Dabrafenib plus trametinib in patients with BRAF V600E-mutant anaplastic thyroid cancer: updated analysis from the phase II ROAR basket study.

BACKGROUND: Combined therapy with dabrafenib plus trametinib was approved in several countries for treatment of BRAF V600E-mutant anaplastic thyroid cancer (ATC) based on an earlier interim analysis of 23 response-assessable patients in the ATC cohort of the phase II Rare Oncology Agnostic Research (ROAR) basket study. We report an updated analysis describing the efficacy and safety of dabrafenib plus trametinib in the full ROAR ATC cohort of 36 patients with ∼4 years of additional study follow-up. PATIENTS AND METHODS: ROAR (NCT02034110) is an open-label, nonrandomized, phase II basket study evaluating dabrafenib plus trametinib in BRAF V600E-mutant rare cancers. The ATC cohort comprised 36 patients with unresectable or metastatic ATC who received dabrafenib 150 mg twice daily plus trametinib 2 mg once daily orally until disease progression, unacceptable toxicity, or death. The primary endpoint was investigator-assessed overall response rate (ORR) per Response Evaluation Criteria in Solid Tumors version 1.1. Secondary endpoints were duration of response (DOR), progression-free survival (PFS), overall survival (OS), and safety. RESULTS: At data cutoff (14 September 2020), median follow-up was 11.1 months (range, 0.9-76.6 months). The investigator-assessed ORR was 56% (95% confidence interval, 38.1% to 72.1%), including three complete responses; the 12-month DOR rate was 50%. Median PFS and OS were 6.7 and 14.5 months, respectively. The respective 12-month PFS and OS rates were 43.2% and 51.7%, and the 24-month OS rate was 31.5%. No new safety signals were identified with additional follow-up, and adverse events were consistent with the established tolerability of dabrafenib plus trametinib. CONCLUSIONS: These updated results confirm the substantial clinical benefit and manageable toxicity of dabrafenib plus trametinib in BRAF V600E-mutant ATC. Dabrafenib plus trametinib notably improved long-term survival and represents a meaningful treatment option for this rare, aggressive cancer.

Interleukin-4 receptor-directed cytotoxin therapy of AIDS-associated Kaposi's sarcoma tumors in xenograft model.

The elusive and enigmatic origin of AIDS-associated Kaposi's sarcoma (AIDS-KS) makes it a complex tumor and therefore difficult to treat. Here we demonstrate that AIDS-KS cells express surface interleukin-4 (IL-4) receptors, and that IL-4 toxin (IL-4(38-37)-PE38KDEL) is specifically cytotoxic to these cells. Intratumoral, intraperitoneal and intravenous administration of IL-4 toxin in nude mice with established subcutaneous AIDS-KS tumors caused considerable anti-tumor activity in a dose-dependent manner, with highest dose producing durable complete responses. Metabolic changes, including cachexia and lymphopenia, induced by KS tumors were prevented by IL-4 toxin treatment. This report establishes IL-4(38-37)-PE38KDEL as an experimental therapeutic agent for the treatment of AIDS-KS.

Synaptic integration mediated by striatal cholinergic interneurons in basal ganglia function.

The physiological role of striatal cholinergic interneurons was investigated with immunotoxin-mediated cell targeting (IMCT). Unilateral cholinergic cell ablation caused an acute abnormal turning behavior. These mice showed gradual recovery but displayed abnormal turning by both excess stimulation and inhibition of dopamine actions. In the acute phase, basal ganglia function was shifted to a hyperactive state by stimulation and suppression of striatonigral and striatopallidal neurons, respectively. D1 and D2 dopamine receptors were then down-regulated, relieving dopamine-predominant synaptic perturbation but leaving a defect in controlling dopamine responses. The acetylcholine-dopamine interaction is concertedly and adaptively regulated for basal ganglia synaptic integration.

Elevation of soluble CD307 (IRTA2/FcRH5) protein in the blood and expression on malignant cells of patients with multiple myeloma, chronic lymphocytic leukemia, and mantle cell lymphoma.

CD307 is a differentiation antigen expressed in B-lineage cells. One soluble and two membrane-bound forms have been predicted and an enzyme-linked immunosorbent assay (ELISA) for soluble CD307 established. Our goal was to determine if CD307 is expressed on the surface of cells from patients with multiple myeloma (MM), chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL) and other B-cell malignancies and if soluble CD307 levels are elevated in the blood of patients with these B-cell malignancies. Cells and blood were collected from patients. Expression of CD307 was measured by flow cytometry and blood levels of soluble CD307 by ELISA. High soluble CD307 levels were detected in 21/43 (49%) of patients with MM, 36/46 (78%) with CLL and 9/24 (38%) with MCL. Soluble CD307 levels correlated with plasma cell percentages in bone marrow aspirates in MM and total white blood cells in CLL. CD307 on the cell membrane was detected by flow cytometry in 8/8 MM, 23/29 CLL and 4/5 MCL samples. Because CD307 is present on malignant cells from patients with MM, CLL and MCL, CD307 may be a useful therapeutic target for the treatment of these diseases.

Immunotoxins in cancer therapy.

Immunotoxins are composed of a protein toxin connected to a binding ligand such as an antibody or growth factor. These molecules bind to surface antigens (which internalize) and kill cells by catalytic inhibition of protein synthesis within the cell cytosol. Immunotoxins have recently been tested clinically in hematologic malignancies and solid tumors and have demonstrated potent clinical efficacy in patients with malignant diseases that are refractory to surgery, radiation therapy and chemotherapy - the traditional modalities of cancer treatment. This therapy is thus evolving into a separate modality of cancer treatment, capable of rationally targeting cells on the basis of surface markers. Efforts are underway to obviate impediments to clinical efficacy, including immunogenicity and toxicity to normal tissues. Immunotoxins are now being developed to new antigens for the treatment of cancer.

Essential role for Bim in mediating the apoptotic and antitumor activities of immunotoxins.

Protein synthesis is crucial for regulating cell homeostasis and, when unrestricted, it can lead to tumorigenesis. Immunotoxins derived from Pseudomonas exotoxin are antibody-toxin fusion proteins that inhibit protein synthesis of mammalian cells via ADP-ribosylation of the eukaryotic elongation factor-2. Here we investigate the role of the Bcl-2 family proteins in the response of cancer cells to immunotoxin challenge. Besides the well-known reduction of the prosurvival Bcl-2 family member, Mcl-1, following inhibition of protein synthesis, we show for the first time that immunotoxins also reduce the levels of selected proapoptotic BH-3-only proteins. Among these, only Bim protein levels correlated with the ability of immunotoxins to induce an apoptotic response. To support our findings, we verified that a Bim knockout completely abolished immunotoxin-mediated apoptosis. Further, mice bearing either wild-type or Bid knockout tumors responded to immunotoxin treatment with a decrease in growth kinetics, whereas mice engrafted with Bim knockout tumors showed no reduction in tumor size or prolongation of survival following immunotoxin treatment. From these results, we conclude that Bim expression is a major susceptibility factor for tumor cell death and, as such, constitutes a potential biomarker that could be evaluated before immunotoxin treatment. In support of this hypothesis, clinically, we analyzed patient cells before immunotoxin treatment and report that samples of hairy cell leukemia with high levels of Bim protein responded with a greater decrease in leukemic cell count compared with those samples expressing a low level of Bim.

SV40 Pseudovirion gene delivery of a toxin to treat human adenocarcinomas in mice.

SV40 vectors packaged in vitro (pseudovirions) are an efficient delivery system for plasmids up to 17.7 kb, with or without SV40 sequences. A truncated Pseudomonas exotoxin gene (PE38) was delivered into various human cells (HeLa, KB-3-1, human lymphoblastoids, and erythroleukemia cells), in vitro using pseudovirions. The number of viable cells was reduced significantly in the PE38-transduced cells. Human KB adenocarcinomas growing in mice were treated with intratumoral injection of PE38 packaged in vitro, and tumor size decreased significantly. Intraperitoneal treatments were as effective in reducing tumor size as intratumoral treatments. To check the viability of mock- or PE38-treated mice, every 4 days they were weighed, their blood was tested, and various tissues were screened for pathology. All parameters showed that the in vitro-packaged vectors, injected into tumors or intraperitoneally, caused no abnormalities in mice. The combined treatment of doxorubicin with in vitro-packaged PE38 reduced tumor size slightly more than each of the treatments separately. However, the combined treatment did not cause the weight loss seen with doxorubicin alone. These results indicate that SV40 in vitro packaging is an effective system for cancer gene delivery using two different routes of injection and in combination with chemotherapy.

Accumulation of a recombinant immunotoxin in a tumor in vivo: fewer than 1000 molecules per cell are sufficient for complete responses.

Recombinant immunotoxins have been shown to cure human tumor xenografts in mice, but their biodistribution to both tumors and normal organs has not been reported. Anti-Tac(Fv)-PE38 is a single-chain recombinant immunotoxin composed of the variable heavy and light domains of the anti-Tac monoclonal antibody that reacts with the primate interleukin 2 (IL2) receptor alpha subunit (IL2R alpha or CD25) fused to a truncated form of Pseudomonas exotoxin (PE). 125I-labeled anti-Tac(Fv)-PE38 was given i.v. to immunodeficient mice each bearing two A431 tumors, one that expresses IL2R alpha (ATAC-4) and one that does not (A431, parental). A single i.v. dose of 4 microg/mouse caused complete regression of the IL2R alpha + tumor. At 6 h, over 6% of the injected dose/g was found in the ATAC-4 tumor, and 2% was in the A431 tumor. Uptake in the ATAC-4 tumor was higher than in any other tissue. Sections of tumor examined by autoradiography indicated that anti-Tac(Fv)-PE38 was distributed throughout the entire tumor, with some portions having higher uptake than others. By subtracting uptake in tumors without receptor (A431) from uptake in receptor-containing tumors (ATAC-4), we calculated that at least 400 molecules/cell specifically bound to IL2R alpha-positive tumor cells at 90 min and 750 molecules/cell bound at 360 min. This is similar to the 400-870 molecules/cell required for >99.9% killing of ATAC-4 cells growing as a monolayer. The results show that solid tumors in mice can be eradicated like cells in tissue culture, and that delivery of less than 1000 molecules/cell is sufficient to cause complete tumor regressions.

Increased antitumor activity of a circularly permuted interleukin 4-toxin in mice with interleukin 4 receptor-bearing human carcinoma.

We reported previously that circularly permuted interleukin-4 (IL4), composed of amino acids 38-129 of IL4 connected by a linker peptide GGNGG to amino acids 1-37, is preferable to native IL4 for fusing to the amino terminus of truncated Pseudomonas exotoxin (PE) to make a recombinant toxin, because the new ligand-toxin junction results in improved IL4 receptor (IL4R)-binding (R. J. Kreitman et al., Proc. Natl. Acad. Sci. USA, 91: 6889-6893, 1994). We now report that the improved binding of circularly permuted IL4-toxin is associated with improved antitumor activity in tumor-bearing mice. For in vivo testing, we made an improved circularly permuted IL4-toxin, termed IL4(38-37)-PE38KDEL. It contains an N38D mutation at the amino terminus, allowing improved expression and large-scale production in Escherichia coli. It also contains the truncated toxin PE38KDEL, which is composed of amino acids 253-364 and 381-608 of PE, followed by KDEL. To evaluate antitumor activity, nude mice carrying s.c. tumors composed of IL4R-bearing human A431 epidermoid carcinoma cells were injected with recombinant toxins i.v. every other day for three doses. IL4(38-37)-PE38KDEL induced complete remissions in 80% of mice receiving 50 micrograms/kg x 3 and 100% of mice receiving 100 micrograms/kg x 3, while only 70% of mice receiving 200 micrograms/kg x 3 of the native IL4-toxin IL4-PE38KDEL obtained complete remission. Disease-free survival after obtaining complete remissions was higher in mice treated with IL4(38-37)-PE38KDEL 50 micrograms/Kg QOD x 3 than with IL4-PE38KDEL 200 micrograms/Kg QOD x 3 (P < 0.03). IL4(38-37)-PE38KDEL and IL4-PE38KDEL exhibited similar toxicity and pharmacokinetics in the mice, indicating that the improved antitumor activity of the circularly permuted IL4-toxin was due to its improved binding to the IL4R on the target cells.

Rapid and specific uptake of anti-Tac disulfide-stabilized Fv by interleukin-2 receptor-bearing tumors.

The disulfide-stabilized Fv (dsFv) is a novel form of a variable-region fragment (Fv) of an antibody which is stabilized by an interchain disulfide bond. As a consequence, it is more stable than its Fv analogue. Anti-Tac(dsFv) is derived from anti-Tac(IgG) which specifically binds to the p55 subunit of the interleukin-2 receptor (IL2R alpha). The receptor is found in large numbers on activated T cells and many T-cell leukemias. The biodistribution patterns of 125I-anti-Tac(dsFv) and 125I-anti-Tac(IgG) were determined in athymic nude mice bearing two s.c. tumors, one expressing a stably transfected plasmid encoding IL2R alpha (ATAC4) and one composed of parental untransfected A431 epidermoid carcinoma cells. Anti-Tac(dsFv), which has a molecular weight of 25,000, was specifically captured by the ATAC4 tumors but not by control A431 tumors. The antigen-specific tumors accumulated > 2% of the injected dose/g within 15-45 min after i.v. injection. The level of radioactivity in the ATAC4 tumors was maintained at > 1% of the injected dose/g for nearly 6 h, at which time the ATAC4 tumors contained 11-fold more 125I-anti-Tac(dsFv) than did the A431 tumors. Unbound 125I-anti-Tac(dsFv) was rapidly cleared from the blood with apparently biphasic pharmacokinetics (alpha t 1/2 = < 10 min; beta t 1/2 = approximately 5.5 h). Initially, the bulk of the 125I-anti-Tac(dsFv) appeared in the kidneys. In contrast, 125I-anti-Tac(IgG) showed no tumor- or tissue-specific uptake over the 24-h time course of the experiments and remained primarily in the blood stream (blood clearance t 1/2 = approximately 12 h). This is the first report of the biodistribution of a dsFv fragment. Because of its rapid uptake by IL2 receptor-bearing tumors, short serum half-life, and increased stability, radiolabeled anti-Tac(dsFv) may be useful for the imaging and therapy of neoplasias expressing the IL2 receptor.

Antitumor effects of interleukin 6-Pseudomonas exotoxin chimeric molecules against the human hepatocellular carcinoma, PLC/PRF/5 in mice.

IL6-PE40 and IL6-PE664Glu are chimeric molecules composed of interleukin 6 (IL6) fused to a truncated form (PE40) or a full-length mutated form (PE664Glu) of Pseudomonas exotoxin. Both forms of IL6-Pseudomonas exotoxin are cytotoxic to IL6 receptor-bearing tumor cell types in culture. In this report, we show that both IL6-PE40 and IL6-PE664Glu have antitumor activity against the hepatocellular carcinoma PLC/PRF/5 implanted s.c. in nude mice. The PLC/PRF/5 tumor contains about 2300 IL6 receptors per cell. IL6-PE664Glu showed improved therapeutic efficacy when released continuously for 7 days by an osmotic pump planted i.p. than when administered by multiple daily i.p. injections. Both forms of IL6 toxin exhibited a schedule-dependent antitumor effect. These results demonstrate that IL6-Pseudomonas exotoxin can suppress the growth of cancer which overexpresses cell surface IL6 receptors.

Immunotoxins made with a recombinant form of Pseudomonas exotoxin A that do not require proteolysis for activity.

We used recombinant DNA technology to construct a mutant form of Pseudomonas exotoxin A (PE) called cysPE35 that contains amino acids 280-364 and 381-613 of PE. cysPE35 begins at the native PE proteolytic cleavage site and contains a single cysteine residue at position 287 that can be used to conjugate the toxin to monoclonal antibodies (MAbs). Unlike immunotoxins containing larger mutant forms of PE, such as PE40 or PE38, immunotoxins containing cysPE35 linked through a disulfide bond do not require proteolysis to generate a toxin fragment able to translocate to the cytosol. cysPE35 was conjugated to several MAbs and their activities were studied in vitro and in vivo. The concentration of toxin that inhibited protein synthesis as measured by a decrease in [3H]leucine incorporation of 50% of cysPE35 conjugated through a disulfide bond to the MAb HB21, which targets the human transferrin receptor, was 1 ng/ml on A431 cells. The MAb HB21 conjugated through a thioether bond to cysPE35 was much less active (concentration of toxin that inhibited protein synthesis as measured by a decrease in [3H]leucine incorporation of 50%, 200 ng/ml). An immunotoxin containing PE38 conjugated through either a disulfide or thioether bond to the MAb HB21 had a concentration of toxin that inhibited protein synthesis as measured by a decrease in [3H]leucine incorporation of 50% of 5 ng/ml, indicating that proteolysis of PE38 may be rate limiting in the action of these immunotoxins. Two other MAbs, LL2 and B3, were also conjugated through a disulfide bond to cysPE35. Both immunotoxins were also more active against cultured cells than conjugates using PE38 or PE40, and caused complete regression of human tumor xenografts growing in nude mice. In conclusion, we have constructed a mutant form of PE which must be coupled to MAbs through a disulfide bond to produce fully active immunotoxins that do not require proteolysis to generate a toxin fragment able to reach the cell cytosol.

Complete regression of established human glioblastoma tumor xenograft by interleukin-4 toxin therapy.

No curative therapy is available for malignant gliomas. We have discovered that human glioblastoma cells express high affinity interleukin-4 receptor (IL-4R), which is an attractive target for receptor-directed IL-4 toxin therapy. The IL-4 toxin, IL-4(38-37)-PE38KDEL, is a fusion protein containing translocation and enzymatic domains of Pseudomonas exotoxin and a circularly permuted human IL-4. The IL-4 toxin binds specifically to the IL-4R and is highly cytotoxic to glioblastoma cells, as determined by clonogenic and protein synthesis inhibition assays. Intratumoral administration of the IL-4 toxin given on alternate days for 3-4 doses into U251 glioblastoma flank tumors in nude mice, showed a complete remission of small (approximately 13 mm3) and large (approximately 60 mm3) tumors in all animals, without any evidence of toxicity. A significant antitumor activity was also observed when the IL-4 toxin was administered via i.p. and i.v. routes. These results demonstrate that the IL-4 toxin may be a new therapeutic drug for the treatment of human glioblastoma. Therefore, we have begun a Phase I clinical trial with IL-4(38-37)-PE38KDEL for treatment of human glioblastoma.

Pseudomonas exotoxin-based immunotoxins containing the antibody LL2 or LL2-Fab' induce regression of subcutaneous human B-cell lymphoma in mice.

We have produced immunotoxins using LL2, a monoclonal antibody which binds to human B-cell lymphomas and which, in a radioiodinated form, induced responses in lymphoma patients (D.M. Goldberg et al., J. Clin. Oncol., 9: 548-564, 1991). We have coupled LL2 to Lys-PE38KDEL, a derivative of Pseudomonas exotoxin (PE) which does not bind to the PE receptor. LL2-PE38KDEL was cytotoxic toward several Burkitt's lymphoma lines, with 50% inhibitory concentration values ranging from 2 to 6 ng/ml (10-30 pM). Another immunotoxin, LL2-Fab'-PE38KDEL, was produced by chemically coupling the Fab' fragment of LL2 to Lys-PE38KDEL. LL2-Fab'-PE38KDEL also was cytotoxic toward the Burkitt's cells, with a 50% inhibitory concentration of 1-2 ng/ml (13-24 PM). The antibody LL2 alone had no cytotoxicity toward the malignant cells, and excess LL2 prevented the cytotoxicity of LL2-PE38KDEL and LL2-Fab'-PE38KDEL. Control immunotoxins UPC-10-PE38KDEL and Mu-9-Fab'-PE38KDEL were not cytotoxic. LL2-PE38KDEL and LL2-Fab'-PE38KDEL bound to cells with 50% and 17% of the affinity of LL2, respectively. Both immunotoxins, but not UPC-10-PE38KDEL, prevented the development of CA-46 tumors in nude mice. LL2-PE38KDEL and LL2-Fab'-PE38KDEL, but not the control immunotoxins, led to complete regressions of measurable s.c. CA-46 tumors in nude mice, when given at 50% and 35% of the 50% lethal dose, respectively. LL2 alone significantly retarded the growth of CA-46 tumors but did not cause complete tumor regressions. Immunotoxins containing derivatives of Pseudomonas exotoxin can be targeted to human B-cell lymphoma and merit further study as potential therapeutic agents.

Preclinical development of a recombinant toxin containing circularly permuted interleukin 4 and truncated Pseudomonas exotoxin for therapy of malignant astrocytoma.

Effective treatment is lacking for malignant glioblastoma/astrocytoma. We have identified interleukin-4 receptors (IL-4R) on human malignant astrocytoma. We demonstrate that 16 of 21 surgical samples of high-grade astrocytoma and glioblastoma but not normal brain tissues expressed IL-4R as assessed by reverse transcriptase PCR. We further demonstrate that human malignant astrocytoma cell lines express high-affinity IL-4R. Using a chimeric protein composed of circularly permuted IL-4 and a truncated form of Pseudomonas exotoxin A, we observed that this toxin IL4(38-37)-PE38KDEL) is highly cytotoxic to IL-4R-bearing glioblastoma cells. Compared with a previously reported IL4-PE chimeric protein (IL-PE4E), IL4(38-37)-PE38KDEL bound with higher affinity and was 3-30-fold more cytotoxic to glioblastoma cell lines. Upon intrathecal administration in monkeys, high cerebrospinal fluid IL4(38-37)-PE38KDEL levels were achieved using 2- and 6-microg/kg doses without any central nervous system or other abnormalities. IL4(38-37)-PE38KDEL levels were not detectable in the serum of any monkey studied. When IL4(38-37)-PE38KDEL was injected into the right frontal cortex of rats, localized necrosis was observed at 1000-ng/ml doses but not at < or = 100-ng/ml doses. We conclude that by localized administration, nontoxic levels of IL4(38-37)-PE38KDEL can be achieved, which may have significant cytotoxic activity against malignant astrocytoma.

Cytotoxicity of recombinant Fab and Fv immunotoxins on adult T-cell leukemia lymph node and blood cells in the presence of soluble interleukin-2 receptor.

Single-chain immunotoxins anti-Tac(Fv)-PE40 and anti-Tac(Fv)-PE40KDEL, composed of variable domains of the anti-Tac monoclonal antibody and truncated forms of Pseudomonas exotoxin, have shown potent cytotoxic activity against malignant peripheral blood mononuclear cells (PBMCs) from adult T-cell leukemia (ATL) patients originating from the Caribbean. However, several clinically important issues have not previously been addressed. These include the potential of soluble interleukin 2 receptor in ATL patients to block immunotoxin effectiveness, the relative sensitivity of malignant lymph node cells (LNCs) versus PBMCs, the effect of an immunotoxin with a prolonged half-life, and finally whether ATL cells from patients in Japan have toxin sensitivity equal to those of the Caribbean patients. To resolve these questions, we studied 32 malignant PBMC and LNC samples from 30 ATL patients from Japan. PBMCs from 27 of 27 patients were very sensitive with 50% inhibition of protein synthesis achieved with 0.02-0.85 ng/ml (0.3-13 pM) of anti-Tac(Fv)-PE40KDEL or anti-Tac(Fv)-PE40. LNCs had sensitivity very similar to that of PBMCs in the five patients tested. The fully recombinant immunotoxin, anti-Tac(Fab)-PE40, which has 8-10 times the t1/2 alpha and beta compared to the Fv-immunotoxins, was also very cytotoxic toward cells from 27 of 27 patients tested with 50% inhibition of protein synthesis of 0.08-25 ng/ml. It was found that purified soluble interleukin 2 receptor added to the cytotoxicity assay decreased the cytotoxic activity of anti-Tac(Fv)-PE40KDEL or anti-Tac(Fab)-PE40, but that 1 x 10(4) units/ml or less had minimal competitive effects. It was found that ATL patients who have responded even incompletely to conventional chemotherapy have soluble interleukin 2 receptor levels lower than this at posttreatment. We conclude that recombinant immunotoxins containing anti-Tac(Fv) are effective against Japanese ATL PBMCs or LNCs and might be most effective if used in vivo after conventional chemotherapy. If it is found in humans that the effectiveness of single-chain recombinant toxins is limited by short half-life, anti-Tac(Fab)-PE40 should be considered as an alternative agent.

Lowering the isoelectric point of the Fv portion of recombinant immunotoxins leads to decreased nonspecific animal toxicity without affecting antitumor activity.

Recombinant immunotoxins are genetically engineered proteins in which the Fv portion of an antibody is fused to a toxin. Our laboratory uses a 38-kDa form of Pseudomonas exotoxin A termed PE38 for this purpose. Clinical studies with immunotoxins targeting CD25 and CD22 have shown that dose-limiting side effects are attributable to liver damage and other inflammatory toxicities. We recently showed that mutating exposed surface neutral residues to acidic residues in the framework region of the Fv portion of an immunotoxin targeting CD25 [anti-Tac(scFv)-PE38] lowered its isoelectric point (pI) and decreased its toxicity in mice without impairing its cytotoxic or antitumor activities. We have now extended these studies and made mutations that change basic residues to neutral or acidic residues. Initially the pI of the mutant Fv (M1) of anti-Tac(scFv)-PE38 was decreased further. Subsequently, mutations were made in two other immunotoxins, SS1(dsFv)-PE38 targeting ovarian cancer and B3(dsFv)-PE38 targeting colon and breast cancers. We have found that all these mutant molecules fully retained specific target cell cytotoxicity and antitumor activity but were considerably less toxic to mice. Therefore, lowering the pI of the Fv may be a general approach to diminish the nonspecific toxicity of recombinant immunotoxins and other Fv fusion proteins without losing antitumor activity.

Phase I trial of recombinant immunotoxin anti-Tac(Fv)-PE38 (LMB-2) in patients with hematologic malignancies.

PURPOSE: To evaluate the toxicity, pharmacokinetics, immunogenicity, and antitumor activity of anti-Tac(Fv)-PE38 (LMB-2), an anti-CD25 recombinant immunotoxin that contains an antibody Fv fragment fused to truncated Pseudomonas exotoxin. PATIENTS AND METHODS: Patients with CD25(+) hematologic malignancies for whom standard and salvage therapies failed were treated with LMB-2 at dose levels that ranged from 2 to 63 microg/kg administered intravenously over 30 minutes on alternate days for three doses (QOD x 3). RESULTS: LMB-2 was administered to 35 patients for a total of 59 cycles. Dose-limiting toxicity at the 63 microg/kg level was reversible and included transaminase elevations in one patient and diarrhea and cardiomyopathy in another. LMB-2 was well tolerated in nine patients at the maximum-tolerated dose (40 microg/kg QOD x 3); toxicity was transient and most commonly included transaminase elevations (eight patients) and fever (seven patients). Only six of 35 patients developed significant neutralizing antibodies after the first cycle. The median half-life was 4 hours. One hairy cell leukemia (HCL) patient achieved a complete remission, which is ongoing at 20 months. Seven partial responses were observed in cutaneous T-cell lymphoma (one patient), HCL (three patients), chronic lymphocytic leukemia (one patient), Hodgkin's disease (one patient), and adult T-cell leukemia (one patient). Responding patients had 2 to 5 log reductions of circulating malignant cells, improvement in skin lesions, and regression of lymphomatous masses and splenomegaly. All four patients with HCL responded to treatment. CONCLUSION: LMB-2 has clinical activity in CD25(+) hematologic malignancies and is relatively nonimmunogenic. It is the first recombinant immunotoxin to induce major responses in cancer. LMB-2 and similar agents that target other cancer antigens merit further clinical development.

Pediatric acute myelogenous leukemia cells express IL-6 receptors and are sensitive to a recombinant IL6-Pseudomonas exotoxin.

We have studied IL-6 receptor (IL-6R) expression on AML cells from 15 pediatric patients by immunocytochemistry/flow cytometry, reverse-transcription polymerase chain reaction, and Scatchard analysis. High-affinity IL-6R were detected on leukemic cells from 12 (80%) patients. Binding sites per cell ranged from 140 to 3580 (median 920; mean 1240), with dissociation constants of 0.26 to 0.71 nM. We therefore assessed the in vitro sensitivity of IL-6R+ AML cells to treatment with a recombinant IL6-Pseudomonas exotoxin fusion protein (IL6-PE4E), using the XTT cytotoxicity assay. Leukemic cells from eight patients had ID50 values (concentration of IL6-PE4E producing a 50% decrease in cell viability) of <1000 ng/ml (median, 87 ng/ml; mean, 262 ng/ml). Sensitivity to IL6-PE4E correlated significantly with receptor number. Normal bone marrow mononuclear cells had undetectable IL6-R expression (<20 receptors/cell) and were relatively resistant to IL6-PE4E. To test the efficacy of IL6-PE4E for ex vivo purging in an autologous stem cell transplantation setting, we incubated primary IL-6R+ AML cells with 10(3) ng/ml IL6-PE4E for 24 h, followed by inoculation into SCID mice. Mice receiving treated cells showed no leukemic engraftment, while all mice receiving untreated or control-treated cells developed leukemia with a median presymptomatic interval of 55 days. In recipients of IL6-PE4E treated cells, no evidence of occult leukemia was detected by PCR analysis of blood and bone marrow cells at 185 days postinoculation. These data suggest that IL-6R are expressed on leukemic cells from a substantial percentage of pediatric AML patients. Furthermore, leukemic cells expressing high numbers of IL6-R may be sensitive to IL6-PE4E in an ex vivo purging protocol.

DT388-GM-CSF, a novel fusion toxin consisting of a truncated diphtheria toxin fused to human granulocyte-macrophage colony-stimulating factor, prolongs host survival in a SCID mouse model of acute myeloid leukemia.

Despite significant advances in the treatment of acute myeloid leukemia (AML), the majority of patients will succumb to drug-resistant AML. To overcome this resistance, we have developed a novel fusion toxin consisting of the catalytic and translocation subunits of diphtheria toxin (DT388) linked to human granulocyte-macrophage colony-stimulating factor (GM-CSF). In vitro, DT388-GM-CSF demonstrated significant activity against numerous AML cell lines and fresh AML blasts. To determine its in vivo efficacy, we developed an in vivo model of human AML in severe combined immunodeficiency (SCID) mice injected intravenously with 1 x 10(7) HL-60 cells (AML-M2 cell line). The SCID mice developed abdominal masses, infiltration of the liver and bone marrow, and peripheral blasts with a median survival of 42.5 days. We tested DT388-GM-CSF, ara-C, human GM-CSF, and DAB389IL-2, which were injected intraperitoneally on days 2-6 in this model. DT3-GM-CSF significantly improved survival of the SCID mice over Ara-C, DAB389IL-2, or control (P < 0.001). DT388-GM-CSF-treated mice who developed leukemia exhibited no difference in the number of GM-CSF receptors (P = 0.39), ligand affinity (P = 0.77), or sensitivity (P = 0.56) to DT388-GM-CSF as compared to the controls. Frank leukemia in DT388-GM-CSF-treated mice may be due to incomplete penetration of drug into tissues rather than cellular resistance. DT388-GM-CSF is an active therapeutic agent in our SCID mouse model of AML with a unique mechanism of action and differing toxicities than current cytotoxic agents.