Dual dehydrosqualene/squalene synthase inhibitors: leads for innate immune system-based therapeutics.

With the rapid rise in bacterial drug resistance, there is great interest in developing innovative approaches to anti-infective therapy. For example, in the United States, more people die from Staphylococcus aureus infections than die from human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS). Alternative approaches are thus of interest, and as discussed in a recent National Research Council (US) report, these include targeting virulence factors, as well as boosting innate immunity. In recent work, we showed that blocking formation of the carotenoid virulence factor staphyloxanthin with BPH-652 (1, Figure 1) rendered staph bacteria noninfective and susceptible to immune system clearance mediated by reactive oxygen species, and in other work, we showed that inhibiting squalene biosynthesis in neutrophils with 2 generated antibacterial, neutrophil extracellular traps (NETs). The targets, dehydrosqualene synthase (CrtM, for 1) and squalene synthase (SQS, for 2), are both involved in the first committed steps in carotenoid and sterol biosynthesis, the conversion of two farnesyl diphosphate molecules to form presqualene diphosphate (Figure 2a). These findings led us to contemplate that it might be possible to develop dual activity CrtM/SQS inhibitors that simultaneously block formation of the S. aureus virulence factor staphyloxanthin—removing the golden protective shell of the bacterium (Figure 2b)—while at the same time stimulating host antimicrobial NET formation (Figure 2c). This would represent a novel, dual-targeting approach to antibacterial therapy. Here, we report the discovery of such lead compounds. First, we carried out an in silico high-throughput screen (HTS) for new CrtM inhibitor leads, since current inhibitors such as compound 1 are poor NET inducers (Table 1). Likewise, the use of statins to block pigment formation is not feasible since the Ki value for inhibition of S. aureus HMG-CoA reductase by statins is approximately 10 larger than for inhibition of