The moonlighting enzyme CD13: old and new functions to target

Quantitative profiling of CD13 on single acute myeloid leukemia cells by super-resolution imaging and its implication in targeted drug susceptibility assessment

Quantitative profiling of membrane proteins on the cell surface is of great interest in tumor targeted therapy and single cell biology. However, the existing technologies are either of insufficient resolution, or unable to provide precise information on the localization of individual proteins. Here, we report a new method that combines the use of quantum dot labeling, super-resolution microscopy (structured illumination microscopy, SIM) and software modeling. In this proof-of-principle study, we assessed the biological effects of Bestatin on individual cells from different AML cell lines expressing CD13 proteins, a potential target for tumor targeted therapy. Using the proposed method, we found that the different AML cell lines exhibit different CD13 expression densities, ranging from 0.1 to 1.3 molecules per μm2 cell surface, respectively. Importantly, Bestatin treatment assays shows that its effects on cell growth inhibition, apoptosis and cell cycle change are directly proportional to the density of CD13 on the cell surface of these cell lines. The results suggest that the proposed method advances the quantitative analysis of single cell surface proteins, and that the quantitative profiling information of the target protein on single cells has potential value in targeted drug susceptibility assessment.

Production of porcine aminopeptidase N (pAPN) site-specific edited pigs

Porcine viral diarrhea is an acute and highly contagious enteric disease in pigs which causes huge economic losses in pig industry worldwide. Transmissible gastroenteritis virus (TGEV) is main pathogens responsible for piglets viral diarrhea. Knockout the host cellular surface receptor for TGEV may be an effective way to accelerate the breeding of resistant pigs. In this study, we applied site‐specific editing pAPN which is effective in swine testis (ST) cells. Site‐specific editing of pAPN reduced TGEV proliferation in ST cells by 96%–99% at different time periods post‐infection. Next, the site‐specific editing of pAPN porcine fetal fibroblasts were produced, and then the cell colonies were used as donor cells to generate the site‐specific editing of pAPN pigs. Our research findings will not only offer a more thorough understanding of the pathogenesis of piglet diarrhea and lay the foundation for breeding TGEV‐resistant piglets, but also understanding the molecular mechanisms involved in coronaviral infections.

Aminobenzosuberone Scaffold as a Modular Chemical Tool for the Inhibition of Therapeutically Relevant M1 Aminopeptidases

The synthesis of racemic substituted 7-amino-5,7,8,9-tetrahydrobenzocyclohepten-6-one hydrochlorides was optimized to enhance reproducibility and increase the overall yield. In order to investigate their specificity, series of enzyme inhibition assays were carried out against a diversity of proteases, covering representative members of aspartic, cysteine, metallo and serine endopeptidases and including eight members of the monometallic M1 family of aminopeptidases as well as two members of the bimetallic M17 and M28 aminopeptidase families. This aminobenzosuberone scaffold indeed demonstrated selective inhibition of M1 aminopeptidases to the exclusion of other tested protease families; it was particularly potent against mammalian APN and its bacterial/parasitic orthologues EcPepN and PfAM1.

Characterization of the interaction between recombinant porcine aminopeptidase N and spike glycoprotein of porcine epidemic diarrhea virus

Porcine epidemic diarrhea (PED) has caused huge economic losses to the global pork industry. Infection by its causative agent PED virus (PEDV), an Alpha-coronavirus, was previously proven to be mediated by its spike (S) glycoprotein and a cellular receptor porcine aminopeptidase N (pAPN). Interestingly, some recent studies have indicated that pAPN is not a functional receptor for PEDV. To date, there is a lack of a direct evidence for the interaction between pAPN and PEDV S protein in vitro. Here, we prepared pAPN ectodomain and the truncated variants of PEDV S protein in Drosophila S2 cells. These recombinant proteins were homogeneous after purification by metal-affinity and size-exclusion chromatography. We then assayed the purified target proteins through immunogenicity tests, PEDV binding interference assays, circular dichroism (CD) measurements, pAPN activity assay and structural determination, demonstrating that they were biologically functional. Finally, we characterized their interactions by gel filtration chromatography, native-polyacrylamide gel electrophoresis (PAGE) and surface plasmon resonance (SPR) analyses. The results showed that their affinities were too low to form complexes, which suggest that pAPN may be controversial as the genuine receptor for PEDV. Therefore, further research needs to be carried out to elucidate the interaction between PEDV and its genuine receptor.

CD13 expression in B cell malignancies is a hallmark of plasmacytic differentiation

The diagnosis of Waldenström Macroglobulinaemia (WM)/lymphoplasmacytic lymphoma (LPL) remains one of exclusion because other B-cell lymphoproliferative disorders (B-LPD), such as marginal zone lymphoma (MZL), can fulfil similar criteria, including MYD88 L265P mutation. It has been suggested that expression of the myeloid marker CD13 (also termed ANPEP) is more frequent in LPL than in other B-LPD and has also been described on normal and malignant plasma cells. Here, CD13 expression was tested in a cohort of 1037 B-LPD patients from 3 centres by flow cytometry. The percentage of CD13-expressing cells was found to be variable among B-LPD but significantly higher in WM/LPL (median 31% vs. 0% in non-WM/LPL, P < 0·001). In multivariate linear regression, CD13 expression remained significantly associated with a diagnosis of WM/LPL (P < 0·001). A cut-off value of 2% of CD19⁺ cells co-expressing CD13 yielded the best diagnostic performance for WM/LPL assertion. This was further improved by association with the presence or absence of IgM paraprotein. Finally, given that previously published transcriptomic data revealed no difference in CD13 (also termed ANPEP) mRNA between normal and pathological B-cells, the hypothesis of some post-transcriptional regulation must be favoured. These results suggest that testing for CD13 expression in routine flow cytometry panels could help to discriminate WM/LPL from other B-LPD.

Neutral metalloaminopeptidases APN and MetAP2 as newly discovered anticancer molecular targets of actinomycin D and its simple analogs

The potent transcription inhibitor Actinomycin D is used with several cancers. Here, we report the discovery that this naturally occurring antibiotic inhibits two human neutral aminopeptidases, the cell-surface alanine aminopeptidase and intracellular methionine aminopeptidase type 2. These metallo-containing exopeptidases participate in tumor cell expansion and motility and are targets for anticancer therapies. We show that the peptide portions of Actinomycin D and Actinomycin X₂ are not required for effective inhibition, but the loss of these regions changes the mechanism of interaction. Two structurally less complex Actinomycin D analogs containing the phenoxazone chromophores, Questiomycin A and Actinocin, appear to be competitive inhibitors of both aminopeptidases, with potencies similar to the non-competitive macrocyclic parent compound (K_i in the micromolar range). The mode of action for all four compounds and both enzymes was demonstrated by molecular modeling and docking in the corresponding active sites. This knowledge gives new perspectives to Actinomycin D's action on tumors and suggests new avenues and molecules for medical applications.

Molecular Imaging of Aminopeptidase N in Cancer and Angiogenesis

This review focuses on recent advances in the molecular imaging of aminopeptidase N (APN, also known as CD13), a zinc metalloenzyme that cleaves N-terminal neutral amino acids. It is overexpressed in multiple cancer types and also on the surface of vasculature undergoing angiogenesis, making it a promising target for molecular imaging and targeted therapy. Molecular imaging probes for APN are divided into two large subgroups: reactive and nonreactive. The structures of the reactive probes (substrates) contain a reporter group that is cleaved and released by the APN enzyme. The nonreactive probes are not cleaved by the enzyme and contain an antibody, peptide, or nonpeptide for targeting the enzyme exterior or active site. Multivalent homotopic probes utilize multiple copies of the same targeting unit, whereas multivalent heterotopic molecular probes are equipped with different targeting units for different receptors. Several recent preclinical cancer imaging studies have shown that multivalent APN probes exhibit enhanced tumor specificity and accumulation compared to monovalent analogues. The few studies that have evaluated APN-specific probes for imaging angiogenesis have focused on cardiac regeneration. These promising results suggest that APN imaging can be expanded to detect and monitor other diseases that are associated with angiogenesis.

Effect of Aminopeptidase N on functions and fertility of mouse spermatozoa in vitro

Aminopeptidase N (APN) is defined as a multifunctional enzyme, which regulate cellular physiology of a wide variety of cells in human. Earlier studies reported that mammalian semen shares this common enzyme as a major protein of seminal plasma that has correlation with male fertility, while the regulatory mechanisms of APN in spermatozoa are still far from being well understood. Present study was designed to investigate the role of APN in biological and chemical functions of spermatozoa using an in vitro antagonistic approach. Results showed that lower APN activity in sperm culture medium significantly increased sperm motility and the percentage of high speed spermatozoa and decreased the percentage of slow speed spermatozoa after a dose dependent inhibitor treatment (10, 100, and 1000 μM leuhistin) on epididymal mouse spermatozoa in a capacitating media for 90 min. Both 100 μM and 1000 μM decreased APN activity, while only 1000 μM decreased cell viability and increased PKA activity significantly compared to control. Nonetheless capacitation status, acrosome reaction status, and lactate dehydrogenase activity were not affected. Intriguingly, the treatment affected embryonic development through decreasing tyrosine phosphorylation of proteins and increasing reactive oxygen species levels. Further in silico analysis revealed associated regulatory proteins, which have critical functional role for male fertility.

Broad receptor engagement of an emerging global coronavirus may potentiate its diverse cross-species transmissibility

Porcine deltacoronavirus (PDCoV), identified in 2012, is a common enteropathogen of swine with worldwide distribution. The source and evolutionary history of this virus is, however, unknown. PDCoV belongs to the Deltacoronavirus genus that comprises predominantly avian CoV. Phylogenetic analysis suggests that PDCoV originated relatively recently from a host-switching event between birds and mammals. Insight into receptor engagement by PDCoV may shed light into such an exceptional phenomenon. Here we report that PDCoV employs host aminopeptidase N (APN) as an entry receptor and interacts with APN via domain B of its spike (S) protein. Infection of porcine cells with PDCoV was drastically reduced by APN knockout and rescued after reconstitution of APN expression. In addition, we observed that PDCoV efficiently infects cells of unusual broad species range, including human and chicken. Accordingly, PDCoV S was found to target the phylogenetically conserved catalytic domain of APN. Moreover, transient expression of porcine, feline, human, and chicken APN renders cells susceptible to PDCoV infection. Binding of PDCoV to an interspecies conserved site on APN may facilitate direct transmission of PDCoV to nonreservoir species, including humans, potentially reflecting the mechanism that enabled a virus, ancestral to PDCoV, to breach the species barrier between birds and mammals. The APN cell surface protein is also used by several members of the Alphacoronavirus genus. Hence, our data constitute the second identification of CoVs from different genera that use the same receptor, implying that CoV receptor selection is subjected to specific restrictions that are still poorly understood.

Distinct Epitopes on CD13 Mediate Opposite Consequences for Cell Adhesion

CD13 is a membrane glycoprotein with aminopeptidase activity, expressed on several cell types, including myeloid cells (dendritic cells, monocytes, macrophages, neutrophils, etc.). CD13 participates in several functions such as proteolytic regulation of bioactive peptides, viral receptor, angiogenesis, and tumor metastasis. CD13 has also been proposed to participate in cell adhesion, as crosslinking of CD13 by certain CD13-specific antibodies induces homotypic aggregation of monocytes and heterotypic adhesion of monocytes to endothelial cells. We generated two monoclonal antibodies (mAbs C and E) that block homotypic aggregation of U-937 monocytic cells induced by CD13-specific mAb 452. Moreover, the mAbs cause detachment of cells whose aggregation was induced by CD13 crosslinking. Both mAbs also inhibit heterotypic adhesion of U-937 monocytes to endothelial cells. mAbs C and E recognize membrane CD13 but bind to epitopes different from that recognized by mAb 452. Crosslinking of CD13 by mAb C or E is required to inhibit adhesion, as monovalent Fab fragments are not sufficient. Thus, C and E antibodies recognize a distinct epitope on CD13, and binding to this epitope interferes with both CD13-mediated cell adhesion and enzymatic activity. These antibodies may represent important tools to study cell-cell interactions mediated by CD13 in physiological and pathological conditions.

Fungal lectin MpL enables entry of protein drugs into cancer cells and their subcellular targeting

Lectins have been recognized as promising carrier molecules for targeted drug delivery. They specifically bind carbohydrate moieties on cell membranes and trigger cell internalization. Fungal lectin MpL (Macrolepiota procera lectin) does not provoke cancer cell cytotoxicity but is able to bind aminopeptidase N (CD13) and integrin α3β1, two glycoproteins that are overexpressed on the membrane of tumor cells. Upon binding, MpL is endocytosed in a clathrin-dependent manner and accumulates initially in the Golgi apparatus and, finally, in the lysosomes. For effective binding and internalization a functional binding site on the α-repeat is needed. To test the potential of MpL as a carrier for delivering protein drugs to cancer cells we constructed fusion proteins consisting of MpL and the cysteine peptidase inhibitors cystatin C and clitocypin. The fused proteins followed the same endocytic route as the unlinked MpL. Peptidase inhibitor-MpL fusions impaired both the intracellular degradation of extracellular matrix and the invasiveness of cancer cells. MpL is thus shown in vitro to be a lectin that can enable protein drugs to enter cancer cells, enhance their internalization and sort them to lysosomes and the Golgi apparatus.

In vitro and in vivo anti-leukemic activity of the peptidase-potentiated alkylator melflufen in acute myeloid leukemia

The novel aminopeptidase potentiated alkylating agent melflufen, was evaluated for activity in acute myeloid leukemia in a range of in vitro models, as well as in a patient derived xenograft study. All tested AML cell lines were highly sensitive to melflufen while melphalan was considerably less potent. In the HL-60 cell line model, synergy was observed for the combination of melflufen and cytarabine, an interaction that appeared sequence dependent with increased synergy when melflufen was added before cytarabine. Also, in primary cultures of AML cells from patients melflufen was highly active, while normal PBMC cultures appeared less sensitive, indicating a 7-fold in vitro therapeutic index. Melphalan, on the other hand, was only 2-fold more potent in the AML patient samples compared with PBMCs. Melflufen was equally active against non-malignant, immature CD34⁺ progenitor cells and a more differentiated CD34⁺ derived cell population (GM14), whereas the stem cell like cells were less sensitive to melphalan. Finally, melflufen treatment showed significant anti-leukemia activity and increased survival in a patient derived xenograft of AML in mice. In conclusion, melflufen demonstrates high and significant preclinical activity in AML and further clinical evaluation seem warranted in this disease.

Preclinical activity of melflufen (J1) in ovarian cancer

Ovarian cancer carries a significant mortality. Since symptoms tend to be minimal, the disease is often diagnosed when peritoneal metastases are already present. The standard of care in advanced ovarian cancer consists of platinum-based chemotherapy combined with cytoreductive surgery. Unfortunately, even after optimal cytoreduction and adjuvant chemotherapy, most patients with stage III disease will develop a recurrence. Intraperitoneal administration of chemotherapy is an alternative treatment for patients with localized disease. The pharmacological and physiochemical properties of melflufen, a peptidase potentiated alkylator, raised the hypothesis that this drug could be useful in ovarian cancer and particularily against peritoneal carcinomatosis. In this study the preclinical effects of melflufen were investigated in different ovarian cancer models. Melflufen was active against ovarian cancer cell lines, primary cultures of patient-derived ovarian cancer cells, and inhibited the growth of subcutaneous A2780 ovarian cancer xenografts alone and when combined with gemcitabine or liposomal doxorubicin when administered intravenously. In addition, an intra- and subperitoneal xenograft model showed activity of intraperitoneal administered melflufen for peritoneal carcinomatosis, with minimal side effects and modest systemic exposure. In conclusion, results from this study support further investigations of melflufen for the treatment of peritoneal carcinomatosis from ovarian cancer, both for intravenous and intraperitoneal administration.

Ubenimex, an APN inhibitor, could serve as an anti‑tumor drug in RT112 and 5637 cells by operating in an Akt‑associated manner

Bladder cancer, a common urinary tract tumor, has high mortality and recurrence rates associated with metastasis. Aminopeptidase N (APN) expression and metastasis have been indicated to be associated with one another. Ubenimex may function as an APN inhibitor to inhibit the degradation of the extracellular matrix during tumorigenesis. Furthermore, APN has been widely used as an adjuvant therapy for the treatment of tumors; however, little information is available regarding the impact of ubenimex on patients. Autophagy is suggested to be important in the transformation and progression of cancer. Additionally, apoptosis, which leads to the rapid demolition of cellular organelles and structures, has also been suggested as an important factor. Thus, the present study investigated the role of ubenimex in inhibiting migration and invasion by downregulating APN expression levels to induce autophagic cell death and apoptosis in bladder cancer cells. RT112 and 5637 cell lines were treated with varying doses of ubenimex. Cell viability was measured by CCK8 colorimetry and flow cytometry. Using fluorescence microscopy, autophagic cell death was assessed using acridine orange/ethidium bromide staining. Furthermore, apoptotic cell death was assessed using flow cytometry and Trypan blue staining was used to evaluate the cell death rate. Protein expression was determined by western blot analysis. Matrigel invasion assays were exploited to assess the invasion capabilities of 5637 cells. Wound-healing migration assays and Matrigel migration assays were exploited to assess the migratory abilities of 5637 cells. Treatment with ubenimex was accompanied by decreased Akt expression, indicating that ubenimex may have similar functions to Akt inhibitors. Results also indicated that ubenimex inhibited cell migration and invasion in bladder cancer cells. Furthermore, ubenimex also induced autophagic cell death and apoptosis, which suggested that mixed programmed cell death occurred in ubenimex-treated bladder cancer cells. The results from the present study suggest that ubenimex may be a potential adjuvant therapy for the treatment of bladder cancer.

Synthesis and biological characterization of ubenimex-fluorouracil conjugates for anti-cancer therapy

Previously a novel ubenimex-fluorouracil (5-FU) conjugate, BC-01 was identified and validated as a potent CD13 inhibitor with marked in vitro and in vivo antitumor potency. Herein, further structural modifications of the linker part of BC-01 was carried out to get more potent and stable ubenimex-fluorouracil conjugates. It was striking that most of these conjugates showed even more potent CD13 inhibitory activities than BC-01 and the approved CD13 inhibitor ubenimex. One representative compound 12a displayed significant in vitro anti-proliferation, pro-apoptosis, anti-metastasis, anti-angiogenesis and CD13+ cell elimination effects. In vitro stability and in vivo pharmacokinetic study revealed that compound 12a could release ubenimex and 5-FU slowly, which could act as a mutual prodrug of ubenimex and 5-FU. Compared with 5-FU or 5-FU plus ubenimex, 12a exhibited superior in vivo antitumor growth efficiency, even in our mice model of 5-FU-resistant liver cancer. Moreover, 12a exhibited more potent in vivo anti-metastasis and lifespan extension effects compared to the approved 5-FU prodrug capecitabine. Collectively, these results suggest that further optimization and evaluation of 12a as a promising anticancer candidate are warranted to develop effective therapeutic agents for human liver cancer.

The bulky and the sweet: How neutralizing antibodies and glycan receptors compete for virus binding

Numerous viruses rely on glycan receptor binding as the initial step in host cell infection. Engagement of specific glycan receptors such as sialylated carbohydrates, glycosaminoglycans, or histo-blood group antigens can determine host range, tissue tropism, and pathogenicity. Glycan receptor-binding sites are typically located in exposed regions on viral surfaces-sites that are also generally prone to binding of neutralizing antibodies that directly interfere with virus-glycan receptor interactions. In this review, we examine the locations and architecture of the glycan- and antibody-binding sites in four different viruses with stalk-like attachment proteins (reovirus, influenza virus, norovirus, and coronavirus) and investigate the mechanisms by which antibodies block glycan recognition. Those viruses exemplify that direct molecular mimicking of glycan receptors by antibodies is rare and further demonstrate that antibodies often partly overlap or bind sufficiently close to the receptor-binding region to hinder access to this site, achieving neutralization partially because of the epitope location and partly due to their sheer size.

Nano-delivery system targeting to cancer stem cell cluster of differentiation biomarkers

Cancer stem cells (CSCs) are one of the most important origins of cancer progression and metastasis. CSCs have unique self-renewal properties and diverse cell membrane receptors that induced the resistance to the conventional chemotherapeutic agents. Therefore, the therapeutic removal of CSCs could result in the cancer cure with lack of recurrence and metastasis. In this regard, targeting CSCs in accordance to their specific biomarkers is a talented attitude in cancer therapy. Various CSCs surface biomarkers have been described, which some of them exhibited similarities on different cancer cell types, while the others are cancer specific and have just been reported on one or a few types of cancers. In this review, the importance of CSCs in cancer development and therapeutic response has been stated. Different CSCs cluster of differentiation (CD) biomarkers and their specific function and applications in the treatment of cancers have been discussed, Special attention has been made on targeted nano-delivery systems. In this regard, several examples have been illustrated concerning specific natural and artificial ligands against CSCs CD biomarkers that could be decorated on various nanoparticulated drug delivery systems to enhance therapeutic index of chemotherapeutic agents or anticancer gene therapy. The outlook of CSCs biomarkers discovery and therapeutic/diagnostic applications was discussed.

A Novel Function for the Streptococcus pneumoniae Aminopeptidase N: Inhibition of T Cell Effector Function through Regulation of TCR Signaling

Streptococcus pneumoniae (Spn) causes a variety of disease states including fatal bacterial pneumonia. Our previous finding that introduction of Spn into an animal with ongoing influenza virus infection resulted in a CD8⁺ T cell population with reduced effector function gave rise to the possibility of direct regulation by pneumococcal components. Here, we show that treatment of effector T cells with lysate derived from Spn resulted in inhibition of IFNγ and tumor necrosis factor α production as well as of cytolytic granule release. Spn aminopeptidase N (PepN) was identified as the inhibitory bacterial component and surprisingly, this property was independent of the peptidase activity found in this family of proteins. Inhibitory activity was associated with reduced activation of ZAP-70, ERK1/2, c-Jun N-terminal kinase, and p38, demonstrating the ability of PepN to negatively regulate TCR signaling at multiple points in the cascade. These results reveal a novel immune regulatory function for a bacterial aminopeptidase.

Autophagy flux inhibition, G2/M cell cycle arrest and apoptosis induction by ubenimex in glioma cell lines

This study aimed to investigate whether ubenimex could work as an anti-tumor drug alone in glioma cells and figure out the underlying potential mechanisms. Ubenimex is widely used as an adjunct therapy in multiple solid cancers. However, it is rarely used to treat glioblastoma. The function of ubenimex in enhancing JQ1 treatment sensitivity of glioma cells by blocking autophagic degradation of HEXIM1 was previously studied. However, the detailed mechanism of autophagy regulation by ubenimex remains unclear. The U87 and U251 cell lines were treated with different doses of ubenimex. Cell viability was measured by using the WST-8 assay. Cell death was assessed using trypan blue staining and flow cytometry. The migration and invasive ability of glioma cells were examined by transwell migration/invasion assay. LC3-GFP-RFP was used to measure autophagic flux. Protein expression was assessed by Western blot analysis. Autophagosomes were evaluated using the transmission electron microscopy. Moreover, cell cycle arrest (PI Staining) was measured by flow cytometry. Results revealed that ubenimex inhibited cell proliferation as well as migration/invasion in glioma cells. Besides, ubenimex increased glioma cell death via autophagic flux inhibition. Meanwhile, ubenimex induced G2/M phase arrest and apoptosis, and this effect was accompanied by the decreased levels of p-Akt, indicating the role of ubenimex in the regulation of glioma cell proliferation and metastasis. To sum up, this study concluded that ubenimex could work as an anti-tumor drug alone in the glioma cells via inhibiting autophagic flux and inducing G2/M arrest as well as apoptosis.

Efficient targeting of CD13 on cancer cells by the immunotoxin scFv13-ETA' and the bispecific scFv [13xds16]

PURPOSE

Treatment of cancer using standard chemotherapy still offers a poor prognosis combined with severe side effects. Novel antibody-based therapies have been shown to overcome low efficiency and lack of selectivity by targeting cancer-associated antigens, such as aminopeptidase CD13.

METHODS

We isolated a high-affinity CD13-specific single-chain fragment variable (scFv13) from a phage display library of V-genes from mice immunized with soluble antigen. An immunotoxin comprising the scFv13 and a truncated version of the exotoxin A of Pseudomonas aeruginosa (ETA', scFv13-ETA') and a bispecific scFv targeting CD13 and CD16 simultaneously (bsscFv[13xds16]) was generated and investigated for their therapeutic potential.

RESULTS

Both fusion proteins bound specifically to target cells with high affinity. Furthermore, scFv13-ETA' inhibited the proliferation of human cancer cell lines efficiently at low concentrations (IC50 values of 408 pM-7 nM) and induced apoptosis (40-85% of target cells). The bsscFv triggered dose-dependent antibody-dependent cell-mediated cytotoxicity, resulting in the lysis of up to 23.9% A2058 cells, 18.0% MDA-MB-468 cells and 19.1% HL-60 cells.

CONCLUSION

The provided data demonstrate potent therapeutic activity of the scFv13-ETA' and the bsscFv[13xds16]. The CD13-specific scFv is therefore suitable for the direct and specific delivery of both cytotoxic agents and effector cells to cancer-derived cells, making it ideal for further therapeutic evaluation.

Surface marker profiling of SH-SY5Y cells enables small molecule screens identifying BMP4 as a modulator of neuroblastoma differentiation

Neuroblastoma is the most common extra-cranial solid tumor in children. Its broad spectrum of clinical outcomes reflects the underlying inherent cellular heterogeneity. As current treatments often do not lead to tumor eradication, there is a need to better define therapy-resistant neuroblastoma and to identify new modulatory molecules. To this end, we performed the first comprehensive flow cytometric characterization of surface molecule expression in neuroblastoma cell lines. Exploiting an established clustering algorithm (SPADE) for unbiased visualization of cellular subsets, we conducted a multiwell screen for small molecule modulators of neuroblastoma phenotype. In addition to SH-SY5Y cells, the SH-EP, BE(2)-M17 and Kelly lines were included in follow-up analysis as in vitro models of neuroblastoma. A combinatorial detection of glycoprotein epitopes (CD15, CD24, CD44, CD57, TrkA) and the chemokine receptor CXCR4 (CD184) enabled the quantitative identification of SPADE-defined clusters differentially responding to small molecules. Exposure to bone morphogenetic protein (BMP)-4 was found to enhance a TrkA^high/CD15⁻/CD184⁻ neuroblastoma cellular subset, accompanied by a reduction in doublecortin-positive neuroblasts and of NMYC protein expression in SH-SY5Y cells. Beyond yielding novel marker candidates for studying neuroblastoma pathology, our approach may provide tools for improved pharmacological screens towards developing novel avenues of neuroblastoma diagnosis and treatment.

Discovery of novel non-competitive inhibitors of mammalian neutral M1 aminopeptidase (APN)

Neutral metallo-aminopeptidase (APN) catalyzes the cleavage of neutral and basic amino acids from the N-terminus of protein or peptide substrates. APN expression is dysregulated in inflammatory diseases as well as in several types of cancer. Therefore, inhibitors of APN may be effective against cancer and inflammation. By virtual screening and enzymatic assays, we identified three non-competitive inhibitors (α > 1) of the porcine and human APN with K_i values in the μM range. These non-peptidic compounds lack the classical zinc-binding groups (ZBG) present in most of the APN inhibitors. Molecular docking simulations suggested the novel inhibitors suppress APN activity by an alternative mechanism to Zn coordination: they interacted with residues comprising the S1 and S5′ subsites of APN. Of note, these compounds also inhibited the porcine aminopeptidase A (pAPA) using a competitive inhibition mode. This indicated differences in the binding mode of these compounds with APN and APA. Based on sequence and structural analyses, we predicted the significance of targeting human APN residues: Ala-351, Arg-442, Ala-474, Phe-896 and Asn-900 for improving the selectivity of the identified compounds. Remarkably, the intraperitoneal injection of compounds BTB07018 and JFD00064 inhibited APN activity in rat brain, liver and kidney indicating good bio-distribution of these inhibitors in vivo. These data reinforce the idea of designing novel APN inhibitors based on lead compounds without ZBG.

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We identified three non-competitive inhibitors of the human and porcine APN.

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These compounds lack the classical zinc-binding groups of the APN inhibitors.

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We proposed these molecules block APN by an alternative mechanism to Zn chelation.

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All the inhibitors interact with APN residues comprising the S1 and S5′ subsites.

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Two compounds blocked the APN activity in the brain, liver and kidney of rats.

Expression and regulation of the neutral amino acid transporter B0AT1 in rat small intestine

Absorption of neutral amino acids across the luminal membrane of intestinal enterocytes is mediated by the broad neutral amino acid transporter B0AT1 (SLC6A19). Its intestinal expression depends on co-expression of the membrane-anchored peptidase angiotensin converting enzyme 2 (ACE2) and is additionally enhanced by aminopeptidase N (CD13). We investigated in this study the expression of B0AT1 and its auxiliary peptidases as well as its transport function along the rat small intestine. Additionally, we tested its possible short- and long-term regulation by dietary proteins and amino acids. We showed by immunofluorescence that B0AT1, ACE2 and CD13 co-localize on the luminal membrane of small intestinal villi and by Western blotting that their protein expression increases in distal direction. Furthermore, we observed an elevated transport activity of the neutral amino acid L-isoleucine during the nocturnal active phase compared to the inactive one. Gastric emptying was delayed by intragastric application of an amino acid cocktail but we observed no acute dietary regulation of B0AT1 protein expression and L-isoleucine transport. Investigation of the chronic dietary regulation of B0AT1, ACE2 and CD13 by different diets revealed an increased B0AT1 protein expression under amino acid-supplemented diet in the proximal section but not in the distal one and for ACE2 protein expression a reverse localization of the effect. Dietary regulation for CD13 protein expression was not as distinct as for the two other proteins. Ring uptake experiments showed a tendency for increased L-isoleucine uptake under amino acid-supplemented diet and in vivo L-isoleucine absorption was more efficient under high protein and amino acid-supplemented diet. Additionally, plasma levels of branched-chain amino acids were elevated under high protein and amino acid diet. Taken together, our experiments did not reveal an acute amino acid-induced regulation of B0AT1 but revealed a chronic dietary adaptation mainly restricted to the proximal segment of the small intestine.

Elevated aminopeptidase N affects sperm motility and early embryo development

Aminopeptidase N (APN) is a naturally occurring ectopeptidase present in mammalian semen. Previous studies have demonstrated that APN adversely affects male fertility through the alteration of sperm motility. This enzyme constitutes 0.5 to 1% of the seminal plasma proteins, which can be transferred from the prostasomes to sperms by a fusion process. In the present study, we investigated the molecular mechanism of action of APN and its role in regulating sperm functions and male fertility. In this in vitro study, epididymal mouse spermatozoa were incubated in a capacitating media (pH 7) containing 20 ng/mL of recombinant mouse APN for 90 min. Our results demonstrated that the supplementation of recombinant APN in sperm culture medium significantly increased APN activity, and subsequently altered motility, hyperactivated motility, rapid and medium swimming speeds, viability, and the acrosome reaction of mouse spermatozoa. These effects were potentially caused by increased toxicity in the spermatozoa. Further, altered APN activity in sperm culture medium affected early embryonic development. Interestingly, the effect of elevated APN activity in sperm culture medium was independent of protein tyrosine phosphorylation and protein kinase A activity. On the basis of these results, we concluded that APN plays a significant role in the regulation of several sperm functions and early embryonic development. In addition, increased APN activity could potentially lead to several adverse consequences related to male fertility.

Silencing FLI or targeting CD13/ANPEP lead to dephosphorylation of EPHA2, a mediator of BRAF inhibitor resistance, and induce growth arrest or apoptosis in melanoma cells

A majority of patients with BRAF-mutated metastatic melanoma respond to therapy with BRAF inhibitors (BRAFi), but relapses are common owing to acquired resistance. To unravel BRAFi resistance mechanisms we have performed gene expression and mass spectrometry based proteome profiling of the sensitive parental A375 BRAF V600E-mutated human melanoma cell line and of daughter cell lines with induced BRAFi resistance. Increased expression of two novel resistance candidates, aminopeptidase-N (CD13/ANPEP) and ETS transcription factor FLI1 was observed in the BRAFi-resistant daughter cell lines. In addition, increased levels of the previously reported resistance mediators, receptor tyrosine kinase ephrine receptor A2 (EPHA2) and the hepatocyte growth factor receptor MET were also identified. The expression of these proteins was assessed in matched tumor samples from melanoma patients obtained before BRAFi and after disease progression. MET was overexpressed in all progression samples while the expression of the other candidates varied between the individual patients. Targeting CD13/ANPEP by a blocking antibody induced apoptosis in both parental A375- and BRAFi-resistant daughter cells as well as in melanoma cells with intrinsic BRAFi resistance and led to dephosphorylation of EPHA2 on S897, previously demonstrated to cause inhibition of the migratory capacity. AKT and RSK, both reported to induce EPHA2 S897 phosphorylation, were also dephosphorylated after inhibition of CD13/ANPEP. FLI1 silencing also caused decreases in EPHA2 S897 phosphorylation and in total MET protein expression. In addition, silencing of FLI1 sensitized the resistant cells to BRAFi. Furthermore, we show that BRAFi in combination with the multi kinase inhibitor dasatinib can abrogate BRAFi resistance and decrease both EPHA2 S897 phosphorylation and total FLI1 protein expression. This is the first report presenting CD13/ANPEP and FLI1 as important mediators of resistance to BRAF inhibition with potential as drug targets in BRAFi refractory melanoma.

Ubenimex suppresses Pim-3 kinase expression by targeting CD13 to reverse MDR in HCC cells

Hepatocellular carcinoma (HCC) is one of the most serious cancers, with rapid progression and high mortality. However, chemotherapy of HCC is hindered by multi-drug resistance (MDR). It is urgent, therefore, to explore new approaches for overcoming MDR of HCC cells. Ubenimex, an inhibitor of CD13, has been used as an immuno-enhancer for treating hematological neoplasms and other solid tumors. Here, we demonstrate that Ubenimex can also reverse MDR in the HCC cell lines HepG2/5-FU and Bel7402/5-FU. Ubenimex inhibits the expression of the proto-oncogene, Pim-3, which is accompanied by decreased expression of BCL-2 and BCL-XL, decreased phosphorylation of Bad, and increased tumor apoptosis.

Moreover, Ubenimex decreases expression of the MDR-associated proteins P-gp, MRP3 and MRP2 to enhance intracellular accumulation of Cisplatin, for which down-regulation of Pim-3 is essential. Our results reveal a previously uncharacterized function of Ubenimex in mediating drug resistance in HCC, which suggests that Ubenimex may provide a new strategy to reverse MDR and improve HCC sensitivity to chemotherapeutic drugs via its effects on Pim-3.

Origin and characterization of small membranous vesicles present in the venom of Crotalus durissus terrificus

Small membranous vesicles are small closed fragments of membrane. They are released from multivesicular bodies (exosomes) or shed from the surface membrane (microvesicles). They contains various bioactive molecules and their molecular composition varies depending on their cellular origin. Small membranous vesicles have been identified in snake venoms, but the origin of these small membranous vesicles in the venom is controversial. The aim of this study was to verify the origin of the small membranous vesicles in venom of Crotalus durissus terrificus by morphological analyses using electron microscopy. In addition, the protein composition of the vesicles was analyzed by using a proteome approach. The small membranous vesicles present in the venom were microvesicles, since they originated from microvilli on the apical membrane of secretory cells. They contained cytoplasmic proteins, and proteins from the plasma membrane, endoplasmic reticulum (ER), and Golgi membrane. The release of microvesicles may be a mechanism to control the size of the cell membrane of the secretory cells after intense exocytosis. Microvesicle components that may have a role in envenoming include ecto-5'-nucleotidase, a cell membrane protein that releases adenosine, and aminopeptidase N, a cell membrane protein that may modulate the action of many peptides.

Cancer Stem Cells in Hepatocellular Carcinoma

BACKGROUND

Hepatocellular carcinoma is one of the most common cancers and the second leading cause of cancer-related deaths worldwide. Only a small proportion of patients benefit from curative treatment and the prognosis is very poor for the majority of cases due to late presentation, resistance to chemotherapy and high recurrence rate. In recent years, progress in stem cell biology allowed us to explain that hierarchically organized cancer stem cells (CSCs) drive histological and functional heterogeneity of hematological malignancies and solid tumors.

METHODS AND RESULTS

Also referred to as tumor-initiating cells, CSCs have been isolated from both hepatocellular carcinoma (HCC) cell lines and primary tumors by using hepatic progenitor markers. Although there is still no consensus on cancer stem cell phenotype in HCC, single or combined use of CSC markers defines a minor population of tumor cells with the capacity of self-renewing and the ability to recapitulate the original tumor heterogeneity.

CONCLUSIONS

This review focuses on the biological features of CSCs and their potential as diagnostic/prognostic tools and therapeutic targets in HCC.

Melflufen - a peptidase-potentiated alkylating agent in clinical trials

Aminopeptidases like aminopeptidase N (APN, also known as CD13) play an important role not only in normal cellular functioning but also in the development of cancer, including processes like tumor cell invasion, differentiation, proliferation, apoptosis, motility, and angiogenesis. An increased expression of APN has been described in several types of human malignancies, especially those characterized by fast-growing and aggressive phenotypes, suggesting APN as a potential therapeutic target.

Melphalan flufenamide ethyl ester (melflufen, previously denoted J1) is a peptidase-potentiated alkylating agent. Melflufen readily penetrates membranes and an equilibrium is rapidly achieved, followed by enzymatic cleavage in aminopeptidase positive cells, which results in trapping of less lipophilic metabolites. This targeting effect results in very high intracellular concentrations of its metabolite melphalan and subsequent apoptotic cell death. This results in a potency increase (melflufen vs melphalan) ranging from 10- to several 100-fold in different in vitro models. Melflufen triggers a rapid, robust, and an irreversible DNA damage which may account for its ability to overcome melphalan-resistance in multiple myeloma cells. Furthermore, anti-angiogenic properties of melflufen have been described.

Consequently, it is hypothesized that melflufen could provide better efficacy but no more toxicity than what is achieved with melphalan, an assumption so far supported by experiences from hollow fiber and xenograft studies in rodents as well as by clinical data from patients with solid tumors and multiple myeloma. This review summarizes the current preclinical and clinical knowledge of melflufen.

The Rational Design of Therapeutic Peptides for Aminopeptidase N using a Substrate-Based Approach

The M1 family of metalloproteases represents a large number of exopeptidases that cleave single amino acid residues from the N-terminus of peptide substrates. One member of this family that has been well studied is aminopeptidase N (APN), a multifunctional protease known to cleave biologically active peptides and aide in coronavirus entry. The proteolytic activity of APN promotes cancer angiogenesis and metastasis making it an important target for cancer therapy. To understand the substrate specificity of APN for the development of targeted inhibitors, we used a global substrate profiling method to determine the P1-P4' amino acid preferences. The key structural features of the APN pharmacophore required for substrate recognition were elucidated by x-ray crystallography. By combining these substrate profiling and structural data, we were able to design a selective peptide inhibitor of APN that was an effective therapeutic both in vitro and in vivo against APN-expressing prostate cancer models.

A preliminary study on the interaction between Asn-Gly-Arg (NGR)-modified multifunctional nanoparticles and vascular epithelial cells

Previously developed Asn-Gly-Arg (NGR) peptide-modified multifunctional poly(ethyleneimine)–poly(ethylene glycol) (PEI–PEG)-based nanoparticles (TPIC) have been considered to be promising carriers for the co-delivery of DNA and doxorubicin (DOX). As a continued effort, the aim of the present study was to further evaluate the interaction between TPIC and human umbilical vein endothelial cells (HUVEC) to better understand the cellular entry mechanism. In the present investigation, experiments relevant to co-localization, endocytosis inhibitors and factors influencing the internalization were performed. Without any treatment, there was no co-localization between aminopeptidase N/CD13 (APN/CD13) and caveolin 1 (CAV1). However, co-localization between CD13 and CAV1 was observed when cells were incubated with an anti-CD13 antibody or TPIC. As compared with antibody treatment, TPIC accelerated the speed and enhanced the degree of co-localization. TPIC entered HUVEC not only together with CD13 but also together with CAV1. However, this internalization was not dependent on the enzyme activity of CD13 but could be inhibited by methyl-β-eyclodextfin (MβCD), further identifying the involvement of caveolae-mediated endocytosis (CvME). This conclusion was also verified by endocytosis inhibitor experiments.

Allosteric inhibition of aminopeptidase N functions related to tumor growth and virus infection

Cell surface aminopeptidase N (APN) is a membrane-bound ectoenzyme that hydrolyzes proteins and peptides and regulates numerous cell functions. APN participates in tumor cell expansion and motility, and is a target for cancer therapies. Small drugs that bind to the APN active site inhibit catalysis and suppress tumor growth. APN is also a major cell entry receptor for coronavirus, which binds to a region distant from the active site. Three crystal structures that we determined of human and pig APN ectodomains defined the dynamic conformation of the protein. These structures offered snapshots of closed, intermediate and open APN, which represent distinct functional states. Coronavirus envelope proteins specifically recognized the open APN form, prevented ectodomain progression to the closed form and substrate hydrolysis. In addition, drugs that bind the active site inhibited both coronavirus binding to cell surface APN and infection; the drugs probably hindered APN transition to the virus-specific open form. We conclude that allosteric inhibition of APN functions occurs by ligand suppression of ectodomain motions necessary for catalysis and virus cell entry, as validated by locking APN with disulfides. Blocking APN dynamics can thus be a valuable approach to development of drugs that target this ectoenzyme.

Aminopeptidase N is not required for porcine epidemic diarrhea virus cell entry

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Overexpression of porcine APN in cells does not confer susceptibility to PEDV.

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Knockout APN expression in PEDV-susceptible cells has no effect on PEDV infection.

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Results demonstrate that APN is not essential for PEDV cell entry.

Porcine epidemic diarrhea virus (PEDV) is an emerging pathogenic coronavirus that causes a significant economic burden to the swine industry. The virus infects the intestinal epithelium and causes villous atrophy, resulting in diarrhea and dehydration. Interaction of the viral spike (S) surface glycoprotein − through its S1 subunit − with the host cell receptor is the first step in infection and the main determinant for virus tropism. As for several other alphacoronaviruses including the porcine transmissible gastroenteritis virus (TGEV) and the human coronavirus 229E (HCoV-229E), the aminopeptidase N (APN) protein was reported to be a functional receptor for PEDV. In this study we examined the role of APN as a receptor. We show that overexpression of porcine APN renders MDCK cells susceptible to TGEV, but not to PEDV. Consistently, unlike TGEV-S1, PEDV-S1 exhibited no binding to cell-surface expressed APN or to a soluble version of APN. Moreover, preincubation of these viruses with soluble APN or pretreatment of APN expressing ST cells with soluble TGEV-S1 blocked TGEV infection, but had no effect on infection by PEDV. The combined observations indicated that APN is not required for PEDV infection. To definitively prove this conclusion, we applied CRISPR/Cas9 genome engineering to knock out APN expression in PEDV-susceptible porcine (ST) and human cell lines (Huh7 and HeLa). As a consequence these cells no longer bound TGEV-S1 and HCoV-229E-S1 at their surface and were resistant to infection by the corresponding viruses. However, genetic ablation of APN expression had no effect on their infectability by PEDV, demonstrating that APN is not essential for PEDV cell entry.

Macrophage Polarization Modulates FcγR- and CD13-Mediated Phagocytosis and Reactive Oxygen Species Production, Independently of Receptor Membrane Expression

In response to microenvironmental cues, macrophages undergo a profound phenotypic transformation acquiring distinct activation phenotypes ranging from pro-inflammatory (M1) to anti-inflammatory (M2). To study how activation phenotype influences phagocytosis and production of reactive oxygen species (ROS) mediated by receptors for IgG antibodies (Fcγ receptors) and by CD13, human monocyte-derived macrophages were polarized to distinct phenotypes using IFN-γ (Mϕ-IFN-γ), IL-4 (Mϕ-IL-4), or IL-10 (Mϕ-IL-10). Phenotypically, Mϕ-IFN-γ were characterized as CD14⁺CD80⁺CD86⁺ cells, Mϕ-IL-4 as CD209^highCD206⁺CD11b⁺CD14^low, and Mϕ-IL-10 as CD16⁺CD163⁺ cells. Compared to non-polarized macrophages, FcγRI expression increased in Mϕ-IFN-γ and Mϕ-IL-10 and FcγRIII expression increased in Mϕ-IL-10. None of the polarizing cytokines modified FcγRII or CD13 expression. Functionally, we found that cytokine-mediated activation significantly and distinctively affected FcγR- and CD13-mediated phagocytosis and ROS generation. Compared to non-polarized macrophages, FcγRI-, FcγRII-, and CD13-mediated phagocytosis was significantly increased in Mϕ-IL-10 and decreased in Mϕ-IFN-γ, although both cytokines significantly upregulated FcγRI expression. IL-10 also increased phagocytosis of Escherichia coli, showing that the effect of IL-10 on macrophage phagocytosis is not specific for a particular receptor. Interestingly, Mϕ-IL-4, which showed poor FcγR- and CD13-mediated phagocytosis, showed very high phagocytosis of E. coli and zymosan. Coupled with phagocytosis, macrophages produce ROS that contribute to microbial killing. As expected, Mϕ-IFN-γ showed significant production of ROS after FcγRI-, FcγRII-, or CD13-mediated phagocytosis. Unexpectedly, we found that Mϕ-IL-10 can also produce ROS after simultaneous stimulation through several phagocytic receptors, as coaggregation of FcγRI/FcγRII/CD13 induced a belated but significant ROS production. Together, these results demonstrate that activation of macrophages by each cytokine distinctly modulates expression of phagocytic receptors, FcγR- and CD13-mediated phagocytosis, and ROS production.

M1 aminopeptidases as drug targets: broad applications or therapeutic niche?

M1 aminopeptidase enzymes are a diverse family of metalloenzymes characterized by conserved structure and reaction specificity. Excluding viruses, M1 aminopeptidases are distributed throughout all phyla, and have been implicated in a wide range of functions including cell maintenance, growth and development, and defense. The structure and catalytic mechanism of M1 aminopeptidases are well understood, and make them ideal candidates for the design of small‐molecule inhibitors. As a result, many research groups have assessed their utility as therapeutic targets for both infectious and chronic diseases of humans, and many inhibitors with a range of target specificities and potential therapeutic applications have been developed. Herein, we have aimed to address these studies, to determine whether the family of M1 aminopeptidases does in fact present a universal target for the treatment of a diverse range of human diseases. Our analysis indicates that early validation of M1 aminopeptidases as therapeutic targets is often overlooked, which prevents the enzymes from being confirmed as drug targets. This validation cannot be neglected, and needs to include a thorough characterization of enzymes’ specific roles within complex physiological pathways. Furthermore, any chemical probes used in target validation must be carefully designed to ensure that specificity over the closely related enzymes has been achieved. While many drug discovery programs that target M1 aminopeptidases remain in their infancy, certain inhibitors have shown promise for the treatment of a range of conditions including malaria, hypertension, and cancer.

Acute myeloid leukemia targets for bispecific antibodies

Despite substantial gains in our understanding of the genomics of acute myelogenous leukemia (AML), patient survival remains unsatisfactory especially among the older age group. T cell-based therapy of lymphoblastic leukemia is rapidly advancing; however, its application in AML is still lagging behind. Bispecific antibodies can redirect polyclonal effector cells to engage chosen targets on leukemia blasts. When the effector cells are natural-killer cells, both antibody-dependent and antibody-independent mechanisms could be exploited. When the effectors are T cells, direct tumor cytotoxicity can be engaged followed by a potential vaccination effect. In this review, we summarize the AML-associated tumor targets and the bispecific antibodies that have been studied. The potentials and limitations of each of these systems will be discussed.

Peptides for tumor-specific drug targeting: state of the art and beyond

This review outlines the most recent advances in peptide-mediated tumor-targeting and gives insight into the direction of the field.

Design, synthesis and application of a near-infrared fluorescent probe for in vivo imaging of aminopeptidase N

We have, for the first time, developed a near-infrared fluorescent probe for aminopeptidase N by combining a hemicyanine and an alanyl residue.

Discovery of a novel chimeric ubenimex-gemcitabine with potent oral antitumor activity

Herein, a novel mutual prodrug BC-A1 was discovered by integrating ubenimex and gemcitabine into one molecule. Biological characterization revealed that compound BC-A1 could maintain both the anti-CD13 activity of ubenimex and the cytotoxic activity of gemcitabine in vitro. Further characterization also demonstrated that compound BC-A1 exhibited significant anti-invasion and anti-angiogenesis effects in vitro. The preliminary stability test of BC-A1 revealed that it could release gemcitabine in vitro. The in vivo anti-tumor results in liver cancer showed that at the same dosage, oral administration of BC-A1 was as potent as intraperitoneal administration of gemcitabine. This warranted the further research and development of the orally active prodrug BC-A1 because gemcitabine can not be orally administrated in clinic.