The growth factor fusion construct containing B-lymphocyte stimulator (BLyS) and the toxin rGel induces apoptosis specifically in BAFF-R-positive CLL cells

Advances on Delivery of Cytotoxic Enzymes as Anticancer Agents

Cancer is one of the most serious human diseases, causing millions of deaths worldwide annually, and, therefore, it is one of the most investigated research disciplines. Developing efficient anticancer tools includes studying the effects of different natural enzymes of plant and microbial origin on tumor cells. The development of various smart delivery systems based on enzyme drugs has been conducted for more than two decades. Some of these delivery systems have been developed to the point that they have reached clinical stages, and a few have even found application in selected cancer treatments. Various biological, chemical, and physical approaches have been utilized to enhance their efficiencies by improving their delivery and targeting. In this paper, we review advanced delivery systems for enzyme drugs for use in cancer therapy. Their structure-based functions, mechanisms of action, fused forms with other peptides in terms of targeting and penetration, and other main results from in vivo and clinical studies of these advanced delivery systems are highlighted.

Toxic proteins application in cancer therapy

Ribosome inactivating proteins (RIPs) as family of anti-cancer drugs recently received much attention due to their interesting anti-cancer mechanism. In spite of small drugs, RIPs use the large-size effect (LSE) to prevent the efflux process governed by drug resistance transporters (DRTs) which prevents inside of the cells against drug transfection. There are many clinical translation obstacles that severely restrict their applications especially their delivery approach to the tumor cells. As the main goal of this review, we will focus on trichosanthin (TCS) and gelonin (Gel) and other types, especially scorpion venom-derived RIPs to clarify that they are struggling with what types of bio-barriers and these challenges could be solved in cancer therapy science. Then, we will try to highlight recent state-of-the-arts in delivery of RIPs for cancer therapy.

Advances on Tumor-Targeting Delivery of Cytotoxic Proteins

Great attention has been paid to cytotoxic proteins (e.g., ribosome-inactivating proteins, RIPs) possessing high anticancer activities; unlike small drugs, cytotoxic proteins can effectively retain inside the cells and avoid drug efflux mediated by multidrug resistance transporters due to the large-size effect. However, the clinical translation of these proteins is severely limited because of various biobarriers that hamper their effective delivery to tumor cells. Hence, in order to overcome these barriers, many smart drug delivery systems (DDS) have been developed. In this review, we will introduce two representative type I RIPs, trichosanthin (TCS) and gelonin (Gel), and overview the major biobarriers for protein-based cancer therapy. Finally, we outline advances on the development of smart DDS for effective delivery of these cytotoxic proteins for various applications in cancer treatment.

Aptamers in hematological malignancies and their potential therapeutic implications

Aptamers are short DNA/RNA oligonucleotides selected by the process called Systematic Evolution of Ligands by Exponential Enrichment (SELEX). Due to their functional similarity to monoclonal antibodies with some superior characters, such as high specificity and affinity, flexible modification and stability, and lack of toxicity and immunogenicity, they are promising alternative and complementary targeted therapy for hematologic malignancies. The trends in aptamer technology including production, selection, modifications are briefly discussed in this review. The key aspect is to illustrate aptamers against cancer cells in hematologic malignancies especially those that have entered clinical trials. We also discuss some challenges remain in the application of aptamers.

Methods for assembling B-cell lymphoma specific and internalizing aptamer-siRNA nanoparticles via the sticky bridge

Structured functional RNA entities, including aptamers and siRNAs, have amazing versatility in structure and function. These molecules can serve as powerful, attractive building blocks for the bottom-up assembly of complex nanostructures. Here, we describe novel cell-type specific and internalizing B-cell activating factor receptor (BAFF-R) aptamer-siRNA delivery systems for B-cell lymphoma therapy, in which both the aptamer and the Dicer substrate siRNA (DsiRNA) portions are conjugated through a "sticky bridge." The BAFF-R is overexpressed on the surface of B-cell malignancies, allowing binding and internalization of the aptamer-stick-siRNA nanoparticles. STAT3 siRNAs are encapsulated within the nanoparticles delivered by the BAFF-R aptamers and are localized to the cytoplasm, resulting in robust gene silencing of STAT3 mRNAs in a variety of B-cell lines. Moreover, these nanoparticles do not induce cell proliferation and apoptosis. Collectively, aptamer-mediated delivery strategies provide a toolset to become a more widely used therapeutic modality for the treatment of diseases.

Role of BAFF/BAFF-R axis in B-cell non-Hodgkin lymphoma

B-cell activating factor (BAFF), as a member of the tumor necrosis factor (TNF) ligand family, plays important roles in B-cell homeostasis, tolerance, and malignancy. BAFF binds to three receptors of TNF, TACI, BCMA and BAFF-receptor (BAFF-R). In particular, the BAFF/BAFF-R pathway is crucial to the survival and growth of mature normal and malignant B-cells. BAFF is displayed on the cell surface or is released in a soluble form after cleavage from the plasma membrane. BAFF-R as the main BAFF receptor is expressed mainly on B-cells. Aberrant BAFF expression was found in malignant B-cells from B-cell non-Hodgkin lymphoma (B-NHL) patients, which protects these cells from spontaneous or drug-induced apoptosis and stimulated NF-κB activation via autocrine and/or paracrine pathways. However, the mechanisms involved in the gene expression and regulation of BAFF or BAFF-R has not been elucidated. More importantly, the design of reagents able to counteract BAFF/BAFF-R pathways may be of therapeutic value for B-NHL. Results of ongoing clinical trials with BAFF or BAFF-R antagonists are eagerly awaited.

B cell activation through CD40 and IL4R ligation modulates the response of chronic lymphocytic leukaemia cells to BAFF and APRIL

The two tumour necrosis factor family proteins BAFF (TNFSF13B) and APRIL (TNFSF13) and their receptors [BAFF-R (TNFRSF13C), TACI (TNFRSF13B), BCMA (TNFRSF17)] play a critical role in the survival of normal B cells. The sensitivity of normal B cells to BAFF and APRIL can be modulated by signals regulated by their receptors. This modulation, however, has not been extensively investigated in chronic lymphocytic leukaemia (CLL) cells. We evaluated the expression, regulation and signalling of BAFF and APRIL receptors in normal and in CLL cells upon stimulation through CD40+IL4R and BCR. We further analysed the prognostic value of BAFF and APRIL receptors expression in patients with CLL. BCMA expression was significantly higher on CLL cells than on normal B cells. BCR and CD40+IL4R stimulation promoted an increase in TACI and BCMA expression, cell viability and activation in normal B cells. A similar effect was observed in CLL cells after CD40+IL4R but not BCR stimulation. BCMA expression correlated with unmutated IGHV genes, poor-risk cytogenetics, and short progression-free survival. These findings further characterize the link between CD40+IL4R regulatory signals, BAFF, APRIL and their receptors and the survival of leukaemic cells and clinical features of CLL.

Dual functional BAFF receptor aptamers inhibit ligand-induced proliferation and deliver siRNAs to NHL cells

The B-cell–activating factor (BAFF)-receptor (BAFF-R) is restrictedly expressed on B-cells and is often overexpressed in B-cell malignancies, such as non-Hodgkin’s lymphoma. On binding to its ligand BAFF, proliferation and cell survival are increased, enabling cancer cells to proliferate faster than normal B-cells. Nucleic acid aptamers can bind to target ligands with high specificity and affinity and may offer therapeutic advantages over antibody-based approaches. In this study, we isolated several 2′-F–modified RNA aptamers targeting the B-cell–specific BAFF-R with nanomolar affinity using in vitro SELEX technology. The aptamers efficiently bound to BAFF-R on the surface of B-cells, blocked BAFF-mediated B-cell proliferation and were internalized into B-cells. Furthermore, chimeric molecules between the BAFF-R aptamer and small interfering RNAs (siRNAs) were specifically delivered to BAFF-R expressing cells with a similar efficiency as the aptamer alone. We demonstrate that a signal transducer and activator of transcription 3 (STAT3) siRNA delivered by the BAFF-R aptamer was processed by Dicer and efficiently reduced levels of target mRNA and protein in Jeko-1 and Z138 human B-cell lines. Collectively, our results demonstrate that the dual-functional BAFF-R aptamer–siRNA conjugates are able to deliver siRNAs and block ligand mediated processes, suggesting it might be a promising combinatorial therapeutic agent for B-cell malignancies.

Diphtheria Toxin/Human B-Cell Activating Factor Fusion Protein Kills Human Acute Lymphoblastic Leukemia BALL-1 Cells: An Experimental Study

OBJECTIVE

This study aimed to express a fusion protein of diphtheria toxin and human B cell-activating factor (DT388sBAFF) in Escherichia coli (E. coli) and investigate its activity in human B-lineage acute lymphoblastic leukemia 1 cells (BALL-1).

METHODS

A fragment of DT388sBAFF fusion gene was separated from plasmid pUC57-DT388sBAFF digested with Nde I and Xho I, and inserted into the expression vector pcold II digested with the same enzymes. Recombinants were screened by the colony polymerase chain reaction (PCR) and restriction map. The recombinant expression vector was transformed into BL21 and its expression was induced by isopropyl β-D-1-thiogalactopyranoside (IPTG). The recombinant protein was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot, and then purified by Ni(2+)-NTA affinity chromatography. The expression level of B cell-activating factor receptor (BAFF-R) on BALL-1 cells was assessed by real-time PCR. The receptor binding capacity of recombinant protein was determined by cell fluorescent assay. The specific cytotoxicity of recombinant protein on BALL-1 cells was detected by 3-(4,5-dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) assay.

RESULTS

The expression level of recombinant protein was 50% of total bacterial proteins in E. coli, and the recombinant protein could bind to BAFF-R-positive BALL-1 cells and thereby produce a cytotoxic effect on the cells.

CONCLUSION

The fusion protein expression vector DT388sBAFF was successfully constructed and the recombinant protein with selective cytotoxicity against BALL-1 cells was obtained, providing foundation for further study of the therapy of human B-lineage acute lymphoblastic leukemia.

Fusion toxin BLyS-gelonin inhibits growth of malignant human B cell lines in vitro and in vivo

B lymphocyte stimulator (BLyS) is a member of the TNF superfamily of cytokines. The biological activity of BLyS is mediated by three cell surface receptors: BR3/BAFF-R, TACI and BCMA. The expression of these receptors is highly restricted to B cells, both normal and malignant. A BLyS-gelonin fusion toxin (BLyS-gel) was generated consisting of the recombinant plant-derived toxin gelonin fused to the N-terminus of BLyS and tested against a large and diverse panel of B-NHL cell lines. Interestingly, B-NHL subtypes mantle cell lymphoma (MCL), diffuse large B cell lymphoma (DLBCL) and B cell precursor-acute lymphocytic leukemia (BCP-ALL) were preferentially sensitive to BLyS-gel mediated cytotoxicity, with low picomolar EC₅₀ values. BLyS receptor expression did not guarantee sensitivity to BLyS-gel, even though the construct was internalized by both sensitive and resistant cells. Resistance to BLyS-gel could be overcome by treatment with the endosomotropic drug chloroquine, suggesting BLyS-gel may become trapped within endosomal/lysosomal compartments in resistant cells. BLyS-gel induced cell death was caspase-independent and shown to be at least partially mediated by the “ribotoxic stress response.” This response involves activation of p38 MAPK and JNK/SAPK, and BLyS-gel mediated cytotoxicity was inhibited by the p38/JNK inhibitor SB203580. Finally, BLyS-gel treatment was shown to localize to sites of disease, rapidly reduce tumor burden, and significantly prolong survival in xenograft mouse models of disseminated BCP-ALL, DLBCL, and MCL. Together, these findings suggest BLyS has significant potential as a targeting ligand for the delivery of cytotoxic “payloads” to malignant B cells.

Cell-targeting fusion constructs containing recombinant gelonin

Therapeutic agents capable of targeting tumor cells present as established tumors and micrometastases have already demonstrated their potential in clinical trials. Immunotoxins targeting hematological malignancies and solid tumors have additionally demonstrated excellent clinical activity. This review focuses on our design and characterization studies of constructs composed of recombinant gelonin toxin fused to either growth factors or single-chain antibodies targeting solid tumor cells, tumor vasculature or hematological malignancies. These agents demonstrate cytotoxicity at nanomolar or sub-nanomolar levels. All of these constructs display impressive selectivity and specificity for antigen-bearing target cells in vitro and in vivo and are excellent clinical trial candidates.

In vitro and in vivo anti-tumor activities of anti-EGFR single-chain variable fragment fused with recombinant gelonin toxin

PURPOSE

Epidermal growth factor receptor (EGFR) plays an important role in the growth and metastasis of many solid tumors. Strategies that target EGFR hold promising therapeutic potential for the treatment for non-small cell lung cancer (NSCLC), as EGFR is normally overexpressed in these tumors. This study was designed to determine whether an anti-EGFR immunotoxin has anti-tumor activity against NSCLC, and if so, to further investigate the possible mechanisms of cytotoxicity.

METHODS

A fusion protein of anti-EGFR single-chain variable fragment (anti-EGFR scFv) and the plant toxin gelonin (rGel) was constructed, expressed in bacteria, and purified to homogeneity. Cytotoxicity of anti-EGFR scFv/rGel (E/rG) immunotoxin was assessed on A549, HCC827, and H1975 cells (EGFR-overexpressing NSCLC-derived cell lines) and A549 xenografts in nude mice.

RESULTS

Cytotoxicity experiments using E/rG on A549, HCC827, and H1975 cells demonstrated that E/rG can specifically inhibit proliferation of these cells, whereas it did not affect the proliferation of Raji cells that do not express EGFR. Treatment for A549 xenografts in nude mice with E/rG resulted in significant suppression of tumor growth compared to controls. Immunofluorescence in frozen tissue sections confirmed that E/rG could specifically bind to tumor tissues in nude mice bearing A549 tumor xenografts, while rGel alone showed no binding activity. Furthermore, E/rG inhibited the growth of A549 cells by cytotoxic effects that blocked tumor proliferation, and the immunotoxin-induced cell death may be mediated by autophagy.

CONCLUSIONS

These results showed that E/rG might have significant potential as a novel clinical therapeutic agent against human NSCLC.

Treatment of acute lymphoblastic leukemia with an rGel/BLyS fusion toxin

Acute lymphoblastic leukemia (ALL) is the most common malignancy affecting children and a major cause of mortality from hematopoietic malignancies in adults. A substantial number of patients become drug resistant during chemotherapy, necessitating the development of alternative modes of treatment. rGel (recombinant Gelonin)/BlyS (B-lymphocyte stimulator) is a toxin-cytokine fusion protein used for selective killing of malignant B-cells expressing receptors for B-cell-activating factor (BAFF/BLyS) by receptor-targeted delivery of the toxin, Gelonin. Here, we demonstrate that rGel/BLyS binds to ALL cells expressing BAFF receptor (BAFF-R) and upon internalization, it induces apoptosis of these cells and causes downregulation of survival genes even in the presence of stromal protection. Using an immunodeficient transplant model for human ALL, we show that rGel/BLyS prolongs survival of both Philadelphia chromosome-positive and negative ALL-bearing mice. Furthermore, we used AMD3100, a CXCR4 antagonist, to mobilize the leukemic cells protected in the bone marrow (BM) microenvironment and the combination with rGel/BLyS resulted in a significant reduction of the tumor load in the BM and complete eradication of ALL cells from the circulation. Thus, a combination treatment with the B-cell-specific fusion toxin rGel/BLyS and the mobilizing agent AMD3100 could be an effective alternative approach to chemotherapy for the treatment of primary and relapsed ALL.

Biodistribution, pharmacokinetics, and nuclear imaging studies of 111In-labeled rGel/BLyS fusion toxin in SCID mice bearing B cell lymphoma

PURPOSE

We examined the biodistribution and pharmacokinetics of (111)In-labeled rGel/BLyS, a gelonin toxin (rGel)-B lymphocyte stimulator (BLyS) fusion protein.

MATERIALS AND METHODS

rGel/BLyS was labeled with In-111 through DTPA with a labeling efficiency >95%. Biodistribution/imaging studies were obtained in severe-combined immunodeficiency mice bearing diffuse large B cell lymphoma OCI-Ly10. Pharmacokinetic studies were performed in BALB/c mice.

RESULTS

In vitro, DTPA-conjugated rGel/BLyS displayed selective cytotoxicity against OCI-Ly10 cells and mantle cell lymphoma JeKo cells. In vivo, rGel/BLyS exhibited a tri-exponential disposition with a rapid initial mean distribution followed by an extensive mean distribution and a long terminal elimination phase. At 48 h after injection, uptake of the radiotracer in tumors was 1.25 %ID/g, with a tumor-to-blood ratio of 13. Tumors were clearly visualized at 24-72 h post-injection. Micro-SPECT-CT images and ex vivo analyses confirmed the accumulation of rGel/BLyS in OCI-Ly10 tumors.

CONCLUSIONS

(111)In-DTPA-rGel/BLyS are distributed to B cell tumors and induce apoptosis in tumors. Preclinical antitumor studies using rGel/BLyS should use a twice-per-week treatment schedule.

p52 Activation in monomorphic B-cell posttransplant lymphoproliferative disorder/diffuse large B-cell lymphoma without BAFF-R expression

Posttransplantation lymphoproliferative disorders (PTLD) are associated with Epstein-Barr virus (EBV) and activate the NF-κB pathway. B-cell activating factor (BAFF) modulates cell growth and survival in non-Hodgkin's lymphomas. However, there are few studies of EBV, BAFF/BAFF-R signaling, and NF-κB1 and NF-κB2 pathway activation in PTLD. Diffuse large B-cell lymphomas (DLBCL) in two different clinical contexts, immunocompetent patients (DLBCL/IC; n = 30) or posttransplantation solid-organ recipients (DLBCL/PTLD; n = 21), were characterized histogenically as germinal center (GC) or non-germinal center (NGC). Expression of BAFF, BAFF-R, and NF-κB proteins p50 and p52 and the presence or absence of EBV were compared in these clinical contexts. Regardless of the GC or NGC pattern of DLBCL, BAFF-R was expressed in 37% of DLBCL/IC but in only 4.8% of DLBCL/PTLD. p52 was expressed in DLBCL/PTLD/NGC (12 of 19 cases) as compared with DLBCL/IC/NGC (0 of 18 cases). This pattern might be related to the presence of EBV and latent membrane protein 1 because p52 expression was observed primarily in EBV-positive DLBCL/PTLD cases expressing latent membrane protein 1. Thus, the activation profile or NGC pattern of DLBCL/PTLD was not associated with BAFF/BAFF-R expression, whereas nuclear p52 related to NF-κB2 pathway activation might be linked to EBV.

Selective killing of Burkitt's lymphoma cells by mBAFF-targeted delivery of PinX1

Increased expression of BAFF (B cell-activating factor belonging to the TNF family) and its receptors has been identified in numerous B-cell malignancies. A soluble human BAFF mutant (mBAFF), binding to BAFF receptors but failing to activate B-lymphocyte proliferation, may function as a competitive inhibitor of BAFF and may serve as a novel ligand for targeted therapy of BAFF receptor-positive malignancies. Pin2/TRF1-interacting protein X1 (PinX1), a nucleolar protein, potently inhibits telomerase activity and affects tumorigenicity. In this study, we generated novel recombinant proteins containing mBAFF, a polyarginine tract 9R and PinX1 (or its C/N terminal), to target lymphoma cells. The fusion proteins PinX1/C–G₄S–9R–G₄S–mBAFF and PinX1/C–9R–mBAFF specifically bind and internalize into BAFF receptor-positive cells, and subsequently induce growth inhibition and apoptosis. The selective cytotoxicity of the fusion proteins is a BAFF receptor-mediated process and depends on mBAFF, PinX1/C and 9R. Moreover, the fusion proteins specifically kill BAFF receptor-expressing Burkitt's lymphoma (BL) cells by inhibiting telomerase activity and the consequent shortening of telomeres. Therapeutic experiments using PinX1C–G₄S–9R–G₄S–mBAFF in severe combined immunodeficient (SCID) mice implanted with Raji cells showed significantly prolonged survival times, indicating the in vivo antitumor activity of the fusion protein. These results suggest the potential of PinX1/C–G₄S–9R–G₄S–mBAFF in targeted therapy of BL.

Ribosome-inactivating proteins: from plant defense to tumor attack

Ribosome-inactivating proteins (RIPs) are EC3.2.32.22 N-glycosidases that recognize a universally conserved stem-loop structure in 23S/25S/28S rRNA, depurinating a single adenine (A4324 in rat) and irreversibly blocking protein translation, leading finally to cell death of intoxicated mammalian cells. Ricin, the plant RIP prototype that comprises a catalytic A subunit linked to a galactose-binding lectin B subunit to allow cell surface binding and toxin entry in most mammalian cells, shows a potency in the picomolar range. The most promising way to exploit plant RIPs as weapons against cancer cells is either by designing molecules in which the toxic domains are linked to selective tumor targeting domains or directly delivered as suicide genes for cancer gene therapy. Here, we will provide a comprehensive picture of plant RIPs and discuss successful designs and features of chimeric molecules having therapeutic potential.

The rGel/BLyS fusion toxin inhibits diffuse large B-cell lymphoma growth in vitro and in vivo

Diffuse large B-cell lymphoma (DLBCL) is an aggressive subtype of B-cell non-Hodgkin lymphoma (NHL) and accounts for 30%to 40%of NHL. Molecules targeting nuclear factor-kappaB (NF-kappaB) are expected to be of therapeutic value in those tumors where NF-kappaB seems to play a unique survival role such as activated B-cell (ABC)-subtype DLBCL. We previously generated a rGel/BLyS fusion toxin for receptor-mediated delivery of the rGel toxin specifically to malignant B cells. In this study, we examined this fusion toxin for its ability to suppress DLBCL growth in vitro and in vivo. rGel/BLyS was specifically cytotoxic to DLBCL lines expressing all three BLyS receptors and constitutively active NF-kappaB. Treatment with rGel/BLyS induced down-regulation of the phosphorylation of inhibitory subunit of NF-kappaB (IkappaB-alpha), inhibition of NF-kappaB DNA-binding activity, and accumulation of IkappaB-alpha. In agreement with these results, we additionally found that rGel/BLyS downregulated levels of several NF-kappaB targets including Bcl-xL, Mcl-1, survivin, and x-chromosome linked inhibitor-of-apoptosis. Treatment also induced up-regulation of Bax and apoptosis through caspase-3 activation and poly ADP-ribose polymerase cleavage. Importantly, rGel/BLyS significantly inhibited tumor growth (P < .05) in a DLBCL xenograft model. Thus, our results indicate that rGel/BLyS is an excellent candidate for the treatment of aggressive NHLs that are both dependent on NF-kappaB and are resistant to conventional chemotherapeutic regimens.

The rGel/BLyS fusion toxin inhibits STAT3 signaling via down-regulation of interleukin-6 receptor in diffuse large B-cell lymphoma

Aberrant signal transducer and activator of transcription (STAT)3 signaling participates in the development and progress of human cancers. We previously generated a highly cytotoxic fusion toxin designated rGel/BLyS for receptor-mediated delivery of the rGel toxin to malignant B-cells. In this study, we examined this fusion toxin for its ability to impact STAT3 signaling in diffuse large B-cell lymphoma (DLBCL). The activated B cell-like DLBCL lines were found to express higher levels of interleukin-6 receptor (IL-6R) and STAT3 than did the germinal center B cell-like DLBCL lines. Treatment of DLBCL cells with rGel/BLyS resulted in down-regulation of IL-6R and inhibited STAT3 phosphorylation, STAT3-DNA binding activity, and IL-6-inducible STAT3 reporter gene activity. In agreement with these results, we additionally found that rGel/BLyS down-regulated levels of several STAT3 targets (c-Myc, p21, Mcl-1, and Bcl-x(L)) and p-SYK, a positive regulator of STAT3. Inhibition of IL-6R-mediated STAT3 signaling by rGel/BLyS led to growth inhibition, triggered accumulation of cells in the sub-G(1) phase of the cell cycle, and induced apoptosis. Our results indicate that rGel/BLyS is an excellent candidate for the treatment of aggressive DLBCL which is resistant to conventional chemotherapeutic regimens and STAT3 signaling pathway may be an attractive therapeutic target for non-Hodgkin's lymphoma.

Antitumor activity of fibroblast growth factor receptor 3-specific immunotoxins in a xenograft mouse model of bladder carcinoma is mediated by apoptosis

Human single-chain Fv directed against fibroblast growth factor receptor 3 (FGFR3) have been shown to block proliferation of RT112 bladder carcinoma cells in vitro. Here, we examined the ability of the recombinant gelonin toxin (rGel) to enhance this inhibitory effect in vitro and in vivo on the bladder cancer cell line RT112 and the corresponding xenografts. Immunotoxins were genetically engineered by fusing FGFR3-specific Fv fragments (3C) to the NH(2) terminus of rGel and expressed as a soluble protein in Escherichia coli. The 3C/rGel fusion construct showed an IC(50) of 200 nmol/L against log-phase RT112 cells compared with 1,500 nmol/L for free rGel. Immunofluorescence studies showed that the 3C/rGel construct internalized rapidly into the cytoplasm of RT112 cells within 1 h of exposure. The mechanism of immunotoxin-induced cell death was found to be mediated by apoptosis. RT112 tumor xenografts in severe combined immunodeficient mice treated with 50 mg/kg 3C/rGel exhibited considerable growth delay relative to control tumors and a significant reduction of 55% to 70% in mean tumor size. Immunohistochemical analysis showed that tumors from mice treated with 3C/rGel displayed considerable apoptotic damage compared with control groups. Subcellular location of FGFR3 in immunotoxin-treated tumors indicated a translocation of FGFR3 to the nuclear membrane in contrast to tumors from saline-treated controls. These results show that FGFR3-driven immunotoxins may be an effective therapeutic agent against human bladder and other tumor types overexpressing FGFR3.

Antibody targeting of B-cell maturation antigen on malignant plasma cells

B-cell maturation antigen (BCMA) is expressed on normal and malignant plasma cells and represents a potential target for therapeutic intervention. BCMA binds to two ligands that promote tumor cell survival, a proliferation inducing ligand (APRIL) and B-cell activating factor. To selectively target BCMA for plasma cell malignancies, we developed antibodies with ligand blocking activity that could promote cytotoxicity of multiple myeloma (MM) cell lines as naked antibodies or as antibody-drug conjugates. We show that SG1, an inhibitory BCMA antibody, blocks APRIL-dependent activation of nuclear factor-kappaB in a dose-dependent manner in vitro. Cytotoxicity of SG1 was assessed as a naked antibody after chimerization with and without Fc mutations that enhance FcgammaRIIIA binding. The Fc mutations increased the antibody-dependent cell-mediated cytotoxicity potency of BCMA antibodies against MM lines by approximately 100-fold with a > or = 2-fold increase in maximal lysis. As an alternative therapeutic strategy, anti-BCMA antibodies were endowed with direct cytotoxic activity by conjugation to the cytotoxic drug, monomethyl auristatin F. The most potent BCMA antibody-drug conjugate displayed IC(50) values of < or = 130 pmol/L for three different MM lines. Hence, BCMA antibodies show cytotoxic activity both as naked IgG and as drug conjugates and warrant further evaluation as therapeutic candidates for plasma cell malignancies.