Preclinical Assessment of Tungsten Pentacarbonyl-Organophosphine Complexes as Novel Antibiotic Candidates

Antibiotic resistance must be addressed by developing effective antibiotics. The tungsten-organophosphine phosphopyricin (PPR) was designed as a modifiable antibiotic architecture to target the ESKAPE pathogens. PPR and its analogs, PPR-1 and PPR-2, display strong in vitro antibiotic activity against ESKAPE pathogens, but drug-like characteristics are unknown. This work seeks to establish physicochemical and pharmacokinetic information for this compound class and determine whether PPR, PPR-1, and/or PPR-2 should be prioritized as lead compound(s). In silico predictions characterized PPR as moderately lipophilic (median logP 2.18), moderately to highly permeable across the skin and intestinal epithelium, a CYP1A2 and CYP2C9 inhibitor, and poorly eliminated from the body. A validated 4 °C rapid equilibrium dialysis assay to determine the unbound (fu) and bound fractions (fb) of PPR, PPR-1, and PPR-2 in human plasma failed due to poor compound stability in buffer samples; however, compound concentrations in plasma could approximate relative extents of plasma protein binding (PPR ≈ PPR-2 > PPR-1). The cytotoxicity of PPR, PPR-1, and PPR-2 in Caco-2 and HepG2 cells were assessed via SYTOX Deep Red and MTT assays to investigate changes to membrane permeability and mitochondrial activity, respectively. The effect of PPR on membrane permeability was indeterminate due to poor assay performance and PPR precipitation. MTT assay results suggest that PPR-2 is a more potent mitochondrial inhibitor than PPR or PPR-1. An fu-independent human liver microsomal metabolic stability assay determined the in vitro intrinsic clearance of PPR (CLint,u) as 909.09 ± 113.7 μL/min/mg. This value is likely overestimated due the heat instability and/or non-specific binding of PPR. PPR depletion from the Transwell system prevented determination of its apparent permeability across Caco-2 monolayers; it is unclear whether PPR loss was caused by heat instability or its accumulation and/or metabolism by Caco-2 cells. This work concludes that PPR, PPR-1, and PPR-2 should not be pursued as lead compounds. Compound incompatibility with standard drug development assays raises concerns regarding development cost and time and the appropriateness of pharmacokinetic determinations. The tungsten-organophosphine class should be further investigated, as these may be developed into potent, synthetic antibiotics to counter resistant ESKAPE pathogens.