Three rejections and still trying. That sums up U.S. drugmaker AbbVie's pursuit of the London listed bio-pharmaceutical giant, Shire. After AbbVie's latest unsuccessful $46 billion bid to gain control of Shire, AbbVie upped it's offer to $51 billion. Shire CEO Flemming Ornskov previously said that he was not against a buyout, but needed the right price. So is $51 billion the right price? In an interview in London on Wednesday after the new offer was revealed, Ornskov said he is "not entrenched." He said, "I would imagine that if you have a premium company, you would expect -- without commenting on any expectation of price -- you would also expect a premium." Shire manufactures a number of well-known drugs. Its star drug is Vyvanse, an amphetamine to treat attention deficit hyperactivity disorder, or ADHD. A U.S. court recently ruled in Shire's favor to block the generic version of Vyvanse until its patents expire in 2023. Doctors prescribe $1.2 billion worth of Vyvanse to treat ADHD every year. AbbVie wants to access Vyvanse's stable growth and reach more types of patients. Currently, AbbVie is very depended on the rheumatoid arthritis drug Humira. Acquiring Shire could mean a lower tax rate for AbbVie since Shire its headquartered outside of the U.S.