The film will stars Ranbir Kapoor in the titular role.
Gossip mills are abuzz that Deepika Padukone will be a part of the film too.
"We have not finalized anyone. Only the script is complete apart from Ranbir on board. Only this much is finalized. Once "Saala Khadoos" releases on January 29, then I will work on it," Hirani told reporters here at an event.
Dutt, sentenced to five-year imprisonment after he was convicted for possessing an illegal weapon, part of the cache meant to be used during the 1993 Mumbai serial blasts, is set to be released next month.
Besides the Dutt biopic, Hirani is also working on the third installment of his superhit "Munnabhai" series.
When asked about the status of Dutt biopic and "Munnabhai" series, Hirani says, "Not in touch with him (Sanjay Dutt) for long time as I was busy with "Saala Khadoos" and its promotions. I have heard he will come out at the end of next month. Once he is out then we will take it ahead."