1. Chapter 7. Solubility 2013. 10. 25. Chun Jong-Soo

2. Overview  Solubility is the maximum dissolved concentration under given solution conditions.  Solubility is a determinant of intestinal absorption and oral bioavailability.  Solubility is increased by adding ionizable groups or reducing Log P and MW.  Salt forms increase dissolution rate.

3. Many negative effects can occur for low-solubility compounds  Poor absorption and bioavailability after oral dosing  Insufficient solubility for IV dosing  Artificially low activity values from bioassays  Erratic assay results (biological and property methods)  Development challenges (expensive formulations and increased development time)  Burden shifted to patient (frequent high-dose administrations)

4. low-solubility compounds too high of a reliance on advanced formulation and delivery technologies lead a discovery project team toward a clinical candidate wise to solve solubility insufficiencies with structural modifications

5. Solubility is determined by many factors  Compound structure  Physical state of compound that is introduced into solution Solid: Amorphous, crystalline, polymorphic form Liquid: Predissolved in solvent (e.g., DMSO)  Composition and physical conditions of solvent(s) Types of solvents Amount (%) of co-solvents Solution components (e.g., salts, ions, proteins, lipids, surfactants) pH Temperature  Methods of measurement Equilibration time Separation techniques (e.g., filter, centrifuge) Detection (e.g., ultraviolet, mass spectrometry, turbidity)

6. Structural Properties Affect Solubility  Lipophilicity : Determined by van der Waals, dipolar, hydrogen bonds, ionic interactions  Size : Molecular weight, shape  pKa : Determined by functional group ionizability  Crystal lattice energy : Determined by crystal stacking, melting point

7. Solubility equation of Yalkowsky and Banerjee - A demonstration of the effect of lipophilicity and crystal lattice energy on solubility Log S = 0.8−Log P ow −0.01(MP−25) S; solubility Log P ow ; octanol/water partition coefficient MP; melting point Log P 1 unit ↑ MP 100°C ↑ Solubility 10-fold ↓

8. Solubility equation at a particular pH - A mathematical derivation of the Henderson-Hasselbalch equation S = S 0 (1+10 (pH−pKa) ) (Acid) S = S 0 (1+10 (pKa−pH) ) (Base) S 0 ; intrinsic solubility HA+H 2 O = H 3 O+A − (Acid) B+H 2 O = OH − +HB + (Base) S = [HA]+[A − ] (Acid) S = [B]+[HB + ] (Base) - Drugs ionize according to the reactions : - At equilibrium, the solubility :

9. more acid x6 x4000

10. Inflection point (pH=pka) S = S 0 (1+10 (pH−pKa) )

11. Kinetic and Thermodynamic Solubility KineticThermodynamic Characteristics (a) the compound initially is fully dissolved in an organic solvent (e.g., DMSO) then added to the aqueous buffer (b) equilibrium is not reached between dissolved compound and solid compound  metastable ppt (amorphous, mixture) (a) the addition of aqueous solvent directly to solid crystalline material (b) establishment of equilibrium between the dissolved and solid material  original ppt Use  Alert teams to potential absorption or bioassay liabilities  Diagnose erratic bioassay results  Develop structure-solubility relationships  Select compounds for NMR-binding and x-ray co-crystallization experiments  Develop generic formulations for animal dosing  Guide formulation development  Diagnose in vivo results  Plan development strategy  File regulatory submissions drug discovery late discovery Early development

12. Consequences of Chirality on Solubility - Chirality affects solubility because of the crystal form - Wallach’ rule : Racemate crystals are more stable and dense than their chiral counterparts racemic crystal  more stable  higher MP  thermodynamic solubility ↓ S-ketoprofen MP 72°C RS-ketoprofen MP 94°C Thermodynamic solubility 2.3mg/mL 1.4mg/mL

13. Effects of Solubility - Low Solubility Limits Absorption and Causes Low Oral Bioavailability

14. (Merck; Antiviral)

15. Effects of Solubility - Good Solubility is Essential for IV Formulation : compounds must have sufficient solubility in the vehicle to deliver the expected dose in a restricted volume - Acceptance Criteria for Solubility “What is the minimum solubility required for a compound?” MAD = S ∗ K a ∗ SIWV ∗ SITT MAD ; maximum absorbable dose at a certain dose S ; solubility (mg/mL, pH 6.5) K a ; intestinal absorption rate constant (min −1 ; permeability in rat intestinal perfusion experiment, quantitatively similar to human K a ) SIWV ; small intestine water volume ( ∼ 250 mL) SITT ; small intestine transit time (min; ∼ 270 min)

17. - The solubility classification ranges suggested for medicinal chemists <10 μg/mL Low solubility 10–60 μg/mL Moderate solubility >60 μg/mL High solubility human oral absorption much higher than 60 μg/mL

18. - Biopharmaceutics Classification System (BCS) used in drug development  Class I : ideal class for oral absorption.  Class II : Formulation typically is used to enhance solubility of compounds in this class.  Class III : Prodrug strategies typically are used for these compounds.  Class IV : Development of this class of compounds can be risky and costly. (No in vitro/in vivo correlations are expected)

19. - Molecular Properties for Solubility and Permeability Often are Opposed ↑  Perm ↑, Sol ↓ ↑  Sol ↑, Perm ↓

20. - Physiology of the Gastrointestinal Tract basic drugs acidic drugs

21. - Species Differences in Gastrointestinal Tract bioavailability more increases earlier absorption

22. Chapter 25. Solubility Method Overview  Solubility can be estimated using pKa, pH, Log P, and melting point.  Commercial software calculates equilibrium solubility for comparing series analogs.  High-throughput (HT) kinetic solubility is most relevant in discovery because it mimics discovery conditions.  HT kinetic methods include direct UV, nephelometry, and turbidimetry.  Custom solubility methods are used to mimic a specific project issue.  Equilibrium solubility is relevant for animal dosing and development studies.

23. Literature Solubility Calculation Methods - The total solubility “the Henderson-Hasselbalch equation” S tot = S HA (1+10 (pH−pKa) ) S tot ; total solubility S HA ; intrinsic solubility of the neutral acid - The intrinsic solubility “the Yalkowsky equation” Log S = 0.8−Log P ow −0.01(MP−25) S; solubility Log P ow ; octanol/water partition coefficient MP; melting point

24. - Kinetic Solubility Methods ▶ first dissolve the solid compound in DMSO ▶ then add an aliquot of DMSO solution to the aqueous buffer ▶ most appropriate for drug discovery - Thermodynamic Solubility Methods ▶ add aqueous buffer directly to compound solid ▶ most appropriate for late drug discovery and development

25. Direct UV Kinetic Solubility Method  The concentration of the compound is proportional to the UV absorbance  useful to measure solubility at various pHs to simulate physiological and assay conditions

26. Nephelometric Kinetic Solubility Method Well plate  turning point = maximum concentration = solubility  sparingly soluble ( 100 μg/mL)

27. Turbidimetric In Vitro Solubility Method  concentration↑  precipitate↑  Light transmission ↓  turning point = solubility

28. Customized Kinetic Solubility Method - cases where generic solubility method conditions are not adequate to accurately assess the solubility of compounds under specific conditions - performed using the same buffer composition, dilution procedure, incubation time, and temperature as the biological assay Compounds are added to the bioassay buffer under the conditions of the biological assay incubated according to bioassay time schedules Precipitate is removed by filtration analyzed by LC/UV/MS techniques

29. Equilibrium Shake Flask Thermodynamic Solubility Method

30. Potentiometric In Vitro Thermodynamic Solubility Method  used to determine the intrinsic solubility (S 0 )  suitable only for compounds with ionization centers