Vitreous and aqueous penetration of orally administered moxifloxacin in humans

Sir,

We would like to comment on the above paper by Vedantham et al.¹ The authors state that ‘the spectrum of coverage does not appropriately encompass the most common causative organisms in endophthalmitis, especially Staphylococcus epidermidis’. This is based on the assumption that the MIC₉₀ of moxifloxacin for Staph. epidermidis is 2 μg/ml (Table 1).² This is contrary to other publications on MIC₉₀ tables for moxifloxacin.^{3, 4, 5} Published susceptibility tables for moxifloxacin include pathogens isolated from systemic infections and therefore are not representative of endophthalmitis pathogen susceptibilities. When these were examined, the great majority of Staphylococci were susceptible to moxifloxacin.^{6, 7} The authors also state that they decided to test moxifloxacin because of its low MICs against pathogens implicated in endophthalmitis (including Gram-positive bacteria), which is contrary to the MICs in Table 1 that they use for their analysis.

Of notice also are the wide variations of moxifloxacin concentrations in aqueous and vitreous samples in their series. These are contrary to other studies,^{8, 9, 10, 11} where moxifloxacin achieved very steady concentrations for at least 12 h after oral administration. Their serum concentrations also are variable and in a few cases extremely low, which is not in accordance with bibliography on serum moxifloxacin levels,^{3, 4, 5} although the usual dose is the same as the one used by the authors (400 mg OD). The authors attribute (in part) low levels of moxifloxacin to the fact that only one dose of 400 mg was administered to ‘simulate the clinical scenario’. In general, whenever intraocular penetration of a systemically administered antibiotic is to be determined, higher loading doses are used and in the case of moxifloxacin no hazardous side effects were noted previously.^{8, 9, 10, 11}

In conclusion, we mention that the results of this paper should be interpreted with caution and that there are numerous data suggesting that moxifloxacin may be a very useful systemic addition in endophthalmitis treatment.