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Plasma and albumin‐free recombinant factor VIII: pharmacokinetics, efficacy and safety in previously treated pediatric patients

2008; Elsevier BV; Volume: 6; Issue: 8 Linguagem: Inglês

10.1111/j.1538-7836.2008.03032.x

1538-7933

Victor S. Blanchette, Amy D. Shapiro, Ri Liesner, Felipe Navarro, Indira Warrier, P Schroth, Gerald Spotts, Bruce M. Ewenstein,

Blood Coagulation and Thrombosis Mechanisms

Journal of Thrombosis and HaemostasisVolume 6, Issue 8 p. 1319-1326 Free Access Plasma and albumin-free recombinant factor VIII: pharmacokinetics, efficacy and safety in previously treated pediatric patients V. S. BLANCHETTE, V. S. BLANCHETTE Hospital for Sick Children, University of Toronto, ON, CanadaSearch for more papers by this authorA. D. SHAPIRO, A. D. SHAPIRO Indiana Hemophilia and Thrombosis Center, Indianapolis, IN, USASearch for more papers by this authorR. J. LIESNER, R. J. LIESNER Great Ormond Street Hospital for Children NHS Trust, London, UKSearch for more papers by this authorF. HERNÁNDEZ NAVARRO, F. HERNÁNDEZ NAVARRO Hospital Universitario La Paz Servicio de Hematologia, Madrid, SpainSearch for more papers by this authorI. WARRIER, I. WARRIER Children’s Hospital of Michigan, Detroit, MISearch for more papers by this authorP. C. SCHROTH, P. C. SCHROTH Baxter BioScience, Westlake Village, CA, USASearch for more papers by this authorG. SPOTTS, G. SPOTTS Baxter BioScience, Westlake Village, CA, USASearch for more papers by this authorB. M. EWENSTEIN, B. M. EWENSTEIN Baxter BioScience, Westlake Village, CA, USASearch for more papers by this authorFOR THE rAHF-PFM CLINICAL STUDY GROUP, FOR THE rAHF-PFM CLINICAL STUDY GROUP The rAHF-PFM Clinical Study Group is given in the Appendix.Search for more papers by this author V. S. BLANCHETTE, V. S. BLANCHETTE Hospital for Sick Children, University of Toronto, ON, CanadaSearch for more papers by this authorA. D. SHAPIRO, A. D. SHAPIRO Indiana Hemophilia and Thrombosis Center, Indianapolis, IN, USASearch for more papers by this authorR. J. LIESNER, R. J. LIESNER Great Ormond Street Hospital for Children NHS Trust, London, UKSearch for more papers by this authorF. HERNÁNDEZ NAVARRO, F. HERNÁNDEZ NAVARRO Hospital Universitario La Paz Servicio de Hematologia, Madrid, SpainSearch for more papers by this authorI. WARRIER, I. WARRIER Children’s Hospital of Michigan, Detroit, MISearch for more papers by this authorP. C. SCHROTH, P. C. SCHROTH Baxter BioScience, Westlake Village, CA, USASearch for more papers by this authorG. SPOTTS, G. SPOTTS Baxter BioScience, Westlake Village, CA, USASearch for more papers by this authorB. M. EWENSTEIN, B. M. EWENSTEIN Baxter BioScience, Westlake Village, CA, USASearch for more papers by this authorFOR THE rAHF-PFM CLINICAL STUDY GROUP, FOR THE rAHF-PFM CLINICAL STUDY GROUP The rAHF-PFM Clinical Study Group is given in the Appendix.Search for more papers by this author First published: 17 July 2008 https://doi.org/10.1111/j.1538-7836.2008.03032.xCitations: 115 Victor S. Blanchette, Hospital for Sick Children, Division of Hematology/Oncology, 555 University Avenue, Toronto, ON, M5G 1X8, Canada.Tel.: +1 416 813 5852; fax: +1 416 813 5327.E-mail: victor.blanchette@sickkids.ca AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Summary. Background: The pharmacokinetics of factor VIII replacement therapy in preschool previously treated patients (PTPs) with hemophilia A have not been well characterized. Objectives: To assess the pharmacokinetics, efficacy and safety of a plasma-free recombinant FVIII concentrate, ADVATE [Antihemophilic Factor (Recombinant), Plasma/Albumin-Free Method, rAHF-PFM], in children < 6 years of age with severe hemophilia. Patients/methods: Fifty-two boys, one girl, mean (± SD) age 3.1 ± 1.5 years and ≥ 50 days of prior FVIII exposure, were enrolled in a prospective study of ADVATE rAHF-PFM at 23 centers. Results: The mean terminal phase half-life (t1/2) was 9.88 ± 1.89 h, and the mean adjusted in vivo recovery (IVR) was 1.90 ± 0.43 IU dL−1 (IU kg−1)−1. Over the 1–6-year age range, t1/2 of rAHF-PFM increased by 0.40 h year−1. IVR increased by 0.095IU dL−1(IU kg−1)−1 (kg m−2)−1 in relation to body mass index (BMI). Patients primarily received prophylaxis. Median (range) annual joint bleeds were 0.0 (0.0–5.8), 0.0 (0.0–6.1) and 14.2 (0.0–34.5) for standard prophylaxis, modified prophylaxis and on-demand treatment, respectively. Bleeds were managed in 90% (319/354) of episodes with one or two rAHF-PFM infusions; response was rated excellent/good in 93.8% of episodes. Over a median 156 exposure days, no FVIII inhibitors were detected and no related severe adverse events or unusual non-serious adverse events were seen. Conclusions: Children < 6 years of age appear to have shorter FVIII t1/2 and lower IVR values than older subjects. However, these parameters increased with age (t1/2) and BMI (adjusted IVR), respectively. rAHF-PFM was clinically effective and well tolerated, with no signs of increased immunogenicity in previously treated young children with hemophilia A. Introduction Pharmacokinetic (PK) studies of infused factor VIII in very young boys with hemophilia A and how the results relate to optimal FVIII replacement therapy remains poorly characterized. A goal of primary prophylaxis is the elimination and/or minimization of bleeding episodes from the earliest age possible. Developing both optimal and feasible prophylaxis regimens remains a challenge. Early prophylaxis is widely accepted as preventing or minimizing disabling arthropathy [1-5]. Several studies have demonstrated that continuous long-term prophylaxis is superior to on-demand treatment in reducing bleeding frequency and preventing or delaying arthropathy [6-9]. Primary prophylaxis is now recommended as the optimal treatment for young boys with severe hemophilia [1, 2]. PK data may be fundamental in devising optimal individualized dosing regimens [10]. However, PK parameters post-FVIII administration display large interindividual variation [11] and appear to differ between age groups [12]. The majority of FVIII PK studies have been conducted in older patients because of the difficulty in obtaining frequent blood samples from small children. FVIII PK data for preschool children are primarily anecdotal, and the influence of age and body weight on individual PK parameters remains relatively unexplored in small children. ADVATE [Antihemophilic Factor (Recombinant), Plasma/Albumin Free Method (rAHF-PFM)] is a full-length recombinant FVIII (rFVIII) concentrate developed on the basis of RECOMBINATE (rAHF) technology [human rFVIII and recombinant von Willebrand factor (rVWF) coexpression in CHO cells, prior to rFVIII purification and concentration]. The rAHF-PFM process, however, excludes all human or animal blood-derived additives throughout cell culture, purification and formulation, virtually eliminating the risk of blood-borne disease transmission [13]. As part of a comprehensive rAHF-PFM clinical study program, this multicenter, prospective study evaluated the pharmacokinetics, efficacy, safety and immunogenicity of rAHF-PFM, in a cohort of 53 children with severe hemophilia A < 6 years of age. Materials and methods A multicenter, open-label, prospective cohort study of 53 pediatric previously treated patients (PTPs) was undertaken at 12 North American and 11 European sites. The study, conducted between June 2002 and January 2005, involved an initial PK evaluation of rAHF-PFM (Baxter BioScience, Westlake Village, CA, USA), followed by an efficacy and safety assessment for ≥ 50 exposure days or 6 months. Participants had severe or moderately severe hemophilia A (baseline FVIII ≤ 2%), were < 6 years of age at enrollment and had ≥ 50 prior days of exposure to other FVIII concentrates. Patients with inhibitors [≥ 1 Bethesda Unit (BU)], a history of inhibitors or an inherited or acquired hemostatic defect other than hemophilia A were excluded. The study protocol and informed consent were approved by the Institutional Review Board, Research Ethics Board or Independent Ethics Committee of each participating institution. Written informed consent was obtained from the parent or legally authorized guardian prior to study enrollment. Pharmacokinetics A 50 IU kg−1 intravenous bolus dose of rAHF-PFM (one lot used in this phase) in 5 mL was administered over approximately 5 min (maximum infusion rate 10 mL min−1). Potency determinations for labeling and dosing purposes were performed at central Baxter BioScience laboratories using a one-stage activated partial thromboplastin time (APTT)-based assay. A minimum washout period of 72 h after any prior FVIII infusion was required before PK determinations. PK results were separately assessed for both intent-to-treat (ITT) (n = 52) and per-protocol populations without deviations (n = 47). One subject was excluded from PK analyses due to several outlying data points, determined by Dixon’s outlier test. At study termination, patients were also evaluated for adjusted in vivo recovery (IVR) 1 h after infusion of 50 IU kg−1 rAHF-PFM. Standard PK parameters were assessed in general accordance with the prevailing Scientific and Standardization Committee (SSC) of the International Society on Thrombosis and Haemostasis guidelines for pediatric patients [14]. The SSC proposed an abbreviated sampling schedule of five blood draws in children < 5 years of age to obtain PK information prior to prophylaxis initiation [15]. Consistent with these recommendations, blood samples were drawn < 30 min prior to the FVIII infusion and at 1 h ±5 min, 9 ± 1 h, 24 ± 2 h and 48 ± 2 h postinfusion. All FVIII levels were measured at a central laboratory (Haemophilia Centre and Haemostasis Unit, Royal Free Hospital, Hampstead, London, UK) using an APTT-based assay (≥ 3 plasma dilutions each). Terminal phase half-life (t1/2), adjusted IVR, area under the concentration vs. time curve (AUC0–48 h), maximal plasma concentration (Cmax), area under the moment curve (AUMC), clearance (CL), mean residence time (MRT) and volume of distribution at steady state (Vss) were evaluated. Preinfusion FVIII levels were adjusted for and PK parameters were computed using actual blood draw times when possible. Efficacy Post-PK phase treatment regimens were determined individually by each physician. Patients received either ‘standard’ long-term prophylaxis (25–50 IU kg−1, three to four times per week), modified long-term prophylaxis or on-demand treatment. Long-term prophylaxis was defined as a regimen in which subjects receive treatment ≥ 46 weeks year−1 [5]. Modified prophylaxis was classified as any long-term prophylactic regimen that differed in dose or frequency from study-defined standard prophylaxis. The number of bleeding episodes, etiology and anatomic site of bleeds were recorded. Switching regimens during study was permitted. Bleeds were assigned to the recorded prescribed regimen at the time of the event. Study visits were scheduled at 3-month intervals. Blood samples for genetic mutation testing were collected at the first visit. Two bleed management efficacy measures were evaluated at periodic study visits: the number of infusions used, and an efficacy rating by the caregiver of ‘excellent’ (abrupt pain relief and/or unequivocal improvement in objective bleeding signs within approximately 8 h after one infusion), ‘good’ (definite pain relief and/or improvement in bleeding signs within approximately 8 h after one infusion, possibly requiring > 1 infusion), ‘fair’ (probable/slight beneficial effect within approximately 8 h after first infusion, usually requiring > 1 infusion), or ‘none’ (no improvement or worsened condition). The number of rAHF-PFM infusions administered was at the discretion of the investigator. Investigators also determined hemostatic cover for surgical procedures. Bleeds documented and managed during the protocol-defined observation period were included in annual bleed rate calculations and efficacy assessment. Safety Safety was assessed on an ITT basis for all patients receiving rAHF-PFM. Assessment included physical examinations and clinical laboratory measurements, including levels of antibody to FVIII, Chinese hamster ovary (CHO) cell proteins, murine IgG, and human rVWF. Changes in hematologic parameters (hemoglobin, hematocrit, red blood cell count, white blood cell count with differential, and platelet count), as well as changes in clinical chemistry parameters (sodium, potassium, chloride, CO2, glucose, total protein, albumin, total bilirubin, alanine aminotransferase, alkaline phosphatase, blood urea nitrogen, and creatinine), were evaluated prior to and 48 h post-PK determination, during periodic study visits, and at the termination visit. Changes in these parameters were also evaluated preoperatively and 24 h postsurgery. FVIII inhibitor testing was performed at screening, at 3-month visits, at study termination, and if clinically indicated. Blood was drawn ≥ 48 h after patients’ last infusion at study visits, and ≥ 72 h after screening and termination visits. Assays were performed using the Bethesda method; if titers were ≤ 1 BU, the Nijmegen modification was performed at the central laboratory [16]. On the recommendation of the Data Safety Monitoring Board, inhibitor levels ≥ 0.4 BU were considered to be positive. Antibodies to CHO cell proteins, anti-murine IgG and anti-human rVWF were measured at screening, periodic study visits, and termination. Hematology, clinical chemistry and hepatitis C assays were performed at local laboratories. FVIII inhibitor assays were performed both locally and in a central laboratory (Technology Resources, Applied Sciences, Pathobiology, Clinical Laboratory Services, Baxter Healthcare Corporation, Round Lake, IL, USA). Assays for antibodies to CHO cell proteins, murine IgG and human rVWF were performed at the Baxter laboratory in Round Lake, IL, USA. FVIII gene mutation assays were performed at Royal Hallamshire Hospital, Sheffield, UK. Statistical analysis The presented summary statistics consist of mean, median, standard deviation (SD), range and interquartile range. For paired differences and regression slopes, 95% confidence intervals were calculated. In PK analyses, the t1/2 of rAHF-PFM was determined as previously described [17], using robust estimation methodology. AUC0–48 h was computed by a linear trapezoidal method. The total area under the curve was estimated as the sum of AUC0–48 h and an additional area extrapolated by linear regression. AUMC was defined as area under the curve for the product of AUC and time vs. time. CL was computed as dose divided by total AUC; MRT was computed as total AUMC divided by total AUC. Vss was calculated as CL × MRT. Cmax (the observed peak FVIII concentration) was measured 1 h postinfusion. The definition of adjusted recovery was (Cmax – FVIIIbaseline)/IU kg−1 infused, as detailed elsewhere [18]. Plasma FVIII concentration values were screened for outliers prior to PK analysis. Relationships between PK and demographic variables were evaluated by linear regression and linear correlation. Results Baseline data Table 1 summarizes baseline patient data. The mean (± 1 SD) age of the cohort was 3.1 ± 1.5 years. Forty-eight patients (90.6%) were Caucasian and three (5.6%) were African-American. Race was unspecified in two patients (3.8%). Table 1. Baseline patient data Characteristic n % Gender Male 52 98.1 Female 1 1.9 Age (years) < 3 24 45.3 3–5 29 54.7 FVIII regimen in preceding 6 months On demand 5 9.4 Prophylaxis 39 73.6 Both 9 17.0 FVIII gene mutation testing was performed for 45/53 patients. Intron 22 inversions were identified in 18 patients (40%), missense point mutations in 12 (27%), nonsense point mutations in six (13%), frameshift mutations in five (11%), deletions in two (5%), intron 1 inversion in one (2%), and a splice defect in one (2%). Patients primarily received prophylaxis before and during the study. In the 6 months prior to study entry, the majority of patients were on standard prophylaxis (three to four infusions weekly). During the study treatment phase, 14 patients exclusively received standard prophylaxis, 27 modified prophylaxis, and two on-demand treatment. In addition, six patients switched from standard to modified prophylaxis, two from on-demand to modified prophylaxis, one from modified to standard prophylaxis, and one from modified prophylaxis to on-demand treatment. All treated patients completed the study. Pharmacokinetics For the initial PK determination, patients received a mean of 50.4 ± 2.26 IU kg−1 rAHF-PFM (range 45.3–55.1 IU kg−1). Postinfusion FVIII levels exhibited a typical biphasic decay pattern, with a mean t1/2 of 9.88 ± 1.89 h (range 6.8–15.4 h) and mean residual levels of 3.5% at 48 h in the per-protocol population (Fig. 1). PK parameters for the ITT and per-protocol populations were very similar (Fig. 1). One hour after infusion of a mean rAHF-PFM dose of 50.41 ± 2.26 IU kg−1, the mean adjusted IVR was 1.90 ± 0.43 IU dL−1 (IU kg−1)−1 [range 1.19–3.39 IU dL−1 (IU kg−1)−1]. The IVR was at least 1.5 IU dL−1 (IU kg−1)−1 in 87.2% of the per-protocol population (n = 47) and 86.5% (n = 52) of the ITT population. For 46 patients with termination visit IVR determinations, the mean rAHF-PFM dose was 51.8 ± 16.0 IU kg−1 and the adjusted IVR was 1.76 ± 0.49 IU dL−1 (IU kg−1)−1. A small but statistically significant (P = 0.039) mean decline of adjusted IVR of 0.18 IU dL−1 (IU kg−1)−1 was observed at the termination visit as compared with the initial PK determination. Figure 1Open in figure viewerPowerPoint Time course of change in mean FVIII level [± standard deviation (SD)] after Antihemophilic Factor (Recombinant), Plasma/Albumin-Free Method (rAHF-PFM) infusion in the intent-to-treat (ITT) population and tabulated pharmacokinetic parameters in ITT and per-protocol populations. AUC, area under the curve; AUMC, area under the moment curve; Cmax, maximum concentration; CL, clearance; MRT, mean residence time; t1/2, terminal phase half-life; Vss, volume of distribution at steady state; IVR, in vivo recovery. Patient characteristics such as age and body mass index (BMI) were found to influence PK parameters. BMI was a significant anthropometric predictor of FVIII distribution, as measured by adjusted IVR and Vss. Adjusted IVR increased linearly with rising BMI, whereas Vss decreased linearly (Fig. 2). Over the study BMI range, adjusted IVR increased by 0.095 IU dL−1 (IU kg−1)−1 (kg m−2)−1. BMI exhibited no significant association with t1/2, MRT or clearance. Age, however, was a significant predictor of t1/2 and MRT, both measures of FVIII plasma persistence (Fig. 3). Over the study age range, t1/2 of rAHF-PFM increased at a rate of 0.40 h year−1. Age had no significant correlation with adjusted IVR, Vss or clearance. Figure 2Open in figure viewerPowerPoint Linear regression analysis of (A) adjusted in vivo recovery and (B) volume of distribution at steady state (Vss) as predicted by body mass index. Dashed lines indicate 95% confidence interval (CI) of the regression. Absence of zero from slope CI indicates a statistically significant relationship (P < 0.05). Dotted lines depict 95% prediction limits for new observations. Figure 3Open in figure viewerPowerPoint Linear regression analysis of (A) Antihemophilic Factor (Recombinant), Plasma/Albumin-Free Method, rAHF-PFM (rAHF-PFM) terminal phase half-life and (B) mean residence time as a function of age. Graphic conventions as in Fig. 2. CI, 95% confidence interval. Efficacy rAHF-PFM dose frequency and size during standard and modified prophylaxis are summarized in Table 2. In 86% of treatment weeks under standard prophylaxis, patients received three infusions per week. In the majority (56%) of treatment weeks under modified prophylaxis, one or two weekly infusions were given, whereas three infusions per week were given for another 39.3%. Mean and median prophylactic dose per infusion during modified prophylaxis (48.8 and 45.5 IU kg−1, respectively) were higher than those during standard prophylaxis (34.2 and 33.8 IU kg−1, respectively). However, mean and median cumulative weekly doses were similar between the two prophylaxis regimens. Thus, the distinguishing feature of modified prophylaxis was the administration of fewer infusions per week at a higher per-infusion dose on average. Table 2. Prophylaxis dosing patterns Prophylaxis Dose frequency Number of infusions/week Treatment weeks Number of treatment weeks (% of total) % of treatment weeks per regimen Standard (n = 21)* 3 625 (25.4) 86.0 4 102 (4.1) 14.0 Modified (n = 37)* 1 267 (10.8) 15.4 2 704 (28.6) 40.6 3 681 (27.7) 39.2 ≥ 4 83 (3.4) 4.8 Dose size IU kg−1/infusion IU kg week−1 Mean (SD) Median (IQR) Mean (SD) Median (IQR) Standard 34.2 (5.3) 33.8 (30.0–37.9) 107.3 (19.7) 103.8 (93.1–118.3) Modified 48.8 (21.6) 45.5 (32.1–57.3) 117.0 (71.8) 100.1 (60.9–153.1) SD, one standard deviation; IQR, interquartile range, represents 50% of the distribution. *Number of patients on regimen. Total n exceeds 53 due to switching between regimens. A total of 430 bleeds occurred in 44 subjects, 409 of which were evaluable per-protocol; nine subjects experienced no bleeds (eight on prophylaxis, one on-demand). Among all episodes, etiologies were secondary to trauma for 198 (48.4%) bleeds, spontaneous for 73 (17.8%), postoperative for two (0.5%), and not indicated for 136 (33.3%). Among 112 joint bleeds, 37 (33.0%) were secondary to trauma, 20 (17.9%) spontaneous, and 55 (49.1%) of uncertain etiology. For all episodes, the most frequent sites involved were non-muscle soft tissue in 201 bleeds (49.1%), joints in 112 (27.4%), and muscle in 79 (19.3%). Bleeds secondary to trauma most commonly occurred in soft tissues (124, 62.3%), muscle (38, 19.1%), and joints (37, 18.6%). Some individual bleeds involved more than one site, and two intracranial hemorrhages, secondary to trauma, were treated on study. The effect of regimen on annual bleeding incidence is shown in Table 3. The median numbers of annual bleeds for all body sites were 4.0, 4.4 and 24.4 for patients on standard prophylaxis, modified prophylaxis and on-demand treatment, respectively. The median annual rate of joint bleeding episodes was zero for all prophylaxis regimens (range 0–6.1), as compared with 14.2 (range 0–34.5) for on-demand treatment. Joint bleeds occurred in seven of 21 patients (33.3%) receiving standard prophylaxis, 12/37 (32.4%) receiving modified prophylaxis, and four of five (80.0%) receiving on-demand treatment. Joint bleeds in patients receiving on-demand treatment were primarily attributed to non-traumatic causes (Table 3). Table 3. Effect of regimen on annual incidence of bleeding episodes Site Median annual bleeds (range) Standard prophylaxis, n = 21* Modified prophylaxis, n = 37* On demand, n = 5* All sites 4.0 (0.0–27.1) 4.4 (0.0–37.7) 24.4 (8.9–53.2) Joint Traumatic 0.0 (0.0–1.4) 0.0 (0.0–6.0) 1.2 (0.0–14.4) Non-traumatic 0.0 (0.0–4.3) 0.0 (0.0–5.3) 13.0 (0.0–24.4) Total for joint 0.0 (0.0–5.8) 0.0 (0.0–6.1) 14.2 (0.0–34.5) *Numbers of patients on regimen. Total n exceeds 53 due to switching between regimens. Of the 409 reported bleeds, 354 (86.6%) were treated with rAHF-PFM; for 55 (13.4%), no treatment was administered or no treatment record was provided. Of episodes not treated with FVIII, 13 were managed using the adjunctive hemostatic medications aminocaproic acid or tranexamic acid. In 319/354 treated bleeding episodes (90.1%), one to two rAHF-PFM infusions were used (Table 4). The number of infusions used to treat episodes did not differ between regimens. Hemostatic efficacy was rated excellent/good for 332 (93.8%) treated bleeding episodes (Table 4). In no case was rAHF-PFM rated ineffective. The overall median dose per infusion used to treat bleeds was 34.7 (8–161) IU kg−1. The overall median cumulative treatment dose administered per bleeding episode was 46.6 (16–410) IU kg−1. In bleeding episodes involving trauma, the median treatment dose per episode was 48.7 (21–353) IU kg−1. The median treatment dose per spontaneous bleeding episode was 31.6 (21–150) IU kg−1. Table 4. Hemostatic efficacy for treatment of bleeding episodes Infusions Bleeding episodes treated n % of total Cumulative % 1 277 78.2 78.2 2 42 11.9 90.1 3 19 5.4 95.5 ≥ 4 16 4.5 100.0 Efficacy rating Excellent or good 332 93.8 93.8 Fair 17 4.8 98.6 Ineffective 0 0.0 98.6 Not determined 5 1.4 100.0 Seven patients underwent surgical procedures, five of which were evaluable per-protocol. Procedures included two indwelling port removals, indwelling port removal and circumcision, excision of an ocular cyst, and closed reduction of a nasal fracture. The ratios of actual/predicted blood loss were unremarkable; intraoperative and postoperative hemostatic efficacy was rated excellent/good in the three subjects with assessments. Safety Fourteen lots of rAHF-PFM were used for a total of 7980 infusions. The median number of days of exposure to rAHF-PFM was 156 days (range 14–384 days) out of a median 386 days on study (range 141–933 days). An estimated total of 6.24 × 106 IU was infused, and the estimated median cumulative dose of rAHF-PFM per subject was 6.64 × 103 IU kg−1 (range 0.67–19.9 × 103 IU kg−1). Of the 537 non-serious and 15 serious adverse events reported in 52 patients, six non-serious events in two patients were judged to be related to rAHF-PFM administration. One patient experienced laryngitis, eye inflammation, and influenza; the other developed memory impairment, tremor, and pallor. All rAFH-PFM-related non-serious events resolved completely. No patient was withdrawn from study because of an adverse event. Hematology and clinical chemistry parameters and vital signs following rAHF-PFM infusion did not reveal any evidence of rAHF-PFM-associated toxicity. Evaluation of anti-CHO protein, anti-murine IgG, and anti-human rVWF antibody values over the course of the study showed no evidence of allergic or hypersensitivity responses. Over the median 156 exposure days on study, no patient developed a detectable FVIII inhibitor. Discussion The marked variation in PK parameters of infused FVIII in boys with severe hemophilia suggests that the PK profile of individual subjects may be useful in developing optimal, individualized prophylaxis regimens in young boys with severe hemophilia A [10-12]. The limited evidence available suggests that PK parameters in individuals with hemophilia A vary with age. Following the bolus administration of the same dose of rAHF-PFM (50 IU kg−1), mean adjusted IVR was lower [1.9 IU dL−1 (IU kg−1)−1] in boys < 6 years of age as than that reported in patients 12–65 years of age [2.4 IU dL−1 (IU kg−1)−1] [13]. This age-related difference in IVR is consistent with prior reports [12, 19, 20]. In the present study, a small decrease [0.18 IU dL−1 (IU kg−1)−1] in adjusted IVR was noted between initial and termination PK assessments. This was not clinically significant, as there was no apparent association between bleeding rate and termination recovery. Similar findings have been previously reported [21]. In this study, adjusted IVR increased linearly with BMI (Fig. 2). Vss, a model-independent PK measure that is inversely proportional to IVR, confirmed this relationship, significantly declining with increasing BMI. The potential impact of body size on recovery of infused FVIII has been previously reported [22]. As previously reported in older subjects with hemophilia A, there was wide interpatient variation in t1/2 [11, 12, 23, 24]. The per-protocol mean t1/2 of rAHF-PFM in the present study was 9.9 h (range 6.8–15.4 h). The corresponding mean t1/2 for rAHF-PFM in individuals with hemophilia A ≥ 10 years was 12.0 h (range 6.7–24.7 h) [13]. In this study, age was predictive of t1/2 (Fig. 3). Over the age range 1–6 years, t1/2 increased by approximately 24 min year−1. The model-independent measure of MRT, which is, like t1/2, an indicator of FVIII plasma persistence, also significantly increased in the age range studied. In other rFVIII PK studies, t1/2 has also been reported as being shorter in children than in older individuals [12, 20]. In parallel with these t1/2 findings, clearance of rAHF-PFM was accelerated in these young children compared with older individuals [12]. This finding is consistent with a report of 41 patients aged 7–70 years demonstrating an age-related decrease in weight-adjusted clearance [25]. However, age had no significant correlation with adjusted IVR, Vss or clearance in the present study. Physiologic differences between children and adults may explain some of these differences. Plasma volume in relation to weight is greater in children than in adults, which may affect PK assessments. Children also have a larger volume of distribution than adults, due to greater extracellular and total body water spaces. Moreover, children up to 6 years of age have a larger liver to body weight ratio, which could affect clearance, as catabolism of FVIII is primarily mediated by two hepatic receptors [26, 27]. Other potential biological differences, such as plasma VWF levels, may also contribute to these age differences. Interindividual variation in t1/2 of infused rFVIII is related to preinfusion VWF levels [10]. Reflecting this variable, blood group O individuals may also exhibit shorter FVI