TargetHunter: an in silico target identification tool for predicting therapeutic potential of small organic molecules based on chemogenomic database.

Target identification of the known bioactive compounds and novel synthetic analogs is a very important research field in medicinal chemistry, biochemistry, and pharmacology. It is also a challenging and costly step towards chemical biology and phenotypic screening. In silico identification of potential biological targets for chemical compounds offers an alternative avenue for the exploration of ligand-target interactions and biochemical mechanisms, as well as for investigation of drug repurposing. Computational target fishing mines biologically annotated chemical databases and then maps compound structures into chemogenomical space in order to predict the biological targets. We summarize the recent advances and applications in computational target fishing, such as chemical similarity searching, data mining/machine learning, panel docking, and the bioactivity spectral analysis for target identification. We then described in detail a new web-based target prediction tool, TargetHunter (http://www.cbligand.org/TargetHunter). This web portal implements a novel in silico target prediction algorithm, the Targets Associated with its MOst SImilar Counterparts, by exploring the largest chemogenomical databases, ChEMBL. Prediction accuracy reached 91.1% from the top 3 guesses on a subset of high-potency compounds from the ChEMBL database, which outperformed a published algorithm, multiple-category models. TargetHunter also features an embedded geography tool, BioassayGeoMap, developed to allow the user easily to search for potential collaborators that can experimentally validate the predicted biological target(s) or off target(s). TargetHunter therefore provides a promising alternative to bridge the knowledge gap between biology and chemistry, and significantly boost the productivity of chemogenomics researchers for in silico drug design and discovery.

Epithelial Na channels and short-term renal response to salt deprivation.

To test the role of epithelial Na channels in the day-to-day regulation of renal Na excretion, rats were infused via osmotic minipumps with the Na channel blocker amiloride at rates that achieved drug concentrations of 2-5 microM in the lumen of the distal nephron. Daily Na excretion rates were unchanged, although amiloride-treated animals tended to excrete more Na in the afternoon and less in the late evening than controls. When the rats were given a low-Na diet, Na excretion rates were elevated in the amiloride-treated group within 4 h and remained higher than controls for at least 48 h. Adrenalectomized animals responded similarly to the low-Na diet. In contrast, rats infused with polythiazide at rates designed to inhibit NaCl transport in the distal tubule were able to conserve Na as well as did the controls. Injection of aldosterone (2 microg/100 g body wt) decreased Na excretion in control animals after a 1-h delay. This effect was largely abolished in amiloride-treated rats. On the basis of quantitative analysis of the results, we conclude that activation of amiloride-sensitive channels by mineralocorticoids accounts for 50-80% of the immediate natriuretic response of the kidney to a reduction in Na intake. Furthermore, the channels are necessary to achieve minimal rates of Na excretion during more chronic Na deprivation.

Characterization of the thiazide-sensitive Na(+)-Cl(-) cotransporter: a new model for ions and diuretics interaction.

The thiazide-sensitive Na(+)-Cl(-) cotransporter (TSC) is the major pathway for salt reabsorption in the apical membrane of the mammalian distal convoluted tubule. When expressed in Xenopus laevis oocytes, rat TSC exhibits high affinity for both cotransported ions, with the Michaelis-Menten constant (K(m)) for Na(+) of 7.6 +/- 1.6 mM and for Cl(-) of 6.3 +/- 1.1 mM, and Hill coefficients for Na(+) and Cl(-) consistent with electroneutrality. The affinities of both Na(+) and Cl(-) were increased by increasing concentration of the counterion. The IC(50) values for thiazides were affected by both extracellular Na(+) and Cl(-). The higher the Na(+) or Cl(-) concentration, the lower the inhibitory effect of thiazides. Finally, rTSC function is affected by extracellular osmolarity. We propose a transport model featuring a random order of binding in which the binding of each ion facilitates the binding of the counterion. Both ion binding sites alter thiazide-mediated inhibition of transport, indicating that the thiazide-binding site is either shared or modified by both Na(+) and Cl(-).

Physiologic effects of prazosin HCl: consequences of diuretic combination therapy.

The physiologic action of prazosin in man was studied in 23 patients including 10 treated with prazosin plus polythiazide. Responders to prazosin alone (blood pressure fell greater than 10 mm Hg) had a decrease in peripheral resistance (p less than 0.02) and no significant change in plasma volume, while nonresponders' plasma volume rose significantly (p less than 0.02). 10 patients treated with prazosin and polythiazide (blood pressure not normalized on prazosin alone) were analyzed at baseline (Rx 0), after prazosin (Rx 1), and after prazosin plus polythiazide (Rx 2). Mean supine blood pressure fell from 184/118 +/- 8/5 (SE) mm Hg at Rx 0 to 161/106 +/- 7/4 mm Hg at Rx 1 (p less than 0.005) and 129/89 +/- 4/4 mm Hg at Rx 2 (p less than 0.001). Supine renin increased in each group, but the increase with prazosin alone was not significant. Stimulated renin activity increased significantly after Rx 2 (p less than 0.0025). Cardiac output fell, plasma volume returned to baseline values, and peripheral resistance fell by 467 dyn s/cm-5 during thiazide treatment (p less than 0.025).

Calcium and sodium excretion in rats in response to prolonged treatment with polythiazide.

The hypocalciuric response to prolonged polythiazide (PTZ) administration has been investigated in intact and thyroparathyroidectomized (TPTX) rats. After 3 control days, PTZ was given for 6 days (0.1 mg/100 g body weight . 24 h p.o.). In both groups, calcium excretion fell on day 1 of PTZ and remained depressed thereafter. In the intact rats, this decrease was associated with increased sodium excretion and urine output on treatment day 1. In the TPTX rats, hypocalciuria occurred without change in sodium excretion or body weight; urine output increased on the first 2 days. Thus PTZ can reduce calcium excretion in the absence of parathyroid hormone, changes in sodium excretion, and changes in body weight (an estimate of body fluid balance with constant food intake).

Effect of single dose of diuretics on renal magnesium excretion in man, with special reference to their site of action.

The administration of a single dose of furosemide, ethacrynic acid and polythiazide to healthy individuals under conditions of maximum water diuresis produces a significant increase in renal magnesium excretion. Elevated Mg excretion displayed a direct correlation to renal sodium excretion after furosemide (r=0.689, p less than 0.001), ethacrynic acid (r=0.869, p less than 0.001) and polythiazide (r=0.586, p less than 0.01). The slopes of the various regression lines did not differe significantly from each other or from the slope of the regression line characterizing this correlation for mannitol (r= 0.603, p less than 0.01). A significant linear correlation was likewise found between the excretion of Mg and total osmotically active substances after furosemide (r=0.783, p less than 0.001), ethacrynic acid (r=0.88, p less than 0.001) and polythiazide (r=0.646, p less than 0.01). The regression lines of the given correlations did not differ significantlyfrom each other, but their slopes were significantly higher than that of the regression line for the correlation after mannitol (r=0.454, p less than 0.01). The findings indicate that tubular Mg transport is influenced both by a decrease in tubular Na resorption in the diluting segment (polythiazide) and by an effect on Na resorption in the parts of the nephron proximal to the diluting segment of the nephron (furosemide, ethacrynic acid).

Changes in urea clearance after a single dose of diuretics during water diuresis.

The action of various diuretics on the excretion fraction of urea (Curea/Ccr) was studied in 91 healthy volunteers under conditions of maintained maximal water diuresis. On the basis of previous data on transtubular transport or urea it should be expected that with an increase in the excretion fraction of water (V/Ccr) under the given experimental conditions we should find an increase in Curea/Ccr. After administration of polythiazide, which has a predominantly distal localisation of action in the nephron, Curea/Ccr did not change. After administration of chlorothiazide, furosemide and after i.v. administration of ethacrynic acid there was a marked increase in V/Ccr, ranging from 5.4 to 18%. Despite this there was no significant increase in Curea/Ccr. After i.v. administration of acetazolamide or oral administration of ethacrynic acid Curea/Ccr even showed a statistically significant decrease. These findings suggest that a decrease in proximal tubular reabsorption of water after administration of diuretics during maximal water diuresis is not a decisive factor for the renal excretion of urea. The data suggest that the diuretics affected the permeability of the distal segment of the nephron for urea in the sense of an increase of tubular reabsorption.