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A Novel Approach for Analyzing the Dissolution Mechanism of Solid Dispersions

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ABSTRACT

Purpose

To analyze the dissolution mechanism of solid dispersions of poorly water-soluble active pharmaceutical ingredients (APIs), to predict the dissolution profiles of the APIs and to find appropriate ways to improve their dissolution rate.

Methods

The dissolution profiles of indomethacin and naproxen from solid dispersions in PVP K25 were measured in vitro using a rotating-disk system (USP II). A chemical-potential-gradient model combined with the thermodynamic model PC-SAFT was developed to investigate the dissolution mechanism of indomethacin and naproxen from their solid dispersions at different conditions and to predict the dissolution profiles of these APIs.

Results

The results show that the dissolution of the investigated solid dispersions is controlled by dissolution of both, API and PVP K25 as they codissolve according to the initial API loading. Moreover, the dissolution of indomethacin and naproxen was improved by decreasing the API loading in polymer (leading to amorphous solid dispersions) and increasing stirring speed, temperature and pH of the dissolution medium. The dissolution of indomethacin and naproxen from their amorphous solid dispersions is mainly controlled by the surface reaction, which implies that indomethacin and naproxen dissolution can be effectively improved by formulation design and by improving their solvation performance.

Conclusions

The chemical-potential-gradient model combined with PC-SAFT can be used to analyze the dissolution mechanism of solid dispersions and to describe and predict the dissolution profiles of API as function of stirring speed, temperature and pH value of the medium. This work helps to find appropriate ways to improve the dissolution rate of poorly-soluble APIs.

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Abbreviations

a :

Activity (−)

A :

Surface area (m2)

A f :

Fraction of the area of the interface for the transport of molecules from the solution (−)

c :

Concentration mol/m3

Δc SL p,0API :

Difference in solid and liquid heat capacities of the API (J/(mol K))

Δh SL0API :

Heat of fusion of pure API (kJ/mol)

J :

Dissolution rate (mol/(m2 s))

k B :

Boltzmann’s constant 1.38065*10−23 (J/K)

k d :

Diffusion rate constant (mol/(m2 s))

k e :

Equilibrium-exchange rate between the solid and liquid phase (mol/(m2 s))

k ij :

Binary interaction parameter (−)

k S :

Surface reaction rate constant (mol/(m2 s))

k t :

Total rate constant of dissolution (mol/(m2 s))

m seg :

Segment number (−)

n exp :

Number of experimental data points (−)

T SL0API :

Melting temperature of API (K)

V :

Volume (m3)

w :

API loading (−)

w :

Axial fluid speed (m/s)

x :

Concentration or solubility in mole fraction (−)

a :

Helmholtz energy (J)

A− :

Ionized form of monoprotic weak-acidic API

API:

Active pharmaceutical ingredient

ARD:

Average relative deviation

calc:

Calculated results

exp:

Experimental data

H3O+ :

Hydronium ion

HA:

Monoprotic weak-acidic API

IND:

Indomethacin

Ka :

Acid dissociation equilibrium constant

LLE:

Liquid-liquid equilibrium

M :

Average molar mass (g/mol)

N:

Total number of particles (−)

NAP:

Naproxen

N assoc :

Number of association sites (−)

PVP:

Polyvinylpyrrolidone

R:

Universal ideal gas constant (J/K/mol)

SLE:

Solid–liquid equilibrium

t:

Time (s)

T:

Temperature (Kelvin) or (°C)

Z:

Compressibility factor (−)

α :

Proportionality constant (−)

β API :

Degree of API crystallinity (−)

γ :

Activity coefficient (−)

θ :

Ratio of the dissolution rate of API to that of polymer

\( {\varepsilon}_{hb}^{A_i{B}_i}/{k}_B \) :

Association-energy parameter (K)

φ :

Fugacity coefficient (−)

∅:

Constant fraction of molecules that strike the solid surface (−)

ω :

Stirring speed (round/s)

ν :

Kinematic viscosity of the solution (m2/s)

δ:

Thickness of diffusion layer (m)

u/k B :

Dispersion-energy parameter (K)

\( {\kappa}_{hb}^{A_i{B}_i} \) :

Association-volume parameter (−)

µ :

Chemical potential (J/mol)

ν c :

Enhancement of equilibrium-exchange rate resulting from convection (mol/(m2 s))

ν r :

Collision frequency (mol/(m2 s))

ρ :

Density (mol/Å3) or (mol/L)

σ :

Segment diameter (Å)

API:

Active pharmaceutical ingredient

i j :

Component indexes

polymer:

Polymer

0:

Pure substance

A i B i :

Association sites A and B of molecule i

assoc:

Association

B:

Bulk phase

dipole:

Dipole-dipole interactions

disp:

Dispersion

elec:

Ionic interactions

hc:

Hard chain

I:

Solid–liquid interface

L:

Liquid phase

L1:

Amorphous API-rich phase

L2:

Water-rich phase

res:

Residual

S:

Solid phase

SD:

Solid dispersion

seg:

Segment

SL:

Solid–liquid

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ACKNOWLEDGMENTS AND DISCLOSURES

The authors would like to acknowledge the financial support from the Alexander von Humboldt Foundation (Yuanhui Ji) as well as that from the CLIB Graduate Cluster Industrial Biotechnology (Anke Prudic). They would like to thank Monika Meuris for assisting with the PXRD and SEM experiments. They would also like to thank the reviewers for their helpful advice.

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Authors and Affiliations

  1. TU Dortmund, Department of Biochemical and Chemical Engineering, Laboratory of Thermodynamics, Emil-Figge-Str. 70, 44227, Dortmund, Germany

    Yuanhui Ji, Raphael Paus, Anke Prudic, Christian Lübbert & Gabriele Sadowski

Authors
  1. Yuanhui Ji
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  2. Raphael Paus
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  3. Anke Prudic
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  4. Christian Lübbert
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  5. Gabriele Sadowski
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Corresponding author

Correspondence to Yuanhui Ji.

Electronic Supplementary Material

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Fig. S1

The absorbance-wavelength curves of the aqueous solutions of indomethacin at 30 mg/L and PVP K25 at 125 mg/L. (GIF 13 kb)

High resolution image (TIFF 17440 kb)

Fig. S2

The absorbance-wavelength curves of the aqueous solutions of naproxen at 10 mg/L and PVP K25 at 125 mg/L. (GIF 12 kb)

High resolution image (TIFF 17440 kb)

Fig. S3

The total rate constant kt for the dissolution of indomethacin dependent on the stirring speed ω (in rpm). Gray symbols represent the model results by the chemical-potential-gradient model. Full line represents a linear fitting between kt and stirring speed ω with a function of kt/ (mol/(m2 s)) = 1.441 ∙ 10-9∙ ω/(rpm) +3.66968 ∙ 10-7. (GIF 11 kb)

High resolution image (TIFF 17440 kb)

Fig. S4

The total rate constant kt for the dissolution of indomethacin dependent on the temperature (in K). Gray symbols represent the model results by the chemical-potential-gradient model. Full line represents a linear fitting between kt and temperature with a function of kt/ (mol/(m2 s)) = 6.178 ∙ 10-9 ∙ T/(K) -1.477135 ∙ 10-6. (GIF 11 kb)

High resolution image (TIFF 17440 kb)

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Ji, Y., Paus, R., Prudic, A. et al. A Novel Approach for Analyzing the Dissolution Mechanism of Solid Dispersions. Pharm Res 32, 2559–2578 (2015). https://doi.org/10.1007/s11095-015-1644-z

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  • DOI: https://doi.org/10.1007/s11095-015-1644-z

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