This is a Phase 1b/2, double-blind (study participants and Investigators), placebo-controlled, randomized, single-ascending dose, multi-center study to assess the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of DS-1040b in participants with Acute Ischemic Stroke (AIS).

Inclusion Criteria:

• Has a clinical diagnosis of acute ischemic stroke (including lacunar stroke/infarct) supported by computed topography or magnetic resonance imaging to rule out alternative cause for presenting symptoms
• Has onset of stroke symptoms within 4.5 to 12 hours before initiation of study drug administration - for subjects with a stroke upon waking, time of symptom onset is the last time the subject was known to be well
• Has a NIHSS score of ≥ 2 (for Cohorts 1-5) and ≥ 5 (for Cohort 6)
• Has Low dose heparin or low molecular weight heparin at a preventive dose are allowed from 24 hours after treatment start completion and after confirmation of no intracranial bleeding on the 24-hours repeat brain imaging.
• Is a Cohort 6 participant who is treated or anticipated to be treated with intra-arterial therapy (IAT) for ischemic stroke at the time of randomization (for enrollment in the IAT subgroup)
• Has given written informed consent to participate in the study prior to participating in any study-related procedures - depending on country-specific practice, written informed consent may be acceptable from legally authorized representative
• Has given a separate written informed consent for collecting a blood sample for genotyping

Exclusion Criteria:

• Is a Cohort 1-5 participant who has been treated or is going to be treated with tissue plasminogen activator (tPA) and/or endovascular thrombectomy during current stroke
• Is a Cohort 6 participant treated or anticipated to be treated with tPA during current stroke
• Has evidence of intracranial hemorrhage on non-contrast computed tomography (CT/CAT) scan or magnetic resonance imaging (MRI)
• Has symptoms of subarachnoid hemorrhage, even with normal imaging
• Has an Alberta Stroke Program Early CT Score (ASPECTS) <6
• Has prior non-traumatic intracranial hemorrhage (excluding microhemorrhages observed in imaging)
• Has known arteriovenous malformation or aneurysm
• Has evidence of active bleeding
• Has platelet count less than 100,000
• Has International Normalized Ratio greater than 1.7
• Has used unfractionated heparin within 24 hours prior to treatment and has an elevated partial thromboplastin time
• Has used a non-vitamin K antagonist oral anticoagulant such as dabigatran, rivaroxaban, apixaban, or other factor Xa inhibitors within 24 hours before treatment
• Has used fondaparinux or low molecular weight heparin at an anticoagulation dose within 24 hours prior to treatment
• Has anticipated use of an anticoagulation dose of heparin, or fondaparinux or low molecular weight heparin, or nonvitamin K antagonist oral anticoagulant such as dabigatran, rivaroxaban, apixaban, or other factor Xa inhibitors within 48 hours after completion of study drug treatment (low dose heparin or low molecular weight heparin at a preventive dose are allowed from 24 hours after treatment completion and after confirmation of no intracranial bleeding on the 24 hours repeat brain imaging. In Cohort 6, heparin treatment associated with IAT is allowed.)
• Has blood pressure > 185/110 mmHg, or requires aggressive medication to maintain blood pressure below this limit (routine medical treatment including IV drug treatment is allowed to lower the blood pressure below this limit)
• Has had intracranial surgery, clinically significant head trauma (in the opinion of Principal Investigator), Alteplase treatment, or a previous stroke within 1 month
• Has had major surgery within 14 days
• Has had gastrointestinal or genitourinary bleeding in the last 21 days
• Has had a lumbar puncture (or epidural steroid injection) within 14 days
• Has had a preexisting disability classified by modified Rankin Scale (mRS) > 2
• Has an estimated glomerular filtration rate < 60 mL/min/1.73 m^2
• Has baseline hemoglobin < 10.5 g/dL
• Has a positive pregnancy test
• Is currently participating in another investigational study or has participated in an investigational drug study within 30 days or 5 half-lives of that investigational drug prior to administration of the study drug
• Is an employee or an immediate family member of an employee of the Sponsor, the Contract Research Organization (CRO), or the Site
• Has any other condition the investigator determines would preclude participation in the study

De-identified individual participant data (IPD) and applicable supporting clinical trial documents may be available upon request at https://vivli.org/. In cases where clinical trial data and supporting documents are provided pursuant to our company policies and procedures, Daiichi Sankyo will continue to protect the privacy of our clinical trial participants. Details on data sharing criteria and the procedure for requesting access can be found at this web address: https://vivli.org/ourmember/daiichi-sankyo/

Studies for which the medicine and indication have received European Union (EU) and United States (US), and/or Japan (JP) marketing approval on or after 01 January 2014 or by the US or EU or JP Health Authorities when regulatory submissions in all regions are not planned and after the primary study results have been accepted for publication.

Formal request from qualified scientific and medical researchers on IPD and clinical study documents from clinical trials supporting products submitted and licensed in the United States, the European Union and/or Japan from 01 January 2014 and beyond for the purpose of conducting legitimate research. This must be consistent with the principle of safeguarding study participants' privacy and consistent with provision of informed consent.