Correlation of in vitro MIC and MFC against isavuconazole, voriconazole and amphotericin B of Aspergillus with in vivo outcome in mice with disseminated aspergillosis

Abstract number: P1915

Majithiya J.B., Sharp A., Parmar A., Denning D.W., Warn P.A.

Background: Assessment of susceptibility of Aspergillus isolates is commonly performed using the CLSI M38A protocol. In this protocol breakpoints are assigned using the MIC whilst in many cases there are large differences between the MIC and MFC. In this study we examined strains with large differences between the MIC and MFC using time-kill experiments and murine models of disseminated aspergillosis to determine which parameter better reflects the response to antifungal therapy.

Methods: MICs and MFCs (99.9%) were determined according to the CLSI M38A guidelines and a range of strains in which MIC were similar to MFC and also MIC [Gt] MFC were selected. In vitro time-kill curves using RPMI 1640 + 2% glucose were set up using 9 strains of Aspergillus (table 1) exposed to isavuconazole (ISA), voriconazole (VCZ) and amphotericin B (AmB) and followed for 48 h. Four strains of A. fumigatus (AF) one flavus (AFL) and one terreus (AT) were used to study the in vivo effect. Male CD1 mice 22–24g were immunosuppressed with a single dose of 200 mg/kg cyclophosphamide IV then 3 days later infected IV with a sub-lethal inoculum of Aspergillus (AF4, A1163, AF210, AF293, AT49, AFL8). Both ungerminated and partially germinated conidia were used for AF293. 4 hours after infection mice were treated once daily with either vehicle, ISA (oral), VCZ (IV) plus grapefruit juice (oral) AmB (IP), or caspofungin (CAS) (IV) for 3–4 days. 72–96 h post infection mice were euthanised and the kidneys removed for quantitative culture.

Results: MICs and MFCs are shown in table 1. Four strains of AF with low MIC and MFCs (AF4, AF82, A1163 and AF210) were rapidly killed in vitro. Other strains with low MIC but high MFCs demonstrated much slower clearance using in vitro time-kills (for ISA p = 0.046 for MICgeqslant R: gt-or-equal, slanted4 mg/L AmB and VCZ NS). In vivo strains generated moderately severe non-lethal infections with heavy and reproducible tissue burdens in the kidneys. The strains with low MIC and MFCs (A1163 and AF210) had very low residual tissue burdens following treatment with all antifungals. In contrast strains with high MFCs had terminal tissue burdens very similar to control animals.

Table 1

Conclusion: In this study all strains had similar MICs but demonstrated a wide range of MFCs. The MFC better correlated with the rate of kill in vitro. Additionally the high MFCs of two strains (AF293 and AT49) were generally reflected by higher terminal tissue burden in murine models of disseminated aspergillosis.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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