Minke A. E. Rab

Hydroxyurea (HU) is the most widely prescribed disease‐modifying treatment in sickle cell disease (SCD), though treatment responses vary due to metabolism and adherence. We examined HU blood levels and treatment response in patients with homozygous sickle cell disease (HbSS). We measured HU in whole blood of 955 SCD patients enrolled in GenoMed4All using mass spectrometry. HU detection was defined as Z‐score > 2.5 relative to untreated healthy controls. Among HbSS patients not receiving recent transfusion therapy ( n = 539), HU was detected in 39% (143/369) of HU‐prescribed patients ≥ 10 years (median age 31.8 [10.0–69.9]). In only 24% (23/109) of young patients (< 10 years, median age 5.9 [1.2–9.9]), HU was detected, despite comparable HU dosing (18.8 vs. 20.3 mg/kg/day). Detectable HU was associated with increased MCV ( p < 0.001), and in patients ≥ 10 years also with increased HbF, despite low HbF (< 10%) in 25%. Neutropenia occurred in 8.0% of patients with detectable HU, 4.5% with undetected HU. Only one patient had severe neutropenia (< 0.5 × 10e9/l). In young patients, neutropenia occurred in 1.4% with undetected HU, and in 8.7% with detected HU. This is the largest study to date that reports data on HU levels in HbSS patients. Most HU‐prescribed patients, particularly young patients, had undetectable HU levels. Low MCV, HbF, and absent toxicity in children and limited toxicity in adults suggest that treatment for patients with undetectable HU is suboptimal. These findings warrant the need to optimize dosing and adherence to improve treatment in SCD, especially in young patients.

Summary: Hereditary spherocytosis (HS) is a hereditary haemolytic anaemia, caused by pathogenic variants in genes encoding red blood cell membrane proteins. Osmotic gradient ektacytometry evaluates red cell deformability and hydration and is increasingly used in the diagnosis of HS. We retrospectively evaluated laboratory data from 233 HS patients focusing on osmotic gradient ektacytometry parameters, including two novel parameters, O _min ‐width and O _max ‐width. We found that the maximum elongation index (EI _max ), representing the maximum deformability, was decreased in SPTB relative to SLC4A1 and SPTA1 (0.509 vs. 0.557 and 0.564, both p < 0.01). Hydration was most affected in SLC4A1 , with the lowest median O _hyper (416 mOsm/kg). The novel parameters also showed differences: O _min ‐width was higher in SPTB and ANK1 compared to SLC4A1 and SPTA1 ( p < 0.05). O _max ‐width was lowest in SLC4A1 (92 mOsm/kg). We found that non‐missense variants, opposed to missense variants, were associated with decreased deformability in SPTB and ANK1 subgroups as well as with decreased hydration in the ANK1 subgroup specifically (O _hyper missense 472 mOsm/kg vs. non‐missense 436 mOsm/kg). Lastly, when classifying disease severity based on reticulocyte production index, we found that O _min , EI _max , area under the curve (AUC), O _min ‐width and O _max ‐width differed between mild and moderate patients (all p < 0.05). Our findings suggest that osmotic gradient ektacytometry provides additional information on HS pathophysiology and clinical severity.

Pyruvate kinase (PK) is a key enzyme involved in the final step of glycolysis, essential to produce adenosine triphosphate (ATP). Relatively decreased red blood cell (RBC) PK activity (reflected by a lower PK/hexokinase [HK] ratio) and PK thermostability (PK activity after exposure to heat) were recently identified as pathophysiological features of sickle cell disease (SCD). In this study, we investigated whether impaired PK function is associated with sickle RBC properties and SCD‐related clinical manifestations. This study included 97 non‐transfused patients with SCD (88 HbSS, 9 HbS/β0 thalassemia). PK thermostability was correlated with RBC parameters such as reticulocyte count (r = −0.402, p < 0.0001) and hemoglobin F (r = 0.394, p < 0.0001), and indicators of impaired functional properties of sickle RBCs, such as the point of sickling (r = −0.417, p < 0.0001), oxygen affinity (r = 0.408, p < 0.001) and RBC adhesion to laminin (r = −0.322, p = 0.024). Additionally, a low PK/HK ratio correlated with decreased PK thermostability (r = 0.308, p = 0.002), decreased RBC deformability (r = 0.268, p = 0.009), and elevated 2,3‐diphosphoglycerate levels (r = −0.244, p = 0.016). Multivariate Poisson regression analysis demonstrated that reduced PK thermostability and PK/HK ratio were associated with a higher incidence of SCD‐related clinical complications. For every 10‐unit decrease in PK thermostability and 1‐unit decrease in PK/HK ratio, the incidence of SCD‐related clinical complications increased by 11% (p = 0.012) and 10% (p = 0.019), respectively. Altogether, these findings indicate that impaired PK function is related to compromised sickle RBC properties and SCD‐related clinical manifestations. This supports the relevance and underlines the potential of PK activation therapy.