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Background"Nubiotics" are synthetic oligonucleotides and nucleotides with nuclease-resistant backbones, and are fully protonated for enhanced ability to be taken up by bacterial cells. Nu-3 [butyl-phosphate-5'-thymidine-3'-phosphate-butyl], one of the family members of Nubiotics was efficacious in the treatment of burn-wound infections by Pseudomonas aeruginosa in mice. Subsequent studies revealed that Nu-3 had a favorable toxicological profile for use as a pharmaceutical agent. This study evaluated the antibacterial activity of Nu-3 in vitro and its efficacy as a topical antibiotic. In addition, we investigated the possible mechanisms of Nu-3 action at the levels of DNA synthesis and bacterial membrane changes.MethodsAntimicrobial minimum inhibitory concentrations (MIC) experiments with Nu-3 and controls were measured against a range of Gram-positive and Gram-negative bacteria, including some hospital isolates according to Clinical and Laboratory Standards Institute (CLSI) guidelines. Analysis of the killing kinetics of Nu-3 was also performed against two strains (Staphylococcus aureus cvcc 2248 and Pseudomonas aeruginosa cvcc 5668). The mouse skin suture-wound infection model was used to evaluate the antibacterial activity of Nu-3. We used a 5-Bromo-2'-deoxy-uridine Labeling and Detection Kit III (Roche, Switzerland) to analyze DNA replication in bacteria according to the manufacturer's instruction. The BacLight™ Bacterial Membrane Potential Kit (Invitrogen) was used to measure the bacterial membrane potential in S. aureus.ResultsNu-3 had a wide antibacterial spectrum to Gram-positive, Gram-negative and some resistant bacteria. The MIC values of Nu-3 against all tested MRSA and MSSA were roughly in a same range while MICs of Oxacillin and Vancomycin varied between the bacteria tested. In the mouse model of skin wound infection study, the treatment with 5% Nu-3 glycerine solution also showed comparable therapeutic effects to Ciprofloxacin Hydrochloride Ointment. While Nu-3 had no effect on DNA synthesis of the tested bacteria as demonstrated in a BrdU assay, it could cause bacterial cell membrane depolarization, as measured using a BacLight™ Bacterial Membrane Potential Kit.ConclusionsThese results provide additional experimental data that are consistent with the hypothesis that Nu-3 represents a new class of antibacterial agents for treating topical infections and acts via a different mechanism from conventional antibiotics.

Nu-3 [butyl-phosphate-5′-thymidine-3′-phosphate-butyl] is a modified nucleotide that has been shown to have antimicrobial activity against a range of bacteria including Pseudomonas aeruginosa. However, data on the toxicological profile of Nu-3 are still lacking. In the present study, the toxicity of Nu-3 was evaluated by the following studies: acute oral toxicity, dermal and mucous membrane irritation, multiple-dose toxicity and genotoxicity in vivo and vitro. The acute oral toxicity test in mice showed that Nu-3 had an LD50 of 2001mg/kg body weight. The irritation tests on rats revealed that Nu-3 was not irritant, with an irritation scoring of 0. The multiple-dose toxicity study in rats showed that Nu-3 did not cause significant changes in histology, selected serum chemistry, and hematological parameters compared to the controls. Rats administrated with multiple-doses of Nu-3 showed no visible toxic symptoms. Both in vitro and in vivo, Nu-3 exhibited no notable genetic toxicity. Overall, the data suggest that Nu-3 is hypotoxic or nontoxic antimicrobial compound that warrants being further developed for treating Pseudomonas aeruginosa infection.