US-based clinical-stage biopharmaceutical firm Ardelyx has received a $25m milestone payment from AstraZeneca for the start of a Phase IIb clinical trial evaluating tenapanor to treat hyperphosphatemia, or elevated serum phosphorus, in patients with end-stage renal disease on hemodialysis (ESRD-HD).

Also called AZD1722 and RDX5791, tenapanor is a small-molecule, orally administered, non-systemic inhibitor of the NHE3 sodium transporter being studied to treat patients with ESRD-HD and chronic kidney disease (CKD), as well as for constipation-predominant irritable bowel syndrome (IBS-C).

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So far, Ardelyx has received a total of $75m in upfront and milestone payments from AstraZeneca under an exclusive licence agreement signed in October 2012.

"We believe that tenapanor could be a paradigm shift for the treatment of hyperphosphatemia in patients with ESRD if the drug is successfully developed and commercialised."

Around 150 patients will be enrolled in the trial, which will evaluate the efficacy, safety and tolerability of tenapanor for the treatment of hyperphosphatemia.

Ardelyx president and chief executive officer Mike Raab said AstraZeneca’s on its first study specifically designed to evaluate the ability of tenapanor to reduce hyperphosphatemia in ESRD patients, which remains one of the key challenges in the management of patients on dialysis.

"We believe that tenapanor, as a small molecule inhibitor of phosphate absorption with a significantly reduced pill burden, could be a paradigm shift for the treatment of hyperphosphatemia in patients with ESRD if tenapanor is successfully developed and commercialised," Raab said.

The trial is part of a broad development programme established pursuant to the company’s licence deal with AstraZeneca, which it paid Ardelyx $35m upfront, and could pay up to $237.5m in total potential development milestones, as well as milestones related to launch and commercialisation.

The randomised, double-blind, placebo-controlled, multicentre, dose-finding trial is being conducted in the US, UK, Poland and Slovakia with results expected in the first half of 2015.

So far, Ardelyx and AstraZeneca have evaluated tenapanor in eight human clinical trials in more than 750 people.

In Phase I and Phase II clinical trials, tenapanor was well-tolerated and has shown the ability to divert sodium into the stool in both healthy adult subjects and patients with ESRD-HD.