Pharmacotherapeutic group: Other drugs for disorders of the musculo-skeletal system.
ATC code: M09AX09.
Pharmacology: Pharmacodynamics: Mechanism of action: Onasemnogene abeparvovec is a gene therapy designed to introduce a functional copy of the survival motor neuron gene (
SMN1) in the transduced cells to address the monogenic root cause of the disease. By providing an alternative source of SMN protein expression in motor neurons, it is expected to promote the survival and function of transduced motor neurons.
Onasemnogene abeparvovec is a non-replicating recombinant AAV vector that utilizes AAV9 capsid to deliver a stable, fully functional human SMN transgene. The ability of the AAV9 capsid to cross the blood brain barrier and transduce motor neurons has been demonstrated. The
SMN1 gene present in onasemnogene abeparvovec is designed to reside as episomal DNA in the nucleus of transduced cells and is expected to be stably expressed for an extended period of time in post-mitotic cells. The AAV9 virus is not known to cause disease in humans. The transgene is introduced to target cells as a self-complementary double-stranded molecule. Expression of the transgene is driven by a constitutive promoter (cytomegalovirus enhanced chicken-β-actin-hybrid), which results in continuous and sustained SMN protein expression. Proof of the mechanism of action has been supported by non-clinical studies and by human biodistribution data.
Clinical efficacy and safety: AVXS-101-CL-303 Phase 3 study in patients with Type 1 SMA: AVXS-101-CL-303 (Study CL-303) is a Phase 3 open-label, single-arm, single-dose study of intravenous administration of onasemnogene abeparvovec at the therapeutic dose (1.1 x 10
14 vg/kg). Twenty-two patients were enrolled with Type 1 SMA and 2 copies of
SMN2. Before treatment with onasemnogene abeparvovec, none of the 22 patients required non-invasive ventilator (NIV) support, and all patients could exclusively feed orally (i.e., did not need non-oral nutrition). The mean Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND) score at baseline was 32.0 (range, 18 to 52). The mean age of the 22 patients at the time of treatment was 3.7 months (0.5 to 5.9 months).
Of the 22 enrolled patients, 21 patients survived without permanent ventilation (i.e., event-free survival) to ≥10.5 months of age, 20 patients survived to ≥14 months of age (co-primary efficacy endpoint), and 20 patients survived event-free to 18 months of age.
Three patients did not complete the study, of which 2 patients had an event (death or permanent ventilation) leading to 90.9% (95% CI: 79.7%, 100.0%) event-free survival (alive without permanent ventilation) at 14 months of age (see Figure 1).
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For the 14 patients in Study CL-303 that achieved the milestone of independent sitting for at least 30 seconds at any visit during the study, the median age when this milestone was first demonstrated was 12.6 months (range: 9.2 to 18.6 months). Thirteen patients (59.1%) confirmed the milestone of independent sitting for at least 30 seconds at the 18-month visit (co-primary endpoint, p<0.0001). One patient achieved the milestone of sitting independently for 30 seconds at 16 months of age, but this milestone was not confirmed at the Month 18 visit. The video-confirmed developmental milestones for patients in Study CL-303 are summarised in Table 2. Three patients did not achieve any motor milestones (13.6%) and another 3 patients (13.6%) achieved head control as the maximum motor milestone before the 18 months of age final study visit. (See Table 2.)
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One patient (4.5%) could also walk with assistance at 12.9 months. Based on the natural history of the disease, patients who met the study entry criteria would not be expected to attain the ability to sit without support. In addition, 18 of the 22 patients were independent of ventilatory support at 18 months of age.
Motor function improvements were also observed as measured by the CHOP-INTEND (see Figure 2). Twenty-one patients (95.5%) achieved a CHOP-INTEND score ≥40, 14 patients (63.6%) had achieved a CHOP-INTEND score ≥50, and 9 patients (40.9%) had achieved a CHOP-INTEND score ≥58. Patients with untreated SMA Type 1 almost never achieve a CHOP-INTEND score ≥40. Motor milestone achievement was observed in some patients despite plateauing of CHOP-INTEND. No clear correlation was observed between CHOP-INTEND scores and motor milestone achievement.
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AVXS-101-CL-302 Phase 3 study in patients with Type 1 SMA: AVXS-101-CL-302 (Study CL-302) is a Phase 3, open-label, single-arm, single-dose study of intravenous administration of onasemnogene abeparvovec at the therapeutic dose (1.1 x 10
14 vg/kg). Thirty-three patients were enrolled with Type 1 SMA and 2 copies of
SMN2. Before treatment with onasemnogene abeparvovec, 9 patients (27.3%) reported ventilatory support and 9 patients (27.3%) reported feeding support. The mean CHOP-INTEND score of the 33 patients at baseline was 27.9 (range, 14 to 55). The mean age of the 33 patients at the time of treatment was 4.1 months (range, 1.8 to 6.0 months).
Of the 33 enrolled patients (Efficacy Completers population), one patient (3%) was dosed outside of protocol age range and was therefore not included in the intent-to-treat (ITT) population. Of the 32 patients in the ITT population, one patient (3%) died during the study, due to disease progression.
Of the 32 patients in the ITT population, 14 patients (43.8%) achieved the milestone of sitting without support for at least 10 seconds at any visit up to and including the 18 month visit (primary efficacy endpoint). The median age when this milestone was first achieved was 15.9 months (range, 7.7 to 18.6 months). Thirty-one patients (96.9%) in the ITT population survived without permanent ventilation (i.e., event-free survival) to ≥14 months of age (secondary efficacy endpoint).
The additional video-confirmed developmental milestones for patients in the Efficacy Completers population in Study CL-302 at any visit up to and including the 18 month visit are summarised in Table 3. (See Table 3.)
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One patient (3%) achieved the motor milestones of crawling, standing with assistance, stands alone, walking with assistance, and walking alone all by the age of 18 months.
Of the 33 enrolled patients, 24 patients (72.7%) achieved a CHOP-INTEND score ≥40, 14 patients (42.4%) achieved a CHOP-INTEND score ≥50, and 3 patients (9.1%) achieved a CHOP-INTEND score ≥58 (see Figure 3). Patients with untreated SMA Type 1 almost never achieve a CHOP-INTEND score ≥40.
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AVXS-101-CL-101 Phase 1 study in patients with Type 1 SMA: The results seen in Study CL-303 are supported by study AVXS-101-CL-101 (Study CL-101) a phase 1 study in patients with Type 1 SMA, in which onasemnogene abeparvovec was administered as a single intravenous infusion in 12 patients from 3.6 kg to 8.4 kg (0.9 to 7.9 months of age). At 14 months of age, all treated patients were event-free; i.e. survived without permanent ventilation, compared to 25% in the natural history cohort. At the end of the study (24 months post-dose), all treated patients were event-free, compared to less than 8% in the natural history (see Figure 1).
At 24 months of follow up post-dose, 10 out of 12 patients were able to sit without support for ≥10 seconds, 9 patients were able to sit without support for ≥30 seconds and 2 patients were able to stand and walk without assistance. One out of 12 patients did not achieve head control as the maximum motor milestone before the age of 24 months. Ten of 12 patients from Study CL-101 continue to be followed in a long-term study (for up to 6.6 years after dosing) and all 10 patients were alive and free of permanent ventilation as of 23 May 2021. All patients have either maintained previously attained milestones or gained new milestones such as sitting with support, standing with assistance and walking alone. Five of the 10 patients received concomitant nusinersen or risdiplam treatment at some point during the long-term study. Maintenance of efficacy and achievement of milestones can therefore not be solely attributed to onasemnogene abeparvovec in all patients. The milestone of standing with assistance was newly acquired by 2 patients who had not received nusinersen or risdiplam at any point prior to the time this milestone was achieved.
AVXS-101-CL-304 Phase 3 study in patients with pre-symptomatic SMA: Study CL-304 is a global, Phase 3, open-label, single-arm, single-dose study of intravenous administration of onasemnogene abeparvovec in pre-symptomatic newborn patients up to 6 weeks of age with 2 (cohort 1, n=14) or 3 (cohort 2, n=15) copies of
SMN2.
Cohort 1: The 14 treated patients with 2 copies of
SMN2 were followed to 18 months of age. All patients survived event-free to ≥14 months of age without permanent ventilation.
All 14 patients achieved independent sitting for at least 30 seconds at any visit up to the 18 months of age visit (primary efficacy endpoint), at ages ranging from 5.7 to 11.8 months, with 11 of the 14 patients who achieved independent sitting at or before 279 days of age, the 99th percentile for development of this milestone. Nine patients achieved the milestone of walking alone (64.3%). All 14 patients achieved a CHOP-INTEND score ≥58 at any visit up to the 18 months of age visit. No patients required any ventilatory support or any feeding support during the study.
Cohort 2: The 15 treated patients with 3 copies of
SMN2 were followed to 24 months of age. All patients survived event-free to 24 months of age without permanent ventilation.
All 15 patients were able to stand alone without support for at least 3 seconds (primary efficacy endpoint), at ages ranging from 9.5 to 18.3 months, with 14 of the 15 patients who achieved standing alone at or before 514 days of age, the 99th percentile for development of this milestone. Fourteen patients (93.3%) were able to walk at least five steps independently. All 15 patients achieved a scaled score of ≥4 on Bayley-III Gross and Fine Motor Subtests within 2 standard deviations of the mean for age at any post-baseline visit up to 24 months of age. No patients required any ventilatory support or any feeding support during the study.
Onasemnogene abeparvovec has not been studied in patients with a bi-allelic mutation of the
SMN1 gene and only one copy of
SMN2 in clinical studies.
The European Medicines Agency has deferred the obligation to submit the results of studies with onasemnogene abeparvovec in one or more subsets of the paediatric population in spinal muscular atrophy for the granted indication (see Dosage & Administration for information on paediatric use).
Pharmacokinetics: Onasemnogene abeparvovec vector shedding studies, which assess the amount of vector eliminated from the body through saliva, urine and faeces were performed.
Onasemnogene abeparvovec was detectable in shedding samples post-infusion. Clearance of onasemnogene abeparvovec was primarily via faeces and the majority is cleared within 30 days after dose administration.
Biodistribution was evaluated in 2 patients who died 5.7 months and 1.7 months, respectively, after infusion of onasemnogene abeparvovec at the dose of 1.1 x 10
14 vg/kg. Both cases showed that the highest levels of vector DNA were found in the liver. Vector DNA was also detected in the spleen, heart, pancreas, inguinal lymph node, skeletal muscles, peripheral nerves, kidney, lung, intestines, gonads, spinal cord, brain, and thymus. Immunostaining for SMN protein showed generalized SMN expression in spinal motor neurons, neuronal and glial cells of the brain, and in the heart, liver, skeletal muscles, and other tissues evaluated.
Toxicology: Preclinical safety data: Following intravenous administration in neonatal mice, vector was widely distributed, with the highest vector DNA levels generally detected in the heart, liver, lungs and skeletal muscle. The expression of transgene mRNA showed similar patterns. Following intravenous administration in juvenile non-human primates, vector was widely distributed with subsequent expression of transgene mRNA, with the highest concentrations of vector DNA and transgene mRNA tending to occur in the liver, muscle, and heart. Vector DNA and transgene mRNA in both species was detected in the spinal cord, brain, and gonads.
In pivotal 3-month mouse toxicology studies, the main target organs of toxicity identified were the heart and liver. Onasemnogene abeparvovec-related findings in the ventricles of the heart were comprised of dose-related inflammation, oedema and fibrosis. In the atria of the heart, inflammation, thrombosis, myocardial degeneration/necrosis and fibroplasia were observed. A No Adverse Effect Level (NoAEL) was not identified for onasemnogene abeparvovec in mouse studies as ventricular myocardial inflammation/oedema/fibrosis and atrial inflammation were observed at the lowest dose tested (1.5 x 10
14 vg/kg). This dose is regarded as the Maximum Tolerated Dose and approximately 1.4-fold the recommended clinical dose. Onasemnogene abeparvovec-related mortality was, in the majority of mice, associated with atrial thrombosis, and observed at 2.4 x 10
14 vg/kg. The cause of the mortality in the rest of the animals was undetermined, although microscopic degeneration/regeneration in the hearts of these animals was found.
Liver findings in mice were comprised of hepatocellular hypertrophy, Kupffer cell activation, and scattered hepatocellular necrosis. In long-term toxicity studies with intravenous and intrathecal (not indicated for use) administration of onasemnogene abeparvovec in juvenile non-human primates, liver findings, including single cell necrosis of hepatocytes and oval cell hyperplasia, demonstrated partial (IV) or complete (IT) reversibility.
In a 6-month toxicology study conducted in juvenile non-human primates, administration of a single dose of onasemnogene abeparvovec at the clinically recommended intravenous dose, with or without corticosteroid treatment, resulted in acute, minimal to slight mononuclear cell inflammation and neuronal degeneration in the dorsal root ganglia (DRG) and trigeminal ganglia (TG), as well as axonal degeneration and/or gliosis in the spinal cord. At 6 months, these non-progressive findings resulted in full resolution in the TG, and partial resolution (decreased incidence and/or severity) in the DRG and spinal cord. Following intrathecal administration of onasemnogene abeparvovec (not indicated for use), these acute, non-progressive findings were noted with minimal to moderate severity in juvenile non-human primates with partial to full resolution at 12 months. These findings in non-human primates had no correlative clinical observations, therefore the clinical relevance in humans is unknown.
Genotoxicity, carcinogenicity and reproduction toxicity studies have not been conducted with onasemnogene abeparvovec.