HA22 (R490A) is a recombinant immunotoxin with increased antitumor activity without an increase in animal toxicity

Emerging drugs for rarer chronic lymphoid leukemias

BACKGROUND

Rarer indolent lymphoid leukemias include well defined mature B-cell and T-cell neoplasm with widely varying natural history and specific morphological, immunophenotypic and molecular characteristics. Among these are prolymphocytic leukemia (PLL), hairy cell leukemia (HCL) and its variants, large granular lymphocyte leukemia (LGLL) and adult T-cell leukemia/lymphoma (ATLL).

OBJECTIVE

To present current therapies and emerging drugs potentially useful in the treatment of rarer chronic lymphoid leukemias.

METHODS

After searching MEDLINE, PubMed and the Current Contents database, and conference proceedings from the previous 3 years of the American Society of Hematology (ASH), the European Society of Hematology (EHA) and the American Society of Clinical Oncology (ASCO) were searched manually; articles written in English and additional relevant publications were then selected.

RESULTS/CONCLUSION

New drugs including monoclonal antibodies (mAbs), new purine analogs, small molecules targeting specific molecular targets and other agents are included. Future research should focus on the novel therapeutic strategies based on the molecular pathogenic mechanisms and the development of new targeted therapies for each distinct chronic lymphoid leukemia.

Anti-PSMA immunotoxin as novel treatment for prostate cancer? High and specific antitumor activity on human prostate xenograft tumors in SCID mice

BACKGROUND

Expression of the prostate specific membrane antigen (PSMA) is highly restricted to prostate epithelial cells. Therefore, toxin-based immunotherapy against this antigen may represent an alternative therapeutic option for prostate cancer. For these purposes, the effects of the recombinant anti-PSMA immunotoxin A5-PE40 on prostate tumor growth were investigated in vitro and in vivo.

METHODS

The in vitro binding and cytotoxicity of A5-PE40 were tested on the PSMA-expressing prostate cancer cell line C4-2 and on the PSMA-negative cell line DU145 by flow cytometry and WST assays. The binding of the immunotoxin to SCID mouse xenografts and to various mouse organs was examined by Western blot analysis. In vivo, the antitumor activity of the immunotoxin was tested by injecting A5-PE40 in mice bearing C4-2 or DU145 xenografts.

RESULTS

In vitro, a specific binding of A5-PE40 to C4-2 cells could be shown with a concentration-dependent cytotoxicity (IC(50) value=220 pM). In the next step, a specific binding of the immunotoxin to C4-2 xenografts could be demonstrated. In contrast, no binding on mouse organs expressing high homologous mouse PSMA was found. The treatment of mice with C4-2 tumors caused a significant inhibition of tumor growth in vivo, whereas DU145 xenografts remained totally unaffected.

CONCLUSIONS

A5-PE40 represents a recombinant anti-PSMA immunotoxin with potent antitumor activity in mice bearing human prostate cancer xenograft tumors. Therefore, A5-PE40 could be a promising candidate for therapeutic applications in patients with prostate cancer.

Modeling recombinant immunotoxin efficacies in solid tumors

Effectiveness of cancer therapy is improved by the use of recombinant immunotoxins (RITs) that target membrane proteins unique to malignant tumor cells. Although RIT antitumor activity in vivo can always be improved with larger doses, clinical restriction on the dose toleration makes it critical to explore how RIT antitumor activity can be maximized without resorting to dose elevation. In this work, a mathematical model was developed to explore functional correlations between the properties of several recombinant immunotoxins and their antitumor efficacies in vivo. Simulations were compared with experimental data of human tumor xenografts grown on nude mice to assess parameters critical to optimal antitumor activity. We dissected out or held constant as many parameters of the model as possible to investigate the effect of the remaining parameters on the behavior of the system as a whole. Empirical correlations between immunotoxin binding affinity and the target binding site density were obtained for several recombinant immunotoxins targeting either human A431 carcinoma or CD46 Burkitt's lymphoma. Simulations reinforced the idea of binding site barrier for drug diffusion and suggested that optimal antitumor activity was achieved when the binding affinity is logarithmically dependent on the target binding site density.

Synergistic antitumor activity of taxol and immunotoxin SS1P in tumor-bearing mice

PURPOSE

To investigate the combined antitumor activity in mice of immunotoxin SS1P and Taxol.

METHODS

Immunodeficient mice were implanted with A431/K5 tumors expressing mesothelin. Established tumors were treated i.v. with immunotoxin SS1P alone, i.p. with Taxol alone, or with the two agents together. SS1P was radiolabeled with (111)In and used to study the effect of Taxol on its uptake by A431/K5 tumors.

RESULTS

Using doses at which either agent alone caused stabilization of tumor growth, the combination was synergistic causing long-lasting complete remissions in many animals. In contrast, synergy was not observed when the same cells were treated with these agents in vitro. Tumor uptake of (111)In-SS1P was not affected by treatment with Taxol.

CONCLUSION

The combination of Taxol and SS1P exerts a synergistic antitumor effect in animals but not in cell culture. This effect is not secondary to increased tumor uptake of the immunotoxin. Synergy could be due to improved immunotoxin distribution within the tumor or could involve factors released by other cell types in the tumors.

Small antibody mimetics comprising two complementarity-determining regions and a framework region for tumor targeting

Here we show that fusion of two complementarity-determining regions (CDRs), VHCDR1 and VLCDR3, through a cognate framework region (VHFR2) yields mimetics that retain the antigen recognition of their parent molecules, but have a superior capacity to penetrate tumors. The antigen-recognition abilities of these approximately 3 kDa mimetics surpass those of comparable fragments lacking the framework region. In vivo activities of the mimetics suggests that the structural orientation of their CDRs approximates the conformation of the CDRs in the complex of the parent antibody with antigen. We linked the antibody mimetics to the bacterial toxin colicin Ia to create fusion proteins called "pheromonicins," which enable targeted inhibition of tumor growth. In mice bearing human malignant tumors, pheromonicins directed against tumor-specific surface markers show greater capacity to target and penetrate tumors than their parent antibodies. Rational recombination of selected VH/VL binding sites and their framework regions might provide useful targeting moieties for cytotoxic cancer therapies.

Generation and antitumor effects of an engineered and energized fusion protein VL-LDP-AE composed of single-domain antibody and lidamycin

Type IV collagenase plays a pivotal role in invasion, metastasis and angiogenesis of tumor. Single domain antibodies are attractive as tumor-targeting vehicle because of their much smaller size compared with antibody molecules produced by conventional methods. Lidamycin (LDM) is a potent enediyne-containing antitumor antibiotic. In this study an engineered and energized fusion protein VL-LDP-AE composed of lidamycin and VL domain of mAb 3G11 directed against type IV collagenase was prepared using a novel two-step method. First a VL-LDP fusion protein was constructed by DNA recombination. Secondly VL-LDP-AE was obtained by molecular reconstitution. In MTT assay, VL-LDP-AE showed potent cytotoxicity to HT-1080 cells and KB cells with IC(50) values of 8.55 x 10(-12) and 1.70 x 10(-11) mol/L, respectively. VL-LDP-AE showed antiangiogenic activity in chick chrorioallantoic membrane (CAM) assay and tube formation assay. In in vivo experiments, VL-LDP-AE was proved to be more effective than free LDM against the growth of subcutaneously transplanted hepatoma 22 in mice. Drugs were given intravenously on day 3 and 10 after tumor transplantation. Compared in terms of maximal tolerated doses, VL-LDP-AE at 0.25 mg/kg suppressed the tumor growth by 89.5%, LDM at 0.05 mg/kg by 69.9%, and mitomycin at 1 mg/kg by 35%. Having a molecular weight of 25.2 kDa, VL-LDP-AE was much smaller than other reported antibody-based drugs. The results suggested that VL-LDP-AE would be a promising candidate for tumor targeting therapy. And the 2-step approach could serve as a new technology platform for making a series of highly potent engineered antibody-based drugs for a variety of cancers.

A cleavable molecular adapter reduces side effects and concomitantly enhances efficacy in tumor treatment by targeted toxins in mice

Two of the main problems associated with administration of receptor-targeted toxins in tumor therapy are severe systemic side effects and low transfer of the toxins into the cytosol after binding to the tumor cell surface. To improve chimeric toxins in this respect we have developed a molecular adapter that links the toxic moiety and ligand. The adapter is designed to improve cytosolic uptake, retain the toxin inside the cytosol and detoxify the drug after cell death. The plant toxin saporin linked either directly or via the adapter to epidermal growth factor (EGF) served to evaluate efficacy to inhibit tumor growth and reduce side effects in vivo. The lethal dose for BALB/c mice was three times less for the adapter-containing toxin (SA2E) than for the adapter-free construct (SE). Furthermore, SE only reduced the average weight of induced tumors by 33% whereas SA2E-treated mice exhibited 71% reduction with an almost complete suppression in 60% of the cases. Additionally, severe side effects like hyperalgesia, alopecia and death were drastically reduced in SA2E-treated animals. Tumors without target receptor were only slightly affected by SA2E and the reduction in side effects less pronounced indicating specific depletion from the blood by target receptor expressing cells.

2-CdA in the treatment of hairy cell leukemia: a review of long-term follow-up

Hairy cell leukemia is a rare, indolent, chronic lymphoproliferative disorder characterized by the proliferation of a malignant B-cell clone with irregular cytoplasmic projections. Treatment with purine analogs such as 2-chlorodeoxyadenosine (2-CdA) is associated with excellent remission rates and long-term survival. Although the majority of patients achieve a complete remission (CR), most have minimal residual disease detected by sensitive methods. Despite the long term disease-free survival, approximately 36% of patients relapse and many require further therapy. Repeat administration of 2-CdA is very effective in achieving a second CR. Recently, new agents such rituximab and BL22 were shown to be effective in the treatment of relapsed and refractory disease.

Characterization of the B cell epitopes associated with a truncated form of Pseudomonas exotoxin (PE38) used to make immunotoxins for the treatment of cancer patients

Recombinant immunotoxins composed of an Ab Fv fragment joined to a truncated portion of Pseudomonas exotoxin A (termed PE38) have been evaluated in clinical trials for the treatment of various human cancers. Immunotoxin therapy is very effective in hairy cell leukemia and also has activity in other hemological malignancies; however, a neutralizing Ab response to PE38 in patients with solid tumors prevents repeated treatments to maximize the benefit. In this study, we analyze the murine Ab response as a model to study the B cell epitopes associated with PE38. Sixty distinct mAbs to PE38 were characterized. Mutual competitive binding of the mAbs indicated the presence of 7 major epitope groups and 13 subgroups. The competition pattern indicated that the epitopes are discrete and could not be reproduced using a computer simulation program that created epitopes out of random surface residues on PE38. Using sera from immunotoxin-treated patients, the formation of human Abs to each of the topographical epitopes was demonstrated. One epitope subgroup, E1a, was identified as the principal neutralizing epitope. The location of each epitope on PE38 was determined by preparing 41 mutants of PE38 in which bulky surface residues were mutated to either alanine or glycine. All 7 major epitope groups and 9 of 13 epitope subgroups were identified by 14 different mutants and these retained high cytotoxic activity. Our results indicate that a relatively small number of discrete immunogenic sites are associated with PE38, most of which can be eliminated by point mutations.

Recombinant antibodies: from the laboratory to the clinic

The development of recombinant antibodies has facilitated the exploitation of the Ab-Ag interaction specificity for targeted therapies. A fully human antibody, with custom integrated designs, can be obtained in one-third the time, compared to development of antibodies by hybridoma technology. Recombinant antibodies can be tailored for specific applications, "armed" with cytotoxic agents in a controllable fashion, and used for extracellular and intracellular targeting. Multitargeted and combination therapies are rapidly evolving for the treatment of cancer. Antibody therapeutics, costly to develop and produce, have proven beneficial in the clinic.

Eradication of tumor colonization and invasion by a B cell-specific immunotoxin in a murine model for human primary intraocular lymphoma

Human primary intraocular lymphoma (PIOL) is predominantly a B cell-originated malignant disease with no appropriate animal models and effective therapies available. This study aimed to establish a mouse model to closely mimic human B-cell PIOL and to test the therapeutic potential of a recently developed immunotoxin targeting human B-cell lymphomas. Human B-cell lymphoma cells were intravitreally injected into severe combined immunodeficient mice. The resemblance of this tumor model to human PIOL was examined by fundoscopy, histopathology, immunohistochemistry, and evaluated for molecular markers. The therapeutic effectiveness of immunotoxin HA22 was tested by injecting the drug intravitreally. Results showed that the murine model resembles human PIOL closely. Pathologic examination revealed that the tumor cells initially colonized on the retinal surface, followed by infiltrating through the retinal layers, expanding preferentially in the subretinal space, and eventually penetrating through the retinal pigment epithelium into the choroid. Several putative molecular markers for human PIOL were expressed in vivo in this model. Tumor metastasis into the central nervous system was also observed. A single intravitreal injection of immunotoxin HA22 after the establishment of the PIOL resulted in complete regression of the tumor. This is the first report of a murine model that closely mimics human B-cell PIOL. This model may be a valuable tool in understanding the molecular pathogenesis of human PIOL and for the evaluation of new therapeutic approaches. The results of B cell-specific immunotoxin therapy may have clinical implications in treating human PIOL.

Hairy cell leukemia: towards a curative strategy

Although not all patients who have HCL require therapy at diagnosis, most eventually need treatment. Historically, splenectomy and interferon-alpha resulted in hematologic responses; however, responses tend to be short. The introduction of purine analogs dramatically changed the prognosis for most patients who hae HCL. It is now considered standard of care to use a purine analog, such as 2-CdA or 2'-DCF, as first-line therapy. This approach results in a high CR rate and prolong DFS. Although both agents yield the same rates of CR and survival, 2-CdA seems easier to administer and my be associated with less toxicity. Despite the excellent results with purine analogs, most patients have MRD detected by sensitive techniques; 30% to 40% of patients eventually relapse and most require further therapy. A repeat course of 2-CdA (or 2'-DCF) will result in CR in approximately 70% of patients. For patients who have relapsed or refractory disease, monoclonal antibody-based therapies are emerging options. Rituximab and BL22 are highly active in this setting. Until BL22 becomes widely available, rituximab is a reasonable choice for salvage therapy; however, the dosing schedule needs to be denied further. The roles of rituximab and BL22 as initial therapy for patients who have previously untreated HCL have not been investigated. Fig. 1 is a suggested algorithm for the treatment of HCL. With the introduction of effective new agents, further studies will determine whether the now achievable prolonged survival will translate into cure.

Engineered antibody fragments and the rise of single domains

With 18 monoclonal antibody (mAb) products currently on the market and more than 100 in clinical trials, it is clear that engineered antibodies have come of age as biopharmaceuticals. In fact, by 2008, engineered antibodies are predicted to account for >30% of all revenues in the biotechnology market. Smaller recombinant antibody fragments (for example, classic monovalent antibody fragments (Fab, scFv)) and engineered variants (diabodies, triabodies, minibodies and single-domain antibodies) are now emerging as credible alternatives. These fragments retain the targeting specificity of whole mAbs but can be produced more economically and possess other unique and superior properties for a range of diagnostic and therapeutic applications. Antibody fragments have been forged into multivalent and multi-specific reagents, linked to therapeutic payloads (such as radionuclides, toxins, enzymes, liposomes and viruses) and engineered for enhanced therapeutic efficacy. Recently, single antibody domains have been engineered and selected as targeting reagents against hitherto immunosilent cavities in enzymes, receptors and infectious agents. Single-domain antibodies are anticipated to significantly expand the repertoire of antibody-based reagents against the vast range of novel biomarkers being discovered through proteomics. As this review aims to show, there is tremendous potential for all antibody fragments either as robust diagnostic reagents (for example in biosensors), or as nonimmunogenic in vivo biopharmaceuticals with superior biodistribution and blood clearance properties.

Toxin Conjugate Therapy of Cancer

Standard approaches to the treatment of malignancies include surgery, chemotherapy, and radiation. More recently, with understanding of these processes in molecular detail, biologics have become another important aspect of treatment. Conjugate toxins are biologics that combine a potent toxin with a peptide that will target cancer cells with minimal damage to normal tissues. This review will detail examples of these conjugate toxins and their targeted malignancies.