It has been shown that clinically significant azole resistance in C. albicans is accompanied by increased transcription of the CDR1 and CDR2 genes, encoding ATP-binding cassette transporters Cdr1p and Cdr2p (White, 1997; Coste et al., 2007). We have also demonstrated that Cdr1p contributes more than Cdr2p to the azole resistance phenotype (Holmes et al., 2008). Therefore, inhibitors of Cdr1p have the potential to reverse azole resistance in C. albicans. For example, the immunosuppressant FK506, which is used in cancer chemotherapy, is a Cdr1p substrate that can reverse

fluconazole (FLC) resistance in fungi. It is reported to act on Cdr1p-mediated efflux directly because overexpression of Cdr1p significantly reduces susceptibility to FK506 (Schuetzer-Muehlbauer et al., 2003; Niimi et al., 2004). It can also act indirectly because of the effects on the calcineurin pathway (Cannon et al., click here 2007; Steinbach et al., 2007; Sun et al., 2008; Uppuluri et al., 2008). Unfortunately, the side effects of calcineurin inhibitors can be severe and include predisposition FK506 chemical structure to microbial infection, cardiac damage, hypertension,

blurred vision, and liver and kidney problems (Naesens et al., 2009). As an immunosuppressant, FK506 could also increase the severity of existing fungal, or other infectious, diseases. We have recently developed a d-octapeptide derivative, denoted RC21v3, which is a specific inhibitor of Cdr1p. We have demonstrated that RC21v3 inhibited the efflux activity of Cdr1p and chemosensitized azole-resistant clinical C. albicans cells to FLC in in vitro susceptibility assays (Holmes et al., 2008).

Its ability to chemosensitize C. albicans to azoles in vivo has not been tested. Oral candidiasis is prevalent in the very young, the elderly, terminal cancer patients and in other immunosuppressed individuals. If RC21v3 acts synergistically with azoles delivered orally, it may enable a combination antifungal chemotherapy that could improve the quality of life for oral candidiasis patients. We have developed a reproducible experimental oral candidiasis model using immunosuppressed mice, which has Thymidylate synthase localized lesions characteristic of oral thrush in humans (Takakura et al., 2003). For our study of the effects of the azole resistance reversal agent RC21v3, we selected a pair of isogenic strains, a sensitive parent and its FLC-resistant derivative, in which resistance was associated with overexpression of the Cdr proteins, without contributions from either Mdr1p (which contributes only rarely to clinical resistance) or resistance-conferring mutations in the drug target Erg11p. Using this model, we demonstrate here the efficacy of RC21v3 in combination with azoles against experimental murine oral candidiasis caused by an azole-resistant C. albicans isolate. Candida albicans MML611 (originally named TL1) and MML610 (originally named TL3) (Marr et al.