Donev acquired the shares through a deal on the Bulgarian Stock Exchange on September 9, Sopharma said in a bourse filing.
Donev has been actively acquiring small packages of Sopharma shares since the start of September.
At the end of June, Donev held a stake of below 5% in Sopharma, according to the company's most recent interim financial statement.
However, Ognyan Donev is indirectly the biggest shareholder in Sopharma, as he controls a 25.39% stake through his company Donev Investments Holding. Donev is also a major shareholder in Sopharma's second biggest shareholder - Telecomplect Invest. Telecomplect Invest controls 20.68% interest in Sopharma.
As at 12:15 CET on Tuesday, Sopharma shares traded at 3.34 levs, unchanged from their previous closing price on the Bulgarian Stock Exchange.
(1 euro = 1.95583 levs)