** Drug developer Septerna's shares SEPN.O fall 66.6% to $4.49 premarket

** Co discontinues an early-stage trial of its experimental drug for patients with a hormonal disorder called hypoparathyroidism

** In hypoparathyroidism patients, the body does not produce enough levels of a hormone that regulates calcium and phosphorus levels in the blood

** SEPN says it stopped the trial after two participants showed unexpectedly high levels of a substance called unconjugated bilirubin, which forms when red blood cells break down

** Says it will advance other drugs with distinct and unrelated chemical structures relative to the discontinued treatment

** Stock has fallen nearly 40% since its Nasdaq debut in October

(Reporting by Padmanabhan Ananthan)

((Padmanabhan.Ananthan@thomsonreuters.com;))