Miller F R7 R5 C R8 C O N R1 CO2H R2 Figure 1 Basic 4-quinolone structure (adapted from Domagala 1994). Modiﬁcation of the parent molecule’s core 4-quinolone core, at positions 1, 5, 7, and 8 has engineered novel ﬂuoro- quinolones with enhanced antimicrobial activity, safety and tolerability (Keating and Scott 2004). Substitutions at the N-1 nitrogen atoms are critical for the spectrum of activity and potency of the molecule. The N-1 cyclopropyl substitution in moxiﬂoxacin confers increased activity against gram-positive and anaerobic isolates. Substitutions at the C-5 position also impact the in vitro activity against gram-positive isolates; the larger the molecule the greater the gram positive potency. Addition of a bulky C-7 (diazabicyclononyl ring) side chain and a methoxy group at the C-8 position reduces the poten- tial for selection of resistant bacterial subpopulations and increase the binding/blocking afﬁnity for DNA gyrase and topoisomerase IV, essential bacterial enzymes (Domagala 1994; Ball et al 1998; Appelbaum and Hunter 2000; Peterson 2001; Zhanel et al 2002; Caeiro and Iannini 2003; Saravolatz and Leggett 2003) These modifications were incorporated to meet the challenge of emerging resistance in the older ﬂuoroquinolones among ocular and nonocular isolates (Blondeau 1999; Chaudhry et al 1999; Goldstein et al 1999; Alexandrakis et al 2000; Zhanel and Noreddin 2001; Mather et al 2002; Hwang 2004; Mah 2004; Marangon et al 2004; Van Bambeke et al 2005). Mechanism of action Moxiﬂoxacin is a bactericidal, concentration dependent, anti-infective. It interferes with bacterial survival by bind- ing to DNA gyrase (topoisomerase II) and topoisomerase IV, essential bacterial enzymes involved in the replication, translation, repair and recombination of deoxyribonucleic acid. DNA gyrase is encoded by the genes gyra A and gyr B, while topoisomerase IV is encoded by Par C (grl A) and pare (grl B). Inhibition of either enzyme leads to bacteria death (Zhanel and Noreddin 2001; Hwang 2004; Mah 2004; Van Bambeke et al 2005). All ﬂuoroquinolones bind to DNA gyrase and topoi- somerase enzymes in susceptible organisms. The afﬁnity or strength of the attachment varies; dependent on the class of ﬂuoroquinolone and the bacteria species. Moxiﬂoxacin binds strongly to both DNA gyrase and topoisomerase, but demonstrates preferential binding to DNA gyrase in gram-negative pathogens and Streptococcus pneumoniae. There is controversy as to the preferential target in the staphylococci. Preferential of dual targeting conﬁrmation is dependent on methods used to evaluate the targets, and wild type strains employed to generate the mutants (Hooper 2001a; Oliphant and Green 2002; Ball et al 2004; Keating and Scott 2004). Studies conﬁrmed that for the older ﬂuoroquinolones such as ciproﬂoxacin, topoisomerase IV is the preferred target in gram positive bacteria (Ball et al 1998; Dalhoff and Schmitz 2003; Drlica and Malik 2003; Zhanel et al 2006). In vitro studies supporting the dual activity of moxiﬂoxacin have been mixed. Takei and colleagues using MIC ratios; classiﬁed moxiﬂoxacin as a class three quinolone exhibiting dual activ- ity against the two enzymes in Staphylococcus aureus (Takei, Table 1 Impact of core modiﬁcation and potency to the ﬂuoroquinolones Substitution area Structure/side effects relationship R1 This area is part of the enzyme-DNA binding complex-inappropriate stereochemistry inhibits activity.A cyclopyl substitution here increases topoisomerase binding. R2 Area closes to the gyrase binding site: bulky side chains lowers potency R3-R4 These two groups interact with cleaved or damage DNA; no good substitutions/alternatives have been found R-5 Substitutions here affects topoisomerase afﬁnity, A NH2 or CH3 group adds gram-positive activity R-6 Affects potency; NH2 or H possible, Interactions of (R6+7+8) are key R-7 Interacts with DNA gyrase. Bulky side chain here impedes efﬂux; increases gram positive activity R-8 Affects target afﬁnity. A CH3 or OCH3 increases afﬁnity against anaerobes Adapted from Domagala (1994), Peterson (2001). 78 Clinical Ophthalmology 2008:2(1)