Whereas the use of valproic acid (VPA) as a histone deacetylase inhibitor (HDACi) is relatively new,1–3 it has been used for decades on a large scale in the treatment of epilepsy and psychiatric disorders. In those patients, long-term use is regularly accompanied by hematological toxicity, including concentration-dependent thrombocytopenia, and leucopenia/neutropenia in 5% to 40% and 5% to 26% of patients, respectively.4–7 In addition, comparable to other antiepileptic drugs, such as carbamazepine (CBZ) and phenytoin (PHT), VPA treatment has been associated with toxicity of all 3 myeloid lineages and an increased risk (odds ratio 18.2) of aplastic anemia.4,8–10 This suggests that VPA treatment induces general myelosuppression in susceptible patients and that this toxicity is directed toward the hematopoietic stem cells or myeloid progenitor cells. Together with the increased clinical use of VPA as an HDACi, the knowledge concerning the effects of this compound in patients treated for hematological malignancies increases rapidly. Nonetheless, the effects of VPA on the normal hematopoietic compartment are still largely unknown. Recently, we have demonstrated in vitro that VPA treatment of hematopoietic progenitor cells affects the composition of the myeloid progenitor compartment, resulting in a significant and concentration-dependent inhibition of neutrophil differentiation.11 To investigate whether the in vitro effects of VPA treatment are also observed in a nonhematologic patient population treated with VPA, we conducted a retrospective study within a cohort of outpatients treated with VPA for neurological or psychiatric disorders, to investigate the effects on the composition of the total leukocyte compartment.
MATERIALS AND METHODS
For this study, data were obtained from the Utrecht Patient Oriented Database (UPOD). The UPOD is an infrastructure of relational databases comprising coded electronic data on patients’ characteristics, laboratory test results, hospital discharge diagnoses, medical procedures, and medication orders for all patients treated at the University Medical Center Utrecht (UMC Utrecht) since 2004. The content of UPOD and its setting have been described in detail elsewhere.12 From the UPOD, all outpatients, both adults and children, were identified who were treated with VPA (N = 221) and had at least one hematological blood test together with a VPA plasma level test on the same day from January 2005 until December 2009. Patients treated with CBZ or PHT was used as reference. For all patient groups (VPA, CBZ, and PHT), the mean and standard deviation of total leukocyte count, absolute neutrophil, lymphocyte, monocyte, and eosinophil counts were determined. A one-way analysis of variance followed by a Bonferroni multiple comparison test was performed to compare the differences between the groups. Within VPA, the correlation between VPA plasma concentration and the absolute number and percentage of neutrophils or lymphocytes was evaluated using linear regression analysis. Data analysis was performed using SPSS 17.0 (SPSS Inc, Chicago, Ill). P < 0.05 was considered statistically significant.
Mean total leukocyte count in patients treated with VPA (7.94 × 109/L, SD = 3.00) was comparable to patients treated with PHT (7.85 × 109, SD = 3.32) but was significantly lower in patients treated with CBZ (7.02 × 109/L, SD = 2.63, P = 0.006). Whereas there was no difference in mean absolute leukocyte numbers between the VPA-treated patients and the PHT-treated patients, the absolute neutrophil number was significantly lower in the patients treated with VPA (4.36 × 109/L, SD = 2.67 vs 5.23 × 109/L, SD = 3.06; P = 0.008) and the absolute lymphocyte number was significantly higher in the VPA-treated patients (2.63 × 109/L, SD = 1.21 vs 1.82 × 109/L, SD = 0.92; P < 0.001) (Fig. 1), suggesting that VPA differentially affects the development of leukocyte subsets. Therefore, we further investigated the effect of VPA plasma concentration on distinct leukocyte subsets. Four patients with extremely high VPA levels due to autointoxication were excluded from this analysis. Linear regression analysis in 217 patients treated with VPA revealed a significant correlation between VPA plasma concentration and the absolute number of neutrophils (rp = −0.26; P < 0.001) and lymphocytes (rp = −0.21; P = 0.002) (Fig. 2), which was not accompanied by a significant negative correlation concerning the total leukocyte number (rp = −0.11; P = 0.095; data not shown) in the peripheral blood. Plasma VPA levels also significantly correlated to the percentage of neutrophils (rp = −0.32; P < 0.001) and percentage of lymphocytes (rp = 0.26; P < 0.001) (data not shown). In the patients treated with VPA together with CBZ, PHT, or both, we observed a specific correlation between VPA and the absolute number and percentage of neutrophils and lymphocytes (data not shown). Together, these data suggest that VPA treatment modulates leukocyte subset development, which could be the result of VPA-induced inhibition of neutrophil development, VPA-induced increased lymphocyte development, or both.
Taking into consideration that neutrophil and lymphocyte development is a dynamic process modulated by a variety of factors, including (latent) infections, our data suggest that VPA treatment affects the development of both leukocyte subsets. Consequently, as neutrophils and lymphocytes are derived from a different (lineage) committed progenitor, this suggests that the effects of VPA concern differentiation of the hematopoietic stem cell or multipotent progenitor cell subsets. Previously, it has been demonstrated that VPA treatment induces the expansion and self-renewal of hematopoietic stem cells in vitro.13,14 In addition, Maës et al15 demonstrated that in human hematopoietic stem cells, lymphoid-affiliated genes and more restricted myeloid-affiliated genes are associated with acetylated histone 3 (H3) and histone 4 (H4), and related histone acetylation profiles in hematopoietic progenitors to progenitor function. We have recently demonstrated that VPA treatment of CD34+ myeloid progenitor cells derived from umbilical cord blood modulates the development of neutrophils in a concentration-dependent manner. In this study, we demonstrated that upon continuous exposure of neutrophil progenitors to 500 μmol/L VPA (72.1 mg/L), terminal differentiation was reduced dramatically.11 Whereas the effects of VPA treatment on lymphocyte development are largely unknown, it has been recently demonstrated that treatment with HDACi, including VPA, increases the number and function of regulatory T cells (FOXP3+ Tregs) in vitro and in vivo.16,17 Finally, it has been recently demonstrated that VPA treatment inhibits the proliferation and differentiation capacity of mesenchymal stromal cells18 and thereby potentially affects hematopoietic progenitor cell function.
Valproic acid treatment is regularly accompanied by hematological toxicity, including leukopenia and neutropenia. Hematological manifestations can be encountered directly after initiation of therapy or after prolonged use and occur more frequently when the serum VPA level exceeds 100 μg/mL.4–6,9 Together with the increased usage of VPA as an HDACi for the treatment of hematological malignancies, the knowledge concerning the effects of VPA on hematopoietic cells, including myeloid progenitor cells, is increasing rapidly. In this study, we demonstrate for the first time that VPA treatment significantly affects the proportional and absolute size of distinct leukocyte subsets in a large patient population. Together with our previous findings, these data suggest that VPA treatment affects the differentiation of normal multipotent hematopoietic progenitors. These data contribute to a better understanding of VPA-induced cytopenias and can be of clinical importance for interpreting clinical findings and considering treatment decisions. Whereas, in most patients, hematological toxicity is generally mild, these data can be of great importance for specific patient groups, including patients with epilepsy, as a part of systemic disease (eg, metabolic diseases and genetic syndromes), elderly patients who generally have increased drug-induced toxicity,19 and patients with hematological diseases characterized by disrupted or aberrant myeloid differentiation (eg, hematological malignancies and bone marrow failure syndromes).
AUTHOR DISCLOSURE INFORMATION
The authors declare no conflicts of interest.
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