Anthony Weinstock‡
a,
Natarajan Arumugam‡*b,
Abdulrahman I. Almansourb,
Raju Suresh Kumar
b and
Shankar Thangamani
*c
aArizona College of Osteopathic Medicine, Midwestern University, 19555 N. 59th Ave., Glendale, AZ 85308, USA
bDepartment of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia. E-mail: antarajan@ksu.edu.sa; Fax: +966 4675992; Tel: +966 4675907
cDepartment of Pathology and Population Medicine, College of Veterinary Medicine, Midwestern University, 19555 N. 59th Ave., Glendale, AZ 85308, USA. E-mail: sthang@midwestern.edu; Fax: +1 623 537 6399; Tel: +1 623 537 6378
First published on 20th September 2019
Invasive fungal infections caused by Candida and Cryptococcus species lead to life threating infections in immunocompromised individuals. Furthermore, increasing incidence of fungal strains resistant to FDA-approved antifungal drugs along with the paucity of antifungal drugs warrants novel drugs to treat invasive fungal infections. In this study, we investigated the antifungal activity of a novel series of diazahexa/hepta cyclic cage-like compounds. Results indicate that compounds with unsubstituted and o-methyl substitution on aryl rings exhibit potent broad-spectrum antifungal activity against various fungal strains. In addition, these compounds showed significant inhibitory activity against Candida hyphae and biofilm formation. Collectively, results from this study indicate that these compounds are promising candidates to develop as novel antifungal drugs to treat drug-resistant fungal infections.
In this study, we aim to identify novel compounds against the important human fungal pathogens, Candida and Cryptococcus species. Our results indicate that compounds 5a and 5g exhibit potent inhibitory activity against fungal strains and have the potential to develop as new antifungal drugs.
Entry | Compounds | MIC (μg mL−1) |
---|---|---|
1 | 5a | 4 |
2 | 5b | 128 |
3 | 5c | 32 |
4 | 5d | 128 |
5 | 5e | 128 |
6 | 5f | 128 |
7 | 5g | 4 |
8 | 5h | 256 |
9 | 5i | 16 |
10 | 5j | 16 |
11 | 6a | 16 |
12 | 6c | 64 |
14 | 6d | 128 |
15 | 6g | 64 |
16 | 6i | 16 |
Next, we tested the antifungal activity of compounds 5a and 5g against clinical strains of Candida spp. and Cryptococcus neoformans in order to determine if the compounds possess broad-spectrum activity against a variety of fungal species. After 24 hours of incubation, both 5a and 5g demonstrated potent antifungal activity against C. albicans, Candida parapsilosis, and Candida tropicalis with the MICs ranging from 0.5 to 4 μg mL−1 (Table 2). Further, 5a and 5g also inhibited the growth of these fungal strains even after 48 hours of incubation with the MICs ranging from 2 to 8 μg mL−1 (Table 2). Compounds 5a and 5g also showed excellent antifungal activity against C. neoformans and C. glabrata with the MICs ranging from 0.5 to 2 μg mL−1 after 48 hours of incubation (Table 2).
Strain | Description | MIC Fluconazole (μg mL−1) | MIC 5a (μg mL−1) | MIC 5g (μg mL−1) | |||
---|---|---|---|---|---|---|---|
24 h | 48 h | 24 h | 48 h | 24 h | 48 h | ||
a Not detected (n.d.). | |||||||
C. albicans NR-29434 | Bloodstream isolate from a patient with candidemia from Winnipeg, Manitoba, Canada in 2000 | 0.125 | 32 | 4 | 8 | 2 | 4 |
C. albicans NR-29449 | Vaginal isolate from a patient with vaginitis from Ann Arbor, Michigan, USA between 1990 and 1992 | 0.5 | 32 | 4 | 8 | 4 | 8 |
C. albicans NR-29435 | Bloodstream isolate from a patient with candidemia from Iowa City, Iowa, USA in 2000 | 0.5 | 0.5 | 2 | 4 | 2 | 4 |
C. albicans NR-29448 | Clinical isolate from a person with candidemia from Arizona, USA | 1 | 32 | 2 | 8 | 2 | 4 |
C. albicans NR-29437 | Bloodstream isolate from a patient with candidemia from Brussels, Belgium in 2000 | 0.25 | 32 | 4 | 8 | 2 | 4 |
C. albicans NR-29446 | Bloodstream isolate from a patient with candidemia from Utah, USA | 16 | 32 | 1 | 4 | 0.5 | 4 |
C. albicans NR5-29453 | Clinical isolate from a patient with thrush and HIV from Pretoria, South Africa | 0.0625 | 0.0625 | 4 | 4 | 2 | 4 |
C. albicans NR-29438 | Bloodstream isolate from a patient with candidemia from Tel-Hashomer, Israel, in 2000 | 0.0625 | 0.25 | 4 | 8 | 2 | 4 |
C. albicans NR-29367 | Clinical isolate from China | 0.0625 | 0.0625 | 4 | 4 | 2 | 4 |
C. albicans NR-29439 | Bloodstream isolate from a patient with candidemia from Omaha, Nebraska, USA, in 2000 | 0.5 | 32 | 2 | 8 | 2 | 4 |
C. albicans NR-29440 | Bloodstream isolate from a patient with candidemia from Lille, France, in 2000 | 0.25 | 0.5 | 4 | 8 | 2 | 4 |
C. albicans NR-29441 | Bloodstream isolate from a patient with candidemia from Iowa City, Iowa, USA, in 2000 | 0.25 | 16 | 2 | 8 | 1 | 4 |
C. albicans NR-29442 | Bloodstream isolate from a patient with candidemia from Ottawa, Ontario, Canada, in 2000 | 0.25 | 32 | 4 | 8 | 2 | 4 |
C. albicans NR-29444 | Oral isolate from a patient with vaginitis collected in Ann Arbor, Michigan, USA between 1990 and 1992. | 1 | 16 | 2 | 4 | 2 | 4 |
C. parapsilosis ATCC 22019 | Clinical isolate from a patient with celiac disease from Puerto Rico | 1 | 4 | 0.5 | 2 | 0.5 | 2 |
C. tropicalis ATCC 13803 | FDA provided isolate | 8 | 32 | 4 | 8 | 2 | 4 |
C. glabrata ATCC 90030 | Bloodstream isolate from a patient from Iowa | n.d. | 4 | n.d. | 0.5 | n.d. | 0.5 |
C. albicans ATCC 10231 | Clinical isolate from a patient with bronchomycosis | 2 | 4 | 2 | 4 | 1 | 4 |
C. neoformans NR-41291 | Cerebrospinal fluid isolate from a patient from China in July 2011 | n.d. | 32 | n.d. | 2 | n.d. | 2 |
C. neoformans NR-41292 | Cerebrospinal fluid isolate from a patient from China in February 2012 | n.d. | 32 | n.d. | 1 | n.d. | 1 |
C. neoformans NR-41296 | Cerebrospinal fluid isolate from a patient from China in February 2012 | n.d. | 4 | n.d. | 1 | n.d. | 1 |
C. neoformans NR-41295 | Cerebrospinal fluid isolate from a patient from China in February 2012 | n.d. | 32 | n.d. | 1 | n.d. | 1 |
C. neoformans NR-41294 | Cerebrospinal fluid isolate from a patient from China in June 2011 | n.d. | 4 | n.d. | 0.5 | n.d. | 0.5 |
C. neoformans NR-41297 | Cerebrospinal fluid isolate from a patient from China in February 2012 | n.d. | 8 | n.d. | 1 | n.d. | 1 |
C. neoformans NR-41298 | Cerebrospinal fluid isolate from a patient from China in February 2012 | n.d. | 16 | n.d. | 2 | n.d. | 2 |
C. neoformans NR-41299 | Cerebrospinal fluid isolate from a patient from China in August 2009 | n.d. | 8 | n.d. | 1 | n.d. | 1 |
Fluconazole was used as a metric of comparison, due to its status as FDA-approved antifungal drug with widespread clinical use. Fluconazole inhibited the Candida strains with a wide range of MICs from 0.0625 to 16 μg mL−1 after 24 hours of incubation (Table 2). However, after 48 hours, the MICs of fluconazole for all strains were increased several fold with most strains inhibited at 32 μg mL−1 (Table 2). Surprisingly, unlike fluconazole, both compounds 5a and 5g showed potent inhibitory activity against most of the Candida strains, even after 48 hours of incubation without considerable increase in the MIC values. In addition, 5a and 5g also showed excellent antifungal activity against C. glabrata and C. neoformans after 48 hours of incubation compared to fluconazole (Table 2). Collectively, compounds 5a and 5g exhibit potent broad-spectrum antifungal activity compared to fluconazole against all the fungal strains tested in this study.
Results from this study indicate that compounds 5a and 5g possess potent antifungal and antivirulence activities without causing mammalian host toxicity. In addition, 5a and 5g showed excellent activity compared to fluconazole. Further studies to determine the pharmacokinetic and physicochemical profile of these compounds is essential to move these compounds to the next stage of the drug development pipeline. The structure of 5a and 5g contain the nitrogen (N1) atom in the diazahexa cage-like compounds, a characteristic shared by the benzylamine and allylamine antifungals. These antifungals act as inhibitors of squalene epoxidase, a key enzyme in the synthesis of sterols by fungi. The nitrogen (N12) atom in the pyrrolidine ring shares similar bonding to that of echinocandins, which act as beta-1, 3-D-glucan synthase inhibitors. The carbonyl unit of piperidone at position sixteen shares similarities to that of the flavonoids, which have become a new area of study for their antifungal activity. Therefore, we speculate that these novel compounds may target pathways specific to fungi including fungal 3b-glucan synthase and ergosterol biosynthesis.25 However, future studies will be needed to understand the antifungal mechanism of these compounds. In addition, further structure modifications to enhance the activities of 5a and 5g should be also a promising avenue. Taken together, compounds 5a and 5g have strong potential to develop as novel antifungal drugs.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra05777c |
‡ Co-first authors. |
This journal is © The Royal Society of Chemistry 2019 |