Agios Pharmaceuticals Inc. said its experimental drug for a rare blood disorder boosted production of a critical protein in a mid-stage trial.

Half of 18 patients with pyruvate kinase deficiency in the study saw a sustained increase in hemoglobin production after taking the drug, Agios said Saturday in a statement. The trial didn’t compare those taking the pill, called AG-348, with patients on placebo.

People with the genetic disorder have too few red blood cells because the cells die early. The disease, which often results in severe anemia, stems from the mutation of pyruvate kinase, or PK, an enzyme that helps store energy, and AG-348 is designed to activate PK. Agios is also working on a related experimental PK activator called AG-519. When tested in healthy volunteers, AG-519 caused no serious side effects, but one patient developed low blood platelets, Agios said Thursday, sending the shares down 15 percent on that day.

The AG-348 drug was generally safe, Chief Executive Officer David Schenkein said by phone. The nine patients who responded to the treatment saw their hemoglobin rise significantly within about two weeks, according to the statement. There was no correlation between increases in hemoglobin and ATP, a molecule that cells use to store energy.

“There is no chance that any of these increases to this extent could be spontaneous,” Schenkein said. The company will move either AG-348 or AG-519 into larger final-stage trials soon, the CEO added.

One patient on AG-348 had a severe rise in triglycerides, a type of fat in the blood, that may have been connected to the treatment, Schenkein said, though no one stopped taking the pills early. One patient with a prior bone condition developed osteoporosis, which may or may not have been related to the drug, he said.

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