Mapping the time-dependent effects of paricalcitol on serum calcium, phosphorus and parathyroid hormone levels in 5/6 nephrectomized uremic rats

Preclinical studies of VS-505: a non-absorbable highly effective phosphate binder

BACKGROUND AND PURPOSE

Phosphate imbalance is often present in chronic kidney disease (CKD), and it contributes to a higher cardiovascular mortality rate. A phosphate binder is typically part of a treatment strategy for controlling phosphate imbalance. However, safety concerns and low compliance are two well-recognized disadvantages of on-market phosphate binders. This report describes the preclinical studies of VS-505, a non-absorbable, calcium- and aluminum-free, plant-derived polymer currently being evaluated in haemodialysis patients in Australia.

EXPERIMENTAL APPROACH

Normal Sprague Dawley (SD) rats or uraemic SD rats induced by 5/6 nephrectomy fed a high-phosphate diet were treated with VS-505 or sevelamer (0.05-10% in food) for 5 and 28 days respectively.

KEY RESULTS

Urinary and serum phosphate levels were significantly elevated in untreated rats, and were decreased by VS-505 and sevelamer. VS-505 increased faecal phosphate levels in a dose-dependent manner. High-phosphate diet also caused an increase in serum FGF-23 and parathyroid hormone in nephrectomized (NX) rats, effects prevented by VS-505 or sevelamer. Significant aortic calcification was observed in NX rats treated with 5% sevelamer, whereas VS-505 at all doses tested did not show effects. VS-505 had no effects on small intestine histomorphology and intestinal sodium-dependent phosphate cotransporter gene expression. In vitro characterizations showed that VS-505 has a relatively high density and low expansion volume when exposed to simulated gastric fluid.

CONCLUSIONS AND IMPLICATIONS

VS-505 is a safe and effective phosphate binder and may offer the advantage of having a reduced pill burden and minimal GI side effects for CKD patients.

Vitamin D receptor agonist VS-105 improves cardiac function in the presence of enalapril in 5/6 nephrectomized rats

Vitamin D receptor (VDR) agonists (VDRAs) are commonly used to manage hyperparathyroidism secondary to chronic kidney disease (CKD). Patients with CKD experience extremely high risks of cardiovascular morbidity and mortality. Clinical observations show that VDRA therapy may be associated with cardio-renal protective and survival benefits in patients with CKD. The 5/6 nephrectomized (NX) Sprague-Dawley rat with established uremia exhibits elevated serum parathyroid hormone (PTH), hypertension, and abnormal cardiac function. Treatment of 5/6 NX rats with VS-105, a novel VDRA (0.05 and 0.5 μg/kg po by gavage), once daily for 8 wk in the presence or absence of enalapril (30 mg/kg po via drinking water) effectively suppressed serum PTH without raising serum calcium. VS-105 alone reduced systolic blood pressure (from 174 ± 6 to 145 ± 9 mmHg, P < 0.05) as effectively as enalapril (from 174 ± 6 to 144 ± 7 mmHg, P < 0.05). VS-105 improved cardiac functional parameters such as E/A ratio, ejection fraction, and fractional shortening with or without enalapril. Enalapril or VS-105 alone significantly reduced left ventricular hypertrophy (LVH); VS-105 plus enalapril did not further reduce LVH. VS-105 significantly reduced both cardiac and renal fibrosis. The lack of hypercalcemic toxicity of VS-105 is due to its lack of effects on stimulating intestinal calcium transport and inducing the expression of intestinal calcium transporter genes such as Calb3 and TRPV6. These studies demonstrate that VS-105 is a novel VDRA that may provide cardiovascular benefits via VDR activation. Clinical studies are required to confirm the cardiovascular benefits of VS-105 in CKD.

VS-501: A NOVEL, NON-ABSORBED, CALCIUM- AND ALUMINUM-FREE, HIGHLY EFFECTIVE PHOSPHATE BINDER DERIVED FROM NATURAL PLANT POLYMER

Inadequate control of serum phosphate in chronic kidney disease can lead to pathologies of clinical importance. Effectiveness of on-market phosphate binders is limited by safety concerns and low compliance due to high pill size/burden and gastrointestinal discomfort. VS-501 is a non-absorbed, calcium- and aluminum-free, chemically-modified, plant-derived polymer. In vitro studies show that VS-501 has a high density and a low swell volume when exposed to simulated gastric fluid (vs. sevelamer). When male Sprague Dawley (SD) rats on normal diet were treated with VS-501 or sevelamer, serum phosphate was not significantly altered, but urinary phosphate levels decreased by >90%. VS-501 had no effect on serum calcium (Ca) or urinary Ca, while 3% sevelamer significantly increased serum and urine Ca. In 5/6 nephrectomized (NX) uremic SD rats on high-phosphate diet, increasing dietary phosphate led to an increase in serum and urine phosphate, which was prevented in rats treated with VS-501 or sevelamer (0.2-5% in food). High phosphate diet also increased serum FGF-23 and parathyroid hormone in 5/6 NX rats, which was prevented by VS-501 or sevelamer. VS-501 or sevelamer increased fecal phosphate in a dose-dependent manner. More aortic calcification was observed in 5/6 NX rats treated with 5% sevelamer, while VS-501 and sevelamer did not show significant effects on cardiac parameters, fibrosis, intestine histology and intestinal sodium-dependent phosphate cotransporter gene expression. These results suggest that VS-501 is effective in binding phosphate with no effects on calcium homeostasis, and may have improved pill burden and gastrointestinal side effects.

Synthesis of VS-105: A novel and potent vitamin D receptor agonist with reduced hypercalcemic effects

We have synthesized a novel vitamin D receptor agonist VS-105 ((1R,3R)-5-((E)-2-((3αS,7αS)-1-((R)-1-((S)-3-hydroxy-2,3-dimethylbutoxy)ethyl)-7α-methyldihydro-1H-inden-4(2H,5H,6H,7H,7αH)-ylidene)ethylidene)-2-methylenecyclohexane-1,3-diol). Preparation of a-ring phenylphosphine oxide 11, followed by Wittig-Horner coupling of 11 with the protected 25-hydroxy Grundmann's ketone 22 generated the precursor 12. Deprotection of the TBDMS groups of 12 produced the target compound VS-105. The biological profiles of VS-105 were evaluated using in vitro assays (VDR receptor binding, VDR reporter gene and HL-60 differentiation) in comparison to calcitriol (the endogenous hormone) or paricalcitol. Furthermore, the PTH suppressing potency and hypercalcemic side effects of VS-105 were evaluated in the 5/6 nephrectomized uremic rats in comparison to paricalcitol. Combining various changes at 20-epi, 22-oxa, 24-methyl, and 2-methylene yielded VS-105 that not only is highly potent in inducing functional responses in vitro, but also effectively suppresses PTH in a dose range that does not affect serum calcium in the 5/6 nephrectomized uremic rats.