ALBERT

Description: Fetal hemoglobin (HbF) is strongly associated with clinical severity in the β-hemoglobinopathies, including sickle cell disease (SCD). In recent years, the three major HbF genetic loci (at BCL11A on chromosome 2p, HMIP-2 on chromosome 6q and Xmn1-HBG on chromosome 11p) have been more clearly characterized and mechanisms of the likely causal variants better defined. In this study, we have combined this new biological understanding with statistical methods to create a genetic "predictor" for HbF in SCD. We chose 7 variants to represent the 3 HbF quantitative trait loci (QTL) to investigate their utility in predicting HbF levels, and, in turn, clinical severity of SCD. For BCL11A, we used 2 markers: rs1427407 (62kb downstream of BCL11A) localizes to an erythroid-specific enhancer (Bauer et al. Science 2013) and rs6545816 tags a second signal 58kb downstream of BCL11A. The genetic architecture of HMIP-2 as a QTL comprises two elements, A and B (Menzel et al. Ann Hum Genet 2014). We have represented HMIP-2A withthe 3bp deletion rs66650371, shown as a causal variant (Stadhouders et al. JCI 2014) plus the ethnicity marker rs9376090. HMIP-2B is less well-characterized, we selected: rs9494142 (near the MYB enhancer) and rs9494145. For the β-globin locus, we used the long-established Xmn1 marker (rs7482144) in the proximal promoter 158kb upstream of HBG2. This is likely not the variant itself, but in tight linkage disequilibrium with the causal element. Of 892 initial patients (516 females, 376 males), we excluded 17 children aged under 5 because of the non-linear relationship between age and HbF at a young age (we confirmed this finding in our cohort). This left: 658 with HbSS, 206 with HbSC, 8 with HbSβ0 thalassemia, and 20 with HbSβ+ thalassemia. We then genotyped 666 patients with HbSS/HbSβ0 thalassemia for the 7 genetic variants. For each patient, we selected 'validated' HbF levels i.e. HbF not influenced by transfusion, drugs (especially hydroxyurea) or pregnancy. HbF levels were log-transformed (Ln). We then used multiple linear regression models to identify variants which were independently associated with Ln-HbF levels. Using only age and sex as covariates revealed predictive power r2~10% which was orthogonal to (i.e. additive) the predictive power of the variants, and so we did not include them in subsequent analysis. Also, by adding α-globin status to the model where known (N=272), the r2 remained unchanged and is not significant for α-globin status. We then normalized the 7 variants to take account of the mean allele count (a strongly predictive but rare variant may not explain much of the total population variance). We performed multiple linear regression to rationalize the 7 variants, and found 4 markers (rs6545816, rs1427407, rs66650371 and rs7482144) independently contributing HbF-boosting alleles (see table). Combining these 4 variants into a genetic risk model, as per the table, allows us to predict 21.8% of variability (r2) of HbF in our HbSS / HbSβ0 thalassemia patients. We validated the 4-variant risk score first with a 5-fold cross-validation within the cohort which demonstrated a mean r2=22% for the 5 folds. We then replicated the findings in the cohort of HbSC patients (N=206) and found the 4-variant model to predict HbF with variability r2=27.5% (i.e. towards r2=44% seen in non-anemic individuals). Thus, our 4-variant model provides a robust approach to genetic prediction of HbF in SCD. The predictive power appears to be larger for HbSC compared to HbSS (r2=27.5% vs 21.8%) which may be related to stress erythropoiesis in HbSS patients releasing immature erythrocytes as a non-genetic factor modifying HbF levels. This process is a first step towards creating a global genetic predictive score in SCD: stratifying patients with SCD early in life would enable us to offer curative therapy (i.e. hematopoietic stem cell transplant) to those identified as genetically severe. Disclosures No relevant conflicts of interest to declare.

Description: New treatments directly targeting polymerization of sickle hemoglobin (HbS), the proximate event in the pathophysiology of sickle cell disease (SCD), are needed to address the severe morbidity and early mortality associated with the disease. Voxelotor (GBT440) is a first-in-class oral therapy specifically developed to treat SCD by modulating the affinity of hemoglobin (Hb) for oxygen, thus inhibiting HbS polymerization and downstream adverse effects of hemolytic anemia and vaso-occlusion. GBT440-001 was a phase 1/2 randomized, double-blind, placebo-controlled, single and multiple ascending dose study of voxelotor in adult healthy volunteers and patients with SCD, followed by a single-arm, open-label extension study. This report describes results of voxelotor (500-1000 mg per day) in patients with sickle cell anemia. The study evaluated the safety, tolerability, pharmacokinetic, and pharmacodynamic properties of voxelotor and established proof of concept by improving clinical measures of anemia, hemolysis, and sickling. Thirty-eight patients with SCD received 28 days of voxelotor 500, 700, or 1000 mg per day or placebo; 16 patients received 90 days of voxelotor 700 or 900 mg per day or placebo. Four patients from the 90-day cohort were subsequently enrolled in an extension study and treated with voxelotor 900 mg per day for 6 months. All patients who received multiple doses of voxelotor for ≥28 days experienced hematologic improvements including increased Hb and reduction in hemolysis and percentage of sickled red cells, supporting the potential of voxelotor to serve as a disease-modifying therapy for SCD. Voxelotor was well tolerated with no treatment-related serious adverse events and no evidence of tissue hypoxia. These trials were registered at www.clinicaltrials.gov as #NCT02285088 and #NCT03041909.

Description: Abstract 261 Introduction: In children with sickle cell disease (SCD), obstructive sleep apnoea (OSA) is common and the degree of overnight hypoxia is closely correlated with disease severity, particularly frequency of painful crises, risk of stroke and presence of pulmonary hypertension. No studies have examined the prevalence or effects of OSA in adults with SCD. Methods: All adults with SCD attending routine sickle cell out-patient clinics were offered screening for OSA using the Epworth Sleepiness Score (ESS) questionnaire. All of those in whom the score was 〉10 or in whom there was a high clinical suspicion of OSA were offered overnight domiciliary oximetry. Oximetry traces were both analysed by computer and manually. Computerised analysis was used to quantify night time hypoxic load (mean nocturnal SpO2 and sleep time with SpO2 below 90%) and frequency of desaturation (number of times per hour the SpO2 dropped by 〉4%, or Overnight Desaturation Index, ODI). Manual analysis of each trace was performed by four independent sleep physicians who assigned a diagnosis of normal, OSA, non-OSA nocturnal hypoxia or inadequate based on the pattern of the oxygen saturation and heart rate traces. These parameters were then correlated with measures of disease severity and presence of complications taken from the patient's medical record. Statistical analysis was performed using the t-test and linear regression. Results: 93 patients completed the ESS, with 34 patients identified for subsequent oximetry (26 ESS〉10, 8 clinical suspicion). 22 patients went on to have subsequent oximetry. 17/22 of recordings (77%) were abnormal of which 11 showed OSA (65%) and 6 showed nocturnal hypoxia without OSA (35%). Nocturnal hypoxic load, measured as mean overnight oxygen saturation, was correlated with glomerular filtration rate (r=0.5; p=0.0008) and pulmonary artery systolic pressure estimated by echocardiography (r=0.71; p=0.0001). Furthermore, urine protein level correlated with ODI (r=0.5 p=0.0007) and mean overnight oxygen saturation (r=0.35; p=0.02). In addition, mean overnight oxygen saturation was 87% in male patients that experienced priapism and 94% in those who did not (p=0.004). Mean ODI was 10/hr in patients with priapism and 4 in those without (p=0.008). There were no correlations demonstrated between either hypoxic load or ODI with the frequency of painful crises, frequency of hospital admissions or avascular necrosis. Lung function test parameters, history of chest syndrome, hydroxyurea therapy and Epworth score did not predict either hypoxic load or ODI. However, daytime SpO2 were highly predictive of nocturnal hypoxic load (r=0.69 p=0.0000007) but not ODI. Conclusion: OSA and non-OSA nocturnal hypoxia are common in adults with sickle cell disease. The degree of nocturnal hypoxia is associated with impaired renal function, proteinuria, priapism and pulmonary hypertension. Daytime SpO2 appears to be a better predictor of night time hypoxic load than daytime somnolence. Further work is needed to investigate the effect of interventions such as nocturnal oxygen therapy or continuous positive airway pressure on the associations reported. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 9 Introduction: The rate of complications after surgery is increased in patients with Sickle Cell Disease (SCD) and pre-operative blood transfusion has historically been used to decrease this risk. Observational studies and one limited Randomised Controlled Trial (RCT) have suggested that in some patients, transfusion can safely be omitted. Since transfusion is associated with complications including alloimmunisation and increased post-operative infections, we performed a RCT to address whether overall peri-operative complications in SCD are reduced by pre-operative transfusion. Methods: TAPS was a Phase III multicentre, pragmatic, randomised controlled trial with a parallel group sequential superiority design, carried out between November 2007 and March 2011 at 22 sites in the UK, Netherlands and Canada. Eligible patients had HbSS or HbSβ°thal, were aged one year or more and were having low risk (eg adenoidectomy, dental surgery) or medium risk (eg joint replacement, cholecystectomy, tonsillectomy) elective surgery. Patients were excluded if they had a haemoglobin (Hb)

Description: Introduction Women with Sickle Cell Disease (SCD) have been reported to have delayed menarche, increased rates of miscarriage & still birth. There is little evidence of the impact of SCD severity on fertility, or of patients’ attitudes towards their fertility. Methods Following IRB approval, an anonymised questionnaire was distributed to female patients over the age of 16 attending an SCD clinic. This included questions on menarche, fertility, reproductive history & clinical severity. We also surveyed attitudes to fertility & the role of fertility treatment. Results 101 women gave informed consent & returned the survey of whom 68 (67%) had sickle cell anaemia (HbSS), 31 (31%) had HbSC & 2 (2%) HbSß0 thalassaemia (HbSß0thal), the group mean age was 38yrs (range 17-74). 46 patients had severe disease phenotype (criteria for severity: ≥3 hospital admissions per year / ≥2 episodes of acute chest crisis/ ≥6 crisis at home per year or on Hydroxycarbamide (HU) therapy), 78% (36/46) had HbSS & 49%(2/46) HbSC. 49% (22/46) were on HU, of whom 19 (86%) had HbSS & 3 (14%) HbSC, the mean duration on HU was 4.7 years (range 2 weeks–15 years). 3 patients with HbSS, were on a chronic transfusion program, 1 as secondary stroke prevention & 2 for disease severity. Mean age of menarche was 14 yrs (range 10-19). HbSC patients’ mean age of menarche was 13yrs (range 10-16), HbSS patients mean age of menarche was 14 yrs (range 10-19). 30% (30/101), (24 with HbSS & 6 with HbSC), had never been pregnant. This group had a mean age of 28yrs (range 17-46), 1 never wanted children, 2 had previously been advised not to conceive, 1 was actively trying for a baby, & 1 patient was awaiting in vitro fertilisation therapy (IVF). There was no difference in the severity of sickle disease in this group compared to the group with pregnancies. 70%(71/101) reported a total of 211 pregnancies, resulting in 111 (52%) successful deliveries (SDs) i.e. a live child/children. Further evaluation of 175 (81%) pregnancies revealed 98 (56%) SDs, (5 sets of twins & 1 early neonatal death equating to 102 live births), 45 (26%) miscarriages, 27 (15%) terminations of pregnancy & 5 (3%) still births (pregnancy loss at 〉 24 weeks gestation). 52% (50/96) & 63% (47/75) of pregnancies in women with HbSS & HbSC resulted in SDs respectively, as did 25%(1/4) of pregnancies in HbSß0thal women. 74% (53/71) had at least 1 miscarriage (mean 1.7, range 1-7) there was no correlation between miscarriage & sickle genotype or age at pregnancy. 46% (20/46) & 63% (36/55) of patients with severe & milder disease phenotype had miscarriages respectively (p = 0.01). The mean age at termination of pregnancy (26yrs) was lower, compared to 30yrs for women who had SDs or miscarriage & stillbirth (31yrs). In total 41% (72/175) pregnancies were unplanned. 96% (26/27) of terminated pregnancies were unplanned, as were 32% (31/98) of SDs & 28% (15/50) of miscarriages. 28% (28/100) of responders were currently using contraception. 12 (16%) of 71 women reporting pregnancies had no children, they had a total of 21 pregnancies (mean 1.75, range 1-7), 8 (67%) had HbSS, 3 (25%) HbSC. 18% (18/101) thought sickle may have affected their fertility, 93% (93/101) reported having children as being important, this included 28 (93%) of 30 women who had reported no pregnancies. 62% (63/101) would consider prenatal diagnoses (PND) if their partner had sickle cell trait, 59% (60/101) would consider pre implantation diagnosis (PIGD) & 24% ( 24/101) would consider termination of an fetus with SCD. Discussion This study confirms that compared to the normal UK population, women with SCD, have a slightly increased rate of miscarriage & a significantly elevated rate of stillbirth 3% vs 0.005%. Menarche is delayed by a year in patients with HbSS & women with HbSC are more likely to have a successful pregnancy. Although 74% of the women had miscarriage & only 56% of pregnancies resulted in a successful delivery, the majority of women were unaware of the effect of SCD on fertility. Most were not currently using any form of contraception, the rate of unplanned pregnancy was lower at 41% than that in the normal UK population of 50%. These women report having a child as being very important to them, & though majority would consider PND & PIGD only a minority (24%) would terminate a foetus diagnosed with SCD. This study is limited by being retrospective & single centred, further multi-centre prospective studies are required to validate these results. Disclosures: Howard: Sangart: Membership on an entity’s Board of Directors or advisory committees.

Description: Background: Sickle cell disease (SCD) is caused by polymerization of Hemoglobin S, resulting in red blood cell (RBC) sickling, RBC destruction, vaso-occlusive episodes and end-organ damage. No direct anti-polymerization, mechanism-based, preventive therapy is available. GBT440 is a novel small molecule hemoglobin modifier which increases hemoglobin oxygen affinity. In vitro and in vivo studies have shown it to be a potent and direct anti-sickling agent with high specificity for hemoglobin, and that 10-30% hemoglobin modification could be both safe (not compromising oxygen delivery) and effective (preventing HbS polymerization). We hypothesized that a potent antisickling hemoglobin modifier should rapidly interrupt RBC hemolysis, improve anemia and potentially become a safe and effective long-term disease-modifying therapy. We therefore explored safety, pharmacokinetics and pharmacodynamics and potential efficacy in healthy volunteers and SCD patients. Methods: This prospective, randomized, placebo-controlled, double blind, parallel group phase I/II study enrolled healthy volunteers (HV) ages 18-55, and homozygous HbSS SCD patients ages 18-60 with baseline Hb levels ³6 g/dL and ²10 g/dL and without vaso-occlusive crisis or transfusion within 30 days of screening. The study was conducted in two parts: part A tested single ascending doses and part B multiple ascending doses of study drug with 6:2 randomization (GBT440:placebo). Doses administered were: part A, HV cohorts 100mg to 2800 mg, SCD cohort 1000 mg; part B, HV cohorts 300 mg to 900 mg once daily for 15 days, SCD cohort 700 mg once daily for 28 days. The primary endpoint was safety. Secondary endpoints included pharmacokinetics (PK) and pharmacodynamics (PD). Clinical indices of hemolysis were prespecified as endpoints and are described for the SCD multiple dose cohort (700 mg QD for 28 days). Descriptive statistics were used to analyze data. Results: As of July 24 2015, 64 healthy volunteers (HVs) and 16 SCD patients had been enrolled. 54 healthy volunteers had completed the study, 2 were discontinued due to mild-moderate nonserious adverse events (headache, rash) and 8 were in follow-up; 8 SCD patients had completed Part A of the study and 8 were in Part B follow-up. No SCD patients were discontinued; one part B subject had a dose reduction from 700 mg to 400 mg (due to abdominal discomfort). GBT440 was generally well-tolerated, most adverse events were mild, there were no deaths, and there was one serious adverse event (AE) of acute painful crisis in a placebo subject. There were no AEs related to tissue hypoxia. GBT440 showed dose proportional PK, a terminal half-life of 1.5-3 days, high partitioning into RBCs (RBC:Plasma 60-90:1), and a dose dependent increase in hemoglobin oxygen affinity in both HV and SCD patients. In the multiple dose SCD cohort, all patients were evaluable at 28 days (age range 21-52 years; 2 were on hydroxyurea; 3 males/5 females). GBT440-treated patients showed increased hemoglobin, hematocrit and erythrocyte counts with corresponding decreases in LDH, unconjugated bilirubin, reticulocytes and erythropoietin levels with trends evident as early as day 8 compared to no changes in the placebo group (Figure 1 and Table 1). Analysis of peripheral blood smears revealed a marked reduction in sickle cells with GBT440 treatment. Conclusions: Single, oral, daily dosing with GBT440 was well tolerated across a wide dose range and demonstrated dose proportional and predictable PK and PD. GBT440 demonstrated proof of mechanism with a dose-dependent increase in hemoglobin oxygen affinity without causing tissue hypoxia. GBT440 demonstrated proof of concept in SCD patients with rapid reduction in RBC hemolysis and improved oxygen delivery to tissues as evidenced by reduced erythropoietin level, and a marked reduction in circulating sickle cells. These results support further clinical investigation of GBT440 as a potential disease-modifying therapy for SCD. Table 1. Mean change to Day 28 (SEM) GBT440 PLA Hemoglogin (g/dL) 0.8 (0.2) -0.5 (0.4) % Sickle cells in peripheral blood (%) -83 (9) 19 (4) Erythrocyte count (1012/L) 0.5 (0.1) -0.1 (0.2) Reticulocyte count (%) -2.2 (1) 0.8 (1.6) LDH (% change from baseline) -12 (7) -9 (1) Unconjugated bilirubin (% change from baseline) -25 (10) -9.6 (16) Erythropoietin (U/L) -29 (16) 21 (33) GBT = GBT440 treated subject; PLA = placebo subject; SEM = standard error of the mean Figure 1. Figure 1. Disclosures Lehrer-Graiwer: Global Blood Therapeutics: Employment, Equity Ownership. Howard:Pfizer: Consultancy; Novartis: Consultancy, Other: Travel Grant; Aes-Rx: Consultancy. Layton:Agios: Consultancy; Novartis: Consultancy; Glaxo Smith Kline: Consultancy. Mant:Quintiles: Employment, Equity Ownership. Dufu:Global Blood Therapeutics: Employment, Equity Ownership. Hutchaleelaha:Global Blood Therapeutics: Employment, Equity Ownership. Koller:Global Blood Therapeutics: Employment, Equity Ownership. Oksenberg:Global Blood Therapeutics: Employment, Equity Ownership. Patel:Global Blood Therapeutics: Employment, Equity Ownership. Ramos:Global Blood Therapeutics: Employment, Equity Ownership.

Description: Background: The clinical picture in sickle cell disease (SCD) is highly heterogeneous. Knowledge of the determinants of a severe disease course will help us to unravel the pathophysiological mechanisms underlying the disease process and provide novel targets for therapeutic interventions. To generate this knowledge well designed etiological studies are needed, using a valid outcome measure for disease severity. Objective: The aim of this systematic review is to identify all indices used to discriminate SCD patients by their current disease severity and to evaluate the methodological foundations of the indices and utility for clinical research. Method: We performed a systematic search in MEDLINE (Pubmed) (1966- February 2006) using the search terms: anaemia, sickle cell/ [MeSH]; health status indicators/ [MeSH]; severity; severe and clinical spectrum. Reference lists of relevant studies and reviews were screened to identify additional articles. Information was extracted in duplicate by two independent reviewers (XT, KF). Results: The Medline search yielded 1346 abstracts from which we selected 91 articles. Reference tracking resulted in 20 additional articles. After evaluation of the full text of these 111 articles, 28 articles (27 in English) were included in the review. These 28 indices contained 50 different items, including age at diagnosis, symptoms, findings at physical examination, laboratory values, organ damage, treatments and socio-economic consequences. The five most frequently used items were painful vaso-occlusive crises (86% of the indices), central nervous system abnormalities (59%), aseptic/avascular necrosis of the bone (52% of the indices), acute chest syndrome (48% of the indices) and leg ulcers (48% of the indices). Many indices (37%) were calculated by adding up scores on individual items without making a difference in the weight of the constituent items. In indices that did differentiate between items by allocating scores differentially, the rationale for weighing of the items was not explained. There were no scores that clearly distinguished devastating complications from complications that leave no sequelae. Most severity indices (89%) were not validated. Conclusion: In order to increase our understanding of the pathofysiology of SCD and identify potential targets for new therapeutic interventions, we need to identify the determinants of a severe phenotype. For this purpose, consensus on the concept of severity in SCD and an instrument or index to measure it are necessary. This review reveals that there is no consensus on a definition of SCD severity and an index to measure it. Given the current state, efforts should be made to reach international consensus on a definition of SCD severity and a core set of items to measure this concept should be developed.

Description: Abstract 2642 We studied 36 patients attending the sickle cell clinic in our large adult population of over 600 patients. These patients presented to the sickle psychology service because they had a past history of a stroke, or presented with concerns about memory. An MRI brain scan and neuropsychological testing (including tests for IQ, processing speed, executive function and memory) were performed on each patient. The data were organised into four groups in terms of severity of MRI abnormalities: normal MRI n=13; silent cerebral infarcts (subcortical punctate small vessel cerebrovascular disease) n= 11; severe infarcts as an adult n=8; and severe infarcts as a child n=4. The majority of patients with a history of stroke had evidence of large vessel infarcts as well as features of deep watershed ischaemia seen in large vessel (distal internal carotid artery) disease. These groups did not differ significantly in terms of gender, phenotype, age and mood. Cognitive impairments were more prevalent in the severe infarct groups but were also found in patients with silent cerebral infarcts and normal MRIs. Executive functioning and processing speed deficits were evident in all groups but were more severe in the silent cerebral infarct and severe infarct groups. Chi-squared tests for trend showed that the following test scores tended to reduce as MRI abnormalities increased: Full-Scale IQ (p=0.016), Processing Speed Index (p=0.015), Trail Making Test A (p=0.014), Trail Making Test B (p=0.018), and FAS Verbal Fluency Test (p=0.006). This suggests that executive functioning, processing speed and full-scale IQ are particularly vulnerable to the effects of MRI abnormalities in this patient population. Although the cognitive impairments were more severe in the groups with abnormal MRIs, there was significant cognitive impairment in some patients with normal MRIs, suggesting that other factors are also causative of cognitive impairments. These factors may include physiological causes such as impaired perfusion, and psychosocial factors such as disruption to education. These results agree with a recent US study (Vichinsky et al, 2010, JAMA, 303, 1823–1831) showing cognitive impairment in patients with normal MRI scans which implies that MRI is not an adequate screening tool to identify patients with cognitive impairment. This study has important clinical implications in terms of how cognitive deficits can affect the effectiveness of patient – health care professional consultations, patients' ability to manage their SCD and adhere to medication and health care advice. It is important therefore to identify patients with SCD who have such cognitive impairments so appropriate support can be offered. Disclosures: No relevant conflicts of interest to declare.

Description: Patients with sickle cell disease (SCD) suffer from frequent and severe episodes of pain that are associated with hospitalizations, impaired quality of life and increased mortality. Current treatment options are scarce. Oxidative stress appears to play a pivotal role in the pathophysiology of SCD. Pilot studies have demonstrated that administration of the antioxidant N-acetylcysteine (NAC) effectively reduces markers of oxidative stress in SCD and may decrease the hospitalization rate for painful crises. NAC is a safe, inexpensive and well-tolerated drug that has been used for years for various indications. The primary aim of this study was to evaluate the effect of NAC on the frequency of daily pain in patients with SCD. We conducted a randomized (1:1), double-blind, placebo-controlled, parallel-group trial at 11 sites across The Netherlands, Belgium and the United Kingdom. Patients were eligible for participation if they were ≥12 years of age, had either HbSS, HbSC, HbSβ⁰- or HbSβ⁺-thalassemia with a history of at least 1 painful crisis per year over the 3 years prior to enrolment. At randomization patients were assigned to receive either oral NAC 600 mg twice daily or placebo for a total duration of 6 months. Patients had monthly checkups during the study. The primary endpoint was the rate of SCD related pain days per patient year, assessed daily with the use of pain diaries. Secondary endpoints included the rate of days with painful crises, admission days, hospitalizations and days with home analgesic use, the severity of pain, the time to first painful crisis and hospitalization, the number of adverse events and the effect on quality of life and various blood markers. The primary intention-to-treat analysis of this study was limited to patients with a minimal completed diary observation time of 110 days. Sensitivity analyses were done in both all randomized patients as well as a subset with ≥80 days of observation time. Lastly, an additional per protocol analysis was performed on patients with adequate adherence (≥80% of tablets used) and ≥110 days of completed diary observation time. A total of 96 patients were randomized of which 67 patients met the minimum observation time of 110 days (27 in the NAC and 40 in the placebo arm). Inclusions were stopped before reaching the estimated sample size of 116 patients due to imposed time restrictions by the main funder of this study. Groups were well balanced for baseline characteristics. The proportion of patients adherent to the assigned study medication regimen was low in both treatment groups (53% in the NAC and 50% in the placebo arm). The rate of SCD related pain days per patient year was 61.4 in the placebo group and 61.6 in the NAC group (rate ratio 0.98; 95% confidence interval [CI] 0.54-1.78; P=.98). Moreover, there were no significant differences in the secondary endpoints between groups. These findings were consistent in the sensitivity analyses. In the total study population, more patients reported gastro-intestinal events in the NAC group (42%) as compared to the placebo group (15%, P