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Peritoneal carcinomatosis (PC) is considered incurable, and more effective therapies are needed. Herein we test the hypothesis that GPA33-directed intracompartmental pretargeted radioimmunotherapy (PRIT) can cure colorectal peritoneal carcinomatosis. Nude mice were implanted intraperitoneally with luciferase-transduced GPA33-expressing SW1222 cells for aggressive peritoneal carcinomatosis (e.g., resected tumor mass 0.369 ± 0.246 g; n = 17 on day 29). For GPA33-PRIT, we administered intraperitoneally a high-affinity anti-GPA33/anti-DOTA bispecific antibody (BsAb), followed by clearing agent (intravenous), and lutetium-177 (Lu-177) or yttrium-86 (Y-86) radiolabeled DOTA-radiohapten (intraperitoneal) for beta/gamma-emitter therapy and PET imaging, respectively. The DOTA-radiohaptens were prepared from S-2-(4-aminobenzyl)-1,4,7, 10-tetraazacyclododecane tetraacetic acid chelate (DOTA-Bn). Efficacy and toxicity of single- versus three-cycle therapy were evaluated in mice 26–27 days post-tumor implantation. Single-cycle treatment ([177Lu]LuDOTA-Bn 111 MBq; tumor dose: 4,992 cGy) significantly prolonged median survival (MS) approximately 2-fold to 84.5 days in comparison with controls (P = 0.007). With three-cycle therapy (once weekly, total 333 MBq; tumor dose: 14,975 cGy), 6/8 (75%) survived long-term (MS > 183 days). Furthermore, for these treated long-term survivors, 1 mouse was completely disease free (microscopic “cure”) at necropsy; the others showed stabilized disease, which was detectable during PET-CT using [86Y]DOTA-Bn. Treatment controls had MS ranging from 42–52.5 days (P < 0.001) and 19/20 mice succumbed to progressive intraperitoneal disease by 69 days. Multi-cycle GPA33 DOTA-PRIT significantly prolongs survival with reversible myelosuppression and no chronic marrow (929 cGy to blood) or kidney (982 cGy) radiotoxicity, with therapeutic indices of 12 for blood and 12 for kidneys. MTD was not reached.

Immuno-positron emission tomography (immuno-PET) is a powerful tool to noninvasively characterize the in vivo biodistribution of engineered antibodies (Abs). Methods L1 cell adhesion molecule (L1CAM)-targeting HuE71 immunoglobulin G1 (IgG1) and immunoglobulin G4 (IgG4) Abs bearing identical variable heavy and light chain sequences but different fragment crystallizable (Fc) portions were radiolabeled with Zirconium-89 (89Zr) and the in vivo biodistribution was studied in SKOV3 ovarian cancer xenografted nude mice. Results In addition to showing uptake in L1CAM-expressing SKOV3 tumors like its parental counterpart HuE71 IgG1, the afucosylated variant having enhanced Fc-receptor (FcR) affinity showed high nonspecific uptake in lymph nodes. On the other hand, aglycosylated HuE71 IgG1 with abrogated FcR binding did not show lymphoid uptake. The use of IgG4 subclass showed high nonspecific uptake in the kidneys, which was prevented by mutating serine at position 228 to proline (S228P) in the hinge region of the IgG4 Ab to mitigate in vivo fragment antigen-binding (Fab) arm exchange. Conclusion Our findings highlight the influence of Fc-modifications and the choice of IgG subclass on the in vivo biodistribution of Abs and the potential outcomes thereof.

BackgroundThe prognosis for metastatic Ewing sarcoma family of tumors (EFT) is still poor despite high-dose chemotherapy and radiation treatment. Immunotherapies hold promise, but cancer antigen-targeting immunotherapies have largely failed to induce effective T cell receptor-mediated antitumor response. However, T cell-engaging bispecific antibodies (T-BsAbs) have yet to be adequately explored.MethodsRehumanized STEAP1-IgG was used to build T-BsAb (named BC261) using the 2+2 IgG-[L]-scFv platform carrying the anti-CD3 huOKT3 scFv as the second specificity. Its binding epitope mapping, species cross-reactivity, tumor cell line staining, and in vitro cytotoxicity were investigated thoroughly. Its potency in driving tumor-infiltrating lymphocytes (TILs) was quantified using bioluminescence, correlated with in vivo antitumor response against cell line-derived or patient-derived xenografts (CDXs or PDXs) and compared with anti-STEAP1 T-BsAbs built on representative antibody platforms.ResultsBC261 binding epitope was mapped to its second extracellular domain of STEAP1 shared among canine and primate orthologs. BC261 induced potent cytotoxicity against panels of EFT, prostate cancer, and canine osteosarcoma cell lines despite their low antigen density. BC261 drove significantly more TILs into tumors (30-fold) and exerted superior antitumor effects compared with the other standard BsAb platforms. The antitumor efficacy of BC261 was consistent against EFT and prostate cancer CDXs and PDXs.ConclusionsBC261 was highly efficient in driving T cell infiltration and tumor ablation. Either as stand-alone therapeutics or for ex vivo armed T cells, this novel anti-STEAP1 T-BsAb BC261 has therapeutic potential.

Clearing agents (CAs) can rapidly remove non-localized targeting biomolecules from circulation for hepatic catabolism, thereby enhancing the therapeutic index (TI), especially for blood (marrow), of the subsequently administered radioisotope in any multi-step pretargeting strategy. Herein we describe the synthesis and in vivo evaluation of a fully synthetic glycodendrimer-based CA for DOTA-based pretargeted radioimmunotherapy (DOTA-PRIT). The novel dendron-CA consists of a non-radioactive yttrium-DOTA-Bn molecule attached via a linker to a glycodendron displaying sixteen terminal α-thio-N-acetylgalactosamine (α-SGalNAc) units (CCA α-16-DOTA-Y(3+); molecular weight: 9059 Da). Pretargeting [(177)Lu]LuDOTA-Bn with CCA α-16-DOTA-Y(3+) to GPA33-expressing SW1222 human colorectal xenografts was highly effective, leading to absorbed doses of [(177)Lu]LuDOTA-Bn for blood, tumor, liver, spleen, and kidneys of 11.7, 468, 9.97, 5.49, and 13.3 cGy/MBq, respectively. Tumor-to-normal tissues absorbed-dose ratios (i.e., TIs) ranged from 40 (e.g., for blood and kidney) to about 550 for stomach.

Background Humanized 3F8-bispecific antibody (hu3F8-BsAb) using the IgG(L)-scFv format (where scFv is single-chain variable fragment), where the anti-CD3 huOKT3 scFv is fused with the carboxyl end of the hu3F8 light chain, has potent antitumor cytotoxicity against GD2(+) tumors. To overcome the insufficient number and function of T cells in cancer patients, they can be rejuvenated and expanded ex vivo before arming with hu3F8-BsAb for adoptive transfer, potentially reducing toxic side effects from direct BsAb administration. Procedure T cells from normal volunteers were expanded and activated ex vivo using CD3/CD28 beads for 8 days. Activated T cells (ATCs) were harvested and co-incubated with a Good Manufacturing Practice grade hu3F8-BsAb at room temperature for 20 min. These armed ATCs were tested for cytotoxicity in vitro and in vivo against human GD2(+) cell lines and patient-derived xenografts in BALB-Rag2-/- IL-2R-γc-KO mice. Results Hu3F8-BsAb armed ATCs showed robust antigen-specific tumor cytotoxicity against GD2(+) tumors in vitro. In vivo, T cells armed with hu3F8-BsAb were highly cytotoxic against GD2(+) melanoma and neuroblastoma xenografts in mice, accompanied by T-cell infiltration without significant side effects. Only zeptomole (10-21 ) quantities of BsAb per T cell was required for maximal antitumor effects. Tumor response was a function of T-cell dose. Conclusion BsAb armed T cells may have clinical utility as the next generation of cytotherapy combined with recombinant BsAb against human tumors for both adult and pediatrics, if autologous T cells can be activated and expanded ex vivo.

Purpose: Many cancer treatments suffer from dose-limiting toxicities to vital organs due to poor therapeutic indices. To overcome these challenges we developed a novel multimerization platform that rapidly removes tumor-targeting proteins from the blood to substantially improve therapeutic index. Experimental Design: The platform was designed as a fusion of a self-assembling and disassembling (SADA) domain to a tandem single-chain bispecific antibody (BsAb, anti-ganglioside GD2 × anti-DOTA). SADA–BsAbs were assessed with multiple in vivo tumor models using two-step pretargeted radioimmunotherapy (PRIT) to evaluate tumor uptake, dosimetry, and antitumor responses. Results: SADA–BsAbs self-assembled into stable tetramers (220 kDa), but could also disassemble into dimers or monomers (55 kDa) that rapidly cleared via renal filtration and substantially reduced immunogenicity in mice. When used with rapidly clearing DOTA-caged PET isotopes, SADA–BsAbs demonstrated accurate tumor localization, dosimetry, and improved imaging contrast by PET/CT. When combined with therapeutic isotopes, two-step SADA-PRIT safely delivered massive doses of alpha-emitting (225Ac, 1.48 MBq/kg) or beta-emitting (177Lu, 6,660 MBq/kg) S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (DOTA) payloads to tumors, ablating them without any short-term or long-term toxicities to the bone marrow, kidneys, or liver. Conclusions: The SADA–BsAb platform safely delivered large doses of radioisotopes to tumors and demonstrated no toxicities to the bone marrow, kidneys, or liver. Because of its modularity, SADA–BsAbs can be easily adapted to most tumor antigens, tumor types, or drug delivery approaches to improve therapeutic index and maximize the delivered dose. See related commentary by Capala and Kunos, p. 377

In recent reports, we have shown that optimized pretargeted radioimmunotherapy (PRIT) based on molecularly engineered antibody conjugates and 177Lu-DOTA chelate (DOTA-PRIT) can be used to cure mice bearing human solid tumor xenografts using antitumor antibodies to minimally internalizing membrane antigens, GPA33 (colon) and GD2 (neuroblastoma). However, many solid tumor membrane antigens are internalized after antibody binding and it is generally believed that internalizing tumor membrane antigens are not suitable targets for PRIT. In this study, we tested the hypothesis that DOTA-PRIT can be performed successfully to target HER2, an internalizing membrane antigen widely expressed in breast, ovarian, and gastroesophageal junction cancers. Methods: DOTA-PRIT was carried out in athymic nude mice bearing BT-474 xenografts, a HER2-expressing human breast cancer, using a three-step dosing regimen consisting of sequential intravenous administrations of: 1) a bispecific IgG-scFv (210 kD) format (BsAb) carrying the IgG sequence of the anti-HER2 antibody trastuzumab and the scFv “C825” with high-affinity, hapten-binding antibody for Bn-DOTA (metal) (BsAb: anti-HER2-C825), 2) a 500 kD dextran-based clearing agent, followed by 3) 177Lu-DOTA-Bn. At the time of treatment, athymic nude mice bearing established subcutaneous BT-474 tumors (medium- and smaller-sized tumors with tumor volumes of 209 ± 101 mm3 and ranging from palpable to 30 mm3, respectively), were studied along with controls. We studied single- and multi-dose regimens. For groups receiving fractionated treatment, we verified quantitative tumor targeting during each treatment cycle using non-invasive imaging with single-photon emission computed tomography/computed tomography (SPECT/CT). Results: We achieved high therapeutic indices (TI, the ratio of radiation-absorbed dose in tumor to radiation-absorbed dose to critical organs, such as bone marrow) for targeting in blood (TI = 28) and kidney (TI = 7), while delivering average radiation-absorbed doses of 39.9 cGy/MBq to tumor. Based on dosimetry estimates, we implemented a curative fractionated therapeutic regimen for medium-sized tumors that would deliver approximately 70 Gy to tumors, which required treatment with a total of 167 MBq 177Lu-DOTA-Bn/mouse (estimated absorbed tumor dose: 66 Gy). This regimen was well tolerated and achieved 100% complete responses (CRs; defined herein as tumor volume equal to or smaller than 4.2 mm3), including 62.5% histologic cure (5/8) and 37.5% microscopic residual disease (3/8) at 85 days (d). Treatment controls showed tumor progression to 207 ± 201% of pre-treatment volume at 85 d and no CRs. Finally, we show that treatment with this curative 177Lu regimen leads to a very low incidence of histopathologic abnormalities in critical organs such as bone marrow and kidney among survivors compared with non-treated controls. Conclusion: Contrary to popular belief, we demonstrate that DOTA-PRIT can be successfully adapted to an internalizing antigen-antibody system such as HER2, with sufficient TIs and absorbed tumor doses to achieve a high probability of cures of established human breast cancer xenografts while sparing critical organs of significant radiotoxicity.

Radioimmunotherapy of solid tumors using antibody-targeted radionuclides has been limited by low therapeutic indices (TIs). We recently reported a novel 3-step pretargeted radioimmunotherapy (PRIT) strategy based on a glycoprotein A33 (GPA33)–targeting bispecific antibody and a small-molecule radioactive hapten, a complex of 177Lu and S-2-(4-aminobenzyl)-1,4,7,10-tetraazacyclododecane tetraacetic acid (177Lu-DOTA-Bn), that leads to high TIs for radiosensitive tissues such as blood (TI = 73) and kidney (TI = 12). We tested our hypothesis that a fractionated anti-GPA33 DOTA-PRIT regimen calibrated to deliver a radiation absorbed dose to tumor of more than 100 Gy would lead to a high probability of tumor cure while being well tolerated by nude mice bearing subcutaneous GPA33-positive SW1222 xenografts. Methods: We treated groups of nude mice bearing 7-d-old SW1222 xenografts with a fractionated 3-cycle anti-GPA33 DOTA-PRIT regimen (total administered 177Lu-DOTA-Bn activity, 167 MBq/mouse; estimated radiation absorbed dose to tumor, 110 Gy). In randomly selected mice undergoing treatment, serial SPECT/CT imaging was used to monitor treatment response and calculate radiation absorbed doses to tumor. Necropsy was done on surviving animals 100–200 d after treatment to determine frequency of cure and assess select normal tissues for treatment-related histopathologies. Results: Rapid exponential tumor progression was observed in control treatment groups (i.e., no treatment or 177Lu-DOTA-Bn only), leading to euthanasia due to excessive tumor burden, whereas 10 of 10 complete responses were observed for the DOTA-PRIT–treated animals within 30 d. Treatment was well tolerated, and 100% histologic cure was achieved in 9 of 9 assessable animals without detectable radiation damage to critical organs, including bone marrow and kidney. Radiation absorbed doses to tumor derived from SPECT/CT (102 Gy) and from biodistribution (110 Gy) agreed to within 6.9%. Of the total dose of approximately 100 Gy, the first dose contributes 30%, the second dose 60%, and the third dose 10%. Conclusion: In a GPA33-positive human colorectal cancer xenograft mouse model, we validated a SPECT/CT-based theranostic PRIT regimen that led to 100% complete responses and 100% cures without any treatment-related toxicities, based on high TIs for radiosensitive tissues. These studies support the view that anti-GPA33 DOTA-PRIT will be a potent radioimmunotherapy regimen for GPA33-positive colorectal cancer tumors in humans.

BackgroundAcute myeloid leukemia (AML) remains one of the most challenging hematological malignancies. Despite progress in therapeutics, majority of patients succumb to this neoplasm. CD33 is a proven therapeutic target, given its expression on most AML cells. Almost all anti-CD33 antibodies target the membrane distal immunoglobulin V (IgV) domain of the CD33 extracellular domain.MethodsIn this manuscript, we present data on three bispecific antibodies (BsAbs) against the CD33 IgV and membrane proximal immunoglobulin C (IgC) domains. We use in vitro binding and cytotoxicity assays to show the effect of these BsAbs on AML cell lines. We also use immunodeficient mice-bearing leukemias from cell lines and patient-derived xenografts to show the effect of these BsAbs in vivo.ResultsIn vitro, the IgV-targeting BsAb had higher binding to AML cell lines using flow cytometry and delivered more potent cytotoxicity in T-cell-dependent cytotoxicity assays; importantly, the IgC domain-targeting outperformed the IgV domain-targeting BsAb in medullary and extramedullary leukemia animal models.ConclusionsThese data support further clinical development of this BsAb for first-in-human phase I clinical trial.

The affinity of therapeutic antibodies for Fcγ receptors (FcγRs) strongly influences their antitumor potency. To generate antibodies with optimal binding and immunologic efficacy, we compared the affinities of different versions of an IgG1 Fc region that had an altered peptide backbone, altered glycans, or both. To produce IgG1 with glycans that lacked α1,6-fucose, we used CHO cells that were deficient in the enzyme UDP-N-acetylglucosamine: α-3-d-mannoside-β-1,2-N-acetylglucosaminyltransferase I (GnT1), encoded by the MGAT1 gene. Mature N-linked glycans require this enzyme, and without it, CHO cells synthesize antibodies carrying only Man5-GlcNAc2, which were more effective in antibody-dependent cell-mediated cytotoxicity (ADCC). Our engineered IgG1, hu3F8-IgG1, is specific for GD2, a neuroendocrine tumor ganglioside. Its peptide mutant is IgG1-DEL (S239D/I332E/A330L), both produced in wild-type CHO cells. When produced in GnT1-deficient CHO cells, we refer to them as IgG1n and IgG1n-DEL, respectively. Affinities for human FcγRs were measured using Biacore T-100 (on CD16 and CD32 polymorphic alleles), their immunologic properties compared for ADCC and complement-mediated cytotoxicity (CMC) in vitro, and pharmacokinetics and antitumor effects were compared in vivo in humanized mice. IgG1n and IgG1n-DEL contained only mannose and acetylglucosamine and had preferential affinity for activating CD16s, over inhibitory CD32B, receptors. In vivo, the antitumor effects of IgG1, IgG1-DEL, and IgG1n-DEL were similar but modest, whereas IgG1n was significantly more effective (P < 0.05). Thus, IgG1n antibodies produced in GnT1-deficient CHO cells may have potential as improved anticancer therapeutics. Cancer Immunol Res; 4(7); 631–8. ©2016 AACR.

Despite progress in the treatment of colorectal cancer, curing metastatic colorectal cancer remains a major unmet medical need worldwide. Here, we describe a T-cell–engaging bispecific antibody (T-BsAb) to redirect polyclonal cytotoxic T cells to eradicate colorectal cancer. A33, a murine antibody specific for GPA33, was humanized to huA33 and reformatted to huA33-BsAb, based on a novel IgG(L)–scFv platform by linking the anti-CD3 huOKT3 scFv to the carboxyl end of the light chain. This T-BsAb was stably expressed in CHO cells and purified as a stable monomer by HPLC, retaining immunoreactivity by FACS through 30 days of incubation at 37°C. In vitro, it induced activation and expansion of unstimulated T cells and elicited potent T-cell–dependent cell-mediated cytotoxicity against colon and gastric cancer cells in an antigen-specific manner. In vivo, huA33-BsAb inhibited the colon and gastric cancer xenografts, in both subcutaneous and intraperitoneal tumor models. More importantly, both microsatellite instable and microsatellite stable colorectal cancer were effectively eliminated by huA33-BsAb. These preclinical results provide further support for the use of IgG(L)–scFv platform to build BsAb, and especially one targeting GPA33 for colorectal cancer. These preclinical results also support further development of huA33-BsAb as a potential immunotherapeutic. Mol Cancer Ther; 17(10); 2164–75. ©2018 AACR.

WT1126 (RMFPNAPYL) is a human leukocyte antigen-A2 (HLA-A2) restricted peptide derived from Wilms tumor protein (WT1), which is widely expressed in a broad spectrum of leukemias, lymphomas and solid tumors. A novel T-cell-receptor (TCR)-like single chain variable fragment (scFv) antibody specific for the T cell epitope consisting of the WT1/HLA-A2 complex was isolated from a human scFv phage library. This scFv was affinity-matured by mutagenesis combined with yeast display, and structurally analyzed using a homology model. This monovalent scFv showed a 100-fold affinity improvement (dissociation constant [KD]= 3nM) and exquisite specificity towards its targeted epitope or HLA-A2+/WT1+ tumor cells. Bivalent scFv-huIgG1-Fc fusion protein demonstrated an even higher avidity (KD = 2pM) binding to the T cell epitope and to tumor targets, and was capable of mediating antibody-dependent cell-mediated cytotoxicity or tumor lysis by chimeric antigen receptor (CAR)-expressing human T or NK-92-MI transfected cells. This antibody demonstrated specific and potent cytotoxicity in vivo towards WT1-positive leukemia xenograft that was HLA-A2 restricted. In summary, T cell epitopes can provide novel targets for antibody-based therapeutics. By combining phage and yeast displays and scFv-Fc fusion platforms, a strategy for developing high affinity TCR-like antibodies could be rapidly explored for potential clinical development.

Acute myeloid leukemia (AML), the most common acute leukemia in adults and the second most common cancer in children, is still a lethal disease in the majority of patients, but immunologic approaches have improved outcome. Bispecific antibodies (BsAbs) are novel immunotherapeutics that can redirect immune cells against AML. We now report a tetravalent (2+2) humanized BsAb in the immunoglobulin G light chain single chain fragment variable [IgG(L)-scFv] format to engage polyclonal T cells to kill CD33+ AML targets. In vitro, this BsAb demonstrated strong antigen-specific T-cell-dependent cell-mediated cytotoxicity (TDCC) with an 50% effective concentration (EC50) in the femtomolar range that translated into treatment of established human AML IV xenografts in vivo. Importantly, it could redirect intraperitoneally injected T cells to ablate established and rapidly growing extramedullary subcutaneous AML xenografts in vivo. Furthermore, internalization of CD33 upon BsAb binding was identical to that of a bivalent (1+1) heterodimer, both being substantially less than anti-CD33 IgG. In contrast to the heterodimer, the tetravalent IgG-scFv BsAb was >10-fold more efficient in TDCC of AML cells in vitro and in vivo. This BsAb did not react with and did not kill CD38-CD34+ hematopoietic stem cells from cord blood. We conclude that the novel anti-CD33 IgG(L)-scFv BsAb construct reported here is a potential candidate for clinical development.

Bispecific antibodies (BsAb) have proven to be useful targeting vectors for pretargeted radioimmunotherapy (PRIT). We sought to overcome key PRIT limitations such as high renal radiation exposure and immunogenicity (e.g., of streptavidin–antibody fusions), to advance clinical translation of this PRIT strategy for diasialoganglioside GD2-positive [GD2(+)] tumors. For this purpose, an IgG-scFv BsAb was engineered using the sequences for the anti-GD2 humanized monoclonal antibody hu3F8 and C825, a murine scFv antibody with high affinity for the chelator 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) complexed with β-particle–emitting radiometals such as 177Lu and 90Y. A three-step regimen, including hu3F8-C825, a dextran-based clearing agent, and p-aminobenzyl-DOTA radiolabeled with 177Lu (as 177Lu-DOTA-Bn; t1/2 = 6.71 days), was optimized in immunocompromised mice carrying subcutaneous human GD2(+) neuroblastoma (NB) xenografts. Absorbed doses for tumor and normal tissues were approximately 85 cGy/MBq and ≤3.7 cGy/MBq, respectively, with therapeutic indices (TI) of 142 for blood and 23 for kidney. A therapy study (n = 5/group; tumor volume, 240 ± 160 mm3) with three successive PRIT cycles (total 177Lu: ∼33 MBq; tumor dose ∼3,400 cGy), revealed complete tumor response in 5 of 5 animals, with no recurrence up to 28 days after treatment. Tumor ablation was confirmed histologically in 4 of 5 mice, and normal organs showed minimal overall toxicities. All nontreated mice required sacrifice within 12 days (>1.0-cm3 tumor volume). We conclude that this novel anti-GD2 PRIT approach has sufficient TI to successfully ablate subcutaneous GD2(+)-NB in mice while sparing kidney and bone marrow. Mol Cancer Ther; 13(7); 1803–12. ©2014 AACR.

GPA33 is a colorectal cancer (CRC) antigen with unique retention properties after huA33-mediated tumor targeting. We tested a pretargeted radioimmunotherapy (PRIT) approach for CRC using a tetravalent bispecific antibody with dual specificity for GPA33 tumor antigen and DOTA-Bn-(radiolanthanide metal) complex.PRIT was optimized in vivo by titrating sequential intravenous doses of huA33-C825, the dextran-based clearing agent, and the C825 haptens (177)Lu-or (86)Y-DOTA-Bn in mice bearing the SW1222 subcutaneous (s.c.) CRC xenograft model.Using optimized PRIT, therapeutic indices (TIs) for tumor radiation-absorbed dose of 73 (tumor/blood) and 12 (tumor/kidney) were achieved. Estimated absorbed doses (cGy/MBq) to tumor, blood, liver, spleen, and kidney for single-cycle PRIT were 65.8, 0.9 (TI 73), 6.3 (TI 10), 6.6 (TI 10), and 5.3 (TI 12), respectively. Two cycles of PRIT (66.6 or 111 MBq (177)Lu-DOTA-Bn) were safe and effective, with a complete response of established s.c. tumors (100 - 700 mm(3)) in nine of nine mice, with two mice alive without recurrence at140 days. Tumor log kill in this model was estimated to be 2.1 - 3.0 based on time to 500-mm(3) tumor recurrence. In addition, PRIT dosimetry/diagnosis was performed by PET imaging of the positron-emitting DOTA hapten (86)Y-DOTA-Bn.We have developed anti-GPA33 PRIT as a triple-step theranostic strategy for preclinical detection, dosimetry, and safe targeted radiotherapy of established human colorectal mouse xenografts.

Cellular heterogeneity is a hallmark of human neuroblastoma tumors and cell lines. Within a single neuroblastoma are cells from distinct neural crest lineages whose relative abundance is significant for prognosis. We postulate that a self-renewing multipotent tumor stem cell, which gives rise to diverse cell lineages, is the malignant progenitor of this cancer. To test this hypothesis, we have established 22 cloned, phenotypically homogeneous populations of the three major cell types from 17 neuroblastoma cell lines. In vitro, malignant neuroblastoma stem cells, termed 1-type (intermediate type), have distinct morphologic, biochemical, differentiative, and tumorigenic properties. I-type cells express features of both neuroblastic (N) cells (scant cytoplasm, neuritic processes, neurofilaments, pseudoganglia, and granin and neurotransmitter enzyme expression) and substrate-adherent (S) cells (extensive cytoplasm and vimentin and CD44 expression). Moreover, they show bidirectional differentiation to either N or S cells when induced by specific agents. I-type cells are significantly more malignant than N- or S-type cells, with four- to five-fold greater plating efficiencies in soft agar and six-fold higher tumorigenicity in athymic mice. Differences in malignant potential are unrelated to N-myc amplification/overexpression or the ability to digest and migrate through the extracellular matrix. Immunocytochemical analyses of a small series of tumors reveal that frequency of cells coexpressing N and S cell markers correlates with poor prognosis. Thus, I-type stem cells may be instrumental in the genesis and growth of tumors in the patient. Their unique biology deserves attention and further investigation.

The 8H9 monoclonal antibody (MAb) is highly reactive with a cell surface glycoprotein expressed on human breast cancers, childhood sarcomas, and neuroblastomas but is not reactive with the cell surface of normal human tissues. This specific reactivity suggests that MAb 8H9 may be useful for targeted cancer therapy. To explore this possibility, we generated two recombinant immunotoxins (ITs) using the single-chain Fv (scFv) of MAb 8H9. Initially the 8H9(scFv) cDNA was fused to a DNA encoding a 38-kDa truncated form of Pseudomonas exotoxin (PE38) to generate the IT 8H9(scFv)-PE38. The fusion gene was expressed in Escherichia coli, and the IT was purified to near homogeneity from inclusion bodies. The purified IT showed specific cytotoxicity on nine different cancer cell lines derived from breast cancer, osteosarcoma, and neuroblastomas, known to react with MAb 8H9. The cytotoxic activity was inhibited by MAb 8H9, showing the cytotoxic activity is specific. The antitumor activity of 8H9(scFv)-PE38 was evaluated in severe combined immunodeficient mice bearing MCF-7 breast cancers or OHS-M1 osteosarcomas. The IT showed a specific dose-dependent antitumor activity at 0.075 and 0.15 mg/kg. Next, a more stable disulfide-linked IT, 8H9(dsFv)-PE38, was constructed. It was produced in high yield (16%) and showed cytotoxic and antitumor activities similar to those of 8H9(scFv)-PE38. 8H9(dsFv)-PE38 was given to two cynomolgus monkeys at doses of 0.1 and 0.2 mg/kg i.v. QOD × 3 and was well tolerated. This shows that a dose that causes significant tumor regressions in mice is well tolerated by monkeys. These results make 8H9(dsFv)-PE38 a candidate for further development as a therapeutic agent for breast cancers, osteosarcomas, and neuroblastomas.

Chimeric immune receptors (CIR) transduced into lymphocytes link target recognition by single chain antibody Fv (scFv) to activation through CD28/TCRzeta signaling. As surrogate antigens, anti-idiotypic antibodies may facilitate gene-transduction and clonal expansion of human lymphocytes for in vivo tumor therapy. The murine monoclonal antibody (MAb) 8H9 reacts with a novel antigen widely expressed on solid tumors. A CIR consisting of human CD8-leader sequence, 8H9-scFv, CD28 (transmembrane and cytoplasmic domains), and TCR-zeta chain was constructed, ligated into the pMSCVneo vector, and used to transfect the packaging line GP + envAM12 bearing an amphotropic envelope. Rat anti-idiotypic MAb 2E9 (IgG2a) was used to clone retroviral producer line as well as to expand gene-modified primary human lymphocytes. Sequential enrichments using either affinity chromatography or cell sorting using anti-idiotypic MAb 2E9 significantly improved the percentage of producer clones positive for surface 8H9-scFv and the efficiency of their supernatant in transducing the indicator cell line K562. By 3 weeks of in vitro culture,95% of transduced primary human lymphocytes were CIR-positive. Upon periodic stimulation with 2E9, these lymphocytes underwent10(6)-fold expansion by 6 months in culture. They mediated antigen-specific non-MHC restricted cytokine release and tumor cytotoxicity, and inhibited human xenograft engraftment in SCID mice. Anti-idiotypic antibody may provide a useful tool for optimizing gene transduction of CIR fusion constructs into primary human lymphocytes and their continual expansion in vitro.

Anti-disialoganglioside GD2 IgG antibodies have shown clinical efficacy in solid tumors that lack human leukocyte antigens (e.g., neuroblastoma) by relying on Fc-dependent cytotoxicity. However, there are pain side effects secondary to complement activation. T-cell retargeting bispecific antibodies (BsAb) also have clinical potential, but it is thus far only effective against liquid tumors. In this study, a fully humanized hu3F8-BsAb was developed, in which the anti-CD3 huOKT3 single-chain Fv fragment (ScFv) was linked to the carboxyl end of the anti-GD2 hu3F8 IgG1 light chain, and was aglycosylated at N297 of Fc to prevent complement activation and cytokine storm. In vitro, hu3F8-BsAb activated T cells through classic immunologic synapses, inducing GD2-specific tumor cytotoxicity at femtomolar EC50 with >105-fold selectivity over normal tissues, releasing Th1 cytokines (TNFα, IFNγ, and IL2) when GD2(+) tumors were present. In separate murine neuroblastoma and melanoma xenograft models, intravenous hu3F8-BsAb activated T cells in situ and recruited intravenous T cells for tumor ablation, significantly prolonging survival from local recurrence or from metastatic disease. Hu3F8-BsAb, but not control BsAb, drove T cells and monocytes to infiltrate tumor stroma. These monocytes were necessary for sustained T-cell proliferation and/or survival and contributed significantly to the antitumor effect. The in vitro and in vivo antitumor properties of hu3F8-BsAb and its safety profile support its further clinical development as a cancer therapeutic, and provide the rationale for exploring aglycosylated IgG-scFv as a structural platform for retargeting human T cells. Cancer Immunol Res; 3(3); 266–77. ©2014 AACR.

Most tumor cells function poorly as antigen-presenting cells in part because they do not express costimulatory molecules. To provide costimulation to T lymphocytes that recognize tumor cells, we constructed a CD28-like receptor specific for GD2, a ganglioside overexpressed on the surface of neuroblastoma, small-cell lung carcinoma, melanoma, and other human tumors. Recognition of GD2 was provided by a single-chain antibody derived from the GD2-specific monoclonal antibody 3G6. We demonstrate that the chimeric receptor 3G6-CD28 provides CD28 signaling upon specific recognition of the GD2 antigen on tumor cells. Human primary T lymphocytes retrovirally transduced with 3G6-CD28 secrete interleukin 2, survive proapoptotic culture conditions, and selectively undergo clonal expansion in the presence of an antiidiotypic antibody specific for 3G6-CD28. Polyclonal CD8+ lymphocytes expressing 3G6-CD28 are selectively expanded when cultured with cells expressing allogeneic major histocompatibility complex class I together with GD2. Primary T cells given such an antigen-dependent survival advantage should be very useful to augment immune responses against tumor cells.

Although metastatic rhabdomyosarcoma (RMS) is chemotherapy and radiotherapy responsive, few patients are cured. 8H9, a murine IgG(1) monoclonal antibody, recognizes a unique cell surface tumor antigen broadly distributed on neuroectodermal, epithelial, and mesenchymal tumors, including RMS. We now report on the in vitro characterization of radiolabeled 8H9 and its in vivo immunotargeting potential in mice with subcutaneous human RMS. Saturation-binding studies carried out to determine (125)I-8H9 affinity to the RMS cell line HTB82 demonstrated that (125)I-8H9 had a K(d) of 10.3nM with an estimated 115,000 binding sites on every HTB82 cell. (125)I-8H9 was retained on the cell surface without significant internalization. Biodistribution of (125)I-8H9 was studied in athymic mice bearing HTB82 xenografts. Following intravenous injection of 4.44MBq of (125)I-8H9, selective tumor uptake was evident 4 to 172 hours after injection. Average tumor uptake was 7.0 +/- 1.8, 11.5 +/- 3.9, 15.1 +/- 3.7, and 5.4 +/- 1.2% injected dose per gram at 4, 24, 48, and 172 hours, respectively. Mean tumor: tissue ratios were maximal at 172 hours (for lung, 4, kidney, 6, liver, 7, spleen, 11, femur, 14, muscle, 18, and brain, 48). Established RMS xenografts treated with a single injection of 18.5 MBq (131)I-8H9 were significantly suppressed compared to controls. Radiolabeled 8H9 effectively targeted RMS xenografts and may have a potential clinical role in radioimmunotherapy.

Single-chain variable fragment (ScFv) is a versatile building block for novel targeting constructs. However, a reliable screening and binding assay is often the limiting step for antigens that are difficult to clone or purify. Anti-idiotypic antibodies may be useful as surrogate antigens for cloning scFv and their fusion proteins. 8H9 is a murine IgG(1) monoclonal antibody (MAb) specific for a novel antigen expressed on the cell surface of a wide spectrum of human solid tumors, but not in normal tissues. Rat anti-8H9-idiotypic hybridomas (clones 2E9, 1E12, and 1F11) were produced by somatic cell fusion between rat lymphocytes and mouse SP2/0 myeloma. In direct binding assays enzyme-linked immunosorbant assay--(ELISA)--they were specific for the 8H9 idiotope. Using 2E9 as the surrogate antigen, 8H9-scFv was cloned from hybridoma cDNA by phage display. 8H9scFv was then fused to human-gamma1-CH2-CH3 cDNA for transduction into CHO and NSO cells. High expressors of mouse scFv-human Fc chimeric antibody were selected. The secreted homodimer reacted specifically with antigen-positive tumor cells by ELISA and by flow cytometry, inhibitable by the anti-idiotypic antibody. The reduced size resulted in a shorter half-life in vivo, while achieving comparable tumor to nontumor ratio as the native antibody 8H9. However, its in vitro activity in antibody-dependent cell-mediated cytotoxicity was modest.

3F8 is a murine IgG 3 monoclonal antibody (MAb) selective for the ganglioside G D2 . Previous studies using 131 I-3F8 have shown great potential in the imaging of neuroectodermal tumors and the therapy of human neuroblastoma. 131 I is commonly used in radioimmunodiagnosis, but its relatively long half-life (8 days) and its high energy γ-emission (364 KeV) are suboptimal for imaging purposes when compared with 99m Tc (6 h and 140 KeV, respectively). To label 3F8 with 99m Tc, the antibody was first coupled with a heterobifunctional linker, succinimidyl-6-hydrazinonicotinate hydrochloride (SHNH), obtaining a hydrazinonicotinamide-antibody conjugate. Using 99m Tc-Tricine as the precursor complex, 3F8-SHNH was coupled efficiently to 99m Tc, resulting in >90% radiometal incorporation, with a specific activity >10 mCi/mg and retaining full immunoreactivity. Immunoscintigraphy at 6, 22, and 46 h after intravenous injection of 1 mCi of 99m Tc-3F8 showed selective neuroblastoma localization in xenografted nude mice, comparable to that obtained with the injection of 100 μCi of 131 I-3F8. Biodistribution studies of 131 I-3F8 and 99m Tc-3F8 in mice demonstrated comparable %ID/g uptake in tumor (with a T/B ratio: ∼2.5 at 24 h and ∼3.5 at 48 h) and normal organs, including blood, except for spleen and liver which had about a three times higher uptake of the 99m Tc conjugate. In conclusion, 99m Tc can be coupled conveniently at high specific activity to 3F8 without compromising immunoreactivity. SHNH appears to be a useful linker for 99m Tc in tumor diagnostic imaging and may have potential utility in coupling other radioisotopes ( e.g., 94m Tc) for positron imaging and therapy.