Formulation and in vitro evaluation of cysteamine hydrochloride viscous solutions for the treatment of corneal cystinosis. 
In the present study, viscous solutions of cysteamine hydrochloride (CH) were prepared by using 0.5%, 1.0%, 1.5% or 3.0% of hydroxypropylmethylcellulose (HPMC) and were evaluated for their in-vitro characteristics and stability.  Osmolalities, pH and viscosity of the formulations were determined.  The influence of benzalkonium chloride and autoclave sterilization on solution characteristics was also investigated.  For stability assessment, the viscous solutions were stored at +4 and +25 degrees C over 12 months.  In-vitro characteristics and CH contents of the stored solutions were monitored.  Irritation tests for the formulations were evaluated on rabbit eyes.  Dialysis sac technique was used to perform in vitro release study of the solutions containing 1.0% and 1.5% HPMC.  All of the viscous solutions tested showed non-newtonian (dilatant) flow behavior.  Osmolality values were ranked between 351.2+/-6.2 and 355.1+/-7.9 mOsm kg(-1), and pH values were between 3.97+/-0.1 and 3.98+/-0.2 for all the solutions.  Furthermore, no significant changes in dilatant behavior, osmolality or pH values of the pure HPMC solutions were observed.  After addition of the excipients or CH-excipients, increased viscosity values were noted in these formulations.  Neither benzalkonium chloride nor autoclave sterilization had any influence on viscosity, pH or osmolality values of the solution containing 1.5% HPMC.  Stability studies showed that a faster decrease in the concentration of CH was observed in the formulations stored at 25 degrees C compared to those kept at 4 degrees C; no changes were determined in osmolality values of the solutions at all storage conditions.  Increased pH and decreased viscosity values were noted in HPMC solutions containing CH and excipients, while no changes in these values were observed for pure HPMC solutions kept at 4 and 25 degrees C. In vitro release tests revealed that 81.2% and 85.3% of CH were released from the viscous solutions containing 1.5% and 1% HPMC, respectively, in 8h.  No irritation was observed when the viscous solutions were tested on rabbit and human eyes.