The In Vitro Efficacy of Antimicrobial Agents Against the Pathogenic Free-Living Amoeba Balamuthia mandrillaris

Challenges and advances in the medical treatment of granulomatous amebic encephalitis

Granulomatous amebic encephalitis, caused by the free-living amebae Balamuthia mandrillaris or Acanthamoeba species, is a rare and deadly infectious syndrome with a current mortality rate of >90%. Much work remains to define the optimal treatment for these infections. Here, we provide a comprehensive overview of the supporting evidence behind antimicrobials currently recommended by the Centers for Disease Control and Prevention (CDC) with updated statistics on survival rates and medication usage from the CDC Free-Living Ameba Database. We also discuss promising treatments, especially the emerging therapeutic agent nitroxoline, and provide recommendations for the next steps in this area.

Treatment of cutaneous Balamuthia mandrillaris infection with diminazene aceturate: a report of 4 cases

Four cases of cutaneous Balamuthia mandrillaris infection were treated with diminazene aceturate. One patient was cured with mainly monotherapy, 2 patients were cured with diminazene aceturate and excision, and 1 patient died of drug induced liver damage. This is the first report of Balamuthia mandrillaris infection treated with diminazene aceturate.

Antiamoebic Properties of Metabolites against Naegleria fowleri and Balamuthia mandrillaris

Naegleria fowleri and Balamuthia mandrillaris are free-living, opportunistic protists, distributed widely in the environment. They are responsible for primary amoebic meningoencephalitis (PAM) and granulomatous amoebic encephalitis (GAE), the fatal central nervous infections with mortality rates exceeding 90%. With the rise of global warming and water shortages resulting in water storage in tanks (where these amoebae may reside), the risk of infection is increasing. Currently, as a result of a lack of awareness, many cases may be misdiagnosed. Furthermore, the high mortality rate indicates the lack of effective drugs available. In this study, secondary metabolites from the plants Rinorea vaundensis and Salvia triloba were tested for their anti-amoebic properties against N. fowleri and B. mandrillaris. Three of the nine compounds showed potent and significant anti-amoebic activities against both N. fowleri and B. mandrillaris: ursolic acid, betulinic acid, and betulin. Additionally, all compounds depicted limited or minimal toxicity to human cells and were capable of reducing amoeba-mediated host cell death. Moreover, the minimum inhibitory concentration required to inhibit 50% of amoebae growth, the half-maximal effective concentration, and the maximum non-toxic dose against human cells of the compounds were determined. These effective plant-derived compounds should be utilized as potential therapies against infections due to free-living amoebae, but future research is needed to realize these expectations.

The transcriptome of Balamuthia mandrillaris trophozoites for structure-guided drug design

Balamuthia mandrillaris, a pathogenic free-living amoeba, causes cutaneous skin lesions as well as granulomatous amoebic encephalitis, a 'brain-eating' disease. As with the other known pathogenic free-living amoebas (Naegleria fowleri and Acanthamoeba species), drug discovery efforts to combat Balamuthia infections of the central nervous system are sparse; few targets have been validated or characterized at the molecular level, and little is known about the biochemical pathways necessary for parasite survival. Current treatments of encephalitis due to B. mandrillaris lack efficacy, leading to case fatality rates above 90%. Using our recently published methodology to discover potential drugs against pathogenic amoebas, we screened a collection of 85 compounds with known antiparasitic activity and identified 59 compounds that impacted the growth of Balamuthia trophozoites at concentrations below 220 µM. Since there is no fully annotated genome or proteome of B. mandrillaris, we sequenced and assembled its transcriptome from a high-throughput RNA-sequencing (RNA-Seq) experiment and located the coding sequences of the genes potentially targeted by the growth inhibitors from our compound screens. We determined the sequence of 17 of these target genes and obtained expression clones for 15 that we validated by direct sequencing. These will be used in the future in combination with the identified hits in structure guided drug discovery campaigns to develop new approaches for the treatment of Balamuthia infections.

Opportunistic free-living amoebal pathogens

Pathogenic free-living amoebae affecting the central nervous system are known to cause granulomatous amoebic encephalitis (GAE) or primary amoebic meningoencephalitis (PAM). Although hosts with impaired immunity are generally at a higher risk of severe disease, amoebae such as Naegleria fowleri and Balamuthia mandrillaris can instigate disease in otherwise immunocompetent individuals, whereas Acanthamoeba species mostly infect immunocompromised people. Acanthamoeba also cause a sight-threatening eye infection, mostly in contact lens wearers. Although infections due to pathogenic amoebae are considered rare, recently, these deadly amoebae were detected in water supplies in the USA. This is of particular concern, especially with global warming further exacerbating the problem. Herein, we describe the epidemiology, presentation, diagnosis, and management of free-living amoeba infections.

Evaluation of the non-clinical toxicity of an antiparasitic agent: diminazene aceturate

The diminazene aceturate (C₁₄H₁₅N₇.2C₄H₇NO₃) is a chemotherapeutic agent with more than six decades of use, however more studies regarding its toxicity still need to be performed. Thus, the present study determined the acute toxicity (14 days) of diminazene acetate (DIZE) in male and female swiss mice by changes in body mass, food consumption, biochemical and hematological parameters, locomotor activity and motor coordination. DIZE was administered at a single dose (1000 and 2000 mg/kg) orally. In addition, in vitro antioxidant capacity, hemolytic activity, toxicity in Artemia salina and in silico evaluation were also performed. The results obtained include several signs of toxicity (hypoactivity, loss of the straightening reflex and tachycardia), reduction of behavioral activity (locomotor activity and motor coordination) and significant changes (p < 0.05) in biochemical and hematological parameters. According to the in silico study, the DIZE can be classified based on the mean lethal dose (LD₅₀) in category 4 (300 mg/kg < LD₅₀ ≤ 2000 mg/kg, ProTox-II) or 3 (50 mg/kg < LD₅₀ ≤ 300 mg/kg, AdmetSAR 1.0). Additionally, DIZE (30.3-969.9 nM) was not toxic to A. salina in the first 48 hours of treatment and was not cytotoxic to rat red blood cells after induced hemolysis. In vitro results indicated low antioxidant capacity against DPPH^• and ABTS^•+ radicals. Therefore, DIZE induces several adverse effects with influence on the central nervous system, changes in hematological and biochemical parameters and even mortality at the highest dose. However, absence of toxicity was observed in A. salina and rats red blood cells.

Miltefosine: A Miracle Drug for Meningoencephalitis Caused by Free-Living Amoebas

Meningoencephalitis caused by free-living amoebas (FLA) has a high mortality rate, and most treatments are ineffective. FLA includes Naegleria, Fowleri, Acanthamoeba, and Balamuthia mandrillaris (M). We explore the use of miltefosine in the treatment of one of these infections. The concerning mortality of the infection obligates us to look for more effective treatments for meningoencephalitis caused by FLA. During this review, we will consolidate the knowledge of using miltefosine in these three infections. We will investigate the mechanism by which the drug is effective in these infections as well. After this comprehensive review, we should assess if miltefosine improves the mortality and prognosis of the infection with the information collected. We used a Medical Subject Headings (MeSH) search on PubMed. Inclusion criteria included papers written in the English language and human subjects research for the past 25 years. Until today, there are no definitive guidelines to be followed when treating such patients. However, miltefosine has demonstrated promising results. Miltefosine decreases the usual mortality rate in the three infections; however, there are few reports due to the low frequency of these infections. Almost all cases we documented have survived. More information needs to be gathered for the use of miltefosine for these infections.

Discovery of repurposing drug candidates for the treatment of diseases caused by pathogenic free-living amoebae

Diseases caused by pathogenic free-living amoebae include primary amoebic meningoencephalitis (Naegleria fowleri), granulomatous amoebic encephalitis (Acanthamoeba spp.), Acanthamoeba keratitis, and Balamuthia amoebic encephalitis (Balamuthia mandrillaris). Each of these are difficult to treat and have high morbidity and mortality rates due to lack of effective therapeutics. Since repurposing drugs is an ideal strategy for orphan diseases, we conducted a high throughput phenotypic screen of 12,000 compounds from the Calibr ReFRAME library. We discovered a total of 58 potent inhibitors (IC50 <1 μM) against N. fowleri (n = 19), A. castellanii (n = 12), and B. mandrillaris (n = 27) plus an additional 90 micromolar inhibitors. Of these, 113 inhibitors have never been reported to have activity against Naegleria, Acanthamoeba or Balamuthia. Rapid onset of action is important for new anti-amoeba drugs and we identified 19 compounds that inhibit N. fowleri in vitro within 24 hours (halofuginone, NVP-HSP990, fumagillin, bardoxolone, belaronib, and BPH-942, solithromycin, nitracrine, quisinostat, pabinostat, pracinostat, dacinostat, fimepinostat, sanguinarium, radicicol, acriflavine, REP3132, BC-3205 and PF-4287881). These compounds inhibit N. fowleri in vitro faster than any of the drugs currently used for chemotherapy. The results of these studies demonstrate the utility of phenotypic screens for discovery of new drugs for pathogenic free-living amoebae, including Acanthamoeba for the first time. Given that many of the repurposed drugs have known mechanisms of action, these compounds can be used to validate new targets for structure-based drug design.

Functional Assessment of 2,177 U.S. and International Drugs Identifies the Quinoline Nitroxoline as a Potent Amoebicidal Agent against the Pathogen Balamuthia mandrillaris

Balamuthia mandrillaris is responsible for hundreds of reported cases of amoebic encephalitis, the majority of which have been fatal. Despite being an exceptionally deadly pathogen, B. mandrillaris is understudied, leaving many open questions regarding epidemiology, diagnosis, and treatment. Due to the lack of effective drugs to fight B. mandrillaris infections, mortality rates remain high even for patients receiving intensive care. This report addresses the need for new treatment options through a drug repurposing screen to identify novel B. mandrillaris inhibitors. The most promising candidate identified was the quinoline antibiotic nitroxoline, which has a long history of safe use in humans. We show that nitroxoline kills B. mandrillaris at pharmacologically relevant concentrations and exhibits greater potency and selectivity than drugs commonly used in the current standard of care. The findings that we present demonstrate the potential of nitroxoline to be an important new tool in the treatment of life-threatening B. mandrillaris infections.

Balamuthia mandrillaris is a pathogenic free-living amoeba that causes a rare but almost always fatal infection of the central nervous system called granulomatous amoebic encephalitis (GAE). Two distinct forms of B. mandrillaris—a proliferative trophozoite form and a nonproliferative cyst form, which is highly resistant to harsh physical and chemical conditions—have been isolated from environmental samples worldwide and are both observed in infected tissue. Patients suffering from GAE are typically treated with aggressive and prolonged multidrug regimens that often include the antimicrobial agents miltefosine and pentamidine isethionate. However, survival rates remain low, and studies evaluating the susceptibility of B. mandrillaris to these compounds and other potential therapeutics are limited. To address the need for more-effective treatments, we screened 2,177 clinically approved compounds for in vitro activity against B. mandrillaris. The quinoline antibiotic nitroxoline (8-hydroxy-5-nitroquinoline), which has safely been used in humans to treat urinary tract infections, was identified as a lead compound. We show that nitroxoline inhibits both trophozoites and cysts at low micromolar concentrations, which are within a pharmacologically relevant range. We compared the in vitro efficacy of nitroxoline to that of drugs currently used in the standard of care for GAE and found that nitroxoline is the most potent and selective inhibitor of B. mandrillaris tested. Furthermore, we demonstrate that nitroxoline prevents B. mandrillaris-mediated destruction of host cells in cultured fibroblast and primary brain explant models also at pharmacologically relevant concentrations. Taken together, our findings indicate that nitroxoline is a promising candidate for repurposing as a novel treatment of B. mandrillaris infections.

Acute encephalitis in the immunocompromised individual

PURPOSE OF REVIEW

This article describes recent advances in the diagnosis and management of encephalitis in immunocompromised individuals.

RECENT FINDINGS

Herpes simplex virus (HSV) and varicella zoster virus (VZV) are common causes of encephalitis in immunocompromised individuals, although clinical manifestations may be atypical, and thus challenging to recognize. Recently, an increased incidence of HSV and VZV central nervous system infections has been reported in association with novel immunosuppressive and immunomodulatory treatments. The free-living ameba Balamuthia mandrillaris causes granulomatous encephalitis predominantly in immunocompromised individuals and is associated with nearly uniform fatality. In the setting of organ transplantation, the recipient's immunocompromised state along with the potential for donor-transmitted infections can result in a unique epidemiology of encephalitis, including infection by human herpes virus-6 and BK virus. Recent studies utilizing next-generation sequencing techniques have identified several pathogens, including Leptospira santarosai and a neurotropic astrovirus, as causes of encephalitis in immunocompromised individuals.

SUMMARY

Diagnosis and management of encephalitis is challenging in immunocompromised individuals, in part because of atypical clinical presentations and the presence of uncommon or novel infectious agents. Unbiased techniques for pathogen discovery are likely to play an increasing role in the diagnosis of central nervous system infections in immunocompromised individuals.

Clinical metagenomic identification of Balamuthia mandrillaris encephalitis and assembly of the draft genome: the continuing case for reference genome sequencing

Background

Primary amoebic meningoencephalitis (PAM) is a rare, often lethal, cause of encephalitis, for which early diagnosis and prompt initiation of combination antimicrobials may improve clinical outcomes.

Methods

In this study, we sequenced a full draft assembly of the Balamuthia mandrillaris genome (44.2 Mb in size) from a rare survivor of PAM, and recovered the mitochondrial genome from six additional Balamuthia strains. We also used unbiased metagenomic next-generation sequencing (NGS) and SURPI bioinformatics analysis to diagnose an ultimately fatal case of Balamuthia mandrillaris encephalitis in a 15-year-old girl.

Results and Discussion

Comparative analysis of the mitochondrial genome and high-copy number genes from six additional Balamuthia mandrillaris strains demonstrated remarkable sequence variation, and the closest Balamuthia homologs corresponded to other amoebae, hydroids, algae, slime molds, and peat moss. Real-time NGS testing of hospital day 6 CSF and brain biopsy samples detected Balamuthia on the basis of high-quality hits to 16S and 18S ribosomal RNA sequences present in the National Center for Biotechnology Information (NCBI) nt reference database. The presumptive diagnosis of PAM by visualization of amoebae on brain biopsy histopathology and NGS analysis was subsequently confirmed at the US Centers for Disease Control and Prevention (CDC) using a Balamuthia-specific PCR assay. Retrospective analysis of a day 1 CSF sample revealed that more timely identification of Balamuthia by metagenomic NGS, potentially resulting in a better clinical outcome, would have required availability of the complete genome sequence.

Conclusions

These results underscore the diverse evolutionary origins of Balamuthia mandrillaris, provide new targets for diagnostic assay development, and will facilitate further investigations of the biology and pathogenesis of this eukaryotic pathogen. The failure to identify PAM from a day 1 sample without a fully sequenced Balamuthia genome in the database highlights the critical importance of whole-genome reference sequences for microbial detection by metagenomic NGS.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-015-0235-2) contains supplementary material, which is available to authorized users.

Management of granulomatous amebic encephalitis: Laboratory diagnosis and treatment

Granulomatous amebic encephalitis is a life-threatening central nervous system (CNS) infection caused by the free-living amoebae Acanthamoeba spp., Balamuthia mandrillaris and Sappinia pedata. The disease has a subacute to chronic onset affecting commonly the immunocompromised population with high mortality rate. The diagnosis of this disease entity requires high suspicion with appropriate sample collection and testing by the laboratory experts. Radiological investigations are nonspecific and commonly confused with CNS tuberculosis, neurocysticercosis, disseminated encephalomyelitis, viral encephalitis etc., delaying the accurate diagnosis of these cases. Early diagnosis plays a crucial role in the survival of these cases since appropriate management can be initiated. No single drug is effective; hence multiple antibiotics targeting various proteins or receptors are required for successful treatment. A combination of surgical and medical interventions involving multiple specialty experts is required to prevent death and morbidity in survivors.