The challenge of managing fusariosis

Abstract

Fusarium is the second most frequent mold involved in fungal infections and is particularly important among immunocompromised patients. Culture methods and microscopy are still routinely used in clinical laboratories to identify Fusarium spp., and more sophisticated, timely, and effective methods for detecting Fusarium spp. in laboratory samples could improve the outcome for the patient. These investigational diagnostic approaches include serological assays and specific nested PCR assays that can yield positive and negative predictive values of over 90%. Other assays in development, such as mass spectroscopy techniques, can provide accurate and consistent results. The treatment of fusariosis in immunocompromised patients remains a challenge and the prognosis of systemic fusariosis in this population remains poor. Successful treatment is highly dependent on the particular Fusarium species involved in the infection. High dose intravenous amphotericin B formulation is recommended as the first line of therapy in management of fusariosis in patients. Voriconazole is also effective in treating fusariosis. Intolerance, contraindication, or failure of the amphotericin B formulation warrants the use of voriconazole as an alternative agent, and posaconazole is licensed as salvage therapy against invasive fusariosis. Adjunctive therapies such as surgical debridement of infected tissue, granulocyte colony stimulating factor (G-CSF) or granulocyte-macrophage colony stimulating factor (GM-CSF) infusions, or granulocyte transfusions are also tools for managing fusariosis. In conclusion, Fusarium infection is considered an emerging problem and should be suspected in immunocompromised patients experiencing systemic infection and should be treated accordingly.

Introduction

The epidemiology of fungal infections among the immunocompromised patient population has changed with the use of antifungal prophylaxis, specifically in hematopoietic stem cell transplantation (HSCT), solid organ transplantation, and patients with hematological malignancies.^1,2 This change includes a drop in the incidence of yeast infections, namely Candida albicans, and an increase in mold infections.^3–6 This shift in fungal etiology in some populations is complicated by an increase in the number of immunosuppressed patients (increase in transplant, chemotherapy, etc.). Fusarium spp. are important infectious filamentous fungi, second only to Aspergillus spp.⁷ These fungi are ubiquitous in the environment and are found in the soil and air. Interestingly, Fusarium spp. are primarily known as plant pathogens but can also infect humans and animals, causing superficial, locally invasive, and disseminated disease.⁷ Infection is mainly through inhalation of airborne conidia or via breaks in the skin due to trauma and/or burns. Fusarium spp. also produce toxins that can cause food poisoning through the consumption of toxin-contaminated food.⁸

Neutropenia is one of the most important risk factors for acquiring fusariosis. Patients undergoing HSCT or solid organ transplantation, those with hematological malignancies or acquired immune deficiency syndrome (AIDS) and those taking pharmacological immunosuppressant therapies are at high risk for fusariosis.^9,10 Patients with Fusarium infections can present with a broad spectrum of symptoms, depending on the organ systems involved and whether the infection is superficial, locally invasive or disseminated at presentation.¹¹ The presentation can range from keratitis and onychomycosis to sinusitis, pneumonia and systemic infection. Fusarium infections can be divided into two major categories depending on the immune status of the patient. Fusarium infections in immunocompetent patients tend to be superficial, such as keratitis and onychomycosis, or locally invasive, while immunocompromised patients are more likely to develop locally invasive and disseminated forms of the disease.⁹ Usually persistent fever and skin lesions that have central necrosis are often evident in immunosuppressed patients with systemic fusariosis. It is important to mention that Fusarium infections in this patient population is associated with a high mortality rate, and the survival rate 90 days after diagnosis is only 13% in HSCT patients.^2,7,12–15 Therefore, prompt and efficient treatment is very important.

Most clinically important isolates of Fusarium belong to three groups: the Fusarium solani species complex, the Fusarium oxysporum species complex, and the Gibberella fujikuroi species complex.⁷ The Fusarium solani species complex is responsible for the majority of Fusarium infections, followed by F. oxysporum isolates. The most notable G. fujikuroi species to cause infection in humans include F. verticillioides (formerly F. moniliforme) and F. proliferatum.⁷

The treatment of Fusarium infections is challenging because of a lack of data from clinical trials.¹⁶ This is partly due to the fact that immunocompromised patients with systemic fusariosis are usually critically ill, and the high mortality rate among this patient population limits clinical studies. Fortunately, there is currently a prospective multi-center study recruiting participants to evaluate the safety of voriconazole in the treatment of invasive aspergillosis and rare molds, including Fusarium in children (ClinicalTrials.gov identifier: NCT00836875). Moreover, Fusarium spp. are often resistant to many antifungal agents.¹⁷ Resistance is usually associated with inherently different antifungal susceptibility profiles, making Fusarium identification at the species level very important for optimal treatment. This review focuses on the treatment of fusariosis and examines the latest tools for detecting Fusarium spp. in the laboratory.

In Vitro and Animal Models Studies

Published reports indicate that amphotericin B, voriconazole, and posaconazole have low minimum inhibitory concentration (MIC) for both F. solani and F. oxysporum (Table 1). Importantly, there are differences in MICs between isolates of the same Fusarium species demonstrating different antifungal susceptibility profiles between Fusarium subspecies, adding to the complexity of the treatment of fusariosis.^18–21

Table 1.

Minimum inhibitory concentration (MIC) for Fusarium isolates treated with amphotericin B, voriconazole, and posaconazole.

						MIC µg/ mL
		amphotericin B			voriconazole			posaconazole			References
Fusarium species	Number of samples	50%	Range	90%	50%	Range	90%	50%	Range	90%
F. solani	32	1	N/A	2	>8	N/A	>8	>8	N/A	>8	76
	2	N/A	4	1	N/A	>8	N/A	1	1	N/A	77
	22	N/A	0.5 to 8	1.33	N/A	4 to 16	14	N/A	16	16	21
	13	2	0.5 to 4	4	8	1 to 16	16	N/A	N/A	N/A	20
	19	1.25	N/A	N/A	9.21	N/A	N/A	15.21	N/A	N/A	18
	15	N/A	1 to 16	N/A	N/A	4 to 16	N/A	N/A	N/A	N/A	22
F. oxysporum	17	1	N/A	2	4	N/A	>8	4	N/A	>8	76
	2	N/A	1 to 4	2.5	N/A	4>8	N/A	0.12	0.125	N/A	77
	14	N/A	0.12 to 2	0.78	N/A	0.5 to 16	4	N/A	0.06 to 16	4.63	21
	9	2.3	N/A	N/A	4	N/A	N/A	2.7	N/A	N/A	18
	7	N/A	1 to >16	N/A	N/A	2 to 16	N/A	N/A	N/A	N/A	22
species not specified	67	8	N/A	32	16	N/A	32	16	N/A	32	14
	20	N/A	N/A	N/A	4	1 >16	16	N/A	N/A	N/A	19
	3	N/A	2 to 4	N/A	N/A	2 to 8	N/A	N/A	N/A	N/A	20

N/A, not applicable

An in vitro study of the efficacy of multiple combinations of treatment against clinical isolates of Fusarium showed that combining voriconazole and terbinafine is usually synergistic against Fusarium.²² On the other hand, the majority of Fusarium isolates tested using the combination of amphotericin B and voriconazole displayed neither a synergistic or antagonistic interaction between these two antifungal agents.²²

Murine models of fusariosis have been used to explore the efficacy of antifungal agents in treatment of fusariosis.^23,24 In one study, two groups of mice were infected with two phylogenitically diverse clinical isolates of F. solani followed by treatment with the antifungal agents amphotericin B, voriconazole, and micafungin. For both isolates, the combination of amphotericin B and voriconazole showed some efficacy in mice survival. Interestingly, amphotericin B alone had some efficacy against one isolate and the combination of voriconazole and micafungin had some efficacy against the second isolate.²³

Searching for new strategies to treat fungal infections has led to identification of a new compound, MGCD290, a Hos2 histone deacetylase inhibitor (HDACi) for combination therapy with antifungal agents.²⁵ MGCD290 inhibits the expression of the genes encoding the lanesterol demethylase target enzyme (ERG11) and Candida drug resistance (CDR) efflux pumps in Candida when exposed to sterol inhibitors.²⁶ The in vitro efficacy of MGCD290 in combination with an azole antifungal agent against eight Fusarium isolates showed higher susceptibilities when the HDACi was combined with the traditional azole.²⁵ For example, fluconazole alone showed no activity against several Fusarium spp., but when combined with MGCD290 there was substantial synergy against the Fusarium spp. leading to a reduction of the fluconazole MIC from >256 µg/mL to 4 µg/mL in one Fusarium strain and to 8 µg/mL in another. There was also a change in the resistance profile of one strain from voriconazole-resistant to susceptible and in two strains from posaconazole-resistant to susceptible.²⁵ These are encouraging results, but the mechanism of synergy of HDACi with the above mentioned antifungal agents against Fusarium has not been explored. This study highlights the importance of using combination therapy to increase antifungal efficacy of current therapies while reducing the risk of toxicity through a reduction of the recommended doses of antifungal agents.^25,27

Clinical Data

There are only a few published anecdotal cases on fusariosis in immunocompromised patients. In one case, a neutropenic patient who underwent allogeneic stem cell transplantation was treated successfully for fusariosis with a combination of liposomal amphotericin B and voriconazole.²⁸ The patient also received granulocyte-colony stimulating factor (G-CSF) to augment the population of white blood cells. Other published cases have revealed the priority of using amphotericin B as the sole therapy or in combination with voriconazole in treatment of fusariosis.^28–41 These results are in contrast to the proposed antagonism that may exist between amphotericin B and triazole antifungal agents.²⁹ This antagonism has been proposed due to the fact that the synthesis of ergosterol, which is the target for amphotericin B, is inhibited by azoles. On the other hand amphotericin B disrupts the fungal cell membrane, which may affect the influx of the azoles.²⁹

A retrospective study examined the response of 44 immunosuppressed patients with fusariosis and hematologic malignancies to several antifungal agents.⁴² The results showed that 12 weeks after diagnosis 41% (18/44) of the patients had complete or partial response to antifungal therapy. The majority of these patients, 84% (37/44), were treated with combination therapy, and from these patients 73% (27/37) were treated with amphotericin B and a triazole. Of the 44 patients in this study 77% (34 /44) received adjunctive G-CSF. This study highlights the importance of in vivo combination therapy, including adjunctive therapies, in the treatment of fusariosis in immunocompromised patients.⁴²

The three most effective antifungal agents in the armamentarium against fusariosis, amphotericin B, voriconazole, and posaconazole, are further discussed in detail.

Amphotericin B.

Amphotericin B is a polyene that acts by binding to ergosterol. This binding disrupts the fungal cell membrane and leads to cell death. Amphotericin B is highly bound by serum proteins in the body and consequently has poor penetration into body fluids and tissues. Intravenous amphotericin B is toxic and side effects are common, including renal toxicity, fevers, malaise, weight loss, headache, hypotension, abdominal pain, nausea, vomiting, diarrhea, normochromic normocytic anemia, and myalgia. Liposomal formulations are available that are significantly less toxic but more expensive. Amphotericin B is fungistatic or fungicidal depending on the drug concentration reached within the tissues and body fluids.

In one retrospective study, the data from the Collaborative Exchange of Antifungal Research database was analyzed, and 28 patients with invasive Fusarium infection (where at least one organ was infected) were identified. Of these patients, 8 (29%) received amphotericin B lipid complex (ABLC) as the first line therapy and 20 (70%) as the second line therapy.⁴³ The median daily dose was 4.5 mg/kg. Complete data was available to evaluate 26 of the patients, where 12 (46%) of the patients treated with amphotericin B lipid complex either improved or were cured and three of the 26 (12%) were stabilized. Eleven of the 26 patients (42%) were refractory to another antifungal therapy and 8/26 were intolerant to other antifungal agents. Six of the 11 patients (55%) that were refractory to other antifungal agents and four of the 8 (50%) patients that were intolerant to other antifungals either improved or were cured on amphotericin B lipid complex. Of note, the creatinine level doubled from baseline levels in 4/28 (14%) of patients.⁴³

In another retrospective study, 84 patients with hematological disease and fusariosis (where at least one organ was infected) were identified. Sixty-nine of the 84 patients (82%) were treated with deoxycholate amphotericin B, 13 (16%) with amphotericin B lipid complex, and two (2%) did not receive any treatment. There was a response to treatment in 27 of the 84 patients (32%), although 30 days following diagnosis of fusariosis 42 (50%) of the patients died. At the time of death 37 of the 42 patients (88%) still had active fusarial infections. Eighteen of the 84 patients (21%) were alive 90 days following the diagnosis of fusariosis and 66 (79%) of patients had died. Fifty-nine of the 66 patients died of fusariosis and seven from the underlying disease.¹³

Voriconazole.

Voriconazole is one of the newer synthetic triazole antifungal agents, and therefore, it inhibits lanosterol 14-α-demethylase during ergosterol biosynthesis. It can be administrated orally and intravenously, and exhibits excellent tissue penetration. Voriconazole is eliminated via the cytochrome P450 hepatic enzymes CYP2C19, CYP2C9, and CYP3A4 with 2% of the dose excreted unaltered through the kidneys. As a result of its hepatic metabolism, the pharmacokinetics of voriconazole are affected by other drugs and vice versa. Side effects of voriconazole include visual disturbances, in rare cases hepatotoxity, and when administered to pregnant women the compound can be teratogenic.^44,45

Voriconazole has demonstrated better efficacy in in vitro and in vivo studies against Fusarium spp. compared to other azole antifungal agents.^1,22 A multi-center, open-label clinical study to test the efficacy and safety profile of voriconazole for treatment of emerging and refractory invasive fungal infections was conducted. Eleven patients with fusariosis were included and voriconazole was successful in achieving 45% of the satisfactory global response with tolerable side effects.⁴⁶ A recent study to evaluate the outcome of patients with fusariosis treated with voriconazole as initial (16/73, 22%) or salvage (57/73, 78%) antifungal therapy, has re-enforced the possibility of considering voriconazole as an option for treatment of fusariosis.⁴⁷ The response to voriconazole therapy was successful in 34/73 (47%) of patients.⁴⁷ Interestingly, there was no significant difference in the outcome of patients treated with voriconazole either as salvage or primary therapy. In addition, the outcome of patients treated with combination therapy of voriconazole and another antifungal agent was not significantly different from the outcome of those who were treated with voriconazole as monotherapy.⁴⁷ It should be noted that in one case the use of voriconazole in prophylaxis for fungal infections in patients with acute lymphoblastic leukemia has led to breakthrough infections by Fusarium.⁴⁸

Posaconazole.

Posaconazole is a broad spectrum third generation triazole antifungal agent.⁴⁹ It is administered orally and achieves a large volume of distribution.⁵⁰ It is metabolized in the liver where it undergoes glucoronidation, and the inactive metabolites are excreted by the kidneys.⁵⁰ The parent compound is excreted in the feces.⁵⁰ Side effects of posaconazole include fever, headache, fatigue, peripheral edema, anorexia, hyper- or hypotension, tachycardia, anemia, neutropenia, thrombocytopenia, diarrhea, nausea, vomiting, abdominal pain, constipation, electrolyte abnormalities, cough, dyspnea, rash, and pruritis. The oral formulation limits the use of posaconazole in critically ill patients but the medication can be crushed and administered via a nasogastric feeding tube.^49–51

A retrospective study of 21 patients with proven or probable fusariosis was conducted using data from three independent multi-center open-label clinical trials. The infections were either refractory (n = 17, 81%) or intolerant (n = 4, 19%) to conventional antifungal therapy, which was primarily amphotericin B formulation based (n = 20, 95%). Monotherapy with posaconazole was administered as salvage therapy, and no other antifungal agent was permitted over the course of the study period. A complete or partial response to posaconazole was observed in 10 (48%) patients.⁵²

Therapeutic Approach

Fusarium spp. have different susceptibilities to antifungal agents. Susceptibility testing should be carried out if possible to establish the most effective pharmacological treatment. However, in practice this data is usually not available when making a decision on initial antifungal therapy, and Fusarium is not always identified as the causative agent of infection let alone identifying to the species level. In addition, immunocompromised patients with fusariosis generally have a poor overall outcome. High dose amphotericin B and especially its liposomal and lipid complex preparations are the antifungal therapies used in fusariosis treatment. In general, voriconazole also has in vitro and in vivo activity against Fusarium spp.¹ Both voriconazole and posaconazole are licensed by the United States Food and Drug Administration (FDA) as salvage therapy for fusariosis in immunocompromised patients.⁵³ Posaconazole is an oral medication dosed at 200 mg three times a days.⁵⁴ Other azole antifungal agents such as itraconazole, albaconazole, and ravuconazole have shown limited efficacy against Fusarium spp.⁵⁵ Isavuconazole, a promising novel broad-spectrum antifungal agent in the late stages of clinical development, has shown some promise against some Fusarium species but more research is required before establishing a conclusion about its activity in treating fusariosis in immunocompromised patients.⁵⁶ Combination therapy is a very important and potent alternative to monotherapy, and should be considered when monotherapy is not effective.⁷ The effect of therapy is monitored by clinical signs, such as the resolution of fever and other signs and symptoms of disease,⁷ and there should be a resolution of fungemia or other radiological signs of the disease.

Complimentary therapies can be delivered alongside pharmacological therapies for fusariosis in patients.⁷ Surgical debridement should be considered for localized superficial infections and central venous catheters should be removed if fungemia is present.^7,57,58 Other adjunctive therapies in immunocompromised patients include boosting the immune system with infusions of G-CSF, GM-CSF, or with granulocyte transfusions.⁵⁹ Lowering the dose of immunosuppressants is also of value in the treatment of fusariosis.⁷ Prior to starting immunosuppressive therapy patients should undergo a careful skin examination to look for any skin lesion or onychomycosis that could turn into a source of fusariosis after starting the therapy.^7,60 These lesions require treatment prior to starting immunosuppressive therapy. Patients with prior recorded Fusarium infections should have their dose of immunosuppressive therapy decreased to reduce their level of neutropenia.^7,61

Expedited Diagnosis

One way to improve therapy is by expedited diagnosis. Immunocompromised patients with fusariosis are usually critically ill, and therefore rapid and efficient diagnosis is very important both in terms of identifying the etiology of infection and finding the most appropriate treatment.⁴ This diagnosis is based on the clinical scenario and radiological and clinical laboratory investigations.⁶² The radiology findings may be helpful; however they are not specific, as many times the findings include alveolar and interstitial infiltrates, nodules, and cavities that carry numerous possibilities for a wide range of diseases.^7,29

Diagnosing Fusarium on microscopy is challenging because of its morphological similarities to Aspergillus. Preliminary identification of Fusarium infection does not determine the species, an important issue as different species have various susceptibilities to antifungal agents.³⁴ Blood cultures are useful for isolating Fusarium infection because they are often positive in disseminated disease (positive in fifty percent of disseminated fusariosis cases).²⁹ However, culture methods require serial subcultures and microscopic examination which may require over a week to yield results. Cultures from other samples such as broncho-alveolar lavage in case of lung involvement and biopsy from skin lesions for staining and culturing could be considered.³⁴ Galactomannan and 1,3-β-D-glucan assays are of limited value in diagnosing Fusarium infection. Galactomannan is usually negative in Fusarium infection, while 1,3-β-D-glucan is non-specific and positive for a variety of fungal infections.²⁹ However, in cases where there is a proven mold infection with a negative galactomannan test and a positive 1,3-β-D-glucan test, Fusarium should be considered.³⁴

Molecular assays with high sensitivity and specificity for detecting invasive fungal infections, including Fusarium infection, have been developed. Polymerase chain reaction (PCR) assays use the internal transcribed spacer (ITS) region between the 18S rDNA and 28S rDNA or other highly conserved sequences for detecting Fusarium infection and species identification.^63–67 Mass spectroscopy using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) is a technique that is a cost-effective assay and generates results within one hour. At this time, MALDI-TOF is an experimental tool and not widely available in clinical laboratories. There is still no validated protocol for its use in the identification of mycoses.^68–73

Other diagnostic tools for potential use in the clinical laboratory include analysis of the length of the internal transcribed spacer 2 (ITS2) by capillary electrophoresis, microarray technology, hybridization assays for specific parts of the ITS2 sequence, and DNA sequencing assays.^71,72

Conclusion

Fusarium is emerging as a more common fungal infectious agent in immunocompromised patients, especially in patients with hematologic malignancy or those undergoing HSCT and solid organ transplantation, and is an important pathogen in patients suffering from severe burns.^74,75 In the appropriate clinical scenario, fusariosis should be considered in patients exhibiting signs of a systemic infection and not responding to conventional antimicrobial agents. If Fusarium is isolated in the laboratory, species identification should be undertaken as Fusarium species have inherently different antifungal susceptibilities while in vitro antifungal susceptibility testing should be performed to ensure selection of an optimal antifungal regimen to treat the infection.

At the moment, the main diagnostic method of identifying Fusarium mycosis in clinical laboratories is via microscopic identification and culture methods. Obtaining tissue samples for microscopy is not always possible because immunocompromised patients with fusariosis may be too ill to undergo tissue sampling. Serological tests, such as 1,3-β-D-glucan and galactomannan have limited usefulness because they are nonspecific. In cases where there is a proven mold infection and there is a negative galactomannan test and a positive 1,3-β-D-glucan test, Fusarium infection should be considered as highly likely.³⁴

More research is required into Fusarium and its pathogenicity. An important and urgent issue is the need for new antifungal agents, through investing more time and efforts in studying the mechanism of infection and the pathogenesis of Fusarium. Knowing more about the genes contributing to virulence, metabolism, cell wall components, and signaling pathways of these fungi will give a clear idea about the cellular components that may be exploited as potential targets for compounds that can serve clinically for alternative treatment therapies. Only then will the outcome of fusariosis in immunocompromised patients be more favorable.