Introduction

Mucormycosis is an aggressive, angio-invasive disease caused by fungi of the order Mucorales. The number of species in this Order is large and expanding, but the most common are Rhizopus, Mucor, Lichtheimia (formerly Absidia), Rhizomucor, and Cunninghamella. Five major manifestations of infection occur, including rhino-orbital-cerebral, pulmonary, disseminated, cutaneous, and gastrointestinal. Mucorales are ubiquitous in the environment, and aerosolized spores can enter the human host through inhalation, percutaneous inoculation, or ingestion. Invasive disease primarily affects patients in immunocompromised states such as with hematological malignancies, severe neutropenia, high-dose corticosteroids, uncontrolled diabetes mellitus, solid organ transplants, and deferoxamine therapy [1].

The diagnosis of invasive mucormycosis can be difficult and requires a high degree of clinical suspicion, timely diagnostics, such as radiographic imaging, and histological examination of tissue. Histopathologically, angioinvasion with surrounding tissue infarction is seen along with perineural invasion [2]. In tissue, Mucorales typically appear as broad, ribbon-like hyphae with right-angle branching.

There are evolving strategies for the treatment of mucormycosis. Herein, we highlight a case of invasive mucormycosis successfully treated with dual salvage therapy and discuss the potential role and benefit of combination salvage therapy as well as novel agents in the management of mucormycosis.

Case Review

A 62-year-old man with refractory acute myeloid leukemia (AML) underwent induction and consolidation chemotherapy. His initial course was complicated by disseminated fusariosis secondary to fungal pneumonia with a cavitary nodule that required surgical resection and a prolonged course of posaconazole. After clinical remission, he received a hematopoietic stem cell transplant from a haploidentical, related donor, which was complicated by cytokine release syndrome requiring tocilizumab therapy. He developed chronic graft-versus host disease of the skin, eyes, and oropharynx and was initially treated with tacrolimus and high-dose steroids. Later, ibrutinib was added as a steroid-sparing strategy. He was maintained on posaconazole antifungal prophylaxis during this time.

One month following the initiation of ibrutinib, he was admitted to the hospital with new left eye pain, swelling, and redness and was diagnosed with orbital cellulitis. He reported actively participating in the construction of his own house that involved the demolition of walls. Laboratory workup revealed a white blood cell count of 8.7 K/mcL and creatinine of 1.12 mcg/dL; a serum posaconazole level later returned in the therapeutic range at 2.3 μg/mL. He was initially started on liposomal amphotericin (LAmB) at 3 mg/kg and ampicillin-sulbactam; ibrutinib and steroids were held, and tacrolimus was continued at a reduced dose in order to decrease his overall level of immunosuppression. Magnetic resonance imaging (MRI) demonstrated ill-defined enhancement on the left facial retromaxillary fat, cranial nerve V2, in the medial aspect of the left masticator space, and at the left orbital apex; the posterior aspect of the left ethmoid sinus and left middle and superior turbinates and in the left pterygopalatine fossa and pterygoid bone were also involved; finally, there was enhancement of the left optic nerve sheath with signal abnormality extending into left foramen rotundum.

Although initial endoscopic exam was inconclusive, and histopathologic frozen section was negative for invasive fungal disease, he was ultimately brought to the operating room for re-evaluation and debridement, where poorly perfusing necrotic tissue and positive margins with fungal hyphae were found. Fungal cultures from the operating room were negative, but 18S PCR on paraffin embedded tissue later identified Rhizopus oryzae. LAmB was increased to 5 mg/kg and isavuconazole and micafungin 100 mg daily were added for salvage therapy. The patient declined aggressive debridement with left eye enucleation, and antifungal salvage therapy was continued. In addition, hyperbaric oxygen therapy (HBOT) was started, and immunosuppression was weaned. Interval MRI demonstrated progression of disease, and isavuconazole was held due to concerns of antagonism. After a 1-month hospital stay, he was discharged on AmB 5mg/kg daily and micafungin 100 mg daily, as well as HBOT twice weekly.

Two months after discharge, repeat MRI demonstrated further progression of disease. He was briefly started on deferasirox 720 mg twice daily and sargramostim 250 mg subcutaneous daily. His antifungal regimen was continued with LAmB and micafungin. His course was complicated by acute kidney injury requiring adjustment in LAmB dose (5 mg/kg every other day). After approximately 9 months of LAmB and micafungin dual therapy, oral step down to isavuconazole 372 mg daily was made with confirmation of therapeutic drug level of 6.7 mcg/mL. The patient had persistent vision loss in the left eye, and follow-up MRIs demonstrated stable disease. He remains alive and clinically stable 18 months after diagnosis on isavuconazole therapy.

Primary Antifungal Therapy for Mucormycosis

Polyenes and certain azoles along with aggressive surgical debridement have been the established treatment for mucormycosis [3•]. When the lipid formulations, LAmB or amphotericin B lipid complex (ABLC) are not available, standard therapy with amphotericin B deoxycholate (AmB) is the backbone of currently recommended treatment. The optimal dosages for polyenes in the treatment of mucormycosis are not known; however, the recommended starting dose for AmB is 1.0–1.5 mg/kg/day and the recommended starting dose for LAmB and ABLC is 5 mg/kg/day with possible escalation to 10 mg/kg/day in early treatment or if CSF penetration is needed [4•]. A phase I–II study was conducted to determine the maximally tolerated dosage of LAmB in proven or probable fungal infections. Dosage cohorts consisted of 7.5, 10.0, 12.5, and 15 mg/kg of LAmB. A nonlinear relationship was demonstrated between dosage and plasma pharmacokinetic parameters, which reached maximum values with the 10 mg/kg/day dosage and declined at the 12.5 and 15 mg/kg/day dosages [5]. In the AmbiZygo study, 40 patients were enrolled in a prospective pilot study of high-dose LAmB for initial treatment of mucormycosis. Patients received 10 mg/kg/day of LAmB in combination with surgery in 71% of cases, which was associated with an overall response rate of 36% at week 4 and 45% at week 12. Creatinine doubled in 40% of patients but returned to normal within 12 weeks in 63% of these patients [6]. In patients who develop nephrotoxicity secondary to LAmB, changing the dose to every 48 or 72 h is a reasonable strategy [7]. Finally, the pharmacodynamics of 1, 5, and 10 mg/kg/day of ABLC and LAmB were compared in a neutropenic murine model of invasive pulmonary mucormycosis. At a dose of 10 mg/kg/day, both LAmB and ABLC were effective at reducing fungal lung burden. However, mice treated with ABLC at 5 mg/kg/day had significantly higher AmB lung concentrations than the LAmB-treated mice, which shows higher initial doses of LAmB than ABLC may be required during treatment of invasive pulmonary mucormycosis [8].

After a response to parenteral LAmB for at least 3–4 weeks, patients can be transitioned to an oral azole for consolidation or step-down therapy. Response is generally defined as stability or improvement on repeat imaging, but there are not clear guidelines for this assessment. Of the azoles, posaconazole and isavuconazole have the greatest in vitro activity against most Mucorales [9]. Posaconazole has traditionally been limited by poor bioavailability in earlier formulations, especially for a patient population with erratic absorption due to mucositis, gastrointestinal graft-versus-host disease, poor appetite, nausea/diarrhea, or on acid suppressing agents. However, delayed release tablets have been commercially available since Food and Drug Administration (FDA) approval for fungal prophylaxis in November 2013 [10]. Clinical effectiveness was demonstrated in several small studies. In vitro susceptibility of posaconazole was compared to that of AMB, itraconazole, voriconazole, and fluconazole against 37 clinical isolates of Zygomycetes. The MICs of posaconazole at which 50% and 90% of the isolates were inhibited 0.25 and 4 μg/mL, respectively [11]. Contrary to the in vitro data, published reports of murine models raise concerns about the clinical efficacy of posaconazole. Three isolates of Zygomycetes were used to produce a disseminated infection in immunocompetent mice. Posaconazole had no beneficial effects against R. oryzae, partial activity against Absidia corymbifera, and dose-dependent efficacy against Rhizopus microsporus [12].

Isavuconazole, a newer triazole, has been shown to have good bioavailability, fewer adverse effects, and a proven therapeutic option in the setting of renal impairment. In vitro studies that demonstrate sufficient inhibition of certain isolates of Zygomycetes align with clinical data. In the VITAL study, 37 patients received isavuconazole for a median of 84 days for the treatment of invasive fungal disease. Day-42 crude all-cause mortality in 7 out of the 21 primary-treatment isavuconazole cases was similar to 13 amphotericin B-treated match controls [13••], demonstrating that isavuconazole may be an effective option for the treatment of Mucor disease.

Recommendations:

  • AmB, LAmB, and ABLC are primary antifungal agents for mucormycosis. However, LAmB and ABLC are preferred over AmB due to less nephrotoxicity.

  • The recommended starting dose for LAmB and ABLC is 5 mg/kg/day.

  • Isavuconazole is an orally available triazole commonly used for step-down therapy for mucormycosis.

Combination Salvage Antifungal Therapy for Mucormycosis

In patients who are intolerant to or have invasive mucormycosis that is refractory to monotherapy, combination salvage therapy with a polyene backbone of ABLC or LAmB should be considered [14]. Potential advantages to combination therapy include broader coverage against Mucorales species and the possibility for a synergistic therapeutic effect. The disadvantages include the possibility of increased toxicity, other drug-drug interactions (especially through cytochrome P450 metabolism with azoles), the possibility of antagonistic interaction between the paired antifungal agents, and increased cost [15].

There are numerous case reports and small case series that have been published suggesting a possible benefit from this combination strategy. For example, in a retrospective cohort study compromised of 10 cases of mucormycosis, 6 patients died but all of the 4 surviving patients had received combination therapy with LAmB and posaconazole (2 of the 6 nonsurvivors also received combination therapy) [16]. Also, Vasquez et al. [17] described a patient with AML and zygomycosis who was initially unresponsive to monotherapy with LAmB but had a favorable outcome with the addition of caspofungin. Similarly, Gargouri et al. [18] reported a case of a 52-year-old woman with invasive mucormycosis treated with surgical debridement and a combination AmB and caspofungin with a good outcome.

There are no prospective, randomized clinical trials to establish efficacy of these strategies. In fact, there is a 20-year retrospective single-institution study where 106 patients with hematological malignancies were diagnosed with mucormycosis. In this study, 44% received monotherapy with either LAmB or posaconazole while 56% received combination treatment with LAmB plus posaconazole or LAmB plus an echinocandin or triple therapy with LAmB, posaconazole, and an echinocandin as initial therapy. The primary outcome was 6-week mortality after treatment initiation. There were no differences in mortality between monotherapy and combination treatment groups, but the combination group with LAmB plus posaconazole did have a higher rate of survival compared to the monotherapy group [19]. This was also evident in a 10-year retrospective study that included 64 patients with either hematological malignancies or hematopoietic cell transplants with proven or probable invasive mucormycosis. Sixty-six percent of patients received surgical debridement. Initial antifungal treatment consisted of lipid formulation of AmB alone (44%), AmB plus posaconazole (25%), AmB plus echinocandin (13%), AmB plus isavuconazole (8%), posaconazole alone (5%), and isavuconazole alone (3%). Although treatment with AmB plus posaconazole or isavuconazole was associated with a trend toward lower treatment failure compared with AmB alone, the all-cause 1-year mortality was not statistically different between AmB versus combination AmB plus posaconazole or isavuconazole [20].

Currently, there are 3 types of combination therapies that have been discussed in the literature and are widely used. Each strategy utilizes the polyene backbone of AmB in combination with one of the following: (1) echinocandin, (2) azole, (3) iron chelator. The evidence for each of these strategies has first been demonstrated in murine models and then applied to human studies.

Echinocandins, which act on the enzyme (1,3)-β-D-glucan synthase (GS) to disrupt the fungal cell wall, generally do not have in vitro activity against Mucorales; however, R. oryzae has the genetic homolog of the FSK gene, which encodes for GS. Echinocandins combined with other drugs change the fungal membrane, making this gene more accessible [21]. When combined with ABLC or LAmB, caspofungin therapy was associated with improved outcomes in a small case series of 6 patients with rhino-orbital-cerebral mucormycosis when compared to those who received AmB monotherapy alone [22]. This benefit was first described in the murine model. A combination of LAmB and micafungin at 1 mg/kg/day synergistically improved survival of diabetic ketoacidosis (DKA) mice with mucormycosis compared to either monotherapy arm. Similarly, combination LAmB and anidulafungin 10 mg/kg/day improved survival in neutropenic mice compared to either monotherapy arm alone [23]. Zhang et al. [24] observed a high rate of synergism when caspofungin was combined with AmB, posaconazole, or itraconazole against Mucor irregularis isolates. Additionally, the efficacy of triple therapy using LAmB, micafungin, and deferasirox in a DKA murine model of mucormycosis resulted in significant reductions of brain and kidney tissue fungal burdens compared to monotherapy with each agent or combination therapy with deferasirox and micafungin, deferasirox and LAmB, or LAmB and micafungin [25].

Neither posaconazole nor isavuconazole in combination with a lipid polyene has shown any advantage in salvage therapy for mucormycosis. Posaconazole combined with low dose AmB (0.3 mg/kg/day) prolonged survival and reduced tissue burden in a murine model of R. oryzae infected neutropenic mice compared to 0.3 mg/kg/day of AmB monotherapy, however, combination therapy was not superior to AmB given at 0.8 mg/kg/day alone [26]. Likewise, the efficacy of posaconazole plus LAmB therapy was compared to monotherapy with either drug alone in neutropenic or DKA mice infected with R. oryzae. There was no evidence of an additive benefit of combination treatment in terms of death or fungal tissue burden in the mice [27].

Combination therapy with isavuconazole and AmB has shown antagonistic activity with certain molds such as Aspergillus and Fusarium and with Candida glabrata but indifferent interactions with R. oryzae [28, 29]. Moreover, in an in vitro study comparing the activity of isavuconazole plus micafungin combination therapy against placebo, either drug alone, or standard therapy with LAmB when treating pulmonary murine mucormycosis, the combination of isavuconazole with micafungin did not enhance survival over monotherapy but also did not show antagonism [30].

Deferasirox is an oral iron chelator which has in vitro fungicidal properties against Mucorales [31]. While it has proven to be as efficacious as LAmB therapy in DKA mice with disseminated mucormycosis [32], the double-blinded, randomized, placebo-controlled phase II Deferasirox-AmBisome Therapy for Mucormycosis (DEFEAT Mucor) study failed to demonstrate a benefit to combination therapy and actually showed an increased mortality in patients treated with adjunctive deferasirox [33••]. Moreover, given its oral formulation, patients with malabsorption issues should not be treated with deferasirox. It has also been associated with renal toxicity. For these reasons, the adjunctive use of deferasirox should probably be avoided.

In Figure 1, we propose a treatment algorithm for the management of invasive mucormycosis.

Fig. 1
figure 1

Proposed treatment algorithm for the management of invasive mucormycosis

Full size image

Recommendations:

  • There have been no prospective randomized clinical trials to show differences in mortality between monotherapy and combination therapy.

  • ABLC or LAmB combined with an echinocandin has been associated with improved outcomes in case reports of patients with refractory disease caused by mucormycosis.

  • Deferasirox is not recommended in combination therapy given proven increased mortality and renal toxicity.

Novel Antifungal Therapies for Mucormycosis

Another avenue that has been explored in novel therapy for mucormycosis is oteseconazole (VT-1161). VT-1161 is a metalloenzyme inhibitor that targets the biosynthesis of ergosterol by selectively inhibiting fungal Cyp51. In vitro testing has demonstrated that VT-1161 has intrinsic antifungal activity against Mucorales species, particularly Rhizopus arrhizus var. arrhizus [34]. VT-1161 in prophylaxis and continuous therapy models has been shown to outperform posaconazole in mice infected with Rhizopus arrhizus var. arrhizus [35].

Similarly, VT-1598 is a novel tetrazole-based inhibitor of fungal Cyp51 that disrupts the ergosterol biosynthetic pathway by preventing the conversion of lanosterol to ergosterol. More importantly, VT-1598 is more specific for fungal Cyp51, thus limiting potential drug-drug interactions. In vitro, VT-1598 demonstrated activity against R. arrhizus isolates; however, the in vitro potency was reduced in R. arrhizus var. delemar strains [36].

Fosmanogepix (APX001), the prodrug of Manogepix (E1210), is another novel antifungal drug that inhibits the fungal enzyme Gwt1, which subsequently inactivates posttranslational modification of glycosylphosphatidylinositol (GPI) anchor proteins. When the GPI-anchor synthesis is disrupted, β-1,3-glucan can be exposed allowing the immune system to recognize the fungus [37•]. APX001 has been shown to be efficacious against both Rhizopus arrhizus var. arrhizus and R. arrhizus var. delemar strains. In invasive pulmonary infection models, treatment of mice with 78 mg/kg or 104 mg/kg along with 1-aminobenzotriazole to enhance the serum half-life of E1210 increased median survival time and prolonged survival by day 21 post-infection compared to placebo [38].

MGCD290 is a Hos2 fungal histone deacetylase (HDAC) inhibitor that has synergy against molds when combined with various azoles. Despite complete lack of activity of fluconazole against all filamentous fungal isolates when tested alone, synergy was observed with MGCD290 for 40% of Zygomycetes. When tested in combination with voriconazole, MGCD290 showed synergistic activity against 73% of Zygomycetes isolates and when combined with posaconazole, MGCD290 synergistically potentiated its activity against 93% of the isolates [39].

Although none of these novel antifungal agents are currently FDA approved, they are beginning to advance in human clinical trials. VT-1161 is currently in clinical development and undergoing phase 3 ongoing trial. VT-1598 is in its preclinical trial. APX001 is in its phase 2 clinical trial after phase 1 clinical studies demonstrated > 90% bioavailability. Lastly, MGCD290 completed the phase 2 trial and currently has no ongoing trial.

Recommendations:

  • VT-1161 and VT-1598 have proven antifungal activity against R. arrhizus isolates while limiting potential drug-drug interactions by selectively inhibiting fungal Cyp51.

  • APX001 is an intravenously (i.v.) and orally (p.o.) available prodrug that has been shown to have equivalent efficacy to isavuconazole.

  • MGCD290, when combined with certain azoles, has demonstrated synergy against molds.

Adjunctive Therapies for Mucormycosis

The latest developments in the treatment of mucormycosis stem from the pathogenesis of this invasive fungal infection. Mucorales invasion is facilitated when certain host factors impair the cytotoxic function of phagocytes [40]. This is evident in patients with functional or quantitative neutropenia. Hence, the adjunctive use of cytokines such as granulocyte-macrophage colony-stimulating factor (GM-CSF) has been reported. Gracia-Diaz et al. [41] describe successfully treating 3 nonneutropenic patients with GM-CSF along with surgical and standard therapy. They recommended the subcutaneous administration of GM-CSF 250 micrograms 3 times a week for nonneutropenic patients.

Furthermore, Mucorales adhere to endothelial cells by expressing the spore coat homolog (CotH) proteins and attaching to the endothelial host receptor GRP78. In acidic, elevated iron, or elevated glucose environments, the expression of this protein and receptor increases. Hence, anti-CotH3 antibodies are also potential adjunctive immunotherapeutic option for advanced disease [42].

Lastly, while this is not a novel therapy, there are reports of successful outcomes with HBOT as adjunctive treatment for serious, invasive mucormycosis, including rhino-cerebral disease. During HBOT, 100% oxygen is administered with pressures at 250–280 kPa resulting in a PaO2 of 1200–2000 mmHg. Patients usually undergo 20–40 sessions with each session lasting 90–120 minutes [43]. In vitro, HBOT inhibits fungal growth at 10 atmospheres absolute while correcting lactic acidosis, which promotes the oxidative action of AmB. Moreover, HBOT promotes tissue healing by increasing tissue oxygen levels and growth factors as well as enhancing leukocyte-mediated phagocytosis [44]. A review of 28 published cases of mucormycosis indicated that HBOT showed an 86% overall survival rate with a 94% survival rate among patients with diabetes but only a 33% survival rate among patients with hematological malignancies and bone marrow transplants [45]. The authors concluded that HBOT may provide some benefit in patients with diabetes but had limited additional efficacy in the patients with hematological malignancies.

Recommendations:

  • GM-CSF and anti-CotH3 antibodies are potential immunotherapeutic adjunctive treatment options.

  • HBOT adjunctive therapy increases the survival rate from mucormycosis in patients with diabetes, but has limited additional efficacy in hematological malignancies or bone marrow transplant recipients.

Conclusion

Mucormycosis is an aggressive fungal infection that is becoming more common in at-risk, immunosuppressed populations. While definitive guidelines are hard to produce given the lack of prospective randomized clinical trials, combination therapy with a polyene and echinocandin is likely the most efficacious therapy strategy. There are new antifungals in various stages of development that hold promise for the expansion of effective therapeutics for mucormycosis. Finally, debridement surgery, correction of underlying host factors, and polyene therapy are the current standard of care but as we understand more about these organisms and the hosts they invade, the early detection and treatment of this devastating disease will become more successful.