Structure-activity Relationship Study Of Antifungai Natural Product Sampangine And Structural Optimization Of Azole Antifungai Agents

Due to massive use of broad spectrum antibiotics, organ transplantation, chemotherapy and AIDS, the incidence and mortality rate of invasive fungal infection have increased dramatically during the past two decades. Although significant progress of prevention as well as treatment has been made in the filed of antifungal infection, it still remains as a very challenging task because of emerging toxicity, severe drug resistance and undesirable drug-drug interactions. It is of great significance to develop novel antifungal agents with new chemical structure, high efficiency, low toxicity, broad-spectrum and new mode of action.Natural products provide rich source of biologically active molecules with novel scaffold, excellent activity and low toxicity. Many of them have been found to possess antifungal activity, such as alkaloids, organic acids, saponins, anthraquinone, flavonoid, cyclic peptide, terpenoid, steroid, etc. In order to accelerate the discovery of new generation of antifungal agents, it is a good idea to select natural products with potent antifungal activity, reasonable physicochemical properties and synthetic accessibility for further structural optimization and mechanism study. In this thesis, the antifungal alkaloid sampangine was chosen for SAR study after systemic analyzing more than100antifungal natural products. In addition, in order to improve the metabolic stability for our previously identified triazole lead compounds, a series of new dual-triazole antifungal derivatives were designed and synthesized.Ⅰ. Design, Synthesis and Antifungal Activity of Sampangine Derivatives1. Design and synthesis of sampangine derivatives by scaffold hoppingScaffold hopping strategy was used for structure modification of sampangine in order to identify novel derivatives with potent antifungal activity and good solubility. Firstly, sampangine was synthesized as a reference drug. In order to validate whether the concept of "scaffold hopping design" can be applied in the optimization of sampangine, the first target compound with tetrahydrofuran replacing the phenyl group was synthesized. The in vitro activity result revealed that scaffold hopping was an effective method for structural optimization. Then, several new target compounds were designed and synthesized by the heterocycle-benzene exchange(e.g. furan, thiophene, pyrrole and N-methyl pyrrole). In vitro activity assay indicated that ZG-20(the thiophene analogy) showed better solubility and antifungal activity with a broad antifungal spectrum against most of the tested pathogenic fungi (MIC range:0.25μg/mL to8.0μg/mL). Particularly, ZG-20shows good potency against Aspergillus fumigatus, which is clinically difficult to be treated.2. Structure-activity relationship study of the scaffold hopping derivative ZG-20On the basis of the lead ZG-20,10new compounds were prepared by introducing various substitutions(e.g. halogens, nitro, amines, heterocycles, etc.). The in vitro data revealed that the SAR of ZG-20was very narrow and the substitutions had negative effects on the antifungal activity. Moreover, the solubility of substituted ZG-20derivatives was poorer than that of ZG-20. Interestingly, during the process of nitration reaction, a nitro intermediate (ZG-20-07) was unexpectedly found to possess high antifungal potency with a broad spectrum against8common fungal pathogens (MIC range:0.125-2μg/mL). Comparing with ZG-20, ZG-20-07proves to be a better antifungal lead because of its higher potency, smaller molecule weight and better solubility. Thus, compound ZG-20-07provides a good starting point for further structural optimization.Ⅱ. Design, Synthesis and Antifungal Activity of Nitrobenzo[b]thiophene-4,7-dione derivativesIn the above study, ZG-20-07was identified as a very promising antifungal lead. In order to extensively explore its SAR, a total of72novel derivatives were designed and synthesized.. The structural modification mainly focused on three parts:a) nitro reduction and subsequent amine derivatization; b) quinone carbonyl reduction; c) optimization of4-methyl pyridine ring including the change of side chain length and methyl position, the bioisosteric replacement of pyridine and the oxidation of pyridine to nitro oxides. The in vitro activity indicated that the presence of nitro group is crucial for the antifungal activity. The2-nitro regioisomer exhibited better potency than that of the3-nitro derivative. In addition, the1,4-quinone part was also essential to activity. The introduction of electro-withdrawing groups and aromatic/heterocyclic functional groups on the position8of ZG-20-07led to the decrease or loss of antifungal activity. Moving the methyl from position8to position7resulted in slight increase of the antifungal activity. Among the target compounds,10compounds show good activity with a broad spectrum. In particular, compounds ZG-20-15, ZG-20-30, ZG-20-38, ZG-20-77, ZG-20-78and ZG-20-85were highly active (MIC range:0.25-2μg/mL against three major invasive fungi; MIC range:0.5-8μg/mL for other superficial fungi). During the synthesis, we successfully developed a new route with high efficiency for preparation of the2-nitro key intermediate. The major advantages of the new route are lower material cost, easier separation and higher overall yield.Ⅲ. Structural Simplification and Antifungal Activity of Nitrobenzo[b]thiophene-4,7-dione derivativesWhen the pyridine ring was removed and replaced by substituted amines, alcohols, or triazoles, several amine derivatives including ZG-20-41and ZG-20-45showed good antifungal activity. Further halogenation of the highly potent compounds was performed, but no improvement of the antifungal activity was observed.Ⅳ. Design and Synthesis of Dual-trizole Antifungal AgentsIn our previous studies, we have designed a series of novel azoles with4-benzyloxy piperidinyl side chain. In order to improve their metabolic stability, benzoxyl was replaced by triazole ring and a total of24new dual-triazole target compounds were prepared. The in vitro antifungal assay indicated that most of the compounds showed moderate to good potency. Among them, compounds ZG-40, ZG-54, ZG-55and ZG-57exhibited good antifungal activity with broad spectrum (MIC range:<0.125-2μg/mL against C.a and C.n.). In particular, ZG-57showed good activity against A. fumigatus while Fluconazole is inactive to A. fumigatus.Ⅴ. SummaryIn this study,142novel compounds were designed and synthesized. For the sampangine derivatives,118compounds belonging to7different structural classes were rationally designed and obtained. Among them,12compounds prove to be high potent with a broad antifungal spectrum which are good drug candidates for further evaluation. In the series of azoles,24novel dual-triazole target compounds were designed and synthesized in order to address the issue of metabolic stability. Four target compounds showed good activity with a broad spectrum which can be further evaluated to improve the pharmacokinetic parameters.