In vitro activities of antifungal agents alone and in combination against fluconazole-susceptible and -resistant strains of Candida dubliniensis

BRIEF COMMUNICATION

In vitro activities of antifungal agents alone and in combination against fluconazole-susceptible and -resistant strains of Candida dubliniensis

Liliane Alves Scheid; Débora Alves Nunes Mario^*; Thaís Felli Kubiça; Janio Morais Santurio; Sydney Hartz Alves

Postgraduate Course in Pharmaceutical Sciences, Department of Microbiology and Parasitology, Universidade Federal de Santa Maria, Santa Maria, RS, Brazil

ABSTRACT

In the present study we used two groups of Candida dubliniensis strains: one containing fluconazole-susceptible clinical isolates and another containing fluconazole-resistant laboratory derivative from the former to examine the changes on susceptibility accompanying the development of resistance to fluconazole. Our findings confirmed the ability of C. dubliniensis isolates to become resistant to fluconazole and indicated that this resistance was crossed with ketoconazole, itraconazole, ravuconazole and terbinafine. We also tested combinations of terbinafine, amphotericin B, itraconazole and voriconazole against both groups of isolates in a checkerboard assay. Surprisingly, most combinations evidenced indifferent interactions, and the best synergism appeared when terbinafine and itraconazole were combined against the fluconazole-resistant group.

Keywords: Candida; Antifungal agents; Fluconazole; Drug resistance, fungal

The increasing incidence of fungal infections without a satisfactory response to the current antifungal therapy and the slow development of new agents with novel mechanisms of action have produced significant interest on associations between antifungal agents.¹ Achievement of synergy is one of the major theoretical justifications for combination therapy, since it may enable practitioners to diminish drug dosages, expand the coverage in seriously ill patients with mixed infections and delay emergence of resistant mutants. On the other hand, cost of therapy may increase, the chance of drug reactions is greater, one drug may antagonize the effect of the other, and the combination may accomplish no more than one effective drug.²

Since Candida spp. are the most common cause of fungal infections, it stands to reason that Candida albicans has been the Candida spp. most commonly evaluated in the majority of in vitro antifungal combination studies. While the combination of azoles with amphotericin B has provided controversial results,³ most of studies have found additive or synergistic effect when azoles were combined with terbinafine against C. albicans.^4-6 Other Candida spp. have also been evaluated,⁵ but little is known about the effects of antifungal combinations against Candida dubliniensis.

C. dubliniensis resembles C. albicans in many phenotypic aspects and assumes importance since it is often associated with mucocutaneous candidiasis especially in HIV-infected patients, showing a pathogenic character of this group.⁷ Although previous studies indicate that most strains of C. dubliniensis are susceptible to the same antifungal spectrum, clinical fluconazole-resistant isolates have been reported.⁸ In order to search for new alternative therapies, the aim of this study was to evaluate in vitro effects of the combination terbinafine and amphotericin B with azoles against fluconazole-sensitive (FS) and -resistant (FR) strains of C. dubliniensis.

We used two groups of C. dubliniensis strains: the first included clinical fluconazole-susceptible isolates recovered from AIDS-patients. The second group included fluconazole-resistant derivatives obtained from the first through an in vitro method of induction of fluconazole resistance described by Fekete-Forgács et al.⁹ as follows: a 10 mL culture of the FS strain was grown overnight in Sabouraud glucose broth (SDB). Cells were added to flasks containing 20 mL of SDB to achieve a final absorbance of 0.1 (λ = 640 nm). The culture was incubated at 30ºC for 10h, and then fluconazole was added at a final concentration of 8 µg/mL (higher than the MIC, Table 1). After 14h of further incubation the cells of the fluconazole-containing culture were subcultured three times consecutively into fresh SDB containing 8 µg/mL fluconazole and in each case were incubated at 30ºC with shaking for 24h. After the third incubation cells were added to flasks containing 20 mL of SDB containing fluconazole 8 µg/mL to achieve a final absorbance of 0.1. After 10h incubation the fluconazole was added at a final concentration of 16 µg/mL, and after 14h of further incubation the cells of this culture were subcultured three times into fresh SDB containing fluconazole 16 µg/mL and incubated in each case at 30ºC with shaking for 24h. The concentration of fluconazole was always duplicated under the procedure; the final concentration was 64 µg/mL. Cells from this culture were plated, and single colony was designated isolate FR.

Voriconazole (Pfizer Inc. - New York, NY, USA), itraconazol (Janssen-Cilag Pharmaceutica - Belgium) amphotericin B and fluconazole (Sigma Chemical Co. - St. Louis, MO, USA) and terbinafine (Novartis) were obtained as standard powders and prepared according to Clinical and Laboratory Standards Institute guidelines.¹⁰

Antifungal combinations against the two groups of isolates were tested in duplicate using checkerboard method. After determination of the MIC of each drug alone,10 dilutions were prepared in order to obtain four-fold the final concentrations: 50 mL- aliquots of each azole dilution were combined with other 50 mL of either amphotericin B or terbinafine dilutions. Then, 100 mL of inoculum were transferred to each well. Inocula preparation, time and temperature of incubation, and reading were similar to those described for MIC determination.¹⁰

To evaluate the interaction of agents, the fractional inhibitory concentration index (FICI) was calculated for each combination. Fractional inhibitory concentration (FIC) was calculated for each agent by dividing the inhibitory concentration of each antifungal agent when used in combination by its MIC. FIC values were then added together to define the interaction of the combination. Synergy was defined as an FICI < 0.5, additive effect when 0.5 < FICI < 1.0, indifference when 1.0 < FICI < 4.0, and antagonism when FICI > 4.0.

Broth microdilution MICs of antifungals alone were determined against the isolates before and after 15 days of increased exposure to fluconazole. Geometric means are presented in Table 1. Results demonstrate that, besides fluconazole, resistant derivatives were less susceptible to the rest of antifungals when compared to sensitive strains.

Table 2 depicts the interactions of azoles with terbinafine or amphotericin B by checkerboard method. The majority of combinations had indifferent activity against FS and FR C. dubliniensis isolates. However, when interactions were detailed for each isolate, it was possible to note that almost 67% of FS isolates evidenced antagonism for the TRB+ITZ association. Against TRB+VRZ, FS group showed similar percentages for synergy and indifference, but antagonism still remained the most frequent. On the other hand, more positive interactions were obtained when the same azoles were combined with AMB. While 46.67% of isolates presented antagonism against AMB+VRZ, only 20% evidenced the same effect when AMB and VRZ were combined.

Against FR isolates, antagonistic interactions decreased to 20% when TRB and ITZ were combined, whereas synergy and indifference interactions became more prevalent. TRB+VRZ resulted on 53.33% of indifference and synergy decreased to zero. However, when VRZ was combined with AMB, 60% of isolates showed an additive effect. Against AMB+ ITZ, 13.33% of synergy was obtained, but indifference was the most frequent (56.67%).

The increased use of antifungal agents may have contributed to the development of highly resistant microorganisms or those which are more prone to develop resistance to drugs such fluconazole.¹¹ Although most C. dubliniensis clinical isolates are susceptible to azole derivatives, resistant strains have been recovered from oral cavities of human immunodeficiency virus (HIV)-infected patients with oropharyngeal candidiasis and prior exposure to fluconazole.^8,12 Thus, combining antifungal agents has been suggested as an alternative strategy, and the use of terbinafine in combination with azoles has been suggested as a potential therapeutic option.^4-6,13

Despite promising results obtained through some antifungal associations against Candida spp., only the combination of caspofungin with terbinafine has already been tested against C. dubliniensis.¹⁴ Differential effects were detected when C. albicans and C. dubliniensis were exposed to the same antifungal combination; it is acceptable the evidence of negative effects against C. dubliniensis. The combination of caspofungin and terbinafine appears to result in positive interactive effects against C. albicans, whereas against C. dubliniensis it did not.¹⁴

In this study, we investigated in vitro interactions of amphotericin B and terbinafine with azoles against FS and FR groups of C. dubliniensis. Against the FS group, the best activity was obtained for the combination of amphotericin B with voriconazole, which showed 6.66% of synergistic effect and 60% of additive effect. Although the best synergy rates (16.66%) were obtained for both TRB+VRZ and AMB+ITZ, the latter seemed to be better since it showed half of the antagonism seen with the first. Finally, because 66.66% of isolates evidenced antagonism when terbinafine and itraconazole were combined, it was considered the worst association. Synergy rates (16.66%) were low when compared with results obtained by Perea et al.,¹⁵ Barchiesi et al.,⁴ Cantón et al.,⁵ and Weig & Muller,⁶ who found synergistic or addictive interactions when terbinafine and azoles were combined against C. albicans strains.

Against the FR group, TRB+ITZ was the association that most frequently resulted in synergy (30%). When terbinafine was combined with voriconazole, synergy decreased to zero and indifference became the most frequent (53.33%) result. AMB+ITZ resulted in indifference in 56.67% of interactions. But when amphotericin was combined with voriconazole, an additive effect was the most frequent result (60%).

Although the majority of reports mention antagonism between amphotericin B and azoles, data from in vitro studies remain controversial, and indifferent and addictive inter-actions are also described.^3,16 Here, combinations of amphotericin B and azoles mostly resulted in indifferent interactions against C. dubliniensis in both groups of isolates.

Among the combinations tested, terbinafine with itraconazole deserves more attention because this association has shown disparate activities: it was the worst combination for FS group (66.6% of antagonism) and the best combina- tion for FR group (30%). On the other hand, on previous studies, terbinafine has improved the in vitro activity of fluconazole and itraconazole against FS or FR C. albicans isolates. The combination was also very synergistic against clinical isolates of Candida glabrata and no antagonism was detected.¹⁵

In fact, susceptibility of C. dubliniensis to antifungal associations was disappointing and frightening if we consider those interactions obtained on previous studies when C. albicans strains were tested. If we consider clinical situations in which the resistance phenomenon is emergent, careful identification of such Candida species and determination of fluconazole susceptibility are necessary before practitioners choose to treat many mycotic infections with these combinations. Since checkerboard method is a preliminary in vitro test, further studies are required in order to provide clearer information about susceptibility differences between these species when exposed to azole-polyene or azole-allylamine combination.

Acknowledgements

This study received financial support from CNPq.

Conflict of interest

All authors declare to have no conflict of interest.

Received 3 June 2011

Accepted 1 August 2011

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Publication Dates

• Publication in this collection
  16 Feb 2012
• Date of issue
  Feb 2012

History

• Received
  03 June 2011
• Accepted
  01 Aug 2011