Comparing the effectiveness of moxifloxacin and povidone–iodine with povidone–iodine alone

Halachimi-Eyal et al.¹ analyzed prophylactic therapy of eyes that received moxifloxacin, povidone–iodine, and up to 5 other drugs. Specific interactions between moxifloxacin and the drugs used by Halachimi-Eyal et al. have not been investigated, resulting in unknown variables unaccounted for by controls.

While clearly ineffective as a topical antibiotic, diclofenac has minimum inhibitory concentrations reported to be between 5 mg/mL and 100 μg/mL for Staphylococcus aureus.^2,3 Diclofenac inhibits bacterial DNA synthesis,^2,3 causing a bacteriostatic effect. Moxifloxacin kills bacteria through the disruption of gyrase and topoisomerase during DNA replication. Halachimi-Eyal et al.¹ applied diclofenac (1 mg/mL) to eyes before moxifloxacin, resulting in DNA synthesis inhibition and elimination of topoisomerase and gyrase functions. Thus, diclofenac is expected to exert an antagonistic effect on the bactericidal action of moxifloxacin.

Halachimi-Eyal et al.¹ attempted to assess the effectiveness of moxifloxacin and povidone–iodine by comparing the number of culture-positive eyes before and after treatments. However, bactericidal effectiveness is most accurately measured by reduction of the total number of bacteria. The authors assessed cultures as positive or negative and failed to quantify viable bacteria (colony-forming units). A culture-positive eye statistically counts as 1 whether there are 1 or 10 000 bacteria. Therefore, the actual number of total bacteria in the moxifloxacin-treated group could have been significantly lower than in the group treated with povidone–iodine alone.

Another point to consider is the pharmacokinetics of moxifloxacin and povidone–iodine. Moxifloxacin has proven penetration of all ocular tissues in bactericidal quantities. However, povidone–iodine does not penetrate well into tissues.^4,5 Therefore, moxifloxacin has the capability to kill bacteria that reach the cornea or anterior chamber during intraocular surgery.

Halachimi-Eyal et al.¹ conclude that moxifloxacin with povidone–iodine had no significant added effect, but they failed to consider drug interactions, possible antagonism of diclofenac on moxifloxacin, and an appropriate method for determining bactericidal effectiveness.