Results of a small crossover RCT indicated that Procysbi (RP103) is noninferior to Cystagon based on WBC cystine levels after three weeks of open-label treatment. A noninferiority boundary of 0.3 nmol half cystine/mg protein was selected but the minimal clinically important change in WBC cystine is not known. There were no clinically significant differences observed between the two cysteamine formulations for other outcomes such as adherence to therapy, swallowing ability, or quality of life, but the trial was not designed to show differences in these outcomes. The rates of serious adverse events, as well as non-serious gastrointestinal and non-gastrointestinal adverse events were higher during treatment with Procysbi compared with the rates observed during treatment with Cystagon.