Cephalostatin 1 Inactivates Bcl-2 by Hyperphosphorylation Independent of M-Phase Arrest and DNA Damage

Cephalostatins and ritterazines: Distinctive dimeric marine-derived steroidal pyrazine alkaloids with intriguing anticancer activities

Cephalostatins and ritterazines represent fascinating classes of dimeric marine derived steroidal alkaloids with unique chemical structures and promising biological activities. Originally isolated from marine tube worms and the tunicate Ritterella tokioka collected off the coast of Japan, cephalostatins and ritterazines display potent anticancer effects by inducing apoptosis, disrupting cell cycle progression, and targeting multiple molecular pathways. This review covers the chemistry and bioactivities of 45 cephalostatins and ritterazines from 1988 to 2024, highlighting their complex structures and medicinal contributions. With insights into their structure activity relationships (SAR). Key structural elements, such as the pyrazine ring and 5/6 spiroketal moieties, are found crucial for their biological effects, suggesting interactions with lipid membranes or hydrophobic protein domains. Additionally, the formation of oxocarbenium ions from spiroketal cleavage may enhance their potency by covalently modifying DNA. The pharmacokinetics, ADMET and Drug likeness properties of these steroidal alkaloids are thoroughly addressed. Drug likeness analysis shows that these compounds fit well with the Rule of 4 (Ro4) for Protein-Protein Interaction Drugs (PPIDs), underscoring their potential in this area. Ten compounds (20, 27, 33, 34, 39, 40, 41, 42, 43, and 45) have demonstrated favourable pharmacokinetic and ADMET profiles, making them promising candidates for further research. Future efforts should focus on alternative administration routes, structural modifications, and innovative delivery systems, such as prodrugs and nanoparticles, to improve bioavailability and therapeutic effects. Advances in synthetic chemistry, mechanistic insights, and interdisciplinary collaborations will be essential for translating cephalostatins and ritterazines into effective anticancer therapies.

Biological activity and apoptotic signaling pathway of C11-functionalized cephalostatin 1 analogues

Cephalostatin 1, a potent anti-cancer agent, is a natural bis-steroidal alkaloid that causes cell death in the subnanomolar to picomolar ranges via an atypical apoptosis pathway. Although cephalostatin 1 is a highly effective anticancer drug, its availability limits its utilization. We previously reported the synthesis of two 12'α-hydroxy derivatives of cephalostatin 1 that induce cell death by activating the ER stress apoptosis signaling pathway. For the current work, we synthesized six C₁₁-functionalized cephalostatin 1 analogues (CAs) to evaluate their biological activity. For the cytotoxic compounds, the induced apoptotic pathway was investigated. The C₁₁-functionalized cephalostatin 1 analogues 5 and 6 (CA5 and CA6) were found to exhibit cytotoxic activity against K-562 leukemia cells, MCF-7 breast cancer cells and DU-145 prostate cancer cells, while the remaining four analogues did not show anti-tumor activities against any of the cell lines. Our results indicated that CA5 and CA6 induced cell death via the atypical ER-dependent apoptosis pathway; they increased the expression of Smac/DIABLO, an inhibitor of inhibitors of apoptosis (IAPs), which in turn facilitated the activation of different caspases including the ER-caspase 4 without cytochrome c release from mitochondria. CA5 and CA6 are promising anticancer agents due to their low GI₅₀, the remarkable apoptosis pathway they induce which can overcome chemoresistance, and their very low toxicity to normal cells making them cephalostatin 1 utilizable alternatives.

Synthesis and bioactivity of bis-steroidal pyrazine 23-deoxy-25-epi ritterostatin GN1N

Cephalostatins, ritterazines and their hybrid bis-steroidal pyrazine analogs such as 25-epi-rittereostatin G_N1_N show unusually high potency against a wide range of cancer cell lines. Herein, we report the synthesis and bioactivity of 23-deoxy-25-epi ritterostatin G_N1_N, which lacks the 23-hydroxyl group of 25-epi rittereostatin G_N1_N. The less oxygenated bis-steroidal pyrazine was ∼50- to 1000-fold less potent than 25-epi ritterostatin G_N1_N, highlighting the importance of the 23-hydroxyl group for the antiproliferative activity of the cephalostatin/ritterazine class of drugs.

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.

Synthesis of 23-deoxy-25-epi north unit of cephalostatin 1 via reductive and oxidative modifications of hecogenin acetate

An efficient synthesis of the 23-deoxy-25-epi north unit of cephalostatin 1 has been achieved in 17 steps via reductive and oxidative functionalizations of hecogenin acetate with an overall yield of 3.8%. This synthesis features transetherification-mediated E-ring opening, D-ring oxidation, hemiketalization-mediated E-ring closure, and stereoselective 5/5-spiroketalization.

The Cephalostatins. 23. Conversion of Hecogenin to a Steroidal 1,6-Dioxaspiro[5.5]nonane Analogue for Cephalostatin 11

Cephalostatin 1 (1) has proved to be a remarkably potent cancer cell growth inhibitor. Since this steroidal alkaloid constituent of the marine worm Cephalodiscus gilchristi possesses a complex structure, providing preclinical supplies by total synthesis continues to be challenging. Therefore, syntheses of less complex structural modifications of this important pyrazine have also received substantial attention. Herein are summarized the synthesis of [5.5]spiroketal 5, a simplified right-side steroidal unit of 1, in seven steps from hecogenin acetate (11) with an overall yield of 4.6%. Consistent with other SAR studies, such reduction in structural complexity compared to 1 led to loss of cancer cell growth inhibitory activity against the P388 lymphocytic leukemia cell line.

Oxysterol-binding protein (OSBP)-related protein 4 (ORP4) is essential for cell proliferation and survival

Oxysterol-binding protein (OSBP) and OSBP-related proteins (ORPs) comprise a large gene family with sterol/lipid transport and regulatory activities. ORP4 (OSBP2) is a closely related paralogue of OSBP, but its function is unknown. Here we show that ORP4 binds similar sterol and lipid ligands as OSBP and other ORPs but is uniquely required for the proliferation and survival of cultured cells. Recombinant ORP4L and a variant without a pleckstrin homology (PH) domain (ORP4S) bind 25-hydroxycholesterol and extract and transfer cholesterol between liposomes. Two conserved histidine residues in the OSBP homology domain ORP4 are essential for binding phosphatidylinositol 4-phosphate but not sterols. The PH domain of ORP4L also binds phosphatidylinositol 4-phosphate in the Golgi apparatus. However, in the context of ORP4L, the PH domain is required for normal organization of the vimentin network. Unlike OSBP, RNAi silencing of all ORP4 variants (including a partial PH domain truncation termed ORP4M) in HEK293 and HeLa cells resulted in growth arrest but not cell death. ORP4 silencing in non-transformed intestinal epithelial cells (IEC)-18 caused apoptosis characterized by caspase 3 and poly(ADP-ribose) polymerase processing, DNA cleavage, and JNK phosphorylation. IEC-18 transformed with oncogenic H-Ras have increased expression of ORP4L and ORP4S proteins and are resistant to the growth-inhibitory effects of ORP4 silencing. Results suggest that ORP4 promotes the survival of rapidly proliferating cells.

Synthesis of three OSW-1 analogs with maltose side chains bearing different protection groups

In order to simplify the synthesis of OSW-1's disaccharide side chain and explore the structure-activity relationship of OSW-1, three 16α-O-maltose OSW-1 analogs carrying three maltose side chains bearing different protections were designed and synthesized.

Finding chemo: the search for marine-based pharmaceutical drugs active against cancer

OBJECTIVES

Cancer affects the health of many people globally. The most common treatment that is used for cancer is chemotherapy, which has shown promising results but not without side effects. Some of these side effects jeopardise further treatment, and this eventually leads to advanced stages of malignancy and mortality. As a result, there is a need for better and safer anticancer compounds such as those found naturally. One of the most abundant natural environments to find such compounds is the sea, and this vast resource has been biomined since the 1950s.

KEY FINDINGS

There are currently three marine anticancer agents marketed (Yondelis, Cytosar-U and Halaven), with several others undergoing clinical trials. This review discusses marine-derived products in clinical use and in clinical trials, and discusses available literature on the growth suppression or pro-apoptotic properties of these compounds, and the molecular mechanisms underpinning these cell biological phenomena.

SUMMARY

The marine environment may hold promising anticancer compounds within its depths, warranting further research to be performed in this area, albeit with respect for the natural ecosystems that are being explored for drug discover and subsequently used for drug development.

Synthesis of C17-OH-north unit of ritterazine G via "Red-Ox" modifications of hecogenin acetate

The C17-OH-north unit of ritterazine G was prepared in 13 steps from hecogenin acetate. This synthesis features a highly efficient and stereoselective introduction of the C17-OH via E-ring cleavage/F-ring formation, D-ring oxidation, and F-ring cleavage/E-ring formation.

Synthesis of 14',15'-dehydro-ritterazine Y via reductive and oxidative functionalizations of hecogenin acetate

An analog of ritterazine Y was synthesized from hecogenin acetate in 23 steps via functional group manipulations of hecogenin acetate. Preparation of the north G and south Y units and the late stage Guo-Fuchs asymmetric coupling of the both units afforded the ritterazine Y analog.

Cephalostatins and ritterazines

This review article is a tribute to the numerous chemists whose relentless effort for the last quarter of a century resulted in the isolation, identification, and finally the chemical synthesis of a family of bis-steroidal pyrazine alkaloids of marine origin. In the task of defeating cancer, the search for bioactive substances among the naturally occurring compounds is, without any doubt, a preferential approach. The remarkable contribution of Petitt, Fusetani, and their coworkers allowed to discover this family of marine alkaloids that emerge as potential therapeutic anticancer agents, although there is still a long way to go. The challenging and dangerous task of collecting living organisms from deep-waters was followed by a laborious isolation, elucidation of the complicated structures and biological tests. The outcome of this paramount effort was the identification of 45 compounds that stand, to date, as some of the most potent anticancer agents. The intriguing structures of the isolated alkaloids drew the attention of synthetic chemists, valiant enough to undertake the challenging task of synthesizing some of the most active members of the family. Fuchs, Heathcock, Winterfeldt, Suarez, Shair, and their associates pioneered in the establishment of feasible synthetic routes for the preparation of some of the naturally occurring compounds and a large number of synthetic analogs, allowing to establish SAR criteria that have guided the design of new synthetic analogs. Numerous analogs have been prepared to investigate the mechanism of action of bis-steroidal pyrazines, e.g. cephalostatin analogs bearing a strained spiroketal moiety. However, the mechanism of action and the biological target of these compounds remain far from being understood. Therefore, the rational design of simpler, yet highly active analogs seems at the current stage elusive. It is still 1 to clear why these compounds need to be dimeric to show high biological activity. Furthermore, it is not known whether the central pyrazine ring is simply a linker or has some additional function. This could be tested by examining the biological activity of steroidal dimers with other linkers, e.g. with a benzene ring. Such analogs have been actually prepared but without functional groups necessary for biological activity. The clinical trials of cephalostatins have got stuck due to a shortage of material. There is an urgent need to provide highly active, yet not too complex analogs, which could be available in substantial amounts for advanced pharmacological studies.

The cephalostatins. 22. synthesis of bis-steroidal pyrazine pyrones (1)

Cephalostatin 1 (1), a remarkably strong cancer cell growth inhibitory trisdecacyclic, bis-steroidal pyrazine isolated from the marine tube worm Cephalodiscus gilchristi, continues to be an important target for practical total syntheses and a model for the discovery of less complex structural modifications with promising antineoplastic activity. In the present study, the cephalostatin E and F rings were greatly simplified by replacement at C-17 with an α-pyrone (in 12), typical of the steroidal bufodienolides, and by a dihydro-γ-pyrone (in 16). The synthesis of pyrazine 12 from 5α-dihydrotestosterone (nine steps, 8% overall yield) provided the first route to a bis-bufadienolide pyrazine. Dihydro-γ-pyrone 16 was synthesized in eight steps from ketone 13. While only insignificant cancer cell growth inhibitory activity was found for pyrones 12 and 16, the results provided further support for the necessity of more closely approximating the natural D-F ring system of cephalostatin 1 in order to obtain potent antineoplastic activity.

Functional evaluation of a fluorescent schweinfurthin: mechanism of cytotoxicity and intracellular quantification

Schweinfurthins are potent inhibitors of cancer cell growth, especially against human central nervous system tumor lines such as SF-295 cells. However, the mechanisms through which these compounds impede cell growth are not fully understood. In an effort to understand the basis for the effects of schweinfurthins, we present a fluorescent schweinfurthin, 3-deoxyschweinfurthin B-like p-nitro-bis-stilbene (3dSB-PNBS), which displays biological activity similar to that of 3-deoxyschweinfurthin B (3dSB). These two schweinfurthins retain the unique differential activity of the natural schweinfurthins, as evidenced by the spindle-like morphological changes induced in SF-295 cells and the unaltered appearance of human lung carcinoma A549 cells. We demonstrate that incubation with 3dSB or 3dSB-PNBS results in cleavage of poly-ADP-ribose polymerase (PARP) and caspase-9, both markers of apoptosis. Coincubation of 3dSB or 3dSB-PNBS with the caspase-9 inhibitor (Z)-Leu-Glu(O-methyl)-His-Asp(O-methyl)-fluoromethylketone prevents PARP cleavage. Therapeutic agents that induce apoptosis often activate cellular stress pathways. A marker for multiple stress pathways is the phosphorylation of eukaryotic initiation factor 2α, which is phosphorylated in response to 3dSB and 3dSB-PNBS treatment. Glucose-regulated protein 78 and protein disulfide isomerase, both endoplasmic reticulum chaperones, are up-regulated with schweinfurthin exposure. Using the fluorescent properties of 3dSB-PNBS and dimethoxyphenyl-p-nitro-bis-stilbene (DMP-PNBS), a control compound, we show that the intracellular levels of 3dSB-PNBS are higher than those of Rhodamine 123 or DMP-PNBS in SF-295 and A549 cells.

Transetherification-mediated E-ring opening and stereoselective "Red-Ox" modification of furostan

We have developed a novel E-ring opening method for furostan, and applied it to prepare D-ring modified steroids, which can be used to synthesize cephalostatin analogs.

Enantioselective synthesis of (+)-cephalostatin 1

This Article describes an enantioselective synthesis of cephalostatin 1. Key steps of this synthesis are a unique methyl group selective allylic oxidation, directed C-H hydroxylation of a sterol at C12, Au(I)-catalyzed 5-endo-dig cyclization, and a kinetic spiroketalization.

Overexpression of Pygopus2 protects HeLa cells from vinblastine-induced apoptosis

Pygopus, a very important component of the Wnt signaling transcriptional complex, has multiple functions in both Wnt-dependent and -independent pathways. Human Pygopus2 (Pygo2) is expressed in many cancers and plays an important role in tumor growth. In the present study, we generated human carcinoma (HeLa) cell lines stably expressing Pygo2, which counteracts vinblastine- induced apoptosis. The anti-apoptotic function was determined by DNA fragmentation, sub-G1 appearance, loss of mitochondrial membrane potential (Deltapsim) and the activation of caspase-9 and caspase-3. In addition, we found that Pygo2 effectively blocks vinblastineinduced c-Jun and AP-1 activation, maintains the anti-apoptotic protein Bcl-2 in an unphosphorylated state, and thus can render cells resistant to apoptosis. However, Pygo2 does not alter the vinblastine-induced cell cycle changes. Here, we describe an anti-apoptotic activity exerted by Pygo2 through blocking activation of the JNK/AP-1 signaling pathway induced by vinblastine.

Role of Smac in cephalostatin-induced cell death

Cephalostatin 1 is a natural compound isolated from a marine worm that induces apoptosis in tumor cells via an apoptosome-independent but caspase-9-dependent pathway and through an endoplasmic reticulum stress response that is accompanied by caspase-4 activation. Here, we show that cephalostatin evokes mitochondrial Smac (second mitochondria-derived activator of caspases) but not cytochrome c release in various carcinoma cell lines. We also show that Smac is critically involved in caspase-9 activation as evidenced by gene silencing experiments. Remarkably, caspase-2 appears to be a major target for cephalostatin-induced cytosolic Smac. Using biochemical and genetic inhibition experiments, we demonstrate that caspase-2 participates in the apoptotic machinery induced by cephalostatin. Cephalostatin-activated caspase-2 appears to act as initiator caspase and is not involved in the activation of caspase-9. Importantly, experiments immunoprecipitating PIDD (p53-induced protein with a DD), RAIDD (RIP-associated ICH-1/CED-3-homologous protein with DD) and caspase-2 identify cephalostatin as an experimental drug that induces the formation of the PIDDosome. The bis-steroid cephalostatin proves to be both a helpful tool to investigate apoptotic signaling and a promising chemotherapeutic agent.

Asymmetric synthesis of naturally occurring spiroketals

Spiroketals are widely found as substructures of many naturally occurring compounds from diverse sources including plants, animals as well as microbes. Naturally occurring spiroketals are biologically active and most of them are chiral molecules. This article aims at reviewing the asymmetric synthesis of biologically active spiroketals for last 10 years (1998-2007).

Marine pharmacology in 2005-2006: antitumour and cytotoxic compounds

During 2005 and 2006, marine pharmacology research directed towards the discovery and development of novel antitumour agents was reported in 171 peer-reviewed articles. The purpose of this article is to present a structured review of the antitumour and cytotoxic properties of 136 marine natural products, many of which are novel compounds that belong to diverse structural classes, including polyketides, terpenes, steroids and peptides. The organisms yielding these bioactive marine compounds included invertebrate animals, algae, fungi and bacteria. Antitumour pharmacological studies were conducted with 42 structurally defined marine natural products in a number of experimental and clinical models which further defined their mechanisms of action. Particularly potent in vitro cytotoxicity data generated with murine and human tumour cell lines were reported for 94 novel marine chemicals with as yet undetermined mechanisms of action. Noteworthy is the fact that marine anticancer research was sustained by a global collaborative effort, involving researchers from Australia, Belgium, Benin, Brazil, Canada, China, Egypt, France, Germany, India, Indonesia, Italy, Japan, Mexico, the Netherlands, New Zealand, Panama, the Philippines, Slovenia, South Korea, Spain, Sweden, Taiwan, Thailand, United Kingdom (UK) and the United States of America (USA). Finally, this 2005-2006 overview of the marine pharmacology literature highlights the fact that the discovery of novel marine antitumour agents continued at the same active pace as during 1998-2004.

The cephalostatin way of apoptosis

The cephalostatins, bis-steroidal natural products from the marine tube worm Cephalodiscus gilchristi, were isolated by Dr. G. R. Pettit and his group. These compounds show a unique cytotoxicity profile in the in vitro screen of the National Cancer Institute, suggesting a novel mechanism of action. Indeed, cephalostatin 1 ( 1) is an extremely powerful agent that acts via an unusual apoptosis pathway. It induces selective Smac/DIABLO, but no cytochrome c release from mitochondria. Nevertheless, caspase-9 is required for apoptosis induction. Interestingly, caspase-9 is activated without the participation of the apoptosome, leading to the question of its mechanism of activation. We found that endoplasmic reticulum stress-associated caspase-4 contributes to nonclassical cephalostatin-mediated caspase-9 activation, additionally pointing out the unusual pathway used by this substance. Cephalostatin 1 ( 1), therefore, provides a very good tool to discover novel apoptotic pathways, which might be important in the understanding and treatment of chemo-resistant cancer.

Comprehensive study of okaspirodiol: characterization, total synthesis, and biosynthesis of a new metabolite from Streptomyces

The new spiro[4.5]acetal okaspirodiol (4) was isolated from Streptomyces sp. Gö TS 19 as a secondary metabolite in yields up to 380 mg/L. The structure of this cryptic ketotetrol was elucidated by different methods including X-ray analysis, and its equilibration under mildly acidic conditions furnishing three additional isomers was thoroughly studied. Although metabolite 4 is not the thermodynamically favored isomer, a high-yielding total synthesis was accomplished comprising a stereoselective spiroacetalization under equilibrium conditions. This approach benefits from the important influence of an intramolecular hydrogen bond on the stabilization of the spiro[4.5]acetal moiety. The biosynthesis of 4 was investigated by feeding experiments with 13C-labeled precursors proving its origin from a new type of the rare mixed acetate-glycerol biosynthetic pathway. All results are discussed on the basis of the structural diversity of spiroacetals in nature and their chemical properties.

Syntheses of the Eastern Halves of Ritterazines B, F, G, and H, Leading to Reassignment of the 5,5-Spiroketal Stereochemistry of Ritterazines B and F

The ritterazine class of natural products comprises 26 compounds-all of which are spiroketal-containing steroidal heterodimers-that inhibit the proliferation of cultured human cancer cell lines with IC50 values in the low nanomolar range. Little is known about their chemistry, cellular target(s), or mechanism(s) of growth inhibition, due primarily to the small amount of material available from natural sources. In this paper we report syntheses of the eastern halves of ritterazines B, F, G, and H and address the energetic and mechanistic aspects of spiroketal equilibration for each. These studies have led to reassignment of the 5,5-spiroketal stereochemistry of ritterazines B and F, and they have enabled us to propose a quantitative description of the natural distribution of these ritterazine compounds.

Computationally guided organometallic chemistry: preparation of the heptacyclic pyrazine core of ritterazine N

Diels-Alder cycloaddition of 10 followed by Wittig homologation and intramolecular diene cyclozirconation of the resulting triene under equilibrating conditions led to the tricyclic 6-6-5 ketone 5 with high diastereocontrol. The derived alpha-azido ketone 16 cyclized efficiently to the heptacyclic pyrazine core of ritterazine N.

Synthesis and structure-activity studies of schweinfurthin B analogs: Evidence for the importance of a D-ring hydrogen bond donor in expression of differential cytotoxicity

The synthesis and biological evaluation of several enantioenriched schweinfurthin B analogs were undertaken to develop structure-activity relationships and guide design of probes for their putative molecular target. The desired stilbenes contain a common left-half hexahydroxanthene ring system and an aromatic right-half with varied substituents. The synthesis involves penultimate Horner-Wadsworth-Emmons coupling of one of several right-half phosphonates with the aldehyde comprising the left-half of 3-deoxyschweinfurthin B. Preparation of the requisite phosphonates, and the respective stilbenes, as well as the cytotoxicity profiles of these new compounds in the National Cancer Institute's 60 cell-line anticancer screen is described. Several of these analogs displayed cytotoxicity patterns well-correlated with the natural product and differences in activity of approximately 10(3) across the various cell lines. Together, these assay results indicate the importance of at least one free phenol group on the aromatic D-ring of this system for differential cytotoxicity.