1 Sackler School of Medicine, Tel-Aviv University, Tel-Aviv, Israel; 2 Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel
Received 30 December 2003; returned 24 January 2004; revised 4 February 2004; accepted 8 February 2004
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Abstract |
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Methods: Pure allicin was prepared by reacting synthetic alliin with a stabilized preparation of the garlic enzyme alliinase. We tested the in vitro efficacy of pure allicin against 31 clinical isolates of Aspergillus spp. using a microdilution broth method and following the NCCLS guidelines (document M-38P). Subsequently, the in vivo efficacy of allicin was tested in immunocompetent mice infected intravenously (iv) with Aspergillus fumigatus conidia. Allicin (5 mg/kg body weight) was administered iv once daily for 5 days post-infection or orally (po) (9 mg/kg body weight) for 5 days pre-infection and 10 days post-infection. No ill effects were observed in allicin-treated uninfected mice.
Results: The in vitro MICs and MFCs of allicin were between 8 and 32 mg/L, indicating that allicin in its pure form may be an effective fungicide in vitro. Timekill studies indicate that allicin exerts its fungicidal activity within 212 h of administration in vitro. Allicin treatment significantly prolonged survival of infected mice (P < 0.01) from mean survival time (MST) = 7.7 days in untreated mice to MST = 21.3 and 13.9 days for allicin iv and po treated mice, respectively. Allicin iv treatment led to a significant (P < 0.001) 10-fold reduction in fungal burden in A. fumigatus infected mice as evaluated by quantitative fungal cultures of kidney tissue samples.
Conclusions: These favourable results, despite the short half-life of this compound in vivo, support further studies of controlled sustained release or more prolonged administration of allicin as a treatment for aspergillosis.
Keywords: antifungal treatment, susceptibility testing, mouse models
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Introduction |
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Invasive aspergillosis (IA) is an emerging, highly lethal infection of the immunocompromised host.13 Even with the best antifungal drugs, mortality is extremely high, reaching 94%.14 In this report we demonstrate the inhibitory effect of pure allicin on the growth of Aspergillus spp. in vitro, and in a murine model of IA.
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Materials and methods |
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Allicin was produced by passing the synthetic substrate alliin [(+)S-2-propenyl L-cysteine S-oxide] through an immobilized alliinase column.15 The concentration of allicin in the solution was regularly confirmed by sample analysis using high-performance liquid chromatography.3 Allicin was kept in a dark tightly closed flask at 4°C and remained stable during all periods of the experiment (>3 months).
In vitro assays
The in vitro efficacy of pure allicin was tested against 31 clinical isolates of Aspergillus spp. [A. fumigatus (n = 13), Aspergillus niger (n = 6), Aspergillus flavus (n = 6), Aspergillus terreus (n = 6)] by a microdilution broth method and following NCCLS guidelines (document M-38P). The minimal fungicidal concentration (MFC) was the lowest drug concentration resulting in a 99.9% loss of viability as assessed by plating. MICs and minimal effective concentrations (MECs) were calculated as the lowest drug concentration resulting in complete inhibition of hyphal growth and lowest drug concentration resulting in aberrant hyphal growth,16 respectively. Values were assessed after 48 h of incubation in standard 96-well sterile flat-bottomed polystyrene plates (Corning, USA). Control strains (A. fumigatus strain AF293, A. niger strain ATCC 16404) were tested in every experiment as internal controls. Each well received 100 µL of the diluted drug concentrations. Dilutions were made in RPMI 1640 medium containing 0.165 M MOPS buffer at pH 7.0 (RPMI/MOPS medium). Conidial inocula were counted using a haemocytometer, prepared at a concentration of 2.5 x 104 cfu/mL in RPMI/MOPS medium and added at 100 µL/well (final volume of each well, conidia and allicin, 200 µL).
For timekill studies, representative strains Afm-13, An-1, At-3 and Afl-3 were grown as described above, in the presence of 16 mg/L allicin. This concentration was selected because it is at or 2-fold above the MFC determined for these strains in the microdilution broth method described above. After 1, 2, 4, 8, 12 and 24 h conidia and germlings in culture were removed by scraping, serially diluted in 2-fold dilutions and plated on Sabouraud agar plates. Colonies were counted after incubation for 48 h at 37°C. The MFC was calculated as the drug concentration that resulted in a 99.9% loss of viability after 124 h in the presence of allicin as compared with untreated viable spore counts at 0 h. Control measurements of untreated conidia over time cannot be accurately defined using this method because of clumping and the formation of a mycelium, while all the conidia grown in the presence of allicin did not undergo clumping and were amenable to plating and counting of colonies.
Murine model of aspergillosis
For the murine model of aspergillosis, female ICR mice, 46 weeks old, were inoculated intravenously via the lateral tail vein with A. fumigatus strain AF293 (Afm-13) using an inoculum of 5 x 106 freshly harvested conidia/mouse. Three means of administration, intraperitoneal (ip), intravenous (iv) and oral (po) were tested for allicin. For ip administration, allicin was diluted in saline and injected at 1.25, 2.5 and 5 mg/kg/day. The two higher concentrations caused discomfort to the animals and were discontinued. The lower concentration was ineffective when administered once daily for 5 days starting 1 h post-infection (data not shown). For iv administration for the dose-range experiment, allicin was administered iv once daily for 5 days starting 1 h post-infection, at 1.25, 2.5, 5 mg/kg body weight/day, 0.2 mL/mouse, diluted in saline, with 10 mice per group. Higher concentrations of allicin administered iv (9 mg/kg/day) caused discomfort to the animals and were discontinued. For po treatment, a regimen of 5 mg/kg/day for 5 days post infection was ineffective (data not shown). We therefore tested a modified version of the allicin po regimen described previously for the treatment of hyperlipidaemic rabbits;3 allicin was administered po at 9 mg/kg body weight/day, 0.2 mL/mouse, diluted in saline and 20 mice per group, for 5 days pre-infection and 10 days post-infection. Amphotericin B (AMB) was also tested as a comparative standard. AMB was administered ip once daily for 5 days, starting 1 h post-infection at a concentration of 0.5 mg/kg/day, 0.2 mL/mouse, with 10 mice per group. Infection was followed up for 28 days and evaluated in terms of mortality.
For studies of tissue burden, two groups of 20 mice were infected with A. fumigatus strain AF293 using an inoculum of 5 x 106 freshly harvested conidia/mouse. One group was mock treated with saline iv, and the other was treated with allicin iv at 5 mg/kg/day for 5 days as described above. Two randomly selected mice were sacrificed from each group on days 2, 4, 7, 10 and 14 after infection. From each mouse, both kidneys were aseptically removed, homogenized separately in 1 mL of saline and cultured for quantitative analysis in serial 10-fold dilutions as described previously.17 Permission for the animal experiments described in this study was granted by the institutional care and use committee of the Faculty of Medicine, Tel Aviv University.
Statistical analysis
Statistical analysis of mouse survival was performed by the log rank test using the Graphpad Prism 4 software package (Graphpad Software Inc., San Diego, CA, USA). P values of <0.05 were considered significant in these analyses. Error bars denote standard deviation.
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Results |
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The mean MECs and MICs obtained for each of the Aspergillus spp. at 48 h were: A. fumigatus (n = 13), 4 and 8 mg/L; A. flavus (n = 6), 16 and 16 mg/L; A. niger (n = 6), 16 and 32 mg/L; and A. terreus (n = 6), 8 and 16 mg/L, respectively. In pilot experiments, allicin exhibited fungicidal activity against all four Aspergillus spp. tested at MIC concentrations. The mean MECs, MICs and MFCs of allicin for A. fumigatus were 2- to 4-fold lower than for A. niger, A. flavus and A. terreus.
Timekill study results show that conidia grown from strains A. fumigatus 13, A. niger 1, A. terreus 3 and A. flavus 2 displayed a loss in viability after 812 h of incubation in the presence of 16 mg/L allicin (Figure 1).
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We tested three routes of administration for allicin, ip, iv and po:
(i) ip treatment caused discomfort to the animals and was not effective (data not shown).
(ii) iv treatment: control infected mice receiving saline iv showed 100% mortality [mean survival time (MST) = 7.2 days]. Mice treated with allicin iv at 1.25, 2.5 and 5 mg/kg/day gave survival rates of 30% (MST = 13.5 days), 30% (MST = 18.1 days) and 70% (MST = 23.7 days), respectively (Figure 2). Survival time was significantly increased in mice treated with 2.5 and 5 mg/kg/day of allicin compared with the saline-treated mice (P < 0.01, 0.005, respectively). The allicin iv regimen at 5 mg/kg/day was subsequently retested in a larger group of animals and compared with the gold standard treatment with amphotericin B (Figure 3). Control infected mice (20 animals) receiving saline iv, showed 100% mortality (MST = 7.7 days). The survival curve for mice treated with allicin iv (5 mg/kg/day, 20 animals) showed 50% survival (MST iv = 21.3 days, P < 0.01 compared with the controls). Treatment with amphotericin B (0.5 mg/kg/day, 10 animals) gave an 80% survival rate (MST = 25.1 days, P<0.005 compared with the controls) (Figure 3). Treatment with amphotericin B was significantly more effective than treatment with allicin iv (5 mg/kg/day) (P < 0.05).
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We selected the most effective treatment regimen, allicin iv at 5 mg/kg/day, for further analysis of fungal burden. Two groups of mice (n = 20) were infected. One group was mock treated iv with saline, and the second was treated with allicin iv at 5 mg/kg/day for 5 days. On days 2, 4, 7, 10 and 14, two mice from each group were randomly selected and sacrificed. Fungal burden was determined as described in Materials and methods. Renal colony counts of mice infected with AF293 and mock treated with saline showed a dramatic increase in fungal load during the 2 weeks of infection (Table 1). Renal fungal load measured on days 7, 10 and 14 post-infection in mice treated with allicin (5 mg/kg/day, iv) was reduced 10-fold as compared with the saline-treated mice (P < 0.001 compared with the controls).
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Discussion |
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Previous reports describing the in vitro efficacy of allicin against fungi were compromised by the fact that they used allicin that was produced from freshly crushed garlic cloves, which contains many additional untested compounds.1,8,9,11,12 The potency and efficacy of such extracts were variable and difficult to reproduce reliably. In this report, we used pure allicin, prepared by passing the synthetic substrate alliin through an immobilized alliinase column. The allicin prepared by this method is chemically well defined; it is quantified by analysis and gives reliable, reproducible results. We show that pure allicin possesses effective (832 mg/L) and rapid (812 h) fungicidal activity against Aspergillus spp. in vitro. Isolates of A. fumigatus (responsible for >80% of IA infections) were notably more sensitive to allicin: the MFC for 12 of 13 isolates was 8 mg/L or lower. Our results are within the range of those seen previously for Aspergillus spp. using allicin extracts.9
Importantly, allicin is toxic to mammalian cells in culture at significantly higher concentrations (>60 mg/L) than those shown for Aspergillus spp.2 This difference in sensitivity between fungal and mammalian cells may be explained by the higher concentrations of glutathione which the mammalian cells possess, protecting them from the thiolation activity of allicin.6,18
We show, for the first time, that allicin can significantly reduce mortality, prolong survival and reduce fungal load in mice infected with A. fumigatus. The rate of killing of the fungus was shown by us to be very rapid and though the half-life of allicin in blood, solvents and simulated physiological fluids has been reported to be rather short (50 min)19 some of the degradation products, such as allyl mercaptan, may provide additional antifungal activity.4,10 Nevertheless, even the most effective allicin treatment was not as successful as treatment with AMB, necessitating more study to find a better treatment modality. The in vivo antifungal activity of allicin could be improved perhaps by controlled sustained release using prolonged iv infusion. Targeted production of allicin at the site of Aspergillus infection may also hold promise. Such a targeted production was recently achieved against cancer cells by the injection of a conjugate consisting of a monoclonal antibody that was chemically ligated to the enzyme alliinase, followed by injection of the substrate alliin.20 A similar approach could further increase the efficacy of this compound in the treatment of aspergillosis.
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Acknowledgements |
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This work was presented in part at the 43rd Interscience Conference on Antimicrobial Agents and Chemotherapy, Chicago 15 September 2003, Abstract M-960.
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Footnotes |
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References |
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