risdiplam

Introduction

Risdiplam (Evrysdi) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

Risdiplam (brand name Evrysdi) is a small molecule smn2 splicing modifier developed by Roche/Genentech in collaboration with the SMA Foundation and PTC Therapeutics for the treatment of spinal-muscular-atrophy (SMA). Approved by the U.S. Food and Drug Administration (FDA) in August 2020, risdiplam was the first orally administered disease-modifying therapy for SMA and the third overall SMA treatment to receive regulatory approval, following nusinersen (Spinraza, 2016) and onasemnogene abeparvovec (Zolgensma, 2019). Risdiplam functions by promoting the inclusion of exon 7 during SMN2 pre-mRNA splicing, thereby increasing the production of functional full-length survival motor neuron (SMN) protein throughout the body (Ratni et al., 2018). Its oral route of administration, systemic distribution, and favorable safety profile have made it a transformative option for patients with all types of SMA across a wide range of ages (Dhillon, 2020). ^{1Citation1995 · PMID 7824938Open reference}

In February 2025, the FDA approved a new tablet formulation of risdiplam, expanding the dosage form options beyond the original oral solution (FDA, 2025). ^{2Citation2015 · PMID 26515623Open reference}

Spinal Muscular Atrophy Background

Disease Pathophysiology

spinal-muscular-atrophy is an autosomal recessive [neurodegenerative disease caused by homozygous loss-of-function mutations in the SMN1 gene on chromosome 5q13. [The SMN1 gene encodes the survival motor neuron (SMN) protein, which is critical for the maintenance and function of alpha motor neurons in the anterior horn of the spinal-cord. Loss of SMN protein leads to progressive degeneration of lower motor neurons, resulting in symmetrical proximal muscle weakness, atrophy, respiratory failure, and in severe forms, death in early childhood (Lefebvre et al., 1995; Kolb & Kissel, 2015). ^{3'(2002). Quantitative analyses of SMN1 and SMN2 based on real-time LightCycler PCR: fast and highly reliable carrier testing and prediction of severity of Spinal Muscular Atrophy. *American Journal2002 · PMID 11791208Open reference}

SMA Types and Classification

SMA is classified into five clinical subtypes based on age of onset and maximum motor function achieved: ^{4Lorson, C.L., Hahnen, E., Androphy, E.J., & Wirth, B. (1999). A single nucleotide in the SMN gene regulates splicing and is responsible for Spinal Muscular Atrophy. *Proceedings of the National1999 · PMID 10330346Open reference}

• Type 0: Prenatal onset, severe hypotonia at birth, death within weeks

• Type 1 (Werdnig-Hoffmann disease): Onset before 6 months, inability to sit independently, historically fatal by age 2 without intervention

• Type 2 (Dubowitz disease): Onset 6–18 months, ability to sit but not walk independently

• Type 3 (Kugelberg-Welander disease): Onset after 18 months, ability to walk independently (may be lost later)

• Type 4: Adult onset (>21 years), mildest form with ambulatory function preserved

The number of SMN2 gene copies is the primary genetic modifier of disease severity, with more copies associated with milder phenotypes (Feldkötter et al., 2002). ^{5Citation2017 · PMID 28709770Open reference}

The SMN2 Gene as Therapeutic Target

Humans carry a nearly identical paralog, SMN2, which differs from SMN1 primarily by a C-to-T transition at position 6 of exon 7. [This single nucleotide change disrupts an exonic splicing enhancer and creates an exonic splicing silencer, causing approximately 85–90% of SMN2 transcripts to exclude exon 7 during pre-mRNA splicing. The resulting truncated SMNΔ7 protein is unstable and rapidly degraded. Only 10–15% of SMN2 transcripts produce functional full-length SMN protein, which is insufficient to prevent motor neuron degeneration but provides the rationale for splicing modifier therapies like risdiplam (Lorson et al., 1999; Singh et al., 2017). ^{6Citation2019 · PMID 31792448Open reference}

Mechanism of Action

Molecular Target and Binding Sites

Risdiplam is a pyridazine-derived small molecule that modifies the splicing of SMN2 pre-mRNA by binding to two distinct sites within and flanking exon 7. The drug interacts with: ^{7Citation2017 · PMID 28574513Open reference}

1. The exonic splicing enhancer 2 (ESE2) within exon 7: Risdiplam binding at this site displaces the heterogeneous nuclear ribonucleoprotein G (hnRNP G), a splicing factor that normally interacts with this region. This displacement facilitates the binding of the U1 small nuclear ribonucleoprotein (U1 snRNP) complex, which is essential for proper exon 7 recognition and inclusion (Campagne et al., 2019).

2. The 5’ splice site of exon 7: Risdiplam stabilizes the interaction between the 5’ splice site and U1 snRNA, promoting the recognition of exon 7 by the spliceosome. This dual binding mechanism is highly specific for SMN2 exon 7, distinguishing risdiplam from earlier, less selective splicing modulators (Sivaramakrishnan et al., 2017).

Functional Consequences

By promoting exon 7 inclusion, risdiplam shifts the ratio of SMN2 splicing from predominantly SMNΔ7 production to predominantly full-length SMN protein production. In clinical studies, risdiplam treatment led to a greater than 2-fold increase in blood SMN protein levels within four weeks of treatment initiation, with sustained elevation maintained over at least 24 months of continuous dosing (Poirier et al., 2018). ^{8Citation2018 · PMID 30247634Open reference}

Systemic Distribution

Unlike nusinersen, which requires intrathecal administration and is largely confined to the central nervous system, risdiplam distributes systemically after oral administration, crossing the blood-brain-barrier to reach motor neurons in the spinal cord while also increasing SMN protein levels in peripheral tissues. This systemic bioavailability is significant because SMA is increasingly recognized as a multi-system disorder with peripheral organ involvement, including cardiac, hepatic, pancreatic, and metabolic abnormalities (Hamilton & Gillingwater, 2013). ^{9Citation2013 · PMID 23592536Open reference}

Pharmacology

Pharmacokinetics

Risdiplam demonstrates favorable pharmacokinetic properties: ^{10Citation2021 · PMID 33485454Open reference}

• Absorption: Rapidly absorbed after oral administration with time to maximum plasma concentration (t_max) of 1–5 hours. Food does not significantly affect bioavailability

• Distribution: Distributed evenly throughout the body, including the central nervous system. The volume of distribution is approximately 6.3 L/kg, indicating extensive tissue distribution

• Metabolism: Primarily metabolized by flavin monooxygenase (FMO1 and FMO3) and cytochrome P450 enzymes (CYP1A1, CYP2J2, CYP3A4, CYP3A7)

• Elimination: Terminal half-life of approximately 50 hours in adults, supporting once-daily dosing. Elimination is primarily through metabolism with renal excretion of metabolites

• Steady state: Achieved within approximately 7–14 days of once-daily dosing

(Ratni et al., 2018; Roche, 2020) ^{2Citation2015 · PMID 26515623Open reference}0

Dosing

Risdiplam is administered orally once daily, with dosing based on age and body weight: ^{2Citation2015 · PMID 26515623Open reference}1

• Infants 2 months to <2 years: 0.2 mg/kg once daily

• Children ≥2 years weighing <20 kg: 0.25 mg/kg once daily

• Patients ≥2 years weighing ≥20 kg: 5 mg once daily

The drug is available as an oral solution (reconstituted from powder) and, since 2025, as a tablet formulation. It should be administered after a meal at approximately the same time each day (FDA Label, 2022). ^{2Citation2015 · PMID 26515623Open reference}2

Clinical Trials

FIREFISH Trial (Type 1 SMA)

The FIREFISH study was an open-label, two-part Phase 2/3 trial evaluating risdiplam in infants aged 1–7 months with symptomatic Type 1 SMA: ^{2Citation2015 · PMID 26515623Open reference}3

• Part 1 (dose-finding): Enrolled 21 infants to identify the optimal dose

• Part 2 (confirmatory): Enrolled 41 infants at the selected dose (0.2 mg/kg/day)

• Primary endpoint: Proportion of infants sitting without support for ≥5 seconds at 12 months of treatment, as assessed by the Bayley Scales of Infant and Toddler Development, Third Edition (BSID-III)

• Results at 12 months: 29% (12/41) of infants achieved sitting without support, compared to 0% in the natural history. Survival rate was 92.7% (38/41), with 85.4% not requiring permanent ventilation and 89% maintaining oral feeding ability

• Results at 24 months: Motor function continued to improve, with 61% achieving head control and 44% sitting without support for ≥5 seconds

(Baranello et al., 2021; Masson et al., 2022) ^{2Citation2015 · PMID 26515623Open reference}4

SUNFISH Trial (Types 2 and 3 SMA)

The SUNFISH study was a two-part, randomized, double-blind, placebo-controlled Phase 2/3 trial in non-ambulatory patients aged 2–25 years with Type 2 or Type 3 SMA: ^{2Citation2015 · PMID 26515623Open reference}5

• Part 1 (dose-finding): Enrolled 51 patients

• Part 2 (confirmatory): Randomized 180 patients (2:1 risdiplam:placebo)

• Primary endpoint: Change from baseline in Motor Function Measure 32 (MFM-32) total score at 12 months

• Results: Patients receiving risdiplam showed a statistically significant 1.36-point improvement in MFM-32 versus placebo (p=0.0156). In the 2–5 year age group, 78.1% of risdiplam-treated patients had clinically meaningful motor function improvement versus 52.9% on placebo. Among patients aged 18–25, 57.1% on risdiplam maintained stable motor function versus 37.5% on placebo

(Mercuri et al., 2022)

JEWELFISH Trial (Previously Treated Patients)

The JEWELFISH study was an open-label, exploratory Phase 2 trial evaluating risdiplam in patients aged 1–60 years who had previously been treated with other SMA therapies, including nusinersen, onasemnogene abeparvovec, or investigational agents:

• Results: SMN protein levels increased by approximately 2-fold from baseline, similar to increases observed in treatment-naïve patients. This demonstrated that risdiplam can effectively increase SMN protein in patients who have received prior treatment

(Genentech/Roche, 2021)

RAINBOWFISH Trial (Presymptomatic Infants)

The RAINBOWFISH study is an ongoing, open-label Phase 2 trial in presymptomatic infants (≤6 weeks of age at enrollment) with genetically confirmed SMA. Preliminary results indicate that early treatment initiation, before symptom onset, leads to substantial motor milestones including independent sitting, standing, and walking, highlighting the importance of newborn screening programs for SMA.

Comparison with Other SMA Therapies

Three FDA-approved therapies are currently available for SMA, each with distinct mechanisms and routes of administration:

Meta-analyses of clinical trial and real-world data suggest that all three therapies significantly improve survival and motor function compared to natural history, with the greatest benefits observed when treatment is initiated early, ideally in the presymptomatic period. Onasemnogene abeparvovec showed the highest survival rate (95%) in meta-analyses, followed by risdiplam (86%) and nusinersen (60%), though direct comparison is complicated by differences in patient populations and study designs (Stettner & Bastian, 2023).

Safety Profile

Common Adverse Events

In clinical trials across all SMA types, the most commonly reported adverse events included:

• Upper respiratory tract infections (43–48%)

• Pneumonia (in infantile-onset patients, 28%)

• Fever/pyrexia (41%)

• Diarrhea (10–15%)

• Rash (14%)

• Oral ulcers/aphthous stomatitis (6–11%)

• Arthralgia (in later-onset patients, 11%)

• Urinary tract infections (8%)

• Constipation (in infantile-onset patients, 8%)

Safety Monitoring

Based on preclinical findings showing potential effects on rapidly dividing cells, specific monitoring recommendations include:

• Pregnancy testing: Women of childbearing potential should have pregnancy testing before initiating therapy. Risdiplam may cause fetal harm based on animal data

• Ophthalmologic monitoring: Ophthalmological assessments are recommended at initiation and periodically during treatment, due to retinal toxicity observed in animal studies at supratherapeutic doses

• Hematologic monitoring: Complete blood counts should be monitored, as risdiplam may affect rapidly dividing cells

(FDA Label, 2022; EMA, 2021)

Drug Discovery and Development

Historical Context

The discovery of risdiplam emerged from a collaborative effort between Roche, PTC Therapeutics, and the SMA Foundation. The program began with high-throughput screening campaigns to identify small molecules capable of modifying SMN2 splicing:

1. Early screening (2008–2012): PTC Therapeutics identified initial hit compounds through cell-based reporter assays

2. Lead optimization (2012–2015): Roche medicinal chemistry teams optimized the pyridazine scaffold for potency, selectivity, metabolic stability, and CNS penetration

3. Clinical development (2016–2020): The FIREFISH, SUNFISH, JEWELFISH, and RAINBOWFISH trials established efficacy and safety across all SMA types

4. FDA approval (August 2020): Risdiplam received FDA approval for patients ≥2 months of age with SMA

5. EMA approval (March 2021): European Medicines Agency approved risdiplam

(Ratni et al., 2018; Poirier et al., 2018)

Selectivity Challenges

A major challenge in developing SMN2 splicing modifiers was achieving selectivity. The human genome contains approximately 200,000 splice sites, and modifying splicing at one site without unintended effects at others is critical. Earlier compounds from related chemical series showed off-target splicing effects, particularly on the FOXM1 and htt genes. Extensive medicinal chemistry optimization of the pyridazine scaffold ultimately yielded risdiplam with high selectivity for SMN2 exon 7, as confirmed by genome-wide transcriptome profiling (Sivaramakrishnan et al., 2017).

Current Research and Future Directions

Combination Therapy

Research is actively exploring whether combining risdiplam with other SMA therapies could provide additive or synergistic benefits. Real-world studies have shown that patients switching from nusinersen to risdiplam or adding risdiplam after onasemnogene abeparvovec can achieve additional motor function improvements, supporting the concept that addressing SMN deficiency through complementary mechanisms may optimize outcomes (Harada et al., 2025).

Expanded Indications

Risdiplam’s mechanism of modifying pre-mRNA splicing has broader implications for other diseases caused by aberrant splicing, including:

• huntington-pathway (targeting htt exon skipping)

• ftd with GRN mutations

• myotonic-dystrophy

Long-Term Outcomes

Ongoing extension studies are assessing the long-term safety and efficacy of continuous risdiplam treatment over 5+ years, with particular attention to durability of motor function improvements, disease stabilization in older patients, and potential effects on non-motor organ systems.

See Also

• antisense-oligonucleotide-therapy

• gene-therapy

• nusinersen

Background

The study of Risdiplam (Evrysdi) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

External Links

• PubMed - Biomedical literature

• Alzheimer’s Disease Neuroimaging Initiative - Research data

• Allen Brain Atlas - Brain gene expression data

Mechanism of Action

flowchart TD
    A["SMN2 Pre-mRNA"]  -->  B["Risdiplam"]
    B  -->  C["Pyrimidine Ring Binding"]
    C  -->  D["Exon 7 Inclusion"]
    D  -->  E["Full-length SMN Protein"]
    E  -->  F["Increased SMN Protein Levels"]
    F  -->  G["Motor Neuron Survival"]
    G  -->  H["Improved Muscle Function"]

Risdiplam is a small molecule that modifies the splicing of SMN2 pre-mRNA to include exon 7, promoting production of functional SMN protein.