Purely in Silico BCS Classification: Science Based Quality Standards for the World’s Drugs

BCS
classification is a vital tool in the development of both generic
and innovative drug products. The purpose of this work was to provisionally
classify the world’s top selling oral drugs according to the
BCS, using in silico methods. Three different in silico methods were
examined: the well-established group contribution (CLogP) and atom
contribution (ALogP) methods, and a new method based solely on the
molecular formula and element contribution (KLogP). Metoprolol was
used as the benchmark for the low/high permeability class boundary.
Solubility was estimated in silico using a thermodynamic equation
that relies on the partition coefficient and melting point. The validity
of each method was affirmed by comparison to reference data and literature.
We then used each method to provisionally classify the orally administered,
IR drug products found in the WHO Model list of Essential Medicines,
and the top-selling oral drug products in the United States (US),
Great Britain (GB), Spain (ES), Israel (IL), Japan (JP), and South
Korea (KR). A combined list of 363 drugs was compiled from the various
lists, and 257 drugs were classified using the different in silico
permeability methods and literature solubility data, as well as BDDCS
classification. Lastly, we calculated the solubility values for 185
drugs from the combined set using in silico approach. Permeability
classification with the different in silico methods was correct for
69–72.4% of the 29 reference drugs with known human jejunal
permeability, and for 84.6–92.9% of the 14 FDA reference drugs
in the set. The correlations (r2) between
experimental log P values of 154 drugs and their
CLogP, ALogP and KLogP were 0.97, 0.82 and 0.71, respectively. The
different in silico permeability methods produced comparable results:
30–34% of the US, GB, ES and IL top selling drugs were class
1, 27–36.4% were class 2, 22–25.5% were class 3, and
5.46–14% were class 4 drugs, while ∼8% could not be
classified. The WHO list included significantly less class 1 and more
class 3 drugs in comparison to the countries’ lists, probably
due to differences in commonly used drugs in developing vs industrial
countries. BDDCS classified more drugs as class 1 compared to in silico
BCS, likely due to the more lax benchmark for metabolism (70%), in
comparison to the strict permeability benchmark (metoprolol). For
185 out of the 363 drugs, in silico solubility values were calculated,
and successfully matched the literature solubility data. In conclusion,
relatively simple in silico methods can be used to estimate both permeability
and solubility. While CLogP produced the best correlation to experimental
values, even KLogP, the most simplified in silico method that is based
on molecular formula with no knowledge of molecular structure, produced
comparable BCS classification to the sophisticated methods. This KLogP,
when combined with a mean melting point and estimated dose, can be used to provisionally classify potential drugs from just molecular formula, even before synthesis. 49–59% of the world’s top-selling drugs are highly soluble (class 1 and class 3), and are therefore candidates for waivers of in vivo bioequivalence studies. For these drugs, the replacement of expensive human studies with affordable in vitro dissolution tests would ensure their bioequivalence, and encourage the development and availability of generic drug products in both industrial and developing countries.