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A structural powder diffraction study of two polymorphic forms of nortriptyline hydrochloride.

Chiara Vladiskovic1, Norberto Masciocchi, Antonio Cervellino

  • 1Dipharma Francis s.r.l., Baranzate, Milan 20021, Italy. chiara.vladiskovic@dipharma.it

Journal of Pharmaceutical Sciences
|September 19, 2012
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Nortriptyline hydrochloride exhibits two polymorphic forms, α and β. Form α is more thermodynamically stable and crystallizes with homochiral molecules, unlike form β, preventing solid-state interconversion.

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Area of Science:

  • Solid-state chemistry
  • Crystallography
  • Pharmaceutical science

Background:

  • Nortriptyline hydrochloride, a tricyclic antidepressant, exists in multiple crystalline forms.
  • Only the β polymorphic form had been previously characterized by single-crystal analysis.

Purpose of the Study:

  • To characterize the α polymorphic form of nortriptyline hydrochloride using powder diffraction.
  • To determine the thermodynamic relationship and structural differences between the α and β forms.

Main Methods:

  • Structural powder diffraction (laboratory and synchrotron radiation)
  • Differential scanning calorimetry
  • Variable-temperature thermodiffractometry

Main Results:

  • The α form crystallizes in the monoclinic P2/c space group.
  • The α and β forms are monotropically related, with α being more stable.
  • Differences in supramolecular arrangement (homochiral vs. enantiomeric chains) explain the lack of solid-solid interconversion.

Conclusions:

  • The α form of nortriptyline hydrochloride is thermodynamically more stable than the β form.
  • Structural analysis reveals distinct supramolecular arrangements leading to different chiral packing.
  • Understanding these polymorphic differences is crucial for pharmaceutical formulation and stability.