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Cilengitide--exceptional pseudopolymorphism of a cyclic pentapeptide.

C Saal1, M Lange1, C Kuehn1

  • 1Merck KGaA, Frankfurter Strasse 250, 64293 Darmstadt, Germany.

European Journal of Pharmaceutical Sciences : Official Journal of the European Federation for Pharmaceutical Sciences
|February 15, 2015
PubMed
Summary

Cilengitide anhydrate (A1) is surprisingly more stable than its tetrahydrate form in water. Higher solubility pseudo-polymorphs (S1, S2) can be obtained from methanol/ethanol mixtures, avoiding low-solubility A1.

Keywords:
Crystal structureOligopeptidesPolymorphismSelection of solid-state formsSolubilitySolvates

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Area of Science:

  • Pharmaceutical Sciences
  • Solid-State Chemistry
  • Crystallography

Background:

  • Cilengitide (Cil), a cyclic pentapeptide (cyclo-(Arg-Gly-Asp-D-Phe-N-MeVal)), exists as an anhydrate (A1) and a tetrahydrate (Cil1(H2O)4).
  • Understanding the thermodynamic stability and solubility of different crystalline forms is crucial for drug formulation and development.

Purpose of the Study:

  • To investigate the thermodynamic stability and solubility of Cilengitide anhydrate (A1) and tetrahydrate (Cil1(H2O)4) forms.
  • To explore the formation and properties of non-stoichiometric water-alcohol solvates (S1, S2) of Cilengitide.
  • To identify optimal crystallization strategies for obtaining more soluble Cilengitide forms.

Main Methods:

  • Competitive slurry experiments to assess thermodynamic stability.
  • Thermodynamic solubility measurements.
  • Crystallization studies using aqueous and mixed solvent systems (methanol, ethanol).

Main Results:

  • The anhydrate form (A1) demonstrated greater thermodynamic stability in aqueous environments compared to the tetrahydrate form (Cil1(H2O)4).
  • Lower solubility of A1 is attributed to specific hydrogen bonding motifs in its crystal structure.
  • Non-stoichiometric solvates (S1, S2) incorporating methanol or ethanol exhibit higher solubility than A1 but rapidly convert to A1 in aqueous media.
  • S1 and S2 can be obtained via crystallization from methanol/ethanol mixtures, circumventing A1 formation.

Conclusions:

  • Cilengitide anhydrate (A1) is the most thermodynamically stable form in aqueous environments.
  • While non-stoichiometric solvates (S1, S2) offer enhanced solubility, their instability in water limits their direct use.
  • Crystallization from solvent mixtures containing methanol and ethanol is a viable strategy to produce highly soluble pseudo-polymorphs of Cilengitide.