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Monday, June 30, 2008

More on rivaroxaban trials

Recently, I referenced two clinical trials published in the NEJM which compared a new oral anticoagulant,rivaroxaban, with the low molecular weight heparin enoxaparin for thromboprophylaxis in patients undergoing hip arthroplasty and knee replacement surgery.Both studies showed an advantage for rivaroxaban over the comparison drug in terms of fewer thrombotic events and showed no real difference in bleeding.

Dr. RW
pointed out that the dose of enoxaparin was lower than the standard recommended dose,that is the dose recommended in the United States. In both studies, the dose of enoxaparin was 40 mg. daily.This is in contrast to the 30 mg. every 12 hours which is the standard dose used. Could the use of the lower dose of the drug to which rivaroxaban was compared be an instance of "stacking the deck"? In the hip surgery study, there were 33 patients out of 1678 with major venous thromboembolism events versus 4 in 1686 patients in the rivaroxaban arm seemingly indicating the superiority of "riva".

Would the higher enoxaparin dose that is typically given resulted in fewer cases making the comparison less favorable to rivaroxaban? Maybe, but in a 1994 dose finding study the outcomes with 40 mg once daily and 30 mg twice daily were not statistically different.There were 27/199 (14%) patients with clots in the 40 mg dose and 22/208 (11%) in the 30 mg twice a day dose, a difference not statistically significant. Admitedly this was not a mega trial so a false negative could have occurred but the 40 mg dose has been used in other trials.The PDR gives the 30 mg. twice daily dose as the recommended dose but also suggests that in hip surgery patients the 40 mg regimen is reasonable alternative.After all of that, it seems to me that the 40 mg. used is not a unreasonable dose and has been used in other trials with enoxaparin .

In regard to the knee surgery study, the authors stated that the enoxaparin dose used was the dose approved in Europe and not the 30 mg twice daily dose approved in the U.S. for that application. Further, they started that a trail is currently in progress with the 30 mg. twice daily dose.

With those considerations in mind my speculation or veiled accusation that the deck was stacked has little evidential support.It may be best to shoot after aiming.

In a thoughtful followup to his first posting Dr RW makes the important fact that you need to look at the facts of how the trial was designed and executed rather than basing any decision regarding the validity of the trial by simply looking at who paid for it. With some of the drug trials scandals (the Ketex studies come to mind)and the possibility that some data has been withheld in some trials I have become so jaded and distrustful that my objectivity titer has diminished and his suggestion to look at trial critically and then look at the funding makes sense.

1 comment:

Anonymous said...

Good points about funding -- one has to trust the integrity of the authors. They do not attept to hide the sources of funding. I agree also that a close reading (especially Intent to Treat, exclusion criteria, and the associated statistics) will let readers evaluate the quality of the report.