An offer from Amgen Inc. to buy out Onyx Pharmaceuticals Inc. may have helped traders with inside knowledge make $4.8 million in illegal profits, the Securities and Exchange Commission has alleged. Onyx, a South San Francisco cancer drug developer, announced on June 30 it had received a buyout offer from the Thousand Oaks biopharmaceutical giant of $120 a share, prompting shares of Onyx to spike more than 50 percent. But the Onyx board rejected the offer, saying the price significantly undervalued the company, even though it was a 38 percent premium over the previous day’s closing price. The SEC alleged that in the three trading days before the announcement, unnamed traders operating through brokerage accounts in the Canary Islands and Lebanon bought call options that allowed them to buy the stock at a lower price and resell it after the spike. In court filings, the SEC called the timing and volume of the call purchases “highly suspicious.” The SEC’s Los Angeles and New York offices worked together last week to obtain an emergency court order to freeze the accounts. “This action demonstrates that the SEC will not hesitate to freeze the assets of suspicious foreign traders when the timing and size of their trades indicate that they were misusing inside information,” said Michele Wein Layne, director of the SEC’s Los Angeles office, said in a statement. Shares of Onyx closed down 23 cents, or a fraction of a percent, to $135.80 on Monday. Shares of Amgen closed up 53 cents, or half a percent, to $98.17. Both issues trade on the Nasdaq.