Long-Term Efficacy and Safety of Luspatercept for Anemia Treatment in Patients With Lower-Risk Myelodysplastic Syndromes: The Phase II PACE-MDS Study.

Clinical trials frequently include multiple end points that mature at different times. The initial report, typically based on the primary end point, may be published when key planned co-primary or secondary analyses are not yet available. Clinical Trial Updates provide an opportunity to disseminate additional results from studies, published in JCO or elsewhere, for which the primary end point has already been reported.Luspatercept has high clinical activity in patients with transfusion-dependent lower-risk myelodysplastic syndromes (LR-MDS) and ring sideroblasts (RS) relapsed or refractory to erythropoietin. We report long-term luspatercept safety and efficacy in 108 patients with LR-MDS in the PACE-MDS study, including 44 non-RS and 34 non-transfusion-dependent or previously untreated patients. The primary end point was safety. Secondary end points included rates of hematologic improvement (HI) erythroid (HI-E), HI neutrophil, and HI platelet. Exploratory end points included erythropoiesis biomarker quantitation and mutation data. Median duration of luspatercept exposure was 315 days (range, 21-1,934 days). No new safety signals emerged. HI-E was observed in 53.7% of patients, including 36.4% of non-RS and 70.6% of non-transfusion-dependent patients. HI neutrophil and HI platelet were observed in 33.3% and 9.5% of patients, respectively. An almost three-fold increase in bone marrow late to early progenitor cell ratio accompanied HI-E response, irrespective of RS status. Lower baseline erythropoietin levels in non-RS patients (69.6 v 623.3 IU/L; P = .0077) and higher late to early erythroid progenitor cell ratio (10.44 v 4.48; P = .0106) in RS patients were associated with HI-E. This study highlights luspatercept's effects across LR-MDS subtypes, including untreated MDS-RS, serving as a platform for future trials.

Population Pharmacokinetics and Exposure-Response Relationship of Luspatercept, an Erythroid Maturation Agent, in Anemic Patients With ß-Thalassemia.

ß-Thalassemia is an inherited blood disorder resulting from defects in hemoglobin production, leading to premature death of red blood cells (RBCs) or their precursors. Patients with transfusion-dependent ß-thalassemia often need lifelong regular RBC transfusions to maintain adequate hemoglobin levels. Frequent transfusions may lead to iron overload and organ damage. Thus, there is a large unmet need for alternative therapies. Luspatercept, a first-in-class erythroid maturation agent, is the first approved therapy in the United States for the treatment of anemia in adult patients with ß-thalassemia who require regular RBC transfusions. The population pharmacokinetics and exposure-response relationship of luspatercept were evaluated in 285 patients with ß-thalassemia. Luspatercept displayed linear and time-invariant pharmacokinetics when administered subcutaneously once every 3 weeks. Body weight was the only clinically relevant covariate of luspatercept clearance, favoring weight-based dosing. Magnitude and frequency of hemoglobin increase, if not influenced by RBC transfusions, was positively correlated with luspatercept area under the serum concentration-time curve (AUC), 0.2-1.25 mg/kg, whereas a significant reduction in RBC units transfused was observed in frequently transfused patients. The probability of achieving ≥33% or ≥50% reduction in RBC transfusion burden was similar across the time-averaged AUC (0.6-1.25 mg/kg), with the 1 mg/kg starting dose sufficient for most early responders (71%-80%). Increasing luspatercept AUC (0.2-1.25 mg/kg) did not increase incidence or severity of treatment-emergent adverse events. These results provide a positive benefit-risk profile for the recommended luspatercept doses (1-1.25 mg/kg) in treating adult patients with ß-thalassemia who require regular RBC transfusions.

Population Pharmacokinetics and Exposure-Response of Luspatercept, an Erythroid Maturation Agent, in Anemic Patients With Myelodysplastic Syndromes.

Luspatercept is a recombinant fusion protein that enhances late-stage erythroid maturation. This report describes the population pharmacokinetics and exposure-response relationship of luspatercept in 260 patients with anemia due to myelodysplastic syndromes. Luspatercept displayed linear and time-invariant pharmacokinetics over a dose range of 0.125-1.75 mg/kg administered subcutaneously once every 3 weeks. Body weight was the only clinically relevant covariate of luspatercept exposure, supporting the weight-based dosing. The probability of achieving transfusion independence ≥ 8 weeks increased with time-averaged luspatercept serum exposure, reaching the plateau at doses 1.0-1.75 mg/kg. The probability of achieving multiple efficacy end points increased with slower luspatercept clearance, independent of effects of luspatercept exposure or disease characteristics. The probability of experiencing severe treatment-emergent adverse events decreased with increasing luspatercept exposure, especially during long-term treatment. These results provide a positive benefit-risk profile for the titration-to-response dose regimen (1.0-1.75 mg/kg) recommended for this population.

HERMIONE: a randomized Phase 2 trial of MM-302 plus trastuzumab versus chemotherapy of physician's choice plus trastuzumab in patients with previously treated, anthracycline-naïve, HER2-positive, locally advanced/metastatic breast cancer.

BACKGROUND: Human epidermal growth factor receptor 2 (HER2)-positive breast cancer is a particularly aggressive form of the disease, and ultimately progresses in patients with metastases on standard therapies. Anthracyclines, such as doxorubicin, are an effective treatment for HER2-positive breast cancer, particularly when administered in combination with trastuzumab - however, doxorubicin-related cardiotoxicity has limited its use. Many patients are therefore never treated with anthracyclines, even upon disease progression, despite the potential for benefit. MM-302 is a novel, HER2-targeted antibody-liposomal doxorubicin conjugate that specifically targets HER2-overexpressing cells. Preclinical and Phase 1 data suggest that MM-302, as a monotherapy or in combination with trastuzumab, could be effective for managing previously treated, anthracycline-naïve, HER2-positive breast cancer, without the cardiotoxicity observed with free doxorubicin formulations. METHODS/DESIGN: HERMIONE is an open-label, multicenter, randomized (1:1) Phase 2 trial of MM-302 plus trastuzumab versus chemotherapy of physician's choice (gemcitabine, capecitabine, or vinorelbine) plus trastuzumab planned to enroll 250 anthracycline-naïve patients with locally advanced/metastatic HER2-positive breast cancer. Key inclusion criteria are: previous treatment with trastuzumab (with or without pertuzumab) in any setting; refractory or intolerant to pertuzumab (refractory to pertuzumab defined as progression in the locally advanced or metastatic setting, or disease recurrence during or within 12 months of completing pertuzumab-containing neoadjuvant and/or adjuvant therapy); and disease progression on, or intolerant to, ado-trastuzumab emtansine for locally advanced or metastatic disease. The trial is currently being conducted at sites in the USA, Canada, and Western Europe. Treatment will be administered in 21-day cycles, and will be continued until disease progression or unacceptable toxicity. The primary endpoint is independently assessed progression-free survival (PFS). Tumor response will be assessed every 6 weeks, and defined according to RECIST v1.1. Secondary endpoints include investigator-assessed PFS, overall survival (OS), OS rates at 6 months and 1 year, objective response rates, safety and tolerability, quality of life, and the pharmacokinetic profile of MM-302 plus trastuzumab. DISCUSSION: The HERMIONE study will evaluate the efficacy and safety of MM-302 plus trastuzumab in patients with refractory HER2-positive advanced/metastatic breast cancer for whom there are no standard of care therapies with a proven survival advantage. TRIAL REGISTRATION: Clinicaltrials.gov identifier: NCT02213744 . Registration date: 06AUG2014.

Impact of tumor HER2/ERBB2 expression level on HER2-targeted liposomal doxorubicin-mediated drug delivery: multiple low-affinity interactions lead to a threshold effect.

Numerous targeted nanotherapeutics have been described for potential treatment of solid tumors. Although attention has focused on antigen selection and molecular design of these systems, there has been comparatively little study of how cellular heterogeneity influences interaction of targeted nanoparticles with tumor cells. Antigens, such as HER2/ERBB2, are heterogeneously expressed across different indications, across patients, and within individual tumors. Furthermore, antigen expression in nontarget tissues necessitates optimization of the therapeutic window. Understanding the performance of a given nanoparticle under different regimens of antigen expression has the ability to inform patient selection and clinical development decisions. In this work, HER2-targeted liposomal doxorubicin was used as a model-targeted nanoparticle to quantitatively investigate the effect of HER2 expression levels on delivery of doxorubicin to the nucleus. We find quantitatively greater nuclear doxorubicin delivery with increasing HER2 expression, exhibiting a threshold effect at approximately 2 × 10(5) HER2 receptors/cell. Kinetic modeling indicated that the threshold effect arises from multiple low-affinity interactions between the targeted liposome and HER2. These results support previous data showing little or no uptake into human cardiomyocytes, which express levels of HER2 below the threshold. Finally, these results suggest that HER2-targeted liposomal doxorubicin may effectively target tumors that fall below traditional definitions of HER2-positive tumors, thereby expanding the potential population of patients that might benefit from this agent.

HER2-targeted liposomal doxorubicin displays enhanced anti-tumorigenic effects without associated cardiotoxicity.

Anthracycline-based regimens are a mainstay of early breast cancer therapy, however their use is limited by cardiac toxicity. The potential for cardiotoxicity is a major consideration in the design and development of combinatorial therapies incorporating anthracyclines and agents that target the HER2-mediated signaling pathway, such as trastuzumab. In this regard, HER2-targeted liposomal doxorubicin was developed to provide clinical benefit by both reducing the cardiotoxicity observed with anthracyclines and enhancing the therapeutic potential of HER2-based therapies that are currently available for HER2-overexpressing cancers. While documenting the enhanced therapeutic potential of HER2-targeted liposomal doxorubicin can be done with existing models, there has been no validated human cardiac cell-based assay system to rigorously assess the cardiotoxicity of anthracyclines. To understand if HER2-targeting of liposomal doxorubicin is possible with a favorable cardiac safety profile, we applied a human stem cell-derived cardiomyocyte platform to evaluate the doxorubicin exposure of human cardiac cells to HER2-targeted liposomal doxorubicin. To the best of our knowledge, this is the first known application of a stem cell-derived system for evaluating preclinical cardiotoxicity of an investigational agent. We demonstrate that HER2-targeted liposomal doxorubicin has little or no uptake into human cardiomyocytes, does not inhibit HER2-mediated signaling, results in little or no evidence of cardiomyocyte cell death or dysfunction, and retains the low penetration into heart tissue of liposomal doxorubicin. Taken together, this data ultimately led to the clinical decision to advance this drug to Phase I clinical testing, which is now ongoing as a single agent in HER2-expressing cancers.

Myospryn is a calcineurin-interacting protein that negatively modulates slow-fiber-type transformation and skeletal muscle regeneration.

The calcium-calmodulin-regulated protein phosphatase calcineurin plays an important regulatory role in muscle differentiation, fiber-type determination, hypertrophy, and muscle regeneration. Because calcineurin functions in numerous processes in muscle, multiple mechanisms are likely necessary to ensure that the activity of this phosphatase is appropriately regulated. Here we demonstrate that the muscle-specific scaffolding protein myospryn modulates calcineurin signaling by inhibiting calcineurin-dependent transcriptional activity in C2C12 myoblasts through direct interaction with the enzyme via its noncanonical tripartite motif (TRIM-like). Consistent with these data, transgenic mice overexpressing both the TRIM-like domain of myospryn and constitutively active calcineurin displayed a severe attenuation in the ability of calcineurin to induce a slow-fiber phenotype. Furthermore, transgenic mice overexpressing the TRIM-like domain of myospryn displayed attenuated muscle regeneration after cardiotoxin-induced muscle injury. These results indicate that myospryn functions as a novel inhibitor of the calcineurin signaling pathway in skeletal muscle.

Deregulated protein kinase A signaling and myospryn expression in muscular dystrophy.

Alterations in signaling pathway activity have been implicated in the pathogenesis of Duchenne muscular dystrophy, a degenerative muscle disease caused by a deficiency in the costameric protein dystrophin. Accordingly, the notion of the dystrophin-glycoprotein complex, and by extension the costamere, as harboring signaling components has received increased attention in recent years. The localization of most, if not all, signaling enzymes to this subcellular region relies on interactions with scaffolding proteins directly or indirectly associated with the dystrophin-glycoprotein complex. One of these scaffolds is myospryn, a large, muscle-specific protein kinase A (PKA) anchoring protein or AKAP. Previous studies have demonstrated a dysregulation of myospryn expression in human Duchenne muscular dystrophy, suggesting a connection to the pathophysiology of the disorder. Here we report that dystrophic muscle exhibits reduced PKA activity resulting, in part, from severely mislocalized myospryn and the type II regulatory subunit (RIIalpha) of PKA. Furthermore, we show that myospryn and dystrophin coimmunoprecipitate in native muscle extracts and directly interact in vitro. Our findings reveal for the first time abnormalities in the PKA signal transduction pathway and myospryn regulation in dystrophin deficiency.

Identification and mapping of protein kinase A binding sites in the costameric protein myospryn.

Recently we identified a novel target gene of MEF2A named myospryn that encodes a large, muscle-specific, costamere-restricted alpha-actinin binding protein. Myospryn belongs to the tripartite motif (TRIM) superfamily of proteins and was independently identified as a dysbindin-interacting protein. Dysbindin is associated with alpha-dystrobrevin, a component of the dystrophin-glycoprotein complex (DGC) in muscle. Apart from these initial findings little else is known regarding the potential function of myospryn in striated muscle. Here we reveal that myospryn is an anchoring protein for protein kinase A (PKA) (or AKAP) whose closest homolog is AKAP12, also known as gravin/AKAP250/SSeCKS. We demonstrate that myospryn co-localizes with RII alpha, a type II regulatory subunit of PKA, at the peripheral Z-disc/costameric region in striated muscle. Myospryn interacts with RII alpha and this scaffolding function has been evolutionarily conserved as the zebrafish ortholog also interacts with PKA. Moreover, myospryn serves as a substrate for PKA. These findings point to localized PKA signaling at the muscle costamere.

Myospryn is a direct transcriptional target for MEF2A that encodes a striated muscle, alpha-actinin-interacting, costamere-localized protein.

The full repertoire of proteins that comprise the striated muscle Z-disc and peripheral structures, such as the costamere, have yet to be discovered. Recent studies suggest that this elaborate protein network, which acts as a structural and signaling center for striated muscle, harbors factors that function as mechanosensors to ensure coordinated contractile activity. Mutations in genes whose products reside in this region often result in skeletal and cardio myopathies, demonstrating the importance of this macromolecular complex in muscle structure and function. Here, we describe the characterization of a direct, downstream target gene for the MEF2A transcription factor encoding a large, muscle-specific protein that localizes to the costamere in striated muscle. This gene, called myospryn, was identified by microarray analysis as a transcript down-regulated in MEF2A knock-out mice. MEF2A knock-out mice develop cardiac failure during the perinatal period with mutant hearts exhibiting several cardiac abnormalities including myofibrillar disarray. Myospryn is the mouse ortholog of a partial human cDNA of unknown function named cardiomyopathy-associated gene 5 (CMYA5). Myospryn is expressed as a single, large transcript of approximately 12 kilobases in adult heart and skeletal muscle with an open reading frame of 3739 amino acids. This protein, belonging to the tripartite motif superfamily of proteins, contains a B-box coiled-coil (BBC), two fibronectin type III (FN3) repeats, and SPRY domains and interacts with the sarcomeric Z-disc protein, alpha-actinin-2. Our findings demonstrate that myospryn functions directly downstream of MEF2A at the costamere in striated muscle potentially playing a role in myofibrillogenesis.

Mutations of tumor necrosis factor alpha-responsive serine residues within the C-terminal transactivation domain of human transcription factor REL enhance its in vitro transforming ability.

The human c-rel gene (REL), encoding an NF-kappaB transcription factor, is amplified or mutated in several human B-cell lymphomas and can transform chicken lymphoid cells in vitro. We have previously shown that certain deletions of C-terminal transactivation sequences enhance REL's transforming ability in chicken spleen cells. In this report, we have analysed the effect of single amino-acid changes at select serine residues in the C-terminal transactivation domain on REL's transforming ability. Mutation of either of two TNFalpha-inducible serine residues (Ser460 and Ser471) to nonphosphorylatable residues (alanine, asparagine, phenylalanine) made REL more efficient at transforming chicken spleen cells in vitro. In contrast, mutation of Ser471 to a phosphorylation mimetic aspartate residue impaired REL's transforming ability, even though it increased REL's inherent transactivation ability as a GAL4-fusion protein. Alanine mutations of several other serine residues within the transactivation domain did not substantially affect REL's transforming ability. Transactivation by GAL4-REL fusion proteins containing either transformation enhancing or nonenhancing mutations at serine residues was generally similar to wild-type GAL4-REL. However, more transforming mutants with mutations at either Ser460 or Ser471 differed from wild-type REL in their ability to transactivate certain kappaB-site reporter genes. In particular, the SOD2 promoter, encoding manganese superoxide dismutase, was activated less strongly by the more transforming REL mutant REL-S471N in transient assays, but REL-S471N-transformed chicken spleen cells had increased levels of MnSOD protein as compared to wild-type REL-transformed cells. Taken together, our results show that mutations of certain serine residues can enhance REL's transforming ability in vitro and suggest that these mutations increase REL-mediated transformation by altering REL's ability to modulate the expression of select target genes. Furthermore, phosphorylation of Ser471 may be involved in REL-mediated modulation of transformation-specific target gene expression. Lastly, these results suggest that similar mutations in the REL transactivation domain contribute to the development of certain human B-cell lymphomas.

Deletion of either C-terminal transactivation subdomain enhances the in vitro transforming activity of human transcription factor REL in chicken spleen cells.

The REL gene is amplified in many human B-cell lymphomas and we have previously shown that expression of REL from a retroviral vector can malignantly transform chicken spleen cells in vitro. To identify REL protein functions necessary for malignant transformation, we have performed deletion analysis on REL sequences encoding residues of two C-terminal subdomains that are involved in transcriptional activation. We find that deletion of both C-terminal transactivation subdomains abolishes the ability of REL to transform chicken spleen cells in vitro. In contrast, deletion of either transactivation subdomain alone, which reduces the transactivation ability of REL, enhances the transforming activity of REL. Transforming REL mutants missing C-terminal sequences can also be selected at a low frequency in vitro. The REL transactivation domain can be functionally replaced in transformation assays by a portion of the VP16 transactivation domain that activates at a level similar to REL-transforming mutants. We also find that deletion of 29 C-terminal amino acids causes the subcellular localization of REL to change from cytoplasmic to nuclear in chicken embryo fibroblasts. In contrast, wild-type REL and all transforming REL mutants are located primarily in the cytoplasm of transformed spleen cells. Nevertheless, treatment of transformed spleen cells with leptomycin B causes wild-type REL and two REL mutants to relocalize to the nucleus, and nuclear extracts from these transformed cells contain REL DNA-binding activity. Taken together, these results suggest the following: (1) that REL must activate transcription to transform cells in vitro; (2) that a reduced level of transactivation enhances the oncogenicity of REL; (3) that REL shuttles from the cytoplasm to the nucleus in transformed chicken spleen cells; and (4) that mutations in REL, in addition to amplifications, could activate its oncogenicity in human lymphomas.