CLINICAL PHARMACOLOGY
Mechanism Of Action
SPINRAZA is an antisense oligonucleotide (ASO) designed
to treat SMA caused by mutations in chromosome 5q that lead to SMN protein
deficiency. Using in vitro assays and studies in transgenic animal models of
SMA, SPINRAZA was shown to increase exon 7 inclusion in SMN2 messenger
ribonucleic acid (mRNA) transcripts and production of full-length SMN protein.
Pharmacodynamics
Autopsy samples from patients (n=3) had higher levels of SMN2
messenger ribonucleic acid (mRNA) containing exon 7 in the thoracic spinal cord
compared to untreated SMA infants.
Cardiac Electrophysiology
Across the sham-controlled studies in 247 patients with
spinal muscular atrophy who received either SPINRAZA or sham-control, QTcF
values >500 ms and change from baseline values >60 ms were observed in 4
(2.4%) patients receiving SPINRAZA. Compared to the sham-control, there was no
increase in the incidence of cardiac adverse reactions associated with delayed ventricular
repolarization in patients treated with SPINRAZA.
Pharmacokinetics
Absorption
Intrathecal injection of SPINRAZA into the cerebrospinal
fluid (CSF) allows nusinersen to be distributed from the CSF to the target
central nervous system (CNS) tissues. Following intrathecal administration,
trough plasma concentrations of nusinersen were relatively low, compared to the
trough CSF concentration. Median plasma Tmax values ranged from 1.7 to 6.0 hours.
Mean plasma Cmax and AUC values increased approximately dose-proportionally up
to a dose of 12 mg.
Distribution
Autopsy data from patients (n=3) showed that SPINRAZA
administered intrathecally was distributed within the CNS and peripheral
tissues, such as skeletal muscle, liver, and kidney.
Elimination
Metabolism
Nusinersen is metabolized via exonuclease (3’- and 5’)-mediated
hydrolysis and is not a substrate for, or inhibitor or inducer of CYP450
enzymes.
Excretion
The mean terminal elimination half-life is estimated to
be 135 to 177 days in CSF, and 63 to 87 days in plasma. The primary route of
elimination is likely by urinary excretion for nusinersen and its
chain-shortened metabolites. At 24 hours, only 0.5% of the administered dose
was recovered in the urine.
Clinical Studies
The efficacy of SPINRAZA was demonstrated in two
double-blind, sham-procedure controlled clinical trials in symptomatic
infantile-onset and later-onset SMA patients (Study 1 and Study 2) and was
supported by open-label clinical trials conducted in presymptomatic and
symptomatic SMA patients. The overall findings from these trials support the
effectiveness of SPINRAZA across the range of SMA patients, and appear to
support the early initiation of treatment with SPINRAZA.
Infantile-Onset SMA
Study 1 was a multicenter, randomized, double-blind,
sham-procedure controlled study in 121 symptomatic infants ≤ 7 months of
age at the time of first dose, diagnosed with SMA (symptom onset before 6
months of age). Patients were randomized 2:1 to receive either 12 mg SPINRAZA or
sham injection as a series of loading doses administered intrathecally followed
by maintenance doses administered every 4 months. Patients in this study were
deemed most likely to develop Type 1 SMA.
A planned interim efficacy analysis was conducted based
on patients who died, withdrew, or completed at least 183 days of treatment. Of
the 82 patients included in the interim analysis (52 patients in the
SPINRAZA-treated group and 30 in the sham-control group), 44% were male, 87%
were Caucasian, 2% were Black, and 4% were Asian. Age at first treatment ranged
from 30 to 262 days (median 181). Length of treatment ranged from 6 to 442 days
(median 261 days). Baseline demographics were balanced between the SPINRAZA and
control groups with the exception of age at first treatment (median age 175 vs.
206 days, respectively). The SPINRAZA and control groups were balanced with
respect to gestational age, birth weight, disease duration, and SMN2 copy
number. Median disease duration was 14 weeks. There was some imbalance in age
at symptom onset with 88% of subjects in the SPINRAZA group and 77% in the
control group experiencing symptoms within the first 12 weeks of life.
The primary endpoint assessed at the time of interim
analysis was the proportion of responders: patients with an improvement in
motor milestones according to Section 2 of the Hammersmith Infant Neurologic
Exam (HINE). This endpoint evaluates seven different areas of motor milestone
development, with a maximum score between 2-4 points for each, depending on the
milestone, and a total maximum score of 26. A treatment responder was defined
as any patient with at least a 2-point increase (or maximal score of 4) in
ability to kick (consistent with improvement by at least 2 milestones), or at
least a 1-point increase in the motor milestones of head control, rolling,
sitting, crawling, standing or walking (consistent with improvement by at least
1 milestone). To be classified as a responder, patients needed to exhibit
improvement in more categories of motor milestones than worsening. Of the 82
patients who were eligible for the interim analysis, a statistically
significantly greater percentage of patients achieved the definition of a motor
milestone responder in the SPINRAZA group (40%) compared to the shamcontrol group
(0%). Results from the final analysis were consistent with those from the
interim analysis (Table 3). Fifty-one percent of patients in the SPINRAZA group
achieved the definition of a motor milestone responder compared to 0% of
patients in the sham-control group. Figure 1 is a descriptive display of the
distribution of net change from baseline in the total motor milestone score for
Section 2 of the HINE for patients in the final efficacy set who did not die or
withdraw from the study.
The primary endpoint assessed at the final analysis was
time to death or permanent ventilation (≥ 16 hours ventilation/day
continuously for > 21 days in the absence of an acute reversible event or tracheostomy).
Statistically significant effects on event-free survival and overall survival
were observed in patients in the SPINRAZA group compared to those in the
sham-control group (Table 4). A 47% reduction in the risk of death or permanent
ventilation was observed in the SPINRAZA group (p=0.005) (Figure 2). Median
time to death or permanent ventilation was not reached in SPINRAZA group and
was 22.6 weeks in the sham-control group. A statistically significant 63%
reduction in the risk of death was also observed (p=0.004).
At the final analysis, the study also assessed treatment
effects on the Children’s Hospital of Philadelphia Infant Test of Neuromuscular
Disorders (CHOP-INTEND), which is an evaluation of motor skills in patients
with infantile-onset SMA. The CHOP-INTEND results are displayed in Table 3.
Table 3: Motor Milestone Response and CHOP-INTEND
Results of the Final Analysis of Patients with Infantile-Onset SMA (Study 1)
Endpoint |
SPINRAZA-treated Patients
(n=73) |
Sham-control Patients
(n=37) |
Motor function |
Motor milestones1 |
Proportion achieving pre-defined motor milestone responder criteria (HINE section 2)2,3 |
37 (51%) |
0 (0%) |
P<0.0001 |
|
CHOP-INTEND1 |
Proportion achieving a 4-point improvement |
52 (71%) |
1 (3%) |
p<0.0001 |
|
Proportion achieving a 4-point worsening4 |
2 (3%) |
17 (46%) |
1At the final analysis, CHOP-INTEND and motor
milestone analyses were conducted using the Efficacy Set (SPINRAZA n=73; Sham-control
n=37).
2Assessed at the later of Day 183, Day 302, and Day 394 Study Visit
3According to HINE section 2: ≥2 point increase [or maximal
score] in ability to kick, OR ≥1 point increase in the motor milestones
of head control, rolling, sitting, crawling, standing or walking, AND
improvement in more categories of motor milestones than worsening), defined as
a responder for this primary analysis.
4Not statistically controlled for multiple comparisons |
Table 4: Survival Results of Patients with Infantile-Onset
SMA (Study 1 )
Endpoint |
SPINRAZA-treated Patients (n=80) |
Sham-control Patients (n=41) |
Survival |
Event-free survival1 |
Number of patients who died or received permanent ventilation |
31 (39%) |
28 (68%) |
Hazard ratio (95% CI) |
0.53 (0.32 -0.89) |
p-value2 |
p=0.005 |
Overall survival1 |
Number of patients who died |
13 (16%) |
16 (39%) |
Hazard Ratio (95% CI) |
0.37 (0.18 - 0.77) |
p-value2 |
p=0.004 |
1At the final analysis, event-free survival
and overall survival were assessed using the Intent to Treat population (ITT
SPINRAZA n=80; Sham-control n=41).
2Based on log-rank test stratified by disease duration |
Figure 1: Percent of Patients Who Died and Net Change
from Baseline in Total Motor Milestone Score (HINE) Among Patients Alive in the
Final Efficacy Set of Study 1 *
*For subjects who were alive and ongoing in the study,
the change in total motor milestone score was calculated at the later of Day
183, Day 302, or Day 394.
Figure 2: Event-Free Survival in the Intent to Treat Set
Later-Onset SMA
Study 2 was a multicenter, randomized, double-blind,
sham-procedure controlled study in 126 symptomatic children with later-onset
SMA (symptom onset after 6 months of age). Patients were randomized 2:1 to
either SPINRAZA 12 mg or sham injection as a series of loading doses administered
intrathecally followed by maintenance doses administered every 6 months.
The median age at screening was 3 years (range 2-9
years), and the median age of onset of clinical signs and symptoms of SMA was
11 months (range 6-20 months). Of the 126 patients included in the study, 47%
were male, 75% were Caucasian, 2% were Black, and 18% were Asian. Length of
treatment ranged from 324 to 482 days (median 450 days). At baseline, patients had
a mean Hammersmith Functional Motor Scale – Expanded (HFMSE) score of 21.6, all
had achieved independent sitting, and no patients had achieved independent
walking. Patients in this study were deemed most likely to develop Type 2 or 3
SMA.
The primary endpoint assessed was the change from
baseline score at Month 15 on the HFMSE. The HFMSE evaluates motor function in
patients with SMA who have limited ambulation, comprising of 33 scored
activities that give objective information on motor ability and clinical progression,
such as the ability to sit unassisted, stand, or walk. Each item is scored from
0-2, with a maximum total score of 66. Higher scores indicate better motor
function. The primary analysis was conducted in the Intent to Treat (ITT)
population, which included all subjects who were randomized and received at
least 1 dose of SPINRAZA or at least one sham procedure. At the final analysis,
a statistically significant improvement in HFMSE scores from baseline to Month
15 was observed in the SPINRAZA-treated group compared to the sham-control
group (Table 5).
Table 5: HFMSE Results in Patients with Later-Onset SMA (Study 2)
Endpoint |
SPINRAZA-treated Patients
(n=84) |
Sham-control Patients
(n=42) |
HFMSE score |
Change from baseline in total HFMSE score at 15 months1,2,3 |
3.9
(95% CI: 3.0, 4.9)
p=0.0000001 |
-1.0
(95% CI: -2.5, 0.5) |
Proportion of patients who achieved at least a 3-point improvement from baseline to Month 151 |
56.8%
(95% CI: 45.6, 68.1)
p=0.00064 |
26.3%
(95% CI: 12.4, 40.2) |
1Assessed using the Intent to Treat population
who received at least one dose of SPINRAZA or at least one sham procedure
(SPINRAZA n=84; Sham-control n=42); data for patients without a Month 15 visit
were imputed using the multiple imputation method
2Least squares mean
3Negative value indicates worsening, positive value indicates
improvement.
4Based on logistic regression with treatment effect and adjustment
for each subject's age at screening and HFMSE score at baseline |
Figure 3: Mean Change from Baseline in HFMSE Score
Over Time in the Intent to Treat Set1, 2(Study 2)
1Data for patients without a Month 15 visit
were imputed using the multiple imputation method
2Error bars denote +/- standard error
Presymptomatic SMA
The results of the sham-controlled trial in
infantile-onset and later-onset SMA patients were supported by an open-label
uncontrolled trial conducted in presymptomatic SMA patients, who ranged in age
from 3 days to 42 days at the time of first dose. Patients received 12 mg SPINRAZA
as a series of loading doses administered intrathecally followed by maintenance
doses administered every 4 months.
Some patients receiving SPINRAZA before the onset of SMA
symptoms survived without requiring permanent ventilation beyond what would be
expected based on their SMN2 copy number, and some patients also achieved
age-appropriate growth and developmental motor milestones such as the ability
to sit unassisted, stand, or walk.