Target Validation Information
TTD ID	T53601
Target Name	Bacterial 50S ribosomal RNA (Bact 50S rRNA)
Type of Target	Successful
Drug Potency against Target		Drug Info	IC50 = 0.15 mcg/ml	[4]
	Erythromycin	Drug Info	IC50 = 0.36 mcg/ml	[4]
	Telithromycin	Drug Info	MIC50 = 0.004 ug/ml	[3]
	Tobramycin	Drug Info	Ic50 = 21000 nM	[8]
	Troleandomycin	Drug Info	IC50 = 710 nM
Action against Disease Model		Drug Info	All medically useful antibiotics should have the potential to distinguish between target microbes (bacteria) and host cells. Although many antibiotics that target bacterial protein synthesis show little effect on the translation machinery of the eukaryotic cytoplasm, it is unclear whether these antibiotics target or not the mitochondrial translation machinery. We employed anin vitro translation system from bovine mitochondria, which consists of mitochondrial ribosomes and mitochondrial elongation factors, to estimate the effect of antibiotics on mitichondrial protein synthesis. Tetracycline and thiostrepton showed similar inhibitory effects on both Escherichia coli and mitochondrial protein synthesis. The mitochondrial system was more resistant to tiamulin, macrolides, virginiamycin, fusidic acid and kirromycin than the E. coli system. The present results, taken together with atomic structure of the ribosome, may provide useful information for the rational design of new antibiotics having less adverse effects in h uMans and animals.	[6]
		Drug Info	The efficacy of roxithromycin alone or in combination with pyrimethamine or sulphadiazine was examined in vitro and in a murine model of acute toxoplasmosis. In-vitro studies were performed with MRC5 fibroblast tissue cultures, with quantification of toxoplasma growth by an enzyme-linked immunosorbent assay. For in-vivo studies, mice were infected with 10(4) tachyzoites of the virulent RH strain and then treated perorally for 10 days from day 1 after infection. The efficacy of each drug regimen was assessed by determination of survival rates and sequential titration of parasites in blood, brain and lungs, using a tissue culture method. In vitro, roxithromycin inhibited toxoplasma growth at a concentration of > or = 0.02 mg/L; the 50% inhibitory concentration was estimated to be 1.34 mg/L. No synergistic effect was observed when it was combined with pyrimethamine or sulphadiazine. In vivo, roxithromycin alone at 50 or 200 mg/kg/day slightly prolonged survival compared with untreated mice, but a striking synergistic effect was observed when roxithromycin was administered in combination with pyrimethamine or sulphadiazine at subtherapeutic doses, i.e., 12.5 and 100 mg/kg/day, respectively. Combination regimens consistently resulted in a marked reduction fo the parasite burdens in blood and tissue, compared with those in mice treated with any of the agents alone. These results suggest that in-vivo activities of either pyrimethamine or sulphadiazine against T. gondii are reinforced by roxithromycin and such combinations should be considered in development of alternative treatments for h uMan toxoplasmosis	[9]
	Clindamycin	Drug Info	Clindamycin, which has been reported to have no significant in vitro activity against Toxoplasma gondii, actually markedly inhibits the growth of this parasite in infected h uMan fibroblasts. When measured 3 days after treatment, the concentration required to reduce parasite growth by 50% is about 1 ng/mL. Some observers failed to note this inhibition because of its markedlydelayed onset. At 6 ng/mL, clindamycin is parasiticidal, and the rate and extent of parasite killing increase with higher drug concentrations. With the aid of chemical mutagenesis, we isolated a parasite mutant that is approximately 100-fold more resistant to clindamycin than is the wild type. Lincomycin inhibits T. gondii at a higher 50% inhibitory concentration, about 100 ng/mL. The clindamycin-resistant mutant is partially cross-resistant to lincomycin.	[5]
	Dirithromycin	Drug Info	The active efflux of toxic compounds by (multi)drug transporters is one of the mechanisms that bacteria have developed to resist cytotoxic drugs. The authors describe the role of the lactococcal secondary multidrug transporter LmrP in the resistance to a broad range of clinically important antibiotics. Cells expressing LmrP display an increased resistance to the lincosamide,streptogramin, tetracycline and 14- and 15-membered macrolide antibiotics. The streptogramin antibiotic quinupristin, present in the fourth-generation antibiotic RP 59500, can inhibit LmrP-mediated Hoechst 33342 transport, but is not transported by LmrP, indicating that quinupristin acts as a modulator of LmrP activity. LmrP-expressing Lactococcus lactis cells in which a proton-motive force is generated acc uMulate significantly less tetracycline than control cells without LmrP expression. In contrast, LmrP-expressing and control cells acc uMulate equal amounts of tetracycline in the absence of metabolic energy. These findings demonstrate that the increased antibiotic resistance in LmrP-expressing cells is a result of the active extrusion of antibiotics from the cell.	[1]
	Gentamicine sulfate	Drug Info	We have isolated and characterized in vitro mutants of the Lyme disease agent Borrelia burgdorferi that are resistant to spectinomycin, kanamycin, gentamicin, or streptomycin, antibiotics that target the small subunit of the ribosome. 16S rRNA mutations A1185G and C1186U, homologous to Escherichia coli nucleotides A1191 and C1192, conferred >2,200-fold and 1,300-fold resistance to spectinomycin, respectively. A 16S rRNA A1402G mutation, homologous to E. coli A1408, conferred >90-fold resistance to kanamycin and >240-fold resistance to gentamicin. Two mutations were identified in the gene for ribosomal protein S12, at a site homologous to E. coli residue Lys-87, in mutants selected in streptomycin. Substitutions at codon 88, K88R and K88E, conferred 7-fold resistance and 10-fold resistance, respectively, to streptomycin on B. burgdorferi. The 16S rRNA A1185G and C1186U mutations, associated with spectinomycin resistance, appeared in a population of B. burgdorferi parental strain B31 at a high frequency of 6 x 10(-6). These spectinomycin-resistant mutants successfully competed with the wild-type strain during 100 generations of coculture in vitro. The aminoglycoside-resistant mutants appeared at a frequency of 3 x 10(-9) to 1 x10(-7) in a population and were unable to compete with wild-type strain B31 after 100 generations. This is the first description of mutations in the B. burgdorferi ribosome that confer resistance to antibiotics. These results have implications for the evolution of antibiotic resistance, because the 16S rRNA mutations conferring spectinomycin resistance have no significant fitness cost in vitro, and for the development of new selectable markers.	[7]
	Lincomycin	Drug Info	Clindamycin, which has been reported to have no significant in vitro activity against Toxoplasma gondii, actually markedly inhibits the growth of this parasite in infected h uMan fibroblasts. When measured 3 days after treatment, the concentration required to reduce parasite growth by 50% is about 1 ng/mL. Some observers failed to note this inhibition because of its markedlydelayed onset. At 6 ng/mL, clindamycin is parasiticidal, and the rate and extent of parasite killing increase with higher drug concentrations. With the aid of chemical mutagenesis, we isolated a parasite mutant that is approximately 100-fold more resistant to clindamycin than is the wild type. Lincomycin inhibits T. gondii at a higher 50% inhibitory concentration, about 100 ng/mL. The clindamycin-resistant mutant is partially cross-resistant to lincomycin.	[5]
	Retapamulin	Drug Info	IC50 on 50S subunit synthesis in Staphylococcus aureus wild-type strain RN1786: 27 ng/mL	[2]
	Tobramycin	Drug Info	IC50 on luciferase synthesis in Mycobacteri uM smegmatis: 20 nM	[8]
References
REF 1	The lactococcal secondary multidrug transporter LmrP confers resistance to lincosamides, macrolides, streptogramins and tetracyclines. Microbiology. 2001 Oct;147(Pt 10):2873-80.
REF 2	Retapamulin inhibition of translation and 50S ribosomal subunit formation in Staphylococcus aureus cells. Antimicrob Agents Chemother. 2007 Sep;51(9):3385-7.
REF 3	Novel antibacterial agents for the treatment of serious Gram-positive infections. Expert Opin Investig Drugs. 2003 Mar;12(3):379-99.
REF 4	Bacterial ribosomal subunit assembly is an antibiotic target. Curr Top Med Chem. 2003;3(9):929-47.
REF 5	Parasiticidal effect of clindamycin on Toxoplasma gondii grown in cultured cells and selection of a drug-resistant mutant. Antimicrob Agents Chemother. 1992 May;36(5):1091-6.
REF 6	Antibiotic susceptibility of mammalian mitochondrial translation. FEBS Lett. 2005 Nov 21;579(28):6423-7.
REF 7	Mutations conferring aminoglycoside and spectinomycin resistance in Borrelia burgdorferi. Antimicrob Agents Chemother. 2006 Feb;50(2):445-52.
REF 8	Engineering the rRNA decoding site of eukaryotic cytosolic ribosomes in bacteria. Nucleic Acids Res. 2007;35(18):6086-93.
REF 9	In-vitro and in-vivo activities of roxithromycin in combination with pyrimethamine or sulphadiazine against Toxoplasma gondii. J Antimicrob Chemother. 1995 Jun;35(6):821-32.

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