** Shares of drug developer Astria Therapeutics ATXS.O up 38% at $11.39 premarket
** BioCryst Pharmaceuticals BCRX.O says it will buy Astria for $700 million in cash-and-stock deal
** The offer implies a per-share value of $13, a premium of about 54% over Astria's last close
** The deal adds experimental injectable drug navenibart, which is in late-stage testing for hereditary angioedema (HAE), to BioCryst's portfolio
** HAE is a rare genetic condition that causes repeated episodes of severe swelling in areas like the limbs, face, gut, and airway, says U.S. FDA
** Shares of BioCryst fall nearly 7% to $6.55
** Up to last close, BCRX stock down ~6%, ATXS stock down ~5% YTD
(Reporting by Sahil Pandey)
((Sahil.Pandey@thomsonreuters.com))