By Puyaan Singh

(Reuters) -KalVista Pharmaceuticals ( KALV ) said on Monday the U.S. Food and Drug Administration has approved its drug, the first on-demand oral treatment for a type of hereditary swelling disorder, sending its shares up more than 17% in premarket trading.

The regulator's decision follows an extended review of the drug last month due to heavy workloads and limited resources.

Ekterly, which offers a convenient alternative to injectable treatments, will be launched immediately, the company said.

KalVista CEO Ben Palleiko said patients find it "really burdensome" to carry injectable treatments, which are "almost twice the size of an iPhone," while Ekterly is packaged "in a little cardboard wallet, about the size of a credit card."

Other on-demand treatments for hereditary angioedema, which affects about 8,000 people in the United States, include CSL's Berinert, Takeda's Cinryze, and Pharming's Ruconest, which are administered intravenously, as well as injectables such as Takeda's Kalbitor and Firazyr.

Needham analyst Serge Belanger said KalVista previously indicated it would likely price Ekterly at a slight premium to Firazyr, the most commonly used on-demand treatment, which costs $11,000 per unit.

The life-threatening condition causes sudden, dangerous swelling in the body, including the skin, digestive tract and upper respiratory system, due to deficiency in a protein known as C1 inhibitor.

Citizens Bank analyst Jonathan Wolleben estimates peak U.S. sales of Ekterly at around $250 million, adding the early launch could exceed expectations due to potential premium pricing and rapid adoption.

Approval of the drug was based on a trial in which Ekterly achieved the beginning of symptom relief in 1.6 hours, while a later trial showed the drug could treat attacks in 10 minutes, the company said.