GlaxoSmithKline’s fortunes have taken a dip with the news that US regulators want more information before it will approve the firm’s cervical cancer vaccine Cervarix, which means that it is falling further behind rival Merck & Co’s Gardasil.

GSK says it has received a complete response letter from the US Food and Drug Administration relating to Cervarix, which means that the review of a file is completed and “questions remain to be answered prior to approval. The UK drugmaker declined to offer any specific details about the questions raised by the agency. The application filed for the jab included safety, efficacy and immune response data from clinical trials involving almost 30,000 females aged 10 to 55 from ethnically and racially diverse populations.

Barbara Howe, director of North American vaccine development for GSK, said that "we have already started addressing the questions and will be engaged in discussions with the FDA to finalize our responses”. She added that "our discussions with the agency continue to be positive and constructive, and we are working diligently to resolve any outstanding questions to bring Cervarix to the US market.”

All very upbeat but the news is clearly disappointing. Analysts at Collins Stewart noted that once GSK has responded fully to these questions, it may take up to another six months to formally respond with a final decision through the FDA. This means that the Prescription Drug User Fee Act date for Cervarix, ie late January will result in an approvable letter being issued.

On a positive note, however, Navid Malik at Collins Stewart says that given the fact that GSK “has already started to address the questions raised by the FDA, a delay of only six months is more likely”. This means that approval for Cervarix could come through in the USA around the middle of 2008, “a delay of only a few months”.

Mr Malik noted that the same data set submitted to the FDA has generated approvals for Cervarix in Europe and Australia. Thus “we feel this vindicates the strength of the clinical data set for Cervarix and in-market experience with the product”.

Merck’s Gardasil is well-established in the US market but GSK is confident it will be able to build up market share for its own product. Mr Malik noted that the firm will also be in a position in the next year or so to present data from a study which is being conducted in women over 25 which should differentiate the product from Gardasil. There will also be data from a head-to-head trial which could be “a strong marketing tool should the study favour Cervarix, which we believe it will”.