Table 2. Model drugs and their selected physicochemical properties [9–15].

Vendor information is provided in [6].

Drug	Solubility in Water at 25°Ca	pKa	Ionization state at pH 7	Log P	Plasma Protein Binding
Acetaminophen	Sparingly Soluble (10–33 mg/mL)	9.5 (acid)	Neutral	0.2	20–25%
Amitriptyline HCl b	Freely Soluble (100–1000 mg/mL)	9.45 (base)	Positively charged	3.0 (pH 7.4) 5.0 (free base)	~ 95%
Dexamethasone	Practically Insoluble (≤ 0.1 mg/mL)	Non-ionizable	Neutral	1.83	~77%
Nifedipine	Practically Insoluble (≤ 0.1 mg/mL)	-0.9 (base) 13 (acid)	Neutral	2–4	91–98%
Piroxicam	Practically Insoluble (≤ 0.1 mg/mL)c	1.86 (base) 5.46 (acid)	Negatively charged	-1.33 (pH 6.5) 0.72 (pH 6.6)	~ 99%
Prednisolone	Very Slightly Soluble (0.1–1 mg/mL)	Non-ionizable	Neutral	~ 1.6	60–90%

^a The solubility descriptions and ranges are based on USP definitions [16].

^b Amitriptyline was only available as the HCl salt commercially at the time of the study.

^c vvv is pH dependent. At pH 6.8, 37°C, it was reported as 0.47 mg/mL [14].