KW-6356 (Sipagladenant) — Safety, Research, Risks & Legal Status

Not medical advice. KW-6356 (Sipagladenant) is discussed here for informational and harm-reduction purposes only. We do not endorse use, and any dosing context is informational, not a protocol.

Also known as	Sipagladenant; KW-6356
Category	Research Chemical
INN	Sipagladenant
Developer	Kyowa Kirin (Japan)
Drug class	Adenosine A2A receptor antagonist / inverse agonist (non-xanthine)
Highest development stage	Phase 2b (completed; program discontinued July 2022)
Discontinuation reason	Business/regulatory strategy; not safety or efficacy failure
Predecessor compound	Istradefylline (Nourianz) - approved A2A antagonist KW-6356 was designed to succeed
US legal status	Never approved by the FDA or any regulatory authority. Development was discontinued by Kyowa Kirin in July 2022 prior to Phase 3. Not available as a licensed drug, supplement, or approved therapeutic in any market. Would be classified as an unapproved investigational drug in the US; not for human consumption outside of clinical trials, which have since concluded.

What is KW-6356 (Sipagladenant)?

KW-6356 (INN: sipagladenant) is a second-generation, non-xanthine selective adenosine A2A receptor antagonist and inverse agonist developed by Kyowa Kirin for Parkinson's disease. It was investigated as both a monotherapy and as an adjunct to levodopa in patients with motor fluctuations. The development program reached Phase 2b and demonstrated clinical signals before being discontinued in 2022 for business and regulatory strategy reasons, not due to safety or efficacy failure.

How it works

Sipagladenant selectively binds the adenosine A2A receptor and inhibits both agonist-stimulated and constitutive (baseline) receptor activity - the inverse agonist property distinguishing it from earlier antagonists such as istradefylline. In the basal ganglia, A2A receptors are co-expressed with dopamine D2 receptors on striatopallidal neurons. Blockade of A2A signaling is hypothesized to enhance dopaminergic transmission indirectly, reducing motor inhibition. The inverse agonism at constitutive receptor activity is proposed to offer additional functional suppression beyond simple competitive antagonism, though the clinical relevance of this distinction in humans remains unconfirmed.

Background & history

Kyowa Hakko Kirin (now Kyowa Kirin) developed KW-6356 as a successor to istradefylline (approved in Japan 2013, US 2019 as Nourianz), aiming to improve on the pharmacological profile. Preclinical work in MPTP-treated common marmosets demonstrated dose-dependent motor improvement and L-DOPA augmentation without worsening dyskinesia. A Phase 2a monotherapy trial (NCT02939391) in early, untreated Parkinson's disease patients showed improvement in MDS-UPDRS motor scores vs. placebo. A Phase 2b adjunctive trial (NCT03703570) enrolling 502 advanced PD patients met its primary endpoint, showing superior reduction in MDS-UPDRS Part III scores and reduced daily OFF time versus placebo when added to levodopa. Despite these positive results, Kyowa Kirin announced discontinuation of the program in July 2022, citing the global regulatory landscape, development hurdles, and competitive timelines - not safety concerns or efficacy failure.

What the research says

Human clinical data exists at Phase 2 level only. Phase 2a (NCT02939391) was a monotherapy study in early untreated PD; Phase 2b (NCT03703570) was an adjunctive study in 502 advanced PD patients on levodopa, which met its primary endpoint. Additional Phase 1 studies examined pharmacokinetics in healthy volunteers (NCT03830528) and drug-drug interactions (NCT03970798). Preclinical data from MPTP-treated marmoset models supported motor efficacy and L-DOPA augmentation without dyskinesia worsening. In vitro pharmacology has been published characterizing the antagonist/inverse agonist profile. No Phase 3 data exist. The program is discontinued, so no further human trials are anticipated under this developer.

Reported effects

Reduction in Parkinson's disease motor symptoms (MDS-UPDRS Part III improvement vs. placebo observed in Phase 2a and 2b human trials)
Reduction in daily OFF time when used adjunctively with levodopa (Phase 2b signal)
Augmentation of L-DOPA anti-parkinsonian efficacy in primate models without worsening dyskinesia (preclinical, MPTP marmoset)
Dose-dependent motor improvement as monotherapy in primate Parkinson's models (preclinical)
All effects above are based on Phase 2 or preclinical data only; no Phase 3 confirmation exists and the compound is not approved for any indication

Dosing & administration (informational)

Published Phase 2 trial ranges appear in clinical study documentation associated with NCT02939391 and NCT03703570. This information is available for academic reference only. No dosing protocol should be derived from this entry. Consult a licensed clinician for any therapeutic questions regarding Parkinson's disease management.

This is general research/context information, not medical advice or a recommended protocol.

Safety & side effects

Phase 2b tolerability was generally favorable; most common treatment-emergent adverse events were mild constipation (7.3-10.3%) and nasopharyngitis (7.3-8.6%)
No major safety signals were identified across the Phase 1 and Phase 2 clinical trial program
Discontinuation was explicitly attributed to regulatory and business factors, not safety concerns
Long-term safety is unknown - no Phase 3 or long-duration data exist
Drug-drug interaction profile was studied (NCT03970798) but published data are limited in the public domain
Not approved for human use; any off-label or research context carries unknown risk given the absence of comprehensive safety data

Drug & supplement interactions

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Who should avoid it

Do not use outside of a clinical trial or under physician supervision - this compound is not approved for any human indication
No approved formulation or verified supply exists; sourcing from unregulated channels carries serious identity and purity risks
Patients on dopaminergic therapy for Parkinson's disease should not self-administer any investigational A2A antagonist without neurologist oversight
Persons with hypersensitivity to xanthine-class or non-xanthine adenosine receptor ligands should not assume cross-safety given limited published data

How it is commonly combined

KW-6356 was studied in combination with levodopa in Phase 2b and was designed as an adjunct to standard dopaminergic therapy. No data exist on combinations with other investigational compounds. The adjunctive levodopa data represent the only published human combination evidence.

Legal & regulatory. Never approved by the FDA or any regulatory authority. Development was discontinued by Kyowa Kirin in July 2022 prior to Phase 3. Not available as a licensed drug, supplement, or approved therapeutic in any market. Would be classified as an unapproved investigational drug in the US; not for human consumption outside of clinical trials, which have since concluded.

Quality & harm reduction

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Frequently asked questions

Why was KW-6356 discontinued if it showed positive Phase 2b results?

Kyowa Kirin cited the global regulatory landscape, development hurdles, and the timeline required for potential market entry - not a safety finding or efficacy failure. The Phase 2b trial met its primary endpoint, but commercial and regulatory strategy considerations led the company to halt the program in July 2022.

How does sipagladenant differ from istradefylline (Nourianz), the approved A2A antagonist?

Both block the adenosine A2A receptor, but KW-6356 is a second-generation non-xanthine compound that also exhibits inverse agonist activity - meaning it suppresses constitutive (baseline) receptor signaling in addition to blocking agonist-induced activity. Preclinical data suggested superiority over istradefylline in marmoset models, though head-to-head human trial data do not exist.

Is KW-6356 available for purchase or research use?

No approved or commercially licensed formulation exists. The program was discontinued before market authorization in any country. Any material obtained outside of a formal regulated clinical trial cannot be verified for identity or purity and should not be used for human research.

What were the safety findings in human trials?

Across Phase 1 and Phase 2 studies, sipagladenant appeared generally well tolerated. The most commonly reported adverse events in Phase 2b were mild: constipation (7.3-10.3%) and nasopharyngitis (7.3-8.6%). No major safety signals were identified, and discontinuation was not safety-driven. Long-term safety data do not exist.

What is the correct dosing for KW-6356?

This entry does not provide dosing guidance. KW-6356 is an unapproved discontinued investigational compound. Dosing information from clinical trial protocols is available in published literature for academic reference only. Any therapeutic decisions regarding Parkinson's disease should be made with a licensed neurologist.

Could the compound be revived or licensed to another developer?

Theoretically possible but speculative. Kyowa Kirin discontinued the internal program; whether they would out-license it or whether another party would pursue Phase 3 is unknown. As of the available evidence, no such development has been announced.

References & further reading

PubMed: KW-6356 adenosine A2A receptor antagonist inverse agonist pharmacology Parkinson
PubMed: sipagladenant MPTP marmoset anti-parkinsonian L-DOPA dyskinesia
PubMed: KW-6356 population pharmacokinetics
ClinicalTrials.gov: NCT02939391 (Phase 2a monotherapy Parkinson's disease)
ClinicalTrials.gov: NCT03703570 (Phase 2b adjunctive levodopa Parkinson's disease)