In response to a stark stock decline, US biotechnology firm Medarex has leapt to the defence of its investigational melanoma compound ipilimumab which belongs to the same class of drugs as tremelimumab, development of which has been suddenly halted by Pfizer.

Earlier this week, the New York-based drugs giant announced the discontinuation of a Phase III trial of tremelimumab in patients with advanced melanoma after the review of interim data showed that the drug would not demonstrate superiority to standard chemotherapy. This led investors to fear that ipilimumab, which is partnered with Bristol-Myers Squibb, could suffer a similar fate and following Pfizer’s announcement, Medarex shares fell over 18%.

This has prompted the company to speak out and Geoffrey Nichol, senior vice president of product development of Medarex, acknowledged that while tremelimumab and ipilimumab have a similar mechanism of action, “it is natural to attempt to draw parallels between the two molecules”. However, he added, “we feel we need to make certain clarifications”.

Firstly, Dr Nicol said that the molecules are of different IgG antibody subclasses, administered at
different doses and dosing schedules, and with different pharmacokinetic parameters. Second, while results from the Phase II study under special protocol assessment did not meet the primary endpoint, “the three studies in our Phase II programme were suggestive of ipilimumab's potential for clinical anti-tumour activity” based on the totality of the data, “and are under discussion with regulatory agencies”, he added.

Finally, he noted that Medarex's ongoing ipilimumab Phase III programme for front-line treatment of advanced melanoma “is different in design from the Pfizer trial, and at this time, it is too early to draw any clinical conclusions from the Pfizer announcement”. In addition, Dr Nicol said that the aforementioned late-stage trial had been recently reviewed by a data monitoring committee which indicated that the study should continue.

He concluded by saying that Medarex and B-MS “continue to move firmly forward” with ipilimumab, but analysts are not convinced. Banc of America issued a note saying that it is maintaining a neutral rating on the stock but it has lowered its price target to $8 from $10.

Medarex plans to meet with the US Food and Drug Administration in the first half of 2008 to discuss the potential biologics license application filing for MDX-010 in second-line melanoma, with the goal of filing around mid-year, the broker noted. "We continue to believe the data package will be insufficient for approval," it added.