Injection volume and intracameral moxifloxacin dose.

Injection volume and intracameral moxifloxacin dose We read with interest the recently published article of Shorstein and Gardner, which highlights the importance of adequate intracameral (IC) moxifloxacin dosing and injection protocols in achieving consistent bactericidal levels for postoperative endophthalmitis prophylaxis in cataract surgery. In their study, mathematical models of the anterior chamber (AC) concentrations of moxifloxacin and its elimination rates were calculated following laboratory experimentation with 3 concentrations/injected volumes (0.5%/0.05 mL, 0.5%/0.10 mL, and 0.15%/0.50 mL). Two different AC volumes representing the human pseudophakic eye (0.19 mL and 0.33 mL) for each dosing method were used for their calculations. They concluded that larger injection volumes yielded more reliable aqueous concentrations. In our recent publication, we share the authors’ viewpoint that larger injection volumes, with similar total dosing, offer greater precision and reliability.We concluded that an IC injection dose of 600 μg moxifloxacin in 0.4 mL (yielding an AC concentration of about 1200 μg/mL), replacing most of the AC and intracapsular volume as the final step of surgery, enables more consistent antibiotic delivery into the AC. Our proposed IC injection method, after hydration of the main incision to avoid leakage, also enables slight adjustments of the intraocular lens position while injecting. It is important to understand that with IC injection, the IC drug concentration continuously accumulates within the AC by logarithmic growth throughout the injection, gradually approaching the injected solution’s concentration as aqueous is replaced, rather than providing a complete exchange or wash, an overly simplistic concept. We chose our injection technique, because our research led us to results somewhat divergent from those of Shorstein and Gardner. Their model’s assumption of the human pseudophakic AC volume of 0.19 mL and 0.33 mL was derived from Kanellopoulos and Asimellis, whose Scheimpflug imaging measurements were taken 3 months after cataract surgery. This is long after postsurgical equilibration (ie, fibrosis and closure of the capsular bag) has taken place. Our calculation of 0.5 mL volume of the AC and a just-evacuated capsular bag after phacoemulsification was derived by summating the preoperative anterior and posterior chamber volumes with that of the mean preoperative capsular bag. As a result of smaller AC volume estimates, Shorstein and Gardner arrived at a half-life elimination of moxifloxacin from the AC of 1.2 hours. This differs from our calculation of 2.89 hours, as illustrated in Figure 1, which is consistent with the literature. At our calculated abatement rate, the IC moxifloxacin does not dilute to below the bactericidal level of minimum inhibitory concentration greater than 64 μg/mL (the minimum inhibitory concentration of 90% of strains tested of the most moxifloxacin-resistant endophthalmitis pathogens, published in the ARMOR [Antibiotic Resistance Monitoring in Ocular Microorganisms] study) until 7.4 hours (with efficacy to 10.4 hours due to the postantibiotic effect of fluoroquinolones) after injection. This compares favorably with Shorstein and Gardner’s estimation of 5.4 hours for moxifloxacin levels to fall to the same level. Practical and ethical limitations preclude frequent postoperative AC sampling of antibiotic levels in humans. Improved understanding of moxifloxacin’s complex pharmacokinetics as clinical and bacteriological data accumulate will help us refine mathematical models representing IC abatement profiles. We arrived at greater immediate postphacoemulsification AC volume and IC moxifloxacin half-life than Shorstein and Gardner.