In the management of patients with hemophilia A, continuous infusion (CI) of factor VIII (FVIII) is a more cost-effective method than bolus injection (BI), because it prevents unnecessary high peak of FVIII coagulant activity (FVIII:C), to maintain the desired level of FVIII from a pharmacological perspective [1, 2]. In practice, the benefits of using CI of FVIII was confirmed compared to BI, especially in cases of major surgery in patients with hemophilia A.

The nadir level of FVIII in the CI group was higher than that in the BI group [3, 4], and dangerous drops of FVIII levels below 0.3% were more frequent in patients receiving BI than those receiving CI [3]. Hemoglobin levels were also higher with CI than with BI, and the BI group required more blood transfusions than the CI group [3]. Major bleeding complications developed only in the BI group [3]. The CI group required 30-36% lower doses of FVIII than did the patients receiving BI [3, 5]. Although some reports have raised concerns about the possibility of inhibitor development, recent large-scale studies have shown no increased rate of inhibitor development in CI [6, 7]. Despite the aforementioned advantages of CI, there is currently no specific guideline for how frequently a FVIII product should be changed over the course of one day, or of the indoor light-shielding requirements for FVIII-containing vessels or syringes in Korea.

Acquiring absolute hemostasis in patients with hemophilia A depends on maintaining the desired FVIII:C level. There are two crucial factors for determining FVIII:C - individual in vivo recovery of FVIII:C, and in vitro FVIII:C in the syringe prior to injection. In vitro FVIII:C is particularly important in CI, since FVIII concentrate is kept in the syringe for several hours prior to entering the patient's body. Thus in this study, in vitro FVIII:C after reconstitution was estimated by using three commercial FVIII products in Korea; we also discuss the results and how they compare to previous data.

Generally, infusion of FVIII concentrate according to the body weight (IU/kg) is known to raise the patient's FVIII:C by approximately 2 IU/dL. However, the drug should be supplemented according to the patient's response, due to considerable inter- and intra-patient variability in pharmacokinetics (PK) [8, 9]. There has been an emphasis on individual dosing of clotting factors using PK, to avoid the inadequate management and under-treatment of patients who require additional dosage, and to prevent potential overtreatment of the patients who require lower doses, which results in wasting of an expensive product [8, 9]. It is possible to calculate the velocity of CI if we obtain the level of FVIII from patients with hemophilia A during treatment [8, 9].

Several factors influence individual PK in the use of FVIII for treating patients with hemophilia A without inhibitor formation - product type, the patient's age, weight, body surface area, plasma volume, endogenous von Willebrand factor level, blood type, and hematocrit [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Thus, the indicators of PK change with advanced age, and may vary depending on the circumstances of each hemophilia patient. Besides the aforementioned factors, the in vitro FVIII:C that has been stored in a syringe prior to entering the patient's body must be considered for CI. It is a crucial factor because FVIII concentrate is reconstituted and stored in the syringe pump for several hours before infusion.

Several studies have reported the in vitro FVIII:C of various products. In the case of Bioclate, in vitro FVIII:C after reconstitution was maintained at over 75% of baseline activity for a period of 3-7 days [21]. In the case of Recombinate, in vitro FVIII:C remained stable during simulated CI for 96 hours [22]. For ADVATE rAHF-PFM, in vitro FVIII:C retained an average of 92% of its baseline activity over 24 hours after reconstitution [23] and maintained 83% or more of its activity at 24 or 48 hours in simulated CI [24].

In this study, the decline of in vitro FVIII:C was linear from 0-8 hours after reconstitution. In other words, in vitro FVIII:C loss begins immediately after reconstitution and consistently drops during the course of CI. However, the degree of in vitro FVIII:C loss does not seem to be clinically severe in 8 hours, because average FVIII:C was sustained at over 95% and 90% at 4 and 8 hours after reconstitution, respectively. This result suggests that CI with a FVIII concentrate is possible in 8-hour intervals in practice. Additionally, medical personnel should note that FVIII:C can decrease to under 90% of the baseline value after more than 8 hours, particularly because lower levels of FVIII in treatment cannot establish absolute hemostasis in patients with hemophilia A.

In comparison to the results of previous reports [21, 22, 23, 24, 25], the level of in vitro FVIII:C seems to be slightly lower in this study. We suggest that the experimental methods could have influenced the results. 1) FVIII was diluted after reconstitution and subsequently kept for several hours for the experiment, while in previous studies, undiluted reconstituted FVIII was kept for several hours and diluted just prior to experimentation [25]. 2) Estimation of FVIII:C required dilution of FVIII with sterile water in this study, while past reports have used normal saline/plasma of patients with severe hemophilia A [25], or did not dilute the reconstituted FVIII [22, 23, 24]. 3) We also considered whether the efficacy would be affected by the year the product was released or by its expiration date. Further study is needed to determine whether these factors are important.

In a previous report, the activity of reconstituted FVIII was diminished to approximately 30% of its baseline activity after 10 hours of daylight exposure, while shielding in foil wrap prevented the harmful effects of daylight [22]. However, persistent exposure to natural daylight is very rare in a practical clinical environment, especially for patients with hemophilia A who are under CI of FVIII, which is why indoor light was used in this study. There was no difference between FVIII:C in the indoor light-exposed or light-shielded groups during the experimental time frame of 8 hours. This result shows that CI using FVIII concentrate over a period of 8 hours does not require additional protection from indoor light in hospital rooms.

In conclusion, in vitro FVIII:C after reconstitution consistently decreases in the syringe prior to injection into the patient's body during CI, a fact that should be noted by the medical personnel. In vitro FVIII:C decreased after reconstitution, but the activity remained at over 90% of baseline activity, which is not clinically severe over an 8-hour time frame. Further, exposure to indoor light did not accelerate the loss of FVIII:C during 8 hours. These results indicate that FVIII CI is an available treatment option in 8-hour intervals in practice and indoor light-shielding precautions are not needed to safeguard FVIII:C.