Volume intravitreal injection

Volume and location of an intravitreal injection can impact the patterns of ocular distribution and elimination. Friedrich et al. demonstrated a substantial effect of both on vitreal distribution and elimination of fluorescein and fluorescein glucuronide (252), with evaluation of four extreme positions and two injection volumes, 15 or 100 pL. The mean drug concentration remaining in the vitreous 24 hours postdose varied up to 3.8-fold with injection position, and increasing injection volume reduced this effect. 

The effects of INS37217 on subretinal fluid reabsorption in rabbits were made in a similar manner (4). As with the rat studies, a single non-RRD was produced in New Zealand white rabbits by injecting modified PBS ( 50pL administration volume) solution in the subretinal space, which resulted in a detachment that was less than 10% of the retinal surface area of the rabbit eye. Immediately following the creation of the subretinal bleb, an intravitreous injection (50 pL) of saline alone or... 

Once injected into the eye, antibiotics diffuse through the vitreous cavity and are eliminated by either a posterior or an anterior route (23 25). Several factors govern the intravitreal concentration of an antibiotic at any given time. The initial concentration is a result of the extended distribution and the initial dose. Subsequently, the volume of distribution, the dose of the initial injection, and the rate of elimination govern the concentration of the drug at a given time (26). Two parameters characterize the elimination phase of the drug (i) the elimination half-life and (ii) the apparent volume of distribution. 

The vitreous cavity volume affects the half-life of injected intravitreal antibiotics. Maurice has postulated that because of the increase in vitreous cavity volume in the human as compared to the rabbit, the time of the diffusion to the retinal surface is expected to be greater and the half-life of a drug in a human may be 1.7 times longer than in the rabbit vitreous cavity (24,25). Most data for actual concentrations of antimicrobial in the scientific literature do not correct for the difference in vitreous cavity volume (Table 2). The vitreous volume in the human eye is roughly 4 mL. Thus, on a concentration basis, a given amount of drug injected in the human eye will achieve an initial concentration of only 35% of the concentration in the rabbit eye. 

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