoral Erythropodium caribaeorum, and two paclitaxel analogues were found in the stem bark of the tree Ilex macrophylla. The assay was also used for quantitative comparison of the antimitotic activity of different analogues. It revealed the importance of the C-11 to C-13 segment of the diterpene core of eleutherobin for its antimitotic activity. The identification of antimitotic compounds in very low abundance and their high (0.5%) occurrence in natural extracts indicates that drug discovery efforts using this cell-based assay may lead to the identification of structurally novel antimitotic agents.

Topics: Alkaloids; Animals; Antibiotics, Antineoplastic; Antineoplastic Agents; Antineoplastic Agents, Phytogenic; Breast Neoplasms; Diterpenes; Drug Screening Assays, Antitumor; Enzyme-Linked Immunosorbent Assay; Humans; Invertebrates; Lactones; Macrolides; Marine Biology; Paclitaxel; Plant Extracts; Plants, Medicinal; Pseudomonas; Structure-Activity Relationship; Tissue Extracts; Tumor Cells, Cultured