Introduction/Overview
Natural products, as an important source of drug discovery, have always held a central position in the long history of human fight against diseases. Among them, diterpenoid acid compounds have attracted much attention due to their structural diversity and wide range of biological activities. Pseudolaric Acid A-O-beta-D-glucopyranoside (PAAG), as a diterpenoid acid glycoside isolated from traditional Chinese medicine Pseudolaric bark, has increasingly highlighted its research value. This compound is a glucoside derivative of Pseudolaric Acid A (PAA), with a CAS number of 98891-44-2. Early research mainly focused on the anti-tumor and anti fertility activities of its precursor PAA, while in recent years, the outstanding potential of PAAG in the field of antifungal therapy has gradually become a research hotspot. With the increasing incidence rate of invasive fungal infections and the emergence of drug-resistant strains worldwide, it is urgent to develop new, efficient and low toxic antifungal drugs. PAAG exhibits unique antifungal mechanisms and the potential to overcome drug resistance by acting on multiple key targets such as the ergosterol synthesis pathway, efflux pumps, and cell wall synthesis in fungal cell membranes. This article aims to systematically review the chemical structure, plant origin, pharmacological activity, mechanism of action, medicinal properties, and clinical application prospects of PAAG, in order to provide comprehensive scientific references for the in-depth development and transformation research of this natural product.
Chemical structure and physicochemical properties
The molecular formula of oxalic acid glycoside is C29H42O10, with a molecular weight of 550.6010. The core of its structure is a highly oxidized rosin alkane type diterpenoid skeleton, which has a characteristic pentagonal lactone ring (D ring) and multiple chiral centers. The key difference between PAAG and PAA is that their C-2 hydroxyl group is connected to a D-glucopyranose group through a β - glycosidic bond. This glycosylation modification significantly alters the physicochemical properties and biological activity of the parent compound.
From the analysis of physical and chemical properties, the lipid water partition coefficient (LogP) of this compound is 1.1920, indicating moderate lipophilicity. However, compared to its glycoside PAA (with a higher LogP value), glycosylation enhances its hydrophilicity. Its topological polar surface area (TPSA) is as high as 169.0500 Å ², mainly attributed to the abundant oxygen atoms in the molecule (including carboxyl groups, lactone rings, and multiple hydroxyl groups) and the contribution of multiple polar groups on sugar units. Higher TPSA values are usually associated with poorer cell membrane permeability. The predicted value of water solubility is 0.2949 mg/mL, which belongs to the category of slight solubility, which poses certain challenges for its formulation development. Taking into account its molecular weight (>500) and polarity characteristics, PAAG may not conform to the traditional "five principles of drugs", but as a natural product, its unique biological activity often breaks through the limitations of these empirical rules.
Plant sources and extraction methods
PAAG mainly comes from the Pinaceae family, the genus Pinus, and the genus Pinus（Pseudolarix amabilis The dried root bark or near root bark of Nelson Rehd., also known as "Tu Jing Pi" or "Jin Qian Song Pi" in traditional Chinese medicine, is traditionally used to treat skin diseases such as scabies and eczema.
Its extraction and separation is a multi-step refinement process. Usually, alcohol extraction (such as methanol, ethanol) or aqueous alcohol solution is first used to extract the dried and crushed soil bark, and the crude extract is obtained after concentration. Subsequently, liquid-liquid distribution was carried out using solvents such as ethyl acetate and water to preliminarily enrich the target components. Further purification depends on a variety of chromatographic techniques: silica gel column chromatography is often used for preliminary separation, and then combined with reverse phase silica gel (such as ODS), dextran gel (Sephadex LH-20) column chromatography for fine purification. Modern separation methods such as high-performance liquid chromatography (HPLC) or medium pressure liquid chromatography (MPLC) are key technologies for obtaining high-purity PAAG monomers. Due to the complex composition of diterpenoid acids and relatively low PAAG content in the bark of Polygonatum sibiricum, the separation and purification process is time-consuming and the yield is not high. Therefore, exploring efficient extraction processes such as ultrasound assisted and microwave-assisted extraction, as well as obtaining PAAG through biosynthetic or chemical synthesis pathways, are important directions for solving its source problem in the future.
Pharmacological activity research
The pharmacological activity research of PAAG currently mainly focuses on its strong antifungal effect and shows potential superior to its glycoside PAA.
1. Antifungal activity
PAAG exhibits broad-spectrum and significant inhibitory activity against various clinically relevant pathogenic fungi, especially against Candida species（Candida spp.）And dermatophytes. Research has shown that it has an effect on Candida albicans（C. albicans）Smooth Candida albicans（C. glabrata）Candida krusei（C. krusei）And the minimum inhibitory concentration (MIC) value of drug-resistant Candida strains is usually in the micromolar range, with activity comparable to or better than some first-line azole drugs. It is worth noting that PAAG remains effective against azole resistant (such as fluconazole resistant) Candida albicans strains, suggesting that its mechanism of action may be different from classical azole drugs, or it may be able to avoid common resistance pathways. In addition, it also has good effects on skin fungi such as Trichophyton rubrum, which is consistent with the efficacy of traditional Chinese medicine Tujingpi in treating tinea.
2. Other potential activities
Although there is limited research, based on its parent nucleus structure, PAAG may also possess other potential activities. PAA has been proven to have anti-tumor, anti angiogenic, and anti fertility activities. Glycosylation may alter the intensity or targeting of these activities. For example, glycosylation may improve its in vivo distribution by increasing water solubility, but it may also affect its cell penetration ability. At present, further experimental verification is needed regarding the activity of PAAG in anti-tumor, anti-inflammatory and other fields.
Mechanism of action and molecular targets
The antifungal effect of PAAG involves multiple targets and pathways, which is the key to overcoming drug resistance. The potential molecular targets revealed by existing research mainly include:
1. Target of ergosterol synthesis pathway:
* ERG11/CYP51A1/CYP51 This is the main target of azole drugs, namely lanosterol 14 α - demethylase. Preliminary studies suggest that PAAG may interact with this enzyme, interfere with its function, and inhibit the synthesis of ergosterol, leading to damage to fungal cell membrane integrity and increased permeability. But its binding mode may be different from that of azole drugs, making it effective against azole resistant strains (whose resistance is often caused by ERG11 gene mutations or overexpression).
* MLS1 Encoding lanosterol synthase, it is a key upstream enzyme in the ergosterol synthesis pathway. Interference with MLS1 can also block the synthesis of ergosterol.
2. Drug efflux pump targets:
* CDR1 and MDR1 The two are important ATP binding cassette (ABC) transporter superfamily and multidrug resistance (MDR) in Candida, which mainly promote the efflux pump of the subfamily and actively pump drugs out of cells, which is one of the main mechanisms leading to fungal resistance to azole drugs. Research has shown that PAAG may be able to inhibit the function of these efflux pumps or be difficult to pump out by them, thereby effectively accumulating in drug-resistant bacterial cells and restoring their sensitivity to drugs.
3. Cell wall synthesis targets:
* FKS1 Encoding the catalytic subunit of β -1,3-glucan synthase, it is the target of echinocandin drugs such as caspofungin. Inhibiting FKS1 can disrupt the synthesis of fungal cell walls. There is evidence suggesting that PAAG may indirectly affect cell wall related components or pathways, producing synergistic effects with cell membrane targeting, but whether it directly acts on FKS1 remains to be clarified.
Comprehensive mechanism PAAG is likely to exert antifungal effects through a "one stone, many birds" strategy: on the one hand, it directly inhibits ergosterol synthesis and damages cell membranes; On the other hand, inhibiting drug efflux pumps reduces self excretion and may reverse resistance to other drugs; At the same time, it may interfere with the cell wall homeostasis. This multi-target mode of action makes it difficult for fungi to develop drug resistance through a single gene mutation, providing significant advantages for their clinical development.
Evaluation of drug properties and pharmacokinetics
Based on the provided pharmacological parameters and existing knowledge, the preliminary evaluation of the pharmacological properties of PAAG is as follows:
Advantage:
1. Security potential The Ames test result is 0.0, indicating no significant risk of in vitro genetic toxicity. HERG inhibition is' no ', indicating a lower risk of inducing QT interval prolongation in the heart, which is an important cardiac safety advantage.
2. Low blood-brain barrier penetration Although this limits its application in treating fungal infections in the central nervous system, it may also mean a lower risk of central side effects in the peripheral nervous system and heart.
3. Multi-target effect As mentioned earlier, its multi-target mechanism helps to reduce the risk of drug resistance.
Challenge:
1. Solubility and permeability The moderate LogP value but high TPSA and micro solubility suggest that it may belong to Class IV (low solubility, low permeability) in the Biopharmaceutical Classification System (BCS), and its oral bioavailability may be poor.
2. Pharmacokinetic unknown Currently, there is a severe lack of data on the in vivo pharmacokinetics of PAAG, including absorption, distribution, metabolism, and excretion. Glycoside bonds in the body are easily hydrolyzed by glycosidases in the intestine or blood, and then converted back into glycoside PAA, which may alter their original activity profile and metabolic fate. It is necessary to systematically study its in vivo stability, metabolites, and drug time curve.
3. Formulation development To improve its water solubility and bioavailability, advanced formulation technologies such as nanocrystals, liposomes, cyclodextrin inclusion complexes, or prodrug modifications (such as derivatization of sugar or carboxyl groups) may be required.
Clinical application prospects and prospects
PAAG, as a novel antifungal natural product with a novel mechanism of action, has broad clinical application prospects, but the road to transformation is long and arduous.
Potential application directions:
1. Treatment of drug-resistant fungal infections PAAG or its structurally optimized derivatives can be used as monotherapy or in combination with existing drugs to provide new treatment options for the increasingly severe azole resistant Candida infections.
2. External antifungal preparations: In view of the traditional history of external application of the bark of Vitex negundo and the activity of PAAG against dermatophytes, cream, gel or spray can be developed to treat superficial fungal infections such as tinea pedis, tinea corporis and cruris, and its local application can avoid the pharmacokinetic problem of systematic administration.
3. Antifungal sensitizer By using its external pump to inhibit activity, PAAG may be developed as an auxiliary sensitizer, combined with azole drugs such as fluconazole, to restore the sensitivity of drug-resistant strains to existing drugs.
Future research prospects:
1. Structure performance relationship and structural optimization Systematically study the effects of the sugar moiety and various functional groups on the activity, selectivity, and drug formation of diterpenoid parent nuclei. Improve its solubility, metabolic stability, and targeting through chemical modifications such as glycosylation and prodrug preparation.
2. In depth in vitro and in vivo efficacy and safety evaluation Validate its efficacy in more clinically relevant fungal infection models (including systemic infection models) and complete comprehensive preclinical safety evaluations (acute toxicity, chronic toxicity, reproductive toxicity, etc.).
3. Clear and detailed mechanism of action Using chemical biology methods such as photoaffinity labeling and proteomics to directly identify its molecular target proteins and elucidate its precise mode of action.
4. Pharmacokinetic and Formulation Studies Conduct systematic ADME research and actively explore new drug delivery systems suitable for its characteristics.
Conclusion
Tujingpi formate glycoside is a highly distinctive diterpenoid glycoside compound discovered from the traditional Chinese medicine Tujingpi. It not only exhibits broad-spectrum and potent antifungal activity in vitro, especially effective against drug-resistant strains, but also shows great potential to overcome the current clinical antifungal treatment resistance dilemma due to its unique multi-target mechanism of action (simultaneously affecting fungal cell membrane ergosterol synthesis, drug efflux pumps, and possible cell wall synthesis). Although it faces challenges in terms of solubility, permeability, and in vivo metabolic stability in drug development, its good preliminary safety characteristics (no genetic toxicity, no hERG inhibition) lay a positive foundation for its subsequent development. Future research should focus on improving its drug like properties through structural optimization, elucidating its precise molecular mechanisms using modern technology, and validating its therapeutic value in more complex in vivo models. With the continuous deepening of research, PAAG is expected to lead the development of a new generation of multi-target antifungal drugs, contributing Chinese wisdom and solutions derived from natural products to address the global threat of fungal resistance.
