VVC and especially RVVC, the most mucosal infections of the genital tract, proposed one of the major concerning of women heath worldwide [
26]. In this study,
C. glabrata assigned the highest percentage of species isolated from women suffering from VVC, followed by
C. albicans and
C. parapsilosis, which was in agreement some similar studies isolated
C. glabrata, as the most causative agent in infected cases [
27,
28]. In recent decades, we have seen a change in the prevalence of
Candida species in candidiasis infections. As it has been reported in previous epidemiological studies
, Candida albicans is the dominant species in infections. In the present study,
Candida glabrata has replaced
Candida albicans in terms of prevalence. On the other hand, in the studied population, other species of
Candida, including
Candida parapsilopsis, have been detected in limited numbers or in the form of mixed infection with
Candida albicans.
As expected, and according to AFST findings in this work,
Candida isolates showed different pattern of susceptibility to azoles.
C. albicans and
C. glabrata revealed a high rate of resistance to FLU followed by CTZ, whereas approximately only 10% of
C. glabrata isolates were resistant to AMB. In general,
Candida isolates showed lower MIC against ITZ and AMB compared with FLU and VOR. Many studies have highlighted the resistance to azole drugs in
Candida species recovered from VVC, particularly to FLU [
2,
3,
4,
29]. Similarly, some related reports over the world, directed different range of azole resistance among
Candida species. For instance, in a work by Bitew et al., 17.2% of
Candida krusei isolated from vaginal tract were resistant to FLU [
30]. Indeed, Arastehfar et al. showed the high rate of FLU resistant and FLU tolerant phenotypes in
C. albicans strains recovered from Iranian women suffering from VVC and RVVC [
7]. Likewise, similar studies reported the high percentage of
C. albicans (81.5%) and
C. glabrata (83.5%) recovered from Iranian pregnant women were FLU resistant [
31].
Despite being the a first-line azole drug to treatment of VVC, FLU susceptibility has significantly decreased in the last decades, due to various mechanisms of resistance [
22]. As a result, the search for efficient agents antifungal with minimum side effects and low toxicity is highly recommended [
32]. Besides, the efficacy of conventional drugs is often undesirable due to their high toxicity, low tolerability, or narrow spectrum of action [
15]. Hence, the abovementioned issues have encouraged researchers to explore novel antifungal agents, even though combination therapies to overcome therapeutic failure of VVC in women infected with resistant
Candida species which led to recurrent VVC as well as economic burden in health care system. Some of which, farnesol a molecule synthesized by
C. albicans via enzymatic dephosphorylation of farnesyl pyrophosphate [
33] - has gained considerations as a promising antifungal agent in the recent decades. Farnesol exogenously inhibits the conidiation and germination of
Aspergillus niger and
Fusarium graminearum, and also induces apoptotic-like programmed cell death in
Aspergillus flavus,
Aspergillus nidulans and
Fusarium graminearum[
12,
33,
34,
35,
36], besides it has a capability to down regulated of genes expression in
C. albicans which is related to hyphae formation and pathogenesis (
HWP1 &
SAP6) [
37]. Farnesol has also shown to inhibit the biofilm formation against resistant strains of
C. albicans [
38,
39]. Because of the noticeable inhibitory effects of farnesol on fungal cells as well as antifungal activity [
14,
40], we tested combination of farnesol with four common antifungal against 80 clinical isolates of
Candida species recovered from VVC
in vitro. Our finding showed that regarding combination of farnesol-FLU and farnesol-ITZ to resistant clinical isolates of
C. albicans and
C. parapsilopsis, which showed high MIC for those drugs (used alone). These results are expected considering the drug resistance pattern of these two
Candida species and comparison with previous studies. For example in a study in 2020, farnesol in combination with antifungal drugs significantly decreased biofilm formation of 3
C. auris strains and one standard
C. albicans [
41]. Another study, also showed the inhibitory effect of farnesol as a promising molecule on biofilm formation at the beginning stage in 6 isolates of
C. albians from dentures and
C. albicans ATCC10231 [
42]. Morever ,Decanis et al. showed farnesol was able significantly decrease Sap2 secretion, down regulated sap4-6 mRNA expression and changed yeast to hyphae morphogenesis in
C.albicans strain [
43]. In addition, remarkable synergism effects were detected for the combinations of farnesol-VOR and farnesol-AMB against
C. parapsilopsis isolates, which clearly emphasize the potential importance of farnesol as an effective antifungal agent. On the other side, indifferent interactions were observed in combination of farnesol with all antifungals with
C. glabrata isolates. According to the drug sensitivity pattern of
C. galabrata, these isolates have shown resistance to AMB in some cases. Therefore, this high suffering of drug resistance has been attributed to the no synergy. Rodrigues et al. have also tried combining two common antifungal drugs (AMB and posaconazole), and the FICI showed that the combination did not bring a clear advantage for this species [
44]. Also, in agreement with our results, other studies notably confirmed the antifungal effects of farnesol against
Candida species. For example, Cordeiro et al. indicated farnesol ranged from 4.68 to 150 µM concentration significantly reduced MICs of antifungal (FLU, ITZ, AMB, and caspofungin) against drug-resistant
Candida species [
24]. In a study conducted by Xia et al. it has been reported a synergistic effect between farnesol and FLU / 5-flurocytosine, as it reduced the capacity for biofilm in the presence of farnesol [
40]. Liposomal farnesol potentiated the action of FLU against
C. albicans and
C. tropicalis, but the association of unconjugated farnesol with fluconazole resulted in antagonistic effects [
45]. Additionally, Katragkou et al. found that synergistic or additive interaction between farnesol and FLU, AMB, and micafungin related to
C. albicans biofilms [
14]. In 2011 were presented findings, that farnesol at a nontoxic concentration synergized with azoles and this interaction led to reactive oxygen species accumulation (apoptosis) and influenced drug extrusion resulting in shift of MIC [
46]. Later, beside of many other roles, farnesol is able to modulate activity of ABC efflux transporters what can result in changes in susceptibility profile to azoles in
C. albicans (2 standard strains
C. albicans resistant and sensitive to FLU)or
C. auris isolates resistant to FLU [
47,
48]. According to the mechanism of action of farnesol and its derivatives on the fungal cells, the exogenous farnesol leads to alterations in the cell membrane by inhibiting the synthesis of ergosterol [
10] which is the possible mechanism of farnesol in combination therapy. The azole drugs inhibit the biosynthesis of ergosterol by blocking the action of cytochrome P450-depending enzyme 14-alpha-demethylase, resulting in the disruption of plasma membrane that explain the synergistic effect of farnesol and azoles in our study [
49]. So, the farnesol inhibition of the ergosterol biosynthetic pathway might decrease the levels of the intermediates. Hence, its combination with VCZ may result in an indifferent interaction. Farnesol also shows anti-neoplastic activity by down regulation of cell proliferation and enhancement of apoptosis in some human cancer cell lines such as breast cancer, lung cancer, and multiple myeloma with some known mechanism [
38,
42]. Although farnesol has apoptotic influences and chromosomal damage in cancer cell lines such as lung cancer A549 cell line, colon adenocarcinoma (Caco-2) cell line in certain concentrations, it has no apoptotic effect on healthy human lung epithelial BEAS-2B cell line [51]. In line with these conclusions, our flow cytometry findings indicate that farnesol has no apoptosis activity in SW48 cell line, indicating to be a safe agent for mammalian cells for future studies with the purpose of antifungal agent. Because of cells exposed to cytotoxic compounds may undergo necrosis (uncontrolled cell death), apoptosis (programmed cell death), autophagy, or stop actively growing and dividing to decrease cell proliferation. We used apoptosis assay (PI/ Annexin) by flow cytometry to show the effect of farnesol on SW48 as a normal cell line. In agreement with our finding, in a study by Cernakova et al .in 2018,while farnesol at 200µM effectively reduced yeast to hyphae transition in dual biofilm of C.albicans and Streptococcus .mutans ,it did not exhibit cytotoxicity effect on larve Galleria mellonella [52].
Among the limitations of this study, due to Iran's economic conditions, access to flow cytometry kits is very limited and this technique is expensive for us, so it was not possible to perform this test for farnesol/pharmacol. It is worthwhile mention that other related studies previously examined its antifungal activity against standard species of Candida with limited number that strongly highlight the importance of our current study in terms of statistical analysis to confirm farnesol efficacy. This study was conducted for the first time on Candida isolates from clinical samples, and the purpose of selecting drugs was to obtain a basic pattern for future studies and lead to practical solutions in similar populations.