CXCL12 in Pancreatic Cancer: Its Function and Potential as a Therapeutic Drug Target

Simple Summary

Pancreatic cancer is a challenging disease to treat effectively. Fibroblasts associated with pancreatic cancer contribute to disease progression by secreting factors that enhance tumor cell survival and help tumor cells avoid detection by the immune system. This overview focuses on a chemokine, CXCL12, produced by cancer-associated fibroblasts and how CXCL12 signaling enhances pancreatic cancer progression by contributing to various hallmarks of cancer including, but not limited to, tumor growth and evasion of immune response. These pro-oncogenic functions of CXCL12 make it an attractive target in pancreatic cancer. We discuss the different approaches in development to therapeutically target CXCL12 and finally propose a novel approach, the use of the farnesyl transferase inhibitor tipifarnib to inhibit CXCL12 expression in pancreatic fibroblasts.

Abstract

Pancreatic ductal adenocarcinoma (PDAC) is a disease with limited therapeutic options and dismal long-term survival. The unique tumor environment of PDAC, consisting of desmoplastic stroma, immune suppressive cells, and activated fibroblasts, contributes to its resistance to therapy. Activated fibroblasts (cancer-associated fibroblasts and pancreatic stellate cells) secrete chemokines and growth factors that support PDAC growth, spread, chemoresistance, and immune evasion. In this review, we focus on one such chemokine, CXCL12, secreted by the cancer-associated fibroblasts and discuss its contribution to several of the classical hallmarks of PDAC and other tumors. We review the various therapeutic approaches in development to target CXCL12 signaling in PDAC. Finally, we propose an unconventional use of tipifarnib, a farnesyl transferase inhibitor, to inhibit CXCL12 production in PDAC.

Cyclooxygenase-2 Inhibition Potentiates the Efficacy of Vascular Endothelial Growth Factor Blockade and Promotes an Immune Stimulatory Microenvironment in Preclinical Models of Pancreatic Cancer

Resistance to standard therapy remains a major challenge in the treatment of pancreatic ductal adenocarcinoma (PDA). Although anti-VEGF therapy delays PDA progression, therapy-induced hypoxia results in a less differentiated mesenchymal-like tumor cell phenotype, which reinforces the need for effective companion therapies. COX-2 inhibition has been shown to promote tumor cell differentiation and improve standard therapy response in PDA. Here, we evaluate the efficacy of COX-2 inhibition and VEGF blockade in preclinical models of PDA. In vivo, the combination therapy was more effective in limiting tumor growth and metastasis than single-agent therapy. Combination therapy also reversed anti-VEGF-induced epithelial-mesenchymal transition and collagen deposition and altered the immune landscape by increasing tumor-associated CD8+ T cells while reducing FoxP3+ T cells and FasL expression on the tumor endothelium. IMPLICATIONS: Together, these findings demonstrate that COX-2 inhibition enhances the efficacy of anti-VEGF therapy by reducing hypoxia-induced epithelial plasticity and promoting an immune landscape that might facilitate immune activation.Visual Overview: http://mcr.aacrjournals.org/content/molcanres/17/2/348/F1.large.jpg.

Abstract 3885: 11q13 amplification selects for sensitivity to the ERK inhibitor KO-947 in squamous cell carcinomas

The MAPK pathway is a major driver of malignant progression, particularly in cancers arising from mutations in pathway components, and BRAF and MEK inhibitors have been approved for treatment of BRAF-mutant melanoma. ERK1/2 kinases are the final node in the MAPK signaling pathway and offer the possibility of clinical benefit in settings where earlier drugs are ineffective. We have previously reported that KO-947, a potent and selective inhibitor of ERK1/2 with extended target residence time and favorable pharmaceutic properties, displays robust single-agent antitumor activity in PDX models of adenocarcinomas with RAS/RAF mutations, and in squamous cell carcinomas (SCCs) that lack mutations in MAPK pathway components. Here we report the discovery of a novel biomarker that is associated with sensitivity to KO-947 in head and neck SCC and esophageal SCC PDX models. Recurring amplification of chromosome 11 between bands q13.3 and q13.4 is a feature of several tumor types, including ESCC and HNSCC. TCGA reports incidence rates of >50% in ESCC and ~20% in HNSCC, although higher frequencies are seen in some subtypes, such as pharyngeal and HPV-negative SCC. The 11q13 amplicon commonly contains about a dozen genes, including several potential oncogenes with functional linkage to the MAPK pathway, such as CCND1, FADD and the recently described calcium-dependent chloride channel ANO1. PDX campaigns were carried out in 24 ESCC and 18 HNSCC models, including 16 and 7 of each with 11q13 amplification, respectively. Groups of three animals were dosed with KO-947 at 300mg/kg QW, tumor growth was monitored for 3-6 weeks and responses were categorized as progressive disease, stable disease (SD, >80% TGI) or partial/complete response (PR/CR, >30% regression). In ESCC tumor-bearing animals, the overall response rates (ORR = PR+CR) were 33% in the overall population, 51% in the 11q13-amplified (11q-AMP) and 3% in 11q13-WT. The disease control rates (DCR = PR/CR+SD) were 54% overall, 77% in 11q-AMP and 21% in 11q-WT. In HNSCC tumor-bearing animals, the ORRs were 24% in the overall population, 56% in 11q-AMP and 9% in 11q-WT, with DCRs of 40%, 69% and 26% in the three subsets, respectively. 11q-AMP was significantly associated with response to KO-947 in both tumor types. ROC analysis defined the minimum effective copy number as 4 and revealed significant associations between expression levels of several 11q13 amplicon genes and response to KO-947. A key role for ANO1 in driving ERK-dependent tumor growth in 11q-AMP cases was further indicated by the observation that ANO1 expression was silenced in some 11q-AMP models and these tumors failed to respond the ERK inhibition. The results suggest that 11q13 amplification in SCCs can drive tumor growth and survival in a MAPK-dependent manner and that 11q-AMP may be a useful biomarker for predicting clinical response to ERK inhibitors.

Citation Format: Francis J. Burrows, Linda Kessler, Tao Wu, Xin Gao, Jeffrey Chen, Rasmus Hansen, Shuangwei Li, Carol Thach, Shisheng Li, Ke Yu, Jeff Kucharski, Ulf Peters, Jun Feng, Yi Wang, Yvonne Yao, Ata Zarieh, Matt Janes, Jingchuan Zhang, Liansheng Li, Dana Hu-Lowe, Pingda Ren, Yi Liu. 11q13 amplification selects for sensitivity to the ERK inhibitor KO-947 in squamous cell carcinomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3885.

Marizomib activity as a single agent in malignant gliomas: ability to cross the blood-brain barrier

BACKGROUND

The proteasome plays a vital role in the physiology of glioblastoma (GBM), and proteasome inhibition can be used as a strategy for treating GBM. Marizomib is a second-generation, irreversible proteasome inhibitor with a more lipophilic structure that suggests the potential for penetrating the blood-brain barrier. While bortezomib and carfilzomib, the 2 proteasome inhibitors approved for treatment of multiple myeloma, have little activity against malignant gliomas in vivo, marizomib could be a novel therapeutic strategy for primary brain tumors.

METHODS

The in-vitro antitumor activity of marizomib was studied in glioma cell lines U-251 and D-54. The ability of marizomib to cross the blood-brain barrier and regulate proteasome activities was evaluated in cynomolgus monkeys and rats. The antitumor effect of marizomib in vivo was tested in an orthotopic xenograft model of human GBM.

RESULTS

Marizomib inhibited the proteasome activity, proliferation, and invasion of glioma cells. Meanwhile, free radical production and apoptosis induced by marizomib could be blocked by antioxidant N-acetyl cysteine. In animal studies, marizomib distributed into the brain at 30% of blood levels in rats and significantly inhibited (>30%) baseline chymotrypsin-like proteasome activity in brain tissue of monkeys. Encouragingly, the immunocompromised mice, intracranially implanted with glioma xenografts, survived significantly longer than the control animals (P < .05) when treated with marizomib.

CONCLUSIONS

These preclinical studies demonstrated that marizomib can cross the blood-brain barrier and inhibit proteasome activity in rodent and nonhuman primate brain and elicit a significant antitumor effect in a rodent intracranial model of malignant glioma.

Marizomib irreversibly inhibits proteasome to overcome compensatory hyperactivation in multiple myeloma and solid tumour patients

Proteasome inhibitors (PIs) are highly active in multiple myeloma (MM) but resistance is commonly observed. All clinical stage PIs effectively inhibit chymotrypsin‐like (CT‐L) activity; one possible mechanism of resistance is compensatory hyperactivation of caspase‐like (C‐L) and trypsin‐like (T‐L) subunits, in response to CT‐L blockade. Marizomib (MRZ), an irreversible PI that potently inhibits all three 20S proteasome subunits with a specificity distinct from other PIs, is currently in development for treatment of MM and malignant glioma. The pan‐proteasome pharmacodynamic activity in packed whole blood and peripheral blood mononuclear cells was measured in two studies in patients with advanced solid tumours and haematological malignancies. Functional inhibition of all proteasome subunits was achieved with once‐ or twice‐weekly MRZ dosing; 100% inhibition of CT‐L was frequently achieved within one cycle at therapeutic doses. Concomitantly, C‐L and T‐L activities were either unaffected or increased, suggesting compensatory hyperactivation of these subunits. Importantly, this response was overcome by continued administration of MRZ, with robust inhibition of T‐L and C‐L (up to 80% and 50%, respectively) by the end of Cycle 2 and maintained thereafter. This enhanced proteasome inhibition was independent of tumour type and may underlie the clinical activity of MRZ in patients resistant to other PIs.

Therapeutic potential of ERK5 targeting in triple negative breast cancer

Triple negative breast cancers (TNBCs) account for 15% of all breast cancers, and represent one of the most aggressive forms of the disease, exhibiting short relapse-free survival. In contrast to other breast cancer subtypes, the absence of knowledge about the etiopathogenic alterations that cause TNBCs force the use of chemotherapeutics to treat these tumors. Because of this, efforts have been devoted with the aim of incorporating novel therapies into the clinical setting. Kinases play important roles in the pathophysiology of several tumors, including TNBC. Since expression of the MAP kinase ERK5 has been linked to patient outcome in breast cancer, we analyzed the potential value of its targeting in TNBC. ERK5 was frequently overexpressed and active in samples from patients with TNBC, as well as in explants from mice carrying genetically-defined TNBC tumors. Moreover, expression of ERK5 was linked to a worse prognosis in TNBC patients. Knockdown experiments demonstrated that ERK5 supported proliferation of TNBC cells. Pharmacological inhibition of ERK5 with TG02, a clinical stage inhibitor which targets ERK5 and other kinases, inhibited cell proliferation by blocking passage of cells through G₁ and G₂, and also triggered apoptosis in certain TNBC cell lines. TG02 had significant antitumor activity in a TNBC xenograft model in vivo, and also augmented the activity of chemotherapeutic agents commonly used to treat TNBC. Together, these data indicate that ERK5 targeting may represent a valid strategy against TNBC, and support the development of trials aimed at evaluating the clinical effectiveness of drugs that block this kinase.

Potent antimyeloma activity of a novel ERK5/CDK inhibitor

PURPOSE

To analyze the antimyeloma potential of TG02, an ERK5/CDK inhibitory drug.

EXPERIMENTAL DESIGN

Utilizing different multiple myeloma cell lines we determined the effect of TG02 over viability by MTT assays. The apoptotic effect over multiple myeloma patient samples was studied ex vivo by cytometry. The mechanism of action of TG02 was analyzed in the cell line MM1S, studying its effect on the cell cycle, the induction of apoptosis, and the loss of mitochondrial membrane potential by cytometry and Western blot. Two models of multiple myeloma xenograft were utilized to study the in vivo action of TG02.

RESULTS

TG02 potently inhibited proliferation and survival of multiple myeloma cell lines, even under protective bone marrow niche conditions, and selectively induced apoptosis of primary patient-derived malignant plasma cells. TG02 displayed significant single-agent activity in two multiple myeloma xenograft models, and enhanced the in vivo activity of bortezomib and lenalidomide. Signaling analyses revealed that the drug simultaneously blocked the activity of CDKs 1, 2, and 9 as well as the MAP kinase ERK5 in MM1S cells, leading to cell-cycle arrest and rapid commitment to apoptosis. TG02 induced robust activation of both the intrinsic and extrinsic pathways of apoptosis, and depletion of XIAP and the key multiple myeloma survival protein Mcl-1.

CONCLUSIONS

TG02 is a promising new antimyeloma agent that is currently in phase I clinical trials in leukemia and multiple myeloma patients.

Epithelial-mesenchymal transition increases tumor sensitivity to COX-2 inhibition by apricoxib

Although cyclooxygenase-2 (COX-2) inhibitors, such as the late stage development drug apricoxib, exhibit antitumor activity, their mechanisms of action have not been fully defined. In this study, we characterized the mechanisms of action of apricoxib in HT29 colorectal carcinoma. Apricoxib was weakly cytotoxic toward naive HT29 cells in vitro but inhibited tumor growth markedly in vivo. Pharmacokinetic analyses revealed that in vivo drug levels peaked at 2-4 µM and remained sufficient to completely inhibit prostaglandin E(2) production, but failed to reach concentrations cytotoxic for HT29 cells in monolayer culture. Despite this, apricoxib significantly inhibited tumor cell proliferation and induced apoptosis without affecting blood vessel density, although it did promote vascular normalization. Strikingly, apricoxib treatment induced a dose-dependent reversal of epithelial-mesenchymal transition (EMT), as shown by robust upregulation of E-cadherin and the virtual disappearance of vimentin and ZEB1 protein expression. In vitro, either anchorage-independent growth conditions or forced EMT sensitized HT29 and non-small cell lung cancer cells to apricoxib by 50-fold, suggesting that the occurrence of EMT may actually increase the dependence of colon and lung carcinoma cells on COX-2. Taken together, these data suggest that acquisition of mesenchymal characteristics sensitizes carcinoma cells to apricoxib resulting in significant single-agent antitumor activity.

Apricoxib, a novel inhibitor of COX-2, markedly improves standard therapy response in molecularly defined models of pancreatic cancer

PURPOSE

COX-2 is expressed highly in pancreatic cancer and implicated in tumor progression. COX-2 inhibition can reduce tumor growth and augment therapy. The precise function of COX-2 in tumors remains poorly understood, but it is implicated in tumor angiogenesis, evasion of apoptosis, and induction of epithelial-to-mesenchymal transition (EMT). Current therapeutic regimens for pancreatic cancer are minimally effective, highlighting the need for novel treatment strategies. Here, we report that apricoxib, a novel COX-2 inhibitor in phase II clinical trials, significantly enhances the efficacy of gemcitabine/erlotinib in preclinical models of pancreatic cancer.

EXPERIMENTAL DESIGN

Human pancreatic cell lines were evaluated in vitro and in vivo for response to apricoxib ± standard-of-care therapy (gemcitabine + erlotinib). Tumor tissue underwent posttreatment analysis for cell proliferation, viability, and EMT phenotype. Vascular parameters were also determined.

RESULTS

COX-2 inhibition reduced the IC(50) of gemcitabine ± erlotinib in six pancreatic cancer cell lines tested in vitro. Furthermore, apricoxib increased the antitumor efficacy of standard combination therapy in several orthotopic xenograft models. In vivo apricoxib combination therapy was only effective at reducing tumor growth and metastasis in tumors with elevated COX-2 activity. In each model examined, treatment with apricoxib resulted in vascular normalization without a decrease in microvessel density and promotion of an epithelial phenotype by tumor cells regardless of basal COX-2 expression.

CONCLUSIONS

Apricoxib robustly reverses EMT and augments standard therapy without reducing microvessel density and warrants further clinical evaluation in patients with pancreatic cancer.

Abstract 2830: Multikinase inhibition by TG02 is therapeutically effective in two forms of breast cancer

Breast cancer (BC) is a heterogeneous disease in which several distinct genetic and histopathological types have been defined. Triple-negative breast cancer (TNBC) lacking ER/PR and HER-2 is the most aggressive and untreatable form. In the case of HER2+ tumors, and despite the emergence of trastuzumab and lapatinib, in the metastatic setting these tumors remain incurable due to the innate or acquired drug resistance. A number of kinases are implicated in the proliferation and survival of breast cancer cells. Here, we characterize the antitumor activity of the multikinase inhibitor TG02 in TNBC and HER2+ BC. TG02 is a novel drug with a unique kinase inhibitory spectrum, targeting both the cell cycle regulatory CDKs 1/2 and the transcriptional regulators CDKs 7/9, along with the emerging oncogenic MAPK ERK5 and the DNA damage response mediator CDK5, all within a narrow low nanomolar potency range. In panels of breast cancer cell lines, TG02 was potently and consistently antiproliferative at low (100nM) and proapoptotic at moderate (250-1000nM), clinically-achievable concentrations. In TNBC cells, the drug usually induced a G2/M arrest, whereas blockade of cell cycle progression at the G1/S boundary was more common in HER2+ lines. Simultaneous multikinase inhibition was demonstrated in several cellular models, showing that cell cycle arrest induced by inhibition of CDK1 and CDK2 progresses to apoptosis associated with profound depletion of the short-lived survival proteins of the Bcl-2 and IAP families downstream of CDK9-dependent loss of RNA polymerase II-driven transcription. TG02 was synergistic in combination with chemotherapy in TNBC models in vitro and in vivo and with trastuzumab and lapatinib in HER2-overexpressing cell lines. A majority of TNBC and HER2+ lines expressed constitutively active ERK5, which was efficiently inhibited by TG02 and may contribute to the antitumor activity of the drug. A robust DNA damage response including CHK1/2 activation and formation of H2AX foci was induced in TNBC cells, presumably due to inhibition of CDK5 by TG02, so we will also report on the mechanism-based combination of TG02 with PARP inhibitors in this setting. These data support clinical development of this compound for the treatment of TNBC and HER2+ BC, especially in combination with standard of care treatments used in these breast neoplasias.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2830. doi:1538-7445.AM2012-2830

Abstract 3819: COX-2 inhibition with apricoxib drives mesenchymal to epithelial transition and synergizes with anti-VEGF therapy

Resistance to standard therapy remains a challenge in treatment of pancreatic ductal adenocarcinoma (PDAC) resulting in need for novel therapeutic strategies. Anti-VEGF therapy with r84 delays PDAC progression; however, chronic hypoxia ultimately results in transition to a mesenchymal phenotype and rapid progression at a later timepoint. COX-2 inhibition with apricoxib, a novel antagonist in Phase II trials, reverses EMT in PDAC cells. We investigate the relationship between COX-2 and VEGF production in PDAC cell lines to evaluate the efficacy of a combination strategy in preclinical models of PDAC. In vitro, changes in VEGF production by PDAC cells following apricoxib were assessed by ELISA at baseline and following forced induction of EMT. The effect of r84, apricoxib, or combination on tumor growth and metastatic incidence was determined in SCID mice with established orthotopic pancreatic xenografts. Improvement of survival and reduction in tumor burden was assessed in a murine genetic model of PDAC using p48-Cre; KrasG12D;Cdkn2alox/lox mice, tissue was collected at 4 weeks of therapy with control antibody, r84 or apricoxib for analysis. PDAC cell lines grown in the presence of TGF-β and collagen demonstrated increased Zeb1 and decreased ECAD expression by Western blot. Treatment with clinically-achievable doses of apricoxib reversed this change in phenotype. High expression of COX-2 correlated with high levels of VEGF in conditioned media in human PDAC cell lines. VEGF production was initially sensitive to COX-2 inhibition, with complete depletion of VEGF in the first 6 hours, but VEGF levels returned to baseline by 16 hours and continued to rise throughout the first 72 hours post apricoxib. Upon repeat dosing at 24 hours, VEGF production had become independent of COX-2 and no changes were observed. Similar results were seen following induction of EMT, however, baseline VEGF production was dramatically increased, and while there was an initial decrease after apricoxib, production remained more than 2-fold higher than in cells under normal culture conditions. In vivo, apricoxib and r84 had minimal effect on primary tumor growth as single agents; however each agent resulted in a reduction of metastatic incidence. Combination therapy reduced primary tumor size and virtually eliminated metastases. Overall survival was significantly improved with apricoxib as a single agent. Apricoxib treated animals also demonstrated a delay in progression of disease and decreased collagen deposition in the microenvironment compared to r84, which increased collagen deposition.We conclude that COX-2 inhibition by apricoxib delays progression of PDAC and results in mesenchymal to epithelial transition. Combination with anti-VEGF therapy results in potent antitumor and antimetastatic effects in our models and warrants further evaluation as a strategy to augment chemotherapy.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3819. doi:1538-7445.AM2012-3819

Abstract 2542: TG02, a novel multi-kinase inhibitor with potent anti-leukemic activity

Kinase inhibitors have found applications in multiple oncology settings due to their ability to target key signaling pathways in many different cancers. In general, the broad-spectrum kinase inhibitors have yielded better clinical outcomes than more selective ones because they block more than one pathway critical for tumor growth. We describe herein the pharmacological profile of TG02, a multi-kinase inhibitor being developed by Tragara Pharmaceuticals, which combines cell cycle regulatory and transcriptional CDK inhibition with activity against kinase targets important in the etiology of various leukemias.

In vitro potency against the kinases was evaluated using recombinant enzymes with synthetic substrates. Potency on intracellular signaling pathways was assessed by Western blot analyses of the phosphorylated substrates in cell lysates. Functional potency was assessed using proliferation assays performed on human tumor cell lines as well as hematopoietic progenitors expanded from patients' blood. Evaluation of pharmacokinetics, pharmacodynamic biomarkers and anti-tumor efficacy was performed in nude mouse xenograft models of leukemia.

TG02 was shown to be a potent inhibitor of several key CDKs (IC50 = 19, 11, 37 & 10nM vs. CDK1, 2, 7 & 9) as well as two other kinases implicated in hematological malignancies, FLT3 and JAK2, with similar potency. The CDK spectrum includes both cell cycle regulatory and transcriptional CDKs. TG02 potently inhibits proliferation across a broad panel of human liquid tumor cell lines (IC50 from 68 to 230 nM). Comparison with reference inhibitors that block only one of the kinase targets of interest demonstrated the significant added benefit of combined CDK and JAK2 targeting. Inhibition of signaling by the CDK and FLT3-JAK2 pathways in leukemia cell lines was detected at the same range (0.10 to 0.35 μM). TG02 was generally cleared from the blood within 8 hours of oral administration but was retained in tumor masses at supratherapeutic levels for 24-48 hours, depending on dose. Accordingly, pathway-related biomarkers were markedly suppressed for 24-72 hours after dosing. In a murine model of FLT3-mutated leukemia (MV4-11), TG02 induced tumor regression after oral daily dosing (40 mg/kg for 21 days). Furthermore, in an orthotopic model of AML with WT FLT3 and JAK2 (HL60), TG02 significantly prolonged median survival time (59 days vs. 40 days) after treatment at 100 mg/kg p.o. 2d on/5d off. Proliferation assays using hematopoietic progenitors expanded from patients with JAK2 mutations or FLT3 wild-type or mutant acute myeloid leukemia revealed higher efficacy of TG02 compared to reference inhibitors targeting only one of the kinases of interest, again demonstrating the benefits of co-targeting CDKs and additional kinases.

TG02 is a multi-kinase inhibitor with a previously unreported spectrum of targets that shows promising preclinical activity for the treatment of hematological malignancies.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2542.

Heat shock protein 90 inhibitor BIIB021 (CNF2024) depletes NF-kappaB and sensitizes Hodgkin's lymphoma cells for natural killer cell-mediated cytotoxicity

PURPOSE

In Hodgkin's lymphoma, constitutive activation of NF-kappaB promotes tumor cell survival and proliferation. The molecular chaperone heat shock protein 90 (HSP90) has immune regulatory activity and supports the activation of NF-kappaB in Hodgkin's lymphoma cells.

EXPERIMENTAL DESIGN

We analyzed the effect of HSP90 inhibition on viability and NF-kappaB activity in Hodgkin's lymphoma cells and the consequences for their recognition and killing through natural killer (NK) cells.

RESULTS

The novel orally administrable HSP90 inhibitor BIIB021 (CNF2024) inhibited Hodgkin's lymphoma cell viability at low nanomolar concentrations in synergy with doxorubicin and gemcitabine. Annexin V/7-aminoactinomycin D binding assay revealed that BIIB021 selectively induced cell death in Hodgkin's lymphoma cells but not in lymphocytes from healthy individuals. We observed that BIIB021 inhibited the constitutive activity of NF-kappaB and this was independent of IkappaB mutations. Furthermore, we analyzed the effect of HSP90 inhibition on NK cell-mediated cytotoxicity. BIIB021 induced the expression of ligands for the activating NK cell receptor NKG2D on Hodgkin's lymphoma cells resulting in an increased susceptibility to NK cell-mediated killing. In a xenograft model of Hodgkin's lymphoma, HSP90 inhibition significantly delayed tumor growth.

CONCLUSIONS

HSP90 inhibition has direct antitumor activity in Hodgkin's lymphoma in vitro and in vivo. Moreover, HSP90 inhibition may sensitize Hodgkin's lymphoma cells for NK cell-mediated killing via up-regulation of ligands engaging activating NK cell receptors.

Rationally Designed High-Affinity 2-Amino-6-halopurine Heat Shock Protein 90 Inhibitors That Exhibit Potent Antitumor Activity

Heat shock protein 90 (Hsp90) is a molecular chaperone protein implicated in stabilizing the conformation and maintaining the function of many cell-signaling proteins. Many oncogenic proteins are more dependent on Hsp90 in maintaining their conformation, stability, and maturation than their normal counterparts. Furthermore, recent data show that Hsp90 exists in an activated form in malignant cells but in a latent inactive form in normal tissues, suggesting that inhibitors selective for the activated form could provide a high therapeutic index. Hence, Hsp90 is emerging as an exciting new target for the treatment of cancer. We now report on a novel series of 2-amino-6-halopurine Hsp90 inhibitors exemplified by 2-amino-6-chloro-9-(4-iodo-3,5-dimethylpyridin-2-ylmethyl)purine (30). These highly potent inhibitors (IC50 of 30 = 0.009 microM in a HER-2 degradation assay) also display excellent antiproliferative activity against various tumor cell lines (IC50 of 30 = 0.03 microM in MCF7 cells). Moreover, this class of inhibitors shows higher affinity for the activated form of Hsp90 compared to our earlier 8-sulfanylpurine Hsp90 inhibitor series. When administered orally to mice, these compounds exhibited potent tumor growth inhibition (>80%) in an N87 xenograft model, similar to that observed with 17-allylamino-17-desmethoxygeldanamycin (17-AAG), which is a compound currently in phase I/II clinical trials.

The heat-shock protein 90 inhibitor 17-allylamino-17-demethoxygeldanamycin suppresses glial inflammatory responses and ameliorates experimental autoimmune encephalomyelitis

The heat-shock response (HSR), a highly conserved cellular response, is characterized by rapid expression of heat-shock proteins (HSPs), and inhibition of other synthetic activities. The HSR can attenuate inflammatory responses, via suppression of transcription factor activation. A HSR can be induced pharmacologically by HSP90 inhibitors, through activation of the transcription factor Heat Shock Factor 1 (HSF1). In the present study we characterized the effects of 17-allylamino-17-demethoxygeldanamycin (17-AAG), a less toxic derivative of the naturally occurring HSP90 inhibitor geldanamycin, on glial inflammatory responses and the development of experimental autoimmune encephalomyelitis. In primary enriched glial cultures, 17-AAG dose dependently reduced lipopolysaccharide-dependent expression and activity of inducible nitric oxide synthase, attenuated interleukin (IL)-1beta expression and release, increased inhibitor of kappaB protein levels, and induced HSP70 expression. 17-AAG administration to mice immunized with myelin oligodendrocyte glycoprotein peptide prevented disease onset when given at an early time, and reduced clinical symptoms when given during ongoing disease. T cells from treated mice showed a reduced response to immunogen re-stimulation, and 17-AAG reduced CD3- and CD28-dependent IL-2 production. Together, these data suggest that HSP90 inhibitors could represent a new approach for therapeutic intervention in autoimmune diseases such as multiple sclerosis.

7‘-Substituted Benzothiazolothio- and Pyridinothiazolothio-Purines as Potent Heat Shock Protein 90 Inhibitors

We report on the discovery of benzo- and pyridino- thiazolothiopurines as potent heat shock protein 90 inhibitors. The benzothiazole moiety is exceptionally sensitive to substitutions on the aromatic ring with a 7'-substituent essential for activity. Some of these compounds exhibit low nanomolar inhibition activity in a Her-2 degradation assay (28-150 nM), good aqueous solubility, and oral bioavailability profiles in mice. In vivo efficacy experiments demonstrate that compounds of this class inhibit tumor growth in an N87 human colon cancer xenograft model via oral administration as shown with compound 37 (8-(7-chlorobenzothiazol-2-ylsulfanyl)-9-(2-cyclopropylamino-ethyl)-9H- purin-6-ylamine).

Inhibition of neuroblastoma xenograft growth by Hsp90 inhibitors

BACKGROUND

Advanced-stage neuroblastomas are often resistant to chemotherapy. Heat shock protein (Hsp) 90 is a molecular chaperone that maintains the stability of important signal transduction proteins. We have previously reported that geldanamycin (GA), an Hsp90 inhibitor, decreases Raf-1 and Akt protein expressions and induces apoptosis in neuroblastoma cells. We sought to determine the in vivo effects of Hsp90 inhibitor compounds on human neuroblastomas.

MATERIALS AND METHODS

Human neuroblastoma (LAN-1 and SK-N-SH) xenografts (4-mm3 tumor implants) were placed in the flanks of athymic nude mice. The mice received either Hsp90 inhibitors (17-AAG or EC5) or vehicle (control). The tumor dimensions were measured twice weekly. Proteins were extracted for Western immunoblotting.

RESULTS

Hsp90 inhibitor compounds significantly blocked both LAN-1 and SK-N-SH neuroblastoma growth in vivo. Drug-treated tumors showed decreases in Raf-1 and cleaved PARP expressions.

CONCLUSION

Hsp90 inhibitors may prove to be important novel therapeutic agents for patients with advanced-stage neuroblastoma who fail to respond to current treatment regimens.

Orally Active Purine-Based Inhibitors of the Heat Shock Protein 90

Orally active Hsp90 inhibitors are of interest as potential chemotherapeutic agents. Recently, fully synthetic 8-benzyladenines and 8-sulfanyladenines such as 4 were disclosed as Hsp90 inhibitors, but these compounds are not water soluble and consequently have unacceptably low oral bioavailabilities. We now report that water-solubility can be achieved by inserting an amino functionality in the N(9) side chain. This results in compounds that are potent, soluble in aqueous media, and orally bioavailable. In an HER-2 degradation assay, the highest potency was achieved with the neopentylamine 42 (HER-2 IC(50) = 90 nM). In a murine tumor xenograft model (using the gastric cancer cell line N87), the H(3)PO(4) salts of the amines 38, 39, and 42 induced tumor growth inhibition when administered orally at 200 mg/kg/day. The amines 38, 39, and 42 are the first Hsp90 inhibitors shown to inhibit tumor growth upon oral dosage.

ZAP-70 is a novel conditional heat shock protein 90 (Hsp90) client: inhibition of Hsp90 leads to ZAP-70 degradation, apoptosis, and impaired signaling in chronic lymphocytic leukemia

The zeta-associated protein of 70 kDa (ZAP-70) is expressed in patients with aggressive chronic lymphocytic leukemia (CLL). We found that ZAP-70+ CLL cells expressed activated heat-shock protein 90 (Hsp90) with high binding affinity for Hsp90 inhibitors, such as 17-allyl-amino-demethoxy-geldanamycin (17-AAG), whereas normal lymphocytes or ZAP-70- CLL cells expressed nonactivated Hsp90. Activated Hsp90 bound and stabilized ZAP-70, which behaved like an Hsp90 client protein only in CLL cells. Treatment with Hsp90 inhibitors such as 17-AAG and 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG) induced ZAP-70 degradation and apoptosis in CLL cells but not in T cells, and also impaired B-cell receptor signaling in leukemia cells. Transduction of ZAP-70- CLL cells with an adenovirus encoding ZAP-70 activated Hsp90 and specifically rendered the leukemia cells sensitive to 17-AAG. These data indicate that Hsp90 is necessary for ZAP-70 expression and activity; that ZAP-70 is unique among Hsp90 clients, in that its chaperone-dependency is conditional on the cell type in which it is expressed; and also that ZAP-70 is required for cell survival and signaling in CLL. Additionally, ZAP-70 expression in CLL cells confers markedly heightened sensitivity to 17-AAG or 17-DMAG, suggesting that these or other Hsp90 inhibitors could be valuable therapeutically in patients with aggressive CLL.

Therapeutic and diagnostic implications of Hsp90 activation

The molecular chaperone heat-shock protein 90 (Hsp90) is involved in the stabilization and conformational maturation of many signaling proteins that are deregulated in cancers. Hsp90 inhibition results in the proteasomal degradation of these client proteins and leads to potent antitumor activity. The Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG) is presently in clinical trials. Recent work has identified the role of Hsp90 in multiple signal transduction pathways and revealed that the molecular mechanism of tumor selectivity by Hsp90 inhibitors is the result of an activated, high-affinity conformation of Hsp90 in tumors. This review discusses these recent advances in the understanding of tumor Hsp90 for the treatment and diagnosis of cancer. In addition, the role of Hsp90 in non-oncological diseases will also be discussed.

Hsp90 inhibitors as selective anticancer drugs

Extract: Cancer drug discovery has traditionally focused on targeting DNA synthesis and cell division, resulting in drugs that show efficacy but have severe side effects, due to their lack of selectivity for tumor cells over normal cells. One truly tumor-specific protein, the mutant kinase BCR-ABL, single-handedly causes chronic myelogenous leukemia and is the target of the remarkably effective new drug Gleevec. However, BCR-ABL is very much the exception. The majority of new molecularly-targeted drugs, such as kinase inhibitors, aim to exploit the overexpression of a particular kinase in the tumors compared to the normal tissues. However, the pitfall in doing that is that these drugs are aimed at single biological targets, while the vast majority of advanced tumors harbor multiple genetic alterations that drive malignant growth. A newly emerging class of drugs, called heat shock protein 90 (Hsp90) inhibitors, can simultaneously destroy multiple tumor-causing proteins and also have a profound therapeutic selectivity for tumor cells over normal cells. Hsp90 belongs to a family of proteins called molecular chaperones that are involved in the stabilization and folding of many signaling proteins (collectively referred to as Hsp90 "clients") that are dysregulated in cancers. Hsp90 client proteins include key regulators of cell proliferation and survival such as receptor tyrosine kinases, metastable/mutant signaling proteins, transcription factors and cell cycle regulators (Table 1). Hsp90 client proteins are major components of mitogenic signaling pathways that drive cellular proliferation and survival signaling pathways that counteract programmed cell death (apoptosis). Thus, Hsp90 inhibition can concurrently destablize many oncoproteins in numerous signaling pathways, suggesting that Hsp90 inhibitor drugs would be advantageous in destroying cancer cells that can easily overcome the inhibition of a single target or pathway.

A high-affinity conformation of Hsp90 confers tumour selectivity on Hsp90 inhibitors

Heat shock protein 90 (Hsp90) is a molecular chaperone that plays a key role in the conformational maturation of oncogenic signalling proteins, including HER-2/ErbB2, Akt, Raf-1, Bcr-Abl and mutated p53. Hsp90 inhibitors bind to Hsp90, and induce the proteasomal degradation of Hsp90 client proteins. Although Hsp90 is highly expressed in most cells, Hsp90 inhibitors selectively kill cancer cells compared to normal cells, and the Hsp90 inhibitor 17-allylaminogeldanamycin (17-AAG) is currently in phase I clinical trials. However, the molecular basis of the tumour selectivity of Hsp90 inhibitors is unknown. Here we report that Hsp90 derived from tumour cells has a 100-fold higher binding affinity for 17-AAG than does Hsp90 from normal cells. Tumour Hsp90 is present entirely in multi-chaperone complexes with high ATPase activity, whereas Hsp90 from normal tissues is in a latent, uncomplexed state. In vitro reconstitution of chaperone complexes with Hsp90 resulted in increased binding affinity to 17-AAG, and increased ATPase activity. These results suggest that tumour cells contain Hsp90 complexes in an activated, high-affinity conformation that facilitates malignant progression, and that may represent a unique target for cancer therapeutics.

Suicide gene therapy with Herpes simplex virus thymidine kinase and ganciclovir is enhanced with connexins to improve gap junctions and bystander effects

Connexins are proteins that form gap junctions between cells in various mammalian tissues. Because of their role in intercellular communication, connexins are important in the bystander cell death seen in Herpes simplex virus-thymidine kinase (HSV-TK) gene therapy for brain tumors. A selective review of connexin transduction/transfection studies with particular emphasis to central nervous system tumor cells is presented. In addition, specific references to studies with cell types that demonstrate low gap junction intercellular communication are presented. Data are included with the HT-29 colorectal tumor cell line to support the concept that enhancing gap junction protein expression in otherwise low gap junction communicating HT-29 cells increases bystander cell death and reduces tumor burden beyond what might be expected from HSV-TK and ganciclovir (GCV) treatment alone. Maximum in vitro bystander cell death was always produced when GCV treated co-cultures of TK-transduced and non-TK-transduced HT-29 cell lines were also transduced with connexin-43. When connexin was present in only one group of cells in the co-culture, there was more bystander cell death observed with connexin transduced into the non-TK-transduced cells, rather than the TK-transduced cells. The data presented reinforces conclusions made from earlier findings from cell line mixing experiments in which the non-TK-transduced cell population determined the level of bystander cell death (Burrows et al., 2002).

Purified herpes simplex virus thymidine kinase retroviral particles: III. Characterization of bystander killing mechanisms in transfected tumor cells

An important consequence of the suicide gene therapeutic paradigm is the phenomenon of bystander cell killing, the death of adjacent tumor cells not transduced with the thymidine kinase (TK) gene from herpes simplex virus (HSV) after treatment with the antiviral drug, ganciclovir (GCV). Evidence from quantitative in vitro assays of glioma cell lines suggest that both murine and human gliomas are similar in expressing high sensitivity to the bystander effect. In five of six glial tumors examined, the presence of only 5% of HSV-TK-expressing transduced cells in the culture resulted in >90% tumor cell death/stasis after addition of GCV. Several lines of evidence support gap junction intercellular communication (GJIC) as important in the bystander effect. In vitro metabolic assays, performed with GCV in the medium, indicated that more tumor burden was reduced when culture conditions supported cell-cell contact of parental and HSV-TK-transduced cells. Additionally, a double dye transfer assay showed that cell communication through the gap junction is greatest for glioma, less for melanoma, and much less for colorectal carcinoma cell lines. In vitro metabolic assays with mixtures of TK+/TK- homologous tumor cells confirmed that glioma cell lines were more susceptible to bystander killing than melanomas. Assays with chimeric tumor mixtures of TK+/TK - cells showed that the level of the bystander killing obtained was characteristic of the TK-bystander cells. The in vitro findings were confirmed in vivo with GCV-treated homologous and chimeric tumors composed of TK+/TK- cells. Day 21 mean tumor volumes (MTVs) indicated the growths obtained were characteristic of the bystander activity reflective of the nontransduced cell population. Furthermore, nontransduced, high-GJIC cells in a chimeric tumor mass appeared to effectively bridge between transduced tumor cells and poorly communicating nontransduced cells. Finally, the importance of a gap junction protein, such as connexin-43, in facilitating the bystander effect was demonstrated with the HT29 low-GJIC cell line. When the TK-nontransduced cell population expressed connexin-43, a better bystander kill was achieved compared to the parental counterpart.

Survivin and the inner centromere protein INCENP show similar cell-cycle localization and gene knockout phenotype

BACKGROUND

Survivin is a mammalian protein that carries a motif typical of the inhibitor of apoptosis (IAP)proteins, first identified in baculoviruses. Although baculoviral IAP proteins regulate cell death, the yeast Survivin homolog Bir1 is involved in cell division. To determine the function of Survivin in mammals, we analyzed the pattern of localization of Survivin protein during the cell cycle, and deleted its gene by homologous recombination in mice.

RESULTS

In human cells, Survivin appeared first on centromeres bound to a novel para-polar axis during prophase/metaphase, relocated to the spindle midzone during anaphase/telophase, and disappeared at the end of telophase. In the mouse, Survivin was required for mitosis during development. Null embryos showed disrupted microtubule formation, became polyploid, and failed to survive beyond 4.5days post coitum. This phenotype, and the cell-cycle localization of Survivin, resembled closely those of INCENP. Because the yeast homolog of INCENP, Sli15, regulates the Aurora kinase homolog Ipl1p, and the yeast Survivin homolog Bir1 binds to Ndc10p, a substrate of Ipl1p, yeast Survivin, INCENP and Aurora homologs function in concert during cell division.

CONCLUSIONS

In vertebrates, Survivin and INCENP have related roles in mitosis, coordinating events such as microtubule organization, cleavage-furrow formation and cytokinesis. Like their yeast homologs Bir1 and Sli15, they may also act together with the Aurora kinase.

Purified herpes simplex thymidine kinase retroviral particles. II. Influence of clinical parameters and bystander killing mechanisms

High-titer, purified herpes simplex virus thymidine kinase (HSV-TK) retroviral particles, followed with intraperitoneal ganciclovir (GCV) were tested in Fischer rats bearing 1-week established 9L gliosarcomas. 9L cells were infused intracranially through a cannula on day 0, given intracranial infusions of HSV-TK retroviral particles on days 7-12, and given 5 or 10 daily doses of intraperitoneal GCV starting at day 14. Tumor volumetric studies performed on rat brains at day 26 after tumor infusion revealed significant differences in experimental groups receiving HSV-TK retroviral particles plus 10-day GCV or the 100% transduced 9L-TK group receiving GCV versus control groups treated with either buffer, HSV-TK vector, or either 5- or 10-day regimens of GCV alone. The duration of GCV administration and the volume of tumor burden influenced efficacy. Adjuvant dexamethasone did not significantly affect efficacy. Experiments in which 9L rechallenge of animals treated with 9L-TK/GCV or 9L tumors treated with HSV-TK vector/GCV indicated that a host immune response was involved in rejecting the rechallenge tumor. Outcome was dependent upon the site and size of the rechallenge inoculum. In vitro, bystander effects were significant but were especially marked when cell-to-cell contact was maintained. Cumulatively, the data indicate that both the bystander effect and the host immune response are responsible for the efficacy observed in the suicide gene therapy paradigm using purified HSV-TK retroviral particles and GCV to treat smaller tumor burden in rats with 9L gliosarcoma.

Establishment of parameters for optimal transduction efficiency and antitumor effects with purified high-titer HSV-TK retroviral vector in established solid tumors

Suicide gene therapy using the herpes simplex thymidine kinase gene and ganciclovir is an attractive strategy for solid tumors. Early animal studies involved intratumoral injection of retroviral producer cells or unprocessed supernatant to generate an antitumor effect. Xenotransplantation of producer cells proved effective in several models, but the crude supernatants from the same cells were of insufficient titer to produce antitumor effects. We have developed new non-murine producer lines that yield replication-defective retroviral vectors encoding thymidine kinase at high titer which are then further purified and processed, resulting in pharmaceutical grade retroviral vectors with titers of up to 10(8) cfu/ml. Purified, high-titer retroviral preparations were injected directly into solid tumors in two syngeneic mouse tumor models. Significant antitumor responses and some cures were observed following systemic ganciclovir therapy. Assays using monoclonal antibodies to measure thymidine kinase protein expression at the single cell level in vitro and in vivo were developed so that therapeutic transgene expression could be quantified. Intralesional delivery resulted in transduction of over 20% of tumor cells in a protocol designed to maximize transduction on the basis of separate analyses of route, dosage, and schedule of vector administration. A consensus strategy evolved in which the combined effects of increased titer and a longer duration of retroviral vector administration interact to maximize transduction efficiency. These results indicate that purified high-titer retroviral vectors have the potential to transfer effective quantities of therapeutic genes into solid tumors in human subjects and highlight some pharmacologic factors that could be valuable in the design of clinical gene therapy protocols.

Purified herpes simplex thymidine kinase Retrovector particles. I. In vitro characterization, in situ transduction efficiency, and histopathological analyses of gene therapy-treated brain tumors

Replication-defective, highly purified retroviral vectors (Retrovector), at titers of 10(8) colony forming units/mL, were prepared that conferred either beta-galactosidase or herpes simplex thymidine kinase (HSV-TK) activity. 9L gliosarcoma cells, transduced efficiently in vitro, were highly sensitive to ganciclovir (GCV). The mean frequency of in situ transduction, measured by flow cytometry of single-cell tumor suspensions isolated from rat brains, was 3.2 +/- 0.6%; similar assessments were made by staining of beta-galactosidase or by immunohistochemistry with anti-HSV-TK. In vitro HSV-TK-transduced and G418-selected 9L-TK gliosarcoma tumors treated with GCV were eradicated in approximately 53% of the animals (10/19) at day 26, however, 89% (17/19) histologically showed < 1% tumor volume. Histologic evaluation at day 26 of animals with established 9L tumors treated with intralesional injection of HSV-TK vector followed by GCV treatment showed that 29% (4/14) had no tumor; 50% (7/14) had < 1% tumor volume. Regression of tumors proceeded over the time since the complete rate was increased at day 60. Neither HSV-TK vector particles nor GCV alone altered the histological profile of 9L tumors, but substantial numbers of CD4+ and CD8+ lymphocytes infiltrated the tumors of animals treated with both. In cured animals, the former tumor bed contained cell debris, immune cells, and fibroblasts and was without damage to adjacent brain. The efficacy of suicide gene therapy for rat gliosarcoma using highly purified virion vectors approaches that of packaging cell lines.

Up-regulation of endoglin on vascular endothelial cells in human solid tumors: implications for diagnosis and therapy

We have characterized a murine IgM monoclonal antibody, TEC-11, that recognizes endoglin and may be suitable for targeting cytotoxic agents to human tumor vasculature. TEC-11 strongly stains endothelial cells in a broad range of solid human tumors while staining endothelial cells in the majority of normal, healthy adult tissues relatively weakly. Human umbilical vein endothelial cells (HUVECs) in sections of the umbilical vein react weakly with TEC-11, whereas proliferating HUVECs in tissue culture react strongly and uniformly. HUVEC cultures grown to confluence and then rested contain two subpopulations having high and low levels of endoglin expression. Flow cytometry revealed that a significant proportion of cells with high endoglin expression are cycling, having markedly increased levels of cellular protein, RNA, and DNA by comparison to low endoglin-expressing cells, which appear to be noncycling. Taken together, the increased binding of TEC-11 to tumor vasculature and to dividing as opposed to noncycling HUVECs in vitro suggests that endoglin is an endothelial cell proliferation-associated marker. An immunotoxin [TEC-11.deglycosylated ricin A chain (dgA)] composed of TEC-11 and dgA was 3000-fold more potent at inhibiting protein synthesis in proliferating HUVEC cultures than in confluent cultures. The confluent cells were no more sensitive to TEC-11.dgA than they were to an isotype-matched immunotoxin of irrelevant specificity. These findings suggest that TEC-11.dgA might have therapeutic value in the treatment of solid tumors in humans by selectively killing dividing endothelial cells which are prevalent in such tumors.

Antibody-directed targeting of the vasculature of solid tumors

An attractive strategy for the therapy of carcinomas and other solid tumors would be to target cytotoxic agents or host immune effectors to the endothelial cells of the tumor vasculature rather than to the tumor cells themselves. The key advantage of this approach is that the endothelial cells are freely accessible through the blood whereas the tumor cells are, for the most part, inaccessible. Also, endothelial cells are similar in different tumors, making it feasible to develop a single reagent for treating numerous types of cancer. In this chapter, we review progress in this "vascular targeting" approach, from the validation of the concept in a mouse model to the characterization of the TEC-11 antibody against endoglin, an endothelial cell proliferation marker that is upregulated on endothelial cells in miscellaneous human solid tumors. In addition, we review other tumor endothelial cell markers that are candidates for vascular targeting in man.

Vascular targeting--a new approach to the therapy of solid tumors

An attractive approach to the therapy of solid tumors is to attack the endothelial cells of the tumor vascular bed rather than the tumor cells themselves, which circumvents the problem of poor penetration of tumor masses by monoclonal antibodies and other macromolecules. In this review, we will discuss the drawbacks of targeting solid tumors and the advantages of the 'vascular targeting' approach, describe the validation of the concept in a mouse model and summarize the properties of tumor endothelial cell markers, which are candidates for vascular targeting in humans.

Potent antitumor effects of an antitumor endothelial cell immunotoxin in a murine vascular targeting model

Immunotoxins and other antibody-based therapeutic reagents have proved effective agonist lymphomas and leukemias, but results with carcinomas and other solid tumors have thus far been less impressive. A major reason for this difference is that macromolecules penetrate poorly and unevenly into solid tumors. A solution to this problem would be to attack the endothelial cells of the tumor vascular bed rather than the tumor cells themselves. We have developed a murine model of this vascular targeting approach where transfection of the tumor cells with a cytokine gene causes them to induce the expression of an experimental marker (MHC Class II) on tumor endothelium. In this report we show that an anti-Class II-deglycosylated Ricin A-chain immunotoxin kills IFN-gamma-activated endothelial cells in culture and, when injected into tumor-bearing Balb/c nude mice, causes complete thrombosis of the tumor vasculature, widespread infarction, and dramatic regressions of large solid tumors. These findings suggest that immunoconjugates prepared with recently described antibodies against human tumor endothelium could provide a broad-based therapy for variety of solid cancers in humans.

Cytotoxicity of a novel anti-ICAM-1 immunotoxin on human myeloma cell lines

We have generated a murine monoclonal antibody (UV3) which recognizes an epitope on ICAM-1 expressed on myeloma cells. By flow cytometric analysis, the epitope on ICAM-1 recognized by this antibody is strongly expressed on human myeloma cells, pre-B leukemia cells and Burkitt's lymphoma cell lines. Most human T cell lines are weakly positive. The antibody does not react with red blood cells, polymorphonuclear leukocytes (PMNs) or resting B lymphocytes from normal donors, and reacts very weakly with resting T cells. Immunohistochemical assays indicate that the antibody does not react with normal liver, kidney, heart, brain, thymus or lung. An immunotoxin (IT) was prepared by coupling UV3 to deglycosylated ricin A-chain (dgA). In protein synthesis inhibition assays it was highly cytotoxic to the human myeloma cell lines HS-SULTAN (IC50 = 1 x 10(-11)M) and ARH-77 (IC50 = 9 x 10(-11)M), but not to cell lines of T cell lineage or most cell lines of the B lineage. Our results suggest that the UV3-dgA may have therapeutic potential for the treatment of human multiple myeloma.

Eradication of large solid tumors in mice with an immunotoxin directed against tumor vasculature

Antibody-based therapy of solid tumors has met with limited success, chiefly because solid tumors are relatively impermeable to macromolecules. This problem could be circumvented by attacking the readily accessible endothelial cells of the tumor vascular bed. We have developed a model to test this "vascular targeting" approach in which cytokine gene transfection of the tumor cells causes them to induce an experimental marker selectively on tumor vascular endothelium. An anti-tumor endothelial cell immunotoxin caused complete occlusion of the tumor vasculature and dramatic regressions of large solid tumors. By contrast, a conventional anti-tumor cell immunotoxin of equivalent in vitro potency produced only minor, transient antitumor effects but, when combined, the two immunotoxins induced permanent complete remissions in over half of the animals. These experiments indicate that immunotoxins directed against recently described markers on vascular endothelial cells in human tumors could provide a general treatment for solid tumors in humans.

A murine model for antibody-directed targeting of vascular endothelial cells in solid tumors

An attractive approach to the therapy of solid tumors would be to target cytotoxic agents or coagulants to the vasculature of the tumor rather than to the tumor cells themselves. This strategy has 3 advantages: (a) it should be applicable to many types of solid tumors because all require a blood supply for survival and growth; (b) the target endothelial cells are directly accessible through the blood and are normal cells, making the outgrowth of resistant mutants unlikely; and (c) there is an in-built amplification mechanism because thousands of tumor cells are reliant on each capillary for nutrients and oxygen. Despite its theoretical attractions, the approach of tumor vascular targeting has not been testable because antibodies that recognize tumor vascular endothelial cell antigens with adequate specificity are currently not available. In this study, we developed a model system in which to investigate the antibody-directed targeting of vascular endothelial cells in solid tumors in mice. A neuroblastoma transfected with the mouse interferon-gamma gene, C1300(Mu gamma), was grown in antibiotic-treated BALB/c nude mice. The interferon-gamma secreted by the tumor induces the expression of major histocompatibility complex Class II antigens on the tumor vascular endothelium. Class II antigens are absent from the vasculature of normal tissues, although they are present on B-lymphocytes, cells of monocyte/macrophage lineage, and some epithelial cells. Anti-Class II antibody administered i.v. strongly stains the tumor vasculature, whereas an antitumor antibody directed against a major histocompatibility complex Class I antigen of the tumor allograft produces classical perivascular tumor cell staining. This model should enable the theoretical superiority of tumor vascular targeting over conventional tumor cell targeting to be tested.

Influence of tumor-derived interleukin 1 on melanoma-endothelial cell interactions in vitro

Human melanoma cell lines that express high constitutive levels of the metastasis-associated marker intercellular adhesion molecule 1 (ICAM-1) were found to secrete interleukin 1 (IL-1) in vitro. Experiments with neutralizing antibodies showed that this cytokine was responsible for their expression of ICAM-1 but not that of two other progression/metastasis markers, Muc-18 and Gp IIb/IIIa. The IL-1 present in melanoma-conditioned medium induced the expression of vascular cell adhesion molecule 1, endothelial-leukocyte adhesion molecule 1, and ICAM-1 on human umbilical vein endothelial cells (ECs) in culture and increased the rate at which melanoma cells and ECs adhered to each other. IL-1-producing melanoma lines adhered significantly more rapidly to ECs than did non-IL-1-producing lines, and this enhancement was reduced by prior incubation of the melanoma cells with neutralizing anti-IL-1 antibodies. Similarly, endothelial cells treated with conditioned medium from IL-1-producing melanoma lines adhered significantly more rapidly to melanoma cells than did ECs treated with medium from non-IL-1-producing melanoma lines, and this enhancement was abolished by addition of anti-IL-1 antibodies to EC cultures in conditioned medium. Blocking antibodies to endothelial vascular cell adhesion molecule 1, endothelial-leukocyte adhesion molecule 1, and ICAM-1 failed to inhibit melanoma-EC adhesion, but an antibody to tumor cell GpIIb/IIIa did block adhesion by up to 44%. The ability to secrete IL-1 could increase the metastatic potential of melanoma cells by stimulating tumor cell-EC adhesion.

The rat placenta expresses paternal class I major histocompatibility antigens

The expression of paternally inherited class I MHC antigens on the placental trophoblast of the rat has been investigated using a mouse anti-rat monoclonal antibody (MN4-91-6) in an indirect immunoperoxidase labelling assay on cryostat sections. Strong specific staining was obtained on the spongy zone trophoblast of the mature placenta from DA male (RT1a) X PVG female (RT1c) matings. In marked contrast, no staining was observed on the labyrinthine trophoblast nor on the trophoblastic giant cells at any stage of gestation from 8 to 19 days post-coitum. None of the trophoblastic cell populations at any stage of gestation were reactive with an anti-class II monoclonal antibody. Class I positive endovascular cytotrophoblast cells were present in the maternal arterial sinusoids of the decidua. These findings imply that maternal immunoregulatory mechanisms must be essential for the survival of the placenta and fetus.