(updates with analyst comments, background)
By Jana Mlcochova
PRAGUE, Oct 1 (Reuters) - A private shareholder has raised its stake in Czech drug maker Zentiva <
>, challenging French suitor Sanofi-Aventis <SASY.PA> which is bidding to take over the Czech generics firm.Belviport Trading Ltd., a closely-held firm whose owners remain anonymous, raised its holding in Zentiva to 10.06 percent from 6.08 percent, a regulatory filing in Netherlands, where Zentiva is registered, showed on Wednesday.
The announcement that Belviport is building up its stake came a day after Zentiva's second-largest shareholder, financial group PPF and its insurance joint venture with Italy's Generali <GASI.MI>, raised its stake to 21.59 percent from 19.23 percent.
Patented drugsmaker Sanofi, Zentiva's largest shareholder with 24.9 percent, is bidding to gain a controlling stake in the company through a 1,150-crowns-per-share bid that values Zentiva at 43.86 billion crowns ($2.53 billion).
The acquisition would help Sanofi gain a stronger foothold in the field of unpatented medicines, an area previously shunned by large pharmaceutical companies, which is now receiving attention as a way to tap booming emerging markets.
Zentiva, which focuses on cardiovascular, nervous system and pain treatments, is the dominant Czech drug firm and also has factories in Romania and Turkey.
The Sanofi bid will only succeed if the firm raises its stake above 50 percent. Analysts said this could get tough due to the hoarding of stock by Belviport and other shareholders.
"Clearly Sanofi is squeezed into a position in which it will have a problem to gain 50 percent," said Petr Bartek, an analyst at Ceska Sporitelna.
Milan Vanicek, an analyst at Atlantik FT, said Sanofi, which makes a wide range of treatments including blood-thinner Plavix and anti-thrombotic drug Lovenox, may be forced to raise the offer again to succeed.
"I can only speculate, but they are financial groups and a target of any financial group is to make a quick profit, which they can manage by indirectly making Sanofi raise the offer, among other ways," he said.
Another big shareholder in Zentiva is Czech-Slovak financial group J&T's unit Frevent Holdings, which holds a 7.6 percent stake.
PPF, Belviport and J&T have yet to declare their intentions with regard to their respective stakes.
Zentiva's management, which controls 5.9 percent, is backing the Sanofi bid and has pledged to sell its shares in the offering.
PPF RESURFACING
PPF started a bidding war over Zentiva in May, when it said it would bid 950 crowns per share.
It withdrew the offer after Sanofi counterbid 1,050 crowns a share. Sanofi later raised its bid to 1,150 crowns and extended the offer period until Nov. 28.
Sanofi's higher bid looked like it would be successful, but the recent purchases of the other shareholders has raised new question marks.
The Sanofi bid has underpinned the stock at a time when stock markets plunged globally amid the spreading credit crisis. It has been trading just below the bid price at 1,130 on Wednesday.
A Sanofi spokesman said the French company has noticed changes in the shareholdings but had no comment on either Belviport or PPF building up their stakes. (Additional reporting by Jan Lopatka and Jan Korselt; editing by Simon Jessop)