Should have paid more attention during your pharmaceutics class...

Okay, all joking aside... the polarity of a drug and its vehicle is very
often opposite to maximize drug release from the vehicle.  Imagine: if
the drug is highly soluble (or miscible) in its vehicle relative to the
surrounding body fluid, the drug will preferentially remain in the
vehicle and release will be poor.

Also note that in the case eflornithine, it is an amino acid derivative
with an additional ionizable sidechain, so its aqueous solubility will
be strongly dependent on pH.  With a suitable choice of buffer or
cosolvent, you can probably cause it to incorporate equally well into a
hydrophobic or hydrophilic vehicle.

Good luck.