Rhodamine D Q S Q Test Molecule in Intraocular Dynamics

Quantitative measurements of fluorescein in the eye are widely used in studying the function of the bloodretinal barrier, but they also can be of value in measuring the flow rate of the tears and aqueous humor and the barriers offered by the corneal layers and the other anterior structures of the eye. Concentrations down to 10~ g/ml can be determined in the ocular humors, and this allows measurements to be made with systemic doses of low toxicity and with topical application. However, the autofluorescence of the lens and the cornea is similar in color to the light emitted by fluorescein, and this raises the threshold of detection of the dye within these tissues. In consequence, a number of potentially valuable investigative techniques cannot be used in humans. The use of a fluorescent compound similar in chemical properties to fluorescein, but with its excitation and emission spectra shifted toward the red, would remove these limitations. Such a dye is rhodamine B, which is related to fluorescein by the addition of ethylamine groups and the removal of one carboxyl (Fig. 1). We, therefore, studied its behavior in the eye of the rabbit after topical and systemic administration. This was noted to be markedly different from that of fluorescein but similar to that of various drugs whose kinetics have been measured. The physicochemical properties of rhodamine were examined, and its behavior was considered to be a consequence of its high lipid solubility.