Modulation of tyrosine, serine, and threonine phosphorylation and intracellular processing of the epidermal growth factor receptor by antireceptor monoclonal antibody

GABRP sustains the stemness of triple-negative breast cancer cells through EGFR signaling

Effective treatment regimens for triple-negative breast cancer (TNBC) are relatively scarce due to a lack of specific therapeutic targets. Epidermal growth factor receptor (EGFR) signaling is highly active in TNBC and is associated with poor prognosis. Most EGFR antagonists, which significantly improve outcome in lung and colon cancer, have shown limited clinical effects in breast cancer. However, limiting EGFR expression in TNBC is a potential strategy for improving the clinical efficacy of EGFR antagonists. Here, we found that the gamma-aminobutyric acid type A receptor π subunit (GABRP), as a membrane protein enriched in TNBC stem cells, interacted with EGFR and significantly sustained its expression, resulting in stemness maintenance and chemotherapy resistance. Silencing GABRP induced down-regulation of EGFR signaling, which hindered cell stemness and enhanced sensitivity to chemotherapies, including paclitaxel, doxorubicin, and cisplatin. We also identified that retigabine, an FDA-approved drug for adjunctive treatment of seizures, increased the sensitivity of EGFR to gefitinib in gefitinib-resistant cells. Our findings show that GABRP can sustain the stemness of TNBC via modulating EGFR expression, suggesting that GABRP may be a potential therapeutic target that can address EGFR inhibitor resistance in TNBC.

Epidermal growth factor receptor (EGFR) involvement in epithelial-derived cancers and its current antibody-based immunotherapies

The epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein that is part of the family of tyrosine kinase receptors. The binding of EGFR to its cognate ligands leads to its autophosphorylation and subsequent activation of the signal transduction pathways involved in regulating cellular proliferation, differentiation, and survival. Accordingly, this receptor carries out both redundant and restricted functions in the germline development of mammals and in the maintenance of various adult tissues. Correspondingly, the loss of EGFR regulation results in many human diseases, with the most notable cancer. This receptor is overexpressed and/or mutated in multiple epithelial-derived tumors, and associated with poor prognosis and survival in cancer patients. Here, we discuss in detail the role of EGFR in specific epithelial-derived cancer pathologies; these include lung cancer, colorectal cancer, and squamous cell carcinomas. The development of multiple anticancer agents against EGFR diminished the progression and metastasis of tumors. Some of the most versatile therapeutic anti-EGFR agents include the monoclonal antibodies (mAbs), demonstrating success in clinical settings when used in combination with cytotoxic treatments, such as chemotherapy and/or radiation. We thus discuss the development and application of two of the most notable therapeutic mAbs, cetuximab, and panitumumab, currently utilized in various EGFR-related epithelial cancers.

A phase I study of gemcitabine + dasatinib (gd) or gemcitabine + dasatinib + cetuximab (GDC) in refractory solid tumors

PURPOSE

This study was conducted to define the maximum tolerated dose (MTD), recommended phase two dose (RPTD), and toxicities of gemcitabine + dasatinib (GD) and gemcitabine + dasatinib + cetuximab (GDC) in advanced solid tumor patients.

METHODS

This study was a standard phase I 3 + 3 dose escalation study evaluating two combination regimens, GD and GDC. Patients with advanced solid tumors were enrolled in cohorts of 3-6 to either GD or GDC. Gemcitabine was dosed at 1000 mg/m² weekly for 3 of 4 weeks, dasatinib was dosed in mg PO BID, and cetuximab was dosed at 250 mg/m² weekly after a loading dose of cetuximab of 400 mg/m². There were two dose levels for dasatinib: (1) gemcitabine + dasatinib 50 mg ± cetuximab, and (2) gemcitabine + dasatinib 70 mg ± cetuximab. Cycle length was 28 days. Standard cycle 1 dose-limiting toxicity (DLT) definitions were used. Eligible patients had advanced solid tumors, adequate organ and marrow function, and no co-morbidities that would increase the risk of toxicity. Serum, plasma, and skin biopsy biomarkers were obtained pre- and on-treatment.

RESULTS

Twenty-five patients were enrolled, including 21 with pancreatic adenocarcinoma. Three patients received prior gemcitabine. Twenty-one patients were evaluable for toxicity and 16 for response. Four DLTs were observed: Grade (Gr) 3 neutropenia (GDC1, n = 1), Gr 3 ALT (GD2, n = 2), and Gr 5 pneumonitis (GDC2, n = 1). Possible treatment-emergent adverse events (TEAEs) in later cycles included: Gr 3-4 neutropenia (n = 7), Gr 4 colitis (n = 1), Gr 3 bilirubin (n = 2), Gr 3 anemia (n = 2), Gr 3 thrombocytopenia (n = 2), Gr 3 edema/fluid retention (n = 1), and Gr 3 vomiting (n = 3). Six of 16 patients (3 of whom were gemcitabine-refractory) had stable disease (SD) as best response, median duration = 5 months (range 1-7). One gemcitabine-refractory patient had a partial response (PR). Median PFS was 2.9 months (95% CI 2.1, 5.8). Median OS was 5.8 months (95% CI 4.1, 11.8). Dermal wound biopsies demonstrated that dasatinib resulted in a decrease of total and phospho-Src levels, and cetuximab resulted in a decrease of EGFR and ERBB2 levels.

CONCLUSIONS

The MTD/RPTD of GD is gemcitabine 1000 mg/m² weekly for 3 of 4 weeks and dasatinib 50 mg PO BID. The clinical activity of GD seen in this study was modest, and does not support its further investigation in pancreatic cancer.

Delivery of a Proapoptotic Peptide to EGFR-Positive Cancer Cells by a Cyclic Peptide Mimicking the Dimerization Arm Structure of EGFR

A cyclic decapeptide, CQTPYYMNTC (1), which mimics the dimerization arm of the EGF receptor (EGFR), was previously found to be captured into cells. We have sought to investigate the promising potential of this peptide as an intracellular delivery vehicle directed to EGFR-positive cells. The selectivity of peptide 1 to the EGFR was confirmed by a positive correlation between the expression level of the receptor and the cellular uptake of peptide 1 as shown by siRNA knockdown of the EGFR. The proapoptotic domain (PAD) peptide ([KLAKLAK]₂) has limited use due to a deficiency of cell membrane permeability resulting from cationic sequences and lack of specificity for cancer cells. As a proof-of-concept study, the cellular delivery of the PAD peptide was challenged by conjugation with peptide 1. The cellular uptake of a conjugated peptide 2, which was composed of peptide 1, the PAD peptide, and a linker cleavable with a protease, was evaluated by treatment of an EGFR-positive lung carcinoma cell line, A549. Significant suppression of proliferation by peptide 2 was shown in the results of a cell viability assay. The PAD peptide alone had no effect on the cells. The results suggest that peptide 1 is a promising lead compound as a new intracellular delivery vehicle for therapeutically effective peptides.

EGFR Downregulation after Anti-EGFR Therapy Predicts the Antitumor Effect in Colorectal Cancer

Anti-EGFR mAb is reported to induce EGFR internalization in colorectal cancer cells. However, the biological relevance of EGFR internalization with anti-EGFR mAb is unknown. Therefore, the relevance of EGFR downregulation with anti-EGFR mAb to antitumor activity in colorectal cancer cells was investigated. Quantification of EGFR on the cell surface before cetuximab treatment was assessed by flow cytometry, and its growth-inhibitory effects were measured by Trypan blue exclusion, in 10 RAS, BRAF wild-type colorectal cancer cell lines, but there was no significant correlation between EGFR number and its growth-inhibitory effect. However, a significant correlation existed between the percentage decrease in the number of EGFRs after cetuximab treatment and its growth-inhibitory effect in those cell lines. Treatment with TGFα, a ligand for EGFR, induced EGFR internalization in colorectal cancer cells, but most EGFRs subsequently recycled to the cell surface, consistent with previous studies. While cetuximab treatment induced EGFR internalization, most receptors subsequently translocated into the late endosome, leading to lysosomal degradation, as revealed by immunoblotting and double immunofluorescence. Cetuximab-sensitive colorectal cancer cells showed greater EGFR internalization, stronger cell growth inhibition, and more augmented apoptotic signals than nonsensitive cells. IHC for EGFR, performed using an EGFR pharmDx Kit (mouse anti-human EGFR mAb clone 2-18C9), in clinical specimens before and after anti-EGFR mAb therapy in 13 colorectal cancer patients showed a significant correlation between the response to anti-EGFR mAb and decreased staining after therapy.Implications: This report clearly demonstrates that anti-EGFR mAb facilitates internalization and subsequent degradation of EGFRs in lysosomes, which is an important determinant of the efficacy of anti-EGFR mAb treatment for colorectal cancer. Mol Cancer Res; 15(10); 1445-54. ©2017 AACR.

Responsive Contrast Agents: Synthesis and Characterization of a Tunable Series of pH-Sensitive Near-Infrared Pentamethines

The demand for responsive dyes in optical imaging is high to achieve a better signal-to-noise ratio and, more specifically, to visualize acidic compartments of the endocytic pathway. Herein, we present a new synthetic route, with a step-by-step synthesis of water-soluble pH-sensitive cyanine dyes exhibiting pKa values in the region of physiological pH, as confirmed by absorption and fluorescence spectra. Moreover, modification of pKa values was achieved by two different substitution patterns, creating tunable pH-sensitive dyes. We demonstrated the functionality of the pH-sensitive dyes and their suitability as contrast agents for cellular uptake studies by preparing dye-labeled cetuximab and transferrin conjugates. Sulfonated head chains increased water solubility and prevented the formation of dimers, even in the context of dye-labeled bioconjugates. Confocal microscopy images of living cells revealed their pH-responsiveness, as specific fluorescence signal enhancements were observed in acidic compartments of the endocytic pathway (endosomes and lysosomes), although the background signal was low in a pH-neutral environment. Using mixtures of conjugates labeled with either a pH-sensitive or non-pH-sensitive dye for the uptake studies, we could follow the receptor binding and distinguish it from the endocytic uptake process of the conjugates in a simultaneous manner. Moreover, we used flow cytometry to quantify the fluorescence and observed a 3-fold signal enhancement for the pH-sensitive dye conjugates over a period of 3 h.

Functional evaluation of fluorescein-labeled derivatives of a peptide inhibitor of the EGF receptor dimerization

A cyclic decapeptide (1, ), which acts on the extracellular region of the EGF receptor, preventing it from dimerizing, has been developed. Peptide 2, which was labeled with fluorescein at the N-terminus of peptide 1, was synthesized based on structure-activity relationship studies. Peptide 2 essentially retained the inhibitory activity of peptide 1 against the receptor autophosphorylation. Confocal microscopy studies revealed that in carcinoma cells, the fluorescence of peptide 2 was localized inside some vesicles. Treatment of intact cells by peptide 1 in combination with peptide 2 decreased the fluorescence intensity significantly compared to treatment with only peptide 2. These results indicate that peptide 2 competes with peptide 1 for binding to the cellular surface. Six derivatives of peptide 2, in which constituent amino acids, with the exception of two cysteines and proline were randomized, were synthesized and used to treat the cells. Peptides 6 and 9 showed the highest fluorescence intensity in cells. From the results of the EGF receptor autophosphorylation assay, these two derivatives were proven to have higher inhibitory activity than peptide 2, which would therefore be a useful delivery peptide and fluorescent probe to find new inhibitors against the EGF receptor. Peptides 6 and 9 are promising leads for EGF receptor inhibitors.

Enhancement of tumor uptake and therapeutic efficacy of EGFR-targeted antibody cetuximab and antibody-drug conjugates by cholesterol sequestration

Cetuximab, a monoclonal antibody (mAb) targeting the epidermal growth factor receptor (EGFR), has been intensively investigated as a promising cancer treatment strategy. The specific mechanism of cetuximab endocytosis and its influence on cetuximab uptake, biodistribution and efficacy still remain elusive. Recently, statins have been reported to synergize with EGFR-targeting agents. Our prior work established that nystatin, a cholesterol-sequestering antifungal drug, facilitates endocytosis via the clathrin-dependent pathway. This study aimed to investigate whether nystatin regulates the uptake and efficacy of cetuximab and cetuximab-based antibody-drug conjugates (cetuximab-ADCs). In vitro and in vivo efficacies of nystatin on the uptake and activity of cetuximab/cetuximab-ADCs were studied in multiple human carcinoma cell lines and xenograft models, respectively. We identified that cholesterol sequestration by nystatin enhanced cetuximab internalization in EGFR-positive carcinoma cells by regulating EGFR trafficking/turnover and facilitating a switch from lipid rafts to clathrin-mediated endocytosis. Combination treatment with cetuximab and nystatin selectively increased cetuximab uptake by tumor tissues, translating into potentiated antitumor efficacy of cetuximab in vivo (A431 and A549 tumors). Nystatin-enhanced internalization of cetuximab further improved the uptake and potency of cetuximab-doxorubicin and cetuximab-methotrexate conjugates in EGFR-positive cetuximab-resistant tumors. Combination therapy with nystatin plus either cetuximab or cetuximab-ADC further prolonged animal survival and significantly suppressed tumor growth, as compared with single-agent cetuximab or cetuximab-ADC. In summary, our results identify a novel mechanism whereby cholesterol sequestration enhances the uptake of EGFR-targeting mAb and ADCs, therefore providing preclinical proof-of-concept that combination with nystatin can potentiate the delivery and efficacy of these EGFR-targeted agents.

Cetuximab in combination with anti-human IgG antibodies efficiently down-regulates the EGF receptor by macropinocytosis

The monoclonal antibody C225 (Cetuximab) blocks binding of ligand to the epidermal growth factor receptor (EGFR). In addition, it is known that incubation with C225 induces endocytosis of the EGFR. This endocytosis has previously been shown to be increased when C225 is combined with an additional monoclonal anti-EGFR antibody. However, the effects of antibody combinations on EGFR activation, endocytosis, trafficking and degradation have been unclear. By binding a secondary antibody to the C225-EGFR complex, we here demonstrate that a combination of antibodies can efficiently internalize and degrade the EGFR. Although the combination of antibodies activated the EGFR kinase and induced ubiquitination of the EGFR, the kinase activity was not required for internalization of the EGFR. In contrast to EGF-induced EGFR down-regulation, the antibody combination efficiently degraded the EGFR without initiating downstream proliferative signaling. The antibody-induced internalization of EGFR was found not to depend on clathrin and/or dynamin, but depended on actin polymerization, suggesting induction of macropinocytosis. Macropinocytosis may cause internalization of large membrane areas, and this could explain the highly efficient internalization of the EGFR induced by combination of antibodies.

Colorectal cancer in review: the role of the EGFR pathway

IMPORTANCE OF THE FIELD

The overexpression of EGFR has been documented in 30-90% of cases of advanced colorectal cancer (CRC). An increased understanding of the EGFR pathway in CRC has paved the way for the development of other targeted agents to augment therapeutic efficacy as well as for efforts to circumvent tumor resistance to therapy.

AREAS COVERED IN THIS REVIEW

Our aim is to discuss the recent progress in the role of the EGFR pathway, the status of anti-EGFR therapeutic agents currently in use and the rationale for the development of novel agents that work along the pathway for the treatment of CRC.

WHAT THE READER WILL GAIN

The readers will learn about the development and evolution of mAbs directed against EGFR as well as tyrosine kinase inhibitors in the management of CRC patients. In the same vein, determination of optimal dosing and better methods of defining those subsets of patients most likely to benefit will be discussed.

TAKE HOME MESSAGE

All these data must encourage clinicians and basic researches to continue in their efforts to untangle the network behind EGFR and try to focus all that effort towards improving patient's quality of life as well as survival.

Cetuximab/C225-induced intracellular trafficking of epidermal growth factor receptor

The monoclonal antibody C225 interacts with the ectodomain of the epidermal growth factor (EGF) receptor (EGFR) to block ligand binding and initiates receptor endocytosis and intracellular trafficking. The data herein show that C225-dependent EGFR trafficking relocalizes the receptor to the endoplasmic reticulum (ER) and nucleus. This mechanism, which also involves interaction of the C225-internalized receptor with the Sec61 translocon within the ER, is, in most respects, analogous to the pathway previously described for EGF-induced trafficking to the ER and nucleus. However, although inhibition of receptor tyrosine kinase activity blocks EGF-induced nuclear localization of the receptor, the same kinase inhibitors stimulate C225-dependent nuclear localization of EGFR in the nucleus. In contrast, the kinase inhibitor Lapatinib fails to stimulate nuclear accumulation of the receptor in C225-treated cells and does not provoke receptor dimerization as do inhibitors that recognize the open conformation of the receptor kinase. This suggests that inhibitor-dependent receptor dimerization may facilitate C225-induced receptor trafficking.

Epidermal Growth Factor Receptor (EGFR) Antibody Down-regulates Mutant Receptors and Inhibits Tumors Expressing EGFR Mutations

Activating mutations in the kinase domain of the EGF receptor have been reported in non-small cell lung cancer. The majority of tumors expressing these mutants are sensitive to ATP mimetics that inhibit the EGFR tyrosine kinase. The effect of antibodies that bind to the ectodomain of the receptor is less clear. We report herein the effects and mechanisms of action of the antibody cetuximab in lung cancer cells that naturally express receptor mutations and in ErbB-null 32D hematopoietic cells transfected with mutant EGFR. Treatment with cetuximab down-regulated EGFR levels and inhibited cell growth both in vitro and in vivo. This was associated with inhibition of ligand-independent EGFR signaling. These effects were seen in 32D cells arguing the growth inhibitory action was not because of the blockade of autocrine ligand action. Both antibody-induced EGFR down-regulation and inhibition of growth required receptor dimerization as monovalent Fab fragments only eliminated receptor levels or reduced cell proliferation in the presence of antihuman IgG. Finally, cetuximab inhibited growth of H1975 lung cancer cells and xenografts, which expressed L858R/T790M EGFR and were resistant to EGFR tyrosine kinase inhibitors. These data suggest that cetuximab is an effective therapy against mutant EGFR-expressing cancer cells and thus can be considered in combination with other anti-EGFR molecules.

Effect of the anti-receptor ligand-blocking 225 monoclonal antibody on EGF receptor endocytosis and sorting

The anti-receptor antibody, 225 mAb, is known to block binding of ligand to the epidermal growth factor receptor (EGFR). However, the effect of this neutralizing antibody on EGFR endocytosis, trafficking and degradation remains unclear. Here, we demonstrate that endocytosis of (125)I-225 mAb occurs, albeit with a slower rate than that of EGF. Using pulse chase assays, we show that internalized (125)I-225 mAb is recycled to the surface much more efficiently than internalized (125)I-EGF. Also, we found that internalization of (125)I-225 mAb, in contrast to that of EGF, is independent of receptor tyrosine kinase activity, as evidenced by its insensitivity to AG1478, a specific EGFR tyrosine kinase inhibitor. Analysis of the levels of cell surface and total EGFR showed that treatment with 225 mAb results in a 30-40% decrease in surface EGFR and a relatively slow downregulation of total EGFR. Taken together, these data indicate that 225 mAb induces internalization and downregulation of EGFR via a mechanism distinct from that underlying EGF-induced EGFR internalization and downregulation.

Monoclonal antibody therapy of cancer

The most significant recent advances in the application of monoclonal antibodies (mAbs) to oncology have been the introduction and approval of bevacizumab (Avastin), an anti-vascular endothelial growth factor antibody, and of cetuximab (Erbitux), an anti-epidermal growth factor antibody. In combination with standard chemotherapy regimens, bevacizumab significantly prolongs the survival of patients with metastatic cancers of the colorectum, breast and lung. Cetuximab, used alone or with salvage chemotherapy, produces clinically meaningful anti-tumor responses in patients with chemotherapy-refractory cancers of the colon and rectum. In addition, the anti-HER2/neu antibody trastuzumab (Herceptin), in combination with standard adjuvant chemotherapy, has been shown to reduce relapses and prolong disease-free and overall survival in high-risk patients after definitive local therapy for breast cancer. These exciting recent results provide optimism for the development of mAbs that bind novel targets, exploit novel mechanisms of action or possess improved tumor targeting. Progress in the clinical use of radioimmunoconjugates remains hindered by complexity of administration, toxicity concerns and insufficiently selective tumor targeting.

Multicenter phase I/II study of cetuximab with paclitaxel and carboplatin in untreated patients with stage IV non-small-cell lung cancer

PURPOSE

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors have demonstrated antitumor activity in patients with non-small-cell lung cancer (NSCLC). This study examined the safety profile of the monoclonal antibody EGFR inhibitor, cetuximab, when added to paclitaxel and carboplatin in untreated patients with stage IV NSCLC. Secondary objectives included efficacy and paclitaxel and carboplatin pharmacokinetics during cetuximab treatment.

PATIENTS AND METHODS

Patients with tumor evidence of EGFR by immunohistochemistry, performance status of 0 to 2, and measurable disease received paclitaxel 225 mg/m2 with carboplatin area under the curve = 6 on day 1 every 3 weeks. Cetuximab was administered at 400 mg/m2, 1 week before paclitaxel and carboplatin, then weekly at 250 mg/m2. The regimen continued until disease progression or intolerable toxicity.

RESULTS

Thirty-one of 32 enrolled patients were treated. The most common cetuximab toxicity was rash in 84% of patients (grade 3 in 13%). Pharmacokinetic sampling did not reveal an interaction between carboplatin, paclitaxel, and cetuximab. An objective response was observed in eight patients (26%). With a median follow-up of 19 months, the median time to progression was 5 months, median survival was 11 months, and the 1- and 2-year survival rates were 40% and 16%, respectively.

CONCLUSION

The combination of cetuximab, paclitaxel, and carboplatin was safe and well tolerated in this population of stage IV patients. The response rate, time to progression, and median survival were slightly superior to historical controls treated with paclitaxel and carboplatin alone. A randomized phase II trial has completed accrual.

Monoclonal antibody therapy for cancer

Monoclonal antibody therapy has emerged as an important therapeutic modality for cancer. Unconjugated antibodies show significant efficacy in the treatment of breast cancer, non-Hodgkin's lymphoma, and chronic lymphocytic leukemia. Promising new targets for unconjugated antibody therapy include cellular growth factor receptors, receptors or mediators of tumor-driven angiogenesis, and B cell surface antigens other than CD20. Immunoconjugates composed of antibodies conjugated to radionuclides or toxins show efficacy in non-Hodgkin's lymphoma. One immunoconjugate containing an antibody and a chemotherapy agent exhibits clinically meaningful antitumor activity in acute myeloid leukemia. Numerous efforts to exploit the ability of antibodies to focus the activities of toxic payloads at tumor sites are under way and show early promise. The ability to create essentially human antibody structures has reduced the likelihood of host-protective immune responses that otherwise limit the duration of therapy. Antibody structures now can be readily manipulated to facilitate selective interaction with host immune effectors. Other structural manipulations that improve the selective targeting properties and rapid systemic clearance of immunoconjugates should lead to the design of effective new treatments, particularly for solid tumors.

Defective cleavage of membrane bound TGFalpha leads to enhanced activation of the EGF receptor in malignant cells

Transforming growth factor alpha (TGFalpha) is widely expressed in malignant as well as normal cells and is involved in regulating cell growth and differentiation. Although processing of TGFalpha has been extensively studied in normal cells, there is little information regarding TGFalpha cleavage in malignant cells. Therefore, we compared the processing of TGFalpha in two human colon carcinoma cell lines. We found that there was a defective cleavage pattern for the TGFalpha precursor resulting in retention of partially processed TGFalpha on the cell surface of both the HCT116a2alphaS3 and CBS4alphaS2 cell lines. This raised the possibility that signaling from the resulting defective cleavage species could differ from that of soluble TGFalpha. The membrane-associated TGFalpha induced higher phosphorylation of EGFR on the cell surface of adjacent cells than equivalent levels of mature TGFalpha. The interaction of membrane bound TGFalpha precursor with the EGFR caused a slower internalization of activated EGFR relative to the internalization of the soluble TGFalpha/EGFR complexes. In addition, the tethered TGFalpha was resistant to the ability of protein-tyrosine phosphatases (PTPs) to reduce EGFR tyrosine phosphorylation, also contributing to higher activation of EGFR. The enhanced activation of EGFR by the tethered form of TGFalpha was reflected by higher activation of Grb2, SHC and Erk downstream mediators of EGF receptor signaling. The higher activation of EGFR by membrane tethered TGFalpha indicates that defective TGFalpha processing provides a mechanism whereby malignant cells can obtain a growth advantage over normal cells.

Epidermal growth factor receptor inhibition in cancer therapy: biology, rationale and preliminary clinical results

The epidermal growth factor receptor (EGFR), a growth factor receptor involved in the regulation of cellular differentiation and proliferation, is highly expressed by many tumor cells. In light of a relationship between overexpression of EGFR and clinically aggressive malignant disease, EGFR has emerged as a promising target for cancer therapy. In recent years, several molecular strategies have been explored to modulate either the EGFR itself, or the downstream signal beyond the cell surface receptor. One of the most promising current strategies involves the use of anti-EGFR monoclonal antibodies (mAbs), either alone or in combination with conventional cytotoxic modalities such as chemotherapy or radiotherapy. This review focuses primarily on recent progress in the development of anti-EGFR mAbs, and examines their potential in the treatment of cancer.

Antiepidermal growth factor receptor antibodies augment cytotoxicity of chemotherapeutic agents on squamous cell carcinoma cell lines

Recent literature by Fan et al (1993) demonstrated that addition of cisplatin and monoclonal antibody to the epidermal growth factor receptor (EGFR MAb) of the human squamous cell carcinoma (SCC), A431, eradicated gross tumors in nude mice. To determine whether a combination of either cisplatin or 5-fluorouracil (5-FU) with EGFR MAb could affect an SCC other than the A431 tumor model, an assay using 2 human tongue SCC cell lines, BroTo and SCC-25, was performed. Cells were pretreated with 1.25 microgram/mL cisplatin or 10 microgram/mL 5-FU. After a 4-hour incubation period, cisplatin-treated cells were washed with phosphate-buffered saline solution. Various concentrations of EGFR MAb were then added, and after a 24-hour incubation period, an MTT cell growth assay was performed. SCC-25 cells exhibited a greater decrease in growth with the addition of 16 nmol/L EGFR MAb to cisplatin compared with the cytotoxicity of cisplatin alone (P < 0.001). However, this combination did not produce similar results with BroTo cells (P > 0.05). The combination of EGFR MAb and 5-FU produced a growth inhibition versus control (unexposed cells) in both cell lines (P < 0.05). These findings suggest the possible augmentation of the activity of chemotherapeutic agents, cisplatin and/or 5-FU, with the addition of EGFR MAb on SCC cell lines other than A431.

Type I receptor tyrosine kinases as targets for therapy in breast cancer

Breast carcinomas express high levels of type I tyrosine kinase receptors and their ligands. For these reason therapies directed at these receptors have the potential to be useful anticancer agents. A series of monoclonal antibodies (MAbs)3 directed against the EGF receptor and the closely related erbB2/HER2/neu receptor are currently under evaluation. These MAbs have shown promising preclinical activity and "chimeric" and "humanized" MAbs have been produced in order to obviate the problem of host immune reactions. These antibodies are currently being tested in clinical trials either alone or in combination with chemotherapeutic agents. Clinical activity with anti-HER2/neu MAbs has been documented in patients with advanced breast cancer. In addition, compounds that inhibit receptor tyrosine kinases have shown significant preclinical activity and are potential candidates for clinical testing.

Receptor blockade with monoclonal antibodies as anti-cancer therapy

Human tumors express high levels of growth factors and their receptors, and many types of malignant cells appear to exhibit autocrine- or paracrine-stimulated growth. Therefore, antireceptor directed therapies have the potential of being useful anti-cancer agents. A series of murine monoclonal antibodies (MAbs) directed against human growth factor receptors and their corresponding growth factors have been produced. MAbs against the receptors for epidermal growth factor, Her2/Neu, transferrin, insulin-like growth factor, interleukin, (IL)-2 and IL-1 are currently being evaluated. MAbs directed against epidermal growth factor, transforming growth factor-alpha, bombesin, IL-2, and IL-6 also are under study. These MAbs have shown promising preclinical activity, and some of them are being tested in clinical trials. So far, anti-tumor responses have been observed with anti-IL-2 receptor, anti-bombesin and anti-IL-6 MAbs. Further research is focusing in the production of "chimeric" and "humanized" MAbs, in order to obviate the problem of host immune reactions.

The calpain cleavage sites in the epidermal growth factor receptor kinase domain

The proteolysis of the human epidermal growth factor receptor cytoplasmic domain by calpain has been studied in vitro using purified recombinant cytoplasmic domain expressed in insect cells. Limited proteolysis produced kinase that was truncated at either N- or C-termini, as well as in the hinge region. We identified seven sites of calpain proteolysis by N-terminal sequencing of purified fragments. Calpain cleaved between the catalytic and autophosphorylation domains at two sites in the sequence Gln996-Asp1059, in the hinge region. Three new sites were also found in the autophosphorylation domain, preceding each of the major autophosphorylation sites. A fourth new site was located in the juxta-membrane domain, C-terminal to the regulatory Thr654. We purified an active 42-kDa fragment generated by calpain proteolysis between Leu659-Gln660 in the juxta-membrane domain, and in the hinge region. A fifth new site of calpain cleavage was found between the nucleotide binding motif Gly-Xaa-Gly-Xaa-Xaa-Gly and the essential Lys721 in the catalytic core of the kinase. Since both of these features are required for catalysis, calpain cleavage at this site may potentially provide a mechanism for down-regulation of kinase activity in vivo, under conditions of calpain activation. Thus the distribution of calpain cleavage sites along the kinase domain is consistent with a role for calpain both as a processing and as a degradative protease in epidermal growth factor receptor signalling.

The epidermal growth factor receptor as a target for therapy in breast carcinoma

The epidermal growth factor (EGF) receptor and its ligands have an important regulatory role in breast carcinoma. We have produced a series of monoclonal antibodies (MAbs) directed against the external portion of the EGF receptor. These MAbs prevent the binding of the ligands to the receptor, block ligand-induced activation of the receptor, and can inhibit the growth of breast cancer cells both in tissue culture and in human tumor xenografts in nude mice. We have also shown that anti-EGF receptor antibodies greatly enhance the antitumor effects of chemotherapeutic agents active in breast cancer. Phase I clinical trials with single doses of MAb conducted in patients with tumors over-expressing EGF receptors demonstrated favorable pharmacokinetics, good tumor imaging, and a lack of toxicity. A human:murine chimeric antibody has been produced with comparable affinity and antitumor activity that will enable us to administer repeated doses of MAb either alone or in combination with chemotherapy. Our pre-clinical data support the concept that the EGF receptor may be an optimal target for treatment with receptor blocking antibodies, either alone or in combination with chemotherapy.

Effects of epidermal growth factor on invasiveness through the extracellular matrix in high- and low-metastatic clones of RCT sarcoma in vitro

We investigated the invasiveness of tumor cells through the extracellular matrix and the influence of epidermal growth factor (EGF) on tumor cell invasion using in vitro systems in high-[RCT(+)] and low-metastatic [RCT(-)] clones established from poorly differentiated murine RCT sarcoma in C3H/He mice. In the invasion assay using a filter coated with reconstituted basement membrane (Matrigel) in a Boyden chamber, RCT(+) cells were more invasive than RCT(-) cells. The attachment of RCT(+) cells to extracellular matrix components and the degradation of type IV collagen by the cells were significantly greater than with RCT(-) cells. However, there was no significant difference in the migration of cells to the extracellular matrix components between cultured RCT(+) and RCT(-) cells. These findings suggested that the different invasiveness of these clone cells was associated with the difference in the ability of attachment to and degradation of the matrix. The level of laminin receptor expression in RCT(+) cells was about four-fold that in RCT(-) cells and laminin stimulated the type IV collagenolytic activity of RCT(+) cells, suggesting that RCT(+) cell attachment to laminin via laminin receptor on the cell surface induced the production of type IV collagenase by the tumor cells. EGF did not affect the invasiveness of RCT(-) cells. In RCT(+) cells, EGF stimulated the invasiveness through Matrigel, the attachment to extracellular matrix components and the degradation of type IV collagen through high-affinity EGF receptors (EGFR), with Kd of pM order, while the migration to the matrix was not influenced by EGF. These findings suggest that the stimulatory effect of EGF on invasion is related to the acceleration of cell adhesion, and the degradative cascade of the extracellular matrix and high-affinity EGFRs play an important role in the effect of EGF on in vitro invasiveness in this tumor.

Ligand-mediated negative regulation of a chimeric transmembrane receptor tyrosine phosphatase

CD45, a transmembrane protein tyrosine phosphatase (PTPase), is required for TCR signaling. Multiple CD45 isoforms, differing in the extracellular domain, are expressed in a tissue- and activation-specific manner, suggesting an important function for this domain. We report that a chimeric protein in which the extracellular and transmembrane domains of CD45 are replaced with those of the EGF receptor (EGFR) is able to restore TCR signaling in a CD45-deficient cell. Thus, the cytoplasmic domain of CD45 is necessary and sufficient for TCR signal transduction. Moreover, EGFR ligands functionally inactivate the EGFR-CD45 chimera in a manner that is dependent on dimerization of the chimeric protein. Inactivation of EGFR-CD45 chimera function results in the loss of TCR signaling, indicating that CD45 function is continuously required for TCR-mediated proximal signaling events. These results suggest that ligand-mediated regulation of receptor-PTPases may have mechanistic similarities with receptor tyrosine kinases.

Regulation of TGF-alpha expression in human keratinocytes: PKC-dependent and -independent pathways

Transforming growth factor-alpha (TGF-alpha) is an autocrine growth factor for epidermal keratinocytes that can induce its own expression (autoinduction). Because the regulation of this process may be important for the control of epidermal growth, we examined the roles of EGF receptor tyrosine kinase and protein kinase C (PKC) in TGF-alpha autoinduction in cultured human keratinocytes. Antiphosphotyrosine immunoblot analysis demonstrated that EGF and TGF-alpha rapidly and markedly stimulated tyrosine phosphorylation of a 170 kDa protein in growth factor-deprived keratinocytes. This protein was identified as the EGF receptor by immuno-precipitation using anti-EGF receptor mAbs. Tyrosine phosphorylation and TGF-alpha mRNA accumulation in response to EGF and TGF-alpha were both inhibited by a monoclonal antibody against the EGF receptor and by the EGF receptor tyrosine kinase inhibitor RG50864, demonstrating the involvement of the tyrosine kinase activity of the receptor in TGF-alpha autoinduction. The monoclonal antibody inhibited keratinocyte growth and TGF-alpha autoinduction with similar potency (IC50 approximately 0.1 microgram/ml). TGF-alpha and the PKC activator tetradecanoyl phorbol 12-myristyl, 13-acetate (TPA) had similar effects on TGF-alpha steady-state mRNA levels, suggesting that PKC activation might be a downstream mediator of TGF-alpha autoinduction. However, down-regulation of more than 90% of keratinocyte PKC activity by bryostatin pretreatment abrogated the induction of TGF-alpha mRNA in response to TPA without affecting the autoinductive response or EGF-stimulated tyrosine phosphorylation. These results indicate that EGF receptor and PKC stimulate TGF-alpha gene expression by different pathways, and suggest that PKC is not required for TGF-alpha autoinduction in this system. Moreover, the fact that EGF-stimulated tyrosine phosphorylation and TGF-alpha autoinduction were not potentiated after PKC down-regulation suggests that PKC does not exert a tonic inhibitory influence on EGF receptor tyrosine kinase activity in normal human keratinocytes.

Oncogenes

We have discussed oncogenes and their protein products which act at various sites in tumor cells, with special consideration for tumors of the head and neck. In a few systems studied, application of this knowledge has led to preliminary therapeutic interventions. While abnormal expression and function of these genes and proteins are responsible for transformation to the malignant phenotype, further study of these oncogenes will also help illuminate the mechanisms of normal cellular growth and differentiation. Although cancers of the upper aerodigestive tract are heterogeneous in origin and the multi-step process of carcinogenesis is likely to vary in different tumors, the head and neck tumor spectrum is a good model for tumorigenesis and should provide valuable insight into general carcinogenesis and normal cellular growth controls.

Inhibition of prostatic tumor cell proliferation by suramin: alterations in TGF alpha-mediated autocrine growth regulation and cell cycle distribution

Suramin is a trypanocidal drug that has generated recent interest as an antineoplastic agent because of its ability to inhibit the binding of growth factors to their cell surface receptors. Our studies, and others, suggest that the androgen-independent human prostatic carcinoma cell lines PC3 and DU145 proliferate via autocrine growth mechanisms mediated by transforming growth factor alpha (TGFa) and its receptor, the epidermal growth factor (EGF) receptor. The present studies were designed to evaluate the ability of suramin to inhibit PC3 and DU145 proliferation by interfering with TGFa-mediated autocrine growth. Suramin induced a dose-dependent reduction of prostatic tumor cell proliferation which was reversed by removal of suramin from the culture medium. 3H-thymidine release studies showed that suramin had little direct cytotoxicity to either cell line. These findings suggest that the effects of suramin are mediated by cytostatic, rather than cytotoxic, mechanisms. Suramin also interfered with TGFa-mediated growth mechanisms. Specifically, suramin reduced the specific binding of TGFa to PC3 and DU145 cells. Additionally, the inhibitory effect of suramin on DU145 was reversed by cultivation of cells in the presence of excess TGFa. Further investigations revealed that suramin increased the percentage of cells in the S phase of the cell cycle for both cell lines. These studies show that the inhibitory effect of suramin on PC3 and DU145 cell growth is mediated, in part, by alteration of TGFa-mediated autocrine growth mechanisms and cell cycle kinetics.

Ligand induced internalization of epidermal growth factor receptors by A431 cells decreases at high cell densities in culture

Internalization of epidermal growth factor (EGF) receptors by human A431 epidermoid carcinoma cells was studied at various culture densities. The extent of EGF receptor internalization was measured by quantitation of internalized 125I-EGF during incubation at 37 degrees C for 30 min. When cell culture density was below 1 x 10(5) cells/cm2 receptor internalization was active; 30-40% excess moles of ligand over the moles of surface EGF receptors were internalized during this period. However, when culture density increased to above 1.5 x 10(5) cells/cm2 receptor internalization became less extensive, as only 30-50% of ligand bound to the cell surface underwent internalization during 30 min incubation. In parallel with this reduction in receptor internalization, the degradation rate of 35S-methionine labeled EGF receptors was reduced at a high culture density. In contrast with this regulation of receptor internalization, the affinity of EGF receptors for the ligand increased as culture density increased. The extent of EGF-dependent receptor phosphorylation was found to be constant at all culture densities tested. Thus, the observed low level of receptor internalization at high culture densities was not attributable to lower responsiveness of receptors to EGF. These data suggest the presence of an as yet unidentified cell density-dependent mechanism for regulating receptor internalization in cultured A431 cells.

Anti-epidermal growth factor receptor monoclonal antibodies as potential anti-cancer agents

The EGF receptor is a potential target for antitumor therapy, because it is expressed at high levels on many human tumor cells and appears to be involved in autocrine stimulation of cell growth in a number of experimental studies. Anti-EGF receptor MAbs, which block ligand binding, can prevent the growth in culture of cells that are stimulated by EGF or TGF-alpha. Growth of human tumor xenografts bearing high levels of EGF receptors is also inhibited. A Phase I trial in patients with squamous cell carcinoma of the lung has demonstrated the capacity of a single dose of 120 mg anti-EGF receptor MAb to localize in such tumors and to achieve saturating concentrations in the blood for more than 3 days, without causing toxicity.