SIKLOS (hydroxyurea)
Siklos is an antimetabolite indicated to reduce the frequency of painful crises and the need for blood transfusions in adult and pediatric patients (2 years of age and older) with sickle cell anemia who experience recurrent moderate to severe painful crises.
How SIKLOS Works
While the precise mechanism of action is not fully known, hydroxyurea is believed to act as a ribonucleotide reductase inhibitor, resulting in the immediate inhibition of DNA synthesis. In patients with sickle cell anemia, its clinical benefits are associated with increased hemoglobin F levels and red blood cell (RBC) water content, increased deformability of sickled cells, decreased neutrophil counts, and altered adhesion of RBCs to the vascular endothelium.
Development Insights
Details
- Status
- Prescription
- First Approved
- 2017-12-21
- Routes
- ORAL
- Dosage Forms
- TABLET
SIKLOS Approval History
What SIKLOS Treats
1 indicationsSIKLOS is approved for 1 conditions since its original approval in 2017. These indications span multiple therapeutic areas including oncology, immunology, and more.
- Sickle Cell Anemia
SIKLOS Boxed Warning
MYELOSUPPRESSION and MALIGNANCIES WARNING: MYELOSUPPRESSION and MALIGNANCIES See full prescribing information for complete boxed warning . Myelosuppression : SIKLOS may cause severe myelosuppression. Do not give if bone marrow function is markedly depressed. Monitor blood counts at baseline and throughout treatment. Interrupt treatment and reduce dose as necessary. ( 5.1 ) Malignancies : Hydroxyurea is carcinogenic. Advise sun protection and monitor patients for malignancies. ( 5.2 ) Myelosuppre...
WARNING: MYELOSUPPRESSION and MALIGNANCIES WARNING: MYELOSUPPRESSION and MALIGNANCIES See full prescribing information for complete boxed warning . Myelosuppression : SIKLOS may cause severe myelosuppression. Do not give if bone marrow function is markedly depressed. Monitor blood counts at baseline and throughout treatment. Interrupt treatment and reduce dose as necessary. ( 5.1 ) Malignancies : Hydroxyurea is carcinogenic. Advise sun protection and monitor patients for malignancies. ( 5.2 ) Myelosuppression: SIKLOS may cause severe myelosuppression. Monitor blood counts at baseline and throughout treatment. Interrupt treatment and reduce dose as necessary [see Warnings and Precautions (5.1) ] . Malignancies: Hydroxyurea is carcinogenic. Advise sun protection and monitor patients for malignancies [see Warnings and Precautions (5.2) ] .
SIKLOS Competitive Set
ProThree rings of competition based on shared molecular targets and treated indications.
MoA expansion candidates
Same target(s), different indications — where else is this mechanism being explored?
Indication competitors
Same indication, different mechanism — what else might this patient receive?
Filters applied: drops same-active-ingredient (505(b)(2) reformulations), route-mismatch (topical vs systemic), and cross-therapeutic-area matches in same-indication rings.
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Clinical Trial Registry
61 trials| Trial | Sponsor ID | Phase | Status | Title |
|---|---|---|---|---|
| NCT06456346 | 3543-007 2023-505232-36-00, MK-3543-007 | Ph 3 | recruiting | Bomedemstat vs Hydroxyurea for Essential Thrombocythemia (MK-3543-007) |
| NCT05853458 HU-F-AIM | CINC424BDE15 2022-502338-20-00 | Ph 4 | terminated | Evaluation of HU-resistance in Adult Patients With Polycythemia Vera Who Meet PV-AIM Predictors |
| NCT03263559 results posted | BMTCTN1507 2U10HL069294-11, 5U24CA076518 | Ph 2 | completed | Haploidentical Bone Marrow Transplantation in Sickle Cell Patients (BMTCTN1507) |
| NCT03077542 results posted | 170069 17-H-0069 | Ph 1, Ph 2 | active not recruiting | Nonmyeloablative Haploidentical Peripheral Blood Mobilized Hematopoietic Precursor Cell Transplantation for Sickle Cell Disease |
| NCT06171217 REACH | 2013-4221b U01HL133883 | Ph 2 | active not recruiting | Realizing Effectiveness Across Continents With Hydroxyurea |
| NCT07507760 DECISIVO | DECISIVO | Ph 2 | not yet recruiting | Oral Decitabine Plus Ivosidenib as First Line for Older/Unfit Adult AML Patients |
| NCT06093672 GIV-IN-PV | DSC/08/2357/32 2022-502276-23-00 | Ph 3 | recruiting | Study on Efficacy and Safety of Givinostat Versus Hydroxyurea in Patients With Polycythemia Vera |
| NCT06871410 | I-3916824 NCI-2025-01523, I-3916824 | Ph 1 | recruiting | Genetically Engineered Cells (CD83 CAR T Cells) for the Treatment of Relapsed or Refractory Acute Myeloid Leukemia |
| NCT06930352 | OSU-24309 NCI-2025-02414 | Ph 2 | not yet recruiting | Ziftomenib for the Treatment of Patients With NPM1 Mutated or KMT2A Rearranged Acute Myeloid Leukemia Not Eligible for Standard Therapy |
| NCT05357482 | 10000539 000539-H | Ph 1, Ph 2 | active not recruiting | Addition of JSP191 (C-kit Antibody) to Nonmyeloablative Hematopoietic Cell Transplantation for Sickle Cell Disease and Beta-Thalassemia |
| NCT06063317 OASIS | CF33-CD19-101 | Ph 1 | terminated | A Study of onCARlytics (CF33-CD19) in Combination With Blinatumomab in Adults With Advanced or Metastatic Solid Tumors (OASIS) |
| NCT07033598 PROSPERA | PROSPERA ABNL-MARRO 002 | Ph 2 | recruiting | Pacritinib vs. Hydroxyurea in Advanced Proliferative Chronic Myelomonocytic Leukemia |
| NCT06526117 SPRING-2 | 231110 U01NS129143 | Ph 4 | recruiting | Stroke Prevention in Nigeria 2 Trial |
| NCT05526924 | IRB22-0578 | Ph 1 | recruiting | Dosing Study of Radiation Combined With Tislelizumab and Pamiparib in Patients With Previously Treated Head and Neck Cancer |
| NCT03500731 | STUDY19110120 | Ph 1, Ph 2 | recruiting | Lung and Bone Marrow Transplantation for Lung and Bone Marrow Failure |
| NCT05451940 ACHiEvE-SCD results posted | STUDY21120027 | Ph 1, Ph 2 | completed | Hydroxyurea and EPO in Sickle Cell Disease |
| NCT01962415 HSCT+RIC | STUDY19060337 | Ph 2 | recruiting | Reduced Intensity Conditioning for Non-Malignant Disorders Undergoing UCBT, BMT or PBSCT |
| NCT05953584 HIBISCUS 3 | 4202-HEM-204 PACTR202301655446404 | Ph 2 | recruiting | A Phase 2 Open-label Study to Evaluate the Activity of Etavopivat on Transcranial Doppler Velocities in Pediatric Patients With Sickle Cell Disease Who Are at Increased Risk for Primary Stroke |
| NCT03860844 ISAKIDS results posted | ACT15378 PIP - 2018-002697-45, U1111-1202-1096 | Ph 2 | terminated | Isatuximab in Combination With Chemotherapy in Pediatric Patients With Relapsed/Refractory Acute Lymphoblastic Leukemia or Acute Myeloid Leukemia |
| NCT02556099 | 2014-2875 EXTEND | Ph 2 | completed | EXTEND EXpanding Treatment for Existing Neurological Disease |
| NCT03653338 | STUDY19050050 | Ph 1, Ph 2 | recruiting | T-Cell Depleted Alternative Donor Bone Marrow Transplant for Sickle Cell Disease (SCD) and Other Anemias |
| NCT03948867 | SPHERE | Ph 2 | enrolling by invitation | Stroke Prevention With Hydroxyurea Enabled Through Research and Education (SPHERE) |
| NCT03789591 HOPS | CCHMC_HOPS | Ph 3 | completed | Hydroxyurea Optimization Through Precision Study |
| NCT01966731 REACH results posted | 2013-4221 | Ph 1, Ph 2 | active not recruiting | Realizing Effectiveness Across Continents With Hydroxyurea (REACH) |
| NCT03107182 OPTIMA-II results posted | IRB17-0104 | Ph 2 | completed | Chemotherapy and Locoregional Therapy Trial (Surgery or Radiation) for Patients With Head and Neck Cancer |
| NCT04750707 BRAINSAFEII | REC REF 2019-147 R01HD096559 | Ph 3 | completed | Hydroxyurea Therapy for Neurological and Cognitive Protection in Pediatric Sickle Cell Anemia in Uganda ( BRAINSAFE-II ) |
| NCT05285917 PUSHUP | BrUOG 419 U01HL157872 | Ph 3 | recruiting | Promoting Utilization and Safety of Hydroxyurea Using Precision in Africa |
| NCT02258659 OPTIMA results posted | IRB14-0639 NCI-2014-01867, IRB14-0639 | Ph 2 | completed | Nab-paclitaxel and Carboplatin Followed by Response-Based Local Therapy in Treating Patients With Stage III or IV HPV-Related Oropharyngeal Cancer |
| NCT05142254 PIN | 202504 | Ph 2 | completed | A Trial for Prevention of Recurrent Ischemic Priapism in Men With Sickle Cell Disease: A Pilot Study |
| NCT05005182 results posted | MC200805 NCI-2021-08519, 20-013021 | Ph 2 | terminated | Luspatercept With or Without Hydroxyurea for the Treatment of Myelodysplastic/Myeloproliferative Neoplasms With Ring Sideroblasts and Thrombocytosis or Unclassifiable With Ring Sideroblasts |
| NCT01847326 | 12-1554 NCI-2012-02179 | Ph 1 | completed | Paclitaxel Albumin-Stabilized Nanoparticle Formulation and Carboplatin Followed By Chemoradiation in Treating Patients With Recurrent Head and Neck Cancer |
| NCT01389024 HUPrevent results posted | NA_00041623 3UL1RR025005 | Ph 2 | completed | Hydroxyurea to Prevent Brain Injury in Sickle Cell Disease |
| NCT02844673 mDOT | MUPI-001 | Ph 2 | completed | Mobile-Directly Observed Therapy on Adherence to Hydroxyurea |
| NCT06239389 | NIBD/IRB-236/21-2021. | Ph 2 | completed | Comparison Of Efficacy And Safety Of Thalidomide Vs Hydroxyurea In Thalassemia Patients: A Single-Centre Pilot Study. |
| NCT02757885 TRANSFORM results posted | IRB00080955 | Ph 2 | completed | Transplantation for Patients With Sickle Cell Disease From Mismatched Family Donors of Bone Marrow |
| NCT01711541 results posted | NCI-2012-02009 NCI-2012-02009, A091101 | Ph 1, Ph 2 | completed | Combination Chemotherapy With or Without Veliparib in Treating Patients With Stage IV Head and Neck Cancer |
| NCT02149537 | 205449 | Ph 4 | completed | Risk Clinical Stratification of Sickle Cell Disease in Nigeria, Assessment of Efficacy/Safety of Hydroxyurea Treatment |
| NCT02088541 SOPRA results posted | KCP-330-008 | Ph 2 | completed | Selinexor (KPT-330) in Older Patients With Relapsed AML |
| NCT03825341 results posted | HOPE18 | Ph 2 | terminated | Hydroxyurea Therapy: Optimizing Access in Pediatric Populations Everywhere |
| NCT02214407 GFM-DAC-CMML | GFM-DAC-CMML | Ph 3 | completed | Randomized Phase III Study of Decitabine +/- Hydroxyurea (HY) Versus HY in Advanced Proliferative CMML |
| NCT03644953 HAT | Pro00010541 | Ph 2 | completed | Hydroxyurea and Transfusion |
| NCT02522104 results posted | SIK-FR 14-1 | Ph 4 | completed | Evaluation of the Impact of Renal Function on the Pharmacokinetics of SIKLOS ® (DARH) |
| NCT03020615 results posted | HUGKISS R34HL127162 | Ph 2 | completed | Hydroxyurea Management in Kids: Intensive Versus Stable Dosage Strategies |
| NCT04292314 | TQ/Omega-3 on Thalassemia | Ph 2, Ph 3 | completed | Hydroxy Urea, Omega 3, Nigella Sativa,Honey on Oxidative Stress and Iron Chelation in Pediatric Major Thalassemia |
| NCT02336373 SCYTHE results posted | H-35010 SCYTHE | Ph 2 | terminated | Treatment of Hemoglobin SC Disease With Hydroxyurea |
| NCT01425307 TWiTCH results posted | H-28572 TWiTCH R01HL095647 | Ph 3 | terminated | Transcranial Doppler (TCD) With Transfusions Changing to Hydroxyurea |
| NCT03128515 NOHARM-MTD | 00456728 | Ph 3 | completed | Optimizing Hydroxyurea Therapy in Children With SCA In Malaria Endemic Areas |
| NCT02640573 results posted | 37290 IM-SCYTHE | Ph 2 | terminated | Treatment of Adult Patients With Hemoglobin SC Disease (SCYTHE) |
| NCT01964755 results posted | 20090166 | Ph 2 | terminated | Chemotherapy for Relapsed Epstein Barr Virus Associated Lymphoma |
| NCT02225132 results posted | 140172 14-H-0172 | Ph 1, Ph 2 | completed | Assessment of Algorithm-Based Hydroxyurea Dosing on Fetal Hemoglobin Response, Acute Complications, and Organ Function in People With Sickle Cell Disease |
Showing 50 of 61 trials
Active Pipeline
Ongoing clinical trials by development phase
Key Completed Trials
Completed studies with published results, ranked by significance
Trial Timeline
Full development history with FDA approval milestones
Understanding FDA Approval Types
| Count | Type | What it means |
|---|---|---|
| - | ORIG | Original approval - drug first enters market |
| - | SUPPL - Efficacy | New indication (new disease/condition approved) |
| - | SUPPL - Labeling | Label text changes (warnings, dosing updates) |
| - | SUPPL - Manufacturing | Production changes (new facility) |
| - | SUPPL - Chemistry | Formulation changes (new dosage strength) |
Green lines in the timeline show ORIG and Efficacy approvals - the clinically meaningful milestones.
SIKLOS FDA Label Details
Indications & Usage
FDA Label (PDF)SIKLOS is indicated for the treatment of Sickle Cell Anemia.
WARNING: MYELOSUPPRESSION and MALIGNANCIES WARNING: MYELOSUPPRESSION and MALIGNANCIES See full prescribing information for complete boxed warning . Myelosuppression : SIKLOS may cause severe myelosuppression. Do not give if bone marrow function is markedly depressed. Monitor blood counts at baseline...
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Data Sources
Data sourced from official FDA and NIH databases. Click links to verify on original sources.
How We Calculate These Metrics
Trial Activity Stage
Measures the current development activity pattern based on trial phases, status, and trends. Important: This measures R&D activity, not commercial lifecycle.
Trial statuses: "Active" means recruiting or ongoing. "Completed" means reached planned endpoint. "Terminated" means stopped early—often due to safety, efficacy, or business reasons.
- Growth: High proportion of early-phase trials (Phase 1/2), active development
- Expansion: Significant Phase 3 activity, approaching or pursuing approvals
- Mature: Substantial Phase 4 post-marketing studies
- Stable: Mixed phase distribution, steady development
- Declining: Low active trial ratio, reduced R&D investment