Investors and analysts have been pondering over the real value of AstraZeneca after Pfizer invited its board to enter into discussions to provide the US drugmaker with "a better understanding of its business" and fix a final takeover price.

In its sweetened offer on 2 May, Pfizer valued AstraZeneca at £63bn ($106bn, €77bn) or £50 per share, representing a 32% premium to the company's closing price of £37.82 on 17 April, the last trading date preceding the speculation of an offer.

But the UK company rejected the offer, saying it substantially undervalued AstraZeneca.

A number of analysts are of the view that Pfizer should increase the price taking into account the value of drugs in the pipeline.

Questor said AstraZeneca shares are worth more with three of its cancer drugs in the final stage of testing and another 20 at earlier stages of development.

AstraZeneca said its cure for lung cancer called MEDI-4736 could reach revenues of $6.5bn a year. If successful, MEDI-4736 would create a standalone company worth about £35bn or £28 per share.

Savvas Neophytou, analyst at Panmure Gordon, estimates that AstraZeneca shares could be worth up to £105 per share, based on the management's predictions.

In its latest attempt, Pfizer was trying to work with AstraZeneca to help deliver optimal deal terms and structure.

"The possible offer put forward by Pfizer on 2 May 2014 was based solely on public information. Engagement would provide the AstraZeneca management with the opportunity to provide Pfizer a better understanding of the business and its prospects, and the credible basis for their new long-range targets," the company said in a statement.

"Constructive engagement may lead to a transaction that AstraZeneca can recommend. Pfizer will continue to be disciplined on price," it added.

In a response, AstraZeneca said the announcement does not contain any new proposal or substantive information.

"The Board of AstraZeneca believes Pfizer is making an opportunistic attempt to acquire a transformed AstraZeneca, without reflecting the value of its exciting pipeline. This value should accrue fully to AstraZeneca shareholders," it said.

"The Board reiterates its confidence in AstraZeneca's ability to deliver on its prospects as an independent, science led business."