NEW YORK (AP) ¿ Cytokinetics Inc. said Wednesday it ended a deal with GlaxoSmithKline PLC that had focused on developing a potential cancer treatment, sending shares sharply lower in after-hours trading. Cytokinetics said, effective Feb. 28, all rights for the potential cancer treatment GSK-923295 will revert back to the biotechnology company. GlaxoSmithKline will still be responsible for completing an ongoing Phase I clinical trial. The decision is part of Cytokinetics' plan to focus its internal research and development efforts on muscle function and related therapies, the company said. "In our industry, and especially in these challenging times, it is increasingly important to remain focused and execute on a core business strategy," said President and CEO Robert I. Blum, in a statement. "At Cytokinetics, we believe our best opportunities are rooted in our multiple programs directed to the biology of muscle function," he said. "Today's announcement, combined with our company's previously disclosed decisions to discontinue oncology research and phase out related development activities and spending, is further evidence of our commitment to this strategy." Shares of Cytokinetics fell 33 cents, or 10.5 percent, to $2.80 in aftermarket electronic trading. The stock had gained 7 cents to close the regular session at $3.13. Shares of GlaxoSmithKline fell 46 cents to close the regular trading session at $41.59.