Therapy of human B-cell lymphoma bearing SCID mice is more effective with anti-CD19- and anti-CD38-saporin immunotoxins used in combination than with either immunotoxin used alone

Plant-Derived Type I Ribosome Inactivating Protein-Based Targeted Toxins: A Review of the Clinical Experience

Targeted toxins (TT) for cancer treatment are a class of hybrid biologic comprised of a targeting domain coupled chemically or genetically to a proteinaceous toxin payload. The targeting domain of the TT recognises and binds to a defined target molecule on the cancer cell surface, thereby delivering the toxin that is then required to internalise to an appropriate intracellular compartment in order to kill the target cancer cell. Toxins from several different sources have been investigated over the years, and the two TTs that have so far been licensed for clinical use in humans; both utilise bacterial toxins. Relatively few clinical studies have, however, been undertaken with TTs that utilise single-chain type I ribosome inactivating proteins (RIPs). This paper reviews the clinical experience that has so far been obtained for a range of TTs based on five different type I RIPs and concludes that the majority studied in early phase trials show significant clinical activity that justifies further clinical investigation. A range of practical issues relating to the further clinical development of TT’s are also covered briefly together with some suggested solutions to outstanding problems.

A Deep Insight Into CAR-T Cell Therapy in Non-Hodgkin Lymphoma: Application, Opportunities, and Future Directions

Non-Hodgkin's lymphoma (NHL) is a cancer that starts in the lymphatic system. In NHL, the important part of the immune system, a type of white blood cells called lymphocytes become cancerous. NHL subtypes include marginal zone lymphoma, small lymphocytic lymphoma, follicular lymphoma (FL), and lymphoplasmacytic lymphoma. The disease can emerge in either aggressive or indolent form. 5-year survival duration after diagnosis is poor among patients with aggressive/relapsing form of NHL. Therefore, it is necessary to understand the molecular mechanisms of pathogenesis involved in NHL establishment and progression. In the next step, we can develop innovative therapies for NHL based on our knowledge in signaling pathways, surface antigens, and tumor milieu of NHL. In the recent few decades, several treatment solutions of NHL mainly based on targeted/directed therapies have been evaluated. These approaches include B-cell receptor (BCR) signaling inhibitors, immunomodulatory agents, monoclonal antibodies (mAbs), epigenetic modulators, Bcl-2 inhibitors, checkpoint inhibitors, and T-cell therapy. In recent years, methods based on T cell immunotherapy have been considered as a novel promising anti-cancer strategy in the treatment of various types of cancers, and particularly in blood cancers. These methods could significantly increase the capacity of the immune system to induce durable anti-cancer responses in patients with chemotherapy-resistant lymphoma. One of the promising therapy methods involved in the triumph of immunotherapy is the chimeric antigen receptor (CAR) T cells with dramatically improved killing activity against tumor cells. The CAR-T cell-based anti-cancer therapy targeting a pan-B-cell marker, CD19 is recently approved by the US Food and Drug Administration (FDA) for the treatment of chemotherapy-resistant B-cell NHL. In this review, we will discuss the structure, molecular mechanisms, results of clinical trials, and the toxicity of CAR-T cell-based therapies. Also, we will criticize the clinical aspects, the treatment considerations, and the challenges and possible drawbacks of the application of CAR-T cells in the treatment of NHL.

Therapeutic potential of SGN-CD19B, a PBD-based anti-CD19 drug conjugate, for treatment of B-cell malignancies

Patients with relapsed/refractory B-cell malignancies such as non-Hodgkin lymphoma (B-NHL) or acute lymphoblastic leukemia have a poor prognosis. Despite measurable clinical activity with new targeted therapies, many patients do not achieve a complete or durable response suggesting an opportunity to improve upon existing therapies. Here we describe SGN-CD19B, a pyrrolobenzodiazepine (PBD)-based anti-CD19 antibody drug conjugate (ADC) being investigated for treatment of B-cell malignancies, which has improved potency compared with other ADCs. CD19-expressing tumor cells rapidly internalize SGN-CD19B, and the released PBD drug induces DNA damage, resulting in G2/M cell cycle arrest and cell death. SGN-CD19B demonstrated activity against a broad panel of malignant B-cell lines and induced durable regressions in mice bearing xenografts derived from these B-cell malignancies. A single dose of SGN-CD19B induced durable regressions at 300 μg/kg (3 μg/kg drug equivalents); combination with rituximab decreased the curative dose to 100 μg/kg (1 μg/kg drug equivalents). These doses are significantly lower than the level of drug required with other ADC payloads. In cynomolgus monkeys, SGN-CD19B effectively depleted CD20+ B lymphocytes in peripheral blood and lymphoid tissues confirming that SGN-CD19B is pharmacodynamically active at well-tolerated doses. In summary, preclinical studies show SGN-CD19B is a highly active ADC, which releases a DNA cross-linking agent rather than a microtubule inhibitor. The distinct mechanism of action, broad potency, and potential to combine with rituximab suggest that SGN-CD19B may offer unique clinical opportunities in B-cell malignancies. A phase 1 clinical trial is in progress to investigate the therapeutic potential of SGN-CD19B in relapsed/refractory B-NHL. This trial was registered at www.clinicaltrials.gov as #NCT02702141.

CD38 in Hairy Cell Leukemia Is a Marker of Poor Prognosis and a New Target for Therapy

Hairy cell leukemia (HCL) is characterized by underexpression of the intracellular signaling molecule RhoH. Reconstitution of RhoH expression limits HCL pathogenesis in a mouse model, indicating this could represent a new therapeutic strategy. However, while RhoH reconstitution is theoretically possible as a therapy, it is technically immensely challenging as an appropriately functional RhoH protein needs to be specifically targeted. Because of this problem, we sought to identify druggable proteins on the HCL surface that were dependent upon RhoH underexpression. One such protein was identified as CD38. Analysis of 51 HCL patients demonstrated that 18 were CD38-positive. Interrogation of the clinical record of 23 relapsed HCL patients demonstrated those that were CD38-positive had a mean time to salvage therapy 71 months shorter than patients who were CD38-negative. Knockout of the CD38 gene in HCL cells increased apoptosis, inhibited adherence to endothelial monolayers, and compromised ability to produce tumors in vivo. Furthermore, an anti-CD38 antibody proved effective against pre-existing HCL tumors. Taken together, our data indicate that CD38 expression in HCL drives poor prognosis by promoting survival and heterotypic adhesion. Our data also indicate that CD38-positive HCL patients might benefit from treatments based on CD38 targeting.

Daratumumab granted breakthrough drug status

Multiple myeloma (MM) remains incurable despite important recent advances in treatment due to its inherent resistance, characterized by highly complex and heterogeneous molecular abnormalities, as well as the support from myeloma bone marrow (BM) microenvironment. A novel therapeutic strategy that effectively targets specific molecules on myeloma cells and also potentially overcomes tumor microenvironment-mediated drug resistance and the downstream effects of genetic instability is thus urgently needed. Over the last 2 years, an anti-CD38 monoclonal antibody daratumumab (DARA) has emerged as a breakthrough targeted therapy for patients with MM. Early-stage clinical trials have found DARA to be safe and to have encouraging clinical activity as a single agent and in combination with lenalidomide in heavily pretreated, relapsed patients in whom other novel agents (such as bortezomib, thalidomide and lenalidomide) as well as stem cell transplant has already failed. DARA may, therefore, be the first mAb with significant anti-MM activity both as a monotherapy and in combination. It is currently being further evaluated both alone and in combination with conventional and novel anti-MM agents as part of prospective clinical trials. This review discusses the preclinical and clinical development of DARA, its pathophysiological basis, and its prospects for future use in MM.

Immunotoxins and other conjugates containing saporin-s6 for cancer therapy

Ribosome-inactivating proteins (RIPs) are a family of plant toxins that permanently damage ribosomes and possibly other cellular substrates, thus causing cell death. RIPs are mostly divided in two types: Type 1 RIPs that are single-chain enzymatic proteins, and type 2 RIPs that consist of an active A chain (similar to a type 1 RIP) linked to a B chain with lectin properties. RIP-containing conjugates have been used in many experimental strategies against cancer cells, often showing great efficacy in clinical trials. Saporin-S6, a type 1 RIP extracted from Saponaria officinalis L. seeds, has been extensively utilized to construct anti-cancer conjugates because of its high enzymatic activity, stability and resistance to conjugation procedures, resulting in the efficient killing of target cells. This review summarizes saporin-S6-containing conjugates and their application in cancer therapy, considering in-vitro and in-vivo studies both in animal models and in clinical trials. The review is structured on the basis of the targeting of hematological versus solid tumors and on the antigen recognized on the cell surface.

Synergistic and persistent effect of T-cell immunotherapy with anti-CD19 or anti-CD38 chimeric receptor in conjunction with rituximab on B-cell non-Hodgkin lymphoma

Using artificial receptors, it is possible to redirect the specificity of immune cells to tumour-associated antigens, which is expected to provide a useful strategy for cancer immunotherapy. Given that B-cell non-Hodgkin lymphoma (B-NHL) cells invariably express CD19 and CD38, these antigens may be suitable molecular candidates for such immunotherapy. We transduced human peripheral T cells or a T-cell line with either anti-CD19-chimeric receptor (CAR) or anti-CD38-CAR, which contained an anti-CD19 or anti-CD38 antibody-derived single-chain variable domain respectively. Retroviral transduction led to anti-CD19-CAR or anti-CD38-CAR expression in T cells with high efficiency (>60%). The T cell line, Hut78, when transduced with anti-CD19-CAR or anti-CD38-CAR, exerted strong cytotoxicity against the B-NHL cell lines, HT and RL, and lymphoma cells isolated from patients. Interestingly, use of both CARs had an additive cytotoxic effect on HT cells in vitro. In conjunction with rituximab, human peripheral T cells expressing either anti-CD19-CAR or anti-CD38-CAR enhanced cytotoxicity against HT-luciferase cells in xenografted mice. Moreover, the synergistic tumour-suppressing activity was persistent in vivo for over 2 months. These results provide a powerful rationale for clinical testing of the combination of rituximab with autologous T cells carrying either CAR on aggressive or relapsed B-NHLs.

Activated T-cell-mediated immunotherapy with a chimeric receptor against CD38 in B-cell non-Hodgkin lymphoma

T-cell-mediated immunotherapy with a chimeric antigen receptor (CAR) is expected to become a powerful treatment for cancer. CD38, highly expressed in B-cell non-Hodgkin lymphoma (B-NHL) cells, is an attractive target in immunotherapy for B-NHL. We retrovirally transduced a T-cell line, Hut78, expressing little CD38, with an anti-CD38-CAR. Hut78 cells with the anti-CD38-CAR were cocultured with B-NHL cell lines bearing CD38 and also B-NHL cells from patients. Four days later most of the lymphoma cells were killed (the level of cytotoxicity was >95%). By contrast, there was undetectable cytotoxicity against CD38-negative cell lines. Then, we introduced the anti-CD38-CAR into human peripheral T cells. However, the recovery of viable cells was very low, presumably because of an autolytic reaction caused by the association of the anti-CD38-CAR with CD38 on the cell surface. The addition of an anti-CD38 antibody increased the yield of viable transduced T cell probably by blocking the autolytic reaction. We cocultured human peripheral T cells bearing anti-CD38-CAR with B-NHL cells. The median specific cytotoxicity was greater than 90%. These cells were injected 4 times into NOD/SCID mice, which were inoculated with B-NHL cells luciferase. Luciferase activity was not detectable even 30 days after the inoculation in 5 of 6 mice injected. By contrast, it increased in all of the mice injected with the mock vector-transduced T cell. In conclusion, T cell with the anti-CD38-CAR showed powerful cytotoxicity against B-NHL cells in vitro and in vivo. These findings may provide an important clue for improving the methodology of T-cell-mediated immunotherapy.

CD20-specific antibody-targeted chemotherapy of non-Hodgkin's B-cell lymphoma using calicheamicin-conjugated rituximab

Tumor-targeted delivery of a potent cytotoxic agent, calicheamicin, using its immunoconjugates is a clinically validated therapeutic strategy. Rituximab is a human CD20-specific chimeric antibody extensively used in B-NHL therapy. We investigated whether conjugation to calicheamicin can improve the anti-tumor activity of rituximab against human B-cell lymphoma (BCL) xenografts in preclinical models. BCL cells were cultured with rituximab or its calicheamicin conjugates and their in vitro growth was monitored. BCL cells were injected s.c. to establish localized xenografts in nude mice or i.v. to establish disseminated BCL in severe combined immunodeficient (scid) mice. I.p. treatment with rituximab or its calicheamicin conjugates was initiated and its effect on s.c. BCL growth or survival of mice with disseminated BCL was monitored. Conjugation of calicheamicin to rituximab vastly enhanced its growth inhibitory activity against BCL in vitro. Conjugation to calicheamicin had no deleterious effect on the effector functional activity of rituximab. Calicheamicin conjugated to rituximab with an acid-labile linker exhibited greater anti-tumor activity against s.c. BCL xenografts and improved survival of mice with disseminated BCL over that of unconjugated rituximab. Anti-tumor activities of rituximab conjugated to calicheamicin via an acid-stable linker were similar to that of unconjugated rituximab. Superior anti-tumor efficacy exhibited by a calicheamicin immunoconjugate of rituximab with an acid-labile linker over that of rituximab demonstrates the therapeutic potential of CD20-specific antibody-targeted chemotherapy strategy in the treatment of B-NHL.

Potent and specific antitumor efficacy of CMC-544, a CD22-targeted immunoconjugate of calicheamicin, against systemically disseminated B-cell lymphoma

PURPOSE

CMC-544 is a CD22-targeted immunoconjugate of calicheamicin and exerts a potent cytotoxic effect against CD22+ B-cell lymphoma. This study evaluated antitumor efficacy of CMC-544 against systemically disseminated B-cell lymphoma.

EXPERIMENTAL DESIGN

Scid mice received i.v. injections of CD22+ Ramos B-cell lymphoma cells for their systemic dissemination. CMC-544, G5/44, CD33-targeted CMA-676 (control conjugate) or rituximab were given i.p. 3, 9, 15, or 21 days after B-cell lymphoma dissemination. Diseased mice were monitored daily for hind-limb paralysis and death. Histopathological examination of CMC-544-treated and vehicle-treated diseased mice was also performed.

RESULTS

Mice with disseminated B-cell lymphoma developed hind-limb paralysis within 35 days. When given up to 15 days after B-cell lymphoma dissemination, CMC-544 extended survival of the diseased mice to >100 days, and these mice were considered cured. CMC-544 was efficacious when given during both the early initiation phase and the late established phase of the disease. A single dose of CMC-544 was effective in delaying the occurrence of hind-limb paralysis. In contrast, neither CMA-676 nor unconjugated G5/44 was effective. Rituximab was effective when given early in the disease process but not when the disease was established. Histopathological analysis revealed B-cell lymphoma infiltration in brain, spinal cord, bone marrow, and kidney in vehicle-treated but not in CMC-544-treated diseased mice. Consistent with its efficacy against the disseminated B-cell lymphoma, CMC-544 also caused regression of established Ramos B-cell lymphoma xenografts in scid mice.

CONCLUSIONS

CMC-544 confers strong therapeutic activity against systemic disseminated B-cell lymphoma and protects mice from hind-limb paralysis and death. These results support clinical evaluation of CMC-544 in the treatment of CD22+ lymphoid malignancies.

Improved outcome when B-cell lymphoma is treated with combinations of immunoliposomal anticancer drugs targeted to both the CD19 and CD20 epitopes

PURPOSE

We have reported previously that successful immunoliposomal drug therapy with liposomal doxorubicin (DXR) against xenograft B-lymphoma models required targeting against an internalizing B-cell antigen, CD19 (P. Sapra and T. M. Allen. Cancer Res 2002;62:7190-4.). Here we compare targeting of immunoliposomal formulations of DXR with vincristine (VCR) targeted against CD19 versus a noninternalizing (CD20) epitope. We also examine the effect of targeting immunoliposomes with antibody combinations in an attempt to increase the total number of binding sites (apparent antigen density) at the target cell surface.

EXPERIMENTAL DESIGN

Cell association of immunoliposomes (CD19-targeted, CD20-targeted, or combinations of the two) with human B-cell lymphoma (Namalwa) cells were studied using radiolabeled liposomes. Therapeutic efficacy of the same formulations was determined in a severe combined immunodeficient murine model.

RESULTS

Therapeutic results in severe combined immunodeficient mice bearing Namalwa cells administered anti-CD20-targeted liposomal DXR were barely improved over those found for nontargeted liposomal DXR or free DXR but, surprisingly, administration of anti-CD20-targeted liposomal VCR resulted in a significantly improved therapeutic outcome compared with nontargeted liposomal VCR, free VCR, or anti-CD20-targeted liposomal DXR. Treatment of murine B lymphoma with single injections of combinations of anti-CD19- and anti-CD20-targeted liposomal VCR led to cures in 70% of mice. However, mice injected with similar combinations of liposomal DXR did not have improved survival rates over anti-CD19-targeted liposomal DXR by itself.

CONCLUSIONS

The success of immunoliposomal therapy in combination regimens varies with the type of encapsulated drug and the nature of the target epitopes.

Therapy of human T-cell acute lymphoblastic leukaemia with a combination of anti-CD7 and anti-CD38-SAPORIN immunotoxins is significantly better than therapy with each individual immunotoxin

Severe combined immunodeficient (SCID) mice injected i.v. with the human T-ALL cell line CCRF CEM (SCID-CEM mice) develop within 50 days life-threatening multi-organ growth of leukaemia cells. The development of leukaemia in SCID-CEM mice treated with three 10 microg i.v. doses of the anti-CD7 immunotoxin (IT) HB2-SAPORIN or the anti-CD38 IT OKT10-SAPORIN was significantly delayed compared with PBS sham-treated animals but 90% of animals treated with either IT eventually developed disseminated leukaemia cell growth. In contrast treatment of SCID-CEM mice with a combination of both ITs led not only to a significantly greater delay in time to leukaemia development but also in the numbers of animals remaining leukaemia free (60%). The native HB2 and OKT10 antibodies (both murine IgG1antibodies) exerted significant, though relatively weak therapeutic effects, probably mediated through an antibody-dependent cellular cytotoxicity (ADCC) mechanism. Moreover, there was no in vivo additivity of therapeutic effect when both antibodies were used in combination. Apparent, however, was that the combination of HB2-SAPORIN IT with OKT10 antibody led to an intermediate therapeutic effect that was significantly greater than that obtained when either was used alone but significantly less than that obtained when the two IT combination was utilized. This was similarly the case for the combination of OKT10-SAPORIN IT with HB2 antibody though the effect was less pronounced in this instance. This result suggests that the therapeutic effect of IT + antibody treatment results from an additivity between antibody-mediated delivery of saporin combined with a SCID mouse NK cell-mediated ADCC attack on the target cell directed through target cell bound antibody Fc engagement with FcgammaRIII on the NK cell surface. The combination of both ITs however gave the best therapeutic outcome in SCID-CEM mice probably as the result of (i) delivery of greater amounts of saporin to target CEM cells positive for both CD7 and CD38, (ii) delivery of an effective dose of saporin to CEM cells downregulated or negative for one of the target antigens and (iii) through ADCC mechanisms that interact additively with IT action. We have previously proposed that combination IT therapy would be one means of overcoming the problem of heterogeneity of antigen expression within a global tumour cell population and these additional findings support this and provide a further strengthening of the rationale for employing cocktails of ITs for the treatment of human malignancies.

Relationship of the CD22 immunotoxin dose and the tumour establishment in a SCID mice model

Immunotoxins (ITs) may be very potent to erradicate tumour growth in vivo. We investigated the influence of the IT-dose, in relation to the establishment of the tumour, on the anti-tumour activity of CD22-recombinant (rec) ricin A for a disseminated tumour (Ramos) in SCID mice. Furthermore, the enhancement of the IT cytotoxicity in vivo by chloroquine was assessed. CD22-rec ricin A appeared to be highly effective. Paralysis of the hind legs was significantly delayed by a very low IT-dose of 2 microg administered intravenously (i.v.) 7 days after i.v. inoculation of the tumour cells. Even a dose of 30 microg administered 21 days after inoculation of the target cells significantly delayed the onset of paralysis up to 8 days compared with the median paralysis time (MPT) of the control group. The efficacy of treatment was obviously influenced by the establishment of the tumour, the tumour load and localisation. The anti-tumour activity of 10 and 30 microg IT diminished when the IT was administered after increasing the time lag following inoculation of tumour cells. Delaying IT administration resulted in growth of solid tumours. This implies that cells migrate to sanctuaries protected from the IT indicating that the anti-tumour activity was influenced by the accessibility of the IT to the target cells. The in vivo anti-tumour activity of CD22-rec ricin A could not be enhanced by simultaneously administered chloroquine, despite the continuous infusion with an intraperitoneally (i.p.) implanted mini-osmotic pump. Ex vivo experiments revealed that the maximally tolerated serum concentration (3.9 microM) was too low to be effective. In conclusion, CD22-rec ricin A is highly effective for in vivo treatment of B-cell malignancies, in particular if treatment is started when the tumour load is low and before migration takes place to poorly accessible sanctuaries.

Modulation of molecular mechanisms involved in protein synthesis machinery as a new tool for the control of cell proliferation

In the past years, the attention of scientists has focused mainly on the study of the genetic information and alterations that regulate eukaryotic cell proliferation and that lead to neoplastic transformation. All therapeutic strategies against cancer are, to date, directed at DNA either with cytotoxic drugs or gene therapy. Little or no interest has been aroused by protein synthesis mechanisms. However, an increasing body of data is emerging about the involvement of translational processes and factors in control of cell proliferation, indicating that protein synthesis can be an additional target for anticancer strategies. In this paper we review the novel insights on the biochemical and molecular events leading to protein biosynthesis and we describe their involvement in cell proliferation and tumorigenesis. A possible mechanistic explanation is given by the interactions that occur between protein synthesis machinery and the proliferative signal transduction pathways and that are therefore suitable targets for indirect modulation of protein synthesis. We briefly describe the molecular tools used to block protein synthesis and the attempts made at increasing their efficacy. Finally, we propose a new multimodal strategy against cancer based on the simultaneous intervention on protein synthesis and signal transduction.

Immunotoxins against CD19 and CD22 are effective in killing precursor-B acute lymphoblastic leukemia cells in vitro

Monoclonal antibodies (Mabs) conjugated to toxins or their subunits (immunotoxins or ITs) are undergoing clinical testing in adults with a variety of malignancies. The potential impact of this form of therapy in pediatric precursor B-lineage acute lymphoblastic leukemia (pre-B ALL) has yet to be determined. Mabs directed against the cell surface antigens, CD19 and CD22 conjugated to deglycosylated ricin A chain (dgRTA) have been tested in patients with non-Hodgkin's lymphoma (NHL), but not in patients with pre-B ALL. Because of the encouraging performance of these ITs in phase I trials, we evaluated the specific cytotoxicity of anti-CD19 (HD37-dgRTA) and anti-CD22 (RFB4-dgRTA) ITs or their combination (Combotox) on patient-derived pre-B ALL cells maintained in vitro on a stromal feeder layer. After 48 h in culture, cytotoxicity to tumor cells was determined by flow cytometry using propidium iodide (PI) and fluorescein isothiocyanate (FITC)-conjugated anti-CD10, 19, and 22. Both RFB4-dgRTA and HD37-dgRTA induced a statistically significant reduction in the number of viable leukemic cells, and Combotox was even more effective. Our results demonstrate that these ITs are specifically cytotoxic to primary pre-B ALL cells and that they should be further evaluated for the therapy of B-lineage ALL.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.

Comparison of the potency and therapeutic efficacy of the anti-CD7 immunotoxin HB2-saporin constructed with one or two saporin moieties per immunotoxin molecule

Immunotoxins that carry two toxin molecules to the target cell should in theory have a greater anti-tumour effect than those that carry just one. We have investigated the therapeutic efficacy of two anti-CD7-saporin immunotoxins constructed with one saporin (HB2-Sap 1-mer) or two saporin molecules (HB2-Sap 2-mer) per immunotoxin molecule. In vitro, the 2-mer immunotoxin was 5.6 times more effective than the 1-mer immunotoxin at inhibiting protein synthesis in the CD7+ human T-cell acute lymphoblastic leukaemia (T-ALL) cell line HSB-2 and was also more effective at inhibiting HSB-2 cell proliferation. Flow cytometry revealed that the 2-mer immunotoxin had a reduced binding capacity to HSB-2 cells compared with the 1-mer immunotoxin or native HB2 antibody. In therapy studies in SCID mice with disseminated HSB-2 human leukaemia, the 2-mer immunotoxin performed marginally better than the 1-mer immunotoxin, but log-rank analysis did not reveal any significant differences between the two therapy groups. We therefore conclude that, although the 2-mer immunotoxin performed better than the 1-mer immunotoxin against target HSB-2 cells in vitro, this improved performance was not reflected as an improved in vivo therapeutic outcome in the SCID mouse model.