US regulators have told AstraZeneca and Abbott Laboratories that they need more information before they can approve the firm’s investigational drug for mixed dyslipidaemia, Certriad.

The US Food and Drug Administration has issued a complete response letter for the New Drug Application for Certriad, which combines AstraZeneca's blockbuster Crestor (rosuvastatin) with TriLipix (fenofibric acid). The drugmakers provided no details about what the agency wants, merely stating that they are currently evaluating the letter and will continue discussions with the FDA to determine the next steps for the NDA.

The lack of comment from AstraZeneca and Abbott has unsurprisingly led to speculation that the FDA may be asking for more clinical trials. Leerink Swann analyst Rick Wise issued a research note saying that the companies’ joint press release “contains very little information, which could prompt investors to assume a worst-case scenario in which the FDA might require more long-term data and could push approval out by more than 12 months”. He added that “at the very least”, the agency’s letter will likely delay approval beyond a mid-2010 target.

Other observers believe that before it makes a decision, the FDA is waiting to complete a review it is undertaking of the ACCORD trial, which investigated the addition of fenofibrate to statin therapy. Findings from the study were unveiled at the American College of Cardiology meeting in Atlanta earlier this month and showed no reduction in cardiovascular events when a fenofibrate was given in addition to a statin therapy.

However, it should be noted that the drugs used in ACCORD were Abbott’s TriCor (fenofibrate) and Merck & Co’s now off-patent Zocor (simvastatin), not the treatments that make up Certriad. Furthermore, while the primary endpoint of the study was not met, in a subgroup of patients with high triglycerides and low HDL (the 'good' cholesterol), fenofibrate plus simvastatin was associated with a 31% lower rate of heart attack, stroke and cardiovascular death than Zocor alone.

Whatever the reason for the delay, AstraZeneca investors seem concerned and the Anglo-Swedish drugmaker’s shares ended down 1.3% at £29.42.