The combination of a rapidly dissolving and supersaturating "spring" with a precipitation retarding "parachute" has often been pursued as an effective formulation strategy for amorphous solid dispersions (ASDs) to enhance the rate and extent of oral absorption. However, the interplay between these two rate processes in achieving and maintaining supersaturation remains inadequately understood, and the effect of rate of supersaturation buildup on the overall time evolution of supersaturation during the dissolution of amorphous solids has not been explored. The objective of this study is to investigate the effect of supersaturation generation rate on the resulting kinetic solubility profiles of amorphous pharmaceuticals and to delineate the evolution of supersaturation from a mechanistic viewpoint. Experimental concentration-time curves under varying rates of supersaturation generation and recrystallization for model drugs, indomethacin (IND), naproxen (NAP) and piroxicam (PIR), were generated from infusing dissolved drug (e.g., in ethanol) into the dissolution medium and compared with that predicted from a comprehensive mechanistic model based on the classical nucleation theory taking into account both the particle growth and ripening processes. In the absence of any dissolved polymer to inhibit drug precipitation, both our experimental and predicted results show that the maximum achievable supersaturation (i.e., kinetic solubility) of the amorphous solids increases, the time to reach maximum decreases, and the rate of concentration decline in the de-supersaturation phase increases, with increasing rate of supersaturation generation (i.e., dissolution rate). Our mechanistic model also predicts the existence of an optimal supersaturation rate which maximizes the area under the curve (AUC) of the kinetic solubility concentration-time profile, which agrees well with experimental data. In the presence of a dissolved polymer from ASD dissolution, these observed trends also hold true except the de-supersaturation phase is more extended due to the crystallization inhibition effect. Since the observed kinetic solubility of nonequilibrium amorphous solids depends on the rate of supersaturation generation, our results also highlight the underlying difficulty in determining a reproducible solubility advantage for amorphous solids.

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With recent advances in the development of supersaturating oral dosage forms for poorly water-soluble drugs, pharmaceutical scientists are increasingly applying in vitro dissolution testing under non-sink conditions for a direct evaluation of their ability to generate and maintain supersaturation as a predictive surrogate for ensuring product quality and in vivo performance. However, the scientific rationale for developing the appropriate non-sink dissolution methodologies has not been extensively debated. This calls for a comprehensive discussion of recent research efforts on theoretical and experimental considerations of amorphous solubility, liquid-liquid phase separation, and phase transitions of drugs in a supersaturated solution when dissolution testing is performed under supersaturated non-sink conditions. In addition, we outline the concept of "sink index" that quantifies the magnitude of deviations from perfect sink dissolution conditions in the sink/non-sink continuum and some considerations of non-sink dissolution testing for marketed drug products. These factors should be carefully considered in recommending an adequately discriminatory dissolution method in the performance assessment of supersaturating drug delivery systems.