Treatment of drug-resistant Aspergillus infection

Abstract

Triazole antifungals are frontline drugs for the treatment and prophylaxis of infections due to Aspergillus species. Azole resistance is an emerging problem and is associated with treatment failure in several case series. The management of azole-resistant invasive aspergillosis remains a challenge and there are no guidelines with appropriate recommendations. The current clinical practice suggests that liposomal amphotericin B or a combination of voriconazole or posaconazole with an echinocandin may be effective. Although cross-resistance within the azoles seems to be common, the role of azoles in the management of azole-resistant aspergillosis remains unclear, but optimizing drug exposure is critical for treatment success.

Keywords:

• antifungal treatment
• aspergillosis
• Aspergillus
• azole resistance
• posaconazole
• voriconazole

1. Introduction

Aspergillus species are ubiquitous and invasive infections occur primarily in immunocompromised hosts. Invasive fungal diseases affect more than 2 million people worldwide and account for more deaths annually than either tuberculosis or malaria. It is estimated that 2,100,000 Europeans suffer from allergic and 240,000 from chronic aspergillosis; reports calculate 63,250 cases of invasive aspergillosis (IA) annually [1]. Three classes of antifungal agents are available for the treatment of aspergillosis including polyenes, azoles and echinocandins. Appropriate therapy depends on the host’s immune status, organ function, prior therapies and species included. Itraconazole (ITC), voriconazole (VCZ) and posaconazole (PCZ) are widely used for the management of Aspergillus-related infections. VCZ is the drug of choice as first-line treatment for IA followed by liposomal amphotericin B (L-AMB). ITC, PCZ and the echinocandins are preserved for patients who are refractory to or intolerant of primary antifungal therapy (salvage treatment). PCZ is recommended for prophylaxis against fungal infections in patients at risk [2]. PCZ and VRZ are orally available, which is essential for outpatient management. Alarmingly, triazole resistance and pan-azole resistance in Aspergillus species have been reported recently.

2. The burden of azole resistance in Aspergillus fumigatus

In the past, azole resistance was reported sporadically and infrequently with the first case described in 1997 [1]. Reports from the UK and The Netherlands from the late 2000s attracted great attention [3] and meanwhile azole resistance in Aspergillus fumigatus has been detected in several European and Asian countries. Azole-naïve hosts and patients with chronic Aspergillus diseases are at risk, and different resistance mechanisms have been described. Acquired resistance is developed during long-term azole exposure, when a susceptible isolate evolves resistance. Resistance from unrelated single strains showed to be evolved independently and with frequent occurrence in various patients [1,3]. Acquiring primary resistant isolates in azole-naïve patients is given by environmental contamination, and agricultural use of azole compounds is held responsible for the latter. In vitro, prochloraz, an agricultural azole, was shown to induce cross-resistance of medically important azoles [1]. Various mutations within the CYP51A gene, which encodes the fungal target, as well as non-CYP51A-related resistant strains have been characterized. Resistance depends on position and type of amino acid substitution within the CYP51A leading to complete loss or decreased activity of one or all azoles. The amino acid substitution of leucine for histidine at codon 98 in conjunction with a tandem repeat (TR) in the promoter gene causes azole resistance [1]. While TR34/L98H is marked by pronounced ITC resistance with tendency to pan-azole resistance, TR46/Y121F/T289A-mediated resistance leads to high-grade VCZ with only moderate and variable effects on minimal inhibitory concentrations (MICs) of ITC and PCZ [4]. TR34/L98H is the predominant resistance mechanism of environmental origin in A. fumigatus and it is unclear whether TR34/L98H-mediated resistance is attributed to global migration of spores or due to independent local development [1,3,4]. In contrast to acquired resistance, no isogenic isolate with a wild-type phenotype has been detected in patients carrying an Aspergillus strain with TR34/L98H genetic alteration. Although demethylation inhibitor fungicides are also used in the USA, CYP51A mutations are not evident yet, possibly due to less use of fungicides compared to Europe and Asia [5].

ITC resistance varies from 2 to 6% with cross-resistance to VCZ and PCZ in 65 and 74%, respectively. Between 2008 and 2009 azole resistance in A. fumigatus increased from 23 to 31% with 97% ITC, 3% VRC and 78% multi-azole resistance being present [1]. In general, the prevalence varies widely with ranges from 0.8 to 9.5% and may differ between centers within a country [1]. A low prevalence of resistance to azoles in A. fumigatus (0.85%) was identified in a prospective study on patients with hematological disorders [6].

3. Detection of azole resistance in Aspergillus species

Culture-based detection of phenotypically resistant isolates is succeeded by determination of an elevated MIC compared to wild-type isolates. Both the European Committee on Antimicrobial Susceptibility Testing and the Clinical and Laboratory Standards Institute developed broth-based dilution methods for in vitro susceptibility testing; species-specific epidemiological cutoff values for A. fumigatus define resistance showing MICs of ITC and VCZ > 1 mg/l and PCZ > 0.25 mg/l [7,8]. ITC screening plates (agar plates supplemented with 4 mg/l ITC) enhance culture-based resistance detection [9]. PCR for detection of TR alterations and L98H and M220 mutations have shown to be functional [10] but naturally, these molecular-based detections would be unsuitable in regions with high rates of non-CYP51A mutations.

4. The clinical burden of azole resistance in A. fumigatus

Azole resistance in Aspergillus species leads to treatment failure or at least lacks improvement when treated with an azole [1,11]. Mortality-related infections caused by resistant strains rise from 30 – 50 to 88% when compared to wild-type infections. Interestingly, 12% of patients harboring resistant A. fumigatus isolates were reported to suffer from underlying hematologic and oncologic diseases. These immunocompromised patients are significantly more affected than others [12]. Currently, there are no clinical randomized trials available showing how to best treat azole-resistant aspergillosis and in the absence of clinical experience data obtained through in vitro testing and experimental models help to design strategies. However, data obtained from murine studies should be considered with caution as they do not represent the in vivo situation. Limited data indicate that L-AMB or a combination of VCZ or PCZ with an echinocandin may be effective.

The role of azoles in the management of azole-resistant aspergillosis remains unclear, but optimizing drug exposure is critical for treatment success. Pharmacodynamic models recommend plasma levels of VCZ to range between 1.5 (> 85% probability of response) and 4.5 mg/l (< 15% probability of neurotoxicity) [11]. Twice-daily application of 200 mg VCZ orally or 300 mg intravenously succeeds in 49 and 87% or 8 and 37% in obtaining minimum and maximum drug levels, respectively [12]. For A. fumigatus displaying VCZ MICs of 2, 4 and ≥ 4 mg/l, plasma levels of 1.03, 2.65 and 5.3 mg/l should be achieved [11]. In fact, required concentrations may be attainable by azole dose escalation, but are accompanied with significant toxicity. Hence, under VCZ treatment, therapeutic drug monitoring and close clinical observations must be conducted. For PCZ suspension a dose escalation is not recommended because of limited absorption. For successful treatment of an Aspergillus species with PCZ MICs of 0.5 mg/l, therapeutic concentrations should achieve ≥ 3.3 mg/l; PCZ tablets and intravenous formulations are favorable in reaching sufficient drug levels [11]; so far, the role of azoles in treating azole-resistant aspergillosis needs to be further investigated.

Synergism of azoles and echinocandins has been documented in vitro and in vivo [1]. VCZ and caspofungin showed improved survival when compared to VCZ alone in patients who failed initial therapy with AMB [13]. By contrast, the combination of VCZ and anidulafungin did not significantly influence overall mortality [14]. However, the outcome of VCZ and anidulafungin is influenced by the presence of VCZ-susceptible or -resistant strains of A. fumigatus [15]. While synergism was observed for VCZ-susceptible strains, only additive interactions were collected for VCZ-resistant strains. Survival in mice treated with VCZ monotherapy decreased from 100 to 72% with azole resistance being present. Even with maximum doses of anidulafungin (20 mg/kg), survival rates of mice infected with susceptible and resistant strains were 72 and 45%, respectively. Although survival rates were significantly improved by combination therapy, only highest drug doses kept mice alive. The loss of synergism of any combination therapy in treating infections with resistant isolates being involved limits such options.

L-AMB may become a cornerstone in the treatment of azole-resistant aspergillosis. Cross-resistance to AMB in the setting of azole resistance has not been described yet. However, the management of cerebral aspergillosis will be troublesome, as VCZ has a good penetration into the CNS and has shown to be superior to polyene [1]. Echinocandins are unable to completely kill or inhibit Aspergillus species; response rates of 33% in primary treatment of IA show survival to be lower than that obtained with VCZ and AMB [16]. Thus, caspofungin is recommended for salvage treatment of IA [2]. Synergistic or at least additive effects have been demonstrated for amphotericin B and caspofungin in vitro; in vivo, a favorable outcome was obtained for L-AMB and caspofungin when compared to high-dose L-AMB high (10 mg/kg) monotherapy (67 vs 27%) [2]. Nevertheless, data regarding efficacy of combination of AMB and echinocandins in case of azole resistance are lacking.

5. Expert opinion

Antifungal resistance continues to grow and evolve and complicate patient management, despite the introduction of new antifungal agents. A. fumigatus infections not responding to azole monotherapy need immediate consideration. The clinical significance of azole resistance is not well defined, but case series suggest poor outcomes in patients with documented infections. Currently, there are no clinical randomized trials available showing how to best treat azole-resistant aspergillosis. The current practice suggests that L-AMB or a combination of VCZ or PCZ with an echinocandin may be effective. Overall, it seems that azoles will play a limited role in the treatment of azole-resistant invasive Aspergillus infections. In addition, clinical management is complicated by the lack of existing clinical breakpoints for azoles and A. fumigatus; in vitro antifungal susceptibility testing is now standardized internationally and epidemiological cutt off values provide a sensitive tool of detecting resistance but the clinical relevance is lacking yet. The relative contribution of immunomodulation in the management of azole-resistant infections is not known; however, it is supposed that decreasing immune suppression will result in improved outcome. Facing the problem not knowing ‘when’ and ‘how’ to best treat drug-resistant fungi led to a fungal emergency. Thirty to fifty percent of IA patients still fail improvement for reasons that include late diagnosis and drug-resistant strains. Also, other factors may also be involved in treatment failure, such as the fungal burden, underlying disease, reduced drug bioavailability and increased drug metabolism. Since 2006 no new antifungals have been approved and it seems that no major achievements are in prospect. More research is needed about the induction and prevention of resistant Aspergillus infections. The challenge seems to track trends in antifungal resistance by conducting surveillance studies, by developing new laboratory tests to identify and understand specific mutations associated with antifungal resistance, by assessing antifungal use as part of antibiotic stewardship programs, by ensuring adherence to guidelines for treatment and infection control, and by prescribing antifungal medications appropriately. Only understanding of key factors will lead to improved patient management.

Declaration of interests

C Lass-Flörl has received grant support from the Astellas Pharma, Gilead Sciences, Pfizer, Schering Plough and Merck Sharp & Dohme. She has been an advisor/consultant to Gilead Sciences, Merck Sharp & Dohme, Pfizer, Basilea Pharmaceutica and Schering Plough. She has received travel/accommodation expenses and has been paid for talks on behalf of Gilead Sciences, Merck Sharp & Dohme, Pfizer, Astellas and Schering Plough. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.