Engineering microbial cell factories for production of improved polyene antifungal agents

Fungal infections are an escalating global threat to human health. The few
antifungal agents currently available are becoming increasingly ineffective due to emerging
multidrug resistant pathogens (e.g. Aspergillus fumigatus, Candida albicans & Candida auris).
Despite the acute need for alternative treatments, very few new drug candidates have emerged in
recent years, and we remain reliant on a few established antifungals, particularly polyenes
including WHO essential medicines amphotericin B (AmB), nystatin (Nys) and pimaricin (Pim).
Polyenes are complex cyclic polyketide macrolides produced by Actinobacteria. Given polyenes
have potent broad-spectrum activity and the incidence of resistance to them is relatively low, they
remain the most effective and widely used antifungal agents. AmB, which is produced by
Streptomyces nodosus, is the frontline treatment for many common systemic fungal infections.
Recently, AmB was used to combat the outbreak of COVID-19 associated mucormycosis (black
fungus). AmB is also an essential treatment of the parasitic disease leishmaniasis which causes
high mortality in the developing world. Despite their medical importance, polyenes exhibit
severe toxicity, and there is an urgent need for safer, less toxic alternatives. SAR shows that an
exocyclic C16-carboxylate in AmB, which is also present in the majority of the other
polyenes, is a key determinant of toxicity. AmB derivatives lacking this moiety are significantly less
toxic and chemical modification of the C16-carboxylate also affords derivatives with lowered
toxicity.

In addition to C16-modification, conjugation of polyenes with sugars and other moieties that improve solubility and reduce aggregation can also reduce toxicity. However,
chemical synthesis of complex polyenes is very challenging, requiring multiple steps, deleterious
reagents and extensive use of protecting groups, which is expensive, unsustainable and difficult to
scale-up. In this project we aim to engineer microbial cell factories, that can deliver safer polyene
variants with improved activity and lower toxicity via a sustainable single-step fermentation
approach.