Immunotoxins: a clinical review of their use in the treatment of malignancies.

Immunotoxins are a new class of antitumor agents consisting of tumor-selective ligands (generally monoclonal antibodies [MoAbs]) linked to highly toxic protein molecules that have been modified to remove their normal tissue-binding domains. These immuno-conjugates combine the potency of the parent toxin with the specificity of the attached ligand. Toxins used in the construction of immunotoxins belong to a group of peptides that catalytically inhibit the elongation step of protein synthesis, and include ricin, abrin, pokeweed antiviral protein, gelonin, Pseudomonas exotoxin A, diptheria toxin, and alpha-sarcin. To synthesize immunotoxins, the normal cell-binding function must be removed by chemical cleavage or modification, or in the case of toxins that have been cloned, genetic engineering used to delete amino acids critical to cell binding. Covalent linkage of toxin to ligand generally involves a disulfide or thioether bond, though recently, recombinant toxin molecules with ligands that are genetically engineered into the protein have been made. The most successful clinical application of immunotoxins has been in the depletion of T cells from allogeneic bone marrow grafts to prevent graft-versus-host disease (GVHD). Clinical trials have been conducted using immunotoxins for the systemic treatment of chronic lymphocytic leukemia (CLL), GVHD, and selected solid tumors. With the possible exception of GVHD, responses have been limited. Obstacles have included rapid systemic clearance, poor delivery to extravascular tumor deposits, and humoral immune responses to the immunotoxin. Research to overcome these problems is in progress and should lead to a better definition of the role of immunotoxins in the therapy of malignancies.

An anti-CD5 immunotoxin for chronic lymphocytic leukemia: enhancement of cytotoxicity with human serum albumin-monensin.

Five patients with B-cell chronic lymphocytic leukemia (B-CLL) were treated with 6 courses of the anti-CD5 immunotoxin T101-ricin A chain (T101-RTA). Each course consisted of 8 bi-weekly infusions of T101-RTA (7 or 14 mg/m2). The immunotoxin was well tolerated in all cases with no major toxicities. Though saturation of circulating leukemic cell-associated target antigen was demonstrated by FACS analysis in all patients, no intact immunotoxin was detected in bone-marrow or lymph-node aspirates. Pharmacokinetic studies revealed rapid clearance of T101-RTA, with a half-life of 43 min. None of the patients developed detectable titers of antibody against either T101 murine antibody or ricin A chain. Clinical response was limited to a rapid and transient fall in WBC count lasting less than 24 hr, most likely secondary to the antibody portion of the conjugate. In vitro, fresh B-CLL cells were resistant to T101-RTA at concentrations up to 10(-8)M, while fresh malignant T-cells with a 10-fold increase in expression of CD5 antigen were sensitive. In the presence of the enhancing agent human serum albumin-monensin, fresh B-CLL cells were sensitive to T101-RTA, with an ID50 more than 2 logs below the maximal concentration of immunotoxin achieved in vivo. We conclude that T101-RTA is a potentially useful agent in the treatment of T-cell leukemias. In the presence of HSA-monensin, this spectrum of activity may be extended to B-CLL.

An immunotoxin for the treatment of T-acute lymphoblastic leukemic meningitis: studies in rhesus monkeys.

Monoclonal antibody WT1 (anti-CD7), conjugated to ricin A chain, was administered intrathecally to rhesus monkeys to test its suitability for use in the therapy of leukemic meningitis. The WT1-SMPT-dgRTA conjugate was cytotoxic to CEM (T-lymphoblastic leukemia) cells in vitro with an ID50 of 53 pM. Immunoperoxidase testing showed no binding of WT1 to normal human tissues other than lymph nodes. Thirteen animals received one or more intrathecal 60-micrograms doses of WT1-SMPT-dgRTA. All monkeys receiving repeated doses developed a cerebrospinal fluid (CSF) pleocytosis (primarily eosinophils), which was generally resolving by 3-4 weeks after therapy. Pharmacokinetic studies showed a half-life of 99 min, consistent with CSF clearance by bulk flow. Peak CSF immunotoxin concentrations exceeded the ID50 for CEM cells by more than 2 log units and a concentration exceeding the ID50 was maintained for as long as 24 h. All eight monkeys receiving repeated doses of immunotoxin developed serum antibodies against both WT1 and ricin A chain. In six of these monkeys antibodies were also present in the CSF. Both anti-WT1 and anti-(ricin A chain) antibodies were able to inhibit in vitro cytotoxicity of the immunotoxin for CEM cells; however, only anti-WT1 antibodies could block immunotoxin binding to the cell surface. No monkey developed anti-immunotoxin antibodies fewer than 7 days after the initiation of therapy, suggesting that repeated doses could be administered for up to 1 week without inhibition of clinical activity.

A phase I study of T101-ricin A chain immunotoxin in refractory chronic lymphocytic leukemia.

Four patients with chronic lymphocytic leukemia refractory to alkylating agents were treated with T101-ricin A chain immunotoxin (T101-RTA) as part of a phase I study. Over a 4-week period, each patient received eight intravenous infusions of 3 mg/m2 T101-RTA over 1 h. All infusions were well tolerated. Patients had mild fevers but no other systemic toxicities. In vivo binding of T101-RTA was detected by FACS analysis using anti-mouse Ig-FITC or anti-A chain-FITC antibody conjugates. Saturation of circulating leukemic cell-associated target antigen was achieved in three of the patients. Available CD5 sites per cell dropped precipitously at the completion of infusions in all four patients, returning to within 30% of baseline by 24 h. Pharmacokinetic studies revealed rapid clearance of T101-RTA, with wide interpatient variability in peak serum levels (the highest levels in those patients who saturated their circulating CD5 antigen with immunotoxin). Although no patient developed detectable levels of antimurine antibodies, one patient did have a rising titer of anti-ricin A chain antibody associated with declining peak serum levels of immunotoxin. All patients had a rapid fall in WBC count of less than 24-h duration after each T101-RTA infusion, most likely secondary to the antibody portion of immunotoxin. No sustained benefit could be demonstrated in any patient, possibly because in the absence of an enhancing agent the leukemic cells of all four patients were resistent to T101-RTA at concentrations up to 2,000 ng/ml in vitro.

Sarcomas in the elderly.

Though rare, osteosarcomas and soft tissue sarcomas do occur in the geriatric population. When possible, surgical excision is the treatment of choice, with radiation therapy and chemotherapy having largely an adjuvant role. Classic Kaposi's sarcoma is a chronically progressive and ultimately fatal disease of the elderly which can be managed effectively with both radiation therapy and chemotherapy.

Humoral immune response to a ricin A chain immunotoxin in patients with metastatic melanoma.

Immunotoxins, hybrid molecules consisting of a monoclonal antibody linked to a polypeptide toxin have shown anti-tumor activity in both animal models and early clinical trials. However, their potential value in the treatment of human cancer may be limited by the development of host antibodies against the conjugate. Such antibodies could potentially alter immunotoxin pharmacokinetics and pharmacodynamics as well as precipitate serum sickness or anaphylaxis. Using a radioimmunoassay we have measured serial anti-ricin A chain (anti-RTA) and anti-murine immunoglobulin (anti-MIG) titers in 22 patients who received the anti-melanoma immunotoxin XomaZymeR-Mel. Significant titers of anti-RTA and/or anti-MIG were detected in 17 of 21 evaluable patients. Of the four patients not developing antibodies, two were likely immunosuppressed secondary to dexamethasone, and CCNU and dexamethasone respectively. Both patients who received immunotoxin at a time when they had detectable anti-immunotoxin antibodies experienced infusion reactions consistent with immune mediated allergic responses. There was a decrease in peak immunotoxin level in the one patient who had serum immunotoxin levels measured at a time when anti-RTA was present. Strategies to suppress the human immune response to immunotoxins are required before repetitive courses of immunotoxin of this design may be administered.

Isolated anterior chamber relapse in acute monoblastic leukemia.

A case of acute monoblastic leukemia relapsing in the anterior chamber of the eye is described. Despite rigorous evaluation, no systemic or central nervous system involvement was detected. All previously reported cases of anterior chamber infiltrates in AML have had coexistent systemic or central nervous system disease.

Effect of intravenous immunoglobulin on E. coli opsonization in multiple myeloma.

Infections are a major source of morbidity in multiple myeloma; E. coli is now the leading pathogen. Intravenous IgG may be a modality which could ameliorate the opsinopathy of multiple myeloma. We infused 6 patients with multiple myeloma with IgG (100 mg/kg) and compared E. coli opsonophagocytosis pre- and post-IgG infusions. Log phase, broth grown E. coli K12 (5 X 10(6)/ml) and normal, dextran-sedimented human neutrophils (5 X 10(6)/ml) were combined in 10% heat inactivated, pre- or post-IgG infusion multiple myeloma serum with 5% agammaglobulinemic serum as a complement source and incubated at 37 degrees C for 30 min. Phagocytosis was quantified as percentage survival of the inoculum. Control survival in heat inactivated normal serum + complement + neutrophils was 1.7 +/- 0.8%. Pre- and post-IgG infusion sera were equally abnormal opsonin sources with 14.3 +/- 6.5% and 17.5 +/- 3.0% survivals. Individually, patients with poor opsonophagocytosis improved with IgG (e.g., pre = 45.2 +/- 3.7%; post = 29.5 +/- 2.3% survival), whereas patients with good opsonophagocytosis showed a deleterious effect (e.g., pre = 2.3 +/- 0.9%; post 23.3 +/- 6.3% survival). To explain these data, we measured deposited IgM and IgG on E. coli by pre- and post-IgG infusion sera in a fluorescence immunoassay. Pre- and post-IgG infusion sera had equally depressed IgM deposition (pre = 13.7 +/- 2.1%; post = 14.5 +/- 2.6% of normal serum), and also equal IgG deposition (pre = 96.8 +/- 6.5%; post = 94.6 +/- 4.8% of normal serum).(ABSTRACT TRUNCATED AT 250 WORDS)