NEW YORK (AP) ¿ Shares of Cytokinetics Inc. plunged Thursday after the biotechnology company said development of a potential heart failure drug could take a year longer than previously anticipated. The stock slid 76 cents, or 17.8 percent, to $3.51 in afternoon trading. Shares have traded between $1.38 and $5.55 over the last 52 weeks. Late Wednesday, the company said it agreed with partner Amgen Inc. on conducting additional mid-to-late-stage studies on omecamtiv mecarbil for heart failure and renal dysfunction patients in 2011. Lazard Capital Markets analyst Joel Sendek reaffirmed a "Buy" rating on Cytokinetics, but said the delayed timeline could affect milestone payments to Cytokinetics. Cytokinetics also said its third-quarter loss narrowed on more revenue from collaborations. The company recorded a loss of $8.2 million, or 14 cents per share, compared with a loss of $16.3 million, or 33 cents per share, during the same period a year prior. Revenue rose 76 percent to $5.5 million from $3.1 million. Analysts polled by Thomson Reuters expected a loss of 22 cents per share on revenue of $2.9 million.