Targeting DHX9 Triggers Tumor-Intrinsic Interferon Response and Replication Stress in Small Cell Lung Cancer.

Activating innate immunity in cancer cells through cytoplasmic nucleic acid sensing pathways, a phenomenon known as "viral mimicry," has emerged as an effective strategy to convert immunologically "cold" tumors into "hot." Through a curated CRISPR-based screen of RNA helicases, we identified DExD/H-box helicase 9 (DHX9) as a potent repressor of double-stranded RNA (dsRNA) in small cell lung cancers (SCLC). Depletion of DHX9 induced accumulation of cytoplasmic dsRNA and triggered tumor-intrinsic innate immunity. Intriguingly, ablating DHX9 also induced aberrant accumulation of R-loops, which resulted in an increase of DNA damage-derived cytoplasmic DNA and replication stress in SCLCs. In vivo, DHX9 deletion promoted a decrease in tumor growth while inducing a more immunogenic tumor microenvironment, invigorating responsiveness to immune-checkpoint blockade. These findings suggest that DHX9 is a crucial repressor of tumor-intrinsic innate immunity and replication stress, representing a promising target for SCLC and other "cold" tumors in which genomic instability contributes to pathology. SIGNIFICANCE: One promising strategy to trigger an immune response within tumors and enhance immunotherapy efficacy is by inducing endogenous "virus-mimetic" nucleic acid accumulation. Here, we identify DHX9 as a viral-mimicry-inducing factor involved in the suppression of double-stranded RNAs and R-loops and propose DHX9 as a novel target to enhance antitumor immunity. See related commentary by Chiappinelli, p. 389. This article is featured in Selected Articles from This Issue, p. 384.

Quabodepistat (OPC-167832), a Novel Antituberculosis Drug Candidate: Enhancing Oral Bioavailability via Cocrystallization and Mechanistic Analysis of Bioavailability in Two Cocrystals.

Quabodepistat (code name OPC-167832) is a novel antituberculosis drug candidate. This study aimed to discover cocrystals that improve oral bioavailability and to elucidate the mechanistic differences underlying the bioavailability of different cocrystals. Screening yielded two cocrystals containing 2,5-dihydroxybenzoic acid (2,5DHBA) or 2-hydroxybenzoic acid (2HBA). In bioavailability studies in beagle dogs, both cocrystals exhibited better bioavailability than the free form; however, the extent of bioavailability of cocrystals with 2HBA (quabodepistat-2HBA) was 1.4-fold greater than that of cocrystals with 2,5DHBA (quabodepistat-2,5DHBA). Dissolution studies at pH 1.2 yielded similar profiles for both cocrystals, although the percent dissolution differed: quabodepistat-2HBA dissolved more slowly than quabodepistat-2,5DHBA. The poor solubility of quabodepistat-2HBA is likely the primary factor limiting dissolution at pH 1.2. To identify a dissolution method that maintains the bioavailability in beagle dogs, we performed pH-shift dissolution studies that mimic the dynamic pH change from the stomach to the small intestine. Quabodepistat-2HBA demonstrated supersaturation after the pH was increased to 6.8, while quabodepistat-2,5DHBA did not demonstrate supersaturation. This result was consistent with the results of bioavailability studies in beagle dogs. We conclude that a larger quantity of orally administered quabodepistat-2HBA remained in its cocrystal form while being transferred to the small intestine compared with quabodepistat-2,5DHBA.

YBX1 Regulates Satellite II RNA Loading into Small Extracellular Vesicles and Promotes the Senescent Phenotype.

Senescent cells secrete inflammatory proteins and small extracellular vesicles (sEVs), collectively termed senescence-associated secretory phenotype (SASP), and promote age-related diseases. Epigenetic alteration in senescent cells induces the expression of satellite II (SATII) RNA, non-coding RNA transcribed from pericentromeric repetitive sequences in the genome, leading to the expression of inflammatory SASP genes. SATII RNA is contained in sEVs and functions as an SASP factor in recipient cells. However, the molecular mechanism of SATII RNA loading into sEVs is unclear. In this study, we identified Y-box binding protein 1 (YBX1) as a carrier of SATII RNA via mass spectrometry analysis after RNA pull-down. sEVs containing SATII RNA induced cellular senescence and promoted the expression of inflammatory SASP genes in recipient cells. YBX1 knockdown significantly reduced SATII RNA levels in sEVs and inhibited the propagation of SASP in recipient cells. The analysis of the clinical dataset revealed that YBX1 expression is higher in cancer stroma than in normal stroma of breast and ovarian cancer tissues. Furthermore, high YBX1 expression was correlated with poor prognosis in breast and ovarian cancers. This study demonstrated that SATII RNA loading into sEVs is regulated via YBX1 and that YBX1 is a promising target in novel cancer therapy.

Chromatin conformational changes at human satellite II contribute to the senescence phenotype in the tumor microenvironment.

Cellular senescence and senescence-associated secretory phenotype (SASP) in stromal cells within the tumor microenvironment promote cancer progression. Although cellular senescence has been shown to induce changes in the higher-order chromatin structure and abnormal transcription of repetitive elements in the genome, the functional significance of these changes is unclear. In this study, we examined the human satellite II (hSATII) loci in the pericentromere to understand these changes and their functional significance. Our results indicated that the hSATII loci decompact during senescence induction, resulting in new DNA-DNA interactions in distinct genomic regions, which we refer to as DRISR (Distinctive Regions Interacted with Satellite II in Replicative senescent Fibroblasts). Interestingly, decompaction occurs before the expression of hSATII RNA. The DRISR with altered chromatin accessibility was enriched for motifs associated with cellular senescence and inflammatory SASP genes. Moreover, DNA-fluorescence in situ hybridization analysis of the breast cancer tissues revealed hSATII decompaction in cancer and stromal cells. Furthermore, we reanalyzed the single-cell assay for transposase-accessible chromatin with sequencing data and found increased SASP-related gene expression in fibroblasts exhibiting hSATII decompaction in breast cancer tissues. These findings suggest that changes in the higher-order chromatin structure of the pericentromeric repetitive sequences during cellular senescence might directly contribute to the cellular senescence phenotype and cancer progression via inflammatory gene expression.

FOXD1 is associated with poor outcome and maintains tumor-promoting enhancer-gene programs in basal-like breast cancer.

Breast cancer biology varies markedly among patients. Basal-like breast cancer is one of the most challenging subtypes to treat because it lacks effective therapeutic targets. Despite numerous studies on potential targetable molecules in this subtype, few targets have shown promise. However, the present study revealed that FOXD1, a transcription factor that functions in both normal development and malignancy, is associated with poor prognosis in basal-like breast cancer. We analyzed publicly available RNA sequencing data and conducted FOXD1-knockdown experiments, finding that FOXD1 maintains gene expression programs that contribute to tumor progression. We first conducted survival analysis of patients grouped via a Gaussian mixture model based on gene expression in basal-like tumors, finding that FOXD1 is a prognostic factor specific to this subtype. Then, our RNA sequencing and chromatin immunoprecipitation sequencing experiments using the basal-like breast cancer cell lines BT549 and Hs578T with FOXD1 knockdown revealed that FOXD1 regulates enhancer-gene programs related to tumor progression. These findings suggest that FOXD1 plays an important role in basal-like breast cancer progression and may represent a promising therapeutic target.

Effects of combinations of gapmer antisense oligonucleotides on the target reduction.

BACKGROUND: The co-administration of several therapeutic oligonucleotides targeting the same transcript is a beneficial approach. It broadens the target sites for diseases associated with various mutations or splice variants. However, little is known how a combination of antisense oligonucleotides (ASOs), which is one of the major modalities of therapeutic oligonucleotides, affects the potency. In this study, we aimed to elucidate the combination-effects of ASOs and the relationship between the target sites and potency of different combinations. METHOD AND RESULTS: We designed 113 ASOs targeting human superoxide dismutase 1 pre-mRNA and found 13 ASOs that had comparable silencing activity in vitro. An analysis of combination-effects on the silencing potency of 37 pairs of two ASOs on HeLa cells revealed that 29 pairs had comparable potency to that of two ASOs; on the other hand, eight pairs had reduced potency, indicating a negative impact on the activity. A reduced potency was seen in pairs targeting the same intron, exon-intron combination, or two different introns. The sequence distance of target sites was not the major determinant factor of combination-effects. In addition, a combination of three ASOs preserving the potency could be designed by avoiding two-ASO pairs, which had a reduced potency. CONCLUSIONS: This study revealed that more than half of the combinations retain their potency by paring two ASOs; in contrast, some pairs had a reduced potency. This could not be predicted only by the distance between the target sites.

Identification of Novel Senescent Markers in Small Extracellular Vesicles.

Senescent cells exhibit several typical features, including the senescence-associated secretory phenotype (SASP), promoting the secretion of various inflammatory proteins and small extracellular vesicles (EVs). SASP factors cause chronic inflammation, leading to age-related diseases. Recently, therapeutic strategies targeting senescent cells, known as senolytics, have gained attention; however, noninvasive methods to detect senescent cells in living organisms have not been established. Therefore, the goal of this study was to identify novel senescent markers using small EVs (sEVs). sEVs were isolated from young and senescent fibroblasts using three different methods, including size-exclusion chromatography, affinity column for phosphatidylserine, and immunoprecipitation using antibodies against tetraspanin proteins, followed by mass spectrometry. Principal component analysis revealed that the protein composition of sEVs released from senescent cells was significantly different from that of young cells. Importantly, we identified ATP6V0D1 and RTN4 as novel markers that are frequently upregulated in sEVs from senescent and progeria cells derived from patients with Werner syndrome. Furthermore, these two proteins were significantly enriched in sEVs from the serum of aged mice. This study supports the potential use of senescent markers from sEVs to detect the presence of senescent cells in vivo.

Transcriptomic intratumor heterogeneity of breast cancer patient-derived organoids may reflect the unique biological features of the tumor of origin.

BACKGROUND: The intratumor heterogeneity (ITH) of cancer cells plays an important role in breast cancer resistance and recurrence. To develop better therapeutic strategies, it is necessary to understand the molecular mechanisms underlying ITH and their functional significance. Patient-derived organoids (PDOs) have recently been utilized in cancer research. They can also be used to study ITH as cancer cell diversity is thought to be maintained within the organoid line. However, no reports investigated intratumor transcriptomic heterogeneity in organoids derived from patients with breast cancer. This study aimed to investigate transcriptomic ITH in breast cancer PDOs. METHODS: We established PDO lines from ten patients with breast cancer and performed single-cell transcriptomic analysis. First, we clustered cancer cells for each PDO using the Seurat package. Then, we defined and compared the cluster-specific gene signature (ClustGS) corresponding to each cell cluster in each PDO. RESULTS: Cancer cells were clustered into 3-6 cell populations with distinct cellular states in each PDO line. We identified 38 clusters with ClustGS in 10 PDO lines and used Jaccard similarity index to compare the similarity of these signatures. We found that 29 signatures could be categorized into 7 shared meta-ClustGSs, such as those related to the cell cycle or epithelial-mesenchymal transition, and 9 signatures were unique to single PDO lines. These unique cell populations appeared to represent the characteristics of the original tumors derived from patients. CONCLUSIONS: We confirmed the existence of transcriptomic ITH in breast cancer PDOs. Some cellular states were commonly observed in multiple PDOs, whereas others were specific to single PDO lines. The combination of these shared and unique cellular states formed the ITH of each PDO.

Quantitative Model Analysis and Simulation of Pharmacokinetics and Metastasis-Associated Lung Adenocarcinoma 1 RNA Knockdown Effect After Systemic Administration of Cholesterol-Conjugated DNA/RNA Heteroduplex Oligonucleotide Crossing Blood-Brain Barrier of Mice.

The cholesterol-conjugated heteroduplex oligonucleotide (Chol-HDO) is a double-stranded complex; it comprises an antisense oligonucleotide (ASO) and its complementary strand with a cholesterol ligand. Chol-HDO is a powerful tool for achieving target RNA knockdown in the brains of mice after systemic injection. Here, a quantitative model analysis was conducted to characterize the relationship between the pharmacokinetics (PK) and pharmacodynamics (PD), non-coding RNA metastasis-associated lung adenocarcinoma 1 (Malat1) RNA, of Chol-HDO, in a time-dependent manner. The established PK model could describe regional differences in the observed brain concentration-time profiles. Incorporating the PD model enabled the unique knockdown profiles in the brain to be explained in terms of the time delay after single dosing and enhancement following repeated dosing. Moreover, sensitivity analysis of PK exposure/persistency, target RNA turnover, and knockdown potency identified key factors for the efficient and sustained target RNA knockdown in the brain. The simulation of an adequate dosing regimen quantitatively supported the benefit of Chol-HDO in terms of achieving a suitable dosing interval. This was achieved via sufficient and sustained brain exposure and subsequent strong and sustained target RNA knockdown in the brain, even after systemic injection. The present study provides new insights into drug discoveries and development strategies for HDO in patients with neurogenic disorders. SIGNIFICANCE STATEMENT: The quantitative model analysis presented here characterized the PK/PD relationship of Chol-HDO, enabled its simulation under various conditions or assumptions, and identified key factors for efficient and sustained RNA knockdown, such as PK exposure and persistency. Chol-HDO appears to be an efficient drug delivery system for the systemic administration of desired drugs to brain targets.

RNaseH2A downregulation drives inflammatory gene expression via genomic DNA fragmentation in senescent and cancer cells.

Cellular senescence caused by oncogenic stimuli is associated with the development of various age-related pathologies through the senescence-associated secretory phenotype (SASP). SASP is mediated by the activation of cytoplasmic nucleic acid sensors. However, the molecular mechanism underlying the accumulation of nucleotide ligands in senescent cells is unclear. In this study, we revealed that the expression of RNaseH2A, which removes ribonucleoside monophosphates (rNMPs) from the genome, is regulated by E2F transcription factors, and it decreases during cellular senescence. Residual rNMPs cause genomic DNA fragmentation and aberrant activation of cytoplasmic nucleic acid sensors, thereby provoking subsequent SASP factor gene expression in senescent cells. In addition, RNaseH2A expression was significantly decreased in aged mouse tissues and cells from individuals with Werner syndrome. Furthermore, RNaseH2A degradation using the auxin-inducible degron system induced the accumulation of nucleotide ligands and induction of certain tumourigenic SASP-like factors, promoting the metastatic properties of colorectal cancer cells. Our results indicate that RNaseH2A downregulation provokes SASP through nucleotide ligand accumulation, which likely contributes to the pathological features of senescent, progeroid, and cancer cells.

Hepatocyte growth factor derived from senescent cells attenuates cell competition-induced apical elimination of oncogenic cells.

Cellular senescence and cell competition are important tumor suppression mechanisms that restrain cells with oncogenic mutations at the initial stage of cancer development. However, the link between cellular senescence and cell competition remains unclear. Senescent cells accumulated during the in vivo aging process contribute toward age-related cancers via the development of senescence-associated secretory phenotype (SASP). Here, we report that hepatocyte growth factor (HGF), a SASP factor, inhibits apical extrusion and promotes basal protrusion of Ras-mutated cells in the cell competition assay. Additionally, cellular senescence induced by a high-fat diet promotes the survival of cells with oncogenic mutations, whereas crizotinib, an inhibitor of HGF signaling, provokes the removal of mutated cells from mouse livers and intestines. Our study provides evidence that cellular senescence inhibits cell competition-mediated elimination of oncogenic cells through HGF signaling, suggesting that it may lead to cancer incidence during aging.

Pericentromeric repetitive ncRNA regulates chromatin interaction and inflammatory gene expression.

Cellular senescence provokes a dramatic alteration of chromatin organization and gene expression profile of proinflammatory factors, thereby contributing to various age-related pathologies via the senescence-associated secretory phenotype (SASP). Chromatin organization and global gene expression are maintained through the CCCTC-binding factor (CTCF). However, the molecular mechanism underlying CTCF regulation and its association with SASP gene expression remains to be fully elucidated. A recent study by our team showed that noncoding RNA (ncRNA) derived from normally silenced pericentromeric repetitive sequences directly impair the DNA binding of CTCF. This CTCF disturbance increases the accessibility of chromatin at the loci of SASP genes and caused the transcription of inflammatory factors. This mechanism may promote malignant transformation.

Cholesterol-functionalized DNA/RNA heteroduplexes cross the blood-brain barrier and knock down genes in the rodent CNS.

Achieving regulation of endogenous gene expression in the central nervous system (CNS) with antisense oligonucleotides (ASOs) administered systemically would facilitate the development of ASO-based therapies for neurological diseases. We demonstrate that DNA/RNA heteroduplex oligonucleotides (HDOs) conjugated to cholesterol or α-tocopherol at the 5' end of the RNA strand reach the CNS after subcutaneous or intravenous administration in mice and rats. The HDOs distribute throughout the brain, spinal cord and peripheral tissues and suppress the expression of four target genes by up to 90% in the CNS, whereas single-stranded ASOs conjugated to cholesterol have limited activity. Gene knockdown was observed in major CNS cell types and was greatest in neurons and microglial cells. Side effects, such as thrombocytopenia and focal brain necrosis, were limited by using subcutaneous delivery or by dividing intravenous injections. By crossing the blood-brain barrier more effectively, cholesterol-conjugated HDOs may overcome the limited efficacy of ASOs targeting the CNS without requiring intrathecal administration.

Pericentromeric noncoding RNA changes DNA binding of CTCF and inflammatory gene expression in senescence and cancer.

Cellular senescence causes a dramatic alteration of chromatin organization and changes the gene expression profile of proinflammatory factors, thereby contributing to various age-related pathologies through the senescence-associated secretory phenotype (SASP). Chromatin organization and global gene expression are maintained by the CCCTC-binding factor (CTCF); however, the molecular mechanism underlying CTCF regulation and its association with SASP gene expression remains unclear. We discovered that noncoding RNA (ncRNA) derived from normally silenced pericentromeric repetitive sequences directly impairs the DNA binding of CTCF. This CTCF disturbance increases the accessibility of chromatin and activates the transcription of SASP-like inflammatory genes, promoting malignant transformation. Notably, pericentromeric ncRNA was transferred into surrounding cells via small extracellular vesicles acting as a tumorigenic SASP factor. Because CTCF blocks the expression of pericentromeric ncRNA in young cells, the down-regulation of CTCF during cellular senescence triggers the up-regulation of this ncRNA and SASP-related inflammatory gene expression. In this study, we show that pericentromeric ncRNA provokes chromosomal alteration by inhibiting CTCF, leading to a SASP-like inflammatory response in a cell-autonomous and non-cell-autonomous manner and thus may contribute to the risk of tumorigenesis during aging.

DNA Damage Regulates Senescence-Associated Extracellular Vesicle Release via the Ceramide Pathway to Prevent Excessive Inflammatory Responses.

DNA damage, caused by various oncogenic stresses, can induce cell death or cellular senescence as an important tumor suppressor mechanism. Senescent cells display the features of a senescence-associated secretory phenotype (SASP), secreting inflammatory proteins into surrounding tissues, and contributing to various age-related pathologies. In addition to this inflammatory protein secretion, the release of extracellular vesicles (EVs) is also upregulated in senescent cells. However, the molecular mechanism underlying this phenomenon remains unclear. Here, we show that DNA damage activates the ceramide synthetic pathway, via the downregulation of sphingomyelin synthase 2 (SMS2) and the upregulation of neutral sphingomyelinase 2 (nSMase2), leading to an increase in senescence-associated EV (SA-EV) biogenesis. The EV biogenesis pathway, together with the autophagy-mediated degradation pathway, functions to block apoptosis by removing cytoplasmic DNA fragments derived from chromosomal DNA or bacterial infections. Our data suggest that this SA-EV pathway may play a prominent role in cellular homeostasis, particularly in senescent cells. In summary, DNA damage provokes SA-EV release by activating the ceramide pathway to protect cells from excessive inflammatory responses.

Cellular senescence and senescence-associated secretory phenotype via the cGAS-STING signaling pathway in cancer.

Cellular senescence is historically regarded as a tumor suppression mechanism to prevent damaged cells from aberrant proliferation in benign and premalignant tumors. However, recent findings have suggested that senescent cells contribute to tumorigenesis and age-associated pathologies through the senescence-associated secretory phenotype (SASP). Therefore, to control age-associated cancer, it is important to understand the molecular mechanisms of the SASP in the cancer microenvironment. New findings have suggested that the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway, a critical indicator of innate immune response, triggers the SASP in response to accumulation of cytoplasmic DNA (cytoplasmic chromatin fragments, mtDNA and cDNA) in senescent cells. Notably, the cGAS-STING signaling pathway promotes or inhibits tumorigenesis depending on the biological context in vivo, indicating that it may be a potential therapeutic target for cancer. Herein, we review the regulatory machinery and biological function of the SASP via the cGAS-STING signaling pathway in cancer.

Downregulation of cytoplasmic DNases is implicated in cytoplasmic DNA accumulation and SASP in senescent cells.

Accumulating evidence indicates that the senescence-associated secretory phenotype (SASP) contributes to many aspects of physiology and disease. Thus, controlling the SASP will have tremendous impacts on our health. However, our understanding of SASP regulation is far from complete. Here, we show that cytoplasmic accumulation of nuclear DNA plays key roles in the onset of SASP. Although both DNase2 and TREX1 rapidly remove the cytoplasmic DNA fragments emanating from the nucleus in pre-senescent cells, the expression of these DNases is downregulated in senescent cells, resulting in the cytoplasmic accumulation of nuclear DNA. This causes the aberrant activation of cGAS-STING cytoplasmic DNA sensors, provoking SASP through induction of interferon-ß. Notably, the blockage of this pathway prevents SASP in senescent hepatic stellate cells, accompanied by a decline of obesity-associated hepatocellular carcinoma development in mice. These findings provide valuable new insights into the roles and mechanisms of SASP and possibilities for their control.

Platelet-activating factor podoplanin: from discovery to drug development.

Tumor cell-induced platelet aggregation facilitates hematogenous metastasis by promoting tumor embolization, preventing immunological assaults and shear stress, and the platelet-releasing growth factors support tumor growth and invasion. Podoplanin, also known as Aggrus, is a type I transmembrane mucin-like glycoprotein and is expressed on wide range of tumor cells. Podoplanin has a role in platelet aggregation and metastasis formation through the binding to its platelet receptor, C-type lectin-like receptor 2 (CLEC-2). The podoplanin research was originally started from the cloning of highly metastatic NL-17 subclone from mouse colon 26 cancer cell line and from the establishment of 8F11 monoclonal antibody (mAb) that could neutralize NL-17-induced platelet aggregation and hematogenous metastasis. Later on, podoplanin was identified as the antigen of 8F11 mAb, and its ectopic expression brought to cells the platelet-aggregating abilities and hematogenous metastasis phenotypes. From the 8F11 mAb recognition epitopes, podoplanin is found to contain tandemly repeated, highly conserved motifs, designated platelet aggregation-stimulating (PLAG) domains. Series of analyses using the cells expressing the mutants and the established neutralizing anti-podoplanin mAbs uncovered that both PLAG3 and PLAG4 domains are associated with the CLEC-2 binding. The neutralizing mAbs targeting PLAG3 or PLAG4 could suppress podoplanin-induced platelet aggregation and hematogenous metastasis through inhibiting the podoplanin-CLEC-2 binding. Therefore, these domains are certainly functional in podoplanin-mediated metastasis through its platelet-aggregating activity. This review summarizes the platelet functions in metastasis formation, the role of platelet aggregation-inducing factor podoplanin in pathological and physiological situations, and the possibility to develop podoplanin-targeting drugs in the future.

Podoplanin enhances lung cancer cell growth in vivo by inducing platelet aggregation.

Podoplanin/Aggrus, known as a platelet aggregation-inducing factor, is frequently overexpressed in lung squamous cell carcinomas (LSCC) and glioblastomas among other tumours, and its expression has been reported to be correlated with poor prognosis. However, the contribution of podoplanin to malignant progression has been elusive. Here we demonstrate that in podoplanin-positive LSCC cells, their growth was abrogated by podoplanin knockout in vivo but not in vitro. Conversely, ectopic expression of podoplanin promoted cell growth in vivo and facilitated intratumoral platelet activation. Consistently, LSCC cells evoked podoplanin-mediated platelet aggregation (PMPA), and the releasates from platelets during PMPA promoted the growth of LSCC cells in vitro. Phospho-receptor-tyrosine-kinase array analysis revealed that epidermal growth factor receptor (EGFR) phosphorylation of LSCC cells was responsible for the growth promotion induced by platelet releasates. Treatment with an antiplatelet agent or podoplanin-neutralizing antibody depressed the growth of an LSCC tumour xenograft via suppression of EGFR phosphorylation. These results suggested that podoplanin in LSCC enhanced cell growth by inducing PMPA in vivo and contributed to malignant progression.

Development of antibody-siRNA conjugate targeted to cardiac and skeletal muscles.

Despite considerable efforts to develop efficient carriers, the major target organ of short-interfering RNAs (siRNAs) remains limited to the liver. Expanding the application outside the liver is required to increase the value of siRNAs. Here we report on a novel platform targeted to muscular organs by conjugation of siRNAs with anti-CD71 Fab' fragment. This conjugate showed durable gene-silencing in the heart and skeletal muscle for one month after intravenous administration in normal mice. In particular, 1µg siRNA conjugate showed significant gene-silencing in the gastrocnemius when injected intramuscularly. In a mouse model of peripheral artery disease, the treatment with myostatin-targeting siRNA conjugate by intramuscular injection resulted in significant silencing of myostatin and hypertrophy of the gastrocnemius, which was translated into the recovery of running performance. These data demonstrate the utility of antibody conjugation for siRNA delivery and the therapeutic potential for muscular diseases.