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The treatment of rhabdomyosarcoma (RMS) is challenging, and the prognosis remains especially poor for high-grade RMS with metastasis. The conventional treatment of RMS is based on multi-agent chemotherapy combined with resection and radiotherapy, which are often marked by low success rate. Alternative therapeutic options include the combination of standard treatments with immunotherapy. We generated a microtubule-associated protein (MAP)-based fully human cytolytic fusion protein (hCFP) targeting the fetal acetylcholine receptor, which is expressed on RMS cells. We were able to express and purify functional scFv35-MAP from Escherichia coli cells. Moreover, we found that scFv35-MAP is rapidly internalized by target cells after binding its receptor, and exhibits specific cytotoxicity toward FL-OH1 and RD cells in vitro. We also confirmed that scFv35-MAP induces apoptosis in FL-OH1 and RD cells. The in vivo potential of scFv35-MAP will need to be considered in further studies.

Antibody-based immunotherapy of leukemia requires the targeting of specific antigens on the surface of blasts. The Fc gamma receptor (CD64) has been investigated in detail, and CD64-targeting immunotherapy has shown promising efficacy in the targeted ablation of acute myeloid leukemia (AML), acute myelomonocytic leukemia (AMML) and chronic myeloid leukemia cells (CML). Here we investigate for the first time the potential of FcαRI (CD89) as a new target antigen expressed by different myeloid leukemic cell populations. For specific targeting and killing, we generated a recombinant fusion protein comprising an anti-human CD89 single-chain Fragment variable and the well-characterized truncated version of the potent Pseudomonas aeruginosa exotoxin A (ETA'). Our novel therapeutic approach achieved in vitro EC50 values in range 0.2-3 nM depending on the applied stimuli, that is, interferon gamma or tumor necrosis factor alpha. We also observed a dose-dependent apoptosis-mediated cytotoxicity, which resulted in the elimination of up to 90% of the target cells within 72 hr. These findings were also confirmed ex vivo using leukemic primary cells from peripheral blood samples of three previously untreated patients. We conclude that CD89-specific targeting of leukemia cell lines can be achieved in vitro and that the efficient elimination of leukemic primary cells supports the potential of CD89-ETA' as a potent, novel immunotherapeutic agent.

Tumor necrosis factor (TNF) is a pro-inflammatory cytokine that plays a critical role in many inflammatory diseases. Soluble TNF can be neutralized by monoclonal antibodies (mAbs), and this is a widely-used therapeutic approach. However, some patients do not respond to anti-TNF therapy due to the increased expression of CD64 on monocytes and macrophages. A recent study has shown that CD64 captures anti-TNF mAbs via their Fcγ domain, which induces the transcription of pro-inflammatory genes. Specific blocking of CD64 could therefore be a promising strategy to improve the response to anti-TNF therapy. We used the CD64-specific antibody fragment H22(scFv) and tested its activity against the human CD64(+) cell line HL-60. When stimulated with interferon gamma (IFN-γ), these cells represent a pro-inflammatory phenotype of the monocyte/macrophage lineage. We found that H22(scFv) binds selectively to and blocks CD64, preventing the capture of anti-TNF mAb. Importantly, H22(scFv) itself does not induce CD64 activation. We also found that transmembrane TNF on HL-60 cells stimulated with IFN-γ also contributes to the capture of anti-TNF mAb, although via their Fab domain. In conclusion, the specific blocking of CD64 by H22(scFv) could be used a possible anti-inflammatory mechanism for potentiating the effect of anti-TNF antibodies.

The epidermal growth factor receptor (EGFR) is overexpressed in many solid tumors. EGFR-specific monoclonal antibodies (mAbs), such as cetuximab and panitumumab, have been approved for the treatment of colorectal and head and neck cancer. To increase tissue penetration, we constructed single-chain fragment variable (scFv) antibodies derived from these mAbs and evaluated their potential for targeted cancer therapy. The resulting scFv-based EGFR-specific immunotoxins (ITs) combine target specificity of the full-size mAb with the cell-killing activity of a toxic effector domain, a truncated version of Pseudomonas exotoxin A (ETA′). The ITs and corresponding imaging probes were tested in vitro against four solid tumor entities (rhabdomyosarcoma, breast, prostate and pancreatic cancer). Specific binding and internalization of the ITs scFv2112-ETA′ (from cetuximab) and scFv1711-ETA′ (from panitumumab) were demonstrated by flow cytometry and for the scFv-SNAP-tag imaging probes by live cell imaging. Cytotoxic potential of the ITs was analyzed in cell viability and apoptosis assays. Binding of the ITs was proofed ex vivo on rhabdomyosarcoma, prostate and breast cancer formalin-fixed paraffin-embedded biopsies. Both novel ITs showed significant pro-apoptotic and anti-proliferative effects toward the target cells, achieving IC50 values of 4 pM (high EGFR expression) to 460 pM (moderate EGFR expression). Additionally, rapid internalization and specific in vitro and ex vivo binding on patient tissue were confirmed. These data demonstrate the potent therapeutic activity of two novel EGFR-specific ETA′-based ITs. Both molecules are promising candidates for further development toward clinical use in the treatment of various solid tumors to supplement the existing therapeutic regimes.

The treatment of rhabdomyosarcoma (RMS) remains challenging, with metastatic and alveolar RMS offering a particularly poor prognosis. Therefore, the identification and evaluation of novel antigens, which are suitable targets for immunotherapy, is one attractive possibility to improve the treatment of this disease. Here we show that chondroitin sulfate proteoglycan 4 (CSPG4) is expressed on RMS cell lines and RMS patient material. We evaluated the immunotoxin (IT) αMCSP-ETA', which specifically recognizes CSPG4 on the RMS cell lines RD, FL-OH1, TE-671 and Rh30. It is internalized rapidly, induces apoptosis and thus kills RMS cells selectively. We also demonstrate the specific binding of this IT to RMS primary tumor material from three different patients.

Although current cancer treatment strategies are highly aggressive, they are often not effective enough to destroy the collectivity of malignant cells. The residual tumor cells that survived the first-line treatment may continue to proliferate or even metastasize. Therefore, the development of novel more effective strategies to specifically eliminate also single cancer cells is urgently needed. In this respect, the development of antibody-based therapeutics, in particular example immunotoxins, has attracted broad interest. Since the internalization of immunotoxins is essential for their cytotoxic effectivity, it is of crucial importance to study their internalization behavior to assess the potential for their therapeutic use. In this study, we determined the internalization behavior of four different single-chain fragments variable (scFv) when binding to the corresponding target antigen as expressed on solid or non-solid tumor cell lines. The scFvs were recombinantly fused to the SNAP-tag, an engineered variant of the human repair enzyme O(6)-alkylguanine-DNA alkyltransferase that covalently reacts with benzylguanine derivatives. Since a large number of highly sensitive organic fluorescent dyes are already available or can easily be derivatized to react with the self-labeling SNAP-tag, this system provides versatile applications for imaging of intraand extracellular compartments of living cells. The fusion proteins were coupled to SNAP-surface(®) Alexa Fluor(®) 488 or SNAP-surface(®) Alexa Fluor(®) 647 and binding as well as internalization was monitored by flow cytometry and confocal microscopy, respectively. Depending on the respective target antigen, we could distinguish between slow and rapid internalization behavior. Moreover, we detected increased internalization rate for bivalent scFv constructs. Our approach allows for rapid and early stage evaluation of the internalization characteristics of new antibodies designated for further therapeutic development.