DBS Sampling Can be Used to Stabilize Prodrugs in Drug Discovery Rodent Studies without the Addition of Esterase Inhibitors

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Abstract
Prodrugs that exhibit ex vivo instability owing to high levels of esterases in rodent blood, plasma and serum present challenges in the accurate determination of drug exposure in samples from pharmacokinetic, pharmacokinetic/pharmacodynamic, efficacy and toxicology studies in drug discovery. Ensuring the stability of analytes in sample collection, handling, analysis and storage must be established for program progression. Current protocols for the stabilization of prodrugs include the immediate quenching of whole blood with acetonitrile or methanol to stop enzyme activity, or the addition of an esterase inhibitor such as phenylmethanesulfonyl fluoride to the blood collection tubes before serum or plasma is generated. Dried blood spots (DBS) sampling may offer an alternative prodrug stabilization method for sample collection and storage from rodent studies in drug discovery. Two different prodrugs of the same parent compound that were known to exhibit ex vivo instability in rodent blood were selected for the evaluation of DBS for analyte stabilization. Each prodrug was spiked separately into fresh rat EDTA whole blood and prepared three ways: from liquid whole blood, prepared and analyzed as lysate; from whole blood spotted onto Whatman 903(®) Protein Saver untreated cards (903 cards); and from whole blood spotted onto Whatman FTA(®) Elute Micro treated cards, currently known as DMPK-B cards (FTA cards). Samples were extracted by filtration-assisted protein precipitation at 0, 2, 5 and 24 h and 4, 7, 14 and 21 days after spiking and analyzed by UHPLC-MS/MS. For these two prodrugs, stability on DBS cards was observed in rat EDTA whole blood for at least 21 days at room temperature as determined by loss of prodrug and appearance of parent. The Whatman FTA Elute cards, treated with reagents that lyse cells, did not offer more stability for the investigated compounds than the Whatman 903 Protein Saver untreated cards.

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