* Novartis mulls asset disposals of some $50 billion

* Large transaction could fill cancer portfolio holes

* Investors wary of big transaction after Alcon woes

By John Miller

ZURICH, May 30 (Reuters) - As Novartis considers asset sales that could raise $50 billion, investors are worried any cash raised may give the Swiss drugmaker firepower for another unsuccessful megadeal.

Novartis's $52 billion takeover of U.S.-based eye care giant Alcon, completed in 2011, saddled it with a business whose sales and profit have faltered two years running.

Now, Chief Executive Joe Jimenez is reviewing Alcon's surgical devices and contact lens businesses, suggesting they could be valued at $25-$35 billion if he unloads them.

The American CEO is also considering disposal of a roughly $14 billion stake in cross-town rival Roche, as well as his over-the-counter (OTC) drugs venture with GlaxoSmithKline , worth some $10 billion.

Given Alcon missteps, however, investors are wary about arming Novartis with a pile of cash, for fear managers eager to refocus on cancer drugs as they address a sales hit from patent expiries might blunder into a big takeover.

"We would applaud selling those stakes, generally," said Stephen Anness of Invesco Perpetual, Novartis's 23rd largest shareholder, according to Thomson Reuters data.

"But what do you do with that money?" Anness said. "I would be very cautious about selling stakes...in things to raise a war-chest to go and do a massive deal, only for that deal to go and be another poor deal."

To be sure, Jimenez has said Novartis's M&A focus remains on smaller transactions, including lower-risk drug licensing deals, ranging up to $5 billion.

Still, Jimenez has not dismissed the notion of a larger transaction. He suggested last year the Roche stake - amassed during former chairman and CEO Daniel Vasella's unrequited merger aspirations two decades ago - could be sold once another, potentially more significant transaction is lined up to absorb the proceeds.

"We’re always monitoring what’s going on but have not changed our position regarding our M&A strategy or potential disposals," Novartis spokesman Michael Willi told Reuters.

PORTFOLIO HOLES

Novartis, which is holding a two-day investor event in Boston on Tuesday and Wednesday, has portfolio holes a major deal could help fill.

Where rivals including Roche, Merck and Bristol-Myers Squibb have immuno-oncology drugs (I-O) on the market for a range of cancers, Novartis has only investigational molecules in this hot new therapy area.

Vas Narasimhan, Novartis's drug development chief, could be tempted to look outside the company, some analysts said, especially as competitors including AstraZeneca near approval for their own I-O molecules.