The Irish-domiciled company offered $45.23 per share for the group, a 36 per cent premium to the undisturbed share price on 3 August.
Baxalta, a biotech business focused on developing treatments for rare diseases, was spun-out of Baxter International last month.
Shire originally approached the business with an acquisition proposal last month, but on 31 July Baxalta refused to engage in talks with the Dublin-based company, sparking the hostile bid. Shire urged its rival to “engage in a negotiated transaction”, adding that a deal would result in immediate gains for Baxalta shareholders.
Under Shire’s calculation, the firms will deliver $20bn worth of product sales over the next five years between them.
However, Baxalta late last night reaffirmed its objection to the bid, saying that it “significantly undervalues Baxalta and its attractive prospects for growth and value creation”.
Shire’s approach comes in the wake of its collapsed $55bn deal with US giant AbbVie, which would have seen the firm merged into the US firm.
The deal, one of a spate of mooted pharma deals including a proposed Pfizer takeover of Astrazeneca, hit the skids after the US government signalled it would remove alluring tax benefits on offer for US companies buying overseas rivals.