Hien Nguyen,

When mixed in a matrix such as in a tablet with additional excipients, enalapril is unstable and can degrade to an unwanted cyclized diketopiperazine (DKP). Mostly the higher limit is given based on the safety data. BP should have all data and same can be asked through FOI. Also monitor your drug product stability up to 6 Months accelerated Intermediate & RT condition and then can assigned limit as per Qualification Threshold i.e 0.5%.