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Hypotension is the dose-limiting side effect of the radio-protective drug Amifostine and results from relaxation of the vascular smooth muscle, which is directly mediated by the active metabolite, WR-1065, of Amifostine. The route of administration (currently FDA-approved only for intravenous administration) and the rapid metabolic conversion of Amifostine combine to yield high systemic levels of WR-1065 and facilitate the onset of hypotension. Research efforts aiming to optimize the delivery of WR-1065 to maintain efficacy while reducing its peak, systemic concentration below levels that induce hypotension are underway. To fully characterize the effect of reduced dose levels and alternative routes of administration of Amifostine on systemic WR-1065 concentrations, improved analytical techniques are needed. We have developed and evaluated a highly sensitive method for measuring WR-1065 in rat plasma that employs chemical derivatization, protein precipitation and UPLC-MS/MS analysis. The method exhibits a limit of quantification (LOQ) of 7.4 nM in plasma, which is a significant improvement over conventional approaches that utilize LC-electrochemical detection (ECD) (LOQ 150 nM or higher). The method was assessed in a pharmacokinetics study in rats administered Amifostine intravenously and via direct jejunal injection (10 mg/kg each route). The bioavailability of WR-1065 was 61.5% after direct jejunal injection indicating rapid conversion and absorption of the metabolite in the intestinal tract. This demonstrates that an oral formulation of Amifostine designed for site-specific release of the drug in the upper GI tract can deliver systemic absorption/conversion to WR-1065, provided that the formulation protects the therapeutic from gastric decomposition in the stomach., (Copyright © 2018. Published by Elsevier B.V.)