In Fig. 3.17, the anesthetized rabbits were followed for 420 min following the retrobulbar injections. The vitreous and retina fluorescein peaked early: 60 min for vitreous and 30 min for retina. The corneal fluorescence increased and reached a plateau at 120 min. Interestingly, the contralateral eye without the retrobulbar injection of fluorescein mirrored the tissue fluorescence for the injected eye (Fig. 3.17b). This observation supports the vascular connection between the two eyes (Forster et al. 1979) The fluorescein exchange between the two eyes and the loss of fluorescein from the ocular tissue reflects the presence of the blood flushing through the ocular tissue.

In the euthanized rabbit that has received a retrobulbar injection of sodium fluorescein there is a slow uptake of fluorescein in equal concentration in the cornea, vitreous, and retina (Fig. 3.18a). The contralateral eye mirrors the event but at tenfold less concentration (Fig. 3.18b). The bodies were in muscular rigidity by the 420-min examination. There was no indication of the ocular fluorescein concentrations returning to baseline. The results demonstrated uniform cornea, mid-vitreous, and retina uptake for 3 h to a maximum of approximately 2,000 ng/mL. Interestingly, the contralateral control eyes had an uptake to approximately 100 ng/mL.

3.7  Intravenous Fluorescein Injection In Vivo

The retinal/choroid and vitreous uptake and release of fluorescein can be used to investigate in vivo intravenous drug uptake and release from the eye via various modes of drug delivery. Intravenous injections of fluorescein were performed in a third set of experiments. The ocular tissue autofluorescence prior to the injection is plotted in Fig. 3.19.

The fluorescein intravenous injection (14 ng/kg) was in the marginal ear vein. There was a rapid increase in the retinal fluorescein as the fluorescein was flush through the retinal vasculature (Fig. 3.20). Fluorescein leaked from the retinal blood vessels and penetrated into the vitreous but it is partially masked by the tailing concentrations of the retina. As the fluorophotometer sensor window moves through the high retinal fluorescence, the resulting bell-shaped curve extends over the vitreous section of the ocular fluorescence scan. The corneal fluorescein is most likely a result of conjunctival fluorescein leakage into the tear film. The fluorophotometer cannot distinguish between tear fluorescein and corneal fluorescein.

Fig. 3.19  The ocular fluorescence through the full length of the eye is plotted prior to an IV injection of fluorescein. Note all fluorescence values are less than 10 ng/mL

A summary graph (Fig. 3.21) shows in vivo the cornea, vitreous, and retina and release following the intravenous (IV) fluorescein injections in rabbits. The fluorescence values are in Table 3.2. There is a rapid flushing of the retina vasculature­ with fluorescein; peaking 20 min after the injection and then rapidly dropping off. The cornea also peaked in 20 min but did not rapidly release the fluorescein­ . The retinal peak uptake was not achieved for 60 min. The characteristic uptake differences­

between the retina, cornea, and vitreous can be explained by fluorescein being delivered and subsequently removed from the tissue by the vasculature. This resulted in a maximum retinal sodium fluorescein value within 15 min. The cornea and midvitreous maximized at approximately 60 min. The sodium fluorescein peaks in the cornea and mid-vitreous were two times greater in the IV-injected eyes than the contralateral control eyes. The total ocular of sodium fluorescein uptake was <0.0002% of the injected concentration. The retinal and mid-vitreous sodium fluorescein in both the retrobulbar-injected eyes and their control eyes returned to normal after 420 min, while the corneal sodium fluorescein remained elevated at 163 ng/mL in the retrobulbar-injected eyes and 53 ng/mL in the control eyes after 420 min.

The data support the difficulty of significant sodium fluorescein intraocular uptake following periorbital injections. The ocular vasculature rapidly “washes” away the sodium fluorescein and distributes it throughout the circulatory system, including the noninjected contralateral control eyes.

Fig. 3.20  The ocular fluorescence through the full length of the eye is plotted after an IV injection of fluorescein in the anesthetized rabbit. (a) Captures at 1 min after the IV injection and (b) was at 9 min postinjection

Fig. 3.21  Eye of animal with intravenous injection. The baseline values for cornea, vitreous, and retina are indicated by dashed lines (n = 5, mean ± standard deviation)

Table 3.2  Fluorophotometry values for retinal, mid­ -vitreous, and cornea following an IV injection of sodium fluorescein in the anesthetized rabbit

3.8  Ocular Uptake of Fluorescein from Topical Eye Drops

The possibility of delivering a drug to the posterior segment of the eye was evaluated with topical drops of fluorescein applied to a human subject’s eye. Six drops of 10 mL of 7% sodium fluorescein at 15-min intervals was applied. Hourly scans after last fluorescein application was recorded with the fluorophotometer. The contralateral eye did not receive topical applications of fluorescein. The autofluorescence values for the ocular tissue of the treated eye (Fig. 3.22a) and contralateral eye (Fig. 3.22b) show low retinal and corneal fluorescence and a normal high lens fluorescence relative to the 57-year-old subject. Seventy-five minutes after the topical

Fig. 3.22  A fluorophotometer scan of a human subject is scanned for autofluorescent values in (a) to be treated eye and (b) the contralateral untreated eye

application of 7% sodium fluorescein, the treated eye (Fig. 3.23a) corneal fluorescence­has risen without a change in the contralateral eye (Fig. 3.23b). The distribution of fluorescein has not changed after 255 min (Fig. 3.24). Table 3.3 contains a summary of the peak fluorescence values for the cornea and retina following