Combination treatment with tenapanor and sevelamer synergistically reduces urinary phosphorus excretion in rats.

The majority of patients with chronic kidney disease (CKD) receiving dialysis do not achieve target serum phosphorus concentrations, despite treatment with phosphate binders. Tenapanor is a nonbinder, sodium/hydrogen exchanger isoform 3 (NHE3) inhibitor that reduces paracellular intestinal phosphate absorption. This preclinical study evaluated the effect of tenapanor and varying doses of sevelamer carbonate on urinary phosphorus excretion, a direct reflection of intestinal phosphate absorption. We measured 24-h urinary phosphorus excretion in male rats assigned to groups dosed orally with vehicle or tenapanor (0.3 mg/kg/day) and provided a diet containing varying amounts of sevelamer [0-3% (wt/wt)]. We also evaluated the effect of the addition of tenapanor or vehicle on 24-h urinary phosphorus excretion to rats on a stable dose of sevelamer [1.5% (wt/wt)]. When administered together, tenapanor and sevelamer decreased urinary phosphorus excretion significantly more than either tenapanor or sevelamer alone across all sevelamer dose levels. The Bliss statistical model of independence indicated that the combination was synergistic. A stable sevelamer dose [1.5% (wt/wt)] reduced mean ± SE urinary phosphorus excretion by 42 ± 3% compared with vehicle; together, tenapanor and sevelamer reduced residual urinary phosphorus excretion by an additional 37 ± 6% (P < 0.05). Although both tenapanor and sevelamer reduce intestinal phosphate absorption individually, administration of tenapanor and sevelamer together results in more pronounced reductions in intestinal phosphate absorption than if either agent is administered alone. Further evaluation of combination tenapanor plus phosphate binder treatment in patients receiving dialysis with hyperphosphatemia is warranted.

Metabolism and pharmacokinetics of naronapride (ATI-7505), a serotonin 5-HT(4) receptor agonist for gastrointestinal motility disorders.

The absorption and disposition of the serotonin 5-HT(4) receptor agonist, naronapride (6-[(3S,4R)-4-(4-amino-5-chloro-2-methoxy-benzoylamino)-3-methoxy-piperidin-1-yl]-hexanoic acid 1-aza-bicyclo[2,2,2]oct-(R)-3-yl ester dihydrochloride; ATI-7505), were evaluated in healthy males given a single 120-mg oral dose of (14)C-labeled compound. Serial blood samples and complete urine and feces were collected up to 552 h postdose. Naronapride was extensively metabolized, undergoing rapid hydrolysis to 6-[(3S,4R)-4-(4-amino-5-chloro-2-methoxy-benzoylamino)-3-methoxy-piperidin-1-yl]-hexanoic acid (ATI-7500) with stoichiometric loss of quinuclidinol. ATI-7500 was either N-glucuronidated on the phenyl ring or its hexanoic acid side chain underwent two-carbon cleavage, probably through a ß-oxidation metabolic pathway, to form 4-[(3S,4R)-4-(4-amino-5-chloro-2-methoxy-benzoylamino)-3-methoxy-piperidin-1-yl]-butanoic acid (ATI-7400). ATI-7400 underwent further side-chain oxidation to form 2-[(3S,4R)-4-(4-amino-5-chloro-2-methoxy-benzoylamino)-3-methoxy-piperidin-1-yl]-acetic acid (ATI-7100). Quinuclidinol, ATI-7500, ATI-7400, and ATI-7100 were the major metabolites, with plasma area under the curve values approximately 72-, 17-, 8-, and 2.6-fold that of naronapride. Naronapride, ATI-7500, ATI-7400, and ATI-7100 accounted for 32.32, 36.56, 16.28, and 1.58%, respectively, of the dose recovered in urine and feces. ATI-7400 was the most abundant radioactive urinary metabolite (7.77%), and ATI-7500 was the most abundant metabolite in feces (35.62%). Fecal excretion was the major route of elimination. Approximately 32% of the dose was excreted unchanged in feces. Naronapride, ATI-7500, and quinuclidinol reached peak plasma levels within 1 h postdose. Peak ATI-7400 and ATI-7100 concentrations were reached within 1.7 h, suggesting rapid ATI-7500 metabolism. Naronapride plasma terminal half-life was 5.36 h, and half-lives of the major metabolites ranged from 17.69 to 33.03 h. Naronapride plasma protein binding was 30 to 40%. The mean blood/plasma radioactivity ratio indicated minimal partitioning of (14)C into red blood cells.

Evaluation of the Pharmacodynamic Effects of the Potassium Binder RDX7675 in Mice.

INTRODUCTION: Hyperkalemia is a common complication in patients with heart failure or chronic kidney disease, particularly those who are taking inhibitors of the renin-angiotensin-aldosterone system. RDX7675, the calcium salt of a reengineered polystyrene sulfonate-based resin, is a potassium binder that is being investigated as a novel treatment for hyperkalemia. This study evaluated the pharmacodynamic effects of RDX7675 in mice, compared to 2 current treatments, sodium polystyrene sulfonate (SPS) and patiromer. METHODS: Seven groups of 8 male CD-1 mice were given either standard chow (controls) or standard chow containing 4.0% or 6.6% active moiety of RDX7675, patiromer, or SPS for 72 hours. Stool and urine were collected over the final 24 hours of treatment for ion excretion analyses. RESULTS: RDX7675 increased stool potassium (mean 24-hour excretion: 4.0%, 9.19 mg; 6.6%, 18.11 mg; both P < .0001) compared with controls (4.47 mg) and decreased urinary potassium (mean 24-hour excretion: 4.0%, 12.05 mg, P < .001; 6.6%, 6.68 mg, P < .0001; vs controls, 20.38 mg). The potassium-binding capacity of RDX7675 (stool potassium/gram of resin: 4.0%, 1.14 mEq/g; 6.6%, 1.32 mEq/g) was greater (all P < .0001) than for patiromer (4.0%, 0.63 mEq/g; 6.6%, 0.48 mEq/g) or SPS (4.0%, 0.73 mEq/g; 6.6% 0.55 mEq/g). RDX7675 and patiromer decreased urinary sodium (mean 24-hour excretion: 0.07-1.38 mg; all P < .001) compared to controls (5.01 mg). In contrast, SPS increased urinary sodium excretion (4.0%, 13.31 mg; 6.6%, 17.60 mg; both P < .0001) compared to controls. CONCLUSIONS: RDX7675 reduced intestinal potassium absorption and had a greater potassium-binding capacity than patiromer or SPS in mice. The calcium-based resins RDX7675 and patiromer reduced intestinal sodium absorption, unlike sodium-based SPS. These results support further studies in humans to confirm the potential of RDX7675 for the treatment of patients with hyperkalemia.

An Evaluation of the Pharmacodynamics, Safety, and Tolerability of the Potassium Binder RDX7675.

Hyperkalemia is common in patients with heart failure or chronic kidney disease, particularly those taking renin-angiotensin-aldosterone system inhibitors, and can cause arrhythmias and sudden cardiac death. The most widely used treatment, sodium polystyrene sulfonate (SPS), limits gastrointestinal potassium absorption, but has poor palatability. RDX7675 (RDX227675) is the calcium salt of a reengineered polystyrene sulfonate-based resin with improved palatability over SPS. The pharmacodynamic effects and safety of RDX7675 were assessed in a phase 1, single-center, randomized, active-controlled study. Healthy volunteers received nominal active doses of RDX7675 4.6 g twice a day (BID), 4.6 g 3 times a day (TID), 6.9 g BID, 13.7 g daily (QD), 9.2 g TID, or 13.7 g BID (n = 12 each), or equivalent doses of SPS (n = 3 each), for 4 days. RDX7675 dosing increased stool potassium excretion and decreased urinary potassium excretion from baseline. Stool potassium excretion increased by up to 1481 mg/day with RDX7675 (6.9 g BID), and urinary potassium excretion decreased by up to 939 mg/day (13.7 g BID). Similar levels of potassium excretion were observed using QD, BID, or TID dosing of a 13.7 g total daily RDX7675 dose. Few adverse events were reported. In conclusion, repeated oral dosing with RDX7675 over 4 days reduced potassium absorption in healthy volunteers; the results support QD dosing of RDX7675 in future clinical studies.

Inhibition of sodium/hydrogen exchanger 3 in the gastrointestinal tract by tenapanor reduces paracellular phosphate permeability.

Hyperphosphatemia is common in patients with chronic kidney disease and is increasingly associated with poor clinical outcomes. Current management of hyperphosphatemia with dietary restriction and oral phosphate binders often proves inadequate. Tenapanor, a minimally absorbed, small-molecule inhibitor of the sodium/hydrogen exchanger isoform 3 (NHE3), acts locally in the gastrointestinal tract to inhibit sodium absorption. Because tenapanor also reduces intestinal phosphate absorption, it may have potential as a therapy for hyperphosphatemia. We investigated the mechanism by which tenapanor reduces gastrointestinal phosphate uptake, using in vivo studies in rodents and translational experiments on human small intestinal stem cell-derived enteroid monolayers to model ion transport physiology. We found that tenapanor produces its effect by modulating tight junctions, which increases transepithelial electrical resistance (TEER) and reduces permeability to phosphate, reducing paracellular phosphate absorption. NHE3-deficient monolayers mimicked the phosphate phenotype of tenapanor treatment, and tenapanor did not affect TEER or phosphate flux in the absence of NHE3. Tenapanor also prevents active transcellular phosphate absorption compensation by decreasing the expression of NaPi2b, the major active intestinal phosphate transporter. In healthy human volunteers, tenapanor (15 mg, given twice daily for 4 days) increased stool phosphorus and decreased urinary phosphorus excretion. We determined that tenapanor reduces intestinal phosphate absorption predominantly through reduction of passive paracellular phosphate flux, an effect mediated exclusively via on-target NHE3 inhibition.

Development and Characterization of a Human and Mouse Intestinal Epithelial Cell Monolayer Platform.

We describe the development and characterization of a mouse and human epithelial cell monolayer platform of the small and large intestines, with a broad range of potential applications including the discovery and development of minimally systemic drug candidates. Culture conditions for each intestinal segment were optimized by correlating monolayer global gene expression with the corresponding tissue segment. The monolayers polarized, formed tight junctions, and contained a diversity of intestinal epithelial cell lineages. Ion transport phenotypes of monolayers from the proximal and distal colon and small intestine matched the known and unique physiology of these intestinal segments. The cultures secreted serotonin, GLP-1, and FGF19 and upregulated the epithelial sodium channel in response to known biologically active agents, suggesting intact secretory and absorptive functions. A screen of over 2,000 pharmacologically active compounds for inhibition of potassium ion transport in the mouse distal colon cultures led to the identification of a tool compound.

Quinazolinone fungal efflux pump inhibitors. Part 3: (N-methyl)piperazine variants and pharmacokinetic optimization.

Further structure-activity relationships of a novel series of fungal efflux pump inhibitors with respect to potentiation of the activity of fluconazole against strains of C. albicans and C. glabrata over-expressing ABC-type efflux pumps are systematically explored. Rat protein binding and pharmacokinetics of selected analogues are reported.