Leiodermatolide, a Potent Antimitotic Macrolide from the Marine Sponge Leiodermatium sp.

Abstract

Marine macrolides that selectively disrupt cell cycle events continue to occupy a central position as lead compounds in the ongoing search for novel anticancer agents, [1,2] highlighted by the recent FDA approval of Halaven (eribulin mesylate, a fully synthetic analogue of the halichondrins) for the treatment of advanced breast cancer. [3] Lithistid sponges have proven to be a particularly fertile source [4] of such biologically relevant polyketide metabolites, including dictyostatin [5] and discodermolide. [5d, 6] As part of a continued program aimed at the discovery of novel bioactive natural products from deepwater marine invertebrates, we have examined the relatively unexplored [7] lithistid sponge Leiodermatium. A crude extract of Leiodermatium sp. was found to exhibit substantial activity in an assay which identifies antimitotic agents through detection of phosphonucleolin, a marker of mitosis. [8] Bioassay-guided fractionation led to the isolation of leiodermatolide (1, Figure 1), whose unprecedented 16-membered macrolide skeleton, featuring an unsaturated side chain terminating in a d-lactone, has been elucidated through a combination of extensive NMR spectroscopic analysis, comparative DFT GIAO NMR shift calculations, and molecular modeling. Leiodermatolide was found to exhibit potent and selective antimitotic activity (IC 50 < 10 nm) against a range of human cancer cell lines by inducing G2/M cell cycle arrest, and represents a promising new lead for anticancer drug discovery.A specimen of Leiodermatium (1.04 kg) was collected at a depth of 401 m off the coast of Fort Lauderdale, Florida, using the Johnson Sea Link submersible. The frozen sponge was exhaustively extracted using a mixture of EtOAc/EtOH (9:1) followed by partitioning of the dried extract residue between EtOAc and H 2 O. Vacuum column chromatography of the organic partition on a silica gel stationary phase followed by reverse-phase HPLC of the active fraction led to the isolation of leiodermatolide (11.8 mg, 0.0011 % wet weight) as an amorphous white powder with ½a 24 D ¼À84.2 (c = 0.34, MeOH). [9] HRDART MS analysis of leiodermatolide (1) provided a [M+H] + ion at m/z 602.3705, appropriate for a molecular formula of C 34 H 51 NO 8 (calcd for [M+H], 602.3693, D = 1.2 mmu), requiring 10 sites of unsaturation. [10] Optimum NMR signal dispersion was realized in CD 2 Cl 2 ; inspection of the 13 C NMR spectrum revealed a total of 34 resonances in agreement with the HRMS formula. 13 C NMR resonances attributable to two ester groups (d C = 172.4, 170.4 ppm), one carbamate (d C = 157.6 ppm), and 10 olefinic carbons (d C = 137.9, 137.5, 134.2, 131.8, 130.0, 128.8, 128.5, 126.4, 125.9, 124.7 ppm) accounted for eight of the 10 sites of unsaturation, suggesting the presence of two rings.Interpretation of the 2D DQF-COSY spectrum coupled with the edited g-HSQC spectrum led to the assignment of seven isolated spin systems (A-G, Figure 2). These spin systems and their connectivity with the remaining atoms enabled assembly into the final planar struct...

Total Synthesis of the Antimitotic Marine Macrolide (−)‐Leiodermatolide

“…This unique structure incorporates a total of nine stereocenters. In association with the Wright research group,[4b] we elucidated the relative configuration of leiodermatolide by using a combination of homo- and heteronuclear NMR spectroscopic analysis, molecular modeling, and computational DP4 NMR prediction. [8] The resulting assignment for the C1–C16 region was further supported by our synthesis of a macrocyclic fragment with a truncated side chain,[5] whereas an alternative stereostructure could be ruled out on the basis of synthetic studies reported earlier.…”

“…[22] The NMR spectroscopic data for 20 matched well with those for the δ-lactone ring of leiodermatolide, and the stereochemical assignment was further confirmed by single-crystal X-ray crystallography. [4b], [23] Silylation (TMSCl) then afforded 6 (97 %), in readiness for the key Heck reaction. After some experimentation, the exposure of 5 and 6 to Pd(OAc) 2 (10 mol %) and Ag 2 CO 3 in DMF at 80 °C[24] generated adduct 21 (73 %) with exclusively the required 16 E ,18 E geometry.…”

Divergent Total Synthesis of the Antimitotic Agent Leiodermatolide

“…[3] It is against this backdrop that the recent discovery of leiodermatolide has to be seen. [5] Therefore, this particular natural product seems to operate through a mechanism distinct from that of the established antimitotic agents such as vincristine, colchicine, paclitaxel, epothilone, and discodermolide, to mention only the most prominent ones, [1,2] and hence warrants close chemical, biochemical, and, possibly, pharmacological inspection. [6] This concentration also sufficed to effect cell-cycle arrest of the highly responsive A549 lung adenocarcinoma and PANC-1 pancreatic tumor cells at the G2M phase transition, accompanied by abnormal spindle formations in both cell types.…”

“…[5] Approximately tenfold lower concentrations were detected in specimens harvested in the Bahamas, [7] whereas an earlier study on Leiodermatium sponges collected off Palau had only given structurally unrelated secondary metabolites of mixed polyketide/nonribosomal peptide synthetase origin ("leiotolides"). [5] Approximately tenfold lower concentrations were detected in specimens harvested in the Bahamas, [7] whereas an earlier study on Leiodermatium sponges collected off Palau had only given structurally unrelated secondary metabolites of mixed polyketide/nonribosomal peptide synthetase origin ("leiotolides").…”

“…[5] Approximately tenfold lower concentrations were detected in specimens harvested in the Bahamas, [7] whereas an earlier study on Leiodermatium sponges collected off Palau had only given structurally unrelated secondary metabolites of mixed polyketide/nonribosomal peptide synthetase origin ("leiotolides"). [5] However, the segregated stereoclusters within the macrolac-tone [10,11] and the d-lactone terminus could not be correlated with each other, thus leaving it open as to whether structure 1 or 2 represents the natural product (Scheme 1). [5] However, the segregated stereoclusters within the macrolac-tone [10,11] and the d-lactone terminus could not be correlated with each other, thus leaving it open as to whether structure 1 or 2 represents the natural product (Scheme 1).…”