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Physicochemical characterization and membrane binding properties of camptothecin.

Authors
  • Selvi, Bilge
  • Patel, Sanjay
  • Savva, Michalakis
Type
Published Article
Journal
Journal of Pharmaceutical Sciences
Publisher
Elsevier
Publication Date
Oct 01, 2008
Volume
97
Issue
10
Pages
4379–4390
Identifiers
DOI: 10.1002/jps.21314
PMID: 18300302
Source
Medline
License
Unknown

Abstract

The intrinsic solubility of the CPT-lactone free base in acidic pH was determined to be 3.44 and 5.11 microM at 22 and 37 degrees C, respectively. The equilibrium solubility of the drug was found to increase with increasing temperature and decreasing pH. The enhanced solubility of the drug at very low pH is attributed to the protonation of the nitrogen atom in the ring B and the increased solvency of the highly concentrated acidic media. The logarithmic value of the intrinsic partition coefficient P of the free base CPT-lactone form was estimated to be 1.65, characteristic of a molecule suitable for absorption. Association constants K(f) of the drug for bilayers composed of the zwitterionic (DOPC) and the negatively charged (DOPG) were determined at acidic pH by fluorescence anisotropy to be 35.4 +/- 4.5 M(-1) and 93.1 +/- 11.0 M(-1), respectively, indicative of the CPT tendency to preferentially bind to negatively charged membranes. The results indicate that at highly acidic media, CPT is positively charged and exists at its stable lactone form of increased solubility and capacity to bind to negatively charged membranes. These features could be used to invent novel formulation strategies to optimize the antitumor activity of CPT.

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