Novel Binding Mechanisms of Fusion Broad Range Anti-Infective Protein Ricin A Chain Mutant-Pokeweed Antiviral Protein 1 (RTAM-PAP1) against SARS-CoV-2 Key Proteins in Silico

The deadly pandemic named COVID-19, caused by a new coronavirus (SARS-CoV-2), emerged in 2019 and is still spreading globally at a dangerous pace. As of today, there are no proven vaccines, therapies, or even strategies to fight off this virus. Here, we describe the in silico docking results of a novel broad range anti-infective fusion protein RTAM-PAP1 against the various key proteins of SARS-CoV-2 using the latest protein-ligand docking software. RTAM-PAP1 was compared against the SARS-CoV-2 B38 antibody, ricin A chain, a pokeweed antiviral protein from leaves, and the lectin griffithsin using the special CoDockPP COVID-19 version. These experiments revealed novel binding mechanisms of RTAM-PAP1 with a high affinity to numerous SARS-CoV-2 key proteins. RTAM-PAP1 was further characterized in a preliminary toxicity study in mice and was found to be a potential therapeutic candidate. These findings might lead to the discovery of novel SARS-CoV-2 targets and therapeutic protein structures with outstanding functions.

Ribosome-inactivating proteins: from plant defense to tumor attack

Ribosome-inactivating proteins (RIPs) are EC3.2.32.22 N-glycosidases that recognize a universally conserved stem-loop structure in 23S/25S/28S rRNA, depurinating a single adenine (A4324 in rat) and irreversibly blocking protein translation, leading finally to cell death of intoxicated mammalian cells. Ricin, the plant RIP prototype that comprises a catalytic A subunit linked to a galactose-binding lectin B subunit to allow cell surface binding and toxin entry in most mammalian cells, shows a potency in the picomolar range. The most promising way to exploit plant RIPs as weapons against cancer cells is either by designing molecules in which the toxic domains are linked to selective tumor targeting domains or directly delivered as suicide genes for cancer gene therapy. Here, we will provide a comprehensive picture of plant RIPs and discuss successful designs and features of chimeric molecules having therapeutic potential.

Cancer dormancy: lessons from a B cell lymphoma and adenocarcinoma of the prostate

Cancer dormancy delineates a situation in which residual tumor cells persist in a patient with no apparent clinical symptoms. Although the precise mechanisms underlying cancer dormancy have not been explained, experimental models have provided some insights into the factors that might be involved in the induction and maintenance of a tumor dormant state. The authors of the present chapter studied a murine B cell lymphoma that can be made dormant when interacting with antibodies directed against the idiotype on its immunoglobulin Ig receptor. This experimental model of antibody-induced dormancy enabled the isolation and characterization of dormant lymphoma cells. The results indicated that anti-Ig antibodies activate growth-inhibiting signals that induced cycle arrest and apoptosis. This process appeared to be balanced by the growth of the tumor cells such that the tumor did not expand. In contrast, antibodies against HER-2expressed on prostate adenocarcinoma (PAC) cells were not growth inhibitory. However, an immunotoxin (IT) prepared by conjugating HER-2 to the A-chain of ricin (RTA) was internalized by PAC cells, followed by induction of cycle arrest and apoptotic death. Infusion of HER-2-specific IT into PAC-bearing immunodeficient mice did not eradicate the tumor but retained it dormant over an extended period of time. Hence, certain aspects of signaling receptors expressed on cancer can be manipulated by antibodies to induce and maintain a tumor dormant state.

Anti-B4 Blocked Ricin Post Chemotherapy in Patients with Chronic Lymphocytic Leukemia--Long-term Follow-up of a Monoclonal Antibody-based Approach to Residual Disease

Anti-B4-blocked ricin is an immunotoxin consisting of anti-B4 murine monoclonal antibody and "blocked ricin" toxin. The anti-B4 monoclonal antibody is directed against the CD19 antigen, which is expressed on B-lymphocytes. A phase II study of anti-B4 blocked ricin toxin in patients with B-cell chronic lymphocytic leukemia (CLL) with residual disease after chemotherapy was conducted. Eleven patients received anti-B4 blocked ricin at 30 microg/kg lean body mass (LBM) daily by continuous infusion for 7 days followed with repeat infusion administered at 14-day intervals. No patient achieved an objective response. The major reasons for failure to respond were the presence of adenopathy and residual marrow disease. Three patients achieved immunophenotypic response in marrow and peripheral blood. Three of 6 patients with rearranged IgH and/or Ig kappa were germline after anti-B4 blocked ricin. The median follow-up of surviving patients is 8.6 years. The median survival is 5.8 years (range, 0.0-8.8). All patients have progressed. The median time to progression was 0.8 years (range, 0.3-3.0). Infusion-related toxicities were all grade 1-2. The most common toxicity was transaminitis. Human antimouse antibody (HAMA) and/or human antiricin antibody (HARA) development was documented in 2 patients. Anti-B4 blocked ricin was well tolerated but had limited activity in patients with residual CLL after chemotherapy.

Specific targeting to murine myeloma cells of Cyt1Aa toxin from Bacillus thuringiensis subspecies israelensis

Multiple myeloma is currently an incurable cancer of plasma B cells often characterized by overproduction of abnormally high quantities of a patient-specific, clonotypic immunoglobulin "M-protein." The M-protein is expressed on the cell membrane and secreted into the blood. We previously showed that ligand-toxin conjugates (LTC) incorporating the ribosome-inactivating Ricin-A toxin were very effective in specific cytolysis of the anti-ligand antibody-bearing target cells used as models for multiple myeloma. Here, we report on the incorporation of the membrane-disruptive Cyt1Aa toxin from Bacillus thuringiensis subsp. israelensis into LTCs targeted to murine myeloma cells. Proteolytically activated Cyt1Aa was conjugated chemically or genetically through either its amino or carboxyl termini to the major peptidic epitope VHFFKNIVTPRTP (p87-99) of the myelin basic protein. The recombinant fusion-encoding genes were cloned and expressed in acrystalliferous B. thuringiensis subsp. israelensis through the shuttle vector pHT315. Both chemically conjugated and genetically fused LTCs were toxic to anti-myelin basic protein-expressing murine hybridoma cells, but the recombinant conjugates were more active. LTCs comprising the Cyt1Aa toxin might be useful anticancer agents. As a membrane-acting toxin, Cyt1Aa is not likely to induce development of resistant cell lines.

Targeted anticancer immunotoxins and cytotoxic agents with direct killing moieties

Despite the progress of the bioinformatics approach to characterize cell-surface antigens and receptors on tumor cells, it remains difficult to generate novel cancer vaccines or neutralizing monoclonal antibody therapeutics. Among targeted cancer therapeutics, biologicals with targetable antibodies or ligands conjugated or fused to toxins or chemicals for direct cell-killing ability have been developed over the last 2 decades. These conjugated or fused chimeric proteins are termed immunotoxins or cytotoxic agents. Two agents, DAB389IL-2 (ONTAKTM) targeting the interleukin-2 receptor and CD33-calicheamicin (Mylotarg), have been approved by the FDA for cutaneous T-cell lymphoma (CTCL) and relapsed acute myeloid leukemia (AML), respectively. Such targetable agents, including RFB4(dsFv)-PE38 (BL22), IL13-PE38QQR, and Tf-CRM107, are being tested in clinical trials. Several agents using unique technology such as a cleavable adapter or immunoliposomes with antibodies are also in the preclinical stage. This review summarizes the generation, mechanism, and development of these agents. In addition, possible future directions of this therapeutic approach are discussed.

[Ricin--a therapeutic agent and a biological weapon]

Lately there have been several incidents with the plant protein ricin, for instance in the capitol building of the American Senate in February 2004. By focusing on this toxin, the media have spread a fear that ricin will be used in terrorist acts. In this article, we want to give a review of the qualities and potentials of the use of ricin. Ricin is a highly toxic plant protein that is lethal in small doses; it kills cells by inactivating ribosomes but has potential for medical use.

Evaluation of a CD25-specific immunotoxin for prevention of graft-versus-host disease after unrelated marrow transplantation

Donor T cells activated by recipient alloantigens cause graft-versus-host disease (GVHD) after hematopoietic cell transplantation. Activated T cells express CD25, among other components of the interleukin-2 receptor. We conducted a phase I/II study to determine whether administration of CD25-specific antibody conjugated to ricin toxin A could reduce the risk of grade III or IV GVHD after marrow transplantation from HLA-matched unrelated donors. All patients received methotrexate and cyclosporine after the transplantation. The immunotoxin was given to 36 patients for 4 consecutive days beginning approximately 36 hours after the marrow infusion was completed. Fourteen (40%) of the 35 patients who could be evaluated developed grade III or IV GVHD. In a contemporaneous population of 121 patients who received marrow from HLA-matched unrelated donors and were given methotrexate and cyclosporine without the immunotoxin, the incidence of grades III and IV GVHD was 24%. Cyclosporine blocked the induction of CD25 expression on alloactivated T cells in vitro but had no detectable effect on CD25 expression by T-regulatory cells. Taken together, these results are consistent with the hypothesis that cyclosporine protected alloactivated donor T cells from the effects of the immunotoxin, whereas the CD25+ T-regulatory cells remained susceptible, causing an unexpected exacerbation of acute GVHD.

[Ricin--a plant toxin with potential therapeutic use]

Ricin is a very toxic RIP II type lectin. As no effective antidote is known, its toxicity could be misused in terrorist actions. The mechanism of the toxic action is currently studied for the preparation of selective immunotoxins, which could be used in the therapy of various cancer diseases or HIV infection. There are promising in vitro results, but some adverse in vivo effects limit the use of these preparations in real therapy.

Clinical evaluation of ricin A-chain immunotoxins in patients with Hodgkin's lymphoma

BACKGROUND

Immunotoxins (ITs) consist of cell binding ligands coupled to toxins or their subunits. Hodgkin's lymphoma (HL) is an excellent target for ITs since lymphocyte activation markers such as CD25 and CD30 are expressed in large numbers. The ITs RFT5.dgA (anti CD25) and Ki-4.dgA (anti CD30) were constructed by linking the monoclonal antibodies RFT5 and Ki-4 to deglycosylated ricin A-chain (dgA). Both ITs showed potent specific activity against HL cells in vitro and in vivo in animal models, and were subsequently evaluated in phase I/II clinical trials in humans.

PATIENTS AND METHODS

In two separate trials, the ITs were administered i.v. four times every other day over 4 h. The objectives of the phase I trials included the determination of the maximum tolerated dose (MTD), dose-limiting toxicities (DLTs), pharmacokinetics, antitumor activity and immune response against the IT.

RESULTS

Twenty-seven patients with refractory HL were included in the phase I/II study of RFT5.dgA and 17 patients were included in the phase I study of Ki-4.dgA. The MTD of RFT5.dgA was 15 mg/m(2), whereas that of Ki-4.dgA was 5 mg/m(2). DLTs were related to vascular leak syndrome, consisting of edema, tachycardia, dyspnea, weakness and myalgia. Measurement of serum levels of RFT5.dgA demonstrated a C(max) of 0.2-9.7 micro g/ml with a half-life (t()) varying from 4 to 10.5 h. Peak serum concentration of Ki-4.dgA ranged from 0.23 to 1.7 micro g/ml. In both trials approximately 60% of patients developed human anti-mouse and/or anti-dgA antibodies. Seventeen of 18 patients treated at the MTD of RFT5.dgA were evaluable for clinical response. Responses included two partial remissions (PR), one minor response (MR) and five stable diseases (SD). Fifteen of 17 patients treated with Ki-4.dgA were evaluable for clinical response. Responses included one PR, one MR and two SD.

CONCLUSIONS

RFT5.dgA and Ki-4.dgA showed moderate efficacy in heavily pretreated refractory patients with HL. Ki-4.dgA was less well tolerated than RFT5.dgA. This might be due, at least in part, to the formation of Ki-4.dgA/sCD30 complexes.

In vivo efficacy of anti-glycoprotein 41, but not anti-glycoprotein 120, immunotoxins in a mouse model of HIV infection

Immunotoxins (ITs) targeting the HIV envelope protein are among the most efficacious antiviral therapies when tested in vitro. Yet a first-generation IT targeted to gp120, CD4-PE40 (chimeric immunotoxin using CD4 and the translocation and enzymatic domains of Pseudomonas exotoxin A), showed limited promise in initial clinical testing, highlighting the need for improved ITs. We have used a new mouse model of HIV infection to test the comparative efficacy of anti-HIV ITs targeted to gp120 or to gp41. Irradiated SCID/nonobese diabetic mice are injected with a tumor of human CD4(+) cells susceptible to infection and at a separate site persistently HIV-infected cells. The spread of infection from infected to susceptible tumor is monitored by plasma p24 and the presence of HIV-infected cells in the spleen. Anti-gp41 ITs in combination with tetrameric CD4-human Ig fusion protein have pronounced anti-HIV effects. Little if any anti-HIV efficacy was found with either CD4-PE40 or an Ab-targeted anti-gp120 IT. These data support continued exploration of the utility of ITs for HIV infection, particularly the use of anti-gp41 ITs in combination with soluble CD4 derivatives.

Current strategies of antibody-based treatment in Hodgkin's disease

Many new approaches involving antibody-based agents have given promising results in experimental Hodgkin's disease (HD) models. Clinical trials with monoclonal antibodies, immunotoxins, bispecific constructs and radioimmunoconjugates have demonstrated some clinical efficacy in patients with advanced refractory HD. Although it seems unlikely that resistant patients with larger tumor masses will be cured by either of these approaches, it might be feasible to treat bulky disease by conventional therapy and then administer biological agents to kill residual Hodgkin and Reed-Sternberg cells. Future phase III trials will have to prove a possible superior effect of this combined immunochemotherapy. Currently, the evaluation of the most promising approaches continues.

[Ricin--from a Bulgarian umbrella to an optional treatment of cancer]

Ricin toxin, found in the bean of the castor plant, is one of the most toxic and easily produced plant toxin. It is composed of two polypeptide chains linked by a disulfide bond. The toxin irreversibly blocks protein synthesis. Oral intoxication is the most frequent mode of exposure, but direct contact and inhalation have also been described. A single intramuscular exposure was reported, related to an assassination event in 1978. The clinical signs of intoxication begin hours after the exposure, and include gastrointestinal signs, dyspnea, fever, vasculatory collapse, neurological signs, liver necrosis and renal failure. Treatment is mainly supportive. A vaccine has been developed and found effective in animal studies. Ricin may be used as a biological weapon due to its heat stability and worldwide availability. A more friendly use, which has been tested in clinical trials, is the incorporation of ricin-immunotoxin for the treatment of cancer patients.

Phase I trial of anti-B4-blocked ricin in pediatric patients with leukemia and lymphoma

Monoclonal antibodies, specific for antigens expressed on lymphoid malignancies, which have been conjugated to toxins such as ricin, hold promise in the therapy of childhood leukemia and lymphoma. Anti-B4-blocked ricin (anti-B4-bR) is such an agent, and a phase I study of this agent was conducted in children with relapsed or refractory B-lineage leukemia and lymphoma. Anti-B4-bR was given as two 7-day continuous infusions separated by 7 days. Twenty patients were enrolled and 19 received the drug. Two dosage levels (30 and 40 microg/kg per day) were evaluated. Forty micrograms per kilogram per day was the maximally tolerated dose. Dose-limiting toxicity was capillary leak syndrome. Grade 3 reversible elevation in transaminases was also encountered. Human antimouse antibodies or human antiricin antibodies were detected in five patients. No complete remissions or partial remissions were seen.

Cytotoxicity of anti-CD64-ricin a chain immunotoxin against human acute myeloid leukemia cells in vitro and in SCID mice

Blast cells from patients with acute myeloid leukemia (AML) commonly express CD64, the high-affinity receptor for immunoglobulin G (FcgammaRI). An immunotoxin (MDX-44) was constructed by coupling humanized anti-CD64 monoclonal antibody (mAb) H22 via a bivalent linker to deglycosylated ricin A-chain (RA). Human leukemia cell lines were incubated with MDX-44 or H22/free RA. The effect of MDX-44 on the proliferation of leukemia cells was assessed by [(3)H]thymidine incorporation. In the presence of interferon-gamma (IFN-gamma), MDX-44 significantly inhibited the proliferation of CD64(+) HL-60, NB4, and U937 cells in 72-h cultures in a dose-dependent manner. The mechanism of action appeared to be the induction of apoptosis, as measured by propidium iodide staining and flow cytometry analysis. However, CD64(-) KG-1a and Daudi cells were not affected by MDX-44/IFN-gamma. Incubating HL-60 cells with MDX-44/IFN-gamma resulted in a 99% decrease in colony-forming units, whereas colony-forming cells in normal bone marrow were not significantly suppressed by such treatment. Cells from 60% of AML patients (6/10) were inhibited by MDX-44/IFN-gamma, and the inhibition was correlated with CD64 expression on these cells (r = 0.65). In a human AML model in NOD/SCID mice, MDX-44/IFN-gamma inhibited 95-98% of peritoneal exudate AML cell proliferation and 85-90% of solid leukemia masses. The effect of MDX-44 on AML cells was dependent on activation of cells by IFN-gamma. MDX-44/IFN-gamma may have value in the therapy of AML cells expressing cell-surface CD64.

Immunotherapeutic strategies in neuroblastoma: antitumoral activity of deglycosylated Ricin A conjugated anti-GD2 antibodies and anti-CD3xanti-GD2 bispecific antibodies

BACKGROUND

The antigen GD2 is selectively expressed on the surface of neuroblastoma cells, and is detected by the monoclonal antibody BW704. In this study, we describe the antitumoral capacity of the immunotoxin BW704dgA (BW704 conjugated to deglycosylated ricin A), and of anti-CD3xanti-GD2 bispecific antibodies that are capable of redirecting cytotoxic T cells towards neuroblastoma cells. We further investigate the in vivo activity of BW704dgA immunotoxins in a human neuroblastoma model in SCID mice.

PROCEDURE

BW704dgA immunotoxins were injected i.p. as a single close (48 microg/mouse) on day 4 or divided into three doses on day 4, 5, and 6 after i.v. inoculation of the human neuroblastoma cell line IMR5-75.

RESULTS

The mean survival time (MST) of BW704dgA treated animals was significantly increased (MST 49 days) compared to the control animals treated with irrelevant immunotoxin, unconjugated BW704, or control buffer (MST 33 to 39 days, P < 0.0001), without differences in the application schedules. Anti-CD3xanti-NP antibodies and NP-conjugated GD2-antibodies (BW704-NP) were used in a cytotoxicity assay with cytotoxic T-cells as effectors, and tracer labeled neuroblastoma cell line IMR5 as target cells. Anti-CD3xanti-NP antibodies, together with BW704-NP, showed increased cytotoxic activity compared to the incubation with CD3xanti-NP antibodies alone or with unconjugated anti-GD2. Additionally, a dose-dependent effect of NP-conjugated anti-GD2-antibodies upon the lysis of the target cells could be demonstrated. In this report, we describe two immunotherapeutic approaches using GD2 binding BW704 antibodies, modified as immunotoxin and a bispecific antibody, for the targeting and elimination of neuroblastoma cells.

CONCLUSIONS

We envisage a combined immunotherapeutic regimen consisting of BW704dgA mediated stem cell purging, followed by a systemic treatment with anti-CD3xanti-GD2 bispecific antibodies in neuroblastoma.

A combination of anti-CD3 and anti-CD7 ricin A-immunotoxins for the in vivo treatment of acute graft versus host disease

This study evaluated the anti-graft versus host disease (GVHD) potential of a combination of immunotoxins (IT), consisting of a murine CD3 (SPV-T3a) and CD7 (WT1) monoclonal antibody both conjugated to deglycosylated ricin A. In vitro efficacy data demonstrated that these IT act synergistically, resulting in an approximately 99% elimination of activated T cells at 10(-8 )mol/L (about 1.8 microg/mL). Because most natural killer (NK) cells are CD7(+), NK activity was inhibited as well. Apart from the killing mediated by ricin A, binding of SPV-T3a by itself impaired in vitro cytotoxic T-cell cytotoxicity. Flow cytometric analysis revealed that this was due to both modulation of the CD3/T-cell receptor complex and activation-induced cell death. These results warranted evaluation of the IT combination in patients with refractory acute GVHD in an ongoing pilot study. So far, 4 patients have been treated with 3 to 4 infusions of 2 or 4 mg/m(2) IT combination, administered intravenously at 48-hour intervals. The T(1/2) was 6.7 hours, and peak serum levels ranged from 258 to 3210 ng/mL. Drug-associated side effects were restricted to limited edema, fever, and a modest rise of creatine kinase levels. One patient developed low-titer antibodies against ricin A. Infusions were associated with an immediate drop of circulating T cells, followed by a more gradual but continuing elimination of T/NK cells. One patient mounted an extensive CD8 T-cell response directly after treatment, not accompanied with aggravating GVHD. Two patients showed nearly complete remission of GVHD, despite unresponsiveness to the extensive pretreatment. These findings justify further investigation of the IT combination for treatment of diseases mediated by T cells. (Blood. 2000;95:3693-3701)

Combination chemotherapy followed by an immunotoxin (anti-B4-blocked ricin) in patients with indolent lymphoma: results of a phase II study

The purpose of this article was to evaluate the antitumor effects of a combination chemotherapy program based on ProMACE (prednisone, methotrexate, doxorubicin [Adriamycin], cyclophosphamide, etoposide) followed by a B cell-specific immunotoxin in the treatment of patients with advanced-stage indolent histology non-Hodgkin's lymphomas. We performed a prospective phase II clinical trial in a referral-based patient population. After confirmation of diagnosis and staging evaluation, 44 patients (10 small lymphocytic lymphoma, 27 follicular lymphoma, 7 mantle cell lymphoma; 30 without prior therapy, 14 previously treated) received six cycles of ProMACE-CytaBOM (cytarabine, bleomycin, vincristine [Oncovin], mechlorethamine) combination chemotherapy (with etoposide given orally daily for five days) followed by a 7-day continuous infusion of anti-B4-blocked ricin immunotoxin at 30 microg/kg/day given every 14 days for up to six cycles. A complete response was achieved in 25 of 44 patients (57%), 21 from the chemotherapy alone, 3 converted from partial to complete response with the immunotoxin, and 1 patient became a complete responder after a surgical procedure to remove an enlarged spleen that was histologically negative for lymphoma. With a median follow-up of 5 years, 14 of 25 complete responders have relapsed (56%); median remission duration was 2 years, and overall survival was 61%. Forty-two percent of the complete responders have been in continuous remission for more than 4 years. The median number of courses of immunotoxin delivered was two usually because of the development of human anti-ricin antibodies. ProMACE-CytaBOM plus anti-B4-blocked ricin does not produce durable complete remissions in the majority of patients with indolent lymphoma. However, the remissions appear quite durable (> 4 years) in about 40% of the complete responders.

A phase I study of combination therapy with immunotoxins IgG-HD37-deglycosylated ricin A chain (dgA) and IgG-RFB4-dgA (Combotox) in patients with refractory CD19(+), CD22(+) B cell lymphoma

This study used an 8-day continuous infusion regimen of a 1:1 mixture of two immunotoxins (ITs) prepared from deglycosylated ricin A chain (dgA) conjugated to monoclonal antibodies directed against CD22 (RFB4-dgA) and CD19 (HD37-dgA; Combotox) in a Phase I trial involving 22 patients with refractory B cell lymphoma to determine the maximum tolerated dose, clinical pharmacology, and toxicity profile and to characterize any clinical responses. Adult patients received a continuous infusion of Combotox at 10, 20, or 30 mg/m2/192 h. No intrapatient dose escalation was permitted. Patients with > or =50 circulating tumor cells (CTCs)/mm3 in peripheral blood tolerated all doses without major toxicity. The maximum level of serum IT (Cmax) achieved in this group was 345 ng/ml of RFB4-dgA and 660 ng/ml of HD37-dgA (1005 ng/ml of Combotox). In contrast, patients without CTCs (<50/mm3) had unpredictable clinical courses that included two deaths probably related to the IT. Additionally, patients exhibited a significant potential for association between mortality and a history of either autologous bone marrow or peripheral blood stem cell transplants (P2 = 0.003) and between mortality and a history of radiation therapy (P2 = 0.036). In patients with CTCs, prior therapies appeared to have little impact on toxicity. Subsequent evaluation of the ITs revealed biochemical heterogeneity between two lots of HD37-dgA. In addition, HD37-dgA thawed at the study site tended to contain significant particulates, which were not apparent in matched controls stored at the originating site. This suggests that a tendency to aggregate may have resulted from shipping, storage, and handling of the IT that occurred prior to preparation for administration. It is not clear to what extent, if any, the aggregation of HD37-dgA IT was related to the encountered clinical toxicities; however, the potential to aggregate does suggest one possible basis for problems in our clinical experience with HD37-dgA and leads us to the conclusion that non-aggregate-forming formulations for these ITs should be pursued prior to future clinical trials.

An Epstein-Barr virus-infected lymphoblastoid cell line (D430B) that grows in SCID-mice with the morphologic features of a CD30+ anaplastic large cell lymphoma, and is sensitive to anti-CD30 immunotoxins

BACKGROUND AND OBJECTIVE

In this study we describe a newly established CD30+ Epstein Barr virus (EBV)-infected B cell line derived from an EBV-infected B cell culture (utilized, once irradiated, as a feeder) which showed a B clonal rearrangement and strong CD30 antigen expression.

DESIGN AND METHODS

The cells injected into SCID mice were able to grow giving rise to CD30+ solid tumors with the morphologic features of an anaplastic large cell lymphoma (ALCL). Thus we tried to establish a model to investigate the potency of immunoconjugates containing a CD30 monoclonal antibody (Ber-H2) and ribosome-inactivating proteins (saporin, momordin and ricin A-chain) as toxic moieties.

RESULTS

We observed a strong cytotoxic activity of the anti-CD30 immunotoxins on the in vitro growth of D430B cells. High levels of anti-tumor activity were also observed in vivo, in the SCID mouse model.

INTERPRETATION AND CONCLUSIONS

The antitumor immunotoxin therapy was sccessful in our chosen animal model, the effecacy seeming to be associated with strength of CD30 expression. Our data suggest that immunotoxins should be tested (before use) on the tumor cells of the subject to be treated and that immunotoxins should be directed to different tumor-associated antigens to avoid selection of cell populations with different antigenic mosaics.

A Phase II study of adjuvant therapy with anti-B4-blocked ricin after autologous bone marrow transplantation for patients with relapsed B-cell non-Hodgkin's lymphoma

This Phase II trial was undertaken to determine the safety, toxicity, and potential efficacy of the B-cell restricted immunotoxin anti-B4-blocked ricin (Anti-B4-bR) when administered as adjuvant therapy to patients in complete remission (CR) after autologous bone marrow transplantation (ABMT) for B-cell non-Hodgkin's lymphoma (NHL). Forty-nine patients with B-cell NHL in CR 46-202 days (median, 112 days) post-ABMT received Anti-B4-bR at a dose of 30 microg/kg lean body weight/day for 7 days by continuous i.v. infusion. Patients were eligible for up to two additional courses of therapy at 14-day intervals. A total of 83 courses of Anti-B4-bR were administered, with 31 patients receiving two or more courses of therapy. The mean serum level on day 7 of the first course was 0.77+/-0.41 nM. Reversible toxicities included hepatic transaminase elevations, thrombocytopenia, myalgias, fatigue, nausea, hypoalbuminemia, and dyspnea. Human antimouse antibody (HAMA) and/or human antiricin antibody (HARA) responses occurred in 23 patients at a median of 22 days from the initiation of Anti-B4-bR therapy (range, 11-100 days). The 4-year disease-free survival and overall survival are estimated at 56 and 72%, respectively. Twenty-six patients remain in CR after a median follow-up of 54.5 months. This study demonstrates that Anti-B4-bR can be administered safely to patients as adjuvant therapy early after ABMT for B-cell NHL. The toxicities are tolerable and reversible. Although the early estimate of disease-free survival was very encouraging in this single-armed trial, the 4-year follow-up data demonstrate continued relapse.

Mathematical model to predict the need for neodymium: YAG capsulotomy based on posterior capsule opacification rate

PURPOSE

To evaluate a model to project the estimated time required before patients having primary phacoemulsification require neodymium:YAG (Nd:YAG) laser capsulotomy.

SETTING

Eleven private practices in the United States.

METHODS

Projections of time to capsulotomy were based on assessment of the early development of posterior capsule opacification (PCO) over time. The PCO data were collected during a clinical study to evaluate MDX-RA, an investigational immunotoxin designed to limit epithelial cell growth, preventing postsurgical PCO. From the PCO data, the estimated time to Nd:YAG capsulotomy in a placebo-treated group was compared with the actual time to capsulotomy in a cohort of patients from general practice who had had phacoemulsification.

RESULTS

By 6 months, the mean Opacification Index in the MDX-RA group was significantly lower than that in the placebo group (P < .05) and it remained significantly lower at 12 (P < .001), 18 (P < .001), and 24 (P < .016) months. The rate of PCO in the MDX-RA group was approximately 6 times lower than that in the placebo group (P < .0004). Fifty-seven percent in the placebo group and 4% in the MDX-RA group were projected to require an Nd:YAG capsulotomy within 3 years of primary cataract surgery. Projected values for the placebo group were similar to actual values observed in the population-based cohort.

CONCLUSIONS

This technique could be used to predict the need for Nd:YAG capsulotomy using early measurements of PCO.

Inhibition of posterior capsule opacification with an immunotoxin specific for lens epithelial cells: 24 month clinical results

PURPOSE

To assess the safety and effectiveness of an immunotoxin, MDX-RA, designed to inhibit posterior capsule opacification (PCO).

SETTING

Eleven private practices in the United States.

METHODS

This study comprised 63 eyes of 63 patients having extracapsular cataract extraction by phacoemulsification; these patients were enrolled in a Phase I/II clinical investigation of the immunotoxin MDX-RA. At the close of surgery, 21 patients were treated with placebo, 23 patients with 50 units of the immunotoxin, and 19 patients with 175 units of the immunotoxin as an aqueous solution. The patients were monitored for 24 months after primary cataract surgery using external eye and slitlamp examinations, visual acuity assessment, ophthalmoscopy, pachymetry, tonometry, endothelial cell counts, and lens capsule photography. Posterior capsule opacification, recorded on lens capsule photographs, was graded independently by a committee of 3 cataract surgeons. The incidence of neodymium:YAG (Nd:YAG) capsulotomy was projected from the opacification results.

RESULTS

The immunotoxin, at the 50 unit dose, was well tolerated and effective in inhibiting PCO. At the 175 unit dose, there was a trend toward increased postoperative inflammation that was transient with no residua. From 6 to 24 months postoperatively, the 50 unit dose significantly inhibited PCO compared with the placebo (P < .05). This significant reduction in PCO translated into a significantly lower projected need for Nd:YAG capsulotomy in the 50 unit than the placebo group (P < .004). About 60% in the placebo group and 4% in the 50 unit group were projected to need an Nd:YAG capsulotomy by 3 years postoperatively.

CONCLUSION

The immunotoxin was well tolerated and was effective in reducing PCO for up to 24 months after cataract surgery. Although these preliminary results are encouraging, a larger study is underway to determine whether the reduction in PCO by the immunotoxin decreases the need for Nd:YAG capsulotomy.

Phase II clinical trial of bolus infusion anti-B4 blocked ricin immunoconjugate in patients with relapsed B-cell non-Hodgkin's lymphoma

Immunotoxins, composed of a monoclonal antibody conjugated to a protein toxin, mediate cell death through novel cytotoxic mechanisms. Anti-B4-blocked ricin (anti-B4-bR) recognizes CD19-positive cells, which includes most B-cell non-Hodgkin's lymphomas (NHLs). Previous Phase I clinical studies of anti-B4-bR, using both bolus and continuous dosing regimens, demonstrated no safety or efficacy advantage to the continuous infusion regimen. This Phase II trial in 16 patients with relapsed CD19-positive NHL was conducted to evaluate the efficacy of anti-B4-bR when administered at the previously established maximum tolerated dose using a daily bolus for a 5 consecutive days schedule. Serum pharmacokinetics were measured in selected patients. Tissue samples of involved lymph nodes and bone marrow were also obtained from a portion of patients for determination of anti-B4-bR penetration into tissues. Toxicity was similar to what has been described previously for anti-B4-bR and consisted mainly of reversible elevations of hepatic transaminases and mild to moderate thrombocytopenia. No sustained clinical responses were documented. Pharmacokinetic measurements demonstrated that serum levels compatible with 3 logs of cell kill in vitro could be sustained for several hours in most patients. Immunohistochemical analysis of tissue samples provided some insight into the low efficacy. The immunotoxin could be detected in three of the four bone marrow aspirate samples but in only two of the seven lymph node specimens. Thus, anti-B4-bR, using a single daily bolus for a 5 consecutive day schedule, is not an active agent in relapsed NHL. Poor penetration into certain sites of disease may be one explanation for its lack of efficacy.

Lymphoproliferative disorders following allogeneic bone marrow transplantation: the Vancouver experience

Between June 1988 and May 1996, 428 patients underwent allogeneic BMT (288 related donor (RD) and 140 unrelated donor (UD)) at the Vancouver General Hospital. Eight patients (UD six and RD two) developed a post-transplant lymphoproliferative disorder (PTLD). Median age at BMT was 38 years (range 22-51). Five of the six UD allografts were T cell depleted. Cyclosporine+/-methotrexate was used for GVHD prophylaxis. All eight patients developed GVHD; in six this was refractory to treatment with corticosteroids. Rabbit antithymocyte globulin (ATG) or an anti-CD5-ricin A chain immunotoxin (Xomazyme) was used as second-line therapy for GVHD. Presentation with PTLD occurred at median day 90.5 (range 34-282) post BMT. Five of the eight patients had a rapidly progressive course characterized by fever, lymphadenopathy, lung and liver involvement and died within 3-8 days. PTLD was an incidental finding at post mortem examination in two patients. The remaining patient had localized disease and recovered. Pathological analysis revealed two morphological patterns; diffuse large B cell lymphoma (DLBC lymphoma, five patients) and polymorphous B cell hyperplasia (PBCH, three patients). EBV expression was positive in all eight cases and monoclonality was demonstrated in seven cases. In multivariate analysis, T cell depletion of the allograft (P=0.0001, relative risk (RR)=30.5), anti-T cell therapy for GVHD (P=0.006, RR=12.7) and acute GVHD grades 3-4 (P=0.04, RR=7.7) were the significant factors for development of PTLD. In conclusion, we have identified two forms of PTLD after BMT: one is characterized by disseminated disease with a rapidly progressive and often fulminant course and the other by localized, relatively indolent disease. Morphology, EBV positivity and clonality do not appear to correlate with the clinical course. The major risk factors for development of PTLD after BMT are ex vivo T cell depletion of the allograft and in vivo anti-T cell therapy for GVHD.

Cell surface density of p185(c-erbB-2) determines susceptibility to anti-p185(c-erbB-2)-ricin A chain (RTA) immunotoxin therapy alone and in combination with anti-p170(EGFR)-RTA in ovarian cancer cells

Approximately 30% of ovarian and breast cancers overexpress p185(c-erbB-2) with as many as 10(6) receptors/cell. Normal cells have as few as 10(4) receptors/cell. We have examined the susceptibility of SKOv3 human ovarian cancer cells to anti-c-erbB2 antibodies and immunotoxins as a function of c-erbB-2 density on the cell surface. A panel of SKOv3 clones that expressed different densities of p185(c-erbB-2) receptor were generated through transfection with the c-erbB-2 gene. A significant correlation was found between p185(c-erbB-2) density and susceptibility to killing by anti-p185(c-erbB-2)-ricin A chain (anti-p185(c-erbB-2)-RTA) immunotoxins. With 10(5) copies/cell of p185(c-erbB-2), <10% of clonogenic ovarian cancer cells could be eliminated, whereas in clones that expressed 10(6) copies/cell of p185(c-erbB-2), 99.9% of clonogenic tumor cells were killed. In cell lines that overexpressed p185(c-erbB-2) and also expressed p170(EGFR), anti-p185(cerbB-2)-RTA and anti-p170(EGFR)-RTA immunotoxins exerted synergistic cytotoxicity. Treatment with the two immunotoxins could eliminate 99.99% of clonogenic cells. Importantly, tumor cells that had survived first treatment with anti-p185(c-erbB2)-RTA alone still retained sensitivity to repeat treatment with the same immunotoxin and also proved susceptible to the synergistic cytotoxicity of anti-p185(cerbB-2)-RTA in combination with anti-p170(EGFR)-RTA. Growth characteristics of the clones expressing various levels of p185(c-erbB-2) were also studied. No correlation was found between p185(c-erbB-2) expression levels and the rate of anchorage-dependent growth, anchorage-independent growth, or in vivo growth in nude mice.

The use of an anti-CD3 immunotoxin to prevent the development of lymphoproliferative disease in SCID/PBL mice

Severe combined immunodeficient mice (SCID) reconstituted with normal PBLs (SCID/PBL) from Epstein-Barr virus-positive (EBV+) human donors often develop fatal human B lymphomas which resemble the EBV-induced lymphoproliferative disease (LPD) observed in immunosuppressed individuals. This phenomenon appears to be T cell dependent. In this study we used an immunotoxin (IT) prepared by conjugating the monoclonal anti-CD3 antibody, 64.1, to deglycosylated ricin A chain (dgRTA) to prevent LPD in SCID/PBL mice. We show that the incidence of LPD is greatly reduced by either a combination of in vitro treatment of PBLs followed by one in vivo treatment of the xenografted mice with 64.1-dgRTA immunotoxin or by repeated treatments in vivo with the immunotoxin. In contrast, in vitro treatment alone or in vivo treatment with only one injection of 64.1-dgRTA were less effective. As expected, this IT did not have any non-specific cytotoxic effects on already established EBV+ tumors from SCID/PBL mice. The use of this IT, therefore, represents a simple method to avoid LPD when injecting blood-containing tissues into SCID mice.

Clinical trials with an anti-CD25 ricin A-chain experimental and immunotoxin (RFT5-SMPT-dgA) in Hodgkin's lymphoma

Immunotoxins (ITs) consisting of a cell-binding component and a potent toxin were developed as a new class of biological anti-tumor agents to improve adjuvant therapy. Hodgkin's lymphoma (HL) has been demonstrated to be an excellent target for ITs because high concentrations of lymphocyte activation markers such as CD25 and CD30 are expressed on Hodgkin and Reed-Sternberg (H-RS). Several ITs against these antigens have shown potent antitumor effects against H-RS cells in vitro and in different HL animal models. On the basis of its superiority in preclinical models, the anti-CD25 IT RFT5-SMPT-dgA was subsequently evaluated in a phase I study in patients with refractory Hodgkin's lymphoma. The IT was constructed by linking the monoclonal antibody (Moab) RFT5 via a sterically hindered disulfide linker (SMPT) to deglycosylated ricin A-chain (dgA). All 15 patients enrolled in this trial were heavily pretreated with a mean of five different prior therapies. The IT was administered intravenously over four hours on days 1-3-5-7 for total doses per cycle of 5, 10, 15, or 20 mg/m2. Side effects were reversible and related to the vascular leak syndrome (VLS), i.e. decrease in serum albumin, edema, weight gain, hypotension, tachycardia, myalgia, and weakness. In all three patients receiving 20 mg/m2 NCI toxicity grade III was observed. Thus, 15 mg/m2 is the maximal tolerated dose (MTD) of RFT5-SMPT-dgA. 50% of the patients developed human anti-ricin A-chain antibodies (HARA) and/or human anti-mouse antibodies (HAMA). Clinical results included two partial remissions (PR), one minor response (MR), three stable disease (SD) and nine progressive disease (PD). In an extension of the phase I trial, five additional patients have been treated at the MTD.

The emerging role of ricin A-chain immunotoxins in leukemia and lymphoma

Since MRD is the major cause for relapses of malignant diseases, strategies utilizing ITs to target tumor cells surviving conventional treatment have attracted scientific and clinical interest. Many different ITs against various blood-borne as well as solid malignancies have demonstrated specific potent anti-tumor effects in vitro and in animal models. Some of these have already undergone clinical phase I/II-trials. The dose-limiting toxicities of RTA ITs include manifestation of VLS presenting as decreased urinary sodium excretion, hypoalbuminemia, fatigue, hypotonia, myalgia, pulmonary edema, or rhabdomyolysis. Problems encountered clinically include the development of HAMA, HARA, and HACA and the selection of antigen-deficient malignant clones. Most clinical trials performed with ITs so far were conducted in heavily pretreated patients presenting with high tumor burdens. Thus, the responses observed with ITs in these trials are very encouraging and warrant further exploration.

Skeletal muscle-specific immunotoxin for the treatment of focal muscle spasm

Intramuscular injection of botulinum toxin type A (BTX) is used to treat many disorders characterized by muscular spasms. The utility of BTX, however, is limited by its short duration of action, the development of resistance after repeated injections, and cross-reactivity with autonomic neurons. To overcome these limitations, we engineered an immunotoxin (ITX) to damage skeletal muscle fibers selectively by chemically linking a monoclonal antibody against the nicotinic acetylcholine receptor to the toxin ricin. In vitro, the ITX was 20,000-fold more toxic to myotubes than myoblasts, consistent with the degree of acetylcholine receptor expression. The gastrocnemius muscles of 30 rats were unilaterally injected with a series of protein toxins at various concentrations and examined histopathologically 7 and 30 days later. ITX produced destructive myopathic changes at a dose 300-fold less than the maximum tolerated dose. Assessment of rat muscle strength after unilateral gastrocnemius injections showed that ITX was more effective and had a longer duration of action than BTX. ITXs may have potential for the treatment of involuntary muscle spasms.

[The influence of ricin on the course of experimental infections of mice with Trichinella spiralis]

Mice received 5 ng of ricin 24 hours after infection (experiment I) or 1 ng of ricin twice, 24 hours and 17 days of infection (experiment II). Animals were killed in 7, 14, 21, 28, 35, 42 and 60 days after infection. In the jejunum and masseter muscle sections, CD4+ and CD8+ lymphocytes, mast cells and eosinophils were studied. Heavy suppression of CD8+ lymphocytes, strong eosinophils and less pronounced mast cells stimulation was observed in the jejunum of mice received ricin (experiment I). Worm expulsion in intestine was faster than in the control (opposite results in experiment II). The composition of cells infiltration in the muscle was in both experiments similar to the control, however, fewer CD4+ lymphocytes were observed in larva capsule and there were fewer muscle larvae. Therefore CD8+ cells are believed to take part in restricting only the muscle stage of trichinellosis.

Autocrine growth factors and neuroendocrine markers in the development of small-cell lung cancer

Two different clinical trials using biological agents directed against an autocrine growth factor and a surface marker of neuroendocrine differentiation have been used for patients with relapsed small-cell lung cancer. In a phase II trial, an antibody (2A11) directed against the autocrine growth factor gastrin-releasing peptide has been used to treat patients with relapsed small-cell lung cancer. One of 12 evaluable patients treated with 2A11 250 mg/m2 three times weekly for 4 weeks achieved a complete response. An antibody directed against the neural cell adhesion molecule has been linked to a modified ricin molecule. This immunotoxin, N901-bR, has undergone phase I testing, and a recommended phase II dose of 30 micrograms/kg/day for 7 days by continuous infusion has been determined. In the phase I trial, one of 21 patients with relapsed or refractory small-cell lung cancer had a partial response to this treatment. Therefore, it appears that an antibody directed against an autocrine growth factor and an immunotoxin directed against a surface marker of neuroendocrine differentiation can inhibit the growth of small-cell lung cancer in vitro and in vivo; both produced some evidence of antitumor activity in patients. Further studies with agents directed against autocrine growth factors and surface markers of neuroendocrine differentiation appear warranted.

Intraventricular immunotoxin therapy for leptomeningeal neoplasia

OBJECTIVE

The goals of this clinical trial of intraventricular 454A12-rRA therapy were to identify dose-limiting toxicities, to evaluate the pharmacokinetics of single-dose intraventricular 454A12-rRA, and to detect antitumor activity.

METHODS

We performed a pilot study of intraventricular therapy with the immunotoxin 454A12-rRA in eight patients with leptomeningeal spread of systemic neoplasia. The immunotoxin 454A12-rRA is a conjugate of a monoclonal antibody against the human transferrin receptor and recombinant ricin A chain, the enzymatically active subunit of the protein toxin ricin. Patients were treated with single doses of 454A12-rRA ranging from 1.2 to 1200 micrograms.

RESULTS

The early phase half-life of 454A12-rRA in ventricular cerebrospinal fluid (CSF) averaged 44 +/- 21 minutes, and the late phase half-life averaged 237 +/- 86 minutes. The clearance of the immunotoxin was faster than the clearance of coinjected technetium-99m-diethylenetriamine penta-acetic acid, averaging approximately 2.4-fold greater. No 454A12-rRA degradation was detected by Western blot analysis of ventricular CSF for a period of 24 hours, and bioactivity was retained in CSF paralleling the concentration of immunotoxin. No acute or chronic drug toxicity was identified in patients who received less than or equal to 38 micrograms of 454A12-rRA by intraventricular injection. Doses more than or equal to 120 micrograms caused a CSF inflammatory response that was associated with transient headache, vomiting, and altered mental status. This acute syndrome was responsive to steroids and CSF drainage. No systemic toxicity was detected. In four of the eight patients, a greater than 50% reduction of tumor cell counts in the lumbar CSF occurred within 5 to 7 days after the intraventricular dose of 454A12-rRA; however, no patient had their CSF cleared of tumor, and clinical or magnetic resonance imaging evidence of tumor progression was demonstrated in seven of the eight patients after treatment.

CONCLUSION

Tumoricidal concentrations of the immunotoxin 454A12-rRA can be attained safely in the CSF of patients with leptomeningeal tumor spread.

Therapy of patients with T-cell lymphomas and leukemias using an anti-CD7 monoclonal antibody-ricin A chain immunotoxin

Anti-CD7-dgA, DA7, consists of deglycosylated ricin A chain coupled to a mouse monoclonal anti-human CD7 antibody. This study determined the maximally tolerated dose (MTD) of this immunotoxin administered as a one hour infusion over five days to 11 patients with T-cell lymphoma (>30% CD7+ malignant cells). The MTD was 0.2 mg/kg/day or 1 mg/kg/120 hours (maximal toxicity grade 3) with vascular leak syndrome (VLS) as dose-limiting toxicity (DLT). Predictors of severe VLS included age and absence of circulating lymphoma cells. Two partial responses and one minimal response were seen. Patients with minimal lymphoma burden or T-cell large granular lymphocyte (LGL) leukemia showed the best responses. The mean maximal serum concentration of immunotoxin at the MTD was 2.5 ug/ml. The mean alpha-phase half-life was 1.5 hours and the mean beta-phase half-life was 8 hours. Repeated dosing had minimal effects on either peak serum immunotoxin concentrations or serum half-lives. While human antimouse antibodies were observed, they were low in concentration (<55 ng/ml). Human anti-ricin antibody was elevated in one patient (190 ng/ml). VLS presented with hypoalbuminemia, dyspnea, pulmonary edema, aphasia, and peripheral edema and cleared over a two week period. Serum fibronectin levels were measured in three patients and were very low in one patient who developed VLS. No specific binding of DA7 immunotoxin was seen with vascular endothelium in various human tissues.

ATG plus corticosteroid therapy for acute graft-versus-host disease: predictors of response and survival

Innovative treatment strategies for acute graft-versus-host disease (aGVHD) have not replaced corticosteroids as the primary therapy. We retrospectively reviewed 74 patients who received equine antithymocyte globulin (ATG) in addition to corticosteroids as therapy for GVHD, 21 who received primary therapy and 53 who received ATG after progressing or failing to improve with corticosteroids alone. The groups were comparable in clinical characteristics and in timing and severity of GVHD. After primary therapy with ATG 67% of patients' GVHD symptoms were stable or improved by 28 days versus 56% in those receiving secondary ATG (p = 0.57). In univariate analysis the absence of multiple organ, GI, and liver aGVHD and a clinical stage score < or = 4 were predictive of a favorable response, while in a multivariate logistic regression model only a clinical stage score < or = 4 was independently associated with a favorable response (odds ratio 0.08, 95% CI 0.02-0.32, p = 0.003). ATG response rates and 6-month survival (38 vs. 40%, p = 0.89) were similar following primary and secondary ATG. Patients stable or improved 28 days after ATG therapy had a significantly better 6-month survival than those whose aGVHD had progressed (50 vs. 30%, p = 0.02). Further study is required to assess whether some initial presentations of aGVHD would predictably fail corticosteroid therapy and may thus suggest a role for ATG in the primary management of aGVHD. For this determination, formal prospective comparative trials are needed.

The chronic lymphocytic leukemia antigen (cCLLa) as immunotherapy target: pharmacokinetics and biodistribution of two divalent, ricin-based immunotoxins in xenografted athymic mice

The chronic lymphocytic leukemia (CLL) antigen (cCLLa) is potentially suitable for targeted immunotherapy given its restriction to clonal CLL cells and lack of expression by normal lymphocytes. In order to assess the pharmacokinetics and biodistribution of two potent anti-cCLLa immunotoxins (ITs) were examined in the mouse model. The IgG fraction of anti-cCLLa monoclonal antibody CLL2m was conjugated with 125I-labeled intact (RTA) or deglycosylated (dgA) ricin chain A, injected intravenously into athymic mice engrafted with cCLLa-expressing human tumors, and monitored over 120 hours. Blood concentrations of CLL2m/125I-RTA and CLL2m/125I-dgA were best fit to biexponential equations but the latter exhibited a lower alphaT1/2 and betaT1/2 (4.1 and 102 min vs 5.9 and 126 min), a smaller volume of distribution (5.1 g vs 9.7 g), and a lower blood clearance (2.2 g/hr vs 4.6 g/hr). Both ITs exhibited preferential tumor uptake that followed distinct kinetics: rising tumor uptake for 2 hrs post-injection (while tissue uptake decreased), reaching tumor/non-tumoral tissue uptake ratios up to 16.9; and slower dissociation rates of tumor- vs tissue-bound ITs (>45% vs <20% remaining tissue-bound 6 hrs post-injection, respectively). Non-specific liver uptake was not prominent for either IT. In vivo IT deconjugation reached 50% approximately 12 hours pos-injection. The pharmacokinetics and biodistribution data in the mouse model suggest that ricin-based anti-cCLLa ITs are suitable for use in human trials.

Development of a severe combined immunodeficiency (SCID) mouse model consisting of highly disseminated human B-cell leukemia/lymphoma, cure of the tumors by systemic administration of immunotoxin, and development/application of a clonotype-specific polymerase chain reaction-based assay

A new severe combined immunodeficiency (SCID) mouse model consisting of highly disseminated human B-cell leukemia/lymphoma was developed by i.v. inoculation of BALL-1a, an in vivo adapted malignant B-cell line. A 100% transplantability was achieved in nonpreconditioned SCID mice using various BALL-1a doses between 2.5 x 10(4) and 6 x 10(6) cells. Hind-leg paralysis preceded the death of the mice. Utility of the developed tumor model for the therapeutic studies was investigated by i.v. administration of an anti-B-cell monoclonal antibody SN7 (IgG1) and its conjugate with deglycosylated ricin A chain (dgRA). The therapy was initiated 2, 4, or 6 days after tumor inoculation using 4 x 24 microg of SN7-dgRA or 4 x 20 microg of SN7; the total dose (96 microg) of SN7-dgRA corresponded to 14% of the LD50 dose. SN7-dgRA showed a strong antitumor efficacy in all groups of treated mice. All of the day-2 group mice (n = 7) and six (66.7%) of the day-4 group mice (n = 9) survived healthily for as long as followed (240 days), whereas four (57.1%) of the day-6 group mice (n = 7) survived healthily for as long as followed (200 days). Unconjugated SN7 showed a significant antitumor efficacy but was less effective than SN7-dgRA. A PCR-based assay specific for the clonogenic BALL-1a tumor was developed and applied to determine tumors in various organs of BALL-1a-bearing SCID mice. The assay was highly sensitive in screening for trace quantities of residual tumors in various organs of SCID mice, and it could detect 1 malignant cell/2.5 x 10(5) tissue cells. The PCR-based assay was shown to be much more powerful than the conventional histological analysis in detecting residual tumors. Furthermore, we could estimate quantities of the detected tumors by the PCR-based assay. It is remarkable to find that all examined organs of some of the SN7-dgRA-treated mice were tumor-free as determined by the clonotype-specific PCR-based assay. The present results show the usefulness of the newly developed SCID mouse model, SN7-dgRA, and the clonotype-specific PCR-based molecular assay for the study of therapy of human B-cell leukemia/lymphoma.

Immunotoxin therapy of small-cell lung cancer: a phase I study of N901-blocked ricin

PURPOSE

Immunotoxins could improve outcome in small-cell lung cancer (SCLC) by targeting tumor cells that are resistant to chemotherapy and radiation. N901 is a murine monoclonal antibody that binds to the CD56 (neural cell adhesion molecule [NCAM]) antigen found on cells of neuroendocrine origin, including SCLC. N901-bR is an immunoconjugate of N901 antibody with blocked ricin (bR) as the cytotoxic effector moiety. N901-bR has more than 700-fold greater selectivity in vitro for killing the CD56+ SCLC cell line SW-2 than for an antigen-negative lymphoma cell line. Preclinical studies suggested the potential for clinically significant cardiac and neurologic toxicity. We present a phase I study of N901-bR in relapsed SCLC.

PATIENTS AND METHODS

Twenty-one patients (18 relapsed, three primary refractory) with SCLC were entered onto this study. Successive cohorts of at least three patients were treated at doses from 5 to 40 microg/kg/d for 7 days. The initial three cohorts received the first day's dose (one seventh of planned dose) as a bolus infusion before they began the continuous infusion on the second day to observe acute toxicity and determine bolus pharmacokinetics. Toxicity assessment included nerve-conduction studies (NCS) and radionuclide assessment of left ventricular ejection fraction (LVEF) before and after N901-bR administration to fully assess potential neurologic and cardiac toxicity.

RESULTS

The dose-limiting toxicity (DLT) of N901-bR given by 7-day continuous infusion is capillary leak syndrome, which occurred in two of three patients at the dose of 40 microg/kg (lean body weight [LBW])/d. Detectable serum drug levels equivalent to effective in vitro drug levels were achieved at the 20-, 30-, and 40-microg/kg(LBW)/d dose levels. Specific binding of the immunotoxin to tumor cells in bone marrow, liver, and lung was observed. Cardiac function remained normal in 15 of 16 patients. No patient developed clinically significant neuropathy. However, a trend was noted for amplitude decline in serial NCS of both sensory and motor neurons. One patient with refractory SCLC achieved a partial response.

CONCLUSION

N901-bR is an immunotoxin with potential clinical activity in SCLC. N901-bR is well tolerated when given by 7-day continuous infusion at the dose of 30 microg/kg(LBW)/d. Neurologic and cardiac toxicity were acceptable when given to patients with refractory SCLC. A second study to evaluate this agent after induction chemoradiotherapy in both limited- and extensive-stage disease was started following completion of this study.

Decreases in levels of serum fibronectin predict the severity of vascular leak syndrome in patients treated with ricin A chain-containing immunotoxins

The major dose-limiting adverse effect of ricin A chain-containing immunotoxin (IT) therapy is vascular leak syndrome (VLS). Since plasma fibronectin (Fn) plays a role in maintaining microcirculatory integrity and since the gradient between plasma and tissue Fn can be altered in various pathological situations, we determined whether the administration of IT-ricin A chain to patients resulted in changes in the levels of serum Fn and, if so, whether these changes correlated with the severity of VLS. We also measured the serum levels of tumor necrosis factor alpha (TNFalpha), a proinflammatory cytokine which has been implicated in tissue damage and in interleukin 2-mediated VLS. Our results indicate that the most severe manifestations of VLS were associated with the highest pretreatment levels of Fn, the largest decreases in Fn immediately after starting IT therapy, increases in the levels of serum TNFalpha, higher concentrations of circulating IT, and the lowest numbers of circulating tumor cells. These parameters should, therefore, be useful for predicting which patients will have severe VLS.

Combination immunotoxin treatment and chemotherapy in SCID mice with advanced, disseminated Daudi lymphoma

We describe the use of an immunotoxin (IT) cocktail (anti-CD22- and anti-CD19-ricin A chain) and any 1 of 3 chemotherapeutic drugs (doxorubicin, cytoxan or camptothecin) to treat advanced disseminated Daudi lymphoma in SCID mice (SCID/Daudi). In a previous report, we demonstrated that this regimen was curative when given the day following tumor cell inoculation. Here, we show that combination therapy in mice with advanced tumor significantly increased their survival, although it was not curative. Importantly, the outcome of therapy was dependent upon the temporal order in which IT and chemotherapy were administered. Thus, the best anti-tumor effect was achieved when an IT cocktail was given before or at the same time as chemotherapy. When the IT was given after chemotherapy, there was no additional therapeutic benefit. Our results confirm the rationale of using combination therapy in the treatment of advanced B-cell neoplasia and suggest that ITs should be administered prior to or during chemotherapy.