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Mark Vezina, Martin Bussieres, Genevieve Glazier, Marie-Pier Gagnon, Damien Martel; Determination of Injectable Intravitreous Volumes in Rats. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3219.
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A wide range of intravitreal injection volumes have been reported for rats (2 - 20 µL), while reported vitreous volume is 13- 20 µL. To provide dosing guidance on these overlapping ranges, feasible injectable volumes for intravitreal (IVT) injection in rats were evaluated using methylene blue (MB) as a marker.
Using a guide and needle technique, MB (10 mg/mL) was injected into the mid-vitreous of 8 anesthetized 14 wk old Sprague-Dawley rats at dose volumes of 2, 5, 8 and 10 µL (n = 4 eyes/dose volume). Digital photographs were taken to qualitatively assess the amount of reflux following injection. Both eyes were examined using a slit-lamp and indirect ophthalmoscopy immediately post dose. Reflux was subjectively assessed based on the amount of MB found at the injection site on the surface of the eye after needle and guide removal.
Dose volumes of 2, 5 and 8 µL did not result in a complete coloration of the vitreous immediately post dose as did the 10 µL dose volume. An injection volume of 2 µL produced the least amount of reflux, followed by 5, 10 and 8 µL, respectively. The discrepancy between 10 and 8 µL may be due to inherent variability or possibly slightly deeper injection of 10 µL. At 10 µL, only 1 eye had a moderate amount of reflux compared to a minimal amount for the remaining 3 eyes.
Although 10 µL appeared to be a feasible injectable volume for IVT dosing in rats based on reflux alone, it represents ≈50% of the maximum reported vitreal volume, equivalent to a 2 mL injection into a human eye (current human injection volumes are 0.05 to 0.067 mL). The long-term effect on the sensory retina of an increased intraocular pressure related to the high dose volume was not evaluated in this study and requires characterization prior to considering 10 µL as suitable. Numerous reports have shown 2 µL to be safe. For compounds with lower solubility, 5 µL may be a lower risk volume if injected deeper into the vitreous until potential adverse retinal effects are characterized at higher dose volumes.
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