Cephalostatins and ritterazines

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.

Performance of Green Desymmetrization Methods toward Bioactive Cephalostatin Analogues

Since the discovery of cephalostatins, which have shown remarkable activity against human cancer cells, they have attracted the attention of researchers to target the synthesis of such impressive, complicated molecules using the green desymmetrization approach. In the current review, we report the progress in the desymmetrization of symmetrical bis-steroidal pyrazines (BSPs) as an approach toward potentially active anti-- cancer agents, namely cephalostatins/ ritterazines. The achievement of synthesizing a gram-scaled prodrug with comparable activity to the potent natural cephalostatins using green methods is our primary target. These synthetic methods can be scaled up based on the symmetrical coupling (SC) of two steroidal units of the same type. Our secondary target is the discovery of new green pathways that help in structural reconstruction programming toward the total synthesis of at least one potentially active family member. The strategy is based on functional group interconversions with high flexibility and brevity using green selective methods. The introduction of controlling groups using nontrivial reconstruction methodologies forms the backbone of our work. After certain modifications to the symmetrical BSP starting material, the resulting analogs underwent several chemoselective transformations through three main routes in rings F, D, and C. One of these routes is the chemoselective spiroketal opening (ring-F). The second route was the functionalization of the Δ14,15 bond (ring-D), including chlorination/dechlorination, in addition to epoxidation/ oxygenation processes. Finally, the introduction of the C-11 methoxy group as a directing group on ring-C led to several chemoselective transformations. Moreover, certain transformations on C-12 (ring-C), such as methylenation, followed by hydroboration- oxidation, led to a potentially active analog. The alignment of these results directs us toward the targets. Our efforts culminated in preparing effective anti-cancer prodrugs (8, 24, 30, and 31), which are able to overcome cancer drug resistance (chemoresistance) by inducing the atypical endoplasmic reticulum-mediated apoptosis pathway, which works through the release of Smac/Diablo and the activation of caspase-4.

Synthesis, NMR characterization and cytotoxic activity of hybrid spirostanic sapogenins-estradiol dimers

Four hybrid steroid dimers were obtained by BF3·Et2O-catalyzed aldol condensation of acetylated steroid sapogenins with 2-formyl-estradiol diacetate. The structures of the obtained dimers were unambiguously established by NMR. The hybrid dimers 9a (IC50 18.37 μM) and 9c (IC50 9.4 μM) with the 5α configuration at the A/B rings junction showed the higher cytotoxicity against HeLa, with selectivity index of 4.36 and 11.8 respectively. The presence of a carbonyl function at position C-12 produced the highest cytotoxic effect, which is in line with our previous reports.

Montmorillonite Catalyzed Synthesis of Novel Steroid Dimers

The reactions of sterols (androst-5-en-3β-ol-17-one, diosgenin, and cholesterol) and their tosylates with hydroquinone aimed at the synthesis of O,O-1,4-phenylene-linked steroid dimers were studied. The reaction course strongly depended on the conditions used. The study has shown that the major reaction products are the elimination products and unusual steroid dimers resulting from the nucleophilic attack of the hydroquinone C2 carbon atom on the steroid C3 position, followed by an intramolecular addition to the C5-C6 double bond. A different reaction course was observed when montmorillonite K10 was used as a catalyst. The reaction of androst-5-en-3β-ol-17-one under the promotion of this catalyst afforded the O,O-1,4-phenylene-linked steroid dimer in addition to the disteroidal ether. The formation of the latter compound was suppressed by using 3-tosylate as a substrate instead of the free sterol. The reactions of androst-5-en-3β-ol-17-one tosylate and cholesteryl tosylate with hydroquinone catalyzed by montmorillonite K10 carried out under optimized conditions afforded the desired dimers in 31% and 67% yield, respectively.

An unexpected acid-catalyzed rearrangement of diacetoxy benzylidenespirostanes to spirochromene acetals and spiroindenes with radical scavenger activity

23E-diacetoxybenzylidenespirostanes underwent rearrangement when treated with HCl in CH₂Cl₂/CH₃OH. The course of the rearrangement depends on the substitution pattern in the phenyl ring. While compounds bearing an acetoxy group at the ortho position produced spirochromenes, the partners with no substituent at the ortho position led to spiroindenes. All the rearranged compounds exhibited moderate antioxidant activity.

Synthesis and characterization of a fluorescent steroid dimer linked through C-19 by a 1,4-Bis(phenylethynyl)phenylene fragment

The synthesis and characterization of a dimer in which two nuclei of 3β-acetoxy-19-hydroxyandrost-5-en-17-one are linked by the fluorescent 1,4-bis(phenylethynyl)phenylene bridge attached to the oxygenated functions at positions C-19 of each steroid fragment is described. The compound was obtained in five steps and 23 % overall yield and showed a strong blue emission with a quantum yield of 0.66.

Reductive desymmetrization of steroid dimers

NaBH₃CN reduction of symmetric dimers in which two steroid units are linked by a 1,4-dimethylidenebenzene moiety followed two different courses: (a) hydrogenation of the benzylic double bond and (b) reductive F ring opening of the side chain. While courses a and b led to symmetrical dimers, the combination of both pathways produced an unsymmetrical dimer that bears different side chains in each half. The exhaustive NMR characterization of all obtained compounds is presented.

1,5-Hydrogen Atom Transfer/Surzur-Tanner Rearrangement: A Radical Cascade Approach for the Synthesis of 1,6-Dioxaspiro[4.5]decane and 6,8-Dioxabicyclo[3.2.1]octane Scaffolds in Carbohydrate Systems

The 1,5-HAT–1,2-(ester)alkyl radical migration (Surzur–Tanner rearrangement) radical/polar sequence triggered by alkoxyl radicals has been studied on a series of C-glycosyl substrates with 3-C-(α,β-d,l-glycopyranosyl)1-propanol and C-(α-d,l-glycopyranosyl)methanol structures prepared from chiral pool d- and l-sugar. The use of acetoxy and diphenoxyphosphatoxy as leaving groups provides an efficient construction of 10-deoxy-1,6-dioxaspiro[4.5]decane and 4-deoxy-6,8-dioxabicyclo[3.2.1]octane frameworks. The alkoxyl radicals were generated by the reaction of the corresponding N-alkoxyphthalimides with group 14 hydrides [n-Bu₃SnH(D) and (TMS)₃SiH], and in comparative terms, the reaction was also initiated by visible light photocatalysis using the Hantzsch ester/fac-Ir(ppy)₃ procedure. Special attention was devoted to the influence of the relative stereochemistry of the centers involved in the radical sequence on the reaction outcome. The addition of BF₃•Et₂O as a catalyst to the radical sequence resulted in a significant increase in the yields of the desired bicyclic ketals.

Potential of Steroidal Alkaloids in Cancer: Perspective Insight Into Structure-Activity Relationships

Steroidal alkaloids contain both steroidal and alkaloid properties in terms of chemical properties and pharmacological activities. Due to outstanding biological activities such as alkaloids and similar pharmacological effects to other steroids, steroidal alkaloids have received special attention in anticancer activity recently. Substituted groups in chemical structure play markedly important roles in biological activities. Therefore, the effective way to obtain lead compounds quickly is structural modification, which is guided by structure-activity relationships (SARs). This review presents the SAR of steroidal alkaloids and anticancer, including pregnane alkaloids, cyclopregnane alkaloids, cholestane alkaloids, C-nor-D-homosteroidal alkaloids, and bis-steroidal pyrazine. A summary of SAR can powerfully help to design and synthesize more lead compounds.

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.

Natural products as modulators of eukaryotic protein secretion

Covering: up to the end of 2019Diverse natural product small molecules have allowed critical insights into processes that govern eukaryotic cells' ability to secrete cytosolically synthesized secretory proteins into their surroundings or to insert newly synthesized integral membrane proteins into the lipid bilayer of the endoplasmic reticulum. In addition, many components of the endoplasmic reticulum, required for protein homeostasis or other processes such as lipid metabolism or maintenance of calcium homeostasis, are being investigated for their potential in modulating human disease conditions such as cancer, neurodegenerative conditions and diabetes. In this review, we cover recent findings up to the end of 2019 on natural products that influence protein secretion or impact ER protein homeostasis, and serve as powerful chemical tools to understand protein flux through the mammalian secretory pathway and as leads for the discovery of new therapeutics.

Novel double functional protection of cephalostatin analogues using a gas-free chlorination method

Herewith, we report on a method that allows to simultaneously protect both the ∆^14,15 bond and the carbonyl group of the symmetrical bis-steroidal diketone 2. We found that environmentally friendly and gas-free chlorination is ideally suited to achieve this goal. This method was discovered during our efforts to methoxylate 2 in a solution of dichloromethane and basic methanol in the presence of diacetoxy iodobenzene. Unexpectedly, the ∆^14,15 bonds were chlorinated once as well as twice in a statistical manner. Interestingly, the singly dichlorinated desymmetrized product is an ideal precursor for conduction a series of position selective transformations. Importantly, the carbonyl group present in the nonchlorinated hemisphere can be selectively reduced, olefinated or oximated, while the other carbonyl group stays unaltered. A structurally related “monomeric” steroid derivative undergoes ∆^14,15 chlorination and 11-position methoxylation under same conditions. These findings represent a powerful entry for preparing new nonsymmetrical cephalostatin derivatives.

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.

Recent Advances in Drug Discovery from South African Marine Invertebrates

Recent developments in marine drug discovery from three South African marine invertebrates, the tube worm Cephalodiscus gilchristi, the ascidian Lissoclinum sp. and the sponge Topsentia pachastrelloides, are presented. Recent reports of the bioactivity and synthesis of the anti-cancer secondary metabolites cephalostatin and mandelalides (from C. gilchristi and Lissoclinum sp., respectively) and various analogues are presented. The threat of drug-resistant pathogens, e.g., methicillin-resistant Staphylococcus aureus (MRSA), is assuming greater global significance, and medicinal chemistry strategies to exploit the potent MRSA PK inhibition, first revealed by two marine secondary metabolites, cis-3,4-dihydrohamacanthin B and bromodeoxytopsentin from T. pachastrelloides, are compared.