UPDATED: AstraZeneca partner Ardelyx plunges after citing safety issue for lead drug

Four months after getting a promising readout on its lead drug in a Phase IIb study for irritable bowel syndrome, AstraZeneca ($AZN)-partnered Ardelyx ($ARDX) reported another study win today, but investors zeroed in on a disturbing safety issue and ripped up the company's shares soon after the release went out. Tenapanor hit the primary endpoint in its Phase IIb trial in hyperphosphatemic patients with chronic kidney disease on hemodialysis.

It wasn't all clear sailing in the study, though. Fremont, CA-based Ardelyx also reported that investigators tracked an unexpected and troubling spike in the rate of diarrhea patients suffered in the study. And the biotech's shares swiftly plunged by about 30% on the red flag.

Tenapanor is an NHE3 sodium transport inhibitor program which is designed to force the body to flush sodium through feces rather than urine, sparing kidneys and helping patients with IBS, end-stage renal disease and chronic kidney disease. NHE3 is a protein needed to absorb sodium in the gut. AstraZeneca signed a $272 million partnership deal on the drug back in 2012 as it launched a frenzy of dealmaking aimed at repairing a damaged pipeline.

"We are encouraged by tenapanor's ability to significantly decrease elevated serum phosphate levels, although we did not anticipate the high incidence of diarrhea," said Ardelyx CEO Mike Raab about the top-line results. "We look forward to working with our partner, AstraZeneca, to evaluate the future development plans for tenapanor in conjunction with the ongoing CKD trial and the positive Phase 2b results from IBS-C."

- here's the release