Toxicology and pharmacokinetics of DTGM, a fusion toxin consisting of a truncated diphtheria toxin (DT388) linked to human granulocyte-macrophage colony-stimulating factor, in cynomolgus monkeys

Comparative Risk of Human Injury/Exposure While Collecting Blood from Sedated and Unsedated Nonhuman Primates

Collection of blood samples for research or clinical evaluation is one of the most common procedures performed in non-human primates. Several possible methods can be used to obtain samples. In the early days of primate research, manual or physical restraint was used, which was stressful for the animal and risky for the human. As the field developed, chemical immobilization with ketamine or other anesthetics has become the most commonly used method. More recently, training using positive reinforcement has allowed collection of blood samples from unsedated primates that are unrestrained or minimally restrained. Elimination of anesthesia reduces risks to the animal. We wanted to determine whether the risks to humans were different between the sedated or unsedated blood collection. We evaluated injury and near-miss reports in conjunction with blood collection data from 2009 to 2019 at the Washington National Primate Research Center, which houses macaques (M. nemestrina, M. mulatta, and M. fasicularis) and squirrel monkeys (S. sciureus), and has housed baboons (Papio sp.) in the past. Injuries associated with sedated blood collection included those occurring during the sedation procedure and recovery as well as those directly associated with blood collection. Injuries associated with unsedated blood collection included those which occurred both during animal training and during blood collection. Overall, 22 human injury exposures and 5 near misses were associated with 73,626 blood collection procedures. Based on these numbers, 0.026% of sedated blood collections and 0.116% of unsedated blood collections were associated with exposure incidents. In conclusion, our data indicate a very low risk of exposure associated with blood collection. In this data set, the risk was statistically higher for unsedated animals, but the low number of incidents and the variability in the methods of blood collection make the general applicability of this finding questionable.

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

Transcriptional Profiling of Non-Human Primate Lymphoid Organ Responses to Total-Body Irradiation

The global threat of exposure to radiation and its subsequent outcomes require the development of effective strategies to mitigate immune cell injury. In this study we explored transcriptional and immunophenotypic characteristics of lymphoid organs of a non-human primate model after total-body irradiation (TBI). Fifteen middle-aged adult, ovariectomized, female cynomolgus macaques received a single dose of 0, 2 or 5 Gy gamma radiation. Thymus, spleen and lymph node from three controls and 2 Gy (n = 2) and 5 Gy (n = 2) exposed animals were assessed for molecular responses to TBI through microarray-based transcriptional profiling at day 5 postirradiation, and cellular changes through immunohistochemical (IHC) characterization of markers for B and T lymphocytes and macrophages across all 15 animals at time points up to 6 months postirradiation. Irradiated macaques developed acute hematopoietic syndrome. Analysis of array data at day 5 postirradiation identified transcripts with ≥2-fold difference from control and a false discovery rate (FDR) of Padj < 0.05 in lymph node (n = 666), spleen (n = 493) and thymus (n=3,014). Increasing stringency of the FDR to P < 0.001 reduced the number of genes to 71 for spleen and 379 for thymus. IHC and gene expression data demonstrated that irradiated animals had reduced numbers of T and B lymphocytes along with relative elevations of macrophages. Transcriptional analysis revealed unique patterns in primary and secondary lymphoid organs of cynomolgus macaques. Among the many differentially regulated transcripts, upregulation of noncoding RNAs [MIR34A for spleen and thymus and NEAT1 (NCRNA00084) for thymus] showed potential as biomarkers of radiation injury and targets for mitigating the effects of radiation-induced hematopoietic syndrome-impaired lymphoid reconstitution.

Late Effects of Total-Body Gamma Irradiation on Cardiac Structure and Function in Male Rhesus Macaques

Heart disease is an increasingly recognized, serious late effect of radiation exposure, most notably among breast cancer and Hodgkin's disease survivors, as well as the Hiroshima and Nagasaki atomic bomb survivors. The purpose of this study was to evaluate the late effects of total-body irradiation (TBI) on cardiac morphology, function and selected circulating biomarkers in a well-established nonhuman primate model. For this study we used male rhesus macaques that were exposed to a single total-body dose of ionizing gamma radiation (6.5-8.4 Gy) 5.6-9.7 years earlier at ages ranging from ∼3-10 years old and a cohort of nonirradiated controls. Transthoracic echocardiography was performed annually for 3 years on 20 irradiated and 11 control animals. Myocardium was examined grossly and histologically, and myocardial fibrosis/collagen was assessed microscopically and by morphometric analysis of Masson's trichrome-stained sections. Serum/plasma from 27 irradiated and 13 control animals was evaluated for circulating biomarkers of cardiac damage [N-terminal pro B-type natriuretic protein (nt-proBNP) and troponin-I], inflammation (CRP, IL-6, MCP-1, sICAM) and microbial translocation [LPS-binding protein (LBP) and sCD14]. A higher prevalence of histological myocardial fibrosis was observed in the hearts obtained from the irradiated animals (9/14) relative to controls (0/3) (P = 0.04, χ(2)). Echocardiographically determined left ventricular end diastolic and systolic diameters were significantly smaller in irradiated animals (repeated measures ANOVA, P < 0.001 and P < 0.008, respectively). Histomorphometric analysis of trichrome-stained sections of heart tissue demonstrated ∼14.9 ± 1.4% (mean ± SEM) of myocardial area staining for collagen in irradiated animals compared to 9.1 ± 0.9 % in control animals. Circulating levels of MCP-1 and LBP were significantly higher in irradiated animals (P < 0.05). A high incidence of diabetes in the irradiated animals was associated with higher plasma triglyceride and lower HDLc but did not appear to be associated with cardiovascular phenotypes. These results demonstrate that single total-body doses of 6.5-8.4 Gy produced long-term effects including a high incidence of myocardial fibrosis, reduced left ventricular diameter and elevated systemic inflammation. Additional prospective studies are required to define the time course and mechanisms underlying radiation-induced heart disease in this model.

Molecular and cellular profiling of acute responses to total body radiation exposure in ovariectomized female cynomolgus macaques

PURPOSE

The threat of radiation exposure requires a mechanistic understanding of radiation-induced immune injury and recovery. The study objective was to evaluate responses to ionizing radiation in ovariectomized (surgically post-menopausal) female cynomolgus macaques.

MATERIALS AND METHODS

Animals received a single total-body irradiation (TBI) exposure at doses of 0, 2 or 5 Gy with scheduled necropsies at 5 days, 8 weeks and 24 weeks post-exposure. Blood and lymphoid tissues were evaluated for morphologic, cellular, and molecular responses.

RESULTS

Irradiated animals developed symptoms of acute hematopoietic syndrome, and reductions in thymus weight, thymopoiesis, and bone marrow cellularity. Acute, transient increases in plasma monocyte chemoattractant protein 1 (MCP-1) were observed in 5 Gy animals along with dose-dependent alterations in messenger ribonucleic acid (mRNA) signatures in thymus, spleen, and lymph node. Expression of T cell markers was lower in thymus and spleen, while expression of macrophage marker CD68 (cluster of differentiation 68) was relatively elevated in lymphoid tissues from irradiated animals.

CONCLUSIONS

Ovariectomized female macaques exposed to moderate doses of radiation experienced increased morbidity, including acute, dose-dependent alterations in systemic and tissue-specific biomarkers, and increased macrophage/T cell ratios. The effects on mortality exceeded expectations based on previous studies in males, warranting further investigation.

Dopamine inhibits pulmonary edema through the VEGF-VEGFR2 axis in a murine model of acute lung injury

The neurotransmitter dopamine and its dopamine receptor D2 (D2DR) agonists are known to inhibit vascular permeability factor/vascular endothelial growth factor (VEGF)-mediated angiogenesis and vascular permeability. Lung injury is a clinical syndrome associated with increased microvascular permeability. However, the effects of dopamine on pulmonary edema, a phenomenon critical to the pathophysiology of both acute and chronic lung injuries, have yet to be established. Therefore, we sought to determine the potential therapeutic effects of dopamine in a murine model of lipopolysaccharide (LPS)-induced acute lung injury (ALI). Compared with sham-treated controls, pretreatment with dopamine (50 mg/kg body wt) ameliorated LPS-mediated edema formation and lowered myeloperoxidase activity, a measure of neutrophil infiltration. Moreover, dopamine significantly increased survival rates of LPS-treated mice, from 0-75%. Mechanistically, we found that dopamine acts through the VEGF-VEGFR2 axis to reduce pulmonary edema, as dopamine pretreatment in LPS-treated mice resulted in decreased serum VEGF, VEGFR2 phosphorylation, and endothelial nitric oxide synthase phosphorylation. We used D2DR knockout mice to confirm that dopamine acts through D2DR to block vascular permeability in our lung injury model. As expected, a D2DR agonist failed to reduce pulmonary edema in D2DR(-/-) mice. Taken together, our results suggest that dopamine acts through D2DR to inhibit pulmonary edema-associated vascular permeability, which is mediated through VEGF-VEGFR2 signaling and conveys protective effects in an ALI model.

Sensitivity of cancer cells to truncated diphtheria toxin

BACKGROUND

Diphtheria toxin (DT) has been utilized as a prospective anti-cancer agent for the targeted delivery of cytotoxic therapy to otherwise untreatable neoplasia. DT is an extremely potent toxin for which the entry of a single molecule into a cell can be lethal. DT has been targeted to cancer cells by deleting the cell receptor-binding domain and combining the remaining catalytic portion with targeting proteins that selectively bind to the surface of cancer cells. It has been assumed that "receptorless" DT cannot bind to and kill cells. In the present study, we report that "receptorless" recombinant DT385 is in fact cytotoxic to a variety of cancer cell lines.

METHODS

In vitro cytotoxicity of DT385 was measured by cell proliferation, cell staining and apoptosis assays. For in vivo studies, the chick chorioallantoic membrane (CAM) system was used to evaluate the effect of DT385 on angiogenesis. The CAM and mouse model system was used to evaluate the effect of DT385 on HEp3 and Lewis lung carcinoma (LLC) tumor growth, respectively.

RESULTS

Of 18 human cancer cell lines tested, 15 were affected by DT385 with IC(50) ranging from 0.12-2.8 microM. Furthermore, high concentrations of DT385 failed to affect growth arrested cells. The cellular toxicity of DT385 was due to the inhibition of protein synthesis and induction of apoptosis. In vivo, DT385 diminished angiogenesis and decreased tumor growth in the CAM system, and inhibited the subcutaneous growth of LLC tumors in mice.

CONCLUSION

DT385 possesses anti-angiogenic and anti-tumor activity and may have potential as a therapeutic agent.

Recombinant immunotoxins for the treatment of chemoresistant hematologic malignancies

Recombinant immunotoxins are proteins composed of fragments of monoclonal antibodies fused to truncated protein toxins. No agents of this class are approved yet for medical use, although a related molecule, denileukin diftitox, composed of interleukin-2 fused to truncated diphtheria toxin, is approved for relapsed/refractory cutaneous T-cell lymphoma. Recombinant immunotoxins which have been tested in patients with chemotherapy-pretreated hematologic malignancies include LMB-2 (anti-CD25), BL22 (CAT-3888, anti-CD22) and HA22 (CAT-8015, anti-CD22), each containing an Fv fragment fused to truncated Pseudomonas exotoxin. Major responses were observed with LMB-2 in adult T-cell leukemia, chronic lymphocytic leukemia (CLL), cutaneous T-cell lymphoma, Hodgkin's disease, and hairy cell leukemia (HCL). BL22 resulted in a high complete remission rate in patients with HCL, particularly those without excessive tumor burden. HA22, an improved version of BL22 with higher affinity to CD22, is now undergoing phase I testing in HCL, CLL, non-Hodgkin's lymphoma, and pediatric acute lymphoblastic leukemia.

Reducing anti-DT IgG concentrations to improve the efficacy of a diphtheria fusion protein

Preformed antidiphtheria toxin (anti-DT) IgG limits the development of diphtheria fusion proteins because the anti-DT IgG binds and removes the diphtheria fusion protein from the circulation. In our phase I trial of DT-granulocyte macrophage colony stimulating factor (GMCSF), a truncated DT linked to human GMCSF, in relapsed or refractory acute myeloid leukemia, patients with high concentrations of preexisting anti-DT IgG (>2.5 microg/ml) had significantly lower DT-GMCSF concentrations. This study details the fate of anti-DT IgG during the patient's treatment with DT-GMCSF and describes how we could lower anti-DT IgG concentrations and increase the patient's exposure to DT-GMCSF. Using an enzyme immunoassay, we measured anti-DT IgG concentrations before the first cycle of treatment (baseline) and on day 2 (after one dose of DT-GMCSF) and on day 5 (after four doses of DT-GMCSF). Thirty-three patients with relapsed or refractory acute myeloid leukemia in the phase 1 trial received DT-GMCSF at doses from 1 to 5 microg/kg/day intravenously for 5 days. The mean anti-DT IgG concentration pretherapy was 1.3 microg/ml (range: undetectable to 7.8) and significantly decreased to a mean concentration of 0.7 microg/ml on day 2 (P=0.007) and to 0.5 microg/ml on day 5 (P<0.0001). In two individuals in whom we measured DT-GMCSF concentrations on day 1 and day 5, we observed that a decrease in anti-DT IgG concentrations was associated with an increase in DT-GMCSF concentrations. No relationship was observed between dose of DT-GMCSF and the absolute change in anti-DT IgG concentrations on day 2 (r=-0.01, P=0.98) or day 5 (r=-0.12, P=0.53). For patients with high baseline anti-DT IgG concentrations, a single dose of DT-GMCSF could be used to lower the anti-DT IgG concentrations and potentially result in a significant increase in DT-GMCSF concentrations and efficacy.

Preclinical studies in rats and squirrel monkeys for safety evaluation of the bivalent anti-human T cell immunotoxin, A-dmDT390-bisFv(UCHT1)

The bivalent anti-human T cell immunotoxin A-dmDT390-bisFv(UCHT1) for treatment of patients with T cell malignancies is a single chain fusion protein composed of the catalytic domain and translocation domains of diphtheria toxin fused to two tandem sFv molecules reactive with human CD3 epsilon. This immunotoxin selectively kills CD3 epsilon positive T cells. To determine the maximum tolerated dose (MTD), pharmacokinetics and immunogenicity of A-dmDT390-bisFv(UCHT1), rat and squirrel monkey studies were performed. In both animal studies, animals received either 0, 2.5 (low), 25 (medium), or 56.25 microg/kg (high) of A-dmDT390-bisFv(UCHT1) intravenously twice daily for four consecutive days. Although transient elevation of liver transaminases in the high groups was observed, the A-dmDT390-bisFv(UCHT1) administration did not affect liver function, renal function, the hemogram, or produce serious organ histopathology. Adverse events included transient lethargy, inappetence and weight loss in high groups. A-dmDT390-bisFv(UCHT1) plasma half life was 26.95 min in rats and 18.33 min in squirrel monkeys. Immune responses to A-dmDT390-bisFv(UCHT1) were minimal in squirrel monkeys and mild in rats. In vitro cytokine release, T cell activation and CD3 epsilon receptor occupancy assays using human PBMC were further performed since rat and squirrel monkey T cells do not react with A-dmDT390-bisFv(UCHT1). A-dmDT390-bisFv(UCHT1) did not induce cytokine release or T cell activation. The A-dmDT390-bisFv(UCHT1) concentration for 50% CD3 epsilon receptor occupancy was 7.4 nM. The MTD of 200 microg/kg total provides a dose level sufficient for anti-tumor activity in vitro and in a rodent model. Therefore, we propose that this agent is a promising drug for patients with surface CD3+ T cell malignancies.

Safety evaluation of DT388IL3, a diphtheria toxin/interleukin 3 fusion protein, in the cynomolgus monkey

We developed a fusion toxin, DT388IL3, consisting of the catalytic and translocation domains of diphtheria toxin (DT388) linked to interleukin 3 (IL3) for the treatment of patients with acute myeloid leukemia (AML). Our goal in this study was to estimate a range for the maximum tolerated dose (MTD) and to evaluate the dose-limiting toxicity (DLT) of DT388IL3 in cynomolgus monkeys (Macaca fasicularis), which possess cross-reactive IL3 receptors. In our previous study, we administered up to six infusions of DT388IL3 at 40, 60, or 100 microg/kg every other day to three pairs (one male monkey and one female monkey) of young adult monkeys. In five of six monkeys, results showed a dose-dependent increase in malaise and anorexia but no consistent abnormalities in serum chemistries or blood counts. There was no evidence of organ damage by blood tests or histopathology. However, the female treated at 100 microg/kg, died of moderate to severe vasculitis of multiple tissues. Based on these findings, this study repeated the 100 microg/kg group and added a group that received 150 microg/kg in an effort to confirm a dose response. Two female monkeys were treated with up to six infusions of DT388IL3 at 100 microg/kg or 150 microg/kg every other day. One additional female monkey was treated as a negative control. Monkeys in the 100 microg/kg group showed moderate malaise and anorexia, but no consistent abnormalities in blood counts or serum chemistries. Moderate elevations of liver enzymes were noted in the 150 microg/kg group in addition to severe malaise and anorexia. No significant findings were revealed at gross necropsy. The histopathological findings revealed regenerative myeloid hyperplasia and hepatic degeneration and regeneration in the 150 microg/kg group. Similar lesions of less severity were detected in the 100 microg/kg group. DT388IL3 plasma half-life was approximately 20 min with a peak concentration of approximately 2 microg/ml (30,000 pM). The IC50 for AML blasts in vitro was 6 pM. Collectively, our results suggest that DT388IL3 can be tolerated at doses up to 100 microg/kg in a nonhuman primate, which is higher than previously reported for other AML directed diphtheria toxin fusion proteins, and should in principle allow for dose escalation with reduced toxic side effects. Based on these findings a phase I clinical trial has recently been initiated with DT388IL3 for the treatment of AML.

Diphtheria toxin fused to variant human interleukin-3 induces cytotoxicity of blasts from patients with acute myeloid leukemia according to the level of interleukin-3 receptor expression

Leukemic blasts from patients with acute myeloid leukemia (AML) frequently express high levels of the interleukin-3 receptor alpha chain (IL-3Ralpha). In the present study, we have explored the sensitivity of primary leukemic blasts obtained from 34 patients with AML to a diphtheria toxin (DT) composed of the catalytic and translocation domains of DT (DT388) fused to IL-3 (DT388IL-3) and to DT388 fused to a variant IL-3 with increased binding affinity (DT388IL-3[K116W]). On a molar basis, DT388IL-3[K116W] was significantly more active than DT388IL-3 in mediating leukemic cell killing. The rate of cell killing induced by the 2 DT/IL-3 fusion proteins was significantly correlated with the level of IL-3Ralpha/IL-3Rbeta expressed on leukemic blasts. These observations support a potential use of DT388IL-3[K116W] in the treatment of refractory AMLs and provide a simple biochemical parameter for the selection of eligible patients.

Toxicology and pharmacokinetics of DT388IL3, a fusion toxin consisting of a truncated diphtheria toxin (DT388) linked to human interleukin 3 (IL3), in cynomolgus monkeys

The fusion toxin DT388IL3 composed of the catalytic and translocation domains of diphtheria toxin (DT388) linked to interleukin-3 (IL3) was administered to 6 cynomolgus monkeys which possessed cross-reactive IL3 receptors. Groups of 2 animals (1 male and 1 female) received up to 6 every other day slow intravenous infusions of 40, 60, or 100 microg/kg DT388IL3. Monkeys given 40 or 60 microg/kg showed mild or moderate transient malaise and anorexia, respectively, without evidence of organ damage by blood tests or histopathology. Animals treated at 100 microg/kg showed severe malaise and anorexia. The female monkey had moderate to severe vasculitis in multiple tissues. Necropsies were performed on the 40 microg/kg monkeys on day 14 and the 100 microg/kg monkeys on days 6 and 7. DT388IL3 plasma half-life was approximately 30 min with a peak concentration of 0.45 microg/ml or 10,000 pM (IC50 for AML blasts treated in vitro was 6 pM). Immune responses were minimal in 4 animals tested at 12 days and 2 animals tested at 30 days post treatment with anti-DT388IL3 levels < 1 microg/ml. Bone marrow aspirates were obtained on all animals at day 19 or at necropsy and revealed myeloid suppression in the females and myeloid hyperplasia in the males irrespective of dose groups. The maximal tolerated dose of 60 microg/kg for 6 doses is markedly higher than other recombinant diphtheria toxins and provides a dose level sufficient for anti-leukemic activity in vitro and in rodent models. Thus, we propose this agent is a promising drug for AML patients.

The majority of children and adolescents with acute myeloid leukemia have detectable anti-DT388-GMCSF IgG concentrations, but at concentrations that should not preclude in vivo activity

PURPOSE

As a novel approach for the treatment of acute myeloid leukemia (AML), the authors are developing a fusion toxin (DT(388)-GMCSF) consisting of a truncated diphtheria toxin (DT(388)) linked to human granulocyte-macrophage colony-stimulating factor (GMCSF). A critical step in the development of DT(388)-GMCSF for clinical use in childhood and adolescent AML is to determine whether children and adolescents have preexisting antibodies to DT(388)-GMCSF due to childhood immunizations against diphtheria toxoid.

PATIENTS AND METHODS

Sera from 33 children and adolescents with AML and one with juvenile myelomonocytic leukemia were collected. The median age was 11.8 years. All scheduled diphtheria toxoid vaccinations were current except for the child diagnosed at 4 months of age. Anti-DT(388)-GMCSF antibody concentrations were detected by an enzymoimmunoassay and by an in vitro bioassay.

RESULTS

Thirty of 34 (88%) children and adolescents had detectable anti-DT(388)-GMCSF IgG antibody concentrations. The median concentration was 1.5 microg/mL, with a range from undetectable to 191.4 microg/mL. There was a positive correlation between the enzymoimmunoassay and bioassay. There was no difference between the anti-DT(388)-GMCSF IgG concentrations in these children and adolescents with AML and in 43 adults with AML. Preliminary results of the phase 1 trial of DT -GMCSF in adults with AML indicate that patients with baseline anti-DT(388)-GMCSF IgG concentrations of less than 2 microg/mL can achieve circulating DT(388)-GMCSF concentrations and can exhibit antileukemic activity. Twenty-three of 34 (67.6%) children and adolescents had anti-DT(388)-GMCSF IgG concentrations less than 2 microg/mL.

CONCLUSIONS

Despite routine diphtheria toxoid vaccinations, most children and adolescents with AML do not have anti-DT -GMCSF IgG concentrations that preclude in vivo activity of DT -GMCSF.

Antibody response to DT-GM, a novel fusion toxin consisting of a truncated diphtheria toxin (DT) linked to human granulocyte-macrophage colony stimulating factor (GM), during a phase I trial of patients with relapsed or refractory acute myeloid leukemia

We are conducting a Phase I trial of a fusion toxin (DT-GM) for the treatment of relapsed or refractory acute myeloid leukemia (AML). The fusion toxin consists of a truncated diphtheria toxin (DT) linked to human granulocyte-macrophage colony stimulating factor (GM). Prior to beginning the Phase I trial, our first goal was to determine whether healthy controls and adult AML patients had preexisting antibodies able to inhibit DT-GM. Sera from 5 of the 9 controls completely neutralized DT-GM by an in vitro bioassay to assess the inhibition of DT-GM. Sera from 43 patients with AML were tested by bioassay and a specific enzymoimmunoassay (EIA) for anti-DT-GM antibodies. Forty-two of 43 samples were positive by EIA, and 5 patients (11.6%) showed complete neutralization of DT-GM in the bioassay. Anti-DT-GM concentrations were significantly higher in samples demonstrating neutralization than in samples demonstrating no neutralization (P = 0.003). In the Phase I trial of DT-GM prior to therapy, none of 28 patients exhibited neutralization by bioassay, but 89% were positive by EIA. After the first course of DT-GM, 23% developed neutralizing antibodies by the bioassay, and 64% of patients exhibited an increase in their anti-DT-GM antibody concentrations by EIA. Further studies are needed to determine the clinical impact of the anti-DT-GM antibodies and whether the neutralization bioassay can be replaced by our EIA.

I.V. Administration of L-GNRH-PE66 efficiently inhibits growth of colon adenocarcinoma xenografts in nude mice

When developing new anti-cancer therapeutic treatments, it is crucial to find the correct route of administration and timetable for treatment. Recently, we constructed the L-GnRH-PE66 chimeric protein, which can target and kill adenocarcinoma cells both in vitro and in vivo. We examined the ability of the L-GnRH-PE66 chimeric protein to inhibit tumor growth in colon carcinoma xenografted nude mice, using different routes of administration and various timetables of treatment. In addition, we examined the ability of the chimeric protein to inhibit tumor growth of large tumors that resemble those encountered in human patients in the clinical setting. We found that an i.v. dose of 12.5 microg given every 48 hr was the most efficacious in inhibiting tumor growth. Tumors treated with this concentration of the chimeric protein were 4.4 times smaller in volume and 3.4 times smaller in weight than those in the control groups. This protocol of L-GnRH-PE66 treatment is an improvement on our previously suggested treatment for adenocarcinoma in humans. An i.v. injection every 48 hr is effective, less toxic and less painful. Our results further support the use of L-GnRH-PE66 as an effective treatment for adenocarcinoma in humans.

Variable cytotoxicity of diphtheria toxin 388-granulocyte-macrophage colony-stimulating factor fusion protein for acute myelogenous leukemia stem cells

In this study, the utility of DT388-granulocyte-macrophage colony-stimulating factor (GM-CSF) for the ex vivo purging and direct administration to patients with acute myeloid leukemia (AML) is tested using clonogenic assays, long-term cultures (LTC), and NOD/SCID mice as assays for leukemic progenitors. We compare the ability of 24-hour exposure to 0.3 microg/mL (4 nM) DT388-GM-CSF to kill AML colony forming cells (CFC) and the more primitive AML progenitors detected after 6 weeks in stromal cocultures (AML LTC-initiating cells or AML LTC-IC) and after 8 weeks in NOD/SCID mice.AML samples (n = 10), expressing a mean of 35 to 1466 GM-CSF receptors/blast, showed mean (range) percent kills of AML CFC and LTC-IC of 61 (17-98) and 46 (0-94) respectively with a direct correlation (r = 0.69) between the % kills detected in the in vitro assays. Among 5 evaluable samples the percent reduction in AML cell engraftment in NOD/SCID marrow following ex vivo DT388-GM-CSF treatment varied from 38% to 100%. 40% to 56% of normal bone marrow CFC and 31% to 48% of normal LTC-IC survived the same ex vivo treatment (n = 3). In subsequent experiments, NOD/SCID mice received AML blast cell injections intravenously followed in 24 hours by 1.5 microg DT388-GM-CSF daily intraperitoneally for 5 days. A reduction of marrow blast cells was seen with 7 of 9 samples tested 4 to 12 weeks post one course of toxin. Repeating the 5-day course of toxin 2 or 3 times at 4-week intervals did not improve the response, while delaying administration until 4 to 8 weeks post AML cell injection reduced the toxin's effectiveness (n = 5).This fusion toxin may prove useful for in vitro purging of stem cell harvests from selected AML patients and for direct administration to such patients.