Characterization of two new potential impurities of Valsartan obtained under photodegradation stress condition

A photostability study of Valsartan (VAL) is reported. Exposure of the drug to UV–vis radiation (λ > 320 nm) yielded two previously unknown compounds, which were detected by HPLC. Preparative amounts of the new potential degradation products (DP-1 and DP-2) were obtained by submitting VAL bulk drug to extensive photodegradation. The impurities were isolated by preparative normal phase column chromatography. Analytical information from the infrared, nuclear magnetic resonance and mass spectral data of the degradation products revealed their structures as N-[2′-(1H-tetrazol-5-yl)-biphenyl-4-ylmethyl]-N-isobutylpentanamide (DP-1) and N-(diazirino[1,3-f]phenanthridin-4-ylmethyl)-N-isobutylpentanamide (DP-2). DP-1 arose from decarboxylation of VAL, while DP-2 results from further loss of nitrogen from the tetrazole motif of DP-1, with concomitant cyclization to yield a tetracyclic diazacyclopropene derivative.