Can repurposing of existing drugs provide more effective therapies for invasive fungal infections?

KEYWORDS:

• Repurposing
• fungal infections
• efficacious
• anti-fungal

1. Why are invasive fungal infections difficult to cure?

Invasive fungal infections (IFIs) constitute a significant menace of public health. While most people, at least once in their lifetime, will be afflicted by a superficial fungal infection that is usually easy to cure, millions will suffer from an IFI that is difficult to eradicate.[1] IFIs are of great public health concern with alarmingly high mortality rates because they are difficult to diagnose and often refractory to treatment.[1,2] For example, the mortality rates of the deeply IFIs caused by three among the most common species of human fungal pathogens Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans range between 20% and 40%, 50% and 90%, and 20% and 70%, respectively.[3,4] Successful management of IFIs requires early diagnosis; reversal of underlying predisposing risk factors; surgical debridement, when necessary; and prompt administration of effective antifungal agents. However, despite these measures, on many occasions, IFIs are not always amenable to cure. Emerging IFIs such as non-albicans Candida, non-fumigatus Aspergillus, opportunistic yeast-like fungi (Trichosporon and Rhodotorula species), Mucorales, and hyaline molds (Fusarium and Scedosporium species) are more recalcitrant to current antifungal treatment modalities and, thus, associated with higher mortality rates.[3] Some of these emerging IFIs, such as those due to Mucorales, possess unique virulence characteristics and exert distinctive host–pathogen interactions, facilitating, thus, host evasion and disease progression.

The difficulty in the treatment of these IFIs and the subsequent dismal outcomes are associated with the lack of newer efficacious antifungal agents and the expansion of the immunocompromised population, as a result of modern medical interventions and immunosuppressive diseases such as HIV/AIDS.[5] In addition, biofilm formation, which is inevitable with the extensive use of indwelling medical devices, confers high drug resistance and pathogenicity. Numerous clinical observations and experimental studies show that the currently available antifungal agents may not be effective in completely eliminating biofilm-related fungal infections. The current therapeutic options for IFIs are quite limited and include only three different classes of antifungal agents: polyenes, azoles, and echinocandins, which suffer from limitations of toxicity, spectrum of activity, emergence of resistance, drug interactions, and pharmacokinetic/pharmacodynamic properties.

Increasing antifungal drug resistance, pathogenicity of emerging IFIs, and the inherent limitations of conventional antifungal compounds underscore the need for the development of new antifungal agents. The need for effective antifungal therapeutics is apparent in the setting of immunocompromised hosts, where the immune system is ineffective in protecting against invading fungi. However, the development of an entirely new drug is a long and expensive process, which faces fundamental scientific, economic, and regulatory challenges.[6] Indeed, many years are required for a new antifungal agent to progress from the laboratory to clinical practice while, on many occasions, current antifungal treatment is still based on medications discovered 50 years ago.[4] To that end, creative approaches are needed to fill the gap in the pipeline of the depleted antifungal agents.

2. Repurposing of approved drugs against human fungal pathogens

Drug ‘repurposing’, also known as ‘redirecting’, ‘repositioning’, or ‘reprofiling’, has evolved as a strategy to speed and reduce the cost of drug development, as well as to increase treatment success rates. Indeed, while de novo drug discovery and development is a 10–17-year process, and the probability of success is lower than 10%, drug repurposing expedites the introduction of the drug with the new indication into clinical practice. Repurposing builds on previous research and development of established medications that have already been tested in humans in terms of toxicology and pharmacology.[2,4,7]

Since 2010, by the announcement of the NIH’s repurposing initiative by the National Center for Advancing Translational Sciences, the drug-repositioning concept has been expanded, involving considerable interdisciplinary collaboration.[8] The National Cancer Institute/Developmental Therapeutics Program repository maintains approximately >140,000 small molecules and natural products for repurposing evaluation. Interestingly, a large number of these compounds have demonstrated activity against pathogenic fungi, including drug-resistant strains.[2]

3. Expert opinion

In the antifungal arena, drug repurposing as a means to expand the indications of currently used anti-infective agents has been demonstrated in a number of antifungal, antibacterial, and antiviral agents. Namely, terbinafine, an allylamine antifungal agent, was originally indicated for the treatment of superficial skin infections (tinea cruris, tinea pedis, tinea corporis, and onychomycosis usually by a dermatophyte or Candida). Terbinafine has been used effectively alone or in combination with saturated solution of potassium iodide for the treatment of patients with cutaneous or lymphocutaneous sporotrichosis that are unresponsive to itraconazole or when itraconazole is not tolerated, and appears to be effective.[9,10] While there is no consensus for optimal dosage and duration for terbinafine, in small series of patients and case reports, clinical cure was achieved in dosage ranging from 125 to 1000 mg/day given for 4–37 weeks.[9,10] The only randomized controlled treatment trial for sporotrichosis assessed two different dosages of terbinafine (500 mg daily vs. 1000 mg daily) and showed superiority of the higher dosage in the treatment of cutaneous and lymphocutaneous sporotrichosis.[11] The update of the Clinical Practice Guidelines of Sporotrichosis of the Infectious Disease Society of America recommends for cutaneous or lymphocutaneous sporotrichosis terbinafine at a dose of 500 mg orally twice daily in patients who do not respond to the regular dosage of itraconazole (recommendation level A-II).[12]

Trimethoprim–sulfamethoxazole (TMP–SMX), an antibacterial agent with indications and usage involving the treatment and prophylaxis of Pneumocystis jirovecii pneumonia, enteritis caused by Shigella spp., and urinary tract infections due to susceptible strains of Escherichia coli, Klebsiella spp., Enterobacter spp., Morganella morganii, and Proteus spp., has shown striking in vitro activity against Paracoccidioides brasiliensis. The Brazilian guidelines on the diagnosis and treatment of paracoccidioidomycosis recommend itraconazole and TMP–SMX as first- and second-line treatments, respectively.[13] However, there are only two previous studies comparing itraconazole and TMP–SMX in the treatment of paracoccidioidomycosis.[14,15]

Imiquimod is an imidazoquinoline amine analog of guanosine. It has an immunomodulatory effect with powerful antiviral activity, and antitumor action, with enhanced innate and acquired immunity, in particular, activating Toll-like receptor-7.[16] Although there is only a single report on the use of imiquimod against chromoblastomycosis, topical imiquimod 5% applied four to five times per week appeared to be a promising adjunctive agent, when used in combination with itraconazole.[17]

The alkyl phosphocholine drug miltefosine was initially developed as an anticancer agent and was used against Leishmania species and Trypanosoma cruzi. In addition, more recently, it has been shown to have fungicidal activity against Scedosporium, Fusarium, and Mucormycetes.[18–20] In vitro studies have shown that miltefosine demonstrates synergistic interactions when combined with posaconazole or voriconazole against a series of uncommon filamentous fungi [20] and can also cause a significant reduction in amphotericin B and voriconazole minimum inhibitory concentrations when used against Scedosporium spp.[21] In vivo, miltefosine given orally to mice after intravenous infection with C. neoformans delayed the morbidity and mortality and reduced the cryptococcal burden in the brain.[18] The use of miltefosine in patients with IFIs is scarce, being limited to few case reports.

Recently, a number of groups have screened compound libraries of ‘off-patent’ medications and revealed agents previously unknown to have antifungal activity and initially developed as anticancer drugs. Namely, the small molecule bis (1,6-a:5ʹ,6ʹ-g) quinolizinium-8-methyl-salt had greater activity than fluconazole and itraconazole against various fungal pathogens including multidrug-resistant C. albicans and A. fumigatus. The aminopeptidase inhibitor tosedostat, which is currently in a clinical trial phase for anticancer therapy, also displayed broad antifungal activity against different Candida spp. Furthermore, heat shock protein (Hsp90) inhibitors dramatically enhance the efficacy of existing antifungal agents both in vitro and in a model of disseminated candidiasis. They also block the emergence of drug resistance, have fungicidal activity, and demonstrate broad efficacy against diverse fungal pathogens.[2]

Another active antifungal compound primarily used to treat breast cancer is tamoxifen, an estrogen receptor agonist. In a murine model of disseminated candidiasis, tamoxifen treatment decreased kidney fungal burden and provided evidence that at least part of its antifungal activity was due to inhibition of calmodulin. Another estrogen receptor agonist, toremifene, showed in vitro and in vivo synergistic interaction with either caspofungin or amphotericin B against C. albicans and Candida glabrata.[2]

Repurposing as a means to increase the utility of antifungal agents also has been demonstrated with a number of other compounds including statins, fluoroquinolones, nonsteroidal anti-inflammatory agents (ibuprofen, sodium salicylate, propylparaben, and diclofenac sodium), calcineurin inhibitors (cyclosporine or tacrolimus), natural products (farnesol or berberine), immunomodulating factors (granulocyte macrophage-colony stimulating factor or interferon-gamma), and deferasirox. There also are numerous examples of agents of different compounds that when combined with the conventional antifungal agents enhance their in vitro activity even in the setting of antifungal drug resistance.[2] Table 1 summarizes some representative repurposed compounds that could be used as therapeutic compounds for IFIs.

Table 1. Representative repurposed compounds for invasive fungal infections.

Repurposed compound	Initial target	Fungal target	Comment
Terbinafine	N/A	Squalene epoxidase inhibition	Initially used for superficial skin infections and now repurposed for treatment of sporotrichosis
Trimethoprim–sulfamethoxazole	Inhibits successive steps in the folate synthesis pathway	Unknown	Antibacterial agent for PCP pneumonia and UTI due to susceptible Gram-negative bacilli; repurposed for use against paracoccidioidomycosis
Imiquimod	Activates innate immune system through TLR7	Unknown	Initially used as immune response modifier to treat genital warts, superficial basal cell carcinoma, and actinic keratosis; repurposed for use against chromoblastomycosis
Miltefosine	Protein kinase B (Akt) inhibitor	Unknown	Initially used for leishmaniasis, repurposed for use against Cryptococcus neoformans and for synergistic interaction with some triazoles against filamentous fungi, such as infections caused by multidrug-resistant Scedosporium prolificans
BQM	Unknown	Reverses MDR1 resistance by accumulating in cells	Developed as anticancer compound; repurposed for use its broad and potent antifungal activity
Tamoxifen	Estrogen receptor agonist	Calmodulin	Initially used to treat breast cancer; repurposed for use against disseminated candidiasis (see text)
Statins	HMG-CoA reductase inhibitor	HMG-CoA reductase inhibitor (ergosterol synthesis inhibition)	Used as lipid-lowering medication; statins have in vitro activities against Candida spp., C. neoformans, and Mucorales. Synergy with fluconazole has been reported
Fluoroquinolones	Inhibitors of DNA synthesis	Inhibitors of DNA synthesis	Used as a broad-spectrum antibiotic; repurposed for use to enhance the in vitro activity of antifungal agents against Candida and Aspergillus spp.
Nonsteroidal anti-inflammatory agents	Inhibition of cyclooxygenase in the prostaglandin biosynthetic pathway	Reverses the resistance conferred by efflux pumps	Indicated as antipyretics, analgesics; repurposed for use to potentiate fluconazole antifungal activity against Candida albicans
Calineurin inhibitors (cyclosporin and tacrolimus)	Prevent interleukin-2 production by T cells	Calcineurin inhibitor	Indicated for immunosuppression following transplantation, ulcerative colitis, and dermatological uses; repurposed for use in vitro as adjunctive antifungal agents against fungal pathogens
Natural products farnesol	Quorum-sensing molecule	Inhibition of germ tube formation by suppressing both mitogen-activated protein kinase, cAMP–protein kinase A pathways, and RAS protein	Inhibits Candida filamentation, and acts synergistically in vitro with fluconazole against Candida spp.
Deferasirox	Iron chelator	Inhibits iron uptake by fungi	Initially for the treatment of transfusion-dependent iron overload; repurposed for use as adjunctive therapy for mucormycosis
Immunomodulating factors (GM-CSF and IFN-γ)	Stimulate the production and activation of phagocytic host cells	Unknown	GM-CSF, used for the prevention of neutropenia after chemotherapy, IFN-γ used for chronic granulomatous disease and osteopetrosis; repurposed for use as adjunctive immunomodulatory agents against fungal pathogens

BQM: bis (1,6-a:5ʹ,6ʹ-g) quinolizinium-8-methyl-salt; GM-CSF: granulocyte macrophage-colony stimulating factor; IFN-γ: interferon-gamma; N/A, not applicable; PCP, Pneumocystis jirovecii pneumonia; UTI, urinary tract infections; TLR: Toll-like receptor; MDR1: multidrug resistance protein 1; HMG-CoA: hydroxy-methylglutaryl-CoA; cAMP: cyclic adenosine monophosphate; RAS protein: guanosine-nucleotide-binding protein.

In summary, repurposing of existing drugs can potentially provide more effective treatments for IFIs. Whether these approaches would be beneficial for the treatment of invasive mycoses in the clinical setting remains to be verified in vivo and in clinical trials.

Declaration of interests

TJ Walsh receives research grants to Weill Cornell Medicine for experimental and clinical antimicrobial pharmacotherapeutics, new diagnostic systems, and strategies for augmentation of host defense against life-threatening infections in immunocompromised children and adult patients from Save our Sick (SOS) Kids Foundation, Astellas, Novartis, Merck, ContraFect, Cubist, and Pfizer. TJ Walsh has served as consultant to Astellas, ContraFect, iCo, Novartis, Pfizer, Methylgene, SigmaTau, and Trius.

E Roilides has received research grant support from Pfizer, Gilead, Merck, has served as consultant to Gilead, Astellas, Pfizer and has been in the speakers’ bureau of Merck, Pfizer, Gilead and Astellas.

A Katragkou was supported by a 2014-2016 European Society for Pediatric Infectious Disease Fellowship.