** Shares of drug developer Sionna Therapeutics SION.O set to open 33% above their offer price in their Nasdaq debut

** Stock indicated to open at $24 vs $18 IPO price

** SION sold ~10.6 mln shares, 20% more than initially offered, at the top end of its marketed range of $16 to $18 apiece to raise $190.6 mln

** At the IPO price, the Waltham, Massachusetts-based firm fetched a valuation of $765.4 mln

** Founded in 2019 and previously known as Sling Therapeutics, SION is developing drugs to treat cystic fibrosis

** SION is vying for a slice of the cystic fibrosis market that is dominated by drugmaker Vertex Pharmaceuticals VRTX.O

** Vertex booked almost $10 billion in sales in 2023 and has significantly impacted cystic fibrosis treatment

** Prior to the IPO, SION had raised about $330 mln from investors, including RA Capital, TPG's The Rise Fund, Atlas Venture, OrbiMed and Enavate Sciences

** Goldman Sachs, TD Cowen, Stifel and Guggenheim Securities were the underwriters

(Reporting by Arasu Kannagi Basil in Bengaluru)

((ArasuKannagi.Basil@thomsonreuters.com;))