ABSTRACT
The complex and time-consuming process of drug development,
from discovery of a new chemical entity (NCE) to the autho-
rization of marketing for a new drug, can span a period of 12-
20 years.1 Undesirable physicochemical attributes are a major
cause of attrition in the drug development process. In a typical
physicochemical screening process (i.e., pKa, solubility, permeability, stability, and lipophilicity), poor solubility is a key factor limiting
successful development.1,2 Compounds with insufficient solubility
are more likely to fail during discovery and development as
inadequate solubility not only impacts other property assays,
masking additional undesirable properties, but also influences
both pharmacokinetic and pharmacodynamic characteristics of the
Table 2.1 The Biopharmaceutics Classification System (BCS)
Biopharmaceutics class Permeability Solubility
I High High
II High Low
III Low High
IV Low Low
compound.3 The Biopharmaceutics Classification System (BCS) is
the scientific framework that allows the classification of drug sub-
stances based on their dissolution, aqueous solubility, and intestinal
permeability.1−4 A drug’s bioavailability depends primarily on its solubility in the gastrointestinal (GI) tract and its permeability
across the cell membranes upon oral administration.5 This BCS
systemwas proposed by the Food and Drug Administration (FDA) as
a bioavailability/bioequivalence (BA/BE) regulatory guideline and
assigns drugs into four groups illustrated in Table 2.1. According to
the BCS framework, solubility is determined by obtaining the pH-
solubility profile of the drug substance in question in an aqueous
medium of pH range 1-8 at an established temperature of 37± 1◦C. A drug substance according to the solubility classification in the
BCS is thus considered to be highly soluble when its highest dose
strength proves to be soluble in 250 mL or less of an aqueous
medium over the pH range of 1-8.6−8 If not, the drug substance is considered as poorly soluble. The 250 mL volume estimate is
from bioequivalence study protocols which prescribe drug product
administration with a glass of water to fasting volunteers.7 A
drug substance is considered highly permeable when the extent
of intestinal absorption is determined to be 90% or higher; if
not, the drug substance is considered to be poorly permeable.7
Permeability classification is thus based directly on the extent of
intestinal absorption of a drug substance in humans or indirectly
on the measurements of the rate of mass transfer across the human
intestinal membrane.7
