Degradation of O6-benzylguanine in aqueous polyethylene glycol 400 (PEG 400) solutions: concerns with formaldehyde in PEG 400.

The degradation of O6-benzylguanine (BG) in aqueous polyethylene glycol (PEG) 400 solution at room temperature had been investigated using chromatographic and spectrometric methods. The degradation of BG in this solvent appeared to arise from a reaction between BG and formaldehyde. The formaldehyde was present as an impurity in PEG 400 and probably formed through air oxidation of PEG 400. The major product of this reaction was believed to be a methylene-bridged compound containing two BG molecules. This was probably produced via an intermediate imine, a schiff base between one BG molecule and formaldehyde. This degradation reaction was the only observable reaction in the 40% PEG/water solvent (pH 8.0) i.e. degradation of the drug via hydrolysis was minimal under these conditions.