Safety and Efficacy of Apitegromab in Patients With Spinal Muscular Atrophy Types 2 and 3: The Phase 2 TOPAZ Study.

BACKGROUND AND OBJECTIVES: Currently approved therapies for spinal muscular atrophy (SMA) reverse the degenerative course, leading to better functional outcome, but they do not address the impairment arising from preexisting neurodegeneration. Apitegromab, an investigational, fully human monoclonal antibody, inhibits activation of myostatin (a negative regulator of skeletal muscle growth), thereby preserving muscle mass. The phase 2 TOPAZ trial assessed the safety and efficacy of apitegromab in individuals with later-onset type 2 and type 3 SMA. METHODS: In this study, designed to investigate potential meaningful combinations of eligibility and treatment regimen for future studies, participants aged 2-21 years received IV apitegromab infusions every 4 weeks for 12 months in 1 of 3 cohorts. Cohort 1 stratified ambulatory participants aged 5-21 years into 2 arms (apitegromab 20 mg/kg alone or in combination with nusinersen); cohort 2 evaluated apitegromab 20 mg/kg combined with nusinersen in nonambulatory participants aged 5-21 years; and cohort 3 blindly evaluated 2 randomized apitegromab doses (2 and 20 mg/kg) combined with nusinersen in younger participants ≥2 years of age. The primary efficacy measure was mean change from baseline using the Hammersmith Functional Motor Scale version appropriate for each cohort. Data were analyzed using a paired t test with 2-sided 5% type 1 error for the mean change from baseline for predefined cohort-specific primary efficacy end points. RESULTS: Fifty-eight participants (mean age 9.4 years) were enrolled at 16 trial sites in the United States and Europe. Participants had been treated with nusinersen for a mean of 25.9 months before enrollment in any of the 3 trial cohorts. At month 12, the mean change from baseline in Hammersmith scale score was -0.3 points (95% CI -2.1 to 1.4) in cohort 1 (n = 23), 0.6 points (-1.4 to 2.7) in cohort 2 (n = 15), and in cohort 3 (n = 20), the mean scores were 5.3 (-1.5 to 12.2) and 7.1 (1.8 to 12.5) for the 2-mg/kg (n = 8) and 20-mg/kg (n = 9) arms, respectively. The 5 most frequently reported treatment-emergent adverse events were headache (24.1%), pyrexia (22.4%), upper respiratory tract infection (22.4%), cough (22.4%), and nasopharyngitis (20.7%). No deaths or serious adverse reactions were reported. DISCUSSION: Apitegromab led to improved motor function in participants with later-onset types 2 and 3 SMA. These results support a randomized, placebo-controlled phase 3 trial of apitegromab in participants with SMA. TRIAL REGISTRATION INFORMATION: This trial is registered with ClinicalTrials.gov (NCT03921528). CLASSIFICATION OF EVIDENCE: This study provides Class III evidence that apitegromab improves motor function in later-onset types 2 and 3 spinal muscular atrophy.

Advances and limitations for the treatment of spinal muscular atrophy.

Spinal muscular atrophy (5q-SMA; SMA), a genetic neuromuscular condition affecting spinal motor neurons, is caused by defects in both copies of the SMN1 gene that produces survival motor neuron (SMN) protein. The highly homologous SMN2 gene primarily expresses a rapidly degraded isoform of SMN protein that causes anterior horn cell degeneration, progressive motor neuron loss, skeletal muscle atrophy and weakness. Severe cases result in limited mobility and ventilatory insufficiency. Untreated SMA is the leading genetic cause of death in young children. Recently, three therapeutics that increase SMN protein levels in patients with SMA have provided incremental improvements in motor function and developmental milestones and prevented the worsening of SMA symptoms. While the therapeutic approaches with Spinraza®, Zolgensma®, and Evrysdi® have a clinically significant impact, they are not curative. For many patients, there remains a significant disease burden. A potential combination therapy under development for SMA targets myostatin, a negative regulator of muscle mass and strength. Myostatin inhibition in animal models increases muscle mass and function. Apitegromab is an investigational, fully human, monoclonal antibody that specifically binds to proforms of myostatin, promyostatin and latent myostatin, thereby inhibiting myostatin activation. A recently completed phase 2 trial demonstrated the potential clinical benefit of apitegromab by improving or stabilizing motor function in patients with Type 2 and Type 3 SMA and providing positive proof-of-concept for myostatin inhibition as a target for managing SMA. The primary goal of this manuscript is to orient physicians to the evolving landscape of SMA treatment.

Blockade of XBP1 splicing by inhibition of IRE1α is a promising therapeutic option in multiple myeloma.

Multiple myeloma (MM) cells are characterized by high protein synthesis resulting in chronic endoplasmic reticulum (ER) stress, which is adaptively managed by the unfolded protein response. Inositol-requiring enzyme 1α (IRE1α) is activated to splice X-box binding protein 1 (XBP1) mRNA, thereby increasing XBP1s protein, which in turn regulates genes responsible for protein folding and degradation during the unfolded protein response. In this study, we examined whether IRE1α-XBP1 pathway is a potential therapeutic target in MM using a small-molecule IRE1α endoribonuclease domain inhibitor MKC-3946. MKC-3946 triggered modest growth inhibition in MM cell lines, without toxicity in normal mononuclear cells. Importantly, it significantly enhanced cytotoxicity induced by bortezomib or 17-AAG, even in the presence of bone marrow stromal cells or exogenous IL-6. Both bortezomib and 17-AAG induced ER stress, evidenced by induction of XBP1s, which was blocked by MKC-3946. Apoptosis induced by these agents was enhanced by MKC-3946, associated with increased CHOP. Finally, MKC-3946 inhibited XBP1 splicing in a model of ER stress in vivo, associated with significant growth inhibition of MM cells. Taken together, our results demonstrate that blockade of XBP1 splicing by inhibition of IRE1α endoribonuclease domain is a potential therapeutic option in MM.

Genetic background controls tumor development in PTEN-deficient mice.

PTEN is one of the most frequently mutated tumor suppressor genes in human cancers. Germ line mutations of PTEN have been detected in three rare autosomal-dominant disorders. However, identical mutations in the PTEN gene may lead to different symptoms that have traditionally been described as different disorders, such as Cowden disease, Lhermitte-Duclos disease, and Bannayan-Zonana syndromes. This lack of genotype-phenotype correlation prompted us to directly test the possible effects of genetic background or modifier genes on PTEN-controlled tumorigenesis using genetically engineered mouse models. In this study, we generated two animal models in which either exon 5 (Pten(Delta5)) or promoter to exon 3 (Pten(-)) of the murine Pten gene were deleted and compared phenotypes associated with individual mutations on two genetic backgrounds. We found that the onset and spectrum of tumor formation depend significantly on the genetic background but less on the type of mutation generated. Our results suggest that PTEN plays a critical role in cancer development, and genetic background may influence the onset, the spectrum, and the progression of tumorigenesis caused by Pten mutation.

A critical role of erythropoietin receptor in neurogenesis and post-stroke recovery.

Erythropoietin (EPO) is the principal growth factor regulating the production of red blood cells. Recent studies demonstrated that exogenous EPO acts as a neuroprotectant and regulates neurogenesis. Using a genetic approach, we evaluate the roles of endogenous EPO and its classical receptor (EPOR) in mammalian neurogenesis. We demonstrate severe and identical embryonic neurogenesis defects in animals null for either the Epo or EpoR gene, suggesting that the classical EPOR is essential for EPO action during embryonic neurogenesis. Furthermore, by generating conditional EpoR knock-down animals, we demonstrate that brain-specific deletion of EpoR leads to significantly reduced cell proliferation in the subventricular zone and impaired post-stroke neurogenesis. EpoR conditional knockdown leads to a specific deficit in post-stroke neurogenesis through impaired migration of neuroblasts to the peri-infarct cortex. Our results suggest that both EPO and EPOR are essential for early embryonic neural development and that the classical EPOR is important for adult neurogenesis and for migration of regenerating neurons during post-injury recovery.

The soluble extracellular domain of EphB4 (sEphB4) antagonizes EphB4-EphrinB2 interaction, modulates angiogenesis, and inhibits tumor growth.

The receptor tyrosine kinase EphB4 and its ligand EphrinB2 play a crucial role in vascular development during embryogenesis. The soluble monomeric derivative of the extracellular domain of EphB4 (sEphB4) was designed as an antagonist of EphB4/EphrinB2 signaling. sEphB4 blocks activation of EphB4 and EphrinB2; suppresses endothelial cell migration, adhesion, and tube formation in vitro; and inhibits the angiogenic effects of various growth factors (VEGF and bFGF) in vivo. sEphB4 also inhibits tumor growth in murine tumor xenograft models. sEphB4 is thus a therapeutic candidate for vascular proliferative diseases and cancer.

The role of erythropoietin in regulating angiogenesis.

Erythropoietin (EPO) is an essential growth factor that regulates erythrocyte production in mammals. In this study, we demonstrate a novel role of EPO in regulating angiogenesis in vivo. Epo and Epo receptor (EpoR) are expressed in the vasculature during embryogenesis. Deletion of Epo or EpoR leads to angiogenic defects starting at E10.5, 2 days before ventricular hypoplasia and 3 days before the onset of the embryonic lethal phenotype. Overall, angiogenesis was severely affected in the mutant embryos: vascular anomalies included decreased complexity of the vessel networks. However, de novo vasculogenesis remained intact, consistent with the differential expression of Epo and EpoR during the early stages of embryonic development. The aforementioned angiogenesis defect can be partially rescued by expressing human EPO during embryogenesis. Moreover, Ang-1 expression is regulated by EPO/EPOR under normoxic conditions. Taken together, our results suggest important roles of EPO and EPOR in angiogenesis.

PTEN tumor suppressor regulates p53 protein levels and activity through phosphatase-dependent and -independent mechanisms.

We show in this study that PTEN regulates p53 protein levels and transcriptional activity through both phosphatase-dependent and -independent mechanisms. The onset of tumor development in p53(+/-);Pten(+/-) mice is similar to p53(-/-) animals, and p53 protein levels are dramatically reduced in Pten(-/-) cells and tissues. Reintroducing wild-type or phosphatase-dead PTEN mutants leads to a significant increase in p53 stability. PTEN also physically associates with endogenous p53. Finally, PTEN regulates the transcriptional activity of p53 by modulating its DNA binding activity. This study provides a novel mechanism by which the loss of PTEN can functionally control "two" hits in the course of tumor development by concurrently modulating p53 activity.

Cloning and characterization of human and mouse SNRK sucrose non-fermenting protein (SNF-1)-related kinases.

We previously isolated, from the earliest population of CD34+ hematopoietic progenitors that form in the aorta of the human embryo, a partial DNA complementary to RNA (cDNA) sequence that was later identified as the human homologue of rat sucrose non-fermenting protein (SNF-1) related kinase (rSNRK), a novel SNF-1-related kinase previously characterized in the rat. In the present study we report the cloning of the complete human SNF-1 related kinase (hSNRK) cDNA and show that the gene spans 39.8 kb at region 3p21 and contains six exons. Recombinant expression of the hSNRK coding sequence in Escherichia coli led to the production of a functional protein kinase of 85 kDa. Reverse transcriptase-polymerase chain reaction (RT-PCR) analysis of hSNRK expression in fetal CD34+ hematopoietic progenitors revealed its continuous expression throughout human development with higher levels in highly dividing CD34+ CD38+ cells compared to quiescent CD34+ CD38- cells. This observation, together with the expression of hSNRK in numerous human leukemic cell lines, may reflect an implication of hSNRK protein in hematopoietic cell proliferation or differentiation. In the mouse, the SNRK cDNA is 4.6-kb-long and encodes a protein of 748 amino acids with a predicted molecular mass of 81,930 Da. The proteins from human, rat and mouse are strongly conserved and are characterized by the presence of a serine/threonine kinase catalytic domain, a bipartite nuclear targeting signal and an ubiquitin-associated domain. In situ hybridization and RT-PCR analysis of the pattern of mSNRK expression in the mouse reveals that it is temporally and spatially regulated during embryogenesis, and widespread expressed in adult tissues.