Chinese and Malay subjects have been reported to require less maintenance warfarin than Indians that could not be accounted for by cytochrome P450 (CYP) 2C9 variants. Vitamin K epoxide reductase complex 1 (VKORC1) is the target enzyme of warfarin, and VKORC1 intronic variants and haplotypes have recently been shown to influence VKORC1 activity and warfarin requirements.
Pharmacokinetic-pharmacodynamic information regarding warfarin is used to produce a predictive model based on the idea that pharmacodynamic variability is more important than pharmacokinetic variability in the overall dose-response variability to warfarin. A modification of the maximum effect model is tested on a group of patients initiating oral anticoagulation with warfarin. Results indicate that the model can account for at least half of the total variation in maintenance doses observed (sample coefficient of determination, 0.53) and offer the physician a framework for dose requirements at the onset of therapy. The basic prediction equation is as follows: Maintenance dose = (11/international normalized ratio)-1, with a coefficient of correlation of 0.73 (95% confidence limits, 0.46-0.88). Application of this model may improve on the traditional empiric approach to warfarin dose adjustment.
There is preliminary evidence to suggest that some published warfarin dosing algorithms produce biased maintenance dose predictions in patients who require higher than average doses. We conducted a meta-analysis of warfarin dosing algorithms to determine if there exists a systematic under- or overprediction of dose requirements for patients requiring ≥7 mg/day across published algorithms. Medline and Embase databases were searched up to September 2015. We quantified the proportion of over- and underpredicted doses in patients whose observed maintenance dose was ≥7 mg/day. The meta-analysis included 47 evaluations of 22 different warfarin dosing algorithms from 16 studies. The meta-analysis included data from 1,492 patients who required warfarin doses of ≥7 mg/day. All 22 algorithms were found to underpredict warfarin dosing requirements in patients who required ≥7 mg/day by an average of 2.3 mg/day with a pooled estimate of underpredicted doses of 92.3% (95% confidence interval 90.3-94.1, I(2) = 24%).
Specific ethnic genetic backgrounds are associated with the risk of Stevens-Johnson syndrome / toxic epidermal necrolysis (SJS/TEN) especially in Asians. However, there have been no large cohort, multiple-country epidemiological studies of medication risk related to SJS/TEN in Asian populations. Thus, we analyzed the registration databases from multiple Asian countries who were treated during 1998-2017. A total 1,028 SJS/TEN cases were identified with the algorithm of drug causality for epidermal necrolysis. Furthermore, those medications labeled by the US Food and Drug Administration (FDA) as carrying a risk of SJS/TEN were also compared with the common causes of SJS/TEN in Asian countries. Oxcarbazepine, sulfasalazine, COX-II inhibitors, and strontium ranelate were identified as new potential causes. In addition to sulfa drugs and beta-lactam antibiotics, quinolones were also a common cause. Only one acetaminophen-induced SJS was identified, while several medications (e.g., oseltamivir, terbinafine, isotretinoin, and sorafenib) labeled as carrying a risk of SJS/TEN by the FDA were not found to have caused any of the cases in the Asian countries investigated in this study.