Centene Corporation shares tumbled 5.52% in pre-market trading on Tuesday, reversing sharp earlier gains after the company's CEO warned that year-end membership is expected to come in below its previous outlook through the back half of the year.

The sell-off erased an initial rally that had sent shares up as much as 3.7% after Centene reported second-quarter results that handily beat Wall Street expectations. The company posted adjusted earnings of $2.51 per share, well above the $1.08 consensus estimate, while revenue surged 10% to $53.58 billion, exceeding forecasts of $47.62 billion. Centene also raised its full-year adjusted earnings guidance floor to more than $4.80 per share and boosted its total revenue outlook.

However, investor sentiment soured after the conference call, where the CEO noted that the company expects lower year-end membership than previously anticipated. This commentary amplified concerns already present in the earnings report, which showed a 7.6% year-over-year decline in total at-risk membership to 25.89 million, driven by a significant drop in Commercial Marketplace enrollees and a 5.5% decline in Medicaid membership.