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.
Risdiplam (brand name Evrysdi) is a small molecule [SMN2[/genes/[smn2[/genes/[smn2[/genes/[smn2--TEMP--/genes)--FIX-- splicing modifier developed by Roche/Genentech in collaboration with the SMA Foundation and PTC Therapeutics for the treatment of [spinal muscular atrophy[/diseases/[spinal-muscular-atrophy[/diseases/[spinal-muscular-atrophy[/diseases/[spinal-muscular-atrophy--TEMP--/diseases)--FIX-- (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[/treatments/[nusinersen[/treatments/[nusinersen[/treatments/[nusinersen--TEMP--/treatments)--FIX-- (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).
In February 2025, the FDA approved a new tablet formulation of risdiplam, expanding the dosage form options beyond the original oral solution (FDA, 2025).
[spinal muscular atrophy[/diseases/[spinal-muscular-atrophy[/diseases/[spinal-muscular-atrophy[/diseases/[spinal-muscular-atrophy--TEMP--/diseases)--FIX-- is an autosomal recessive [neurodegenerative disease[/[diseases[/[diseases[/[diseases[/[diseases[/[diseases[/diseases 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[/entities/[neurons[/entities/[neurons[/entities/[neurons--TEMP--/entities)--FIX-- in the anterior horn of the [spinal cord[/brain-regions/[spinal-cord[/brain-regions/[spinal-cord[/brain-regions/[spinal-cord--TEMP--/brain-regions)--FIX--. Loss of SMN protein leads to progressive degeneration of lower motor [neurons[/entities/[neurons[/entities/[neurons[/entities/[neurons--TEMP--/entities)--FIX--, resulting in symmetrical proximal muscle weakness, atrophy, respiratory failure, and in severe forms, death in early childhood (Lefebvre et al., 1995; Kolb & Kissel, 2015).
SMA is classified into five clinical subtypes based on age of onset and maximum motor function achieved:
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).
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).
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:
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).
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).
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).
Unlike [nusinersen[/treatments/[nusinersen[/treatments/[nusinersen[/treatments/[nusinersen--TEMP--/treatments)--FIX--, which requires intrathecal administration and is largely confined to the central nervous system, risdiplam distributes systemically after oral administration, crossing the [blood-brain barrier[/entities/[blood-brain-barrier[/entities/[blood-brain-barrier[/entities/[blood-brain-barrier--TEMP--/entities)--FIX-- to reach motor [neurons[/entities/[neurons[/entities/[neurons[/entities/[neurons--TEMP--/entities)--FIX-- 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).
Risdiplam demonstrates favorable pharmacokinetic properties:
(Ratni et al., 2018; Roche, 2020)
Risdiplam is administered orally once daily, with dosing based on age and body weight:
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).
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:
(Baranello et al., 2021; Masson et al., 2022)
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:
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[/treatments/[nusinersen[/treatments/[nusinersen[/treatments/[nusinersen--TEMP--/treatments)--FIX--, onasemnogene abeparvovec, or investigational agents:
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.
Three FDA-approved therapies are currently available for SMA, each with distinct mechanisms and routes of administration:
| Feature | Risdiplam (Evrysdi) | [Nusinersen[/treatments/[nusinersen[/treatments/[nusinersen[/treatments/[nusinersen--TEMP--/treatments)--FIX-- (Spinraza) | Onasemnogene abeparvovec (Zolgensma) |
|---|---|---|---|
| Type | Small molecule splicing modifier | [Antisense oligonucleotide[/treatments/[antisense-oligonucleotide-therapy[/treatments/[antisense-oligonucleotide-therapy[/treatments/[antisense-oligonucleotide-therapy--TEMP--/treatments)--FIX-- | AAV9 [gene therapy[/treatments/[gene-therapy[/treatments/[gene-therapy[/treatments/[gene-therapy--TEMP--/treatments)--FIX-- |
| Route | Oral (daily) | Intrathecal (every 4 months after loading) | Single IV infusion |
| Target | SMN2 pre-mRNA splicing | SMN2 pre-mRNA splicing | SMN1 gene replacement |
| Distribution | Systemic (CNS + peripheral) | Primarily CNS | Primarily CNS (crosses [BBB[/entities/[blood-brain-barrier[/entities/[blood-brain-barrier[/entities/[blood-brain-barrier--TEMP--/entities)--FIX-- in infants) |
| Age range | ≥2 months, all types | All ages, all types | <2 years (Type 1) |
| Approval | 2020 | 2016 | 2019 |
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).
In clinical trials across all SMA types, the most commonly reported adverse events included:
Based on preclinical findings showing potential effects on rapidly dividing cells, specific monitoring recommendations include:
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:
(Ratni et al., 2018; Poirier et al., 2018)
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/[htt[/genes/[htt[/genes/[htt--TEMP--/genes)--FIX-- 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).
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[/treatments/[nusinersen[/treatments/[nusinersen[/treatments/[nusinersen--TEMP--/treatments)--FIX-- 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).
Risdiplam's mechanism of modifying pre-mRNA splicing has broader implications for other diseases caused by aberrant splicing, including:
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.
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.