title: Effects of lyophilization on the physical characteristics and chemical stability of amorphous quinapril hydrochloride.
abstract: OBJECTIVE
  To prepare amorphous quinapril hydrochloride (QHCl) by lyophilization and to compare its physical characteristics and chemical stability as a function of the initial pH of the pre-lyophilized solution.
  
  
  METHODS
  Amorphous QHCl samples were prepared by lyophilization from aqueous solutions. Solid-state characteristics were evaluated by DSC, PXRD, and optical microscopy. Chemical degradation was monitored by an HPLC assay.
  
  
  RESULTS
  Amorphous QHCl samples obtained from lyophilization exhibited variable glass transition temperatures, depending on the pH and/or concentration of the starting aqueous solutions. Neutralized quinapril (Q) in the amorphous form, which has a Tg of 51 degrees C, lower than that of its HCl salt (91 degrees C), was significantly more reactive than QHCl at the same temperature. The Tg of lyophilized samples prepared at various initial pH values correlated well with values predicted for mixtures of QHCl and Q. Their different reaction rates were related to their glass transition temperature, consistent with the results from earlier studies obtained with amorphous samples made by precipitation from an organic solution and grinding of the crystal solvate.
  
  
  CONCLUSIONS
  Lyophilization of different QHCl solutions produces mixtures of amorphous QHCl and its neutralized form Q, with Tg values intermediate to the values of QHCl and Q. As the fraction of Q increases the overall rate of chemical degradation increases relative to QHCl alone, primarily due to the increase in molecular mobility induced by the plasticizing effects of Q.
authors.affiliation: School of Pharmacy, University of Wisconsin-Madison, 53706, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Guo
authors.forename: S R
authors.initials: SR
authors.lastname: Byrn
authors.forename: G
authors.initials: G
authors.lastname: Zografi
mesh_terms: D000465:Algorithms; D000806:Angiotensin-Converting Enzyme Inhibitors / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D004355:Drug Stability; D005612:Freeze Drying; D006371:Helium / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007546:Isoquinolines / Q000737:chemistry*; D007700:Kinetics; D000077583:Quinapril; D044005:Tetrahydroisoquinolines
chemical_list: D000806:Angiotensin-Converting Enzyme Inhibitors; D007546:Isoquinolines; D044005:Tetrahydroisoquinolines; D006371:Helium; D000077583:Quinapril