Design, synthesis and in vitro biological studies of novel triazoles with potent and broad-spectrum antifungal activity

A series of novel triazole derivatives containing aryl-propanamide side chains was designed and synthesised. In vitro antifungal activity studies demonstrated that most of the compounds inhibited the growth of six human pathogenic fungi. In particular, parts of phenyl-propionamide-containing compounds had excellent, broad-spectrum antifungal activity against Candida albicans SC5314, Cryptococcus neoformans 22-21, Candida glabrata 537 and Candida parapsilosis 22-20 with MIC values in the range of ≤0.125 µg/mL-4.0 µg/mL. In addition, compounds A1, A2, A6, A12 and A15 showed inhibitory activities against fluconazole-resistant Candida albicans and Candida auris. Preliminary structure-activity relationships (SARs) are also summarised. Moreover, GC-MS analysis demonstrated that A1, A3, and A9 interfered with the C. albicans ergosterol biosynthesis pathway by inhibiting Cyp51. Molecular docking studies elucidated the binding modes of A3 and A9 with Cyp51. These compounds with low haemolytic activity and favourable ADME/T properties are promising for the development of novel antifungal agents.

Deep amplicon sequencing reveals extensive allelic diversity in the erg11/CYP51 promoter and allows multi-population DMI fungicide resistance monitoring in the canola pathogen Leptosphaeria maculans

Continued use of fungicides provides a strong selection pressure towards strains with mutations to render these chemicals less effective. Previous research has shown that resistance to the demethylation inhibitor (DMI) fungicides, which target ergosterol synthesis, in the canola pathogen Leptosphaeria maculans has emerged in Australia and Europe. The change in fungicide sensitivity of individual isolates was found to be due to DNA insertions into the promoter of the erg11/CYP51 DMI target gene. Whether or not these were the only types of mutations and how prevalent they were in Australian populations was explored in the current study. New isolates with reduced DMI sensitivity were obtained from screens on DMI-treated plants, revealing eight independent insertions in the erg11 promoter. A novel deep amplicon sequencing approach applied to populations of ascospores fired from stubble identified an additional undetected insertion allele and quantified the frequencies of all known insertions, suggesting that, at least in the samples processed, the combined frequency of resistant alleles is between 0.0376% and 32.6%. Combined insertion allele frequencies positively correlated with population-level measures of in planta resistance to four different DMI treatments. Additionally, there was no evidence for erg11 coding mutations playing a role in conferring resistance in Australian populations. This research provides a key method for assessing fungicide resistance frequency in stubble-borne populations of plant pathogens and a baseline from which additional surveillance can be conducted in L. maculans. Whether or not the observed resistance allele frequencies are associated with loss of effective disease control in the field remains to be established.

Novel antifungal triazoles with alkynyl-methoxyl side chains: Design, synthesis, and biological activity evaluation

Previous work led to the rational design, synthesis and testing of novel antifungal triazole analogues bearing alkynyl-methoxyl side chains. Tests of in vitro antifungal activity showed Candida albicans SC5314 and Candida glabrata 537 gave MIC values of ≤0.125 μg/mL for most of the compounds. Among these, compounds 16, 18, and 29 displayed broad-spectrum antifungal activity against seven human pathogenic fungal species, two fluconazole-resistant C. albicans isolates and two multi-drug resistant Candida auris isolates. Moreover, 0.5 μg/mL of 16, 18, and 29 was more effective than 2 μg/mL of fluconazole at inhibiting fungal growth of the strains tested. The most active compound (16) completely inhibited the growth of C. albicans SC5314 at 16 μg/mL for 24 h, affected biofilm formation and destroyed the mature biofilm at 64 μg/mL. Several Saccharomyces cerevisiae strains, overexpressing recombinant Cyp51s or drug efflux pumps, indicated 16, 18, and 29 targeted Cyp51 without being significantly affected by a common active site mutation, but were susceptible to target overexpression and efflux by both MFS and ABC transporters. GC-MS analysis demonstrated that 16, 18, and 29 interfered with the C. albicans ergosterol biosynthesis pathway by inhibition at Cyp51. Molecular docking studies elucidated the binding modes of 18 with Cyp51. The compounds showed low cytotoxicity, low hemolytic activity and favorable ADMT properties. Importantly, compound 16 showed potent in vivo antifungal efficacy in the G. mellonella infection model. Taken together, this study presents more effective, broad-spectrum, low toxicity triazole analogues that can contribute to the development of novel antifungal agents and help overcome antifungal resistance.

Targeted deletion of three CYP51s in Fusarium fujikuroi and their different roles in determining sensitivity to 14α-demethylase inhibitor fungicides

BACKGROUND

Fusarium fujikuroi is the pathogenic agent of rice bakanae disease and has developed serious resistance to prochloraz, a 14α-demethylase inhibitor (DMI). Prochloraz resistance in F. fujikuroi is caused by cooperation between FfCyp51B with Cyp51A and shows cross-resistance only to prothioconazole but not to tebuconazole, difenoconazole, propiconazole, metconazole, hexaconazole, and triadimefon. This study aimed to analyze the functions of the three Cyp51s in F. fujikuroi, especially their role in determining sensitivity to DMIs.

RESULTS

The respective deletion of FfCyp51A, Cyp51B, and Cyp51C had no obvious effect on morphology, conidium germination, or pathogenicity. The involvement of growth, growth and ergosterol biosynthesis, and conidium production and ergosterol biosynthesis was observed for FfCyp51A, Cyp51B, and Cyp51C, respectively. Compared with the sensitive isolate of F. fujikuroi, the effect on sensitivity to the tested DMIs was divided into four groups: (i) both of Cyp51A and Cyp51B positively regulate the sensitivity to prochloraz and prothioconazole; (ii) Cyp51B positively regulate the sensitivity to tebuconazole and metconazole, but negatively regulate the sensitivity to difenoconazole; (iii) Cyp51A and Cyp51B play opposite roles in the sensitivity to triadimefon. Therefore, deletion of Cyp51A in F. fujikuroi confers a higher sensitivity to triadimefon, while deletion of Cyp51B results in a triadimefon-resistant mutant isolate; (iv) deletion of Cyp51B yielded a mutant isolate that was more resistant to propiconazole and hexaconazole.

CONCLUSION

Sophisticated interactions exist within the three Cyp51 genes to DMIs fungicides sensitivity in F. fujikuroi, and Cyp51B probably plays a more critical role than Cyp51A and Cyp51C. © 2022 Society of Chemical Industry.

Analysis of Cyp51 protein sequences shows 4 major Cyp51 gene family groups across fungi

Azole drugs target fungal sterol biosynthesis and are used to treat millions of human fungal infections each year. Resistance to azole drugs has emerged in multiple fungal pathogens including Candida albicans, Cryptococcus neoformans, Histoplasma capsulatum, and Aspergillus fumigatus. The most well-studied resistance mechanism in A. fumigatus arises from missense mutations in the coding sequence combined with a tandem repeat in the promoter of cyp51A, which encodes a cytochrome P450 enzyme in the fungal sterol biosynthesis pathway. Filamentous members of Ascomycota such as A. fumigatus have either 1 or 2 of 3 Cyp51 paralogs (Cyp51A, Cyp51B, and Cyp51C). Most previous research in A. fumigatus has focused on Cyp51A due to its role in azole resistance. We used the A. fumigatus Cyp51A protein sequence as the query in database searches to identify Cyp51 proteins across fungi. We found 435 Cyp51 proteins in 295 species spanning from early-diverging fungi (Blastocladiomycota, Chytridiomycota, Zoopagomycota, and Mucormycota) to late-diverging fungi (Ascomycota and Basidiomycota). We found these sequences formed 4 major Cyp51 groups: Cyp51, Cyp51A, Cyp51B, and Cyp51C. Surprisingly, we found all filamentous Ascomycota had a Cyp51B paralog, while only 50% had a Cyp51A paralog. We created maximum likelihood trees to investigate the evolution of Cyp51 in fungi. Our results suggest Cyp51 is present in all fungi with 3 paralogs emerging in Pezizomycotina, including Cyp51C which appears to have diverged from the progenitor of the Cyp51A and Cyp51B groups.

Damage response protein 1 (Dap1) functions in the synthesis of carotenoids and sterols in Xanthophyllomyces dendrorhous

Cytochrome P450s (P450s) are heme-containing proteins involved in several cellular functions, including biosynthesis of steroidal hormones, detoxification of xenobiotic compounds, among others. Damage response protein 1 (Dap1) has been described as a positive regulator of P450s through protein-protein interactions in organisms such as Schizosaccharomyces pombe. Three P450s in the carotenogenic yeast Xanthophyllomyces dendrorhous have thus far been characterized: Cyp51 and Cyp61, which are involved in ergosterol biosynthesis, and CrtS (astaxanthin synthase), which is involved in biosynthesis of the carotenoid astaxanthin. In this work, we describe the X. dendrorhous DAP1 gene, deletion of which affected yeast pigmentation by decreasing the astaxanthin fraction and increasing the β-carotene (a substrate of CrtS) fraction, which is consistent with the known role of CrtS. We found that the proportion of ergosterol was also decreased in the Δdap1 mutant. However, even though the fractions of the end products of these two pathways (the synthesis of carotenoids and sterols) were decreased in the Δdap1 mutant, the transcript levels of genes from the P450 systems involved were higher than those in the wild-type strain. We demonstrate that Dap1 coimmunoprecipitates with these three P450s, suggesting that Dap1 interacts with these three proteins. We propose that Dap1 regulates the synthesis of astaxanthin and ergosterol in X. dendrorhous, probably by regulating the P450s involved in both biosynthetic pathways at the protein level. This work suggests a new role for Dap1 in the regulation of carotenoid biosynthesis in X. dendrorhous.

Characterization of the Molecular Mechanisms of Resistance against DMI Fungicides in Cercospora beticola Populations from the Czech Republic

Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar pathogen of sugar beet worldwide. Extensive reliance on fungicides to manage CLS has resulted in the evolution of fungicide resistance in C. beticola worldwide, including populations in the Czech Republic. One important class of fungicides used to manage CLS is the sterol demethylation inhibitors (DMI). The aim of our study was to assess DMI resistance in C. beticola from the Czech Republic and elucidate the molecular basis of DMI resistance in this population. A total of 50 isolates were collected in 2018 and 2019 from the major sugar beet growing regions of the Czech Republic and assessed for in vitro sensitivity to the DMI fungicides propiconazole, prochloraz, and epoxiconazole. These analyses identified three strains that exhibited 50% effective concentration (EC50) values > 1.0 μg mL-1 against respective fungicides, which were therefore considered resistant. In contrast, strains that exhibited lowest EC50 values were considered sensitive. To explore the molecular basis of resistance in these three strains, the cytochrome P450-dependent sterol 14α-demethylase (Cyp51) gene was sequenced. Sequence analysis identified a Y464S mutation in all three resistant strains. To assess whether Cyp51 gene expression may play a role in DMI resistance, selected strains were grown in vitro with and without fungicide treatment. These analyses indicated that Cyp51 gene expression was significantly induced after fungicide treatment. Thus, we conclude that Y464S point mutation along with induced Cyp51 gene overexpression is likely responsible for resistance against DMI fungicides in C. beticola from the Czech Republic.

High frequency of fungicide resistance-associated mutations in the wheat yellow rust pathogen Puccinia striiformis f. sp. tritici

BACKGROUND

Reliance on fungicides to manage disease creates selection pressure for the evolution of resistance in fungal and oomycete pathogens. Rust fungi (Pucciniales) are major pathogens of cereals and other crops and have been classified as low-risk for developing resistance to fungicides; no case of field failure of fungicides in a cereal rust disease has yet been recorded. Recently, the Asian soybean rust pathogen, Phakopsora pachyrhizi evolved resistance to several fungicide classes, prompting us to screen a large sample of the globally widespread wheat yellow rust pathogen, Puccinia striiformis f. sp. tritici (Pst), for mutations associated with fungicide resistance.

RESULTS

We evaluated 363 Pst isolates from Europe, the USA, Ethiopia, Chile, China and New Zealand for mutations in the target genes of demethylase inhibitor (DMI; Cyp51) and succinate dehydrogenase inhibitor (SDHI; SdhB, SdhC and SdhD) fungicides. A high proportion of Pst isolates carrying a Y134F DMI resistance-associated substitution in the Cyp51 gene was found among those from China and New Zealand. A set of geographically diverse Pst isolates was also found to display a substitution in SdhC (I85V) that is homologous to that reported recently in P. pachyrhizi and linked to SDHI resistance.

CONCLUSION

The identification of resistance-associated alleles confirms that cereal rusts are not immune to fungicide resistance and that selection for resistance evolution is operating at high levels in certain locations. It highlights the need to adopt fungicide resistance management practices and to monitor cereal rust species for development of resistance. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Molecular Identification, Antifungal Susceptibility Testing, and Mechanisms of Azole Resistance in Aspergillus Species Received within a Surveillance Program on Antifungal Resistance in Spain

Antifungal resistance is one of the major causes of the increasing mortality rates for fungal infections, especially for those caused by Aspergillus spp. A surveillance program was established in 2014 in the Spanish National Center for Microbiology for tracking resistance in the most prevalent Aspergillus species. A total of 273 samples were included in the study and were initially classified as susceptible or resistant according to EUCAST breakpoints. Several Aspergillus cryptic species were found within the molecularly identified isolates. Cyp51 mutations were characterized for Aspergillus fumigatus, Aspergillus terreus, and Aspergillus flavus sensu stricto strains that were classified as resistant. Three A. fumigatus sensu stricto strains carried the TR₃₄/L98H resistance mechanism, while two harbored G54R substitution and one harbored the TR₄₆/Y121F/T289A mechanism. Seventeen strains had no mutations in cyp51A, with ten of them resistant only to isavuconazole. Three A. terreus sensu stricto strains harbored D344N substitution in cyp51A, one of them combined with M217I, and another carried an A249G novel mutation. Itraconazole-resistant A. flavus sensu stricto strains harbored P220L and H349R alterations in cyp51A and cyp51C, respectively, that need further investigation on their implication in azole resistance.

New Insights into the Cyp51 Contribution to Azole Resistance in Aspergillus Section Nigri

Invasive aspergillosis (IA) is a severe condition mainly caused by Aspergillus fumigatus, although other species of the genus, such as section Nigri members, can also be involved. Voriconazole (VRC) is the recommended treatment for IA; however, the prevalence of azole-resistant Aspergillus isolates has alarmingly increased in recent years, and the underlying resistance mechanisms in non-fumigatus species remain unclear. We have determined the in vitro susceptibility of 36 strains from section Nigri to VRC, posaconazole (POS), and itraconazole (ITC), and we have explored the role of Cyp51A and Cyp51B, both targets of azoles, in azole resistance. The three drugs were highly active; POS displayed the best in vitro activity, while ITC and VRC showed MICs above the established epidemiological cutoff values in 9 and 16% of the strains, respectively. Furthermore, expression studies of cyp51A and cyp51B in control condition and after VRC exposure were performed in 14 strains with different VRC susceptibility. We found higher transcription of cyp51A, which was upregulated upon VRC exposure, but no correlation between MICs and cyp51 transcription levels was observed. In addition, cyp51A sequence analyses revealed nonsynonymous mutations present in both, wild-type and non-wild-type strains of A. niger and A. tubingensis Nevertheless, a few mutations were exclusively present in non-wild-type A. tubingensis strains. Altogether, our results suggest that azole resistance in section Nigri is not clearly explained by Cyp51A protein alteration or by cyp51 gene upregulation, which indicates that other mechanisms might be involved.

Identification of 14-α-Lanosterol Demethylase (CYP51) in Scedosporium Species

Scedosporium spp. cause infections (scedosporiosis) in both immunocompetent and immunocompromised individuals and may persistently colonize the respiratory tract in patients with cystic fibrosis (CF). They are less susceptible against azoles than are other molds, such as Aspergillus spp., suggesting the presence of resistance mechanisms. It can be hypothesized that the decreased susceptibility of Scedosporium spp. to azoles is also CYP51 dependent. Analysis of the Scedosporium apiospermum and Scedosporiumaurantiacum genomes revealed one CYP51 gene encoding the 14-α-lanosterol demethylase. This gene from 159 clinical or environmental Scedosporium isolates and three Lomentospora prolificans isolates has been sequenced and analyzed. The Scedosporium CYP51 protein clustered with the group of known CYP51B orthologues and showed species-specific polymorphisms. A tandem repeat in the 5' upstream region of Scedosporium CYP51 like that in Aspergillus fumigatus could not be detected. Species-specific amino acid alterations in CYP51 of Scedosporium boydii, Scedosporiumellipsoideum, Scedosporium dehoogii, and Scedosporiumminutisporum isolates were located at positions that have not been described as having an impact on azole susceptibility. In contrast, two of the three Sapiospermum-specific amino acid changes (Y136F and G464S) corresponded to respective mutations in A. fumigatus CYP51A at amino acid positions 121 and 448 (Y121F and G448S, respectively) that had been linked to azole resistance.

teres Associated with Target Site Modification and Inducible Overexpression of Cyp51

Pyrenophora teres f. sp. teres is the cause of net form of net blotch (NFNB), an economically important foliar disease in barley (Hordeum vulgare). Net and spot forms of net blotch are widely controlled using site-specific systemic fungicides. Although resistance to succinate dehydrogenase inhibitors and quinone outside inhibitors has been addressed before in net blotches, mechanisms controlling demethylation inhibitor resistance have not yet been reported at the molecular level. Here we report the isolation of strains of NFNB in Australia since 2013 resistant to a range of demethylase inhibitor fungicides. Cyp51A:KO103-A1, an allele with the mutation F489L, corresponding to the archetype F495I in Aspergillus fumigatus, was only present in resistant strains and was correlated with resistance factors to various demethylase inhibitors ranging from 1.1 for epoxiconazole to 31.7 for prochloraz. Structural in silico modeling of the sensitive and resistant CYP51A proteins docked with different demethylase inhibitor fungicides showed how the interaction of F489L within the heme cavity produced a localized constriction of the region adjacent to the docking site that is predicted to result in lower binding affinities. Resistant strains also displayed enhanced induced expression of the two Cyp51A paralogs and of Cyp51B genes. While Cyp51B was found to be constitutively expressed in the absence of fungicide, Cyp51A was only detected at extremely low levels. Under fungicide induction, expression of Cyp51B, Cyp51A2, and Cyp51A1 was shown to be 1.6-, 3,- and 5.3-fold higher, respectively in the resistant isolate compared to the wild type. These increased levels of expression were not supported by changes in the promoters of any of the three genes. The implications of these findings on demethylase inhibitor activity will require current net blotch management strategies to be reconsidered in order to avoid the development of further resistance and preserve the lifespan of fungicides in use.

Proposal for a unified nomenclature for target-site mutations associated with resistance to fungicides

Evolved resistance to fungicides is a major problem limiting our ability to control agricultural, medical and veterinary pathogens and is frequently associated with substitutions in the amino acid sequence of the target protein. The convention for describing amino acid substitutions is to cite the wild-type amino acid, the codon number and the new amino acid, using the one-letter amino acid code. It has frequently been observed that orthologous amino acid mutations have been selected in different species by fungicides from the same mode of action class, but the amino acids have different numbers. These differences in numbering arise from the different lengths of the proteins in each species. The purpose of the present paper is to propose a system for unifying the labelling of amino acids in fungicide target proteins. To do this we have produced alignments between fungicide target proteins of relevant species fitted to a well-studied 'archetype' species. Orthologous amino acids in all species are then assigned numerical 'labels' based on the position of the amino acid in the archetype protein. © 2016 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.