Icagen is now a wholly-owned subsidiary of Pfizer. Icagen's stock ceased trading on NASDAQ on Friday.
Icagen and Pfizer announced the $56 million deal in July, but some large investors complained the $6 share price was too low.
Pfizer was able to convince enough shareholders to go along with the deal after extending the deadline twice.
Icagen was founded in 1992 by P. Kay Wagoner, a former GlaxoSmithKline Plc research executive who focused on developing new ways to treat pain.
None of the company's drugs have been approved by the U.S. Food and Drug Administration for sale in the U.S. market.