The extravascular distribution of endogenous factor IX may be a relevant consideration for infused factor IX therapies
Extravascular distribution may have implications on how we evaluate PK activity in hemophilia B, including trough levels1,2
Multiple PK parameters should be considered, including half-life, clearance, and volume of distribution (VOD)3
Trough levels do not account for infused factor IX that has moved outside of the plasma4,5
Data in hemophilia B patients show that plasma factor IX levels decrease quickly following infusion6
Additional research is needed to understand the clinical implications of VOD
Though PK is important, factor IX replacement therapy should ultimately be evaluated based on outcomes, including bleed prevention and control.
PK parameters, such as trough levels, are not patient outcomes3,7
Preclinical data are not correlated with outcomes. Animal studies may not apply to humans.
Additional research for preclinical data and research in humans is needed to confirm these findings.
PK=pharmacokinetic.
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References: 1. Feng D, Stafford KA, Broze GJ, Stafford DW. Evidence of clinically significant extravascular stores of factor IX. Thromb Haemost. 2013;11(12):2176-2178. 2. Berntorp E, Dolan G, Hermans C, Laffan M, Santagostino E, Tiede A. Pharmacokinetics, phenotype and product choice in haemophilia B: how to strike a balance? Haemophilia. 2014;20(suppl 7):1-11. 3. Ragni MV, Croteau SE, Morfini M, et al. Pharmacokinetics and the transition to extended half-life factor concentrates: communication from the SSC of the ISTH. J Thromb Haemost. 2018;16(7):1437-1441. 4. Iorio A, Fischer K, Blanchette V, Rangarajan S, Young G, Morfini M. Tailoring treatment of haemophilia B: accounting for the distribution and clearance of standard and extended half-life FIX concentrates. Thromb Haemost. 2017;117(6):1023-1030. 5. Iorio A, Edginton AN, Blanchette V, et al. Performing and interpreting individual pharmacokinetic profiles in patients with Hemophilia A or B: rationale and general considerations. Res Pract Thromb Haemost. 2018;2:535-548. 6. Shapiro AD, Ragni MV, Valentino LA, et al. Recombinant factor IX-Fc fusion protein (rFIXFc) demonstrates safety and prolonged activity in a phase 1/2a study in hemophilia B patients. Blood. 2012;119(3):666-672. 7. Srivastava A, Brewer AK, Mauser-Bunschoten EP, et al. Guidelines for the management of hemophilia. Haemophilia. 2013;19(1):e1-e47.