Identification, synthesis and structural confirmation of process-related impurities in proparacaine hydrochloride

Proparacaine hydrochloride is an ester-type local anesthetic agent that is extensively used in ophthalmic operations. The process-related impurities of proparacaine hydrochloride were investigated, and seven impurities were detected in the reaction solution of the last step at the level of 0.03-1.08 % by a newly developed HPLC method. Based on the synthetic process and the results of LC-HRMS, the structures of five impurities were proposed as 3-amino-4-propoxybenzoic acid (Imp-A), ethyl 3-amino-4-propoxybenzoate (Imp-B), 2-(ethylamino)ethyl 3-amino-4-propoxybenzoate hydrochloride (Imp-C), 2-(diethylamino)ethyl 3-nitro-4-propoxybenzoate hydrochloride (Imp-F), and 3-nitro-4-propoxybenzoic acid (Imp-G). And the structures were confirmed by synthesis, followed by varieties of spectral and chromatographic analyses. The structures of two impurities with almost same molecular weight in LC-HRMS were elucidated as 2-(diethylamino)ethyl 3-(ethylamino)-4-propoxy-benzoate hydrochloride (Imp-D) and 2-(diethylamino)ethyl 3-formamido-4-propoxybenzoate hydrochloride (Imp-E) by NMR and IR. An HPLC-based method was developed, and validation study demonstrated that the approach was precise, accurate, and sensitive. The impurities information of proparacaine hydrochloride can be used for the quality control of intermediate, raw material drug and its commercial products.