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Int J Pharm. 2004 Mar 19;272(1-2):121-8.

The use of inverse gas chromatography to assess the acid-base contributions to surface energies of cefditoren pivoxil and methacrylate copolymers and possible links to instability.

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Department of Pharmaceutics, School of Pharmacy, University of London, 29-39 Brunswick Square, London WC1N 1AX, UK.


It is known that acids and bases when mixed together have the potential to react, however, there is complexity when materials are in the solid state, as the surface of the solids may not have all functional groups of the molecule expressed in equal proportions. Conceptually a drug that is acidic when in solution could have a crystal surface that is largely basic (if the acid groups are aligned away from the surface). The interaction of cefditoren pivoxil (a basic drug when dissolved) and various Eudragit polymers was studied. The drug was chemically unstable with Eudragit EPO (basic) and stable with Eudragit L100 or S100 (both acidic). Thus, the hypothesis of this work was that the surface of these materials had a different nature to the dissolved molecules, such that solid state reactions do not proceed in line with the expectation from the solution state. Inverse phase gas chromatography (IGC) was used to investigate the cefditoren pivoxil and the Eudragits. The basic to acidic parameter ratios (K(D)/K(A)) on the powder surface of the samples by IGC revealed that surface nature was in keeping with the tendency to react, such that the incompatibility could be due to the acid-base interaction between any carbonyl groups having an amphoteric nature on the surface of cefditoren pivoxil crystal and dimethylaminoethyl groups having a basic nature on the powder surface of Eudragit EPO. This study indicated that IGC would be suitable to elucidate the cause of incompatibility between two different solids.

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