Antifungal potential of the new copper(II)-theophylline/1,10-phenanthroline complex against drug-resistant Candida species

Candida spp. are the commonest fungal pathogens worldwide. Antifungal resistance is a problem that has prompted the discovery of novel anti-Candida drugs. Herein, 25 compounds, some of them containing copper(II), cobalt(II) and manganese(II) ions, were initially evaluated for inhibiting the growth of reference strains of Candida albicans and Candida tropicalis. Eight (32%) of the compounds inhibited the proliferation of these yeasts, displaying minimum inhibitory concentrations (MICs) ranging from 31.25 to 250 μg/mL and minimum fungicidal concentration (MFCs) from 62.5 to 250 μg/mL. Drug-likeness/pharmacokinetic calculated by SwissADME indicated that the 8 selected compounds were suitable for use as topical drugs. The complex CTP, Cu(theo)₂phen(H₂O).5H₂O (theo = theophylline; phen = 1,10-phenanthroline), was chosen for further testing against 10 medically relevant Candida species that were resistant to fluconazole/amphotericin B. CTP demonstrated a broad spectrum of action, inhibiting the growth of all 20 clinical fungal isolates, with MICs from 7.81 to 62.5 μg/mL and MFCs from 15.62 to 62.5 μg/mL. Conversely, CTP did not cause lysis in erythrocytes. The toxicity of CTP was evaluated in vivo using Galleria mellonella and Tenebrio molitor. CTP had no or low levels of toxicity at doses ranging from 31.25 to 250 μg/mL for 5 days. After 24 h of treatment, G. mellonella larvae exhibited high survival rates even when exposed to high doses of CTP (600 μg/mL), with the 50% cytotoxic concentration calculated as 776.2 μg/mL, generating selectivity indexes varying from 12.4 to 99.4 depending on each Candida species. These findings suggest that CTP could serve as a potential drug to treat infections caused by Candida species resistant to clinically available antifungals.