PCS makes McKesson possible takeover target. (McKesson Corp.'s PCS Health Services Inc.)

SAN FRANCISCO - With the Profit margins of third-party prescription plan administrators exploding over the past few years, one of the most widely discussed topics on Wall Street recently has been the possibility of McKesson Corp.'s PCS Health Services Inc. (the nation's largest third-party provider) making the parent company the target of a takeover bid by a drug maker.

Since the announcement of the $6 billion merger of Merck & Co. and Medco Containment Services Inc. last summer, investors have steadily driven up the price of McKesson stock from $46 to $67 on the theory that another drug maker would follow Merck's example and acquire its own customer channel. Indeed, in the aftermath …