Identification of elongation factor G as the conserved cellular target of argyrin B

Argyrins, produced by myxobacteria and actinomycetes, are cyclic octapeptides with antibacterial and antitumor activity. Here, we identify elongation factor G (EF-G) as the cellular target of argyrin B in bacteria, via resistant mutant selection and whole genome sequencing, biophysical binding studies and crystallography. Argyrin B binds a novel allosteric pocket in EF-G, distinct from the known EF-G inhibitor antibiotic fusidic acid, revealing a new mode of protein synthesis inhibition. In eukaryotic cells, argyrin B was found to target mitochondrial elongation factor G1 (EF-G1), the closest homologue of bacterial EF-G. By blocking mitochondrial translation, argyrin B depletes electron transport components and inhibits the growth of yeast and tumor cells. Further supporting direct inhibition of EF-G1, expression of an argyrin B-binding deficient EF-G1 L693Q variant partially rescued argyrin B-sensitivity in tumor cells. In summary, we show that argyrin B is an antibacterial and cytotoxic agent that inhibits the evolutionarily conserved target EF-G, blocking protein synthesis in bacteria and mitochondrial translation in yeast and mammalian cells.

Antibiotic optimization and chemical structure stabilization of thiomuracin A

Synthetic studies of the antimicrobial secondary metabolite thiomuracin A (1) were initiated to improve chemical stability and physicochemical properties. Functional group modifications of 1 included removing the C2-C7 side chain, derivatizing the C84 epoxide region, and altering the C44 hydroxyphenylalanine motif. The resulting derivatives simplified and stabilized the chemical structure and were evaluated for antibacterial activity relative to 1. The simplified structure and improved organic solubility of the derivatives facilitated isolation yields from fermentation broths and simplified the procedures involved for the process. These advancements increased material supply for continued medicinal chemistry optimization and culminated in the identification of 2, a structurally simplified and chemically stable analogue of 1 which retained potent antibiotic activity.

Discovery of LFF571: an investigational agent for Clostridium difficile infection

Clostridium difficile (C. difficile) is a Gram positive, anaerobic bacterium that infects the lumen of the large intestine and produces toxins. This results in a range of syndromes from mild diarrhea to severe toxic megacolon and death. Alarmingly, the prevalence and severity of C. difficile infection are increasing; thus, associated morbidity and mortality rates are rising. 4-Aminothiazolyl analogues of the antibiotic natural product GE2270 A (1) were designed, synthesized, and optimized for the treatment of C. difficile infection. The medicinal chemistry effort focused on enhancing aqueous solubility relative to that of the natural product and previous development candidates (2, 3) and improving antibacterial activity. Structure-activity relationships, cocrystallographic interactions, pharmacokinetics, and efficacy in animal models of infection were characterized. These studies identified a series of dicarboxylic acid derivatives, which enhanced solubility/efficacy profile by several orders of magnitude compared to previously studied compounds and led to the selection of LFF571 (4) as an investigational new drug for treating C. difficile infection.

The thickness of the temporal squama and its implication for transcranial sonography

BACKGROUND AND PURPOSE

The difficult transtemporal ultrasound window is a relatively frequent occurrence. The authors assessed if the thickness of the temporal bone squama as measured in the "bone window" of the head computerized tomography (CT) scan can predict the transtemporal acoustic window.

METHODS

The authors retrospectively reviewed the head CTs on their bone window setting of patients in which nonimaging transcranial Dopplers (TCDs) had been performed. The thickness of the temporal squama in its thinnest portion was measured. The temporal TCD windows were graded in three classes: class 1 as good, class 2 when only a partial study is possible, and class 3 as an impossible ultrasonic window. In a case-control design, for every patient with any class 2 and 3 TCD temporal window, a patient with a class 1 window was randomly included from the same time period.

RESULTS

Fifty-five temporal bones (56%) were class 1, 17 (17%) were class 2, and 27 (27%) were class 3. Bone thicknesses (in mm, mean +/- SD) were greater in those with poorer windows: class 1 = 2.67 +/- 0.70, class 2 = 4.06 +/- 0.56, and class 3 = 5.04 +/- 1.06, P < or = .0001 by Cusick's nonparametric test of trend. Temporal squama thickness of > or = 5 mm portends 86% sensitivity, 90% specificity, 70% positive predictive value, and a positive likelihood ratio of 8.6 for a class 3 transtemporal ultrasound window.

CONCLUSION

Measurement of temporal bone thickness on the bone window setting of the head CT scan may be useful in identifying patients who are poor candidates for transcranial ultrasound.

Preliminary evaluation of iron phytate (inositol hexaphosphate) as a gastrointestinal MR contrast agent

A simple, effective, safe, and well-tolerated contrast agent is needed as a bowel marker for magnetic resonance (MR) imaging. The authors tested a variety of foodstuffs admixed with ferric iron as potential gastrointestinal MR contrast agents. Phytate (inositol hexaphosphate) more than doubled the relaxivity of solutions of ferric iron. Because of the improved relaxivity of iron phytate, the concentration of iron could be reduced substantially relative to free ferric iron (eg, ferric chloride or ferric ammonium citrate). Imaging studies were performed in five volunteers to determine the optimal dose of iron phytate and in five additional volunteers to test its effectiveness. A 200 mg/L concentration of ferric iron with phytate functions as an effective gastrointestinal MR contrast agent for T1-weighted abdominal MR imaging, significantly improving bowel contrast (P < .01). Blood studies after contrast agent administration showed no appreciable increase in serum iron. Compared with standard chelate complexes that decrease the relaxivity of a given paramagnetic ion, phytate not only decreases the absorption of the iron but increases its relaxivity.