S&P 500 top movers: Vertex Pharmaceuticals Inc (VRTX: NASDAQ) shares down nearly 20%

• S&P 500 opened sharply lower but erased majority of daily losses.
• Vertex Pharmaceuticals Inc (VRTX: NASDAQ) shares are suffering heavy losses.
• Energy shares rebound following with Apache Corp (APA: NASDAQ) leading the winners.

The S&P 500 Index (SPX) opened deep in the negative territory on Thursday and touched its lowest level in a week at 3,440. With the market sentiment improving modestly in the late session, the SPX erased a large portion of its daily gains and was last seen losing 0.42% on the day at 3,473.

S&P 500 top movers

Vertex Pharmaceuticals Inc (VRTX: NASDAQ) announced that it will drop the mid-stage trial and development of its drug to treat Alpha-1 Antitrypsin Deficiency amid safety concerns. Following this development, the drug developer's shares fell sharply and VRTX was last seen losing nearly 20% on the day at $219.90.

On the other hand, energy stocks, which were the biggest decliners during the first half of the week, are staging a rebound on Thursday. Apache Corp (APA: NASDAQ) and Cimarex Energy Co (XEC: NYSE) shares are up 5.7% and 4.9%, respectively as the top performers of the day.