Quantification of Phosphonate Drugs by ¹H-³¹P HSQC Shows That Rats Are Better Models of Primate Drug Exposure than Mice

The phosphonate group is a key pharmacophore in many antiviral, antimicrobial, and antineoplastic drugs. Due to its high polarity and short retention time, detecting and quantifying such phosphonate-containing drugs with LC/MS-based methods are challenging and require derivatization with hazardous reagents. Given the emerging importance of phosphonate-containing drugs, developing a practical, accessible, and safe method for their quantitation in pharmacokinetics (PK) studies is desirable. NMR-based methods are often employed in drug discovery but are seldom used for compound quantitation in PK studies. Here, we show that proton-phosphorous (¹H-³¹P) heteronuclear single quantum correlation (HSQC) NMR allows for the quantitation of the phosphonate-containing enolase inhibitor HEX in plasma and tissues at micromolar concentrations. Although mice were shown to rapidly clear HEX from circulation (over 95% in <1 h), the plasma half-life of HEX was more than 1 h in rats and nonhuman primates. This slower clearance rate affords a significantly higher exposure of HEX in rat models compared to that in mouse models while maintaining a favorable safety profile. Similar results were observed for the phosphonate-containing antibiotic, fosfomycin. Our study demonstrates the applicability of the ¹H-³¹P HSQC method to quantify phosphonate-containing drugs in complex biological samples and illustrates an important limitation of mice as preclinical model species for phosphonate-containing drugs.