The present research assessed the
in vitro susceptibility testing of 108 molecularly well-characterized
Aspergillus, and
Candida isolates recovered from otomycosis patients to LUL, a novel imidazole, and EFN, a novel triazole, and comparing them with other antifungal drugs. The LUL and EFN are new imidazole and triazole agents, which their efficacy in the management of dermatophytosis and onychomycosis has been recently confirmed. The literature is indicative of the potent activities of these antifungals against dermatophytes,
C. albicans, dematiaceous fungi,
Malassezia, and azole-resistant and azole-susceptible
A. fumigatus strains (
7,
12,
20-
22). However, to the best of our knowledge, there is no report regarding the resistance of these species to these drugs. As the results of the current research indicated, LUL (0.098 μg/mL) and EFN (0.109 μg/mL) had the lowest GM MICs against
Aspergillus strains. The MIC ranges of LUL against
A. niger and
A. tubingensis isolates were 0.06 - 0.25 and 0.031 - 0.5 μg/mL, respectively. These values for EFN were 0.06 - 0.5 and 0.06 - 0.5 μg/mL, respectively (
Table 1).
In a study performed by Hivary et al., the MICs of LUL against
A. niger and
A. tubingensis, as the predominant agents of otomycosis, were in the ranges of 0.00024 - 0.125 μg/mL and 0.00024 - 0.125 μg/mL, respectively (
20). In addition, the MIC value presented by Tupaki-Sreepurna et al. demonstrated that EFN was more potent than other azoles against different non-dermatophytic fungi (
21). The EFN MIC values was estimated at < 0.25 μg/mL for
Aspergillus (
12,
21,
23). Our results revealed low MIC values for EFN and LUL against
Aspergillus, which is in line with other reports (
12,
21,
23). In the current research, TRB had the GM MICs of 0.040, 0.128, and 0.179 μg/mL against
A. flavus,
A. niger, and
A. tubingensis, respectively. The TRB (Lamisil) is a safe allylamine antifungal used both topically and orally for the management of infections, such as pityriasis versicolor, cutaneous candidiasis, and dermatophytosis (
24). Based on a study by Mosquera et al., this drug had potent activities against
Aspergillus species (
25). In the present study, TRB had a lower GM MIC against
A. flavus and
A. niger compared to the values reported by Moore et al. (
26). The VRC and CLO are other antifungal agents utilized for the treatment of
Aspergillus-induced infections.
The susceptibility of
Aspergillus species to these antifungals was also investigated in the present study. The comparison of VRC and CLO with ITC, FLU, KTO, and AMB revealed a lower GM MIC for VRC and CLO against
Aspergillus species. The results of previous studies are indicative of the resistance of this species to VRC (
27). Sutton et al. suggested VRC as an agent that can be used clinically for the treatment of refractory invasive aspergillosis and should be prioritized over the conventional AMB treatment (
28). There are reports regarding the growing resistance of ITC to
A. fumigatus (
29). On the contrary, black aspergilli (MIC > 16 µg/mL) showed resistance to ITC in more than 50% of cases, without any clear mechanisms explaining this resistance (
30). Our results showed that the common black aspergilli species (e.g.,
A. niger and
A. tubingensis) showed higher ITC resistance.
Ototopical FLU is used for the management of otomycosis, especially against
Aspergillus (
31). The GM MICs of FLU against
A. niger,
A. tubingensis, and
A. flavus were obtained to be 10.2, 2.47, and 1.14 μg/mL, respectively (
Table 1). Our results revealed FLU resistance in 6 isolates (35.29%), which is in line with other reports indicating
in vitro FLU resistance (
32). Evidence is suggestive of the higher susceptibility of
A. niger and
A. tubingensis accounting for mild to systemic aspergillosis to AMB under
in vitro conditions. However, these cases undergo frequent relapses and failures in spite of using antifungal therapy; accordingly, their therapeutic values are challenging (
33).
In the present study, AMB MICs ranged between 0.06 and > 16 μg/mL for
Aspergillus species, which is similar to the results reported by Oakley et al., but in contrast to those obtained by Kaya et al. (
34,
35). Among the tested antifungal agents, KTO, CAS, and NYS had higher GM MICs against
Aspergillus species compared to other drugs. Our data showed that
A. flavus was more susceptible to LUL and EFN than
A. niger and
A. tubingensis. Nevertheless, aspergilli recovered from the external auditory canal demonstrated higher susceptibility to LUL, EFN, and TRB. Therefore, these topical agents can be considered as effective treatments for aspergillosis. In this study, the susceptibility of
Candida isolates recovered from otomycosis was tested for the eleven drugs. The LUL (GM = 0.133μg/mL) and EFN (GM = 0.144 μg/mL) had the lowest MICs for
Candida isolates. The LUL and EFN have an unclear sensitivity, dose dependence, or resistance range because of the lack of an acceptable breakpoint for
Candida species.
The GM MICs of LUL and EFN for
C. glabrata were obtained to be 0.076 and 0.085 μg/mL, respectively. These values were 0.122 and 0.097 μg/mL for
C. albicans and 0.203 and 0.247 μg/mL for
C. parapsilosis, respectively. Our results revealed that
C. glabrata had lower MICs for LUL and EFN than
C. parapsilosis and
C. albicans strains. Niwano et al. (
36) and Koga et al. (
37) were the first researchers reporting a very low MIC for LUL and EFN (MIC range: 0.031 - 0.13 and 0.031 - 0.25 µg/mL, respectively) against
C. albicans. Niwano et al., comparing oral LUL and FLU in a murine model of systemic candidiasis, reported the lower efficacy of LUL compared to that of FLU (
36,
38). In the current study, CAS had a GM MIC of 0.194 μg/mL against
Candida species. The previous studies have also confirmed the potent
in vitro activity of this drug against
Candida strains (
39). Based on our results, NYS and TRB had the highest MIC values against
Candida isolates, which is consistent with the results reported in the literature (
40). However,
Candida isolates also showed resistance to FLU, VRC, and CAS.
5.1. Conclusions
Our findings revealed that LUL and EFN had the highest in vitro activities against Aspergillus (GM MICs of 0.098 and 0.109 μg/mL, respectively) and Candida (GM MICs of 0.133 and 0.144 μg/mL, respectively) strains. In most of the cases, the novel topical triazoles and imidazoles (EFN and LUL) appeared to show good antifungal activities. The findings of the present research could be useful in promoting the significance of antifungal susceptibility testing for appropriate treatment.