4 Discussion This case series highlights the highly variable response to the drug interaction between rifampicin and warfarin amongst rural resource-constrained Talazoparib concentration patients in western Kenya. While much of this variability can be partially explained by the comorbid conditions and other anticoagulation modifying characteristics of patients, this case series highlights the extreme unpredictability of this interaction and need for individualized therapy. Patients tended to require a higher than normal weekly dose (73.1 mg per week (10.4 mg/day). However, the interquartile range for these findings was quite

large, limiting the ability to provide uniform dosing guidance for future patients that may encounter this drug interaction. The TTR for patients receiving rifampicin and warfarin was lower than the TTR for patients not utilizing rifampicin in clinic. Although, VS-4718 in vitro the difference in TTR was not statistically significant, it highlights the added difficulty in managing anticoagulation therapy in these patients. In addition, distinct patient characteristics such as, age, start dates of rifampicin in relation to warfarin, and co-morbid conditions likely play a role in the intricacy of dosing and monitoring requirements of these patients. The findings regarding the impact of age on warfarin dosing are supported by the well-documented physiological

changes that occur in these age groups. In pediatrics, the hemostatic system is a dynamic and evolving entity with both quantitative and qualitative

changes in its components. The changes affect the concentration and functionality of the blood clotting factors. The differences in the system are marked in neonates and infants and continue to mature during childhood until reaching full development during adolescence [24, 25]. These changes affect the response to anticoagulant agents. Also, in studies AUY-922 carried out in children, age has been shown to affect the pharmacokinetic and pharmacodynamic responses to anticoagulants [26, 27]. This may possibly explain the small change in weekly warfarin dose in case 6. On the other extreme, the geriatric population (age >65 years; Case 10) is associated with lower than usual warfarin dose requirements, which may be attributed to impaired enzyme induction in the elderly [2, Phosphoglycerate kinase 28]. Clinicians should be cautious when adjusting warfarin doses in patients at the extremes of age due to the variation in the hemostatic system and drug pharmacokinetics. In addition to the age of the patient, the start date of rifampicin in relationship to warfarin utilization can have a direct impact on the degree of necessary dosing adjustments of the anticoagulant. In patients who started rifampicin therapy within two weeks of starting warfarin, the impact of rifampicin timing was quite pronounced as most patients required large increases in their warfarin dose to compensate for the emerging induction of warfarin metabolism.