In recent years, one of the great leaps in medical advancements has been the development of drugs such as risdiplam, which is used to treat spinal muscular atrophy (SMA). It is the first drug to be approved as a survival motor neuron 2 (SMN2) mRNA splicing modifier. 

Dhillon did an excellent job describing the key features of risdiplam that caused it to win regulatory approval. She wrote, “Risdiplam is an orally administered, SMN2-directed RNA splicing modifier” that is a “small molecule designed to treat SMA caused by mutations in chromosome 5q leading to SMN protein deficiency.”

Dhillon added, “The drug boosts the ability of SMN2 to produce full-length and functional SMN protein.” It is also a truly global drug, used in areas as diverse as the European Union, India, China, Russia, Indonesia, and South Korea. It is the first drug of its kind to be approved for SMA. 

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Its main pharmacodynamic action is by increasing exon 7 inclusion in SMN2 mRNA transcripts in in vitro assays and in transgenic SMA mouse models. The time it takes to peak plasma concentration is around 1 to 4 hours, and steady-state exposures can be reached between 7 and 14 days. In addition, risdiplam is generally well tolerated. The currently recommended dosage for risdiplam varies according to the patient’s age and body weight. 

However, risdiplam is a small molecule drug that, like many other small molecule drugs with the same mechanisms of action, presents complex problems regarding absorption, distribution, metabolism, and elimination (ADME) that make it challenging to investigate as a potential therapeutic. 

In Vitro vs in Vivo 

According to Fowler and colleagues, the key difficulty in acquiring data about small molecules that present ADME properties is that they can yield in vitro findings that are insufficient to describe the situation in vivo. This has the effect of affecting any downstream development decisions. 

To showcase these difficulties in a real-world drug, Fowler et al chose risdiplam, which has gained the approval of the US Food and Drug Administration and the European Medicines Agency. They wrote, “Risdiplam is a low turnover compound whose metabolism is mediated through a non-cytochrome P450 enzymatic pathway.” Among the 4 challenges identified with regard to risdiplam, we will discuss 2, and summarize the arguments for each point: 

  • Predicting in vivo hepatic clearance
  • Determining in vitro metabolites with regard to metabolites in safety testing guidelines

For drug clearance, scientists have tried to make predictions as close to real-life conditions as possible. Fowler and colleagues wrote, “Recently, considerable effort has been invested by many laboratories into validating novel in vitro systems for low clearance measurements” and that “these data were then scaled to predict the in vivo clearance, the result of which was in excellent agreement (34% higher) with that observed in vivo.”

Read more about SMA etiology 

The most abundant metabolite of risdiplam is Nhydroxyl M1. Metabolites in Safety Training guidelines state that drug metabolites present at greater than 10% of total drug-related exposure in humans are a potential safety concern. A study looked into the plasma metabolite profile using pooled plasma samples of volunteers taking risdiplam and found that the main components were the parent drug and Nhydroxyl M1. In that study, N-hydroxyl M1 exceeded 10% of the total drug-related mass-spectrometry signal intensities. 

On the second point raised, Fowler and colleagues wrote, “The metabolism of risdiplam illustrates the challenges drug metabolism scientists face with predicting the relevance of human in vivo metabolites from in vitro incubations of low turnover compounds.” This is because scientists often use in vitro results to predict in vivo conditions, which may not necessarily be accurate. In other words, the quantitative translation from in vitro to in vivo was poor. 

However, researchers managed to finally observe the plasma metabolite profile in plasma circulation, which consisted mainly of risdiplam and Nhydroxyl M1 (with N-hydroxyl M1 exceeding 10% of total drug-related material). For low-turnover compounds like risdiplam, the problem with in vitro systems is that they have limited accuracy in predicting the extent to which metabolites will circulate in vivo. This is because metabolite distribution and excretion are not reflected in vitro. However, in vitro data have other uses: it is useful to reflect qualitatively the principal metabolic biotransformation pathways, and that data could then be used to potentially dictate frontloading activities. 

Finding Out More 

Dhillon highlighted a few major clinical trials that investigated the efficacy of risdiplam in humans. A clinical trial “enrolled infants with Type 1 SMA and two SMN2 gene copies. Part 1 assessed the safety, tolerability, pharmacokinetics, and pharmacodynamics of different risdiplam doses.” Another clinical trial investigated the efficacy of risdiplam in patients (aged 2 to 25 years) who were diagnosed with late-onset type 2 or type 3 SMA. So far, preliminary findings have indicated that risdiplam is promising when used in patients with SMA. 

Getting the drug dosage right, excluding the possibility of unintended drug toxicity, and carefully documenting observable clinical benefits—these are the various tasks that our medical researchers are hard at work with. Undoubtedly, a fortified understanding of drugs like risdiplam will go a long way to reassure and treat patients, as well as form the framework for the development of future therapeutics. 


Fowler S, Brink A, Cleary Y, et al. Addressing today’s ADME challenges in the translation of in vitro absorption, distribution, metabolism and excretion characteristics to human: a case study of the SMN2 mRNA splicing modifier risdiplam. Drug Metab Dispos. Published online October 7, 2021. doi:10.1124/dmd.121.000563

Dhillon S. Risdiplam: first approvalDrugs. 2020;80(17):1853-1858. doi:10.1007/s40265-020-01410-z