Synthesis, Characterization, Mechanism of Decomposition, and Antiproliferative Activity of a Class of PEGylated Benzopolysulfanes Structurally Similar to the Natural Product Varacin

Abstract

Benzopolysulfanes, 4-CH3(OCH2CH2)3NHC(O)-C6H4-1,2-Sx (x = 3–7, and 9) were synthesized with a PEG group attached through an amide bond and examined for water solubility, antitumor activity, and propensity to equilibrate and desulfurate. LCMS and HPLC data show the PEG pentasulfane ring structure predominates, and the tri-, tetra-, hexa-, hepta-, and nonasulfanes were present at very low concentrations. The presence of the PEG group improved water solubility by 50-fold compared to the unsubstituted benzopolysulfanes, C6H4Sx (x = 3, 5, and 7), based on intrinsic solubility measurements. Polysulfur linkages in the PEG compounds decomposed in the presence of ethanethiol and hydroxide ion. The PEG pentathiepin desulfurated rapidly and an S3 transfer reaction was observed in the presence of norbornene, no S2 transfer reaction was observed with 2,3-dimethylbutadiene. The antitumor activities of the PEG-substituted benzopolysulfane mixtures were analyzed against four human tumor cell lines PC3 (prostate), DU145 (prostate), MDA-MB-231 (breast), and Jurkat (T-cell leukemia). The PEG conjugated polysulfanes had IC50 values 1.2–5.8 times lower than the parent “unsubstituted” benzopolysulfanes. Complete cell killing was observed for the PEG polysulfanes with 4 μM for PC3 and DU145 cells, and with 12 μM for MDA-MB-231 cells. The results suggest that solubilization of the polysulfur linkage is a key parameter to the success of these compounds as drug leads.

Synthesis and antiproliferative properties of a new ceramide analog of varacin

“…The synthesis of 4-nitrophenyl benzo[ f ][1,2,3,4,5]pentathiepine-7-carboxylate 9 was carried out as described before (Mahendran et al, 2010), using 3,4-dihydroxybenzoic acid ( 8 ) as a starting material, as well as the use of thiostannole and disulfur dichloride chemistry (Lienard et al 2007; Ogawa et al, 1994; Konstantinova and Rakitin, 2014) (Scheme 2). Briefly, compound 9 was synthesized in seven steps in 2.8% yield, and was reacted with D -erythro-sphingosine (0.035 mmol) in THF to form conjugate 1 .…”

“…Our data on 1 were compared to literature data on antiproliferative effects of 1,2-benzenediothiol 5 , benzopolysulfanes 6 and 7 against DU145 and MDA-MB-231 cells (Mahendran et al, 2010). The comparisons are shown in Figure 2 for DU145 cells and in Figure 3 for MDA-MB-231 cells.…”

“…D -erythro-sphingosine was obtained from Avanti Polar Lipids (Alabaster, AL). Compound 9 was synthesized as described in our previous paper (Mahendran et al, 2010). Purification of the product mixtures was carried out by column chromatography using silica gel with particle sizes of 40–60 Å. TLC was carried out using silica gel 60F 254 TLC-plates (Sigma-Aldrich, St. Louis, MO, USA).…”

“…Cell proliferation and cytotoxicity was assessed by cell proliferation assays as previously described (Bittman et al 2007, Mahendran et al, 2010). Cells were seeded in a 96-well plate at 2000 cells/well density (200 μL of the growth medium per well) and maintained in 5% CO 2 incubator at 37 °C until they were in log phase growth.…”

“…To ameliorate the low water solubility problem, in 2010, Bittman et al synthesized benzopolysulfanes 4-CH 3 (OCH 2 CH 2 ) 3 NHC(O)-C 6 H 4 -1,2-S x (x = 3–7 and 9) with a PEG group attached through an amide bond, where the pentasulfane 7 was the major constituent (Mahendran et al, 2010). Antiproliferative activity of the PEGylated benzopolysulfanes in cancer cells increased in comparison to the parent-unsubstituted benzopolysulfane ( 6 ) due to the PEG substitution.…”