Therapeutic anticancer efficacy of a synthetic diazonamide analog in the absence of overt toxicity Noelle S. Williams*, Anthony W. G. Burgett*, Ashley S. Atkins*, Xiaodong Wang*†, Patrick G. Harran*, and Steven L. McKnight*‡ *Department of Biochemistry and †Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390 Contributed by Steven L. McKnight, December 20, 2006 (sent for review July 14, 2006) Blocking cell division through the inhibition of mitosis is one of the classes of tubulin-binding drugs such as the epothilones, it is most successful clinical strategies for the treatment of cancer. evident that neutropenia remains a substantive toxicological Taxanes and vinca alkaloids are in widespread use and have liability (3, 10). Given this largely unavoidable target-based demonstrated substantive therapeutic efficacy. Both classes of toxicity, new efforts in the antimitotic field have instead focused compounds bind directly to tubulin, a structural component of the on improving dosing schedule, solubility, bioavailability, activity mitotic spindle. The ubiquitous utilization of tubulin in cell division against resistant cell populations, and concomitant administra- in both cancerous and normal cells, however, tempers the broad tion of hematopoietic growth factors or agents that ameliorate spectrum of activity of currently used antimitotics by significant neurotoxic symptoms (11, 12). Thus, agents that inhibit mitosis toxicities in normal dividing tissue. Moreover, peripheral nerve with improved safety profiles, perhaps through cancer-specific cells that rely on microtubules to shuttle cargo along axonal modes of action, are needed. processes are also damaged by tubulin-binding drugs. Thus, neu- Diazonamide A is a natural product originally isolated from tropenia and peripheral neuropathy are the most frequently cited the marine ascidian Diazona angulata (13). Evaluation of the dose-limiting toxicities of this class of chemotherapeutics. Here we compound by the Developmental Therapeutics Program at the report the preclinical assessment of AB-5, a structural and func- National Cancer Institute, including analysis by the COMPARE tional analog of the natural product diazonamide A. AB-5, like algorithm of the resulting differential cytotoxicity pattern (14, taxanes and vinca alkaloids, blocks cell division during mitosis. 15), indicated diazonamide A might represent a tubulin-active However, AB-5 works not by binding tubulin but rather through agent, because its correlation coefficients of toxicity most closely inhibition of a newly discovered role for ornithine-␦-aminotrans- paralleled antitubulin drugs (vinblastine, maytansine, paclitaxel, ferase in mitosis. We hereby report that, unlike other antimitotics, and vincristine). Cells treated with diazonamide A arrest at the AB-5 is extremely well tolerated by mice when administered under G2/M boundary and fail to form appropriately organized bipolar conditions where the drug cures xenografted tumors as effectively mitotic spindles (15, 16). Notably, however, studies by Cruz- as taxanes and vinca alkaloids. AB-5-treated mice show no weight Monserrate et al. (15) indicated that diazonamide A does not loss, no change in overall physical appearance, and no evidence of compete with vinblastine or colchicine for tubulin binding. In neutropenia. These observations raise the possibility that AB-5 fact, our own studies have shown that neither biotinylated nor may have clinical utility for cancer therapy under conditions largely radiolabeled congeners of the natural product interact specifi- devoid of chemotherapeutic toxicity and suggest that further cally with purified tubulin in vitro, raising the possibility that the preclinical evaluation of AB-5 is warranted. compound does not act by binding directly to tubulin (17). Earlier, we developed a successful laboratory synthesis of antimitotic agent ͉ neutropenia ͉ oncology ͉ pharmacokinetics diazonamide A and identified its correct chemical structure (16, 18). Such efforts further enabled the preparation of derivatized he mitotic spindle is a molecular machine composed of forms of diazonamide A useful for biochemical studies of its Tpolymerized tubulin (microtubules) and associated regula- mode of action. These experiments have shown conclusively that tory proteins acting in concert to appropriately segregate rep- tubulin binding does not underpin the antimitotic effects of this licated chromosomes. Compounds that interfere with the mitotic compound. Rather, diazonamide A interacts with a specifically spindle have proven highly effective against both hematological proteolyzed form of ornithine ␦-amino transferase (OAT) crit- malignancies and solid tumors (1, 2). The two most widely used ically involved in a heretofore novel spindle assembly pathway families of antimitotic agents, taxanes and vinca alkaloids, block controlling mitosis (17). Paradoxically, the OAT-mediated spin- cell division by perturbation of microtubule dynamics (reviewed dle assemble pathway appears not to be essential for normal cell in ref. 3). The taxanes stabilize microtubules against depolymer- division. Mice bearing an inactivating mutation in the gene ization, resulting in mitotic cell cycle arrest and eventually cell encoding OAT develop to adulthood if supplemented with death (4). By contrast, the vinca alkaloids inhibit the polymer- ization of tubulin into microtubules, causing G2/M arrest of the cell cycle and subsequent cell death (4). Both families of Author contributions: N.S.W. and S.L.M. designed research; N.S.W. and A.S.A. performed compounds are known to exert their antimitotic activities research; A.W.G.B. and P.G.H. contributed new reagents/analytic tools; N.S.W. and X.W. through direct binding to tubulin (5, 6). Despite the diverse array analyzed data; and N.S.W., P.G.H., and S.L.M. wrote the paper. of spindle proteins that could be used as targets for novel Conflict of interest statement: P.G.H. and X.W. are founders and shareholders of Joyant Pharmaceuticals, a company that has licensed rights to develop diazonomide A analogs for chemotherapeutics, only tubulin-binding agents are currently in commercial use. N.S.W. serves as a paid consultant to Joyant Pharmaceuticals. clinical use, although agents that interfere with the motor Freely available online through the PNAS open access option. protein kinesin are entering clinical trials (3). Abbreviations: OAT, ornithine ␦-amino transferase; GI50, growth inhibitory 50; q2–3dX6, The ubiquitous expression and utilization of tubulin in cell Monday, Wednesday, and Friday dosing schedule for 2 weeks; qd ϫ 6, daily dosing schedule division in both cancerous and normal cells limit the therapeutic for 6 days. utility of tubulin-binding drugs. Bone marrow suppression, in ‡To whom correspondence should be addressed. E-mail: [email protected]. particular neutropenia, is a dose-limiting toxicity of both the This article contains supporting information online at www.pnas.org/cgi/content/full/ taxanes and the vinca family alkaloids (7–9). Despite many 0611340104/DC1. efforts to improve these agents, including enumeration of new © 2007 by The National Academy of Sciences of the USA 2074–2079 ͉ PNAS ͉ February 13, 2007 ͉ vol. 104 ͉ no. 7 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0611340104 Downloaded by guest on September 25, 2021 Fig. 1. Structures of native diazonamide A and a functionally equivalent synthetic analogue, AB-5, and in vitro activity of AB-5 against a panel of human and murine tumors. (A) Structures of diazonamide compounds: (Ϫ)-diazonamide A and AB-5. (B) Activity of AB-5 against a panel of human tumor lines was assessed by addition of 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide (MTT) or Cell Titer-Glo reagent after a 48-h incubation with compound. GI50 values were calculated as described in Materials and Methods. The mean Ϯ SEM for multiple replicate experiments is shown. arginine, the metabolite produced by the mitochondrial biosyn- volume of distribution upon i.v. administration into mice. The thetic pathway in which this enzyme is involved (19). Moreover, compound was formulated in a 1:1 mixture of cremaphor:etha- BIOCHEMISTRY human patients carrying inactivating mutations in the OAT gene nol (10%) and diluted in 5% dextrose (90%) before in vivo also develop to adulthood in the absence of symptoms that would administration. With this formulation, 20 mg/kg could be easily be expected from deficits in mitotic cell division (20). administered to mice as a single-bolus i.v. injection with no The OAT-mediated mechanism of action of diazonamide A in observable acute toxicity. A simple noncompartmental analysis inhibiting cell division implied this compound might show a of plasma levels of AB-5 in mice after a bolus i.v. injection of 20 toxicity profile distinct from those exhibited by other tubulin- mg/kg revealed a low clearance rate and an intermediate termi- binding antimitotics. Herein we describe rigorous studies of the nal half-life that predicted efficacious levels of compound should pharmacokinetics, toxicity, and efficacy of AB-5, a synthetic persist on a daily or every-other-day schedule of dosing (Fig. 2A). variant of diazonamide A, in nude mice bearing xenografted Plasma levels at 24 h after injection were Ϸ15 ng/ml (21 nM), at human tumors. These data reveal substantive therapeutic effi- or above the GI50 value for all tumors examined to date, and cacy of AB-5 in the absence of overt toxicity, suggesting