OSW saponins: facile synthesis toward a new type of structures with potent antitumor activities

Advances in antitumor activity and mechanism of natural steroidal saponins: A review of advances, challenges, and future prospects

BACKGROUND

Cancer, the second leading cause of death worldwide following cardiovascular diseases, presents a formidable challenge in clinical settings due to the extensive toxic side effects associated with primary chemotherapy drugs employed for cancer treatment. Furthermore, the emergence of drug resistance against specific chemotherapeutic agents has further complicated the situation. Consequently, there exists an urgent imperative to investigate novel anticancer drugs. Steroidal saponins, a class of natural compounds, have demonstrated notable antitumor efficacy. Nonetheless, their translation into clinical applications has remained unrealized thus far. In light of this, we conducted a comprehensive systematic review elucidating the antitumor activity, underlying mechanisms, and inherent limitations of steroidal saponins. Additionally, we propose a series of strategic approaches and recommendations to augment the antitumor potential of steroidal saponin compounds, thereby offering prospective insights for their eventual clinical implementation.

PURPOSE

This review summarizes steroidal saponins' antitumor activity, mechanisms, and limitations.

METHODS

The data included in this review are sourced from authoritative databases such as PubMed, Web of Science, ScienceDirect, and others.

RESULTS

A comprehensive summary of over 40 steroidal saponin compounds with proven antitumor activity, including their applicable tumor types and structural characteristics, has been compiled. These steroidal saponins can be primarily classified into five categories: spirostanol, isospirostanol, furostanol, steroidal alkaloids, and cholestanol. The isospirostanol and cholestanol saponins are found to have more potent antitumor activity. The primary antitumor mechanisms of these saponins include tumor cell apoptosis, autophagy induction, inhibition of tumor migration, overcoming drug resistance, and cell cycle arrest. However, steroidal saponins have limitations, such as higher cytotoxicity and lower bioavailability. Furthermore, strategies to address these drawbacks have been proposed.

CONCLUSION

In summary, isospirostanol and cholestanol steroidal saponins demonstrate notable antitumor activity and different structural categories of steroidal saponins exhibit variations in their antitumor signaling pathways. However, the clinical application of steroidal saponins in cancer treatment still faces limitations, and further research and development are necessary to advance their potential in tumor therapy.

A useful strategy for synthesis of the disaccharide of OSW-1

A flexible, efficient, and practical synthesis route was developed to synthesize an OSW-1 disaccharide. The synthesis took 13 steps from l-arabinose and d-xylose derivatives, and the overall yield was 7.2%. The region preferentially protects various d-xylose hydroxides because the TBS group selectively reacts with this hydroxide at low concentrations due to greater activity at the C-4 hydroxyl of d-xylose. Then, high efficiency selectively protects C-2 hydroxyl and C-3 hydroxyl of d-xylose, respectively. The first high yield of glycosylation on an OSW-1 synthesis disaccharide was achieved by taking sulfide donor 4 with β-PMP anomeric l-arabinose acceptor 12. The cytotoxicity reveals that the analogy has a high IC50 for a variety of cell types. This approach should provide a versatile way to modify OSW-1's disaccharide.

Chemical Synthesis of Saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Anticancer Effects and Mechanisms of OSW-1 Isolated From Ornithogalum saundersiae: A Review

For centuries, cancer has been a lingering dark cloud floating on people's heads. With rapid population growth and aging worldwide, cancer incidence and mortality are growing rapidly. Despite major advances in oncotherapy including surgery, radiation and chemical therapy, as well as immunotherapy and targeted therapy, cancer is expected be the leading cause of premature death in this century. Nowadays, natural compounds with potential anticancer effects have become an indispensable natural treasure for discovering clinically useful agents and made remarkable achievements in cancer chemotherapy. In this regards, OSW-1, which was isolated from the bulbs of Ornithogalum saundersiae in 1992, has exhibited powerful anticancer activities in various cancers. However, after almost three decades, OSW-1 is still far from becoming a real anticancer agent for its anticancer mechanisms remain unclear. Therefore, in this review we summarize the available evidence on the anticancer effects and mechanisms of OSW-1 in vitro and in vivo, and some insights for researchers who are interested in OSW-1 as a potential anticancer drug. We conclude that OSW-1 is a potential candidate for anticancer drugs and deserves further study.

Synthesis and antiproliferative activities of OSW-1 analogues bearing 2-acylamino-xylose residues

We synthesized 38 OSW-1 analogues with 2-acylamino xylose residues and found that the antitumor activities could be greatly enhanced.

Synthesis of a fluorescent photoaffinity probe of OSW-1 by site-selective acylation of an inactive congener and biological evaluation

A novel fluorescent photoaffinity probe of OSW-1 was prepared in two steps from a naturally occurring inactive congener by a sequential site-selective acylation strategy using Me₂SnCl₂. It displayed highly potent anticancer activity and a similar intracellular localization property to that of a fluorescently-tagged OSW-1, thereby demonstrating its potential utility in live cell studies.

Influence of intramolecular hydrogen bonds on regioselectivity of glycosylation. Synthesis of lupane-type saponins bearing the OSW-1 saponin disaccharide unit and its isomers

A series of lupane-type saponins bearing OSW-1 disaccharide unit as well as its regio- and stereoisomers were prepared and used for the structure-activity relationships (SAR) study. Unexpected preference for 1→4-linked regioisomers and an unusual inversion of the conformation of the sugar rings were noted. Cytotoxic activity of new lupane compounds was evaluated in vitro and revealed that some saponins exhibited an interesting bioactivity profile against human cancer cell lines. Influence of the protecting groups on the cytotoxicity was investigated. These results open the way to the synthesis of various lupane-type triterpene and saponin derivatives as potential anticancer compounds.

Chemical synthesis of saponins

Saponins are a large family of amphiphilic glycosides of steroids and triterpenes found in plants and some marine organisms. By expressing a large diversity of structures on both sugar chains and aglycones, saponins exhibit a wide range of biological and pharmacological properties and serve as major active principles in folk medicines, especially in traditional Chinese medicines. Isolation of saponins from natural sources is usually a formidable task due to the microheterogeneity of saponins in Nature. Chemical synthesis can provide access to large amounts of natural saponins as well as congeners for understanding their structure-activity relationships and mechanisms of action. This article presents a comprehensive account on chemical synthesis of saponins. First highlighted are general considerations on saponin synthesis, including preparation of aglycones and carbohydrate building blocks, assembly strategies, and protecting-group strategies. Next described is the state of the art in the synthesis of each type of saponins, with an emphasis on those representative saponins having sophisticated structures and potent biological activities.

Fluorescent analog of OSW-1 and its cellular localization

OSW-1 is a steroidal saponin, which has emerged as an attractive anticancer agent with highly cancer cell selective activity. A fluorescent analog was prepared from the natural product to analyze its cellular uptake and localization. We found that the fluorescent analog is rapidly internalized into cells and is primarily distributed in endoplasmic reticulum and Golgi apparatus.

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.

Open-chain steroidal glycosides, a diverse class of plant saponins

Saponins are an important class of plant natural products that consist of a triterpenoid or steroidal skeleton that is glycosylated by varying numbers of sugar units attached at different positions. Steroidal saponins are usually divided into two broad structural classes, namely spirostanol and furostanol saponins. A third, previously unrecognized structural class of plant saponins, the open-chain steroidal saponins, is introduced in this review; these possess an acyclic sidechain in place of the heterocyclic ring/s present in spirostanols and furostanols. Open-chain steroidal saponins are numerous and structurally diverse, with over 150 unique representatives reported from terrestrial plants. Despite this, they have to date been largely overlooked in reviews of plant natural products. This review catalogs the structural diversity of open-chain steroidal saponins isolated from terrestrial plants and discusses aspects of their structure elucidation, biological activities, biosynthesis, and distribution in the plant kingdom. It is intended that this review will provide a point of reference for those working with open-chain steroidal saponins and result in their recognition and inclusion in future reviews of plant saponins.

Effective killing of leukemia cells by the natural product OSW-1 through disruption of cellular calcium homeostasis

3β,16β,17α-Trihydroxycholest-5-en-22-one 16-O-(2-O-4-methoxybenzoyl-β-D-xylopyranosyl)-(1→3)-2-O-acetyl-α-L-arabinopyranoside (OSW-1) is a natural product with potent antitumor activity against various types of cancer cells, but the exact mechanisms of action remain to be defined. In this study, we showed that OSW-1 effectively killed leukemia cells at subnanomolar concentrations through a unique mechanism by causing a time-dependent elevation of cytosolic Ca(2+) prior to induction of apoptosis. A mechanistic study revealed that this compound inhibited the sodium-calcium exchanger 1 on the plasma membrane, leading to an increase in cytosolic Ca(2+) and a decrease in cytosolic Na(+). The elevated cytosolic Ca(2+) caused mitochondrial calcium overload and resulted in a loss of mitochondrial membrane potential, release of cytochrome c, and activation of caspase-3. Furthermore, OSW-1 also caused a Ca(2+)-dependent cleavage of the survival factor GRP78. Inhibition of Ca(2+) entry into the mitochondria by the uniporter inhibitor RU360 or by cyclosporin A significantly prevented the OSW-1-induced cell death, indicating the important role of mitochondria in mediating the cytotoxic activity. The extremely potent activity of OSW-1 against leukemia cells and its unique mechanism of action suggest that this compound may be potentially useful in the treatment of leukemia.

Assembly of naturally occurring glycosides, evolved tactics, and glycosylation methods

Glycosylation of proteins and lipids is critical to many life processes. Secondary metabolites (or natural products), such as flavonoids, steroids, triterpenes, and antibiotics, are also frequently modified with saccharides. The resulting glycosides include diverse structures and functions, and some of them have pharmacological significance. The saccharide portions of the glycosides often have specific structural characteristics that depend on the aglycones. These molecules also form heterogeneous "glycoform" mixtures where molecules have similar glycosidic linkages but the saccharides vary in the length and type of monosaccharide unit. Thus, it is difficult to purify homogeneous glycosides in appreciable amounts from natural sources. Chemical synthesis provides a feasible access to the homogeneous glycosides and their congeners. Synthesis of a glycoside involves the synthesis of the aglycone, the saccharide, the connection of these two parts, and the overall manipulation of protecting groups. However, most synthetic efforts to date have focused on the aglycones, treating the attachment of saccharides onto the aglycones as a dispensable topic. The synthesis of the aglycone and the synthesis of the saccharide belong to two independent categories of chemistry, and different types of the aglycones and saccharides pose as specific synthetic subjects in their own disciplines. The only reaction that integrates the broad chemistry of glycoside synthesis is the glycosidic bond formation between the saccharide and the aglycone. Focusing on this glycosylation reaction in this Account, we string together our experience with the synthesis of the naturally occurring glycosides. We briefly describe the synthesis of 18 glycosides, including glycolipids, phenolic glycosides, steroid glycosides, and triterpene glycosides. Each molecule represents a prototypical structure of a family of the natural glycosides with interesting biological activities, and we emphasize the general tactics for the synthesis of these diverse structures. We provide a rationale for four tactics for the synthesis of glycosides, based on the stage at which the glycosidic bond is formed between the saccharide and the aglycone. This choice of tactic determines the success or failure of a synthesis, and the flexibility and the overall efficiency of the synthesis as well. Toward the synthesis of heterogeneous glycoform mixtures, we discuss successive and random glycosylation reactions. Finally, we have developed two new glycosylation protocols that address the challenges in the glycosylation of aglycones that are poorly nucleophilic, extremely acid labile, or extremely electrophilic. One of these new protocols takes advantage of glycosyl trifluoroacetimidate donors, and a second protocol uses gold(I)-catalyzed glycosylation with glycosyl ortho-alkynylbenzoate donors.

Synthesis of cholestane saponins as mimics of OSW-1 and their cytotoxic activities

To fulfill the structure-activity relationship (SAR) of OSW-1, and aim at finding the simplest structural part while maintaining most of the biological activities, six cholestane saponins were synthesized by introducing OSW-1 disaccharide (2-O-4-methoxybenzoyl-β-D-xylopyranosyl-(1→3)-2-O-acetyl-α-L-arabinopyranosyl) and its 1→4-linked analogue to the 7-hydroxy or 16-hydroxy of steroidal sapogenins. Cytotoxic activities of the products were tested. Compounds 1 and 3 exhibited potent cytotoxicities against five types of human tumor cells, with minimum IC(50) of 2.0 and 75 nM, respectively. And due to its high activity and easy accessibility compound 1 could be a potential candidate for new anti-tumor agents.

Synthesis of biotinylated OSW-1

OSW-1 is a highly potent anticancer natural saponin with an unknown mode of action. To determine its cellular target(s) biotinylated OSW-1 was successfully synthesized in nine steps.

A total synthesis of OSW-1

A new and practical method was developed to synthesize OSW-1, a natural saponin with potent antitumor activities, from (+)-dehydroisoandrosterone, l-arabinose, and D-xylose on gram scale. The synthesis was achieved in 10 linear steps with an overall yield of 6.4% starting from (+)-dehydroisoandrosterone.

Anticancer and antiproliferative activity of natural brassinosteroids

Brassinosteroids (BRs) are steroid plant hormones that are essential for many plant growth and developmental processes, including cell expansion, vascular differentiation and stress responses. Up to now the inhibitory effects of BRs on cell division of mammalian cells are unknown. To determine basic anticancer structure-activity relationships of natural BRs on human cells, several normal and cancer cell lines have been used. Several of the tested BRs were found to have high cytotoxic activity. Therefore, in our next series of experiments, we tested the effects of the most promising and readily available BR analogues with interesting anticancer properties, 28-homocastasterone (1) and 24-epibrassinolide (2), on the viability, proliferation, and cycling of hormone-sensitive/insensitive (MCF-7/MDA-MB-468) breast and (LNCaP/DU-145) prostate cancer cell lines to determine whether the discovered cytotoxic activity of BRs could be, at least partially, related to brassinosteroid-nuclear receptor interactions. Both BRs inhibited cell growth in a dose-dependent manner in the cancer cell lines. Flow cytometry analysis showed that BR treatment arrested MCF-7, MDA-MB-468 and LNCaP cells in G(1) phase of the cell cycle and induced apoptosis in MDA-MB-468, LNCaP, and slightly in the DU-145 cells. Our results provide the first evidence that natural BRs can inhibit the growth, at micromolar concentrations, of several human cancer cell lines without affecting the growth of normal cells. Therefore, these plant hormones are promising leads for potential anticancer drugs.

Synthesis of the A,B-ring-truncated OSW saponin analogs and their antitumor activities

The A,B-ring-truncated OSW saponin analogs (1, 18a, and 18b) were synthesized. These greatly simplified trans-hydrindane disaccharides retained considerable inhibitory activity against the growth of HeLa and Jurkat T cells (IC(50)=0.8-21.1 microM).

OSW-1 analogues: modification of the carbohydrate moiety

4 OSW-1 analogues featuring modified carbohydrate moieties were prepared. The purpose of these modifications was to assess the importance of certain chemical functions with respect to biological activity. The synthesis and biological activity of the target molecules are shown.

Synthesis of OSW saponin analogs with modified sugar residues and their antiproliferative activities

Eight monosaccharide analogs of the potent antitumor OSW saponins (2-9) were synthesized. One analog, 2-O-acetyl-alpha-l-arabinopyranoside 3, showed antiproliferative activity against the Jurkat cells (IC(50)=0.078microM) comparable to that of the disaccharide derivative (1).

Steroids: partial synthesis in medicinal chemistry

This article reviews the progress in the chemistry of the steroids that was published between January and December 2005. The reactions and partial synthesis of estrogens, androgens, pregnanes, bile acid derivatives, cholestanes and vitamin D analogues are covered. There are 139 references.