Shares of Iovance Biotherapeutics Inc. /zigman2/quotes/206632254/composite IOVA +3.76% tumbled about 40% in after-hours trading on Thursday. The company had announced new clinical data for its experimental melanoma drug, including a 29% objective response rate in one cohort of patients. In that cohort, there were three complete responses and 22 partial responses, and the median duration of response was 10.4 months. Iovance also reiterated plans to file a biologics application to the Food and Drug Administration in August. The company's stock is down 20.8% this year, while the S&P 500 /zigman2/quotes/210599714/realtime SPX -0.27% has declined 16.5%.