Patent ID: 7375181

Claim:
A method of purifying a compound of formula I: wherein: R 19 is hydrogen; R 20 is —CH 2 CH 2 —NH—(CH 2 ) 9 CH 3 ; R 3 is —OH; R 5 is —CH 2 —NH—CH 2 —P(O)(OH) 2 ; or a pharmaceutically acceptable salt or stereoisomer, thereof; the method comprising the steps of: (a) contacting a polystyrene divinyl benzene resin with a first acidified aqueous solution comprising the compound; (b) eluting the contacted resin with a second acidified aqueous solution comprising a polar organic solvent selected from methanol, ethanol, isopropyl alcohol, acetonitrile, acetone, n-propyl alcohol, n-butyl alcohol, isobutyl alcohol, methyl ethyl ketone, and tetrahydrofuran to form an eluate containing the compound; and (c) isolating the compound from the eluate.