Development and characterization of Tc-99m timolol maleate for evaluating efficacy of in situ ocular drug delivery system

Aim: Gamma scintigraphy is a well established technique for the diagnosis of various diseases in neurology, oncology and cardiology. Evaluation of drug delivery systems/formulations can be carried out either by tagging the active ingredient or the excipient of formulation. Drug delivery to eye is a challenging problem. Poor bioavailability in eye is mainly due to the precorneal loss factors which include rapid tear turnover and non-productive absorption. Frequent dosing results in side effects. These problems may be overcome by the use of in-situ gel-forming systems. Pharmacoscintigraphy technique can be used to evaluate ocular drug delivery system for precorneal retention and lacrimal clearance. So in our present work we describes preparation and characterization of Tc-99m Timolol maleate for evaluating retention and efficacy of developed chitosan in situ gel system.
Material and methods: Timolol maleate was labelled with Tc-99m using SnCl2.2H2O as reducing agent. Ultraviolet spectroscopy of Tc-99m timolol maleate was taken to check any shift/changes from original molecule. Various labelling parameters were checked along with In-vitro stability of the labelled complex. The developed formulation was also characterized for various in-vitro parameters e.g. clarity, gelation etc. Precorneal drainage and ocular retention was studied on New Zealand rabbits by gamma scintigraphy.
Results and discussion: Drug was instantaneously labelled with Tc-99m. Labelling efficiency was checked by ITLC using 100% acetone as mobile phase. The Rf value of free Tc is approx. 0.9. Various labelling parameters were optimized and it was observed that at 100 g SnCl2 concentration and at pH 6.5 the maximum labelling efficiency (98.2%) was obtained. At these conditions minimum colloids (1.1%) were produced. In-vitro stability found stable labelled complex for upto 24h. The observation of the acquired gamma camera images showed a good spreading over the entire precorneal area for the developed formulation immediately after administration as compared to plain drug solution. Plain drug solution cleared very rapidly from the corneal region and reached in to systemic circulation via nasolachrymal drainage system as significant activity was recorded in kidney and bladder after 2 h of ocular administration. Whereas no significant radioactivity was observed in systemic circulation (kidney and bladder) with developed formulation. Ocular irritation of the developed formulation was checked by Hen`s egg chorioallantoic membrane test and found to be practically non-irritant.
Conclusion: Significant retention and precorneal clearance studies with Tc-99m labeled timolol maleate in developed chitosan/HPMC based formulation suggest its efficacy in evaluating ocular drug delivery systems.