**2.2.2 UV/Vis-spectroscopic method**

There are two methods using UV/Vis-spectroscopy for kinetic solubility determination: Method 1 is based on turbidimetry and the other is based on light absorbance intensity as a function of concentration (Pan et al., 2001).

Fig. 2. Schematic representation of turbidimetry.

### **2.2.2.1 UV/Vis-spectroscopic method 1**

The sample preparation is like nephelometry method, but the analyzing is with a 96-well plate UV/Vis-spectroscopy apparatus. This provides a wider range of wavelength to choose for reading the samples turbidity (190-1000 nm) (Pan et al., 2001). The lower the wavelength, the smaller particle is detected. However, in practice, wavelengths greater than 500 nm is

Experimental and Computational Methods Pertaining to Drug Solubility 197

Nephelometric Not required Low Yes No Low High UV/Vis 1 Not required Low Yes <500 nm Low High UV/Vis 2 Required Medium No <250 nm Medium Medium HPLC Required High No No High Low

Kinetic solubility values are valuable source in early stage of drug discovery in place of thermodynamic solubility values where there is good correlation between trends of these two values for a set of compounds (Hoelke et al. 2009). However, because of the amorphic nature of the solutes, in most of the cases the kinetic solubility is higher than thermodynamic solubility values. The effect of 5% DMSO as a cosolvent on the solubility value in kinetic solubility determination methods also should be considered. This is very important where most of the drugs have very low aqueous solubility and very small amounts of solubilizing

Also the effect of time after dilution is important, especially in turbidimetry methods. Hoelke et al. have shown that by increasing the time after dilution and precipitation, the

The collected data could be compared with the previously reported data in order to ensure the accuracy of the experimental procedure employed. Any mistake in the dilution steps, and miscalculations, or using un-calibrated instruments, such as un-calibrated balances, temperature variation and some other factors could be resulted in different solubility values

Computational methods in recent decades have become an important part of drug design and discovery. They are classified as theoretical, semi empirical and empirical equations. Most of models used in pharmaceutical sciences are semi-empirical (which is theoretical correlation of experimentally determined values) or empirical equations (which is mathematical correlation of experimentally determined values). Examples for semiempirical models are those correlations which use physicochemical parameters in their relationships. In other word, it is needed for them to be calculated based on experimental determinations at least for one time. For example in Noyes-Whitney equation, the diffusion coefficient must be determined at least for one time for a solute. So the Noyes-Whitney equation is a semi-empirical model. The quantitative structure property relationships (QSPR) and quantitative structure activity relationships (QSAR) are examples for empirical modelling. The pioneer for this type of equations in pharmaceutical sciences is Prof. Crowin H. Hansch. He has developed a QSPR model for solubility prediction of liquids, based on

log*S P* 1.339log 0.978 (3)

Table 1. The comparison between four kinetic solubility determination methods

agents such as cosolvents (e.g. DMSO) enhance their solubility largely.

determined solubility become smaller (Hoelke et al. 2009).

for a given drug dissolved in a solvent at a fixed temperature.

**3. Computational methods for solubility prediction** 

their partition coefficient (Hansch et al., 1968):

**2.3 Data validation** 

Method Calibration Specificity Easy Cut off LOD Rapidity

used. This is because of the fact that most of organic compounds which have UV absorbance (e.g. contain a benzene ring) also have fluorescence property and might interfere with turbidimetry which reads the amount of reflected light (or fluorescence emission light) (Pan et al., 2001). An example of this is phenol red which has light absorption in 430 and 560 nm and is exited by these wavelengths which results in fluorescence emission (Pan et al., 2001). Another limitation is the UV absorbance of the most plates which are made of plastics (Pan et al., 2001).

Fig. 3. The method for finding kinetic solubility.
