In vitro activity of SF001: a next-generation polyene versus amphotericin B

ABSTRACT

SF001, a next-generation polyene drug, offers broad-spectrum fungicidal activity with less potential for toxicity than classic polyene amphotericin B (AmB). This study compared the in vitro activity of SF001 and amphotericin B against Candida and Aspergillus species. SF001 demonstrated activity comparable to AmB against Candida isolates (MIC_50/90 of 0.25/1 and 0.5/0.5 mg/L, respectively). However, Aspergillus isolates exhibited higher susceptibility to SF001 than AmB (MIC_50/90 of 0.5/1 and 1/4 mg/L, respectively), notably including AmB-resistant species.

INTRODUCTION

Invasive fungal infections (IFIs) have increased in recent decades, largely driven by the rising number of immunosuppressed patients (1). IFIs are notoriously challenging to treat, often requiring systemic antifungal therapies, such as echinocandins, polyenes, and triazoles (2). Amphotericin B (AmB), a polyene antifungal, has long been a cornerstone in treating various systemic fungal infections for the past six decades, demonstrating strong clinical and pharmacological activity (3, 4). Despite the high toxicity of AmB limiting clinical use, it remains the gold standard for treating severe IFIs, where rapid response is crucial as is AmB’s broad spectrum of activity against yeasts and filamentous and dimorphic fungi (5, 6). The resistance rate of AmB is lower than that of other antifungals due to its fungicidal properties, along with the significant fitness cost associated with resistance (7, 8). Given AmB’s strengths, formulations like AmB lipid complex, liposomal AmB, and AmB colloidal dispersion were developed to reduce nephrotoxicity and infusion reactions (3). Still, the use of these formulations is limited, in part due to high rates of infusion-related events (9, 10), highlighting the unmet need for novel, safer options (11).

SF001, a next-generation polyene drug currently in clinical development, was rationally designed to mitigate toxicity (12). Its active moiety, Sfu-AM2-19 (formerly, AM-2-19) (12) has demonstrated potent, broad-spectrum fungicidal activity, including activity against Aspergillus strains resistant to AmB (12, 13). The current study aimed to compare the in vitro potency of SF001 and AmB (Amphotericin B from Streptomyces sp., Sigma-Aldrich, A4888) against representative clinical isolates of two common fungal genera, Aspergillus (n = 41) and Candida (n = 56), including AmB-resistant/non-wild-type strains. The isolates were tested according to the mould and yeast EUCAST guidelines (14, 15). The minimum inhibitory concentrations (MICs) of SF001 and AmB were determined using two criteria: for yeasts, the lowest concentration achieving 90% growth inhibition after 24 hours by spectrophotometry, and for molds, the lowest concentration showing no visible growth to the naked eye after 48 hours of incubation. A. flavus ATCC 204304, C. krusei ATCC 6258, and C. parapsilosis ATCC 22019 served as the quality control strains for each run of the assay (16). MIC ranges and MIC₅₀ and MIC₉₀ values (the concentrations that inhibited 50% and 90% of the isolates, respectively) were determined.

SF001 exhibited strong in vitro activity against both Candida and Aspergillus genera, with distinct variations in activity between the two (Fig. 1; Table 1). As shown in Fig. 1A and B, the MIC range of SF001 against Candida species was comparable to that of AmB (0.125 to 4 mg/L). The MIC₅₀ and MIC₉₀ values for SF001 were 0.25 and 1 mg/L, respectively, while for AmB, they were 0.5 mg/L (Fig. 1A and B). In contrast, against Aspergillus isolates, SF001 demonstrated a lower MIC range (0.125 to 4 mg/L) compared to that of AmB (0.25 to 8 mg/L), along with lower MIC₅₀ and MIC₉₀ values of 0.5 and 1 mg/L, versus 1 and 4 mg/L for AmB, respectively (Fig. 1C and D). Considering the geometric mean (GM) values, SF001 and AmB exhibited similar overall activity against all tested wild-type and AmB-resistant/non-wild-type Candida isolates (GM: 0.34 and 0.39 mg/L, respectively). In contrast, SF001 displayed greater activity against all tested Aspergillus species, with GM values of 0.54 mg/L for SF001 and 1.07 mg/L for AmB (Table 1). Although the overall GM MIC values of SF001 and AmB against Candida species were comparable, a species-specific analysis revealed subtle variations (Table 1). Species such as C. glabrata, C. parapsilosis, C. tropicalis, C. dubliniensis, and C. inconspicua exhibited a slight reduction in the GM MIC of SF001 compared to AmB. Conversely, other species, including C. albicans, C. krusei, C. auris, C. lusitaniae, and C. guilliermondii, showed higher GM MIC of SF001 compared to AmB (Table 1). Considering Aspergillus species separately (Table 1), SF001 demonstrated potent in vitro activity against all tested Aspergillus species, as reflected by lower GM MIC values, particularly for A. terreus (7, 17), A. flavus (17, 18), and A. versicolor (19, 20), which are commonly resistant to AmB. Interestingly, some A. terreus and A. flavus isolates classified as non-wild-type for AmB (with high MICs of 4–8 mg/L) showed much lower MICs (≤2 mg/L) when tested with SF001. If AmB EUCAST epidemiological cutoff values (ECOFFs) were applied to SF001, these isolates would be considered wild type. However, as SF001-specific breakpoints (BPs) and ECOFFs have yet to be established, caution is required when interpreting susceptibility categorization.

Fig 1.

The distribution of minimum inhibitory concentrations of SF001 and amphotericin B against Candida species (A and B, respectively) and Aspergillus species (C and D, respectively). MIC₅₀/MIC₉₀ stand for concentrations inhibiting ≥50% and ≥90% of the tested isolates, respectively.

TABLE 1.

Susceptibility profiles of SF001 and amphotericin B (AmB) against Candida and Aspergillus species, using EUCAST guidelines

Species	SF001 (mg/L)		AmB (mg/L)		AmB
	MIC range	GMa MIC	MIC range	GMa MIC	BPb	ECOFF/TECOFFc
C. albicans (n = 6)	0.25–4	0.79	0.25–4	0.70	1	1
C. glabrata (n = 6)	0.25–0.5	0.28	0.5	0.5	1	1
C. krusei (n = 5)	0.25–1	0.65	0.5	0.5	1	1
C. auris (n = 5)	0.25–1	0.75	0.5	0.5	–d	2
C. lusitaniae (n = 5)	0.25	0.25	0.125–0.25	0.18	–	0.5
C. guilliermondii (n = 5)	0.125–1	0.32	0.125–0.5	0.25	–	0.5
C. parapsilosis (n = 5)	0.25	0.25	0.5	0.5	1	1
C. tropicalis (n = 5)	0.25	0.25	0.25–0.5	0.43	1	1
C. dubliniensis (n = 5)	0.125–0.25	0.21	0.125–0.5	0.25	1	0.25
C. inconspicua (n = 5)	0.125–0.25	0.14	0.25–0.5	0.28	–	–
C. pararugosa (n = 1)	0.25	–	0.5	–	–	–
C. nivariensis (n = 2)	0.25	–	0.5	–	–	–
C. kefyr (n = 1)	1	–	0.5	–	–	1
Candida species	0.125–4	0.34	0.125–4	0.39
A. fumigatus (n = 6)	0.25–0.5	0.35	0.5–1	0.70	1	1
A. flavus (n = 6)	0.5–2	0.62	1–4	1.41	–	4
A. niger (n = 5)	0.125–0.25	0.14	0.25–0.5	0.37	1	0.5
A. nidulans (n = 6)	0.25–1	0.5	0.5–2	1	–	4
A. lentulus (n = 4)	1	1	2	2	–	–
A. fumigatiaffinis (n = 3)	0.5–4	–	1–4	–	–	–
A. versicolor (n = 4)	0.5–1	0.70	1–2	1.18	–	–
A. terreus (n = 7)	0.25–2	0.82	0.25–8	1.34	–	8
Aspergillus species	0.125–4	0.54	0.25–8	1.07

^a MIC ranges and geometric mean (GM) values are reported only for species represented by at least four isolates.

^b BP, breakpoint (21).

^c ECOFF/TECOFF, EUCAST epidemiological cut-off values/tentative ECOFF (21).

^d – indicates that no ECOFF, TECOFF, or BP is available for the specific species.

The active moiety of SF001, Sfu-AM2-19, is a novel analog of AmB rationally designed to reduce toxicity by selectively extracting ergosterol from fungal membranes without binding to cholesterol in mammalian cells (12). This is achieved through C2′ epimerization and C16 amidation, enhancing ergosterol extraction while preventing cholesterol binding (12). The activity of SF001 in this study was consistent with previous findings by Maji et al. (12), which showed potent in vitro activity against fungal pathogens relatively resistant to liposomal AmB and frequently used azoles, i.e., isavuconazole, voriconazole, and posaconazole. However, the extent of activity varied among pathogens, with a minimal reduction in MIC for some strains (e.g., C. krusei) and a more substantial decrease in MICs for others (e.g., A. fumigatus), aligning with our findings. In addition, previous studies demonstrated SF001’s broad in vivo efficacy in candidiasis, aspergillosis, and mucormycosis mouse models, with dose-dependent reductions in fungal burden and reduced toxicity to human cells and mice (12, 13). SF001 demonstrates notable in vitro potency, particularly against common AmB-resistant and non-wild-type Aspergillus species, such as A. terreus and A. flavus, as well as less common species like A. versicolor. Its potent activity against resistant Aspergillus species underscores its potential to address the growing challenge of azole resistance—a significant limitation of current therapies—and the urgent need for new antifungals to combat escalating resistance and associated clinical failures (22–24). Similarly, the activity of SF001 against Candida species (Table 1) was comparable to AmB and is notable in the context of echinocandin- and fluconazole-resistant Candia species, which are also on the rise (23) and for which there are limited treatment options. SF001 safety data to date, combined with its potent fungicidal activity and ability to achieve pharmacokinetic/pharmacodynamic targets, positions SF001 as a strong candidate for further development (12, 13). Its effective in vitro activity, particularly against Aspergillus species, supports interest in further in vivo and clinical studies of SF001 to evaluate its potential role in treating IFIs.

DATA AVAILABILITY

The authors confirm that all protocols are detailed within the article.