Viral protease inhibitors affect the production of virulence factors in Cryptococcus neoformans

Strategies of Pharmacological Repositioning for the Treatment of Medically Relevant Mycoses

Fungal infections represent a worldwide concern for public health, due to their prevalence and significant increase in cases each year. Among the most frequent mycoses are those caused by members of the genera Candida, Cryptococcus, Aspergillus, Histoplasma, Pneumocystis, Mucor, and Sporothrix, which have been treated for years with conventional antifungal drugs, such as flucytosine, azoles, polyenes, and echinocandins. However, these microorganisms have acquired the ability to evade the mechanisms of action of these drugs, thus hindering their treatment. Among the most common evasion mechanisms are alterations in sterol biosynthesis, modifications of drug transport through the cell wall and membrane, alterations of drug targets, phenotypic plasticity, horizontal gene transfer, and chromosomal aneuploidies. Taking into account these problems, some research groups have sought new therapeutic alternatives based on drug repositioning. Through repositioning, it is possible to use existing pharmacological compounds for which their mechanism of action is already established for other diseases, and thus exploit their potential antifungal activity. The advantage offered by these drugs is that they may be less prone to resistance. In this article, a comprehensive review was carried out to highlight the most relevant repositioning drugs to treat fungal infections. These include antibiotics, antivirals, anthelmintics, statins, and anti-inflammatory drugs.

Early Antiretroviral Therapy Not Associated With Higher Cryptococcal Meningitis Mortality in People With Human Immunodeficiency Virus in High-Income Countries: An International Collaborative Cohort Study

BACKGROUND

Randomized controlled trials (RCTs) from low- and middle-income settings suggested that early initiation of antiretroviral therapy (ART) leads to higher mortality rates among people with HIV (PWH) who present with cryptococcal meningitis (CM). There is limited information about the impact of ART timing on mortality rates in similar people in high-income settings.

METHODS

Data on ART-naive PWH with CM diagnosed from 1994 to 2012 from Europe/North America were pooled from the COHERE, NA-ACCORD, and CNICS HIV cohort collaborations. Follow-up was considered to span from the date of CM diagnosis to earliest of the following: death, last follow-up, or 6 months. We used marginal structural models to mimic an RCT comparing the effects of early (within 14 days of CM) and late (14-56 days after CM) ART on all-cause mortality, adjusting for potential confounders.

RESULTS

Of 190 participants identified, 33 (17%) died within 6 months. At CM diagnosis, their median age (interquartile range) was 38 (33-44) years; the median CD4+ T-cell count, 19/μL (10-56/μL); and median HIV viral load, 5.3 (4.9-5.6) log10 copies/mL. Most participants (n = 157 [83%]) were male, and 145 (76%) started ART. Mimicking an RCT, with 190 people in each group, there were 13 deaths among participants with an early ART regimen and 20 deaths among those with a late ART regimen. The crude and adjusted hazard ratios comparing late with early ART were 1.28 (95% confidence interval, .64-2.56) and 1.40 (.66-2.95), respectively.

CONCLUSIONS

We found little evidence that early ART was associated with higher mortality rates among PWH presenting with CM in high-income settings, although confidence intervals were wide.

Peptidases: promising antifungal targets of the human fungal pathogen, Cryptococcus neoformans

Cryptococcus neoformans is a globally important fungal pathogen, primarily inflicting disease on immunocompromised individuals. The widespread use of antifungal agents in medicine and agriculture supports the development of antifungal resistance through evolution, and the emergence of new strains with intrinsic resistance drives the need for new therapeutics. For C. neoformans, the production of virulence factors, including extracellular peptidases (e.g., CnMpr-1 and May1) with mechanistic roles in tissue invasion and fungal survival, constitute approximately 2% of the fungal proteome and cover five classes of enzymes. Given their role in fungal virulence, peptidases represent promising targets for anti-virulence discovery in the development of new approaches against C. neoformans. Additionally, intracellular peptidases, which are involved in resistance mechanisms against current treatment options (e.g., azole drugs), as well as capsule biosynthesis and elaboration of virulence factors, present additional opportunities to combat the pathogen. In this review, we highlight key cryptococcal peptidases with defined or predicted roles in fungal virulence and assess sequence alignments against their human homologs. With this information, we define the feasibility of the select peptidases as “druggable” targets for inhibition, representing prospective therapeutic options against the deadly fungus.

From Naturally-Sourced Protease Inhibitors to New Treatments for Fungal Infections

Proteases are involved in a broad range of physiological processes, including host invasion by fungal pathogens, and enzymatic inhibition is a key molecular mechanism controlling proteolytic activity. Importantly, inhibitors from natural or synthetic sources have demonstrated applications in biochemistry, biotechnology, and biomedicine. However, the need to discover new reservoirs of these inhibitory molecules with improved efficacy and target range has been underscored by recent protease characterization related to infection and antimicrobial resistance. In this regard, naturally-sourced inhibitors show promise for application in diverse biological systems due to high stability at physiological conditions and low cytotoxicity. Moreover, natural sources (e.g., plants, invertebrates, and microbes) provide a large reservoir of undiscovered and/or uncharacterized bioactive molecules involved in host defense against predators and pathogens. In this Review, we highlight discoveries of protease inhibitors from environmental sources, propose new opportunities for assessment of antifungal activity, and discuss novel applications to combat biomedically-relevant fungal diseases with in vivo and clinical purpose.

Repositioning Lopinavir, an HIV Protease Inhibitor, as a Promising Antifungal Drug: Lessons Learned from Candida albicans-In Silico, In Vitro and In Vivo Approaches

The repurposing strategy was applied herein to evaluate the effects of lopinavir, an aspartic protease inhibitor currently used in the treatment of HIV-infected individuals, on the globally widespread opportunistic human fungal pathogen Candida albicans by using in silico, in vitro and in vivo approaches in order to decipher its targets on fungal cells and its antifungal mechanisms of action. Secreted aspartic proteases (Saps) are the obviously main target of lopinavir. To confirm this hypothesis, molecular docking assays revealed that lopinavir bound to the Sap2 catalytic site of C. albicans as well as inhibited the Sap hydrolytic activity in a typically dose-dependent manner. The inhibition of Saps culminated in the inability of C. albicans yeasts to assimilate the unique nitrogen source (albumin) available in the culture medium, culminating with fungal growth inhibition (IC₅₀ = 39.8 µM). The antifungal action of lopinavir was corroborated by distinct microscopy analyses, which evidenced drastic and irreversible changes in the morphology that justified the fungal death. Furthermore, our results revealed that lopinavir was able to (i) arrest the yeasts-into-hyphae transformation, (ii) disturb the synthesis of neutral lipids, including ergosterol, (iii) modulate the surface-located molecules, such as Saps and mannose-, sialic acid- and N-acetylglucosamine-containing glycoconjugates, (iv) diminish the secretion of hydrolytic enzymes, such as Saps and esterase, (v) negatively influence the biofilm formation on polystyrene surface, (vi) block the in vitro adhesion to epithelial cells, (vii) contain the in vivo infection in both immunocompetent and immunosuppressed mice and (viii) reduce the Sap production by yeasts recovered from kidneys of infected animals. Conclusively, the exposed results highlight that lopinavir may be used as a promising repurposing drug against C. albicans infection as well as may be used as a lead compound for the development of novel antifungal drugs.

Re-emerging Aspartic Protease Targets: Examining Cryptococcus neoformans Major Aspartyl Peptidase 1 as a Target for Antifungal Drug Discovery

Cryptococcosis is an invasive infection that accounts for 15% of AIDS-related fatalities. Still, treating cryptococcosis remains a significant challenge due to the poor availability of effective antifungal therapies and emergence of drug resistance. Interestingly, protease inhibitor components of antiretroviral therapy regimens have shown some clinical benefits in these opportunistic infections. We investigated Major aspartyl peptidase 1 (May1), a secreted Cryptococcus neoformans protease, as a possible target for the development of drugs that act against both fungal and retroviral aspartyl proteases. Here, we describe the biochemical characterization of May1, present its high-resolution X-ray structure, and provide its substrate specificity analysis. Through combinatorial screening of 11,520 compounds, we identified a potent inhibitor of May1 and HIV protease. This dual-specificity inhibitor exhibits antifungal activity in yeast culture, low cytotoxicity, and low off-target activity against host proteases and could thus serve as a lead compound for further development of May1 and HIV protease inhibitors.

Aspartic peptidase of Phialophora verrucosa as target of HIV peptidase inhibitors: blockage of its enzymatic activity and interference with fungal growth and macrophage interaction

Phialophora verrucosa causes several fungal human diseases, mainly chromoblastomycosis, which is extremely difficult to treat. Several studies have shown that human immunodeficiency virus peptidase inhibitors (HIV-PIs) are attractive candidates for antifungal therapies. This work focused on studying the action of HIV-PIs on peptidase activity secreted by P. verrucosa and their effects on fungal proliferation and macrophage interaction. We detected a peptidase activity from P. verrucosa able to cleave albumin, sensitive to pepstatin A and HIV-PIs, especially lopinavir, ritonavir and amprenavir, showing for the first time that this fungus secretes aspartic-type peptidase. Furthermore, lopinavir, ritonavir and nelfinavir reduced the fungal growth, causing remarkable ultrastructural alterations. Lopinavir and ritonavir also affected the conidia-macrophage adhesion and macrophage killing. Interestingly, P. verrucosa had its growth inhibited by ritonavir combined with either itraconazole or ketoconazole. Collectively, our results support the antifungal action of HIV-PIs and their relevance as a possible alternative therapy for fungal infections.

Darunavir inhibits Cryptococcus neoformans/Cryptococcus gattii species complex growth and increases the susceptibility of biofilms to antifungal drugs

Introduction. Cryptococcus species are pathogens commonly associated with cases of meningoencephalitis in individuals who are immunosuppressed due to AIDS.Aim. The aim was to evaluate the effects of the antiretroviral darunavir alone or associated with fluconazole, 5-flucytosine and amphotericin B against planktonic cells and biofilms of Cryptococcus species.Methodology. Susceptibility testing of darunavir and the common antifungals against 12 members of the Cryptococcus neoformans/Cryptococcus gattii species complex was evaluated by broth microdilution. The interaction between darunavir and antifungals against planktonic cells was tested by a checkerboard assay. The effects of darunavir against biofilm metabolic activity and biomass were evaluated by the XTT reduction assay and crystal violet staining, respectively.Results. Darunavir combined with amphotericin B showed a synergistic interaction against planktonic cells. No antagonistic interaction was observed between darunavir and the antifungals used. All Cryptococcus species strains were strong biofilm producers. Darunavir alone reduced biofilm metabolic activity and biomass when added during and after biofilm formation (P<0.05). The combination of darunavir with antifungals caused a significant reduction in biofilm metabolic activity and biomass when compared to darunavir alone (P<0.05).Conclusion. Darunavir presents antifungal activity against planktonic cells of Cryptococcus species and synergism with amphotericin B. In addition, darunavir led to reduced biofilm formation and showed activity against mature biofilms of Cryptococcus species. Activity of the antifungals against mature biofilms was enhanced in the presence of darunavir.

Response of Paracoccidioides lutzii to the antifungal camphene thiosemicarbazide determined by proteomic analysis

Aim: To perform the proteomic profile of Paracoccidioides lutzii after treatment with the compound camphene thiosemicarbazide (TSC-C) in order to study its mode of action. Methods: Proteomic analysis was carried out after cells were incubated with TSC-C in a subinhibitory concentration. Validation of the proteomic results comprised the azocasein assay, western blot and determination of the susceptibility of a mutant to the compound. Results: Proteins related to metabolism, energy and protein fate were regulated after treatment. In addition, TSC-C reduces the proteolytic activity of the protein extract similarly to different types of protease inhibitors. Conclusion: TSC-C showed encouraging antifungal activity, working as a protease inhibitor and downregulating important pathways impairing the ability of the fungi cells to produce important precursors.

Secreted aspartyl proteinase (PbSap) contributes to the virulence of Paracoccidioides brasiliensis infection

Paracoccidioidomycosis (PCM) is the most prevalent deep mycosis in Latin America and is caused by fungi from the Paracoccidioides genus. Virulence factors are important fungal characteristics that support the development of disease. Aspartyl proteases (Saps) are virulence factors in many human fungal pathogens that play an important role in the host invasion process. We report here that immunization with recombinant Sap from Paracoccidioides brasiliensis (rPbSap) imparted a protective effect in an experimental PCM model. The rPbSap-immunized mice had decreased fungal loads, and their lung parenchyma were notably preserved. An aspartyl protease inhibitor (pepstatin A) significantly decreased pulmonary injury and reduced fungal loads in the lung. Additionally, we observed that pepstatin A enhanced the fungicidal and phagocytic profile of macrophages against P. brasiliensis. Furthermore, PbSAP expression was highly altered by environmental conditions, including thermal stress, dimorphism switching and low pH. Hence, our data suggest that PbSap is an important virulence regulator in P. brasiliensis.

Present and Future Therapy of Cryptococcus Infections

Cryptococcal infections burden the immunocompromised population with unacceptably high morbidity and mortality. This population includes HIV-infected individuals and those undergoing organ transplants, as well as seemingly immunocompetent patients (non-HIV, non-transplant). These groups are difficult to manage with the current therapeutic options and strategies, particularly in resource-limited settings. New trials aimed at providing the best treatment strategies for resource-limited countries that will reduce costs and adverse reactions have focused on decreasing the length of therapy and using more readily accessible antifungal agents such as fluconazole. Furthermore, the emergence of antifungal resistance poses another challenge for successful treatment and may require the development of new agents for improved management. This review will discuss the principles of management, current and future antifungal agents, as well as emerging techniques and future directions of care for this deadly infection.

The HIV aspartyl protease inhibitor ritonavir impairs planktonic growth, biofilm formation and proteolytic activity in Trichosporon spp

This study evaluated the effect of the protease inhibitor ritonavir (RIT) on Trichosporon asahii and Trichosporon inkin. Susceptibility to RIT was assessed by the broth microdilution assay and the effect of RIT on protease activity was evaluated using azoalbumin as substrate. RIT was tested for its anti-biofilm properties and RIT-treated biofilms were assessed regarding protease activity, ultrastructure and matrix composition. In addition, antifungal susceptibility, surface hydrophobicity and biofilm formation were evaluated after pre-incubation of planktonic cells with RIT for 15 days. RIT (200 μg ml^-1) inhibited Trichosporon growth. RIT (100 μg ml^-1) also reduced protease activity of planktonic and biofilm cells, decreased cell adhesion and biofilm formation, and altered the structure of the biofilm and the protein composition of the biofilm matrix. Pre-incubation with RIT (100 μg ml^-1) increased the susceptibility to amphotericin B, and reduced surface hydrophobicity and cell adhesion. These results highlight the importance of proteases as promising therapeutic targets and reinforce the antifungal potential of protease inhibitors.

Integrated Activity and Genetic Profiling of Secreted Peptidases in Cryptococcus neoformans Reveals an Aspartyl Peptidase Required for Low pH Survival and Virulence

The opportunistic fungal pathogen Cryptococcus neoformans is a major cause of mortality in immunocompromised individuals, resulting in more than 600,000 deaths per year. Many human fungal pathogens secrete peptidases that influence virulence, but in most cases the substrate specificity and regulation of these enzymes remains poorly understood. The paucity of such information is a roadblock to our understanding of the biological functions of peptidases and whether or not these enzymes are viable therapeutic targets. We report here an unbiased analysis of secreted peptidase activity and specificity in C. neoformans using a mass spectrometry-based substrate profiling strategy and subsequent functional investigations. Our initial studies revealed that global peptidase activity and specificity are dramatically altered by environmental conditions. To uncover the substrate preferences of individual enzymes and interrogate their biological functions, we constructed and profiled a ten-member gene deletion collection of candidate secreted peptidases. Through this deletion approach, we characterized the substrate specificity of three peptidases within the context of the C. neoformans secretome, including an enzyme known to be important for fungal entry into the brain. We selected a previously uncharacterized peptidase, which we term Major aspartyl peptidase 1 (May1), for detailed study due to its substantial contribution to extracellular proteolytic activity. Based on the preference of May1 for proteolysis between hydrophobic amino acids, we screened a focused library of aspartyl peptidase inhibitors and identified four high-affinity antagonists. Finally, we tested may1Δ strains in a mouse model of C. neoformans infection and found that strains lacking this enzyme are significantly attenuated for virulence. Our study reveals the secreted peptidase activity and specificity of an important human fungal pathogen, identifies responsible enzymes through genetic tests of their function, and demonstrates how this information can guide the development of high affinity small molecule inhibitors.

Many pathogenic organisms secrete peptidases. The activity of these enzymes often contributes to virulence, making their study crucial for understanding host-pathogen biology and developing therapeutics. In this report, we employed an unbiased, activity-based profiling assay to examine the secreted peptidases of a fungal pathogen, Cryptococcus neoformans, which is responsible for 40% of AIDS-related deaths. We discovered which peptidases are secreted, identified their substrate specificity, and interrogated their biological functions. Through this analysis, we identified a principal enzyme responsible for the extracellular peptidase activity of C. neoformans, May1, and demonstrated its importance for growth in acidic environments. Characterization of its substrate preferences allowed us to identify compounds that are potent substrate-based inhibitors of May1 activity. Finally, we found that the presence of this enzyme promotes virulence in a mouse model of infection. Our comprehensive study reveals the expression, regulation and function of C. neoformans secreted peptidases, including evidence for the role of a novel aspartyl peptidase in virulence.

Antiretroviral drugs saquinavir and ritonavir reduce inhibitory concentration values of itraconazole against Histoplasma capsulatum strains in vitro

Recent studies have shown that some drugs that are not routinely used to treat fungal infections have antifungal activity, such as protease inhibitor antiretroviral drugs. This study investigated the in vitro susceptibility of Histoplasma capsulatum var. capsulatum to saquinavir and ritonavir, and its combination with the antifungal itraconazole. The susceptibility assay was performed according to Clinical and Laboratory Standards Institute guidelines. All strains were inhibited by the protease inhibitor antiretroviral drugs. Saquinavir showed minimum inhibitory concentrations ranging from 0.125 to 1μgmL(-1) for both phases, and ritonavir presented minimum inhibitory concentrations ranging from 0.0312 to 4μgmL(-1)and from 0.0625 to 1μgmL(-1) for filamentous and yeast phase, respectively. Concerning the antifungal itraconazole, the minimum inhibitory concentration values ranged from 0.0019 to 0.125μgmL(-1) and from 0.0039 to 0.0312μgmL(-1) for the filamentous and yeast phase, respectively. The combination of saquinavir or ritonavir with itraconazole was synergistic against H. capsulatum, with a significant reduction in the minimum inhibitory concentrations of both drugs against the strains (p<0.05). These data show an important in vitro synergy between protease inhibitors and itraconazole against the fungus H. capsulatum.