Treatment strategies for cryptococcal infection: challenges, advances and future outlook

Abstract

Cryptococcus spp., in particular Cryptococcus neoformans and Cryptococcus gattii, have an enormous impact on human health worldwide. The global burden of cryptococcal meningitis is almost a quarter of a million cases and 181,000 deaths annually, with mortality rates of 100% if infections remain untreated. Despite these alarming statistics, treatment options for cryptococcosis remain limited, with only three major classes of drugs approved for clinical use. Exacerbating the public health burden is the fact that the only new class of antifungal drugs developed in decades, the echinocandins, displays negligible antifungal activity against Cryptococcus spp., and the efficacy of the remaining therapeutics is hampered by host toxicity and pathogen resistance. Here, we describe the current arsenal of antifungal agents and the treatment strategies employed to manage cryptococcal disease. We further elaborate on the recent advances in our understanding of the intrinsic and adaptive resistance mechanisms that are utilized by Cryptococcus spp. to evade therapeutic treatments. Finally, we review potential therapeutic strategies, including combination therapy, the targeting of virulence traits, impairing stress response pathways and modulating host immunity, to effectively treat infections caused by Cryptococcus spp. Overall, understanding of the mechanisms that regulate anti-cryptococcal drug resistance, coupled with advances in genomics technologies and high-throughput screening methodologies, will catalyse innovation and accelerate antifungal drug discovery.

Fig. 1. Mechanisms governing resistance to current antifungal agents in Cryptococcus spp.

Cryptococcus spp. achieve resistance to therapeutics by several different mechanisms. Current antifungal agents and their cellular targets are depicted. The polyenes directly target ergosterol whereas the azoles target the ergosterol biosynthetic enzyme Erg11. The pyrimidine analogues block DNA and RNA synthesis. The echinocandins target the glucan synthase Fks1, which is crucial for cell wall synthesis and integrity, although Cryptococcus spp. display inherent resistance to this class of antifungal molecules (dotted line). Within the nucleus, genetic plasticity is generated by aneuploidy, formation of hypermutator strains and transposon movement, which can result in rapid and often transient adaptation to antifungal assault. These adaptations can lead to resistance by overexpression or alteration of the drug target, inactivation of proteins required for drug target engagement or increased expression of efflux pumps. Formation of the cell capsule and thickening and alterations to the cell wall (including melanin production) can also increase tolerance to antifungal treatment. Finally, Cryptococcus can form antifungal-resistant titan cells, which are defined by a large cell size (>10 µm), high ploidy (>4C), a thick cell wall and a highly crosslinked capsule.

Fig. 2. Molecules with anti-cryptococcal activity and their mode of action.

The monoclonal antibody 18B7 binds to glucuronoxylomannan (GXM), a key capsular polysaccharide that modulates anti-cryptococcal immune responses. Glucuronoxylomannogalactan (GXMGal) is another major capsular polysaccharide in Cryptococcus spp. Benzothioureas (BTUs) affect the integrity of the cell wall by inhibiting the late post-Golgi secretory pathway. APX001 (also known as fosmanogepix) targets Gwt1, an inositol acyltransferase required for glycosylphosphatidylinositol (GPI)-anchor biosynthesis, and thereby blocks the cell wall localization of GPI-anchored mannoproteins. Clofazimine is a broad-spectrum antifungal compound that induces cell membrane stress, potentiating the activity with clinically used antifungals. The hydrazycin N′-(3-bromo-4-hydroxybenzylidene)-2-methylbenzohydrazide (BHBM) and its derivative 3-bromo-N′-(3-bromo-4-hydroxybenzylidene)benzohydrazide (B0) target the synthesis of the sphingolipid glucosylceramide (GlcCer). APX879 and Compound 112 (CMPD#112) are stress response inhibitors that target the phosphatase calcineurin and the chaperone protein Hsp90, respectively^,. The natural product ibomycin perturbs the multivesicular body (MVB) pathway, and thereby disrupts membrane function. The extent to which these compounds have been investigated varies: the mechanism of action has been predicted using an indirect assay (red), the proximal target has been verified using a direct assay (orange) or the molecule has been tested for safety and/or efficacy in clinical trials (green). The chemical structure of the highlighted molecules is depicted. ER, endoplasmic reticulum.