* Rome prosecutors open initial probe - judicial sources
* No-one officially under investigation - sources
* Watchdog fined firms for collusion over cancer drug use
* Novartis, Roche deny watchdog accusations, will appeal (Recasts lead, adds source comments, background)
ROME, March 6 (Reuters) - Italian prosecutors looking into allegations of collusion between Roche and Novartis over a cancer drug suspect the firms of possible fraud and manipulating the drugs market, judicial sources said on Thursday.
Prosecutors opened a preliminary probe after Italy's competition watchdog fined the two Swiss drugmakers for colluding to try to stop cancer drug Avastin being used to treat a serious eye disease, sources said on Wednesday.
The watchdog said the companies had sought to prevent distribution of Roche's Avastin as a treatment for wet age-related macular degeneration (AMD) in favour of the more expensive drug Lucentis which is made by Novartis.
The judicial sources said on Thursday that prosecutors suspected the companies of working together on campaigns to favour Lucentis, including promoting it to local doctors.
Novartis and Roche strongly denied the regulator's allegations on Wednesday and said they would appeal against their fines of 92 million euros ($126.4 million) and 90.5 million euros respectively.
Asked on Thursday about the prosecutors' move, a Novartis spokesman said: "Novartis cannot speculate on potential actions of the Italian prosecutors in Rome. We have not been contacted by the prosecutor's office." Roche referred to its earlier denial.
At this stage no-one is formally under investigation, the sources said.
The regulator estimated that the alleged collusion had cost the Italian health service an additional 45 million euros in 2012 and the figure could possibly exceed an annual 600 million euros after that. ($1 = 0.7278 euros) (Reporting by Mario Sarzanini, additional reporting by Caroline Copley; Writing by Stephen Jewkes; Editing by Erica Billingham)