Celogentin C

Anti-Cancer Activity

Celogentin C exhibits potent anti-cancer properties through its ability to inhibit tubulin polymerization. Tubulin is a crucial protein involved in cell division, and its inhibition leads to disrupted mitotic processes, ultimately resulting in cancer cell death.

• Mechanism of Action : The compound binds to tubulin similarly to established anti-cancer agents like vincristine and paclitaxel. In studies conducted on human lung cancer cell lines, this compound demonstrated significant cytotoxicity, with an IC50 value of 0.8 µM, surpassing that of vinblastine (IC50 3.0 µM) .
• Research Findings : A study by Weng and Kersten highlighted that this compound not only inhibits tubulin polymerization but also suggests novel mechanisms specific to lung cancer cells . This dual action makes it a promising candidate for further cancer therapy development.

Synthetic Chemistry

The total synthesis of this compound has been a focal point for chemists aiming to explore its structure-activity relationships and develop analogs with enhanced potency or reduced toxicity.

• Synthesis Techniques : Various synthetic methodologies have been employed to create this compound and its analogs. Notably, a left-to-right synthetic strategy was developed that involved intricate chemical reactions such as intermolecular Knoevenagel condensation and radical conjugate addition .
• Analog Development : Researchers have synthesized bicyclic analogs of this compound with modifications in their side chains to explore their efficacy as tubulin inhibitors . These analogs are crucial for understanding the structure-activity relationship of the compound and optimizing its therapeutic potential.

Pain Research

Beyond its anti-cancer applications, this compound serves as a valuable chemical probe for studying pain receptors. The compound's origin from the stinging tree (Dendrocnide moroides), known for causing intense pain upon contact, provides insights into pain perception mechanisms.

• Pain Mechanism Exploration : The unique structural features of this compound may help identify unknown pain receptors and elucidate the biochemical pathways involved in pain sensation . This research could lead to novel analgesics targeting specific pain pathways.

Traditional Medicine Context

Historically, Celosia argentea, from which this compound is derived, has been used in traditional medicine for treating various ailments, including liver and eye diseases . Understanding these traditional uses can guide modern therapeutic applications and enhance the compound's profile in integrative medicine.

Data Table: Summary of Key Studies on this compound

Basic Research Questions

Q. What are the key synthetic challenges in achieving the total synthesis of Celogentin C, and how were they addressed?

The total synthesis of this compound requires constructing two bicyclic peptide systems: the Leu-Trp crosslink and the Trp-His imidazole-indole linkage. A major challenge was optimizing the oxidation coupling for the Trp-His bond, which only succeeded with the use of Pro-OBn as an additive. This additive facilitated the formation of a chloramine intermediate, preventing over-chlorination and enabling regioselective coupling . Additionally, the Knoevenagel condensation-radical conjugate addition sequence for the Leu-Trp bond faced steric hindrance, resolved by iterative substrate design and protecting group strategies (e.g., Pmc and Mtr groups) . Analytical validation via ¹H NMR and reverse-phase HPLC confirmed structural fidelity to the natural product .

Q. How does this compound’s antimitotic activity compare to established chemotherapeutic agents?

this compound exhibits potent inhibition of microtubule polymerization with an IC₅₀ of 0.8 µM, surpassing vinblastine (IC₅₀ = 3.0 µM). This activity is attributed to its bicyclic structure, particularly the right-hand Pro-containing ring. In contrast, moroidin-type compounds (IC₅₀ = 2.0–4.0 µM) and other celogentins (A, B, D; IC₅₀ = 20–30 µM) show weaker activity, highlighting the critical role of stereochemistry and macrocyclic rigidity .

Q. What analytical techniques are essential for validating synthetic this compound?

Key techniques include:

• ¹H NMR : Used to confirm stereochemistry via NOE correlations (e.g., indole NH/imidazole H2 and Trpβ-H/imidazole H5 interactions) .
• Reverse-phase HPLC : Verified co-elution of synthetic and natural this compound, ensuring purity and structural identity .
• Variable-temperature NMR : Resolved discrepancies in imidazole H2 chemical shifts, revealing concentration- and pH-dependent behavior due to hydrogen bonding or proton exchange .

Advanced Research Questions

Q. Why does Pro-OBn uniquely enable successful Trp-His oxidation coupling, and what mechanistic insights explain its role?

Pro-OBn acts as a sacrificial substrate for N-chlorosuccinimide (NCS), forming a transient N-chloroamine intermediate. This regulates NCS concentration, minimizing over-chlorination and favoring mono-chlorinated indole intermediates. Mechanistic studies using ESI-MS and ¹H NMR confirmed Pro-OBn’s nucleophilic amine and acidic α-hydrogen are critical for stabilizing reactive species. Without Pro-OBn, dichlorinated byproducts dominate, underscoring its role in reaction fidelity .

Q. How do environmental factors (pH, temperature) influence the ¹H NMR spectral properties of this compound’s imidazole protons?

The His imidazole H2 proton exhibits a 0.3 ppm downfield shift (9.22 ppm to 8.92 ppm) when temperature increases from 16°C to 44°C in DMSO-d₆. Similarly, adding TFA shifts H2 downfield due to protonation of the imidazole N3 atom. These dynamics suggest intermolecular hydrogen bonding or pH-dependent tautomerism, complicating structural validation. Researchers must standardize NMR conditions (e.g., 25°C, 0.5 mM concentration) to ensure reproducibility .

Q. What alternative strategies exist for constructing the Leu-Trp crosslink, and how do they compare in efficiency?

Beyond the Knoevenagel-radical approach, Pd-catalyzed C(sp³)–H activation offers a regioselective alternative. For example, using a quinoline-directed macrocyclization, researchers achieved Leu-Trp bond formation with 32% yield and retained α-stereochemistry. This method avoids iterative protection/deprotection steps but requires iodine-substituted Trp precursors. Comparative studies show the radical method (83% yield for macrocyclization) is higher-yielding but less tolerant of steric bulk .

Q. What unresolved contradictions exist in the bioactivity data for this compound, and how might they inform future studies?

While this compound’s microtubule inhibition is well-documented, its cytotoxicity in cancer cells remains uncharacterized. In vitro studies show specificity against H1437 lung adenocarcinoma cells, but mechanisms (e.g., apoptosis vs. mitotic arrest) are unclear. Discrepancies between anti-tubulin potency (IC₅₀ = 0.8 µM) and cellular activity suggest off-target effects or uptake limitations. Future work should integrate proteomic profiling and pharmacokinetic assays .

Q. Methodological Considerations

Q. How should researchers design experiments to address concentration-dependent spectral artifacts in peptide characterization?

• Standardize sample preparation : Use consistent solvent systems (e.g., DMSO-d₆ with 0.1% TFA) and concentrations (0.1–1.0 mM).
• Variable conditions : Acquire NMR spectra at multiple temperatures (e.g., 25°C, 37°C) and pH levels to identify dynamic effects.
• Supplement with HDX-MS : Hydrogen-deuterium exchange mass spectrometry can map solvent-accessible regions, clarifying hydrogen-bonding networks .

Q. What statistical and reproducibility criteria are critical for validating this compound’s bioactivity?

• Sample size : Use ≥5 replicates per group to detect 25% differences with 95% confidence (α = 0.05, power = 0.8) .
• Blinding : Implement double-blinding for cell viability assays to minimize bias.
• Positive controls : Include vinblastine or paclitaxel to benchmark microtubule inhibition .

Q. Future Research Directions

Q. What synthetic and biological gaps remain in understanding this compound’s structure-activity relationship (SAR)?

• Synthetic : Explore non-natural analogs (e.g., D-amino acid substitutions) to probe stereochemical requirements.
• Biological : Map binding interfaces with β-tubulin using cryo-EM or X-ray crystallography.
• Translational : Develop prodrug formulations to enhance cellular uptake and bioavailability .