Drug-Polymer Miscibility and the Overlap Concentration (C*) as Measured by Rheology: Variation of Polymer Structure

Anasuya Sahoo, Ronald A. Siegel

Research output: Contribution to journalArticlepeer-review

3 Scopus citations


Objectives: Amorphous solid dispersions (ASDs), wherein a drug is molecularly dispersed in a polymer, can improve physical stability and oral bioavailability of poorly soluble drugs. Risk of drug crystallization is usually averted using high polymer concentrations. However, we demonstrated recently that the overlap concentration, C*, of polymer in drug melt is the minimum polymer concentration required to maintain drug in the amorphous state following rapid quench. This conclusion was confirmed for several drugs mixed with poly(vinylpyrrolidone) (PVP). Here we assess the solid-state stability of ASDs formulated with a variety of polymers and drugs and at various polymer concentrations (C) and molecular weights (MWs). We further test the hypothesis that degree of drug crystallization decreases with increasing C/C* and vanishes when C>C*, where C* depends on polymer MW and strength of drug-polymer interaction. Methods: We test our hypothesis with ASDs consisting of ketoconazole admixed with polyacrylic acid, polymethacrylic acid and poly (methacrylic acid-co-ethyl acrylate); and felodipine admixed with PVP and poly (vinylpyrrolidone-co-vinyl acetate). Values of C* for polymers in molten drug are rheologically determined. Crystallization behavior is assessed by measuring enthalpy of fusion, ΔHf and by X-ray diffraction. Results: We confirm that ΔHf/ΔHf, C = 0 = f(C/C∗), and essentially no crystallization occurs when C>C*. Conclusions: Our findings will aid researchers in designing or selecting appropriate polymers to inhibit crystallization of poorly soluble drugs. This research also suggests that C* as determined by rheology can be used to compare drug-polymer interactions for similar molecular weight polymers. Graphical abstract: [Figure not available: see fulltext.]

Original languageEnglish (US)
Pages (from-to)2229-2237
Number of pages9
JournalPharmaceutical research
Issue number9
StatePublished - Sep 2023

Bibliographical note

Publisher Copyright:
© 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.


  • NMR
  • amorphous solid dispersions
  • overlap concentration
  • rheology
  • x-ray

PubMed: MeSH publication types

  • Journal Article


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