Aqueous solubility of crystalline and amorphous drugs: Challenges in measurement

Abstract

Measurement of drug solubility is one of the key elements of active pharmaceutical ingredient (API) characterization during the drug discovery and development process. This report is a critical review of experimental methods reported in the literature for the measurement of aqueous solubility of amorphous, partially crystalline and crystalline organic compounds. A summary of high-throughput automated methods used in early drug discovery research is also provided in this report. This review summarizes the challenges that are encountered during solubility measurement and the complexities that are often overlooked. Even though there is an advantage in using the amorphous form of a drug due to its higher solubility, measurement of its solubility with useful accuracy is still a practical problem. Therefore, this review provides recommendations of preferred methods and precautions in using these methods to determine the aqueous solubility of amorphous and crystalline new molecular entities, with emphasis on the physico-chemical characterization of the solid state of the test substance.

Keywords::

• Solubility
• kinetic solubility
• thermodynamic solubility
• dissolution rate
• amorphous
• crystal
• partially crystalline
• high-throughput

Appendix

Recommended procedures for solubility measurement

A. Crystalline drug

Here we suggest a material sparing methodology by which one may measure the thermodynamic solubility of a crystalline drug. In general, this methodology will measure the solubility of the thermodynamically most stable form. Steps 1–10 describe a thorough process that has a considerable history of successful use in industry. However, we acknowledge that variations of this procedure may serve equally well in many practical applications.

Temperature cycling method for measurement of thermodynamic solubility:

1. Weigh out a known amount (5–20 mg) of compound.

2. Add 1–2 mL of vehicle of interest (e.g. water).

3. Vortex the sample.

4. Observe if the compound dissolves completely. If the compound is in solution, additional compound should be added before experiment is begun (a known amount, 5–15 mg excess drug is desired).

5. Once suspensions of crystals in aqueous medium have been obtained, place the samples in the temperature controlled bath. The samples are placed on a shaker (for example, LabQuake, Lab Industries, Inc. Berkeley, CA, USA) to ensure mixing via rotation through the entire cycle. Cycle temperature used as per the following method:

   • (1) 25°C for 1 min;

   • (2) 40°C for 8 h;

   • (3) 15°C for 5 h;

   • (4) 25°C for 12 h.

6. Observe samples after temperature cycling. If there is no solid material left in the tube, add more solid material and repeat temperature cycling.

7. If there is solid material left in the tube, centrifuge the sample. Collect the supernatant, measure its pH, and analyze by HPLC for drug concentration.

8. Collect solid material. Analyze the wet solid material by X-ray powder diffraction (XRPD) and examine the diffraction spectrum for any changes relative to the control sample.

9. Dry solid material under vacuum overnight at 40°C (depending on the potential for chemical instability; the solid material can also be dried under vacuum at room temperature).

10. Analyze the dry material by DSC (against control samples) for change in melting point.

11. Run the following solubility procedure at 25°C if significant degradation is observed after temperature cycling:

    • (a) Add 5–20 mg of solid powder in a suitable size glass vial, and add 1–2 mL of aqueous vehicle (e.g. water).

    • (b) Briefly vortex mix for 30 s to ensure adequate dispersion of solid.

    • (c) Rotate vials containing excess drug in aqueous media on a vial shaker for a period of 48 h in the temperature controlled bath at 25°C.

    • (d) Take two time points: 24 and 48 h to ensure that the equilibration has been reached.

    • (e) After 24 h of equilibration, record pH of slurries and either centrifuge or filter aliquots of slurries and assay for drug content by HPLC.

    • (f) Repeat above 24 h later. If the values at 24 h and 48 h are within ± 10% then assume equilibrium has established.

B. Amorphous drug

Based on work in our laboratory we recommend the following procedure for measuring solubility of amorphous solids. If crystallization occurs during the measurement of experimental solubility, the solubility obtained is ‘apparent solubility’ when crystallization occurs rapidly and ‘apparent equilibrium solubility’ when crystallization occurs beyond the clinically relevant time-scale. In the case of an ‘apparent solubility’, the value will be less, often much less than the equilibrium solubility of the amorphous phase.

1. Screen the amorphous drug powder through a 100-mesh screen (150 µ size opening) and select that retained on a 200-mesh screen (75 µ size opening) in order to avoid fines that dissolve faster and larger particles that might cause difficulties in dispersing the drug in water, thereby improving reproducibility of the concentration:time profile measurements.

2. Add a known amount of excess amorphous drug (at least 10× higher amount than corresponding crystalline drug solubility) powder to a dissolution vessel in a 250 mL dissolution medium in a Type II USP apparatus at 25°C.

3. In order to improve the powder dispersion, use paddle speed of at least 300 rpm.

4. Draw samples at different time points and filter using an appropriate filter.

5. Dilute samples immediately in organic solvent to prevent precipitation and/or crystallization of drug.

6. Assay by HPLC for drug concentration.

7. Pull an additional sample of unfiltered solid at each time point and analyze by polarized light microscope (PLM) for appearance of crystallinity.

8. Generate the concentration-time dissolution profile, and note the time at which crystallinity is first detected.

Notes

¹ Using a heat of solution value of 10 kcal/mol, which is typical for most organic compounds, a temperature control of ± 1°C is required to measure solubility value with ± 10% accuracy, and by proportion, 1% accuracy in solubility would require ± 0.1°C temperature control.

² Differences in thermal history do produce differences in heat capacity and enthalpy, and therefore also in free energy and solubility, but relative to the difference between the given amorphous state and the crystalline state, the impact of thermal history variation on free energy of the amorphous state is expected to be quite small.

* Note: Increasing the temperature to 40°C will increase the drug concentration in aqueous medium providing higher degree of supersaturation relative to 25°C to facilitate the nucleation at 25°C or lower temperature. Lowering the temperature to 15°C from 40°C will lower the drug solubility, further increase the degree of supersaturation and promote nucleation of the thermodynamically stable form. Raising the temperature of the sample back to 25°C allows equilibration of thermodynamically stable form to the temperature at which solubility measurement is intended.