Influence of Serum and Albumin on Echinocandin In Vitro Potency and Pharmacodynamics

Fluconazole is not inferior than caspofungin, micafungin or amphotericin B in the presence of 50% human serum against Candida albicans and Candida parapsilosis biofilms

Biofilm formation is a relevant risk factor for mortality in candidemia. Data about serum-based susceptibility testing against Candida biofilms are scant; therefore, the activity of fluconazole, amphotericin B, caspofungin and micafungin was determined against Candida albicans and C. parapsilosis biofilms with or without 50% human serum using XTT-based assays. Serum caused a remarkable adverse effect regarding biofilm structure for both species. Additionally, the ratio of nonviable cells increased for C. parapsilosis biofilms, as confirmed by fluorescent microscopy and flow cytometry. Despite impaired biofilm development, traditionally biofilm-active antifungals, surprisingly, showed decreased activity against C. albicans biofilms in serum at concentrations ranging from 0.5 to 1 mg/l and from 0.015 to 1 mg/l for amphotericin B and echinocandins, respectively (P < .01-.05). However, C. parapsilosis showed higher susceptibility to these antifungals due to reduced biofilm mass and the fungicidal effect of serum at concentrations ranging from 0.015 to 1 mg/l and from 0.015 to 512 mg/l for amphotericin B and echinocandins, respectively (P < .01-.05). Fluconazole exerted better antifungal activity in serum than traditionally biofilm-active antifungals against both examined biofilms. For fluconazole, significant differences were observed in susceptibility between serum-treated and serum-free biofilms at concentrations ranging from 0.015 to 8 mg/l and from 0.03 to 512 mg/l for C. albicans and C. parapsilosis isolates, respectively (P < .01-.05). The high antifungal activity of fluconazole in 50% serum both against C. albicans and C. parapsilosis biofilms supports the utility of fluconazole prophylaxis to reduce the risk of catheter-associated fungal infections.

Comparison of Killing Activity of Micafungin Against Six Candida Species Isolated from Peritoneal and Pleural Cavities in RPMI-1640, 10 and 30% Serum

Currently echinocandins are recommended in Candida peritonitis and pleuritis. We determined micafungin killing rates (k values) at therapeutic concentrations (0.25-2 mg/L) in RPMI-1640 with and without 10 and 30% serum mimicking in vivo conditions against six Candida species isolated from peritoneal and pleural fluid. In RPMI-1640, micafungin was fungicidal against C. glabrata, C. krusei and C. kefyr within 2.27 ± 10.68, 2.69 ± 10.29 and 3.10 ± 4.41 h, respectively, while was fungistatic against C. albicans, C. tropicalis and C. parapsilosis. In 10% serum, ≥ 0.25, ≥ 0.5, ≥ 0.5 and ≥ 1 mg/L micafungin produced positive k values (killing) for all C. albicans, C. glabrata, C. kefyr and C. krusei, respectively. In 30% serum, 2 mg/L micafungin produced killing against all C. albicans, C. glabrata and C. kefyr isolates, but was ineffective against C. krusei, C. parapsilosis and 2 of 3 C. tropicalis. Micafungin exposure should be increased against non-albicans species to eradicate fungi from peritoneal and pleural cavities.

Poor in vivo efficacy of caspofungin, micafungin and amphotericin B against wild-type Candida krusei clinical isolates does not correlate with in vitro susceptibility results

We determined micafungin, caspofungin and amphotericin B (AMB) minimum inhibitory concentration (MICs) and killing rates in RPMI-1640 and in RPMI-1640 with 50% serum against three Candida krusei bloodstream isolates. MIC ranges in RPMI-1640 were 0.125-0.25, 0.25 and 0.125-0.5 mg/L, in RPMI-1640 with 50% serum, MICs were 64-128-, 8- and 4-16-fold higher, respectively. In RPMI-1640 micafungin and caspofungin at 1, 4, 16 and 32 mg/L as well as AMB at 2 mg/L were fungicidal against all isolates in ≤3.96, ≤4.42 and 14.96 h, respectively. In RPMI-1640 with 50% serum, caspofungin was fungicidal for all isolates only at 32 mg/L, micafungin and AMB were fungistatic. In neutropenic mice, 5 mg/kg caspofungin and 1 mg/kg AMB were ineffective against two of the three isolates. Thus, in vivo efficacy of echinocandins and AMB is weak or absent against C. krusei. Prescribers treating C. krusei infections with echinocandins should watch out for clinical resistance and therapeutic failure.

Recent Insights into the Paradoxical Effect of Echinocandins

Echinocandin antifungals represent one of the most important drug classes for the treatment of invasive fungal infections. The mode of action of the echinocandins relies on inhibition of the β-1,3-glucan synthase, an enzyme essentially required for the synthesis of the major fungal cell wall carbohydrate β-1,3-glucan. Depending on the species, echinocandins may exert fungicidal or fungistatic activity. Apparently independent of this differential activity, a surprising in vitro phenomenon called the “paradoxical effect” can be observed. The paradoxical effect is characterized by the ability of certain fungal isolates to reconstitute growth in the presence of higher echinocandin concentrations, while being fully susceptible at lower concentrations. The nature of the paradoxical effect is not fully understood and has been the focus of multiple studies in the last two decades. Here we concisely review the current literature and propose an updated model for the paradoxical effect, taking into account recent advances in the field.

Killing Activity of Micafungin Against Candida albicans, C. dubliniensis and Candida africana in the Presence of Human Serum

We compared killing activity of micafungin in time-kill experiments in RPMI-1640 with and without 50% serum against Candida albicans, Candida dubliniensis and Candida africana reference strains and clinical isolates. Killing rates (k values) were determined for each strain and concentration. In RPMI-1640 MIC ranges were 0.015-0.03, 0.015-0.03 and 0.015 mg/L against C. albicans, C. dubliniensis and C. africana, respectively. In 50% serum MIC values for the three species increased 16- to 64-fold. In RPMI-1640 micafungin was fungicidal against two of three C. albicans isolates at 16 and 32 mg/L within 14.54 h and fungistatic against all C. africana and C. dubliniensis. Fifty per cent serum significantly decreased the growth rate of C. africana, but not of the other two species; weak in vivo replication ability of C. africana was confirmed in murine model. In 50% serum micafungin at 0.25 and 1 mg/L did not inhibit any of the three species (k values were always negative). Micafungin killing rate in 50% serum at 4, 16 and 32 mg/L was significantly decreased for C. albicans, but increased for C. dubliniensis compared to RPMI-1640. Killing activity of micafungin against C. africana was comparable or higher in 50% serum than in RPMI-1640. Although micafungin is a highly protein-bound drug, it was equally effective against the species of the C. albicans complex in 50% serum at therapeutic trough concentration (4 mg/L). Both in vitro and in vivo data confirmed the low virulence of C. africana compared to the two sibling species.

Decreased Killing Activity of Micafungin Against Candida guilliermondii, Candida lusitaniae, and Candida kefyr in the Presence of Human Serum

Currently, echinocandins are first-line drugs for treatment of invasive candidiasis. However, data on how serum influences killing activity of echinocandins against uncommon Candida species are limited. Therefore, the killing activity of micafungin in RPMI-1640 and in 50% serum was compared against Candida guilliermondii, Candida lusitaniae, and Candida kefyr. Minimum inhibitory concentration (MIC) ranges in RPMI-1640 were 0.5-1, 0.12-0.25, and 0.06-0.12 mg/L, respectively. In 50% serum, MICs increased 32- to 256-fold. In RPMI-1640 ≥ 0.25, ≥4, and 32 mg/L micafungin was fungicidal against all four C. kefyr (≤4.04 hours), two of three C. lusitaniae (≤16.10 hours), and two of three C. guilliermondii (≤12.30 hours), respectively. In 50% serum, all three species grew at ≤4 mg/L. Micafungin at 16-32 mg/L was fungicidal against all C. kefyr isolates (≤3.03 hours) and at 32 mg/L was fungistatic against one of three C. lusitaniae isolates. Two C. lusitaniae isolates and all three C. guilliermondii grew at all tested concentrations. Adding human serum to susceptibility test media drew attention to loss of fungicidal or fungistatic activity of micafungin in the presence of serum proteins, which is not predicted by MICs in case of C. kefyr and C. lusitaniae in RPMI-1640. Our results strongly suggest that micafungin and probably other echinocandins should be used with caution against rare Candida species.

Dose escalation studies with caspofungin against Candida glabrata

Echinocandins are recommended as first-line agents against invasive fungal infections caused by Candida glabrata, which still carry a high mortality rate. Dose escalation of echinocandins has been suggested to improve the clinical outcome against C. glabrata. To address this possibility, we performed in vitro and in vivo experiments with caspofungin against four WT C. glabrata clinical isolates, a drug-susceptible ATCC 90030 reference strain and two echinocandin-resistant strains with known FKS mutations. MIC values for the clinical isolates in RPMI 1640 were ≤ 0.03 mg l(-1 ) but increased to 0.125-0.25 mg l(-1 )in RPMI 1640+50% serum. In RPMI 1640+50% serum, the replication of C. glabrata was weaker than in RPMI 1640.Caspofungin in RPMI 1640 at 1 and 4 mg l(-1) showed a fungicidal effect within 7 h against three of the four clinical isolates but was only fungistatic at 16 and 32 mg l(-1) (paradoxically decreased killing activity). In RPMI 1640+50% serum, caspofungin at ≥ 1 mg l(-1) was rapidly fungicidal (within 3.31 h) against three of the four isolates. In a profoundly neutropenic murine model, all caspofungin doses (1, 2, 3, 5 and 20 mg kg(-1) daily) decreased the fungal tissue burdens significantly (P < 0.05-0.001) without statistical differences between doses, but the mean fungal tissue burdens never fell below 105 cells (g tissue)(-1). The echinocandin-resistant strains were highly virulent in animal models and all doses were ineffective. These results confirm the clinical experience that caspofungin dose escalation does not improve efficacy.