Can an old drug be taught new pharmacogenetic tricks?

Create PDF

Elizabeth Silverman

Despite warfarin’s continued presence near the top of the FDA’s list of adverse drug events and the availability of competing agents, the drug continues to be a mainstay of anticoagulant therapy, particularly among general practitioners. Its narrow therapeutic window and intra- and interpatient variability require regular measurement of the international normalized ratio. This, along with the large genetic component to warfarin response, principally contributed by variants in the genes VKORC1 (–1639G ➞ A) and CYP2C9 (*2 and *3), led many to hypothesize that pharmacogenetics could improve warfarin safety.

The results of three clinical trials designed to test this hypothesis were published in the Dec. 12, 2013 New England Journal of Medicine (369[24]: 2283–2293;2294–2303;2304–2312), and their interpretation proved challenging. Choice of patient population, study design, statistical power, the economics of pharmacogenetic testing, and the problems of rare variants, particularly those that may be race based, were among the issues the trials raised and that will likely have an impact on future pharmacogenetic trials.

The U.S.-based COAG trial was a multicenter, double-blind randomized, controlled trial of 1,015 patients assigned to receive a warfarin dose based on a clinical algorithm or on a clinical algorithm that included genotype. After the first three days a dose revision algorithm was used, and after five days doses were adjusted using INR tests. The primary endpoint was the percentage of time the INR was in the therapeutic range from day four or five through day 28.

The second trial (EU-PACT) was conducted in the United Kingdom and Sweden. It enrolled 455 patients and was a single-blind, multicenter, randomized, controlled trial that compared a genotype-guided group with a group that received standard dosing of 10 mg on day one, 5 mg on day two, and 5 mg on day three. Doses thereafter were determined according to local clinical practice. The primary endpoint was percentage of time in the therapeutic range.

The third trial, a component of EU-PACT, did not use warfarin but two other vitamin K antagonists, acenocoumarol and phenprocoumon, that have longer half-lives than warfarin. The 548 patients in this study were from two single-blind randomized trials brought together because of low enrollment. The trial was similar to COAG in that patients in one arm received initial drug doses that were determined using a clinical algorithm that contained genotype information, while patients in the other arm received doses based on the clinical algorithm alone. The endpoint was percentage of time in the therapeutic range in the 12 weeks after initiation of therapy.

There are two other trials: a randomized, controlled trial conducted at the University of North Carolina (Jonas DE, et al. Pharmacogenomics. 2013;14[13]:1593–1603) and the still ongoing GIFT trial, the results of which are expected at the end of this year. The North Carolina trial studied 109 patients randomized to receive dosing determined by an algorithm containing genetic plus clinical information or only clinical information. Primary endpoints were time in the therapeutic range and number of anticoagulation visits over 90 days. Secondary endpoints were time to therapeutic dose, INR>4, emergency visits, hospitalizations, hemorrhagic events, thrombotic events, and mortality.

The GIFT trial is similar in design to COAG, but instead of an adjustment of dose on day four or five based on a dose refinement algorithm, GIFT uses the dose refinement algorithm through day 11. This trial includes an additional gene variant, CYP4F2, that affects the rate of vitamin K metabolism and is associated with warfarin resistance.

Contrary to hopes and expectations, the trials that compared use of a clinical algorithm alone to a clinical algorithm plus genotype showed no statistically significant difference between the two arms. However, in the COAG trial there was a difference between dosing strategy and race, with African American patients, who made up 27 percent of each arm, faring worse when genotyping was included. This was thought to be a reflection of the fact that variants specific to African Americans were not included in the algorithm. Though this trial was not powered to detect differences in bleeding or thromboembolic events, no difference between the control and experimental arms was observed in the four-week follow-up period.

These results were echoed by the similarly designed EU-PACT trial that used alternative vitamin K antagonists, with the exception of the African American component, as the European clinical trial subjects were more than 95 percent Caucasian. In the North Carolina study, genotype-guided dosing did not reduce the number of visits or improve time in the therapeutic range. However, the genotyping patients experienced fewer hospitalizations and hemorrhagic and thrombotic events and lower mortality, though the data were not statistically significant. It was only when genotyping was compared with standard dosing in the warfarin EU-PACT trial that a statistically significant difference was found that favored the use of genotyping. There were also fewer incidents of excessive bleeding in the genotype group, and the mean time to reach therapeutic INR was statistically significant at 21 days versus 29 days in the control group.

Taken together, the results of these studies do not appear to favor a role for pharmacogenetics in managing warfarin. However, Charles Eby, MD, a member of the COAG trial steering committee and professor of pathology and immunology and associate chief of the Division of Laboratory and Genomic Medicine, Washington University School of Medicine, says, “Comparing these two warfarin trials [COAG and EU-PACT], with a purely Caucasian population in Europe, pharmacogenetics dosing is superior to standard-of-practice dosing for time within therapeutic range at 30 days, and that standard-of-practice dosing performed just as well as the clinical algorithm dosing in the COAG trial.” At least for Caucasians, then, “Pharmacogenetic dosing may have some benefit for time within therapeutic range.” Whether that would reduce the number of adverse clinical outcomes, he says, would have to be extrapolated.

Dr. Eby

Given the studies’ limitations, others too feel the jury is still out. Alan Wu, PhD, a COAG investigator and professor of laboratory medicine at the University of California, San Francisco, and chief of the clinical chemistry laboratory at San Francisco General Hospital, says the wrong patient population was enrolled because patients at anticoagulation clinics are not the typical patient population. “Much of warfarin is given in primary care by doctors who don’t have the same degree of expertise,” he says. “Therefore, benefit such as seen in the EU-PACT trial where they just gave 5 mg would be more evident in an all-comers general practice environment than what we see in COAG.”

Dr. Wu notes, too, the high degree of compliance in the COAG patient population. “What if you don’t have a compliant patient? What if somebody is just given one shot to get the dose right? In my practice at San Francisco General, that’s the norm.” In similar indigent populations, pharmacogenetics could be of great utility. Comparing pharmacogenetics with a gold standard, such as a clinical algorithm that clearly works, is not the best way to determine utility, in his view.

The design of the control arm was a subject of much debate within the COAG group itself, given that most physicians do not use the clinical algorithm. Tolerance for trial and error is ingrained in warfarin clinical practice, Dr. Eby notes, adding that the steering committee members for the COAG trial had many discussions about trial design. “Ultimately,” he says, “the commitment was to perform a randomized, controlled trial with as few different variables between the control arm and the pharmacogenetics arm, recognizing this was not the standard of practice but also recognizing it would provide the best scientific information for the additional value of pharmacogenetic testing.”

Dr. Johnson

As a result, says Julie Johnson, Pharm D, an author of the COAG paper and dean and distinguished professor of pharmacy, University of Florida College of Pharmacy, the results are not surprising. “The COAG trial was designed in a way that made it extraordinarily difficult to show benefit from genetic-guided therapy. The clinical algorithm gets you closer than a standard dose—we knew that in 2009 from a New England Journal of Medicine article on which I was the senior author. The trial also employed frequent INR monitoring, probably more frequently than happens anywhere in the U.S., so between the clinical algorithm and the very, very frequent INR monitoring, it made it hard for the pharmacogenetic arm to show a difference.” The external validity of the EU-PACT trial was much higher than the external validity of the COAG trial because it matched a real-world situation much more closely, she agrees. Just as Dr. Wu believes that pharmacogenetic testing for warfarin might give you the biggest bang for the buck in urban settings, Dr. Johnson feels similarly about rural populations, where standard dosing and limited INR testing prevail.

Dr. Weck

University of North Carolina study author Karen Weck, MD, a professor in the UNC Division of Pathology and Laboratory Medicine and director of the medical genetics laboratory, wanted to include a third arm in the UNC study that would include data from historical standard practice. Ultimately, a completely prospective randomized trial design was chosen even though it was recognized that coagulation control is better when a clinical algorithm, rather than standard dosing, is used and when patients are treated in specialized coagulation clinics that perform frequent INR testing. While this level of care does not reflect the experience of most patients, Dr. Weck notes, “The reason our study and the COAG study were designed the way they were is because that’s the best way to directly analyze the impact of genetic testing.” Another limitation is that the initial dosing period during which pharmacogenetics was used was very short, and it was only a few days before INR results guided dosing. Dr. Weck also notes that diet and compliance cannot easily be controlled for, both of which can have an effect on results. Evidence-based research is important, but “we could be shooting ourselves in the foot because of the way the trials are designed,” she says.