Antitumor agent 25-epi Ritterostatin GN1N induces endoplasmic reticulum stress and autophagy mediated cell death in melanoma cells.

Metastatic melanoma is the most aggressive of all skin cancers and is associated with poor prognosis owing to lack of effective treatments. 25-epi Ritterostatin GN1N is a novel antitumor agent with yet undefined mechanisms of action. We sought to delineate the antitumor mechanisms of 25-epi Ritterostatin GN1N in melanoma cells to determine the potential of this compound as a treatment for melanoma. Activation of the endoplasmic reticulum (ER) stress protein glucose-regulated protein 78 (GRP78) has been associated with increased melanoma progression, oncogenic signaling, drug resistance, and suppression of cell death. We found that 25-epi Ritterostatin GN1N induced cell death in melanoma cells at nanomolar concentrations, and this cell death was characterized by inhibition of GRP78 expression, increased expression of the ER stress marker CHOP, loss of mitochondrial membrane potential, and lipidation of the autophagy marker protein LC3B. Importantly, normal melanocytes exhibited limited sensitivity to 25-epi Ritterostatin GN1N. Subsequent in vivo results demonstrated that 25-epi Ritterostatin GN1N reduced melanoma growth in mouse tumor xenografts and did not affect body weight, suggesting minimal toxicity. In summary, our findings indicate that 25-epi Ritterostatin GN1N causes ER stress and massive autophagy, leading to collapse of mitochondrial membrane potential and cell death in melanoma cells, with minimal effects in normal melanocytes. Thus, 25-epi Ritterostatin GN1N is a promising anticancer agent that warrants further investigation.

Ritterostatin GN 1N , a Cephalostatin-Ritterazine Bis-steroidal Pyrazine Hybrid, Selectively Targets GRP78.

Natural products discovered by using agnostic approaches, unlike rationally designed leads or those obtained through high-throughput screening, offer the ability to reveal new biological pathways and, hence, serve as an important vehicle to unveil new avenues in drug discovery. The ritterazine-cephalostatin family of natural products displays robust and potent antitumor activities, with sub-nanomolar growth inhibition against multiple cell lines and potent activity in xenograft models. Herein, we used comparative cellular and molecular biological methods to uncover the ritterazine-cephalostatin cytotoxic mode of action (MOA) in human tumor cells. Our findings indicated that, whereas ritterostatin GN 1N , a cephalostatin-ritterazine hybrid, binds to multiple HSP70s, its cellular trafficking confines activity to the endoplasmic reticulum (ER)-based HSP70 isoform, GRP78. This targeting results in activation of the unfolding protein response (UPR) and subsequent apoptotic cell death.

Quantitative analysis of F-actin redistribution in astrocytoma cells treated with candidate pharmaceuticals.

Actin fibers (F-actin) control the shape and internal organization of cells, and generate force. It has been long appreciated that these functions are tightly coupled, and in some cases drive cell behavior and cell fate. The distribution and dynamics of F-actin is different in cancer versus normal cells and in response to small molecules, including actin-targeting natural products and anticancer drugs. Therefore, quantifying actin structural changes from high resolution fluorescence micrographs is necessary for further understanding actin cytoskeleton dynamics and phenotypic consequences of drug interactions on cells. We applied an artificial neural network algorithm, which used image intensity and anisotropy measurements, to quantitatively classify F-actin subcellular features into actin along the edges of cells, actin at the protrusions of cells, internal fibers and punctate signals. The algorithm measured significant increase in F-actin at cell edges with concomitant decrease in internal punctate actin in astrocytoma cells lacking functional neurofibromin and p53 when treated with three structurally-distinct anticancer small molecules: OSW1, Schweinfurthin A (SA) and a synthetic marine compound 23'-dehydroxycephalostatin 1. Distinctly different changes were measured in cells treated with the actin inhibitor cytochalasin B. These measurements support published reports that SA acts on F-actin in NF1(-/-) neurofibromin deficient cancer cells through changes in Rho signaling. Quantitative pattern analysis of cells has wide applications for understanding mechanisms of small molecules, because many anti-cancer drugs directly or indirectly target cytoskeletal proteins. Furthermore, quantitative information about the actin cytoskeleton may make it possible to further understand cell fate decisions using mathematically testable models.

A convergent total synthesis of the potent cephalostatin/ritterazine hybrid -25-epi ritterostatin GN1N.

The convergent synthesis of 25-epi ritterostatin GN1N is described for the first time, starting from hecogenin acetate (HA). Stereoselective dihydroxylation employing the chiral ligand (DHQ)2PHAL was used as the key step to introduce the C25 epi-stereocenter on the north 1 segment. The title compound was obtained through a coupling reaction between the C3-keto-azide (cstat North 1) and North G.

A general approach to the synthesis of enantiopure 19-nor-vitamin D3 and its C-2 phosphate analogs prepared from cyclohexadienyl sulfone.

A synthesis of enantiopure 19-nor-Vitamin D(3) and its C-2 substituted cyclic phosphate analogs is achieved via in situ trapping of an α-sulfonyl anion with a CD-ring allyl chloride and 1,2-eliminative desulfonylation exploiting the basic properties of TBAF. The A-ring is prepared via anti-selective dithiane addition to vinyl sulfone and LiBH(4) mediated sequential bis reduction of an epoxy vinyl sulfone.

Intramolecular methylation of an allyl sulfone via lithium alkoxyaluminate; application to the enantioselective synthesis of the CD ring of vitamin D3.

Alcohol-directed intramolecular methylation of an enantiopure allyl sulfone using AlMe(3) provides a trans-hydrindane CD ring alcohol. The substrate cis-CD ring allyl sulfone alcohol is prepared via intramolecular allyl sulfonyl anion addition to aldehyde using Ba(OH)(2).

Jacobsen protocols for large-scale epoxidation of cyclic dienyl sulfones: application to the (+)-pretazettine core.

A Jacobsen epoxidation protocol using H2O2 as oxidant was designed for the large-scale preparation of various epoxy vinyl sulfones. A number of cocatalysts were screened, and pH control led to increased reaction rate, higher turnover number, and improved reliability.

Fluoride-mediated elimination of allyl sulfones: application to the synthesis of a 2,4-dimethyl-A-ring vitamin D3 analogue.

A coupling strategy for the synthesis of 2,4-dimethyl-1α,25(OH)(2)D(3) is achieved which involves methylation of a pro-A ring vinyl sulfone and in situ traping of the allyl sulfonyl anion with a CD ring allyl chloride. TBAF-promoted 1,2-eliminative desulfonylation and concomitant silyl ether deprotection gives the vitamin D(3) analogue.

Synthesis of the C1-C20 and C15-C27 segments of aplyronine A.

The synthesis of C1-C20 and C15-C27 segments of Aplyronine A is described. Oxidative cleavage of cyclic vinyl sulfones has been used to prepare key fragments of Aplyronine A. Key precursors are united by Horner-Wadsworth-Emmons and Julia-Kociensky olefination for the respective elaboration of the C1-C20 and C15-C27 segments.

Synthesis and evaluation of a fluorescent ritterazine-cephalostatin hybrid.

The cephalostatin and ritterazine natural products comprise a potent family of bis-steroidal pyrazines that display potent single-digit nanomolar inhibition of tumor cell growth. An active fluorescent ritterazine-cephalostatin hybrid probe was developed using detailed SAR data derived through total synthetic efforts. A combination of time course and confocal imaging studies indicate that this natural product family is rapidly taken up in tumor cells and localizes subcellularly within ER and surrounding the nuclear-ER interface.

Studies on the synthesis of apoptolidin: synthesis of a C1-C27 fragment of apoptolidin D.

Synthesis of a C(1)-C(27) fragment, a key intermediate in the synthesis of apoptolidin D, is reported. The synthesis involves a combination of Heck coupling and Horner-Wadsworth-Emmons reaction for the C(1)-C(7) trienoate portion and an efficient Suzuki cross-coupling protocol for the C(10)-C(13) diene portion.

Noteworthy observations accompanying synthesis of the apoptolidin disaccharide.

A stereoselective synthesis of the apoptolidin disaccharide is reported. The key chemistry features a new transformation utilizing a highly selective tetramethylalkoxyalanate[v]-directed syn-methylation of a vinylogous ester, isolation of a hydrate of a 2-keto sugar, an eco-friendly radical cleavage of a bromomethyl group, and an efficient preparation of a fluorodisaccharide via the use of XtalFluor-E.

S(N)2' addition/1,2-elimination of dimethylsulfonium methylide with epoxy vinyl sulfones: synthesis of exocyclic cross-conjugated dienyl sulfones.

Dimethylsulfonium methylide undergoes S(N)2' addition/1,2-elimination with epoxy vinyl sulfones to generate enantiopure six and seven membered cross-conjugated hydroxy vinyl sulfones. Moderate to excellent yields were obtained for both six and seven membered substrates.

Reagent-directed allylic quadraselection. Chemoselective anti- and syn-Lawton S(N)2' methylation of seven-membered epoxyvinylsulfones.

Methods have been developed for regio- and stereoselective 1,4-syn or 1,4-anti methylation of seven-membered epoxyvinylsulfones. 1,4-Syn addition is achieved via the combination of Me(2)Zn and catalytic Li(2)CuCl(4), a hitherto unexplored reagent combination. The complementary 1,4-anti addition relies on Cu(I) catalyzed methyl Grignard addition or (CH(3))(3)Al assisted CH(3)Cu addition. The methods described were assayed on four diastereomeric stereodiads and on their parent epoxide.

Synthesis of C14,15-dihydro-C22,25-epi north unit of cephalostatin 1 via "red-ox" modifications of hecogenin acetate.

The C14,15-dihydro-C22,25-epi north unit of cephalostatin 1 has been synthesized in 11 operations from commercially available hecogenin acetate via multiple reductions and oxidations. The key transformations include (i) Cr(VI)-catalyzed E-ring opening, (ii) C17 hydroxylation, and (iii) a base-triggered cyclization cascade.

Second-generation synthesis of syn- and anti-cycloheptadienylsulfone polyketide stereodiads.

Stereodiad sulfones 18a and 18b are key intermediates for polyketide synthesis. This note describes the synthesis of 18a and 18b from enantiopure epoxide 2. The two sequences have been optimized for large-scale synthesis to give 80-85% overall yields in one operation (one operation implies that no crystallization or distillation is required throughout the synthesis) while avoiding chromatography.

Conversion of cyclic vinyl sulfones to transposed vinyl phosphonates.

Functionalized cyclic vinyl sulfones were directly converted to the "polarity reversed" vinyl phosphonates through an efficient one pot procedure. Ozonolysis of these vinyl sulfones and vinyl phosphonates furnish complementary sets of termini-differentiated ester-aldehydes. This strategy has been applied for preparation of segments needed for the synthesis of Aplyronine A. The scope and limitations of this transformation were defined.

Lactol-directed osmylation. Stereodivergent synthesis of four C-19,20 apoptolidin diols from a single allylic hemiacetal.

A synthetic approach to prepare four Apoptolidin C-19,20 diastereomeric diol derivatives was developed. Two diastereomers were obtained from the (Z)-form, which is converted to the (E)-form, followed by dihydroxylation to deliver two more diastereomers. The (E)-allylic hemiacetal and methoxyacetal showed opposite diastereoselectivity.