Abstract CN08-03: Targeting topoisomerase II inhibitors to cancer cells via the polyamines transport system

Abstract

Abstract Topoisomerases inhibitors continue to play a major role in cancer chemotherapy. A large number of drug regimens, routinely used for the management of many forms of solid tumors in particular, include a camptothecin analog targeting topoisomerase I or an anthracycline derivative targeting topoisomerase II. This is the case also for the podophyllotoxin derivative etoposide marketed more than 25 years ago and which remains prescribed for the treatment of a variety of malignancies, including leukaemia. These topoisomerases poisons form an irreplaceable class of antitumor agents, almost all derived from natural products isolated from plants or microorganisms (1). For the past 20 years, efforts have been concentrated on the discovery and conception of novel, more potent, topoisomerases inhibitors but with a limited success. Camptothecins are still unique among topoisomerase I poisons used in the clinic. Anthracyclines and podophyllotoxines remain the two most robust classes of topoisomerase II modulators. Novel compounds have been introduced into the clinical pipeline, such as the naphthyridine derivatives ARC-111 and voreloxin (SNS-595), targeting respectively topoisomerases I and II, to cite only these two novel molecules. Novel compounds interfering with the functions of these DNA manipulating enzymes are continuously reported: the alkaloids thaspine and lamellarins, the yeast-derived compound simocyclinone D-8, calothrixins, etc. An alternative approach to the design of novel categories of topoisomerases-based antitumor agents is to maintain a known chemical archetype associated with a marked target selectivity profile (e.g. selectivity of camptothecin for toposiomerase I) but to coupled the pharmacophore with additional chemical groups susceptible to modulate target-independent properties such as cell selectivity, biodistribution, stability, pharmacokinetics, or to facilitate its handling for example. With this idea in mind, we have engineered a novel group of highly potent podophyllotoxin derivatives equipped with a cell delivery vector. Three points were considered in the design strategy: (i) improving the aqueous solubility of the molecule and its pharmaceutical and clinical use, (ii) reinforcing the drug-target interaction, so as to consolidate its DNA-damaging activity, and (iii) conferring a selectivity for tumor cells via a novel delivery strategy. The third point is obviously essential, with a general applicability to transform conventional cytotoxic agents into what we call “targeted cytotoxics”, i.e., cytotoxic molecules endowed with a pronounced selectivity for cancer cells. At the molecular level, the concept consisted to link the pharmacophore to a polyamine guide providing simultaneously the desired three properties. A suitable connector was constructed to associate the two molecular entities, without affecting their intrinsic recognition properties. Polyamines, such as spermine, are highly water-soluble molecules. As cations, they binds to nucleic acid polymers, in particular DNA through minor groove interactions. Polyamine conjugates are usually potent DNA binders. But most importantly, polyamines and in particular the naturally-occurring ones (spermine, spermidine, putrescine), are essential for cell proliferation and differentiation. Their metabolism is frequently exacerbated in tumors. Different types of cancer cells heavily rely on polyamines for growth and survival. Most importantly, an efficient polyamine transport system (PTS) has been functionally characterized in many tumors cells. Polyamine transporters, import and export systems, are poorly defined at the molecular level in human, but the PTS is now well documented functionally. This polyamine-based strategy has open novel horizons to the rational design of topoisomerase II inhibitors, acting as potent DNA damaging drugs. Here we will present the strategy, the design and pharmacological properties of the lead compound in the series, designated F14512 (2). This compound exploits the PTS for entering and accumulating into tumor cells, functions as a potent topoisomerase II poison, triggers cell death and displays remarkable antitumor activities in vivo, in a large panel of xenografts models (2,3). This molecule is now entering clinical development. As far as we know, this is the first molecule exploiting the PTS for a tumor-selective delivery to reach clinical development in oncology. In addition, a functional screen has been set up to identify PTS(+) cancer cells, using fluorescent polyamine derivatives and cytometry assays (4). A clinical procedure has been designed to select patients with PTS(+) tumors eligible for treatment with F14512. This method is particularly well suited to identify leukemia cells expressing an active PTS. Along these lines, recently we showed that human acute myeloid leukemia (AML) animal models and human samples are sensitive to F14512, with a level of activity well superior to that of etoposide. Through this concept, we are now entering into the field of personalized medicine, with a novel generation of cell-targeted cytotoxic agents. The long history of topoisomerases inhibitors is thus revivified and novel opportunities to increase the therapeutic index of these agents can be envisioned. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):CN08-03.