Small molecule drug targeting RNA-Roche Risdiplam (Evrysdi) for treatment of spinal muscular atrophy

Enzymes, receptors, and ion channels are all protein drug targets, and the entire pharmaceutical industry is established by protein regulators
.

RNA is actually very attractive as a therapeutic target
.

As it happens, spinal muscular atrophy (SMA) is a perfect candidate for targeted RNA splicing
.

SMA is one of the most common hereditary neuromuscular diseases, one in every 10,000 live births suffers from this disease
.

In terminology, exons are coding sequences, and introns are non-coding extensions of RNA sequences
.

In the late 1950s, Francis Crick, the co-discoverer of the double helix structure of deoxyribonucleic acid (DNA), proposed the central principle of genetic information flow
.

In essence, after DNA is transcribed into mRNA, the mRNA is translated into protein (in this particular case, SMN protein)
.

Since SMA is characterized by lack of SMN protein, all existing treatment methods work by increasing the level of SMN protein
.

       Biogen's nusinersen (Spinaraza) has been on the market since 2016 and is an antisense oligonucleotide (ASO)
.

       Another way to treat SMA is Novartis's gene therapy onasemnogene (Zolgensma) based on adeno-associated virus 9 (AAV9)
.
This is a recombinant adenovirus vector encoding the full-length SMN protein, which was approved in 2019
.
This method does not theoretically depend on the SMN2 gene
.
Onasemnogene is a one-time intravenous injection of corticosteroids for at least two months to protect the liver
.
Since the cost of each treatment in 2019 was US$2.
15 million, Zolgensma won the honor of being the most expensive drug in the world! 2

       In 2020, the small molecule SMA2 guided RNA cleavage modifier risdiplam (Evrysdi, 1) was approved by the FDA for the treatment of SMA
.
The drug was discovered by PTC Therapeutics in collaboration with Roche and the SMA Foundation
.

       During the splicing process of SMN2 pre-mRNA, PTC interacts with the RNA components of the splicing mechanism to carry out phenotypic high-throughput screening (HTS) activities to identify small molecules that are conducive to the inclusion of exon 7
.
Coumarin 2 is considered a promising product
.
Unfortunately, coumarin 2 and its isocoumarin derivatives were positive in the Ames test, indicating that it has potential mutagenicity
.
To make matters worse, both coumarin and isocoumarin are unstable in plasma proteins, and it is well known that they are related to phototoxicity
.
3

       In cooperation with Roche and the SMA Foundation, PTC has carried out their leading optimization
.
The scaffold jumping led to the discovery of the core structure pyridine pyrimidine-4-1
.
After extensive structure-activity relationship (SAR) research, RG7800(3) became the first small molecule SMN2 shear modifier to enter human clinical trials
.
4

       However, the clinical study of RG7800(3) was temporarily shelved as a preventive measure due to safety issues found in the long-term toxicity study of cynomolgus monkeys
.
In addition, because RG7800(3) is a weak substrate of the human ether-a-go-go (hERG) potassium ion channel, there are some selectivity issues and potential cardiotoxicity issues
.
In addition, the drug may also cause phototoxicity
.
5

       Further optimization of RG7800 (3) reduced the activity of off-target genes and significantly increased the potency on the target, thereby reducing the effective dose and improving the treatment window
.
Finally, their result is that risdiplam (1) has good pharmacokinetic properties and ideal systemic tissue distribution
.
6

       This is a beautiful work of medicinal chemistry and drug design
.

       Risdiplam (1) is a small molecule drug that fully complies with Lipinski?s rule of 5, and has excellent physical and chemical properties
.
risdiplam (1) has been shown to have good bioavailability into the central nervous system (CNS) and peripheral tissues orally
.
risdiplam (1) is a highly effective SMN2 splicing modifier that increases the exon 7 content of SMN2 mRNA transcription in in vitro testing and SMA transgenic mouse models
.
In clinical trials, risdiplam (1) has been proven to increase SMN protein in a consistent dose and time
.
After one month of exposure, the median ratio of SMN protein increased by approximately 2 in patients given the highest dose
.
6

       Risdiplam (Evrysdi, 1) is an important milestone in drug discovery
.
With the emergence of legal proof-of-concept (PoC), more and more small-molecule drugs targeting RNA will certainly follow, conquering many drug targets that we have not yet conquered
.
In fact, several companies including Skyhawk and Arrakis have clearly established platforms for using small molecules as RNA splicing modifiers
.
2

       references

       1.
Falese, JP; Donlic, A.
; Hargrove, AE Targeting RNA with small molecules: from fundamental principles towards the clinic Chem.
Soc.
Rev.
2021, 50, 2224–2243.

       2.
Cormac, S.
First small-molecule drug targeting RNA gains momentum Nat.
Biotechnol.
2021, 39, 6–8.

       3.
Woll, MG; Qi, H.
; Turpoff, A.
; Zhang, N.
; Zhang, X.
; Chen, G.
; Li, C.
; Huang, S.
; Yang, T.
; Moon, Y.
-C.
; et al.
Discovery and Optimization of Small Molecule Splicing Modifiers of Survival Motor Neuron 2 as a Treatment for Spinal Muscular Atrophy J.
Med.
Chem.
2016, 59, 6070–6085.

       4.
Ratni, H.
; Karp, GM; Weetall, M.
; Naryshkin, NA; Paushkin, SV; Chen, KS; McCarthy, KD; Qi, H.
; Turpoff, A.
; Woll, MG; et al.
Specific Correction of Alternative Survival Motor Neuron 2 Splicing by Small Molecules: Discovery of a Potential Novel Medicine to Treat Spinal Muscular Atrophy J.
Med.
Chem.
2016, 59, 6086-6100.

       5.
Pinard, E.
; Green, L.
; Reutlinger, M.
; Weetall, M.
; Naryshkin, NA; Baird, J.
; Chen, KS; Paushkin, SV; Metzger, F.
; Ratni, H.
Discovery of a Novel Class of Survival Motor Neuron 2 Splicing Modifiers for the Treatment of Spinal Muscular Atrophy J.
Med.
Chem.
2017, 60, 4444–4457.

       6.
Ratni, H.
; Ebeling, M.
; Baird, J.
; Bendels, S.
; Bylund, J.
; Chen, KS; Denk, N.
; Feng, Z.
; Green, L.
; Guerard, M.
et al.
Discovery of Risdiplam, a Selective Survival of Motor Neuron-2 (SMN2) Gene Splicing Modifier for the Treatment of Spinal Muscular Atrophy (SMA) J.
Med.
Chem.
2018, 61, 6501–6517.