• Design and Discovery of Novel Small Molecule Inhibitors targeting Heme Oxygenase (HemO) Dependent Iron Acquisition and Heme Signaling in Pseudomonas aeruginosa.

      Robinson, Elizabeth; Wilks, Angela; Xue, Fengtian, Ph.D.; 0000-0002-7770-4060 (2021)
      The recent rise in antibiotic resistance particularly those pertaining to hospital acquired infections has highlighted the need for alternative therapeutic approaches. Alternative approaches include anti-virulence strategies that differ from current treatments targeting essential pathways in pathogens and instead target systems and factors required for virulence and infection. Gram-negative opportunistic multi-drug resistant (MDR) pathogens like Pseudomonas aeruginosa are ranked at a serious threat level by the CDC and are infamous for causing life threatening infections in immunocompromised populations such as patients with ventilator-assisted pneumonia, open surgical wounds, and cystic fibrosis. Pathogenic bacteria including P. aeruginosa, require the essential micronutrient iron for their survival and virulence. It has been reported that during acute and chronic infections P. aeruginosa preferentially utilizes heme as its iron source over siderophore mechanisms. P. aeruginosa encodes two non-redundant heme uptake systems that both utilize the iron regulated heme oxygenase enzyme (HemO) to release iron and the biliverdin (BVIX) metabolites BVIXβ and -δ. It has been shown that HemO catalytic activity is required to drive heme uptake into the cell. Furthermore, the products of extracellular heme metabolism BVIXβ and -δ function as signaling and regulatory molecules in several virulence traits. Therefore, HemO represents an ideal therapeutic target due to its dual function in limiting both iron and the heme metabolites that regulate several virulence traits. We hypothesize such a dual function strategy will also increase the barrier to resistance. The work herein applies a structure-based design and high-throughput screening approach followed by in vitro and in cell characterization of lead compounds. Further computer-aided drug design (CADD) and guided chemical synthesis optimization were employed to design and discover novel small molecule scaffolds and inhibitors of HemO. The approaches resulted in lead compounds with nanomolar binding affinity and inhibition of HemO enzyme activity both in vitro and in vivo. Additionally, I developed a new method for the production and purification of BVIXβ and -δ, in > 500-fold increased yields, to further study their role in P. aeruginosa virulence and infection.
    • Design, Development, and Characterization of Gallium (III) Salophen Metallotherapeutics Targeting Heme Sensing and Iron Acquisition Pathways in Pseudomonas aeruginosa

      Centola, Garrick; Wilks, Angela; Xue, Fengtian, Ph.D.; 0000-0001-5965-9545 (2022)
      The development of new antibiotics is outpaced by the rise in multi-drug resistant (MDR) bacteria, creating a global health problem. Pseudomonas aeruginosa, one such bacterium, is labeled as a “critical priority” pathogen by the WHO for its resistance to treatment and prevalence in hospital-acquired infections and immunocompromised patients where it is often life threatening. Adding to this problem, most new discoveries are derivatives of existing antibiotic classes rather than new strategies. Newer approaches targeting bacterial pathways critical to infection but not survival outside the host are expected to exert less selective pressure and slow resistance onset. One such strategy is interfering with bacterial iron uptake and utilization, as iron is a key micronutrient with several iron-regulated virulence traits used to counter iron-sequestering defense mechanisms of the host. P. aeruginosa can shift between the acquisition of labile iron stores and the more abundant heme-bound iron at various stages of infection, so inhibitors targeting these pathways must account for this adaptability. One such approach to targeting iron utilization in several forms is the use of gallium, which mimics ferric iron in ionic size and charge but cannot undergo critical redox processes, thus causing toxicity in the bacteria that acquire it under the guise of iron. This work describes the synthesis and characterization of Gallium Salophen (GaSal) and subsequent analogs targeting heme and iron acquisition pathways in P. aeruginosa. In this characterization, GaSal binds to a hemophore, HasAp, secreted by P. aeruginosa, and inhibits an extra-cytoplasmic function (ECF) signaling cascade with the outer-membrane receptor HasR, which is critical for sensing and adapting to host heme levels. GaSal is simultaneously a substrate for uptake, independent of its effect on HasAp. Using a combination of cell-based assays as well as in vitro target characterization and finally preliminary animal infection studies, GaSal and subsequent derivatives are shown to be promising new developments targeting several points in the iron uptake and utilization pathways of P. aeruginosa. Continued developments aim to retain such activity and include several routes towards further optimization and development as a therapeutic.