This is a three-part, multicenter, Phase 2/3 study of orally administered GBT021601 in participants with SCD. Part A will evaluate the safety, tolerability, and efficacy of 12 weeks of GBT021601 in adult participants with SCD to determine the optimum dose. Part B will evaluate the efficacy of GBT021601 versus placebo in adult and pediatric participants with SCD for 48 weeks. Part C is a single-arm study which will evaluate the PK and safety of single and multiple doses of open-label GBT021601 administered to pediatric participants. Part B and C will not be initiated until the optimum dose has been selected from Part A. This study will be conducted in up to 50 sites globally including Kenya, with approximately 12 participants projected to be recruited in Kenya.

The GBT021601-021 study is a Phase 2/3, Randomized, Multicenter Study of GBT021601 Administered Orally to Participants with Sickle Cell Disease and an Open Label Pharmacokinetics Study in Pediatric Participants with Sickle Cell Disease. GBT021601 is an HbS polymerization inhibitor being developed by Global Blood Therapeutics for the treatment of SCD.

SCD is an inherited disorder caused by a point mutation in the β-globin gene leading to formation of HbS. A primary and obligatory event in the molecular pathogenesis of SCD is the polymerization of intracellular HbS following deoxygenation in the microvasculature. HbS polymerization leads to decreased red blood cells (RBC), deformability, morphologic sickling of RBCs, decreased RBC survival, and microvascular obstruction. Clinically, SCD is a devastating and debilitating disease marked by the pathophysiologic features of hemolytic anemia, vaso-occlusion, and progressive end-organ damage. Despite current standards of care, including hydroxyurea (HU), blood transfusion, and supportive care with analgesia, patients with SCD continue to suffer serious morbidity and premature mortality.

GBT021601 increases hemoglobin-oxygen (Hb-O2) affinity and stabilizes hemoglobin (Hb) in the oxy-hemoglobin (oxyHb) state thereby inhibiting polymerization of HbS in RBCs. A drug that inhibits HbS polymerization in all RBCs has the potential to provide superior efficacy to available treatments.

GBT021601 has the potential to be a disease-modifying therapy, leading to increased RBC count, reduced hemolysis, and the potential to reduce the end-organ damage resulting from chronic hemolytic anemia. Proof of concept for this Hb modification approach has been provided with voxelotor, an orally administered small molecule inhibitor of HbS polymerization. Voxelotor was approved by the United States (US) Food and Drug Administration (FDA) in 2019, under the trade name Oxbryta®.

The GBT021601-021 is a three-part multicenter study (Part A, B and C). The maximum duration of the study for adult participants in Part A is approximately 28 weeks. This includes a 28-day Screening period, up to a 12-week Treatment period. All participants will be followed by an EOS Visit 8 weeks after completion of treatment. For Part B, the maximum duration of the study for participants is approximately 60 weeks which includes a 28-day Screening period and a 48-week Treatment period. All participants will be followed by an EOS Visit 8 weeks after completion of treatment. For Part C, the maximum duration of the study for pediatric participants is approximately 23 weeks. This includes a 35-day Screening period and up to an approximately 10-week Treatment period. All participants will be followed by an end of study (EOS) Visit 8 weeks after completion of treatment. Parts A, B, and C: Participants may choose to enroll in an open-label extension (OLE) study under a separate protocol after completing all of the study visits. Participants not entering the OLE study will be followed for approximately 8 weeks after completing dosing to further assess safety

Investigational Product:

GBT021601 as 25mg and 100mg tablets for oral administration (Packaged in high density polyethylene bottles with child resistant caps with induction seal and labeled according to local regulations)

Number of Clinical Sites

The study will be conducted in approximately 50 sites globally including 4 sites in Kenya.

Number of Study Participants  

Approximately, 480 participants with sickle cell disease (SCD).

Treatment

Part A

Eligible participants will be randomized based on a 1:1 (GBT021601 100 mg: GBT021601 150 mg) allocation and receive a loading dose regimen of GBT021601 (twice daily, approximately 12 hours apart) over the first 4 days. Participants will then receive maintenance once daily doses through Week 12. After completion of at least 6 weeks of dosing of 6 participants receiving a 150 mg dose from this study or in combination with Study GBT021601-012, safety, tolerability, and available PK and pharmacodynamic (PD) data will be reviewed by a Safety Monitoring Committee (SMC) which will make a recommendation on whether the maintenance dose may be increased up to a maximum of 200 mg. Participant randomization will be balanced between the treatment arms that are enrolling. That is 1:1 randomization between 100 mg and 150 mg at the beginning of enrollment and switching to 1:1:1 randomization or an adaptive randomization method to ensure balance across treatment arms once enrollment in the 200 mg treatment arm opens. Clinic site visits and remote visits (eg, telephone or telehealth) will occur at appropriate intervals to monitor safety and assess the efficacy of treatment. Participants may choose to enroll in an open-label extension (OLE) study under a separate protocol after completing 12 weeks of treatment. Participants not entering the OLE study will be followed for approximately 8 weeks after completing dosing to further assess safety. Participants on any GBT021601 dose from Part A will not be eligible to enroll in Part B of the study

Part B:

 A Phase 3, randomized, double-blind, placebo-controlled study in up to 380 adult and pediatric participants with SCD. Following the selection of the optimal safe and effective dose from Part A of the study, Part B of the study will assess the efficacy and safety of 48 weeks of the optimal dose, compared to placebo for Phase 3. In Part B, after the informed consent/assent form has been signed, participants will be assessed for study eligibility during a 28-day Screening Period. On study Day 1, eligible participants will be randomized based on a 1:1 (GBT021601: placebo) allocation and will receive the loading dose regimen of GBT021601 twice daily over 4 days. Participants will then receive maintenance daily doses for a total of 48 weeks of treatment. Enrollment of pediatric participants 12 to < 18 years of age in Part B may occur after Data Monitoring Committee (DMC) review of safety and PK data from MD dosing of at least 7 participants in Part C Cohort C1. Clinic site visits and remote visits (eg, telephone or telehealth) will occur at appropriate intervals to monitor safety. Additional assessments for safety will be collected when a participant presents with a VOC on a non-study visit day, as feasible. Participants may choose to enroll in an OLE study under a separate protocol after completing Week 48 visits. Participants not entering the OLE study will be followed for approximately 8 weeks after completing dosing to further assess safety.

Part C

A single-arm, open-label, multi-dose PK study in four age-group cohorts of pediatric participants with SCD.The multi-dose portion of Part C of the study will begin after the dose has been selected for Phase 3 (Part B). After the informed consent/assent form has been signed, pediatric participants will be assessed for study eligibility during a 35-day Screening Period. On study Day 1, eligible pediatric participants will receive a single dose of GBT021601 (100 mg dose in cohort C1, dose level for cohorts C2 to C4 to be determined based on emerging data). The DMC will review safety, tolerability, and available PK data approximately 7 weeks after 4 participants have completed SD administration in each cohort and provide a recommendation on proceeding with MD. Pediatric participants will begin the MD portion of the study (MD Day 1) by receiving a loading dose regimen (twice daily, approximately 12 hours apart) over 4 days followed by once daily dosing for a total treatment duration of 14 days. Doses given in the MD portion will be informed by emerging clinical study data and will not exceed dose levels established as safe in adult clinical studies or previous pediatric cohorts. Participants may choose to enroll in an OLE study under a separate protocol after completing MD Week 2 (Day 14) visits. Participants not entering the OLE study will be followed for approximately 8 weeks after completing dosing to further assess safety. The End of Study (EOS) Visit will be completed at approximately Week 18. Clinical site visits and remote visits (e.g., telephone or telehealth) will occur at appropriate intervals to monitor safety. Part C will be composed of four cohorts. Cohort C1 will include approximately 10 participants aged 12 to less than 18 years, at least 30% of whom must be 12 to less than 15 years old. Cohort C2 will include approximately 10 participants aged 6 to less than 12 years, at least 30% of whom must be 6 to less than 9 years old. Cohort C3 will include approximately 10 participants aged 2 to less than 6 years. Cohort C4 will include approximately 7 participants aged 6 months to less than 2 years. Cohorts C2 to C4 will be initiated after completion of 14-day MD dosing and DMC review of the safety, tolerability, and PK data has been evaluated in at least 4 participants of the preceding Cohort. One of the four participants must be from the lower half of the cohort age range in order to evaluate safety of proceeding to younger age cohorts. Emerging data from adult and older pediatric cohorts will inform dose level and time for initiating each cohort. Up to 10 additional participants may be enrolled in each cohort if needed to evaluate additional dose levels and/or to further characterize PK or safety if deemed necessary by the Sponsor. Participants from Part C will not be eligible to enroll in Part B of the study.

Objectives

Primary objectives:

Part A: To assess the effects of GBT021601 in adult participants with SCD as measured by change in hemoglobin

Part B: To assess the effects of an optimum GBT021601 dose compared to placebo in adult and pediatric participants with SCD as measured by hemoglobin response

Part C: To assess the PK of single and multiple dose of GBT021601 in pediatric participants with SCD.

Secondary objectives:

Part A: To evaluate the effects of GBT021601 on total hemoglobin and clinical measures of hemolysis. To evaluate the safety and tolerability as well as the PK and pharmacodynamic (PD) properties of multiple dose GBT021601 administration.

Part B: To evaluate the effects of GBT021601 compared to placebo on total hemoglobin, clinical measures of hemolysis, and relevant clinical outcomes. To evaluate the safety and tolerability of 48 weeks of daily GBT021601 administration.

Part C: To evaluate the safety and change in total hemoglobin and clinical measures of hemolysis after multiple doses of GBT021601.

Exploratory objectives:

Part A: To evaluate the effects of GBT021601 on neurocognitive function, EPO levels, QOL assessments, RBC deformability, RBC mitochondrial content, VOCs, and other biomarkers

Part B: To evaluate the effects of GBT021601 compared to placebo on neurocognitive function, EPO levels, QOL assessments, RBC deformability, RBC mitochondrial content, VOCs, and renal biomarkers.

Part C: To evaluate the effects of GBT021601 on EPO levels, RBC deformability, RBC mitochondrial content, and other exploratory biomarkers.

Please find herewith enclosed the regulatory documents required in support of the application to conduct the GBT021601-021 multiple-dose study.

Study Design

This is a three-part study. Part A: A Phase 2, randomized, open-label, dose-finding study in approximately 60 adult participants with SCD. In Part A, after the informed consent form (ICF) has been signed, adult participants will be assessed for study eligibility during a 28-day Screening Period. On study Day 1, eligible participants will be randomized based on a 1:1 (GBT021601 100 mg: GBT021601 150 mg) allocation and receive a loading dose regimen of GBT021601 (twice daily, approximately 12 hours apart) over the first 4 days. Participants will then receive maintenance once daily doses through Week 12. After completion of at least 6 weeks of dosing of 6 participants receiving a 150 mg dose from this study or in combination with Study GBT021601-012, safety, tolerability, and available PK and pharmacodynamic (PD) data will be reviewed by a Safety Monitoring Committee (SMC) which will make a recommendation on whether the maintenance dose may be increased up to a maximum of 200 mg. Participant randomization will be balanced between the treatment arms that are enrolling. That is 1:1 randomization between 100 mg and 150 mg at the beginning of enrollment and switching to 1:1:1 randomization or an adaptive randomization method to ensure balance across treatment arms once enrollment in the 200 mg treatment arm opens. Clinic site visits and remote visits (eg, telephone or telehealth) will occur at appropriate intervals to monitor safety and assess the efficacy of treatment. Participants may choose to enroll in an open-label extension (OLE) study under a separate protocol after completing 12 weeks of treatment. Participants not entering the OLE study will be followed for approximately 8 weeks after completing dosing to further assess safety. Participants on any GBT021601 dose from Part A will not be eligible to enroll in Part B of the study. Part B: A Phase 3, randomized, double-blind, placebo-controlled study in up to 380 adult and pediatric participants with SCD. Following the selection of the optimal safe and effective dose from Part A of the study, Part B of the study will assess the efficacy and safety of 48 weeks of the optimal dose, compared to placebo for Phase 3. In Part B, after the informed consent/assent form has been signed, participants will be assessed for study eligibility during a 28-day Screening Period. On study Day 1, eligible participants will be randomized based on a 1:1 (GBT021601: placebo) allocation and will receive the loading dose regimen of GBT021601 twice daily over 4 days. Participants will then receive maintenance daily doses for a total of 48 weeks of treatment. Enrollment of pediatric participants 12 to < 18 years of age in Part B may occur after Data Monitoring Committee (DMC) review of safety and PK data from MD dosing of at least 7 participants in Part C Cohort C1. Clinic site visits and remote visits (eg, telephone or telehealth) will occur at appropriate intervals to monitor safety. Additional assessments for safety will be collected when a participant presents with a VOC on a non-study visit day, as feasible. Participants may choose to enroll in an OLE study under a separate protocol after completing Week 48 visits. Participants not entering the OLE study will be followed for approximately 8 weeks after completing dosing to further assess safety. Part C: A single-arm, open-label, multi-dose PK study in four age-group cohorts of pediatric participants with SCD. The multi-dose portion of Part C of the study will begin after the dose has been selected for Phase 3 (Part B). After the informed consent/assent form has been signed, pediatric participants will be assessed for study eligibility during a 35-day Screening Period. On study Day 1, eligible pediatric participants will receive a single dose of GBT021601 (100 mg dose in cohort C1, dose level for cohorts C2 to C4 to be determined based on emerging data). The DMC will review safety, tolerability, and available PK data approximately 7 weeks after 4 participants have completed SD administration in each cohort and provide a recommendation on proceeding with MD. Pediatric participants will begin the MD portion of the study (MD Day 1) by receiving a loading dose regimen (twice daily, approximately 12 hours apart) over 4 days followed by once daily dosing for a total treatment duration of 14 days. Doses given in the MD portion will be informed by emerging clinical study data and will not exceed dose levels established as safe in adult clinical studies or previous pediatric cohorts. Participants may choose to enroll in an OLE study under a separate protocol after completing MD Week 2 (Day 14) visits. Participants not entering the OLE study will be followed for approximately 8 weeks after completing dosing to further assess safety. The End of Study (EOS) Visit will be completed at approximately Week 18. Clinical site visits and remote visits (eg, telephone or telehealth) will occur at appropriate intervals to monitor safety. Part C will be composed of four cohorts. Cohort C1 will include approximately 10 participants aged 12 to less than 18 years, at least 30% of whom must be 12 to less than 15 years old. Cohort C2 will include approximately 10 participants aged 6 to less than 12 years, at least 30% of whom must be 6 to less than 9 years old. Cohort C3 will include approximately 10 participants aged 2 to less than 6 years. Cohort C4 will include approximately 7 participants aged 6 months to less than 2 years. Cohorts C2 to C4 will be initiated after completion of 14-day MD dosing and DMC review of the safety, tolerability, and PK data has been evaluated in at least 4 participants of the preceding Cohort. One of the four participants must be from the lower half of the cohort age range in order to evaluate safety of proceeding to younger age cohorts. Emerging data from adult and older pediatric cohorts will inform dose level and time for initiating each cohort. Up to 10 additional participants may be enrolled in each cohort if needed to evaluate additional dose levels and/or to further characterize PK or safety if deemed necessary by the Sponsor. Participants from Part C will not be eligible to enroll in Part B of the study.

Duration of Study Participation

Part A: The maximum duration of the study for adult participants in Part A is approximately 28 weeks. This includes a 28-day Screening period, up to a 12-week Treatment period, and 8-week follow up. All participants will be followed by an EOS Visit 8 weeks after completion of treatment.

Part B: The maximum duration of the study for participants in Part B is approximately 60 weeks. This includes a 28-day Screening period, a 48-week Treatment period, and an 8-week follow up. All participants will be followed by an EOS Visit 8 weeks after completion of treatment.

Part C: The maximum duration of the study for pediatric participants is approximately 23 weeks. This includes a 35-day Screening period and up to an approximately 10-week Treatment period. All participants will be followed by an EOS Visit 8 weeks after completion of treatment. Open-Label Extension Study (under separate protocol): All eligible participants (adult and pediatric) will be given the option to enroll in a separate open-label extension study to receive GBT021601 after completing all the study visits.

Statistical Methods

Primary Endpoints

Part A: Change from baseline in hemoglobin (Hb) through Week 12.

Part B:  Proportion of participants with an increase from baseline of >1 g/dL in Hb at Week 48.

Part C: Areas under the concentration-time curve (AUC) from time zero to the time of the last quantifiable concentration (AUC0-last) and to infinity (AUC0-inf) of GBT021601 in whole blood and plasma after single dose, and area under the concentration-time curve from time zero to 24-hours (AUC0-24) after multiple dose administration of GBT021601. · Maximum observed concentration (Cmax) after a single dose, and both Cmax and minimum concentration (Cmin) after multiple dose administration.

Secondary Endpoints

Part A:

· Proportion of participants with an increase from baseline of >1 g/dL in Hb through Week 12

· Percent change from baseline in hemolysis measures, including indirect bilirubin, absolute and % reticulocytes, and lactate dehydrogenase (LDH) through Week 12.

 · Incidence of adverse events, changes in laboratory assessments, electrocardiograms (ECGs), and vital signs.

 · Effect on Hb oxygen equilibrium curves (OEC) as measured by p50 through Week 12.

 · AUC0-24, Cmax, time to maximum concentration (Tmax), blood to plasma (B:P) ratios of these PK parameters (except Tmax) after the first dose. Cmin and B:P ratio after multiple dose administration.

 · % Hb occupancy through Week 12.

Part B:

· Annualized rate of vaso-occlusive crisis (VOC) through end of Week 48. A VOC is defined as an acute episode of pain that: o Has no medically determined cause other than a vaso-occlusive event, and o Results in a visit to a medical facility (hospitalization, emergency department, urgent care center, outpatient clinic, or infusion center), and o Requires parenteral narcotic agents, parenteral nonsteroidal anti-inflammatory drugs (NSAIDs), or an increase in treatment with oral narcotics. Complicated VOCs of acute chest syndrome (ACS), hepatic sequestration, splenic sequestration, priapism, and dactylitis that meet the requirements listed above will be included in the secondary endpoint.

 · Proportion of participants with an increase from baseline of >1 g/dL in Hb at Week 24

· Change from baseline in Hb through Weeks 24 and 48.

 · Percent change from baseline in hemolysis measures, including indirect bilirubin, absolute and % reticulocytes, and LDH through Week 48.

· Change from baseline in quality of life (QOL) assessments through Week 48 including Patient’s Global Impression – Change (PGI-C), Clinical Global Impression – Change (CGI-C), and Patient[1]Reported Outcome Measurement Information System (PROMIS-29) physical function, anxiety, depression, fatigue, social function, pain interference, and pain intensity.

· Incidence of adverse events, changes in laboratory assessments, ECGs, and vital signs. · Trough plasma and blood concentrations, B:P ratio.

· Percent Hb occupancy through Week 48.

 · Time to first VOC from randomization.

 · Time to first Hb increase from baseline of >1 g/dL during the study.

Part C:

 · Incidence of adverse events, changes in laboratory assessments, ECGs, and vital signs. · Change from baseline in Hb at MD Week 2 (Day 14).

 · Change from baseline in hemolysis measures, including indirect bilirubin, absolute and % reticulocytes, and LDH at MD Week 2 (Day 14).

 · Red blood cell (RBC) count

 · PK parameters after single dose in both plasma and whole blood: terminal elimination half-life (t1/2), Tmax, clearance (Cl/F), volume of distribution (V/F), and B:P ratios PK parameters after multiple doses in both plasma and whole blood: B:P ratios

Exploratory Endpoint

Part A:

 · Change from baseline through Week 12 in the Executive abilities composite score (using Dimensional Change Card Sort Test, Flanker Inhibitory Control and Attention Test, and Pattern Comparison Processing Speed Test as assessed by the National Institutes of Health (NIH) Toolbox Cognition Module. · Change from baseline in EPO levels through Week 12.

 · Change from baseline in QOL assessments through Week 12 including PGI-C, CGI-C, EQ-5D-5L, and PROMIS-29.

 · Change from baseline in RBC deformability and RBC mitochondrial content through Week 12.

· Annualized rate of VOC through Week 12.

· Time to first VOC from randomization.

 · Change from baseline in other biomarkers at Week 12 (including genomic analysis [optional], membrane fragility, flow adhesion, and protein markers by multiplex assay which may include intracellular adhesion molecules [ICAMs], vascular cell adhesion molecules [VCAMs], P-selectin, E-selectin, albumin-to-creatinine ratio [ACR], kidney injury molecule [KIM], and markers of hemolysis such as levels of haptoglobin, hemopexin, soluble complement component 3 [C3], and cell free hemoglobin).

Part B:

Change from baseline in other biomarkers through Week 48 (including genomic analysis [optional] membrane fragility, flow adhesion, and protein markers by multiplex assay which may include ICAMs, VCAMs, P-selectin, E-selectin, ACR, KIM, and markers of hemolysis such as levels of haptoglobin, hemopexin, soluble C3, and cell free hemoglobin).

· Change from baseline through Week 48 in the Executive abilities composite score (using Dimensional Change Card Sort Test, Flanker Inhibitory Control and Attention Test, and Pattern Comparison Processing Speed Test) as assessed by the NIH Toolbox Cognition Module.

 · Change from baseline in EPO levels through Week 48.

 · Change from baseline in QOL assessments through Week 48 including EQ-5D-5L.

· Change from baseline in RBC deformability and mitochondrial content through Week 48.

Part C:

 Change from baseline in EPO levels at MD Week 2 (Day 14).

 · Change from baseline in RBC deformability and RBC mitochondrial content at MD Week 2 (Day 14).

 · Change from baseline in other biomarkers at MD Week 2 (Day 14; including genomic analysis [optional] membrane fragility, flow adhesion, and protein markers by multiplex assay which may include ICAMs, VCAMs, P-selectin, E-selectin, and markers of hemolysis such as levels of haptoglobin, hemopexin, soluble C3, and cell free hemoglobin).

 Sample Size:

Part A: The sample size for the Part A, the Phase 2 portion of the study, is planned for selecting an optimal dose based on changes in Hb levels, % Hb occupancy, PK, PD, and safety. Participants will initially be randomized based on a 1:1 ratio (GBT021601 100 mg: GBT021601 150 mg). After at least 6 weeks of dosing of at least 6 participants receiving 150 mg from this study or in combination with Study GBT021601-012 and determination of whether a higher dose group may be added, participants will be randomized based on a 1:1:1 ratio (GBT021601 100 mg: GBT021601 150 mg: GBT021601 up to 200 mg) or an adaptive randomization method to ensure balance across treatment groups. Final analysis to determine the optimal maintenance dose will be performed after all enrolled participants have completed treatment. Sample size estimation was performed to enable detecting a reasonable difference in trend of efficacy when treatment arms are compared for change from baseline in Hb and % Hb occupancy. For the endpoint of Hb change from baseline, a total sample size of 60 participants will provide approximately 90% power to detect a mean difference of 1.0 g/dL between doses at alpha level 0.05 (two-sided); assuming standard deviation of 0.8 g/dL in each dose group. The mean % Hb occupancy is assumed to be 70%, 50%, 30% with a standard deviation of 21%, 15%, and 9% for GBT021601 200 mg, GBT021601 150 mg, and GBT021601 100 mg, respectively. Similarly, a total sample size of 60 participants will allow at least 80% power to detect a difference between doses at alpha level 0.05. The above power calculations are calculated for the t-test assuming the data are normally distributed.

 Part B: This study is powered to evaluate both the Hb response rate (defined as proportion of participants with a change from baseline of >1 g/dL in Hb) and the annualized rate of VOCs as primary and key secondary endpoints, respectively. For the primary efficacy endpoint, the Hb response rate at Week 48, assuming the Hb response rate in the placebo group is 10% to 15%, a 1:1 ratio between GBT021601 and placebo arms, with 25% dropout rate, a sample size of 67 participants per arm will provide at least 90% power to detect a 30% to 35% increase in Hb response rate from the placebo group at alpha level of 0.05 (two-sided) using Fisher’s exact test. For the key secondary endpoint of annualized rate of VOCs, a sample size of 190 per arm, a total of 380 participants, will provide approximately 90% power to detect a 37.5% reduction in the annualized rate of VOCs at alpha level of 0.05 (two-sided). The additional assumptions for this estimation, were the mean annualized VOC rate in the placebo group is 3.0 per year, a 1:1 ratio between the GBT021601 and placebo arms, and the number of VOCs per year follows a negative binomial distribution with a dispersion parameter of 1.04 and 25% drop-out rate.

Part C: No formal sample size estimation analysis is performed for this study. Sample size for pediatric PK studies suggested by Wang (2012) and using a 30% CV for PK parameters, as observed in Study GBT021601-012, resulted in a sample size of approximately 7 participants per cohort when comparing one age group to another. Sample size of 10 will be used for Cohorts C1, C2, and C3 and a sample size of 7 will be used for Cohort C4. The planned sample size will account for potential early withdrawal in Cohorts C1, C2, and C3 and feasibility of recruitment for participants 6 months to < 2 years old in Cohort C4. Up to 10 additional participants per cohort (N=20 total) may be enrolled if deemed necessary by the Sponsor or DMC to further characterize safety or PK parameters and/or to evaluate an additional dose level

Efficacy Analyses: Details of all planned analyses will be specified in the SAP. The SAP will be finalized prior to study unblinding. All statistical tests will be conducted at a two-sided alpha level of 0.05 unless otherwise stated. For Part A, statistical testing will be performed between GBT021601 groups at a two-sided α level of 0.05 using Mixed Model Repeated Measures analyses. However, these analyses are intended to serve only as a guide for the final decision. No multiplicity corrections will be made. For Part B, hypothesis testing for the primary efficacy analysis and the secondary efficacy analyses will follow the fixed-sequence hierarchical testing procedure at overall two-sided α level of 0.05. Baseline measurements for efficacy assessments will be the average of pre-treatment values, or the last measurement depending on the type of endpoint. In Part A, the primary endpoint, Hb change from baseline, will be compared between every two dose groups using 2-sample t-test for independent groups. In Part B, the primary endpoint, Hb response, comparisons between GBT021601 and placebo groups will be performed using an exact Cochran-Mantel-Haenszel (CMH) general association test stratifying for the randomization stratification factors. The annualized rate of VOC will be estimated using a negative binomial regression model with the independent variable of treatment group and adjusted for the stratification factors used for randomization. Additional risk factors, including number of VOC occurrences during the 12 months prior to study participation will be explored. The mean cumulative function of VOC events will be presented using recurrent-events analysis methods.

Safety Analyses: Adverse events will be coded to system organ class and preferred term using the Medical Dictionary for Regulatory Activities (MedDRA) and summarized. Vital signs, ECG parameters, laboratory assessments and concomitant medication usage will be summarized at each visit for observed values and changes from Baseline. Laboratory safety assessments determined to be clinically significant will be listed

Pharmacokinetic and Pharmacodynamic Analyses: All plasma and whole blood concentration vs time data will be summarized using descriptive statistics and will be listed and summarized in tabular format. If conducted, the population PK or exposure/response analyses will be specified in a separate analysis plan. Pharmacodynamic markers and computed PD parameters will be summarized descriptively. Individual and mean PD marker data will be presented graphically. The relationships between PK and PD may be explored by PK/PD modeling, if appropriate. Results of these analyses will be presented in a separate report.