US politicians have turned the spotlight onto Vytorin again and have asked Merck & Co and Schering-Plough to explain discrepancies among certain documents relating to a study of the controversial cholesterol drug.

House of Representatives Energy and Commerce Committee chairman John Dingell and Bart Stupak, chairman of the subcommittee on oversight and investigations, has written to the firms’ chief executives, saying that their probe into the withholding of clinical data regarding the ENHANCE trial of Vytorin – a combination of Zetia (ezetimibe) and Zocor (simvastatin).

The study concluded that generic Zocor was just as effective as Vytorin in reducing atherosclerotic plaque in the arteries of the neck, but the Committee’s investigation is focusing on why it took nearly two years to report the findings. The letter to Merck’s Richard Clark and S-P’s Fred Hassan lists 14 questions relating to “some unusual circumstances” surrounding the trial.

For example, the Committee wants to know why minutes from a November meeting of outside expert panels were "created after the fact" and after participants had been told no transcript would be produced. None were, until the US Food and Drug Administration later requested them, and then the minutes suggested that the experts recommended a change in the primary endpoint of ENHANCE, a point disputed by one of the experts, James Stein, a cardiac-imaging expert at the University of Wisconsin. The documents show that he complained about the minutes, saying that they did not reflect all the opinions expressed at the meeting.

Reps Dingell and Stupak also asked Merck and S-P to provide more information on the meeting as well as on marketing and other issues concerning Vytorin. This latest development is keeping the pressure on the two firms who have seen their shares slide dramatically over the negative findings of ENHANCE, coupled with their handling of the data.

The Committee’s letter concludes by saying that the investigation into ENHANCE “is far from concluded” and the firms have two weeks to respond to the questions.