Roche is filing a formal complaint against the US firm’s “poison-pill” defense provision to issue new shares at half price to its shareholders if Roche buys 20% of the company.
In June, Roche made an unsolicited $3-billion bid at $75 per share in cash to acquire Ventana after efforts to negotiate a friendly merger were rebuffed.
Roche says it remains ready to negotiate an amicable agreement with Ventana.