Hydrolytic pathway of 5-fluorouracil in aqueous solutions for clinical use

• Fluorouracil degradation in mildly alkaline solutions as for commercial solutions.
• Structural identification of the hydrolysis products.
• Oxazolidine adducts between Tris buffer and aldehydes from fluorouracil hydrolysis.
• Formation of N-fused bicyclic oxazolidine–pyrrolidine compounds in Tris buffer.

The purpose of the study was to investigate the degradation pathway of 5-fluorouracil (FU) in the situation of commercial formulations for clinical use, namely FU dissolved in sodium hydroxide (NaOH) solutions or Tris buffer at pH 8.5–9. Combination of data from 19F, 1H and 13C NMR and in some cases MS led to the identification of 8 and 13 FU degradation products in NaOH and Tris solutions respectively. In FU NaOH solutions, the first stage of FU degradation is a stereoselective hydration of the C5–C6 double bond leading to 5,6-dihydro-5-fluoro-6-hydroxyuracil, the cis stereoisomer being predominant relative to the trans. The second stage involves either a defluorination step with formation of fluoride ion and 5-hydroxyuracil or the cleavage of the N3–C4 bond giving the two diastereoisomeric 2-fluoro-3-hydroxy-3-ureidopropanoic acids. The subsequent N1–C6 bond breakdown of these compounds releases urea and 2-fluoro-3-oxopropanoic acid (FOPA) which in turn losses easily carbon dioxide leading to the formation of fluoroacetaldehyde (Facet). The degradation pathway in FU–Tris solutions is identical, except that Tris reacts with the aldehydes FOPA and Facet to form oxazolidine adducts stable at pH 8.5 but in equilibrium with the aldehyde forms at physiological pH, whereas the high reactivity of free aldehydes leads to numerous unidentified degradation compounds all in very low amounts. The FOPA diastereoisomeric adducts react with Facet to form four diastereoisomeric fused bicyclic five-membered ring compounds. Facet and FOPA are highly cardiotoxic. In Tris formulations, they are trapped as stable oxazolidine adducts which release the free aldehydes at physiological pH thus explaining the higher cardiotoxicity of FU in Tris solutions compared to that of FU in NaOH solutions.

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