Stabilization of Amorphous Drugs by Polymers: The Role of Overlap Concentration (C*)

Amorphous solid dispersions (ASDs), in which polymers are admixed with a drug, retard or inhibit crystallization of the drug, increasing the drug’s apparent solubility and oral bioavailability. To date, there are no guidelines regarding how much polymer should be added to stabilize the amorphous form of the drug. We hypothesized that only drug that is not within a “sphere of influence” of a polymer chain is able to nucleate and form crystals and that the degree of crystallization should depend primarily on the ratio C/C*, where C is the polymer concentration and C* is the overlap concentration. We tested this hypothesis by quenching dispersions of polyvinylpyrrolidone (PVP) dissolved in molten felodipine (FEL) or indomethacin (IMC) at four molecular weights of PVP. For each molecular weight of PVP, C* in the drug (as solvent) was determined by dynamic light scattering and intrinsic viscosity. The enthalpy of fusion (ΔH_f), determined by DSC, was used to measure the fraction of drug that crystallized in an ASD. It was found, roughly, that ΔH_f/ΔH_f,C=0 = f(C/C*) and that no crystallization occurred when C > C*. XRD also showed that crystallization was completely inhibited up to ∼T_g + 75 °C when the polymer concentration was above C*. Our results suggest that stabilization of amorphous drugs can be achieved by incorporating a polymer just above C*, which is much lower than polymer concentrations customarily used in ASDs. This work reveals the importance of C* in selecting polymer concentrations when formulating drugs as ASDs.