Target- and resistance-based mechanistic studies with TP-434, a novel fluorocycline antibiotic

Abstract

TP-434 is a novel, broad-spectrum fluorocycline antibiotic with activity against bacteria expressing major antibiotic resistance mechanisms, including tetracycline-specific efflux and ribosomal protection. The mechanism of action of TP-434 was assessed using both cell-based and in vitro assays. In Escherichia coli cells expressing recombinant tetracycline resistance genes, the MIC of TP-434 (0.063 μg/ml) was unaffected by tet(M), tet(K), and tet(B) and increased to 0.25 and 4 μg/ml in the presence of tet(A) and tet(X), respectively. Tetracycline, in contrast, was significantly less potent (MIC ≥ 128 μg/ml) against E. coli cells when any of these resistance mechanisms were present. TP-434 showed potent inhibition in E. coli in vitro transcription/translation (50% inhibitory concentration [IC(50)] = 0.29 ± 0.09 μg/ml) and [(3)H]tetracycline ribosome-binding competition (IC(50) = 0.22 ± 0.07 μM) assays. The antibacterial potencies of TP-434 and all other tetracycline class antibiotics tested were reduced by 4- to 16-fold, compared to that of the wild-type control strain, against Propionibacterium acnes strains carrying a 16S rRNA mutation, G1058C, a modification that changes the conformation of the primary binding site of tetracycline in the ribosome. Taken together, the findings support the idea that TP-434, like other tetracyclines, binds the ribosome and inhibits protein synthesis and that this activity is largely unaffected by the common tetracycline resistance mechanisms.

Fig 1

Structures of TP-434 and comparator tetracyclines. The TP-434 structure is labeled using the convention for tetracycline carbon numbering and ring letter assignments.

Fig 2

E. coli-coupled in vitro transcription/translation assays in the presence and absence of purified Tet(M). Compound titrations were assayed for inhibition of translation of a firefly luciferase gene, in the presence and absence of Tet(M) protein, as described in Materials and Methods.

Fig 3

Ribosome binding competition assays with [³H]tetracycline. Compound titrations were incubated with empty ribosomes in the presence a fixed concentration of [³H]tetracycline, and the concentration of compound required to compete for 50% of [³H]tetracycline binding was determined as described in Materials and Methods.