Noncytotoxic Pyrrolobenzodiazepine-Ciprofloxacin Conjugate with Activity against Mycobacterium tuberculosis

Abstract

The development of new antitubercular agents for the treatment of infections caused by multidrug-resistant (MDR) Mycobacterium tuberculosis is an urgent priority. Pyrrolobenzodiazepines (PBDs) are a promising class of antibacterial agents that were initially discovered and isolated from a range of Streptomyces species. Recently, C8-linked PBD monomers have been shown to work by inhibiting DNA gyrase and have demonstrated activity against M. tuberculosis. However, both PBD monomers and dimers are toxic to eukaryotic cells, limiting their development as antibacterial agents. To eliminate the toxicity associated with PBDs and explore the effect of C8-modification with a known antibacterial agent with the same mechanism of action (i.e., ciprofloxacin, a gyrase inhibitor), we synthesized a C8-linked PBD-ciprofloxacin (PBD-CIP, 3) hybrid. The hybrid compound displayed minimum inhibitory concentration values of 0.4 or 2.1 μg/mL against drug-sensitive and drug-resistant M. tuberculosis strains, respectively. A molecular modeling study showed good interaction of compound 3 with wild-type M. tuberculosis DNA gyrase, suggesting gyrase inhibition as a possible mechanism of action. Compound 3 is a nontoxic combination hybrid that can be utilized as a new scaffold and further optimized to develop new antitubercular agents.

Figure 1

Structure of selected fluoroquinolones and pyrrolobenzodiazepine with reported activity against M. tuberculosis.

Figure 2

Design of the fluoroquinolone–pyrrolobenzodiazepine hybrid.

Figure 3

Two-dimensional (2D) and 3D structures of ciprofloxacin (A, B) and compound 3 (C, D) within the binding site of the wild type of DNA gyrase A.

Scheme 1. Synthetic Scheme for PBD–CIP (3)

Figure 4

Effect of compound 3 and GWL-78 against HeLa and WI-38 cell line. The lack of cytotoxicity observed for compound 3 was statistically significant compared to that for GWL-78.

Figure 5

Growth inhibition of M. tuberculosis strains by compound 3 or ciprofloxacin. Dose-dependent growth inhibition of M. tuberculosis strains H37Rv (black), M. tuberculosis 1192/015 (red), and M. tuberculosis 08/00483E (blue) by ciprofloxacin (0.094–6.036 μM) (A) and compound 3 (0.048–3.1 μM) (B). Twofold serial dilutions of each antibiotic were evaluated. The antibiotic activity was ascertained by the addition of resazurin, and the optical density (OD_570nm) was measured. The OD_570nm measurements were converted to a percentage of the OD_570nm measurements for the control that contained no antibiotic. The values plotted are a percentage of the growth without antibiotic and are the mean values of three independent experiments ± standard deviation and analyzed using Student’s t test. Vertical lines indicate modified Gompertz-determined MIC values.