Fluorescent schweinfurthin B and F analogs with anti-proliferative activity

Abstract

The natural tetracyclic schweinfurthins are potent and selective inhibitors of cell growth in the National Cancer Institute’s 60 cell-line screen. At this time, the mechanism or cellular target that underlies this activity has not yet been identified, and efforts to illuminate the schweinfurthins’ mode of action would benefit from development of potent fluorescent analogs that could be readily visualized within cells. This report describes the synthesis of fluorescent analogs of schweinfurthins B and F, and demonstrates that these compounds retain the potent and differentially toxic activities against select human cancer cells that are characteristic of the natural schweinfurthins. In addition, the synthesis of control compounds that maintain parallel fluorescent properties, but lack the potent activity of the natural schweinfurthin is described. Use of fluorescence microscopy shows differences between the localization of the active and relatively inactive schweinfurthin analogs. The active compounds localize in peripheral puncta which may identify the site(s) of activity.

Introduction

Natural products have been a significant source of drug leads in oncology.¹ Indeed several commonly used anti-cancer agents are themselves natural products or are the result of drug development programs based upon a natural product. In some instances, natural products have identified a novel target or mechanism of action useful in treating human cancer. One such agent is paclitaxel, which was found to stabilize micro-tubules causing cell cycle arrest just prior to mitosis.² From this perspective, the discovery of the schweinfurthins and their potent anti-cancer activity at the National Cancer Institute (NCI) may offer a similar opportunity for discovery of a novel drug target and/or mechanism of action.³ Nine compounds are now part of the natural family, including schweinfurthin A–H and vedelianin (Fig. 1).4, 5, 6 Those that contain a hexahydroxanthene substructure, such as schweinfurthin A (1) and B (2), were found to have potent activity in the NCI 60 cell-line screen. Perhaps of even greater importance, the pattern of activity against the 60 cell lines indicated a potentially novel molecular target or mechanism of activity.7, 8 Due to the scarcity of the natural schweinfurthins, we have had an ongoing program aimed at synthesis of natural schweinfurthins and preparation of analogs. This effort has culminated in the reported syntheses of schweinfurthin B (2),⁹ C (3),¹⁰ E (6),⁹ F (7),¹¹ and G (8),¹² as well as the lead compound 3-deoxyschweinfurthin B (3dSB, 10) and more than 50 other analogs.14, 15, 16, 17, 18, 19

Recently the bis-stilbene 11 was prepared as a fluorescent analog of 3dSB (10), with the hope that this probe would aid in elucidation of the mechanism of action of these natural products.¹⁶ Unfortunately, the fluorescence emission maximum of this compound (416 nm) displayed significant overlap with autofluorescence of the SF-295 human-derived glioma cell line, and the compound demonstrated a rapid loss of fluorescence upon irradiation. To overcome these limitations and improve the fluorescent characteristics of potential probes, further synthetic efforts based on this core structure have been undertaken. Here the design, synthesis, and initial biological results of these studies will be reported, along with preparation of some related compounds with similar fluorescence properties and less potent biological activity to serve as controls.