You don’t see hostile bids undertaken by big drugmakers every day, but Roche is determined to build its diagnostic business. So now that Ventana Medical Systems rejected merger talk, the Swiss drugmaker says it will launch a tender offer for $75 a share in cash, which reps a 45 percent premium over Ventana’s $51.74 closing price before trading was halted this afternoon. As of a little while ago, the stock jumped $26.90, or 52 percent, to $78.64 in after-hours trading.

In their statement, Roche officials said they made several attempts to talk to Ventana about a deal, and they remain open to negotiating a friendly takeover. In any event, Roche also says the tender offer remains subject to Ventana pulling its “poison pill,” or shareholder rights plan, which would make an unsolicited acquisition costly and difficult.

Acquiring Ventana, which specializes in tissue-based diagnostics, would help Roche to diversify its diagnostic business. As with others pursuing personalized medicine, Ventana’s technology can help scientists and doc figure out which drugs may be best-suited for specific people.

Ventana employs around 950 people and had sales of $238.2 million in 2006, Reuters notes. Roche said the $1 billion tissue-based testing market is growing at 10 percent annually - or twice the rate of the overall in-vitro diagnostics market. Greenhill & Co. and Citigroup are servicing as financial advisers to Roche.