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Keywords:

  • age at first exposure;
  • antibody;
  • hemophilia A;
  • recombinant factor VIII;
  • risk factors;
  • therapy

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

Context: The development of inhibitory antibodies against infused factor (F) VIII is a major complication of treatment of patients with severe hemophilia A.Objective: This study was set up to examine the effects of treatment-related factors on inhibitor development among previously untreated patients with severe hemophilia A.Design, setting and patients: In this multicenter cohort study, we combined individual patient data obtained from four recombinant FVIII product registration studies (Kogenate®, Kogenate Bayer®, Recombinate®, ReFacto®) that were performed between 1989 and 2001. From the databases we selected all 236 previously untreated patients with severe hemophilia A who were subsequently treated with FVIII on at least 50 days.Main outcome measures: Clinically relevant inhibitor development, defined as the occurrence of at least two positive inhibitor titers and a decreased recovery.Results: 67 patients (28%) developed clinically relevant inhibitors (44 high-titer) at a median of ten exposure days. Age at first exposure was not associated with inhibitor development. Peak treatment moments and surgical procedures were related to an increased inhibitor risk [adjusted relative risk 1.6 (95% confidence interval 1.0–2.6) and 2.7 (95% confidence interval 1.3–5.7), respectively]. A shorter duration between exposure days was associated with an increased risk of inhibitor development. There was a possible association between dosing of FVIII and inhibitor development, which largely disappeared after adjustment for confounding factors.Interpretation: These findings show that intensive treatment periods are associated with an increased risk of inhibitor development in previously untreated patients with severe hemophilia A. Our results do not support the notion that age at first exposure is associated with the risk of developing inhibitors.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

Hemophilia A is an X-linked inherited disease caused by a qualitative or quantitative deficiency of coagulation factor (F) VIII. Characteristic symptoms of severe hemophilia are spontaneous joint and muscle bleeds. During the last decades, hemophilia management has improved dramatically as a result of advances in safety of clotting factor products and the introduction of prophylactic therapy. However, inhibitor development continues to be a major complication of treatment, occurring in 3% to 52% of patients with severe hemophilia A [1]. These polyclonal allo-antibodies against FVIII neutralize infused clotting factor products, leading to increased bleeding complications and difficulties in the surgical management.

Several genetic factors, such as positive family history of inhibitors [2], ethnicity [3,4], and FVIII gene mutation type [5], have repeatedly been associated with inhibitor development in patients with severe hemophilia. Other genetic factors, such as the major histocompatibility complex profile and polymorphisms in the interleukin-10 and tumor necrosis factor-alpha genes, have also been associated with an increased risk of inhibitor development [6–9]. Yet, the prevention of the development of inhibitors in patients with severe hemophilia has been difficult because of the lack of knowledge about potentially modifiable risk factors.

It seems plausible that, in addition to the genetic risk factors, FVIII treatment strategies also have an effect on the development of inhibitors. Some have suggested that the age at which patients are first exposed to FVIII influences their risk of inhibitor development [10–12]. However, this was not confirmed in several other studies [13–17]. In addition, clinicians have long suspected that periods of intensive treatment at the event of major bleeds or surgery may trigger inhibitor development [18]. However, to date this has not been formally studied in a cohort of patients with severe hemophilia A.

We therefore set out to describe the relationship of age at first exposure to FVIII and intensity of treatment with FVIII (periods of peak treatment, surgical procedures, duration between exposure days, dosing of FVIII and regular prophylaxis) with the risk of developing clinically relevant inhibitors in a cohort of previously untreated patients with severe hemophilia A. To this aim we merged individual patient data from four FVIII product registration studies.

Patients and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

Study population

We combined individual patient data of all 272 previously untreated patients with severe hemophilia A (residual FVIII activity <0.02 IU mL−1) who had participated in one of four recombinant FVIII product registration studies (Kogenate®, Kogenate Bayer®, Recombinate®, ReFacto®) [19–27]. Minimally treated patients were not included. The pharmaceutical companies who had performed the studies provided us with their databases. Patients had been recruited between January 1989 and July 2001. The participating center’s institutional review boards had approved the studies and written informed consent had been obtained from all patients. Because the majority of inhibitors develop before the 50th exposure day, we excluded for the present analyses 32 patients who were lost to follow-up before they had received treatment with FVIII on a total of 50 exposure days [12% of the total number of patients; median cumulative number of exposure days, 16 days; interquartile range (IQR), 10–32 days; range: 1–42 days] [1]. These patients were excluded to enable us to directly estimate the cumulative risk of inhibitor development during the first 50 exposure days. Three other patients were excluded because of unknown baseline FVIII activity levels and one other because of advanced age at entrance into the study (date of birth in 1960s), leaving 236 patients for the present analysis. A total of 62 patients originated from the Recombinate® study, 54 from the Kogenate® study, 30 from the Kogenate Bayer® study and 90 from the ReFacto® study.

Measurements

All studies had collected information on patient characteristics, such as date of birth, residual FVIII activity level, ethnicity, family history of hemophilia and inhibitors, and information on all clotting factor infusions up to the 50th day at which the patient was exposed to FVIII, including dates of infusion, doses and reasons for treatment, types of bleeds and surgery. Details were available on all inhibitor tests and recovery measurements, including dates, body weights, infused doses of clotting factor, pre-infusion and postinfusion FVIII activity levels, and time between infusion of clotting factor and blood sampling for postinfusion FVIII activity level. Inhibitor tests and recovery studies were regularly performed as previously described [20,22,23,26]. Plasma samples with an inhibitor level of 0.6 BU mL–1 or more were considered to be positive for FVIII inhibitor.

We calculated the bodyweights of the patients for all days on which the patients were infused with FVIII using the Dutch growth reference data [28].

In the database of the ReFacto® previously untreated patients study, surgery was not encoded as a reason for treatment, but was categorized as ‘other’. In order to enable investigation of the impact of surgery on inhibitor development, high-dosed treatment on at least three consecutive days for an ‘other’ reason in that database was considered as treatment for surgery.

Definition of inhibitor development

We defined inhibitor development in two ways. Firstly, the development of ‘all clinically relevant inhibitors’ was defined as the occurrence of at least two positive antibody titers combined with a decreased FVIII recovery. Secondly, the development of ‘high-titer inhibitors’ occurred when the peak inhibitor titer was at least 5 BU mL−1 [29]. The FVIII recovery was considered to be decreased when it was < 66% of that expected. The expected level of FVIII activity was calculated according to Lee et al. [30].

Ten patients with marginally positive inhibitor titers on only one occasion (0.6–1.0 BU mL−1) with normal recoveries did not meet the definition of clinically relevant inhibitor.

Data analyses

To examine whether potential determinants were associated with inhibitor development, we used Cox proportional hazard regression models with inhibitor development as the event and the cumulative number of ‘exposure days’ as the time variable [31]. ‘Exposure day’ was defined as a calendar day during which one or more infusions of FVIII were given. As 50% of the inhibitors occur before the 15th exposure day and the other inhibitors occur mostly before the 50th exposure day, it is important to take into account the changes in absolute risk to develop inhibitors with increasing exposures to FVIII: if a patient survives exposure days without inhibitor development, his risk of developing inhibitors decreases gradually. Also, at the 50th exposure day the risk of developing inhibitory antibodies is extremely low. This is taken into account in the Cox proportional hazard models we used.

In the analyses with ‘all clinically relevant inhibitors’ as the outcome, censoring occurred at exposure day 50, as all patients either developed inhibitors or were followed until they received FVIII on at least 50 exposure days. In the analyses with ‘high-titer inhibitor development’ as the outcome, censoring occurred both at exposure day 50 in non-inhibitor patients and at the cumulative number of exposure days at inhibitor development in patients with low-titer inhibitors. Relative hazard rates were interpreted as relative risks. Potential risk factors for which values changed over time were entered as time-dependent covariates.

Potential determinants of inhibitor development

Fixed potential risk factors of inhibitor development were baseline FVIII activity, ethnicity, family history of inhibitors, age at first exposure to FVIII and presence of peak treatment moments at the first treatment episode.

Time-varying potential determinants of inhibitor development were peak treatment moments, surgical procedures, duration between exposure days, dosing of FVIII product and treatment regimen (prophylactic vs. on-demand treatment). Definitions were formulated before the start of data analyses. A ‘peak treatment moment’ was defined as an episode of treatment with FVIII for a bleed or surgery on at least three consecutive days. A ‘major peak treatment moment’ was defined as a peak treatment moment during which treatment was given on at least five consecutive days. We studied major surgical procedures for which at least three consecutive days of replacement therapy were given. The corresponding time-dependent variables were defined as ‘a peak treatment moment during the last three months’, ‘a major peak treatment moment during the last three months’ or ‘a major surgical procedure during the last three months’. This period was chosen because inhibitor tests were performed at least every three months, and thus the development of an inhibitor was likely to be recognized within this period. ‘Duration between exposure days’ was our measure for frequency of exposures; it was defined as the time period between the current exposure day and the fifth exposure day prior to the current exposure day (or in the first four exposure days, the time period was converted to a total of five exposure days). For example, if a patient had developed an inhibitor at the 25th exposure day, the time period between the days at which he had his 20th and 25th exposure day was his ‘duration between exposure days’ at the 25th exposure day. This was calculated for all patients at all exposure days. Similarly, ‘dose of FVIII product’ was defined as the mean dose per kilogram bodyweight of the last five exposure days prior to each exposure day (or in the first four exposure days, the mean dose of all previous days). ‘Regular prophylaxis’ was defined as clotting factor infusions at least once a week aimed at preventing bleeds.

The continuous variables, age at first exposure, duration between exposure days and mean dose of FVIII, were categorized into groups in order to also detect non-linear associations. We created categories containing similar numbers of observations and with practical cut points.

In the multivariate analyses we adjusted for other possible determinants that could have confounded the specific association studied, independent of their statistical significance in univariate analyses. We did not adjust ethnicity and family history of inhibitors for each other because they are both indicators of a genetic predisposition to inhibitors. Peak treatment moment at first exposure day, peak treatment moments, major peak treatment moments and major surgical procedures were not adjusted for duration between exposure days or for mean FVIII dose, because the latter factors are not confounders, but rather part of the same causal path. We adjusted duration between exposure days and mean FVIII dose for each other and for peak treatment moments in order to elucidate the separate effects of these individual factors on the relationship between intensive treatment and inhibitor development. Mean FVIII dose was adjusted for the bodyweights of the patients at each exposure day, because due to the fixed file sizes the dose per kilogram bodyweight depends largely on bodyweight. We have specified in the footnotes to the tables for which variables we adjusted each association.

Sensitivity analyses

We repeated all analyses in patients with a baseline FVIII activity level of < 0.01 IU mL−1.

Role of the sponsor

The pharmaceutical companies who had performed the registration studies for their recombinant FVIII products provided us with the databases. The companies did not have a role in the analytical decisions or in the writing of this manuscript. We gave them insight into the manuscript before submitting it. By contract, mutual comparisons of the products were not allowed by one of the companies.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

Baseline characteristics

Table 1 presents characteristics of the patients included in the study. 67 of 236 patients (28%) developed clinically relevant inhibitors against FVIII. High-titer inhibitors developed in 44 patients. Table 2 presents the characteristics of all clinically relevant inhibitors. Inhibitors developed after a median of ten exposure days (IQR 7–15 days) at a median age of 16 months (IQR 11–21 months).

Table 1.   Baseline characteristics
 Number (%) or median (interquartile range)
  1. *In non-inhibitor patients.

Total number of patients236
Baseline factor VIII activity (IU mL−1)
 0.01–0.0231 (13%)
 < 0.01 152 (64%)
 Unknown53 (22%)
Ethnicity
 Caucasian196 (83%)
 Afro-American22 (9%)
 Hispanic11 (5%)
 Other7 (3%)
Family history hemophilia
 Negative84 (36%)
 Positive149 (63%)
 Unknown3 (1%)
Family history inhibitors
 Negative88 (37%)
 Positive26 (11%)
 Negative family history of hemophilia84 (36%)
 Unknown38 (16%)
Age at first exposure day (months)9 (6–13)
Age at 50th exposure day (months)*38 (27–52)
Duration 1st and 50th exposure day (months)*26 (19–40)
Surgery during first 50 exposure days
 No 192 (81%)
 Yes44 (19%)
Treatment regimen during first 50 exposure days
 On demand169 (72%)
 Prophylaxis67 (28%)
Age start prophylaxis, months21 (14–35)
Reason of first exposure
 Bleed 180 (76%)
 Prophylaxis16 (7%)
 Surgery4 (2%)
 Recovery8 (3%)
 Other 26 (11%)
 Unknown2 (1%)
Table 2.   Characteristics of all inhibitor patients
 Clinically relevant inhibitorsRangeHigh-titer inhibitors*Range
  1. Values are numbers (%) or medians (interquartile range). ED indicates exposure day and BU mL−1 is Bethesda Units per mL.

  2. *High-titer inhibitor was defined as a clinically relevant inhibitor with inhibitor titers of at least 5 BU mL−1 at any time.

Number of patients (%)67 (28%) 44 (66%) 
Number of EDs until inhibitor development10 (7–15)3–4910 (7–15)3–49
Age at inhibitor development (months)16 (11–21)2–6214 (10–23)2–55
Duration between first ED and inhibitor development (months)6 (3–12)14 days–365 (3–9)1–35
Peak titer (BU mL−1) 10 (3–39)1–122926 (10–169)6–1229

Inhibitor risk according to patient characteristics

Table 3 presents the cumulative incidences, crude and adjusted relative risks of inhibitor development according to patient characteristics.

Table 3.   Inhibitor development according to patient characteristics
DeterminantsAll inhibitorsHigh-titer inhibitors*
No. inh (%)Crude relative risk (CI)PAdjusted relative risk (CI)PNo. high inh (%)Crude relative risk (CI)PAdjusted relative risk (CI)P
  1. Inh indicates inhibitor and CI is 95% confidence interval. *High-titer inhibitor was defined as a clinically relevant inhibitor with inhibitor titers of at least 5 BU mL–1 at any time.

  2. Adjusted for ethnicity, family history of inhibitors, age at first exposure, all peak treatment moments, duration between exposure days, mean dose and prophylaxis. Adjusted for baseline factor VIII:C, age at first exposure, all peak treatment moments, duration between exposure days, mean dose and prophylaxis. §Adjusted for baseline factor VIII:C, ethnicity, age at first exposure, all peak treatment moments, duration between exposure days, mean dose and prophylaxis.

Baseline factor VIII:C (IU mL−1)
 0.01–0.02 7/31 (23%)1.0 1.0 6/31 (19%)1.0 1.0 
 < 0.01 40/152 (26%)1.2 (0.5–2.7)0.641.6 (0.7–3.8)0.3129/152 (19%)1.0 (0.4–2.5)0.961.5 (0.6–3.9)0.44
Ethnicity
 Caucasian50/196 (26%)1.0 1.0 29/196 (15%)1.0 1.0 
 Black11/22 (50%)2.4 (1.2–4.5)0.012.7 (1.3–3.1)0.0069/22 (41%)3.3 (1.5–6.9)0.0023.7 (1.6–8.4)0.002
 Hispanic6/11 (55%)2.5 (1.1–5.9)0.033.3 (1.3–8.4)0.016/11 (55%)4.4 (1.8–10.6)0.0015.6 (2.1–14.9)0.001
 Other0/7 (0%)0/7 (0%)
Family history hemophilia
 Negative20/84 (24%)1.0 1.0§ 14/84 (17%)1.0 1.0§ 
 Positive46/149 (31%)1.3 (0.8–2.3)0.271.0 (0.6–1.8)0.8730/149 (20%)1.3 (0.7–2.4)0.490.9 (0.5–1.8)0.83
Family history inhibitors
 Negative23/88 (26%)1.0 1.0 14/88 (16%)1.0 1.0 
 Positive 12/26 (46%)2.0 (1.0–4.0)0.051.6 (0.8–1.4)0.178/26 (31%)2.2 (0.9–5.3)0.071.7 (0.7–4.2)0.23

Patients with a baseline FVIII activity below 0.01 IU mL−1 did not have a statistically significant higher risk of developing inhibitors than patients with a FVIII activity of 0.01–0.02 IU mL−1. This is probably because the study included only 31 patients with FVIII activity between 0.01 and 0.02 IU mL−1, which is too low to study the effect of severity on the risk of inhibitors. Afro-American and Hispanic patients respectively had a 2.4 [95% confidence interval (CI) 1.2–4.5] and 2.5 (CI 1.1–5.9) times higher risk of developing inhibitors than Caucasian patients. A positive family history of inhibitors was associated with an increased risk of inhibitor development [relative risk (RR) 2.0; CI 1.0–4.0]. The results were similar for high-titer inhibitor development.

Inhibitor risk according to treatment-related factors

Age at first exposure Table 4 shows that there was no apparent relation between age at first treatment and the risk of developing clinically relevant inhibitors. Also, patients who were first treated before the age of one month had a similar inhibitor risk [eight of 25 (32%)] as patients who were first treated later in life [59 of 211 (28%)].

Table 4.   Inhibitor development according to treatment-related factors
DeterminantsAll inhibitorsHigh-titer inhibitors*
No. inh (%)Crude relative risk (CI)P for trendAdjusted relative risk (CI)P for trendNo. high inh (%)Crude relative risk (CI)P for trendAdjusted relative risk (CI)P for trend
  1. Inh indicates inhibitor, CI is 95% confidence interval and ED is exposure day. Statistical significance of continuous determinants was assessed by the calculation of P for trend values.

  2. *High-titer inhibitor was defined as a clinically relevant inhibitor with inhibitor titers of at least 5 BU mL–1 at any time. All eight patients who developed inhibitors before the fifth ED only received treatment with factor VIII on single days before inhibitor development. Adjusted for baseline factor VIII:C, ethnicity, family history of inhibitors, all peak treatment moments, duration between EDs, mean dose and prophylaxis. §Adjusted for baseline factor VIII:C, ethnicity, family history of inhibitors, age at first exposure and prophylaxis. Adjusted for baseline factor VIII:C, ethnicity, family history of inhibitors, age at first exposure, all peak treatment moments, mean dose and prophylaxis. **Adjusted for baseline factor VIII:C, ethnicity, family history of inhibitors, age at first exposure, all peak treatment moments, duration between exposure days, prophylaxis and bodyweight.

Age at first ED (months)
 More than 1220/72 (28%)1.0 1.0 13/72 (18%)1.0 1.0 
 6–1226/103 (25%)0.9 (0.5–1.6) 0.8 (0.4–1.4) 16/103 (16%)0.8 (0.4–1.7) 0.6 (0.3–1.3) 
 < 621/61 (34%)1.3 (0.7–2.4)0.431.0 (0.5–2.0)1.0015/61 (25%)1.4 (0.7–3.0)0.370.8 (0.4–2.0)0.76
Peak treatment moment at first ED
 No peak treatment moment58/205 (28%)1.0 1.0§ 37/205 (18%)1.0 1.0§ 
 3 or 4 days1/16 (6%)0.2 (0.03–1.4)0.110.2 (0.02–1.2)0.081/16 (6%)0.3 (0.04–2.2)0.240.2 (0.03–1.8)0.17
 At least 5 days 8/15 (53%)2.1 (1.0–4.5)0.042.7 (1.2–5.8)0.016/15 (40%)2.5 (1.0–5.9)0.043.0 (1.2–7.5)0.02
Peak treatment moments (treatment on at least 3 days) 1.6 (1.0–2.6)0.051.6 (1.0–2.6)§0.06 2.0 (1.1–3.7)0.021.9 (1.0–3.4)§0.04
Major peak treatment moments (treatment on at least 5 days) 1.6 (0.9–2.9)0.111.6 (0.9–2.8)§0.14 1.7 (0.8–3.4)0.161.5 (0.7–3.1)§0.25
Major surgical procedures 2.4 (1.2–4.8)0.022.7 (1.3–5.7)§0.009 4.1 (2.0–8.6)<0.0014.5 (2.0–10.1)§<0.001
Duration between EDs (days)
 More than 100 1.0 1.0  1.0 1.0 
 Ten to 100 2.6 (1.4–5.0) 2.5 (1.3–4.9)  2.9 (1.3–6.4) 2.5 (1.1–5.8) 
 < 10 3.4 (1.6–7.2)0.0012.7 (1.1–6.9)0.01 3.9 (1.5–9.9)0.0032.1 (0.7–6.8)0.14
Dose of factor VIII (IU kg−1)
 < 35 1.0 1.0**  1.0 1.0** 
 35–50 1.4 (0.7–3.0) 1.1 (0.5–2.3)  2.1 (0.7–6.3) 1.8 (0.6–5.7) 
 More than 50  1.9 (0.9–4.0)0.061.0 (0.5–2.3)0.96 3.7 (1.3–10.8)0.0062.2 (0.7–7.0)0.19

Peak treatment moments  Patients who received treatment for a bleed or surgery on at least five consecutive days (major peak treatment moment) immediately at the first treatment episode had a 2.1 (CI 1.0–4.5) times higher risk of inhibitor development than patients who only received treatment on a single day or on two consecutive days.

In addition to studying the effect of peak treatment moments immediately at the first treatment episode, we also studied the effect of peak treatment moments at any exposure day during the first 50 exposure days. The effect of peak treatment moments at any exposure day on the risk of inhibitor development was less distinct (RR 1.6; CI 1.0–2.6).

Major surgical procedures  44 patients had undergone at least one surgical procedure during the first 50 exposure days. During the first 50 exposure days, surgical procedures were associated with a 2.4 (CI 1.2–4.8) times higher risk of developing clinically relevant inhibitors. This appeared to be more pronounced when the analyses were restricted to high-titer inhibitor development (Table 4). The findings were similar in the patients originally included in the studies that specifically encoded surgery as a reason of treatment (data not shown).

Duration between exposure days and dose of FVIII  To examine whether the frequency of infusions or the dosing of FVIII, or both, were primarily associated with inhibitor development, we studied these determinants separately.

A shorter duration between exposure days was associated with an increased risk of inhibitor development. Compared to a duration between five consecutive exposure days of more than 100 days, the risk of ‘all inhibitors’ was 2.6 (CI 1.4–5.0) times higher when the duration was between ten and 100 days, and 3.4 (CI 1.6–7.2) times higher when the duration was < 10 days (P for trend 0.001). When adjusted for all covariates, the adjusted relative risks were similar. Furthermore, the analyses restricted to ‘high-titer inhibitors’ revealed similar findings (Table 4).

Compared to mean doses of five consecutive exposure days of < 35 IU kg−1, the risk of ‘all inhibitors’ was 1.4 (CI 0.7–3.0) times higher when the mean dose was between 35 and 50 IU kg−1, and 1.9 (CI 0.9–4.0) times higher when the mean dose was more than 50 IU kg−1 (P for trend 0.06). After adjustment for all covariates including patients’ bodyweights at each exposure day, this relationship was no longer present. Findings for ‘high-titer inhibitor development’ were similar (Table 4).

Regular prophylaxis  A total of 67 patients (28%) started prophylaxis at least once a week within 50 exposure days. Prophylaxis was started at a median age of 21 months and after a median of 22 exposure days (IQR 13–36 days). Four patients on prophylaxis developed inhibitors (all high-titer; RR 0.8; CI 0.3–2.2).

Sensitivity analyses

All findings were similar in the subgroup of patients with a baseline FVIII activity level of < 0.01 IU mL−1 (see the appendix).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

In this study of 236 previously untreated patients with severe hemophilia A, age at first exposure to FVIII was not associated with inhibitor development. Peak treatment moments were associated with a 60% increased risk and surgical procedures were associated with a more than twice increased risk of subsequent inhibitor development. A shorter duration between exposure days and higher dosing of FVIII per kilogram bodyweight were related to an increased risk of inhibitor development. However, the latter association disappeared after adjustment for patients’ bodyweights. Because too few patients started early prophylaxis during the first 50 exposure days, we could not study the effect of prophylaxis on the risk of inhibitor development in this cohort.

Although we observed a slightly higher incidence of inhibitors in patients who were first treated during the first 6 months of life, we did not find a clearly decreasing trend in inhibitor development according to an increasing age at first exposure. This finding was in accordance with several recent reports [13–17]. However, other studies have shown an inverse association between age at first exposure and the risk of developing inhibitors [10–12]. Possibly, the observed relationships between age at first exposure and inhibitor risk in the latter studies were confounded by other determinants of inhibitors, such as intensity of treatment. Alternatively, selection bias could explain the observed absence of an association, as in our study. Patients with negative family histories, patients treated for severe bleeding at a very young age and patients treated prophylactically at birth may have had less chance to be included in registration studies. The exclusion of these patients may have led to an underestimate of the incidence of inhibitors among those first treated at a young age.

We investigated the effect of the intensity of treatment with FVIII in a cohort of previously untreated patients with severe hemophilia A. We observed an association between intensive treatment and an increased risk of inhibitor development. A role for intensive treatment in inhibitor development has been suggested in a small case-series of patients with mild hemophilia who appeared to develop inhibitors more frequently after surgical procedures and continuous infusion [32]. Our findings appear to contrast with the results of a case–control study, in which no statistically significant difference was seen between the frequencies of surgery and central nervous system bleeding in cases and controls before inhibitor development and the 150th exposure day, respectively [13]. However, the power of the latter study may have been too small to find statistically significant differences. An increased risk of inhibitors after intensive treatment seems plausible as during intensive treatment for bleeding and surgery, significant concentrations of circulating FVIII protein with extensive tissue damage and associated inflammation are present. This may cause the release of immunological ‘danger signals’ that may facilitate an antibody response towards FVIII [33].

The strengths of our analyses were, first, that we stratified our findings according to the number of exposure days. This is crucial because inhibitors develop from the second exposure day onwards. Thus, exposure to potential determinants occurring after this exposure day is no longer independent of the outcome, which could lead to spurious associations if analyzed otherwise. Secondly, all 236 patients had been exposed during at least 50 days, providing a valid estimate of the cumulative incidence of clinically relevant inhibitor development among previously untreated patients. Finally, we prevented selection bias by excluding minimally treated patients.

A limitation of our analysis is that in some instances it is impossible to discern whether intensive treatment is the cause or consequence of an inhibitor. Undetected inhibitors will lead to more intensive treatment, possibly before they are detected. This may explain the observed association between treatment intensity and the occurrence of ‘detected’ inhibitors. However, the observation that elective surgery is also followed by a higher incidence of inhibitors clearly indicates that intensive treatment is the cause and not the consequence of inhibitor development; in the case of surgery, intensive treatment with FVIII is planned and not given according to bleeding symptoms.

We could not study whether the association between treatment intensity and the occurrence of inhibitors was independent of mutations in the gene for FVIII. In theory, it is possible that patients with ‘high risk’ FVIII mutations have a more severe phenotype and as a consequence need more intensive treatment. Our other studies into this subject suggest that the association between treatment intensity and the risk for inhibitors is independent of FVIII mutations [34].

Our observations suggest that multiple exposures to FVIII in a short period of time may increase the risk of inhibitor development in previously untreated patients with severe hemophilia A. Whether alternative treatment regimens, such as avoiding early elective surgery, reduce the risk of developing inhibitors should first be evaluated in a study designed for this purpose.

Addendum

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

S. C. Gouw, H. M. van den Berg and J. G. van der Bom take responsibility for the accuracy of the data analyses. Study concept and design: pharmaceutical companies had previously designed and performed the four cohort studies for registration purposes, and had reported their findings separately in the medical literature. S. C. Gouw, H. M. van den Berg and J. G. van der Bom designed the protocol for the present analysis based on individual patient data from the combined data sets. Acquisition of data: the pharmaceutical companies who had collected the data for registration purposes permitted the authors to perform analyses on treatment-related risk factors for inhibitor development. S. C. Gouw, H. M. van den Berg, S. le Cessie and J. G. van der Bom participated in statistical analyses and interpretation of the data. S. C. Gouw, H. M. van den Berg and J. G. van der Bom drafted the manuscript. H. M. van den Berg, S. le Cessie and J. G. van der Bom did the critical revision of the manuscript for important intellectual content. S. C. Gouw involved in administrative, technical, or material support. H. M. van den Berg and J. G. van der Bom conducted study supervision.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

The authors would like to thank J. P. Vandenbroucke, professor of the Department of Clinical Epidemiology, Leiden University Medical Center, the Netherlands for critical revision of the manuscript for important intellectual content, and C. A. Lee, Emeritus Professor of Haemophilia, University of London for critical recommendations. We also would like to thank H. Ehrlich from Baxter, E. Gorina from Bayer and D. Wages from Wyeth for providing us with the original databases of the recombinant previously untreated patients studies.

Disclosure of Conflict of Interests

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

S. C. Gouw, H. M. van den Berg and J. G. van der Bom have received unrestricted research/educational fundings for various projects at the Van Creveldkliniek from Bayer, Baxter, ZLB Behring, Novo Nordisk and Wyeth. S. le Cessie states that she has no conflict of interest.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Patients and methods
  5. Results
  6. Discussion
  7. Addendum
  8. Acknowledgements
  9. Disclosure of Conflict of Interests
  10. References
  11. Supporting Information

Table S1. Inhibitor development according to patient characteristics in patients with baseline factor VIII:C <0.01 IU/mL.

FilenameFormatSizeDescription
JTH2595+Table+S1.doc72KSupporting info item

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