An effective approach to screening for new multicomponent
crystal forms of an API is to simply remove the
constraint that the counterions must have certain ¢pKa
values. By screening a wider variety of potential counterions
and guest molecules (including those with negative ¢pKa
values), a screen is likely to yield a wider array of solid forms
with potentially useful physical properties and valuable
intellectual property. If theophylline were to be screened for
new solid forms based on the prevailing approach to salt
screening, ethylenediamine would not be included in a list
of potential counterions and aminophylline (a successful
commercial form) would not be a targeted solid form. The
traditional approach of selecting counterions based on ¢pKa
would only have led to the following salts: HCl (¢pKa )
9.7), 5-sulfosalicylic acid (¢pKa ) 2.3), and sodium hydroxide
(¢pKa ) 7.4). However, more than 15 additional
salts and at least 32 cocrystals of 2 are known to exist. A
more comprehensive solid form screen of an ionizable API
is one that also applies a crystal engineering approach
focused on selecting counterions and guests based on
complementary intermolecular interactions rather than just
pKa.