GlaxoSmithKline’s (GSK) subsidiary Adriatic Acquisition Corporation (AAC) has extended the expiration date for its tender offer for US-based Tesaro.

The deal, worth $5.1 billion, will expire on the 18th January, four days on from the originally agreed expiration date.

The extension allows for the expiration of the one month statutory review period contemplated by the German Act against Restraints of Competition, which is scheduled to expire on January 18, 2019.

It also allows for the expiration of the four week review period contemplated by the Austrian Cartel Act, which is scheduled to expire on January 15, 2019.

GSK and AAC expect the tender offer to be ‘consummated promptly’ following the expiration date, subject to the satisfaction of the remaining conditions to the consummation of the tender offer.

The companies announced the agreement back in December 2018, stating that GSK was to acquire Tesaro, to significantly strengthen GSK’s pharmaceutical business, and accelerate the build of GSK’s pipeline and commercial capability in oncology.

As of Monday, 46.7 million Tesaro shares have been tendered, representing about 77% of the company's issued share capital.

Shares in GlaxoSmithKline were down 0.3% Tuesday at 1,486.00 pence each.