Highly efficient destruction of squamous carcinoma cells of the head and neck by photochemical internalization of Ranpirnase

INTRODUCTION

Photochemical Internalization is a novel drug delivery technology for cancer treatment based on the principle of Photodynamic Treatment. Using a photosensitizer that locates in endocytic vesicles membranes of tumor cells, Photochemical internalization enables cytosolic release of endocytosed antitumor agents in a site-specific manner. The purpose of the present in-vitro study was to explore whether Photochemical Internalization is able to enhance the efficacy of Ranpirnase, a cytotoxic amphibian ribonuclease, for eradication of squamous cell carcinoma of the head and neck.

METHODS

Cell viability was measured in 8 primary human cell lines of squamous cell carcinoma of the head and neck after treatment with Ranpirnase and Photochemical Internalization. For Photochemical Internalization the photosensitizer disulfonated tetraphenyl porphine was incubated with tumor cells followed by exposure to blue light (435 nm).

RESULTS

Our study demonstrates significant enhancement of antitumor activity of Ranpirnase by Photochemical Internalization. Treatment responses were heterogeneous between the primary cancer cell lines. Combining Photochemical Internalization with Ranpirnase resulted in 4.6 to 1,940-fold increased cytotoxicity when compared with the ribonuclease alone (P < 0.05).

CONCLUSION

Cytotoxicity of Ranpirnase can be markedly enhanced by Photochemical Internalization in squamous cell carcinoma of the head and neck.

An EGF receptor targeting Ranpirnase-diabody fusion protein mediates potent antitumour activity in vitro and in vivo

Cytotoxic ribonucleases such as the leopard frog derivative Ranpirnase (Onconase(®)) have emerged as a valuable new class of cancer therapeutics. Clinical trials employing single agent Ranpirnase in cancer patients have demonstrated significant clinical activity and surprisingly low immunogenicity. However, dose-limiting toxicity due to unspecific uptake of the RNase into non-cancerous cells is reached at relatively low concentrations of > 1 mg/m(2). We have in the present study generated a dimeric anti-EGFR Ranpirnase-diabody fusion protein capable to deliver two Ranpirnase moieties per molecule to EGFR-positive tumour cells. We show that this compound mediated far superior efficacy for killing EGFR-positive tumour cells than a monomeric counterpart. Most importantly, cell killing was restricted to EGFR-positive target cells and no dose-limiting toxicity of Ranpirnase-diabody was observed in mice. These data indicate that by targeted delivery of Ranpirnase non-selective toxicity can be abolished and suggests Ranpirnase-diabody as a promising new drug for therapeutic interventions in EGFR-positive cancers.

Genetic delivery of an immunoRNase by an oncolytic adenovirus enhances anticancer activity

Antibody therapy of solid cancers is well established, but suffers from unsatisfactory tumor penetration of large immunoglobulins or from low serum retention of antibody fragments. Oncolytic viruses are in advanced clinical development showing excellent safety, but suboptimal potency due to limited virus spread within tumors. Here, by developing an immunoRNase-encoding oncolytic adenovirus, we combine viral oncolysis with intratumoral genetic delivery of a small antibody-fusion protein for targeted bystander killing of tumor cells (viro-antibody therapy). Specifically, we explore genetic delivery of a small immunoRNase consisting of an EGFR-binding scFv antibody fragment fused to the RNase Onconase (ONC(EGFR)) that induces tumor cell death by RNA degradation after cellular internalization. Onconase is a frog RNase that combines lack of immunogenicity and excellent safety in patients with high tumor killing potency due to its resistance to the human cytosolic RNase inhibitor. We show that ONC(EGFR) expression by oncolytic adenoviruses is feasible with an optimized, replication-dependent gene expression strategy. Virus-encoded ONC(EGFR) induces potent and EGFR-dependent bystander killing of tumor cells. Importantly, the ONC(EGFR)-encoding oncolytic adenovirus showed dramatically increased cytotoxicity specifically to EGFR-positive tumor cells in vitro and significantly enhanced therapeutic activity in a mouse xenograft tumor model. The latter demonstrates that ONC(EGFR) is expressed at levels sufficient to trigger tumor cell killing in vivo. The established ONC(EGFR)-encoding oncolytic adenovirus represents a novel agent for treatment of EGFR-positive tumors. This viro-antibody therapy platform can be further developed for targeted/personalized cancer therapy by exploiting antibody diversity to target further established or emerging tumor markers or combinations thereof.

Generation of “LYmph Node Derived Antibody Libraries” (LYNDAL) for selecting fully human antibody fragments with therapeutic potential

The development of efficient strategies for generating fully human monoclonal antibodies with unique functional properties that are exploitable for tailored therapeutic interventions remains a major challenge in the antibody technology field. Here, we present a methodology for recovering such antibodies from antigen-encountered human B cell repertoires. As the source for variable antibody genes, we cloned immunoglobulin G (IgG)-derived B cell repertoires from lymph nodes of 20 individuals undergoing surgery for head and neck cancer. Sequence analysis of unselected “LYmph Node Derived Antibody Libraries” (LYNDAL) revealed a naturally occurring distribution pattern of rearranged antibody sequences, representing all known variable gene families and most functional germline sequences. To demonstrate the feasibility for selecting antibodies with therapeutic potential from these repertoires, seven LYNDAL from donors with high serum titers against herpes simplex virus (HSV) were panned on recombinant glycoprotein B of HSV-1. Screening for specific binders delivered 34 single-chain variable fragments (scFvs) with unique sequences. Sequence analysis revealed extensive somatic hypermutation of enriched clones as a result of affinity maturation. Binding of scFvs to common glycoprotein B variants from HSV-1 and HSV-2 strains was highly specific, and the majority of analyzed antibody fragments bound to the target antigen with nanomolar affinity. From eight scFvs with HSV-neutralizing capacity in vitro,the most potent antibody neutralized 50% HSV-2 at 4.5 nM as a dimeric (scFv)2. We anticipate our approach to be useful for recovering fully human antibodies with therapeutic potential.

High-level production of a humanized immunoRNase fusion protein from stably transfected myeloma cells

ImmunoRNases represent a highly attractive alternative to conventional immunotoxins for cancer therapy. Quantitative production of immunoRNases in appropriate expression systems, however, remains a major challenge for further clinical development of these novel compounds. Here we describe a method for high-level production and purification of a fully functional immunoRNase fusion protein from supernatants of stably transfected mammalian cells.

Efficient killing of CD22+ tumor cells by a humanized diabody–RNase fusion protein

We report on the generation of a dimeric immunoenzyme capable of simultaneously delivering two ribonuclease (RNase) effector domains on one molecule to CD22(+) tumor cells. As targeting moiety a diabody derived from the previously humanized scFv SGIII with grafted specificity of the murine anti-CD22 mAb RFB4 was constructed. Further engineering the interface of this construct (V(L)36(Leu-->Tyr)) resulted in a highly robust bivalent molecule that retained the same high affinity as the murine mAb RFB4 (K(D)=0.2 nM). A dimeric immunoenzyme comprising this diabody and Rana pipiens liver ribonuclease I (rapLRI) was generated, expressed as soluble protein in bacteria, and purified to homogeneity. The dimeric fusion protein killed several CD22(+) tumor cell lines with high efficacy (IC(50)=3-20 nM) and exhibited 9- to 48-fold stronger cytotoxicity than a monovalent rapLRI-scFv counterpart. Our results demonstrate that engineering of dimeric antibody-ribonuclease fusion proteins can markedly enhance their biological efficacy.

A Dimeric Angiogenin Immunofusion Protein Mediates Selective Toxicity Toward CD22+ Tumor Cells

To improve selective cytotoxicity and pharmacokinetics of an anti-CD22 antibody single chain Fv (scFv)-ribonuclease fusion protein, a dimeric derivative was generated. Human angiogenin was fused via a (G4S)3 spacer peptide to the carboxy-terminal end of the stable dimeric anti-CD22 VL-VH zero-linker scFv MLT-7. The dimeric fusion protein and a monovalent counterpart were produced as soluble proteins in the periplasm of Escherichia coli. Comparative studies with homogeneously purified fusion proteins revealed that both constructs specifically bound to the target antigen and retained ribonucleolytic activity. However, they exhibited a markedly different capability for killing CD22+ tumor cells. The monomeric construct inhibited protein synthesis of target cells in a dose-dependent manner, but 50% inhibition (IC50) could be achieved only at the highest tested concentration (>350 nM). In contrast, the dimeric fusion protein efficiently killed CD22+ Raji and Daudi tumor cell lines with IC50 values of 74 nM and 118 nM, respectively. These results show that the therapeutic potential of scFv-ANG fusion proteins can be markedly enhanced by engineering dimeric derivatives.

Targeting malignant B-cell lymphoma with a humanized anti-CD22 scFv-angiogenin immunoenzyme

We report on the generation and functional characterization of a humanized immunoenzyme comprising a stable humanized single chain Fv (scFv) with grafted specificity of the anti-CD22 murine monoclonal antibody RFB4 and the human ribonuclease angiogenin (ANG). The fusion protein produced from transiently transfected mammalian Chinese hamster ovary cells could easily be purified to homogeneity, retained full ribonucleolytic activity, and efficiently killed CD22(+) tumour cells with an IC(50) of 56 nmol/l. In contrast, incubation of tumour cells with either ANG or scFv alone did not result in any cytotoxicity. Potent receptor-mediated killing of target cells, expected lack of extracellular toxicity, predictable low immunogenic potential, and ease of production, suggest that this novel immunoenzyme has potential for the immunotherapy of CD22(+) malignancies.

Antigen binding and stability properties of non-covalently linked anti-CD22 single-chain Fv dimers

By varying linker length and domain orientation three multivalent derivatives of a monovalent anti-CD22 single-chain fragment variable (scFv) antibody were generated. Shortening the linker of the V(H)-V(L) oriented scFv to 5 or 0 residues resulted in the formation of diabodies or a mixture of tetramers and trimers, respectively. Unexpectedly, a V(L)-0-V(H) scFv assembled to homogenous dimers, remained substantially more stable than the V(H)-5-V(L) diabody when incubated in human serum at 37 degrees C, and retained its dimeric state when concentrated up to 4 mg/ml. These properties suggest the V(L)-0-V(H) scFv could become an attractive vehicle for the selective delivery of multiple effector molecules to CD22(+) tumor cells.

Impact of antibody framework residue VH-71 on the stability of a humanised anti-MUC1 scFv and derived immunoenzyme

Anti-MUC1 single-chain Fv (scFv) fragments generated from the humanised antibody huHMFG1 had adequate antigen-binding properties but very poor stability irrespective of the applied linker or domain orientation. Mutagenesis of heavy-chain framework residue V_H-71, previously described as a key residue for maintaining the CDR-H2 main-chain conformation and thus important for antigen binding, markedly stabilised the scFv while having only a minor effect on the binding affinity of the molecule. Because of its improved stability, the engineered fragment exhibited immunoreactivity with tumour cells even after 7 days of incubation in human serum at 37°C. It also showed, in contrast to the wild-type scFv, a concentration-dependent binding to the target antigen when displayed on phage. When fusing the scFv to the recombinant ribonuclease rapLRI, only the fusion protein generated with the stable mutant scFv was able to kill MUC1⁺ tumour cells with an IC₅₀ of 80 nM. We expect this novel immunoenzyme to become a promising tool for the treatment of MUC1⁺ malignancies.

Generation of a highly stable, internalizing anti-CD22 single-chain Fv fragment for targeting non-Hodgkin's lymphoma

The generation of a single chain Fv (scFv) fragment derived from the anti-CD22 monoclonal antibody LL2 resulted in a molecule with good antigen binding but very poor stability properties, thus hampering its clinical applicability. Here we report on the construction of an engineered LL2 scFv fragment by rational mutagenesis. The contribution of uncommon wild-type sequence residues for providing stability to the conserved common core structure of immunoglobulins was examined. Aided by computer homology modeling, 3 destabilizing residues within the core of the wild-type VH domain were identified. Owing to the conserved nature of the buried core structure, mutagenesis of these sites to respective consensus residues markedly stabilized the molecule but did not influence its antigen binding properties: the engineered scFv MJ-7 exhibited exceptional biophysical stability with a half-life not reached after 6 days of incubation in human serum at 37 degrees C, while fully retaining the epitope specificity of the monoclonal antibody, and antigen binding affinity of the wild-type scFv. Furthermore, both the monoclonal antibody LL2 and the engineered scFv fragment became fully internalized after only 30 min of incubation at 37 degrees C with CD22+ tumor cells. These properties predict scFv MJ-7 could become a novel powerful tool to selectively deliver cytotoxic agents to malignant CD22+ cells.

Specificity grafting of human antibody frameworks selected from a phage display library: generation of a highly stable humanized anti-CD22 single-chain Fv fragment

A prerequisite for the enrichment of antibodies screened from phage display libraries is their stable expression on a phage during multiple selection rounds. Thus, if stringent panning procedures are employed, selection is simultaneously driven by antigen affinity, stability and solubility. To take advantage of robust pre-selected scaffolds of such molecules, we grafted single-chain Fv (scFv) antibodies, previously isolated from a human phage display library after multiple rounds of in vitro panning on tumor cells, with the specificity of the clinically established murine monoclonal anti-CD22 antibody RFB4. We show that a panel of grafted scFvs retained the specificity of the murine monoclonal antibody, bound to the target antigen with high affinity (6.4-9.6 nM), and exhibited exceptional biophysical stability with retention of 89-93% of the initial binding activity after 6 days of incubation in human serum at 37 degrees C. Selection of stable human scaffolds with high sequence identity to both the human germline and the rodent frameworks required only a small number of murine residues to be retained within the human frameworks in order to maintain the structural integrity of the antigen binding site. We expect this approach may be applicable for the rapid generation of highly stable humanized antibodies with low immunogenic potential.