Early Changes in Circulating Tumor Cells Are Associated with Response and Survival Following Treatment of Metastatic Neuroendocrine Neoplasms

PURPOSE

To investigate posttreatment circulating tumor cell (CTC) counts in patients with neuroendocrine neoplasms (NENs) as a predictive biomarker for disease progression and overall survival (OS).

EXPERIMENTAL DESIGN

Patients with metastatic NENs commencing therapy were prospectively recruited (n = 138). Blood samples were obtained for evaluation of CTCs using the CellSearch platform and for chromogranin A (CgA) at baseline, three to five (median, 4.3) weeks and 10 to 15 (median 13.7) weeks after commencing therapy. Radiologic response and OS data were collected.

RESULTS

There was a significant association between first posttreatment CTC count and progressive disease (PD; P < 0.001). Only 8% of patients with a favorable "CTC response" (0 CTCs at baseline and 0 at first posttreatment time-point; or ≥50% reduction from baseline) had PD compared with 60% in the unfavorable group (<50% reduction or increase). Changes in CTCs were strongly associated with OS (P < 0.001), the best prognostic group being patients with 0 CTCs before and after therapy; followed by those with ≥50% reduction in CTCs [hazard ratio (HR), 3.31]; with those with a <50% reduction or increase in CTCs (HR, 5.07) having the worst outcome. In multivariate analysis, changes in CTCs had the strongest association with OS (HR, 4.13; P = 0.0002). Changes in CgA were not significantly associated with survival.

CONCLUSIONS

Changes in CTCs are associated with response to treatment and OS in metastatic NENs, suggesting CTCs may be useful as surrogate markers to direct clinical decision making.

Increased sensitivity to platinum drugs of cancer cells with acquired resistance to trabectedin

BACKGROUND

In order to investigate the mechanisms of acquired resistance to trabectedin, trabectedin-resistant human myxoid liposarcoma (402-91/T) and ovarian carcinoma (A2780/T) cell lines were derived and characterised in vitro and in vivo.

METHODS

Resistant cell lines were obtained by repeated exposures to trabectedin. Characterisation was performed by evaluating drug sensitivity, cell cycle perturbations, DNA damage and DNA repair protein expression. In vivo experiments were performed on A2780 and A2780/T xenografts.

RESULTS

402-91/T and A2780/T cells were six-fold resistant to trabectedin compared with parental cells. Resistant cells were found to be hypersensitive to UV light and did not express specific proteins involved in the nucleotide excision repair (NER) pathway: XPF and ERCC1 in 402-91/T and XPG in A2780/T. NER deficiency in trabectedin-resistant cells was associated with the absence of a G2/M arrest induced by trabectedin and with enhanced sensitivity (two-fold) to platinum drugs. In A2780/T, this collateral sensitivity, confirmed in vivo, was associated with an increased formation of DNA interstrand crosslinks.

CONCLUSIONS

Our finding that resistance to trabectedin is associated with the loss of NER function, with a consequent increased sensitivity to platinum drugs, provides the rational for sequential use of these drugs in patients who have acquired resistance to trabectedin.

The MEK1/2 Inhibitor Pimasertib Enhances Gemcitabine Efficacy in Pancreatic Cancer Models by Altering Ribonucleotide Reductase Subunit-1 (RRM1)

PURPOSE

Gemcitabine, a nucleoside analogue, is an important treatment for locally advanced and metastatic pancreatic ductal adenocarcinoma (PDAC) but provides only modest survival benefit. Targeting downstream effectors of the RAS/ERK signaling pathway by direct inhibition of MEK1/2 proteins is a promising therapeutic strategy, as aberrant activation of this pathway occurs frequently in PDAC. In this study, the ability of pimasertib, a selective allosteric MEK1/2 inhibitor, to enhance gemcitabine efficacy was tested and the molecular mechanism of their interaction was investigated.

EXPERIMENTAL DESIGN

Cell survival and apoptosis were assessed by MTT and Caspase 3/7 Glo assays in human pancreatic cancer cell lines. Protein expression was detected by immunoblotting. The in vivo sensitivity of gemcitabine with pimasertib was evaluated in an orthotopic model of pancreatic tumor.

RESULTS

Synergistic activity was observed when gemcitabine was combined sequentially with pimasertib, in human pancreatic cancer cells. In particular, pimasertib reduced ribonucleotide reductase subunit 1 (RRM1) protein, and this was associated with sensitivity to gemcitabine. Pretreatment with MG132 impaired reduction of RRM1 protein induced by pimasertib, suggesting that RRM1 is degraded posttranslationally. Immunoprecipitation indicated enhanced MDM2-mediated polyubiquitination of RRM1 through Lys-48-mediated linkage following pimasertib treatment, an effect mediated, in part, by AKT. Finally, the combination treatment with pimasertib and gemcitabine caused significant tumor growth delays in an orthotopic pancreatic cancer model, with RRM1 downregulation in pimasertib-treated mice.

CONCLUSIONS

These results confirm an important role of RRM1 in gemcitabine response and indicate MEK as a potential target to sensitize gemcitabine therapy for PDAC. Clin Cancer Res; 21(24); 5563-77. ©2015 AACR.

Abstract C51: Chemosensitisation to cisplatin by STAT3 inhibitors is mediated through the inhibition of DNA interstrand crosslink unhooking

Signal Transducer and Activator of Transcription 3 (STAT3) is constitutively activated in many cancer cell lines, leading to survival, proliferation, angiogenesis and metastasis. The inhibition of STAT3 by natural or synthetic compounds has been shown to sensitise cancer cell lines to the chemotherapy agent cisplatin, however the molecular mechanisms contributing to this chemosensitisation have not yet been fully elucidated. Therefore we investigated the effect of STAT3 inhibition on cisplatin-induced DNA damage and key DNA repair factors.

STAT3 inhibitors stattic and curcumin were investigated in combination with cisplatin using the Suphorhodamine B cell growth inhibition assay in the DU145 prostate cancer and A549 non-small cell lung cancer cell lines. Combination treatments result in a significant (P&lt;0.0001) reduction in cisplatin GI50 values. These combinations were synergistic as determined by combination index value analysis using the software Calcusyn, confirming that inhibition of STAT3 enhances the cytotoxic effect of cisplatin. Using a modification of the alkaline comet assay we observed that STAT3 inhibitors significantly prevent the unhooking step of repair of cisplatin induced interstrand crosslinks (ICLs). Treatment with cisplatin alone induces formation of ICLs which peak at 9 hours and unhook by 24 hours post cisplatin exposure in both cell lines. Pre-treating cells with 12μM stattic or 48μM curcumin followed by cisplatin results in an equivalent peak formation of ICLs at 9 hours, however 24 and 48 hours post cisplatin no unhooking is observed. Therefore both STAT3 inhibitors used in this study block the unhooking stage of ICL repair. Using RT-PCR and immunoblotting we show that treatment with STAT3 inhibitors dose-dependently down-regulates EME1, BRCA1 and FANCD2 mRNA and protein levels. These three DNA repair proteins have been shown to be involved in the early stages of ICL repair and therefore down-regulation of these genes may be responsible for the impairment of ICL unhooking seen with STAT3 inhibition.

As the effects observed here have been replicated with two STAT3 inhibitors of differing chemical structure, this suggests that STAT3 is directly involved in the regulation of cisplatin-induced ICL repair through modulation of key ICL repair genes. We propose this as a contributing mechanism to the synergy reported between STAT3 inhibitors and cisplatin. The data presented here will inform the design of future drug combination strategies.

Citation Format: Helen Valentine, Emily Dixon, John A. Hartley, Konstantinos Kiakos. Chemosensitisation to cisplatin by STAT3 inhibitors is mediated through the inhibition of DNA interstrand crosslink unhooking. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C51.

AzaHx, a novel fluorescent, DNA minor groove and G·C recognition element: Synthesis and DNA binding properties of a p-anisyl-4-aza-benzimidazole-pyrrole-imidazole (azaHx-PI) polyamide

The design, synthesis, and DNA binding properties of azaHx-PI or p-anisyl-4-aza-benzimidazole-pyrrole-imidazole (5) are described. AzaHx, 2-(p-anisyl)-4-aza-benzimidazole-5-carboxamide, is a novel, fluorescent DNA recognition element, derived from Hoechst 33258 to recognize G·C base pairs. Supported by theoretical data, the results from DNase I footprinting, CD, ΔT(M), and SPR studies provided evidence that an azaHx/IP pairing, formed from antiparallel stacking of two azaHx-PI molecules in a side-by-side manner in the minor groove, selectively recognized a C-G doublet. AzaHx-PI was found to target 5'-ACGCGT-3', the Mlu1 Cell Cycle Box (MCB) promoter sequence with specificity and significant affinity (K(eq) 4.0±0.2×10(7) M(-1)).

Activity of the DNA minor groove cross-linking agent SG2000 (SJG-136) against canine tumours

BACKGROUND

Cancer is the leading cause of death in older dogs and its prevalence is increasing. There is clearly a need to develop more effective anti-cancer drugs in dogs. SG2000 (SJG-136) is a sequence selective DNA minor groove cross-linking agent. Based on its in vitro potency, the spectrum of in vivo and clinical activity against human tumours, and its tolerability in human patients, SG2000 has potential as a novel therapeutic against spontaneously occurring canine malignancies.

RESULTS

In vitro cytotoxicity was assessed using SRB and MTT assays, and in vivo activity was assessed using canine tumour xenografts. DNA interstrand cross-linking (ICL) was determined using a modification of the single cell gel electrophoresis (comet) assay. Effects on cell cycle distribution were assessed by flow cytometry and measurement of γ-H2AX by immunofluorescence and immunohistochemistry. SG2000 had a multi-log differential cytotoxic profile against a panel of 12 canine tumour cell lines representing a range of common tumour types in dogs. In the CMeC-1 melanoma cell line, DNA ICLs increased linearly with dose following a 1 h treatment. Peak ICL was achieved within 1 h and no removal was observed over 48 h. A relationship between DNA ICL formation and cytotoxicity was observed across cell lines. The formation of γ-H2AX foci was slow, becoming evident after 4 h and reaching a peak at 24 h. SG2000 exhibited significant anti-tumour activity against two canine melanoma tumour models in vivo. Anti-tumour activity was observed at 0.15 and 0.3 mg/kg given i.v. either once, or weekly x 3. Dose-dependent DNA ICL was observed in tumours (and to a lower level in peripheral blood mononuclear cells) at 2 h and persisted at 24 h. ICL increased following the second and third doses in a repeated dose schedule. At 24 h, dose dependent γ-H2AX foci were more numerous than at 2 h, and greater in tumours than in peripheral blood mononuclear cells. SG2000-induced H2AX phosphorylation measured by immunohistochemistry showed good correspondence, but less sensitivity, than measurement of foci.

CONCLUSIONS

SG2000 displayed potent activity in vitro against canine cancer cell lines as a result of the formation and persistence of DNA ICLs. SG2000 also had significant in vivo antitumour activity against canine melanoma xenografts, and the comet and γ-H2AX foci methods were relevant pharmacodynamic assays. The clinical testing of SG2000 against spontaneous canine cancer is warranted.

Abstract 637: Preclinical activity of hLL2-PBD, a novel anti-CD22 antibody-pyrrolobenzodiazepine (PBD) conjugate in models of non-Hodgkin lymphoma

CD22 is a type I transmembrane sialoglycoprotein, which expression is restricted to the B-cell lineage. CD22 is also found highly expressed on most malignant mature B cells, including follicular non-Hodgkin lymphoma (NHL), marginal zone lymphoma (MZL), mantle cell lymphoma (MCL), diffuse large B-cell lymphoma (DLBCL), small lymphocytic lymphoma (SLL) and chronic lymphocytic leukemia (CLL).

The differential and favorable expression profile of CD22 in normal and tumor tissues, together with its rapid internalization upon binding with a ligand or antibody, make CD22 an attractive target for an antibody drug conjugate (ADC) mediated treatment of B-cell malignancies. hLL2-PBD is an ADC composed of the humanized monoclonal antibody epratuzumab (hLL2) against human CD22 stochastically conjugated via a valine-alanine cleavable, maleimide linker to a PBD dimer. PBD dimers represent a novel class of payloads, which bind in the DNA minor groove and form highly cytotoxic DNA interstrand cross-links. The average drug to antibody ratio (DAR) of hLL2-PBD is 2.

In vitro, hLL2-PBD showed potent and specific cytotoxicity, as assessed by the MTS assay, in the CD22-positive human Burkitt's lymphoma-derived cell lines Ramos and Daudi and the human diffuse large B-cell lymphoma-derived cell line WSU-DLCL2.

In vivo, hLL2-PBD demonstrated potent anti-tumor efficacy in subcutaneously (s.c.) implanted Daudi and Ramos xenograft models. In the Daudi model, hLL2-PBD, administered as a single dose at either 0.1 or 0.3 mg/kg, achieved dose-dependent anti-tumor activity, resulting in significant increase in survival. At 0.3 mg/kg, hLL2-PBD induced complete tumor regression in all treated animals. In the Ramos xenograft model, a single dose of 1 mg/kg of hLL2-PBD achieved strong tumor regression. Moreover, hLL2-PBD achieved a superior anti-tumor activity compared to Hu10F4-vcMMAE, a CD22-targeting ADC with the auristatin payload vcMMAE and a DAR of 4 (Genentech Inc.), when tested at 1 mg/kg, single dose. Pharmacokinetic (PK) analysis of hLL2-PBD in CB.17 SCID mice showed a favorable PK profile, with a half-life of approximately 12 days.

In conclusion, these data demonstrate the potent in vitro and in vivo anti-tumor activity of hLL2-PBD against CD22-positive hematological tumors and warrant further development of this ADC into the clinic.

Citation Format: Francesca Zammarchi, David Williams, Karin Havenith, Francois D'Hooge, Philip W. Howard, John A. Hartley, Patrick van Berkel. Preclinical activity of hLL2-PBD, a novel anti-CD22 antibody-pyrrolobenzodiazepine (PBD) conjugate in models of non-Hodgkin lymphoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 637. doi:10.1158/1538-7445.AM2015-637

Nuclear Localization and Gene Expression Modulation by a Fluorescent Sequence-Selective p-Anisyl-benzimidazolecarboxamido Imidazole-Pyrrole Polyamide

Synthetic pyrrole (P)-imidazole (I) containing polyamides can target predetermined DNA sequences and modulate gene expression by interfering with transcription factor binding. We have previously shown that rationally designed polyamides targeting the inverted CCAAT box 2 (ICB2) of the topoisomerase IIα (topo IIα) promoter can inhibit binding of transcription factor NF-Y, re-inducing expression of the enzyme in confluent cells. Here, the A/T recognizing fluorophore, p-anisylbenzimidazolecarboxamido (Hx) was incorporated into the hybrid polyamide HxIP, which fluoresces upon binding to DNA, providing an intrinsic probe to monitor cellular uptake. HxIP targets the 5'-TACGAT-3' sequence of the 5' flank of ICB2 with high affinity and sequence specificity, eliciting an ICB2-selective inhibition/displacement of NF-Y. HxIP is readily taken up by NIH3T3 and A549 cells, and detected in the nucleus within minutes. Exposure to the polyamide at confluence resulted in a dose-dependent upregulation of topo IIα expression and enhanced formation of etoposide-induced DNA strand breaks.

A G-quadruplex-binding compound showing anti-tumour activity in an in vivo model for pancreatic cancer

We report here that a tetra-substituted naphthalene-diimide derivative (MM41) has significant in vivo anti-tumour activity against the MIA PaCa-2 pancreatic cancer xenograft model. IV administration with a twice-weekly 15 mg/kg dose produces ca 80% tumour growth decrease in a group of tumour-bearing animals. Two animals survived tumour-free after 279 days. High levels of MM41 are rapidly transported into cell nuclei and were found to accumulate in the tumour. MM41 is a quadruplex-interactive compound which binds strongly to the quadruplexes encoded in the promoter sequences of the BCL-2 and k-RAS genes, both of which are dis-regulated in many human pancreatic cancers. Levels of BCL-2 were reduced by ca 40% in tumours from MM41-treated animals relative to controls, consistent with BCL-2 being a target for MM41. Molecular modelling suggests that MM41 binds to a BCL-2 quadruplex in a manner resembling that previously observed in co-crystal structures with human telomeric quadruplexes. This supports the concept that MM41 (and by implication other quadruplex-targeting small molecules) can bind to quadruplex-forming promoter regions in a number of genes and down-regulate their transcription. We suggest that quadruplexes within those master genes that are up-regulated drivers for particular cancers, may be selective targets for compounds such as MM41.

WASH and Tsg101/ALIX-dependent diversion of stress-internalized EGFR from the canonical endocytic pathway

Stress exposure triggers ligand-independent EGF receptor (EGFR) endocytosis, but its post-endocytic fate and role in regulating signalling are unclear. We show that the p38 MAP kinase-dependent, EGFR tyrosine kinase (TK)-independent EGFR internalization induced by ultraviolet light C (UVC) or the cancer therapeutic cisplatin, is followed by diversion from the canonical endocytic pathway. Instead of lysosomal degradation or plasma membrane recycling, EGFR accumulates in a subset of LBPA-rich perinuclear multivesicular bodies (MVBs) distinct from those carrying EGF-stimulated EGFR. Stress-internalized EGFR co-segregates with exogenously expressed pre-melanosomal markers OA1 and fibrillar PMEL, following early endosomal sorting by the actin polymerization-promoting WASH complex. Stress-internalized EGFR is retained intracellularly by continued p38 activity in a mechanism involving ubiquitin-independent, ESCRT/ALIX-dependent incorporation onto intraluminal vesicles (ILVs) of MVBs. In contrast to the internalization-independent EGF-stimulated activation, UVC/cisplatin-triggered EGFR activation depends on EGFR internalization and intracellular retention. EGFR signalling from this MVB subpopulation delays apoptosis and might contribute to chemoresistance.

Loss of INPP4B causes a DNA repair defect through loss of BRCA1, ATM and ATR and can be targeted with PARP inhibitor treatment

Treatment options for ovarian cancer patients remain limited and overall survival is less than 50% despite recent clinical advances. The lipid phosphatase inositol polyphosphate 4-phosphatase type II (INPP4B) has been described as a tumor suppressor in the PI3K/Akt pathway with loss of expression found most pronounced in breast, ovarian cancer and melanoma. Using microarray technology we identified a DNA repair defect in INPP4B-deficient cells, which we further characterized by comet assays and quantification of γH2AX, RAD51 and 53BP1 foci formation. INPP4B loss resulted in significantly increased sensitivity towards PARP inhibition, comparable to loss of BRCA1 in two- and three-dimensional in vitro models, as well as in in vivo xenograft models. Mechanistically, we discovered that INPP4B forms a protein complex with the key players of DNA repair, ATR and BRCA1, in GST pulldown and 293T overexpression assays, and INPP4B loss affects BRCA1, ATM and ATR protein stability resulting in the observed DNA repair defect. Given that INPP4B loss has been found in 40% of ovarian cancer patients, this study provides the rationale for establishing INPP4B as a biomarker of PARP inhibitor response, and consequently offers novel therapeutic options for a significant subset of patients. Loss of the tumor suppressor inositol polyphosphate 4-phosphatase type II (INPP4B) results in a DNA repair defect due to concomitant loss of BRCA1, ATR and ATM and can be therapeutically targeted with PARP inhibitors.

Direct involvement of retinoblastoma family proteins in DNA repair by non-homologous end-joining

Deficiencies in DNA double-strand break (DSB) repair lead to genetic instability, a recognized cause of cancer initiation and evolution. We report that the retinoblastoma tumor suppressor protein (RB1) is required for DNA DSB repair by canonical non-homologous end-joining (cNHEJ). Support of cNHEJ involves a mechanism independent of RB1's cell-cycle function and depends on its amino terminal domain with which it binds to NHEJ components XRCC5 and XRCC6. Cells with engineered loss of RB family function as well as cancer-derived cells with mutational RB1 loss show substantially reduced levels of cNHEJ. RB1 variants disabled for the interaction with XRCC5 and XRCC6, including a cancer-associated variant, are unable to support cNHEJ despite being able to confer cell-cycle control. Our data identify RB1 loss as a candidate driver of structural genomic instability and a causative factor for cancer somatic heterogeneity and evolution.

Therapeutic Agents Based on DNA Sequence Specific Binding

DNA interactive agents have been used in the clinical setting for the treatment of cancer since the beginning of modern-era chemotherapy. Despite a shift of focus towards molecular targeted therapy, DNA remains a critical macromolecular target for anti-cancer intervention and the next generation of agents must conform to the optimum combination of increased therapeutic activity and reduced off-target toxicity. We evaluate the potential of non-covalent DNA binding small molecules as "gene-control" agents, exploiting inherent or engineered sequence selectivity, to target critical genomic sequences. In addition we review examples of natural products and synthetic derivatives that exert their activity through sequence specific DNA-covalent modification.

Abstract 2659: The MedImmune ADC platform: Building highly potent and specific cancer drugs

MedImmune has made a major commitment to developing antibody-drug conjugates for cancer. Our ADC initiative builds on our previous experience with this area and with other armed antibody technologies, which have generated several drugs currently undergoing clinical development. We have leveraged our expertise in antibody engineering to develop new antibody constructs for ADC development, including variants that facilitate site-specific conjugation of the payload to the antibody. This technology circumvents problems associated with random payload conjugation, resulting in a more homogeneous drug product as well as improving the stability and potency of the ADC. We have developed new, potent payloads, teaming up with Spirogen to apply their expertise in the pyrrolobenzodiazapine (PBD) dimer payload technology. The PBD payloads are versatile and potent, allowing use of multiple types of linkers and adjustment of potency to very high levels (picomolar IC50 range). The mechanism of action of the PBDs is different from other commonly used ADC payloads, inducing DNA damage that may evade DNA repair mechanisms and killing both bulk cancer cells and cancer stem cells. We have also developed other, novel ADC payloads. Our ADC target discovery approach permits rapid identification and validation of targets specifically suited for this technology. This includes the early generation and use of tool ADCs for target evaluation. We are applying this state-of-the-art ADC platform to advancing multiple projects as a major component of our oncology drug development strategy.

Citation Format: Robert E. Hollingsworth, Adeela Kamal, Philip W. Howard, John A. Hartley, David Tice, Changshou Gao, Nazzareno Dimasi, Haihong Zhong, Jay Harper, Zhan Xiao, Dorin Toader, Chris Martin, Herren Wu, Norman Greenberg, Bahija Jallal. The MedImmune ADC platform: Building highly potent and specific cancer drugs. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2659. doi:10.1158/1538-7445.AM2014-2659

Importance of EGFR/ERCC1 interaction following radiation-induced DNA damage

PURPOSE

The epidermal growth factor receptor (EGFR) plays an important role in cellular response to chemotherapy and radiotherapy through modulation of DNA repair. EGFR activates DNA-dependent protein kinase (DNA-PK) stimulating repair of DNA strand breaks (SB) and interstrand crosslinks (ICL). We investigated the role of EGFR in repair of ionizing radiation (IR)-induced SB independently of DNA-PK.

EXPERIMENTAL DESIGN

The EGFR interactome was investigated via mass spectrometry. IR-induced EGFR-ERCC1 binding was validated biochemically and via proximity ligation assay in different cell lines including the M059K and M059J glioma cell lines, proficient and deficient for the expression of DNAPKcs, respectively. EGFR-ERCC1 functional significance following IR-induced SB was investigated in knockdown experiments with the Comet and γH2AX foci assays. The effect of this interaction was tested with EGFR-ERCC1 knockdown in combination with gefitinib and NU7026 using the MTT and apoptosis assays.

RESULTS

This study demonstrates that EGFR inhibition further impairs IR-induced DNA repair in cells lacking expression of DNAPKcs or in combination with the DNAPK inhibitor NU7026. Our data suggest a role for EGFR in DNA repair independent of DNAPKcs but dependent on ERCC1. Alkaline comet and γH2AX foci assays in cells depleted of EGFR, ERCC1, or EGFR-ERCC1 expression demonstrated involvement of this interaction in DNA repair. Cellular survival and apoptosis data correlate with levels of residual DNA damage underlying the importance of this complex following SB.

CONCLUSION

These data emphasize the importance of understanding the various mechanisms by which EGFR modulates DNA repair to optimize targeted therapy for patients with cancer.

Treatment with gefitinib or lapatinib induces drug resistance through downregulation of topoisomerase IIα expression

The EGF receptor (EGFR) is therapeutically targeted by antibodies and small molecules in solid tumors including lung, colorectal, and breast cancer. However, chemotherapy remains important, and efforts to improve efficacy through combination with targeted agents is challenging. This study examined the effects of short and long durations of exposure to the EGFR- and HER2-targeted tyrosine kinase inhibitors (TKI) gefitinib and lapatinib, on induction of cell death and DNA damage by topoisomerase IIα (Topo IIα) poisons, in the SK-Br-3 HER2-amplified breast cancer cell line. Short exposure to either gefitinib or lapatinib for 1 hour did not affect the induction of apoptosis by the Topo IIα poisons doxorubicin, etoposide, and m-AMSA. In contrast, cells treated for 48 hours were resistant to all three drugs. Short exposure (1 hour) to TKI did not alter the number of DNA single- or double-strand breaks (DSB) induced, whereas longer exposure (48 hours) reduced the number of DNA DSBs and the formation of γ-H2AX foci. Both gefitinib and lapatinib reduced the expression and activity of Topo IIα at 48 hours. Studies using a cell line with inducible downregulation of Topo IIα showed that expression of Topo IIα, and not Topo IIβ, determined the number of DNA strand breaks induced by these chemotherapeutic agents. These results indicate that prolonged exposure to TKIs targeting EGFR and HER2 induce resistance to doxorubicin, etoposide, and m-AMSA through downregulation of Topo IIα. This may explain why their addition to chemotherapy regimens have not increased efficacy.

An extended pyrrolobenzodiazepine-polyamide conjugate with selectivity for a DNA sequence containing the ICB2 transcription factor binding site

The binding of nuclear factor Y (NF-Y) to inverted CCAAT boxes (ICBs) within the promoter region of DNA topoisomerase IIα results in control of cell differentiation and cell cycle progression. Thus, NF-Y inhibitory small molecules could be employed to inhibit the replication of cancer cells. A library of pyrrolobenzodiazepine (PBD) C8-conjugates consisting of one PBD unit attached to tri-heterocyclic polyamide fragments was designed and synthesized. The DNA-binding affinity and sequence selectivity of each compound were evaluated in DNA thermal denaturation and DNase I footprinting assays, and the ability to inhibit binding of NF-Y to ICB1 and ICB2 was studied using an electrophoretic mobility shift assay (EMSA). 3a was found to be a potent inhibitor of NF-Y binding, exhibiting a 10-fold selectivity for an ICB2 site compared to an ICB1-containing sequence, and showing low nanomolar cytotoxicity toward human tumor cell lines. Molecular modeling and computational studies have provided details of the covalent attachment process that leads to formation of the PBD-DNA adduct, and have allowed the preference of 3a for ICB2 to be rationalized.

Lack of unique neuropathology in amyotrophic lateral sclerosis associated with p.K54E angiogenin (ANG) mutation

AIMS

Five to 10% of cases of amyotrophic lateral sclerosis are familial, with the most common genetic causes being mutations in the C9ORF72, SOD1, TARDBP and FUS genes. Mutations in the angiogenin gene, ANG, have been identified in both familial and sporadic patients in several populations within Europe and North America. The aim of this study was to establish the incidence of ANG mutations in a large cohort of 517 patients from Northern England and establish the neuropathology associated with these cases.

METHODS

The single exon ANG gene was amplified, sequenced and analysed for mutations. Pathological examination of brain, spinal cord and skeletal muscle included conventional histology and immunohistochemistry.

RESULTS

Mutation screening identified a single sporadic amyotrophic lateral sclerosis case with a p.K54E mutation, which is absent from 278 neurologically normal control samples. The clinical presentation was of limb onset amyotrophic lateral sclerosis, with rapid disease progression and no evidence of cognitive impairment. Neuropathological examination established the presence of characteristic ubiquitinated and TDP-43-positive neuronal and glial inclusions, but no abnormality in the distribution of angiogenin protein.

DISCUSSION

There is only one previous report describing the neuropathology in a single case with a p.K17I ANG mutation which highlighted the presence of eosinophilic neuronal intranuclear inclusions in the hippocampus. The absence of this feature in the present case indicates that patients with ANG mutations do not always have pathological changes distinguishable from those of sporadic amyotrophic lateral sclerosis.

Affinity and kinetic modulation of polyamide-DNA interactions by N-modification of the heterocycles

Synthetic N-methyl imidazole and N-pyrrole containing polyamides (PAs) that can form "stacked" dimers can be programmed to target and bind to specific DNA sequences and control gene expression. To accomplish this goal, the development of PAs with lower molecular mass which allows for the molecules to rapidly penetrate cells and localize in the nucleus, along with increased water solubility, while maintaining DNA binding sequence specificity and high binding affinity is key. To meet these challenges, six novel f-ImPy*Im PA derivatives that contain different orthogonally positioned moieties were designed to target 5'-ACGCGT-3'. The synthesis and biophysical characterization of six f-ImPy*Im were determined by CD, ΔTM, DNase I footprinting, SPR, and ITC studies, and were compared with those of their parent compound, f-ImPyIm. The results gave evidence for the minor groove binding and selectivity of PAs 1 and 6 for the cognate sequence 5'-ACGCGT-3', and with strong affinity, Keq = 2.8 × 10(8) M(-1) and Keq = 6.2 × 10(7) M(-1), respectively. The six novel PAs presented in this study demonstrated increased water solubility, while maintaining low molecular mass, sequence specificity, and binding affinity, addressing key issues in therapeutic development.

Design, synthesis, and DNA binding characteristics of a group of orthogonally positioned diamino, N-formamido, pyrrole- and imidazole-containing polyamides

Orthogonally positioned diamino/dicationic polyamides (PAs) have good water solubility and enhanced binding affinity, whilst retaining DNA minor groove and sequence specificity compared to their monoamino/monocationic counterparts. The synthesis and DNA binding properties of the following diamino PAs: f-IPI (3a), f-IPP (4), f-PIP (5), and f-PPP (6) are described. P denotes the site where a 1-propylamino group is attached to the N1-position of the heterocycle. Binding of the diamino PAs to DNA was assessed by DNase I footprinting, thermal denaturation, circular dichroism titration, biosensor surface plasmon resonance (SPR), and isothermal titration calorimetry (ITC) studies. According to SPR studies, f-IPI (3a) bound more strongly (K(eq)=2.4×10(8) M(-1)) and with comparable sequence selectivity to its cognate sequence 5'-ACGCGT-3' when compared to its monoamino analog f-IPI (1). The binding of f-IPI (3a) to 5'-ACGCGT-3' via the stacked dimer motif was balanced between enthalpy and entropy, and that was quite different from the enthalpy-driven binding of its monoamino parent f-IPI (1). f-IPP (4) also bound more strongly to its cognate sequence 5'-ATGCAT-3' (K(eq)=7.4×10(6) M(-1)) via the side-by-side stacked motif than its monoamino analog f-IPP (2a). Although f-PPP (6) bound via a 1:1 motif, it bound strongly to its cognate sequence 5'-AAATTT-3' (K(eq)=4.8×10(7) M(-1)), 15-times higher than the binding of its monoamino analog f-PPP (2c), albeit f-PPP bound via the stacked motif. Finally, f-PIP (5) bound to its target sequence 5'-ATCGAT-3' as a stacked dimer and it has the lowest affinity among the diamino PAs tested (Keq <1×10(5) M(-1)). This was about two times lower in affinity than the binding of its monoamino analog f-PIP (2b). The results further demonstrated that the 'core rules' of DNA recognition by monoamino PAs also apply to their diamino analogs. Specifically, PAs that contain a stacked IP core structure bind most strongly (highest binding constants) to their cognate GC doublet, followed by the binding of PAs with a stacked PP structure to two degenerate AT base pairs, and finally the binding of PAs with a PI core to their cognate CG doublet.

Abstract 5468: Interaction between cetuximab and chemotherapy in colon cancer

The epidermal growth factor receptor (EGFR) is overexpressed in many tumours and is a target for cancer therapy. Cetuximab is a monoclonal antibody targeting EGFR. It is used for the treatment of metastatic colorectal cancer (mCRC) as monotherapy and in combination with oxaliplatin or irinotecan-based chemotherapy. Several clinical studies have shown no benefit when cetuximab is combined with oxaliplatin as opposed to the positive effects observed with irinotecan. The aim of this study is to investigate these interactions in cellular models of colon cancer.

The antitumour effects of cetuximab in vitro in combination with oxaliplatin or irinotecan were determined in the SW 48 colon cancer cell line and the isogenic KRAS mutant G12A cell line. Combination indices (CI) describing the interaction of a fixed concentration of cetuximab and a range of concentrations of oxaliplatin or irinotecan were determined. Combination of cetuximab (100μg/ml) with oxaliplatin (1μM) indicated antagonism in both cell lines tested (CI=4.7 in the KRAS wild type cell line and CI=19.9 in the KRAS G12A cell line). In contrast, the combination of cetuximab (100μg/ml) with irinotecan (0.1μM) showed synergistic effects (CI=0.28 in the KRAS wild type cell line and CI=0.14 in the KRAS G12A cell line). Preclinical studies have suggested that the cytotoxic efficacy of oxaliplatin may depend partially on the production of Reactive Oxygen Species (ROS). The combination of cetuximab with oxaliplatin resulted in a reduction of ROS levels by 30% ± 2.9 in the KRAS WT and by 40% ± 10 in the KRAS G12A cell line compared to those produced by oxaliplatin alone. Additionally, levels of oxaliplatin-induced apoptosis measured by caspase 3/7 cleavage, were reduced by 70% ± 5.4 in the KRAS WT and by 50% ± 2.7 in the KRAS G12A cell line with the antioxidant N-acetyl-cysteine (NAC), suggesting that oxaliplatin induces apoptosis through the production of ROS. To elucidate the mechanism of the modulation of ROS by cetuximab and oxaliplatin, RT-PCR arrays analyzing 84 genes involved in oxidative stress response were performed following cetuximab, oxaliplatin and their combination treatment. Experiments to determine the contribution of these genes to the antagonism between oxaliplatin and cetuximab are under investigation.

Our results show that alteration of ROS levels by cetuximab affects efficacy of oxaliplatin and suggests a key role for ROS in the observed antagonism between the two drugs.

Citation Format: Valeria Santoro, John A. Hartley, Daniel Hochhauser. Interaction between cetuximab and chemotherapy in colon cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5468. doi:10.1158/1538-7445.AM2013-5468

GC-targeted C8-linked pyrrolobenzodiazepine-biaryl conjugates with femtomolar in vitro cytotoxicity and in vivo antitumor activity in mouse models

DNA binding 4-(1-methyl-1H-pyrrol-3-yl)benzenamine (MPB) building blocks have been developed that span two DNA base pairs with a strong preference for GC-rich DNA. They have been conjugated to a pyrrolo[2,1-c][1,4]benzodiazepine (PBD) molecule to produce C8-linked PBD-MPB hybrids that can stabilize GC-rich DNA by up to 13-fold compared to AT-rich DNA. Some have subpicomolar IC50 values in human tumor cell lines and in primary chronic lymphocytic leukemia cells, while being up to 6 orders less cytotoxic in the non-tumor cell line WI38, suggesting that key DNA sequences may be relevant targets in these ultrasensitive cancer cell lines. One conjugate, 7h (KMR-28-39), which has femtomolar activity in the breast cancer cell line MDA-MB-231, has significant dose-dependent antitumor activity in MDA-MB-231 (breast) and MIA PaCa-2 (pancreatic) human tumor xenograft mouse models with insignificant toxicity at therapeutic doses. Preliminary studies suggest that 7h may sterically inhibit interaction of the transcription factor NF-κB with its cognate DNA binding sequence.

Hx-amides: DNA sequence recognition by the fluorescent Hx (p-anisylbenzimidazole)•pyrrole and Hx•imidazole pairings

Hx-amides are fluorescent hybrids of imidazole (I)- and pyrrole (P)-containing polyamides and Hoechst 33258, and they bind in the minor groove of specific DNA sequences. Synthesis and DNA binding studies of HxII (5) complete our studies on the first set of Hx-amides: Hx-I/P-I/P. HxPP (2), HxIP (3) and HxPI (4) were reported earlier. Results from DNase I footprinting, biosensor-SPR, CD and ΔTM studies showed that Hx-amides interacted with DNA via the anti-parallel and stacked, side-by-side motif. Hx was found to mimic the DNA recognition properties of two consecutive pyrrole units (PP) in polyamides. Accordingly, the stacked Hx/PP pairing binds preferentially to two consecutive AT base pairs, A/T-A/T; Hx/IP prefers C-A/T; Hx/PI prefers A/T-C; and Hx/II prefers C-C. The results also showed that Hx-amides bound their cognate sequence at a higher affinity than their formamido-triamide counterparts.

γ-H2AX foci formation as a pharmacodynamic marker of DNA damage produced by DNA cross-linking agents: results from 2 phase I clinical trials of SJG-136 (SG2000)

PURPOSE

To evaluate γ-H2AX foci as a pharmacodynamic marker for DNA damage induced by DNA interstrand cross-linking drugs.

EXPERIMENTAL DESIGN

γ-H2AX foci formation was validated preclinically in comparison with the Comet assay, and evaluated pharmacodynamically in two phase I studies of different dosing schedules of the novel cross-linking agent SJG-136 (SG2000).

RESULTS

The measurement of γ-H2AX foci in human fibroblasts and lymphocytes in vitro was more than 10-fold more sensitive than Comet assay measurement of cross-linking, with peak γ-H2AX response 24 hours after the peak of cross-linking. In lymphocytes from a phase I study (every three week schedule), γ-H2AX foci were detectable 1 hour following the end of administration, and in all patients, maximum response was observed at 24 hours. Significant levels of foci were still evident at days 8 and 15 consistent with the known persistence of the DNA damage produced by this agent. In two tumor biopsy samples, foci were detected 4 hours postinfusion with levels higher than in lymphocytes. Extensive foci formation was also observed before the third dose in cycle 1 in lymphocytes from a second phase I study (daily × 3 schedule). These foci also persisted with a significant level evident before the second cycle (day 21). An increased γ-H2AX response was observed during the second cycle consistent with a cumulative pharmacodynamic effect. No clear relationship between foci formation and administered drug dose was observed.

CONCLUSION

This is the first use of γ-H2AX as a pharmacodynamic response to a DNA cross-linking agent in a clinical trial setting.

Circulating tumor cells as prognostic markers in neuroendocrine tumors

PURPOSE

To determine the prognostic significance of circulating tumor cells (CTCs) in patients with neuroendocrine cancer.

PATIENTS AND METHODS

In this single-center prospective study, 176 patients with measurable metastatic neuroendocrine tumors (NETs) were recruited. CTCs were measured using a semiautomated technique based on immunomagnetic separation of epithelial cell adhesion molecule-expressing cells.

RESULTS

Overall, 49% patients had ≥ one CTC, 42% had ≥ two CTCs, and 30% had ≥ five CTCs in 7.5 mL blood. Presence of CTCs was associated with increased burden, increased tumor grade, and elevated serum chromogranin A (CgA). Using a 90-patient training set and 85-patient validation set, we defined a cutoff of < one or ≥ one as the optimal prognostic threshold with respect to progression-free survival (PFS). Applying this threshold, the presence of ≥ one CTC was associated with worse PFS and overall survival (OS; hazard ratios [HRs], 6.6 and 8.0, respectively; both P < .001). In multivariate analysis, CTCs remained significant when other prognostic markers, grade, tumor burden, and CgA were included. Within grades, presence of CTCs was able to define a poor prognostic subgroup. For grade 1, HRs were 5.0 for PFS (P = .017) and 7.2 for OS (P = .023); for grade 2, HRs were 3.5 for PFS (P = .018) and 5.2 for OS (P = .036).

CONCLUSION

CTCs are a promising prognostic marker for patients with NETs and should be assessed in the context of clinical trials with defined tumor subtypes and therapy.

Strategy for Imidazotetrazine Prodrugs with Anticancer Activity Independent of MGMT and MMR

The imidazotetrazine ring is an acid-stable precursor and prodrug of highly reactive alkyl diazonium ions. We have shown that this reactivity can be managed productively in an aqueous system for the generation of aziridinium ions with 96% efficiency. The new compounds are potent DNA alkylators and have antitumor activity independent of the O6-methylguanine-DNA methyltransferase and DNA mismatch repair constraints that limit the use of Temozolomide.

Evidence for different mechanisms of 'unhooking' for melphalan and cisplatin-induced DNA interstrand cross-links in vitro and in clinical acquired resistant tumour samples

Background

DNA interstrand cross-links (ICLs) are critical lesions produced by several cancer chemotherapy agents including platinum drugs and nitrogen mustards. We have previously shown in haematological (multiple myeloma) and solid tumours (ovarian cancer) that clinical sensitivity to such agents can result from a defect in DNA ICL processing leading to their persistence. Conversely, enhanced repair can result in clinical acquired resistance following chemotherapy. The repair of ICLs is complex but it is assumed that the ‘unhooking’ step is common to all ICLs.

Methods

Using a modification of the single cell gel electrophoresis (Comet) assay we measured the formation and unhooking of melphalan and cisplatin-induced ICLs in cell lines and clinical samples. DNA damage response in the form of γ-H2AX foci formation and the formation of RAD51 foci as a marker of homologous recombination were also determined. Real-time PCR of 84 genes involved in DNA damage signalling pathways was also examined pre- and post-treatment.

Results

Plasma cells from multiple myeloma patients known to be clinically resistant to melphalan showed significant unhooking of melphalan-induced ICLs at 48 hours, but did not unhook cisplatin-induced ICLs. In ovarian cancer cells obtained from patients following platinum-based chemotherapy, unhooking of cisplatin-induced ICLs was observed at 48 hours, but no unhooking of melphalan-induced ICLs. In vitro, A549 cells were proficient at unhooking both melphalan and cisplatin-induced ICLs. γ-H2AX foci formation closely followed the formation of ICLs for both drugs, and rapidly declined following the peak of formation. RPMI8226 cells unhooked melphalan, but not cisplatin-induced ICLs. In these cells, although cross-links form with cisplatin, the γ-H2AX response is weak. In A549 cells, addition of 3nM gemcitabine resulted in complete inhibition of cisplatin-induced ICL unhooking but no effect on repair of melphalan ICLs. The RAD51 foci response was both drug and cell line specific. Real time PCR studies highlighted differences in the damage response to melphalan and cisplatin following equi-ICL forming doses.

Conclusions

These data suggest that the mechanisms by which melphalan and cisplatin-induced ICLs are ‘unhooked’ in vitro are distinct, and the mechanisms of clinical acquired resistance involving repair of ICLs, are drug specific.

Characterization of the human SNM1A and SNM1B/Apollo DNA repair exonucleases

Human SNM1A and SNM1B/Apollo have both been implicated in the repair of DNA interstrand cross-links (ICLs) by cellular studies, and SNM1B is also required for telomere protection. Here, we describe studies on the biochemical characterization of the SNM1A and SNM1B proteins. The results reveal some fundamental differences in the mechanisms of the two proteins. Both SNM1A and SNM1B digest double-stranded and single-stranded DNA with a 5'-to-3' directionality in a reaction that is stimulated by divalent cations, and both nucleases are inhibited by the zinc chelator o-phenanthroline. We find that SNM1A has greater affinity for single-stranded DNA over double-stranded DNA that is not observed with SNM1B. Although both proteins demonstrate a low level of processivity on low molecular weight DNA oligonucleotide substrates, when presented with high molecular weight DNA, SNM1A alone is rendered much more active, being capable of digesting kilobase-long stretches of DNA. Both proteins can digest past ICLs induced by the non-distorting minor groove cross-linking agent SJG-136, albeit with SNM1A showing a greater capacity to achieve this. This is consistent with the proposal that SNM1A and SNM1B might exhibit some redundancy in ICL repair. Together, our work establishes differences in the substrate selectivities of SNM1A and SNM1B that are likely to be relevant to their in vivo roles and which might be exploited in the development of selective inhibitors.

Evodiamine, a dual catalytic inhibitor of type I and II topoisomerases, exhibits enhanced inhibition against camptothecin resistant cells

DNA topoisomerases are nuclear enzymes that are the targets for several anticancer drugs. In this study we investigated the antiproliferative activity against human leukaemia cell lines and the effects on topoisomerase I and II of evodiamine, which is a quinazolinocarboline alkaloid isolated from the fruit of a traditional Chinese medicinal plant, Evodia rutaecarpa. We report here the anti-proliferative activity against human leukaemia cells K562, THP-1, CCRF-CEM and CCRF-CEM/C1 and the inhibitory mechanism on human topoisomerases I and II, important anti-cancer drugs targets, of evodiamine. Evodiamine failed to trap [Topo-DNA] complexes and induce any detectable DNA damage in cells, was unable to bind or intercalate DNA, and arrested cells in the G(2)/M phase. The results suggest evodiamine is a dual catalytic inhibitor of topoisomerases I and II, with IC(50) of 60.74 and 78.81 μM, respectively. The improved toxicity towards camptothecin resistant cells further supports its inhibitory mechanism which is different from camptothecin, and its therapeutic potential.

Abstract 1766: Persistence of drug-induced DNA interstrand cross-links distinguishes bendamustine from conventional DNA cross-linking agents

Bendamustine has demonstrated substantial clinical efficacy in the treatment of hematologic malignancies and continues to distinguish itself from other alkylating agents. The mechanistic and clinical differences associated with bendamustine may be directly related to its unique structural features, although the precise mechanism of action is still poorly understood. We have undertaken a detailed study of the drug-DNA interactions of bendamustine. It alkylates DNA primarily at guanine-N7 positions with a sequence selectivity similar to other nitrogen mustards. It produces DNA interstrand cross-links (ICLs) in both solid tumor and hematological tumor cells at doses that cause growth inhibition. ICLs peak at 8h following a 1h treatment and persist over 48h in tumor cells including those that are proficient at repairing (unhooking) ICLs formed by melphalan or cisplatin. This persistence of ICLs is also observed in multiple myeloma cells from patients who have relapsed on melphalan therapy and which efficiently repair melphalan-induced ICLs compared to cells from treatment naive patients. The peak γ-H2AX response follows the ICL peak and also persists beyond 48h. Cells defective in ICL unhooking proteins ERCC1 and XPF, or defective in homologous recombination repair, show increased sensitivity to bendamustine, but at a level less than observed for other nitrogen mustards or cisplatin. Real time PCR profiling showed that at equivalent peak levels of DNA ICLs bendamustine induced fewer changes in DNA damage signaling and DNA repair gene expression compared to either cisplatin or melphalan. These data suggest that the critical DNA damage produced by bendamustine induces a different signaling and repair response to conventional cross-linking agents, including other nitrogen mustards, which may be an important contribution to its clinical efficacy.

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 1766. doi:1538-7445.AM2012-1766

Novel diamino imidazole and pyrrole-containing polyamides: Synthesis and DNA binding studies of mono- and diamino-phenyl-ImPy*Im polyamides designed to target 5'-ACGCGT-3'

Pyrrole- and imidazole-containing polyamides are widely investigated as DNA sequence selective binding agents that have potential use as gene control agents. The key challenges that must be overcome to realize this goal is the development of polyamides with low molar mass so the molecules can readily diffuse into cells and concentrate in the nucleus. In addition, the molecules must have appreciable water solubility, bind DNA sequence specifically, and with high affinity. It is on this basis that the orthogonally positioned diamino/dicationic polyamide Ph-ImPy*Im 5 was designed to target the sequence 5'-ACGCGT-3'. Py* denotes the pyrrole unit that contains a N-substituted aminopropyl pendant group. The DNA binding properties of diamino polyamide 5 were determined using a number of techniques including CD, ΔT(M), DNase I footprinting, SPR and ITC studies. The effects of the second amino moiety in Py* on DNA binding affinity over its monoamino counterpart Ph-ImPyIm 3 were assessed by conducting DNA binding studies of 3 in parallel with 5. The results confirmed the minor groove binding and selectivity of both polyamides for the cognate sequence 5'-ACGCGT-3'. The diamino/dicationic polyamide 5 showed enhanced binding affinity and higher solubility in aqueous media over its monoamino/monocationic counterpart Ph-ImPyIm 3. The binding constant of 5, determined from SPR studies, was found to be 1.5 × 10(7)M(-1), which is ∼3 times higher than that for its monoamino analog 3 (4.8 × 10(6)M(-1)). The affinity of 5 is now approaching that of the parent compound f-ImPyIm 1 and its diamino equivalent 4. The advantages of the design of diamino polyamide 5 over 1 and 4 are its sequence specificity and the ease of synthesis compared to the N-terminus pyrrole analog 2.

Human SNM1A and XPF-ERCC1 collaborate to initiate DNA interstrand cross-link repair

One of the major DNA interstrand cross-link (ICL) repair pathways in mammalian cells is coupled to replication, but the mechanistic roles of the critical factors involved remain largely elusive. Here, we show that purified human SNM1A (hSNM1A), which exhibits a 5'-3' exonuclease activity, can load from a single DNA nick and digest past an ICL on its substrate strand. hSNM1A-depleted cells are ICL-sensitive and accumulate replication-associated DNA double-strand breaks (DSBs), akin to ERCC1-depleted cells. These DSBs are Mus81-induced, indicating that replication fork cleavage by Mus81 results from the failure of the hSNM1A- and XPF-ERCC1-dependent ICL repair pathway. Our results reveal how collaboration between hSNM1A and XPF-ERCC1 is necessary to initiate ICL repair in replicating human cells.

Mechanism of action: the unique pattern of bendamustine-induced cytotoxicity

Bendamustine has demonstrated substantial efficacy in the treatment of hematologic malignancies and continues to distinguish itself from other alkylating agents with regard to its activity in tumor cells. The mechanistic and clinical differences associated with bendamustine may be directly related to its unique structural features. Although the precise mechanisms of action are still poorly understood, bendamustine is associated with extensive and durable DNA damage. The increased potency of bendamustine may be due to secondary mechanisms such as inhibition of mitotic checkpoints, inefficient DNA repair, and initiation of p53-dependent DNA-damage stress response, all of which lead to mitotic catastrophe and apoptosis. It has also been hypothesized that the presence of a benzimidazole ring in addition to the nitrogen mustard group may influence the way bendamustine interacts with DNA and/or confer antimetabolite properties. Further elucidation of the mechanisms of action for bendamustine and the signaling pathways involved in the response to bendamustine-induced DNA damage is essential to maximize its therapeutic potential, identify biomarkers for response, and understand the potential for synergy with other agents involved in DNA damage and inhibition of DNA repair. This review will discuss the current understanding and hypotheses regarding bendamustine mechanisms of action and suggest future investigations that would shed light on the many unanswered questions.

Abstract 2536: Investigation of the effect of the sequence selective DNA minor groove cross-linking agent SG2000 (SJG-136) on canine tumor cells in vitro and in vivo

SG2000 (SJG-136) is a novel sequence selective DNA minor groove cross-linking agent that causes minimal distortion of the helical structure of DNA, such that interstrand cross-links (ICLs) persist and are not easily repaired. It has potent antitumor activity against rodent and human tumour cells in vitro and in vivo and is currently in Phase II clinical trials in humans. The aim of this study was to evaluate whether SG2000 is effective against canine cancer cells in vitro and in vivo. Fourteen canine tumour cell lines, representing the main canine cancers, and two canine normal cell lines were screened for their IC50 values following SG2000 treatment using the SRB and MTT assays. A large differential activity was observed (0.33nM to &gt;100nM following a 1 hour exposure) as previously reported in human tumour cells. Formation and persistence of ICLs were measured with the modified single cell gel electrophoresis (comet) assay in six canine tumour (two mast cell and four melanoma) cell lines. DNA damage response to SG2000-induced ICLs was also studied by measuring γH2AX foci formation in two melanoma cell lines. IC50 values were related to the peak levels of ICLs and γH2AX foci/cell. ICLs and γH2AX foci persisted for 48 hours following a 1 hour exposure. SG2000 anti-tumour and pharmacodynamic effects (ICL and γH2AX formation) were measured in vivo in athymic mice carrying a canine oral melanoma xenograft (LMeC). These were treated with 0.15 mg/Kg and 0.30 mg/Kg i.v. either as a single dose or as a weekly dose for 3 consecutive weeks. Dose-dependent tumour growth delay was observed following the single dose treatment. Greater activity was observed in the repeated dose schedule resulting in a 40 day median tumour growth delay in the 0.30 mg/Kg group. Dose-dependent ICLs were detected in both lymphocytes and tumour at 2 hours following SG2000 treatment and were still evident at 24 hours. The level of ICLs measured in peripheral lymphocytes was lower than that measured in the target tumour tissue, indicating a selective ICL formation in vivo. An increasing number of ICLs was observed in lymphocytes following repeated doses of SG2000. Other pharmacodynamic endpoints, including γH2AX (DNA double-strand breaks), caspase 3 (apoptosis), CD34 (blood vessels), Hoechst 33342 (perfusion) and pimonidazole (hypoxia) staining of tumours were also measured. Together these data suggests that SG2000 might be a valuable antitumor agent for the treatment of canine neoplasia and early phase clinical trials against solid tumours in dogs are planned.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2536. doi:10.1158/1538-7445.AM2011-2536

Abstract 665: Targeting the MluI cell cycle box (MCB) sequence 5’-ACGCGT-3’ in the human Dbf4 promoter using the rationally designed polyamide formamido-imidazole-pyrrole-imidazole (f-IPI)

Dbf4 is the regulatory subunit of Cdc7 kinase, which is essential for the initiation of DNA replication throughout S-phase. The level of Dbf4 is critical for the activation of Cdc7. Human Cdc7/Dbf4 (HuCdc7/Dbf4) kinase activity is essential for cell proliferation by activating DNA replication. High levels of the Cdc7 kinase activity have been implicated with a large number of aggressive solid cancers. The MluI cell cycle box (MCB) sequence 5’-ACGCGT-3’ is the critical site for transcription factor binding that activates the expression of the HuCdc7/Dbf4 core gene in mammalian cells. HuDbf4 promoter activity has previously been shown to be significantly reduced by a point mutation within the MCB. We have previously shown that the rationally designed simple polyamide formamido-imidazole-pyrrole-imidazole (f-IPI) binds to its cognate sequence 5’-ACGCGT-3’ as a stacked dimer in an anti-parallel conformation (2:1 ligand/DNA complex) with exceptionally high affinity and selectivity, superior to the natural product distamycin A binding to its cognate sequence. DNAse I footprinting experiments indicated selective binding of f-IPI to the MCB sequence in the HuDbf4 promoter at 0.1µM. Eletrophoretic mobility shift assays using the MCB-containing sequence from the HuDbf4 promoter showed dose-dependent inhibition of protein binding by f-IPI which was complete at 0.5µM. RT-PCR showed decreased expression of HuDbf4 mRNA in response to incubation of exponentially growing MDA-MB231 cells with 10 μM f-IPI. Selective effects on protein expression, DNA replication and cell cycle are currently underway. These data demonstrate the potential of small molecule polyamides as modulators of gene expression.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 665. doi:10.1158/1538-7445.AM2011-665

The development of pyrrolobenzodiazepines as antitumour agents

INTRODUCTION

DNA interacting agents play a major role in cancer chemotherapy, either as single agents, in combination drug regimens, or as components of novel targeted therapies. The search for more selective and efficacious drugs that can deliver critical DNA damage with minimal side effects continues.

AREAS COVERED

The development of the pyrrolobenzodiazepines (PBDs) from their discovery as natural products in the 1960s, through synthetic PBD monomers, PBD hybrids and conjugates, and PBD dimers is described. The latter molecules are capable of forming sequence selective, non-distorting and potently cytotoxic DNA interstrand cross-links in the minor groove of DNA. In particular, the development of PBD dimer SJG-136 (SG2000), currently in Phase II clinical trials, is presented. Potential future cancer therapeutic applications of PBDs, including their use as components of targeting strategies, are also discussed.

EXPERT OPINION

The culmination of over four decades of study on structure-activity relationships of PBDs has led to a detailed understanding of how to introduce structural modification to enhance biological activity and potency. The challenge for the next phase in the development of the PBDs is to harness this activity and potency in a new generation of cancer therapeutics.

Hx, a novel fluorescent, minor groove and sequence specific recognition element: design, synthesis, and DNA binding properties of p-anisylbenzimidazole-imidazole/pyrrole-containing polyamides

With the aim of incorporating a recognition element that acts as a fluorescent probe upon binding to DNA, three novel pyrrole (P) and imidazole (I)-containing polyamides were synthesized. The compounds contain a p-anisylbenzimidazolecarboxamido (Hx) moiety attached to a PP, IP, or PI unit, giving compounds HxPP (2), HxIP (3), and HxPI (4), respectively. These fluorescent hybrids were tested against their complementary nonfluorescent, non-formamido tetraamide counterparts, namely, PPPP (5), PPIP (6), and PPPI (7) (cognate sequences 5'-AAATTT-3', 5'-ATCGAT-3', and 5'-ACATGT-3', respectively). The binding affinities for both series of polyamides for their cognate and noncognate sequences were ascertained by surface plasmon resonance (SPR) studies, which revealed that the Hx-containing polyamides gave binding constants in the 10(6) M(-1) range while little binding was observed for the noncognates. The binding data were further compared to the corresponding and previously reported formamido-triamides f-PPP (8), f-PIP (9), and f-PPI (10). DNase I footprinting studies provided additional evidence that the Hx moiety behaved similarly to two consecutive pyrroles (PP found in 5-7), which also behaved like a formamido-pyrrole (f-P) unit found in distamycin and many formamido-triamides, including 8-10. The biophysical characterization of polyamides 2-7 on their binding to the abovementioned DNA sequences was determined using thermal melts (ΔT(M)), circular dichroism (CD), and isothermal titration calorimetry (ITC) studies. Density functional calculations (B3LYP) provided a theoretical framework that explains the similarity between PP and Hx on the basis of molecular electrostatic surfaces and dipole moments. Furthermore, emission studies on polyamides 2 and 3 showed that upon excitation at 322 nm binding to their respective cognate sequences resulted in an increase in fluorescence at 370 nm. These low molecular weight polyamides show promise for use as probes for monitoring DNA recognition processes in cells.

DNA interstrand cross-linking and in vivo antitumor activity of the extended pyrrolo[2,1-c][1,4]benzodiazepine dimer SG2057

The pyrrolobenzodiazepines (PBDs) are naturally occurring antitumor antibiotics and a PBD dimer (SJG-136, SG2000) is in Phase II trials. SG2000 is a propyldioxy linked PBD dimer which binds sequence selectively in the minor groove of DNA forming DNA interstrand and intrastrand cross-linked adducts, and also mono-adducts depending on sequence. SG2057 is the corresponding dimer containing a pentyldioxy linkage. SG2057 has multilog differential in vitro cytotoxicity against a panel of human tumour cell lines with a mean GI(50) of 212 pM. The agent is highly efficient at producing DNA interstrand cross-links in cells which form rapidly and persist over a 48 h period. Significant antitumor activity was demonstrated in several human tumor xenograft models. Cures were obtained in a LOX-IMVI melanoma model following a single administration and dose-dependent activity, including regression responses, observed in SKOV-3 ovarian and HL-60 promyelocytic leukemia models following repeat dose schedules. In the advanced stage LS174T model, SG2057 administered either as a single dose, or in two repeat dose schedules, was superior to irinotecan. SG2057 is therefore a highly active antitumor agent, with more potent in vitro activity and superior in vivo activity to SG2000, warranting further development.

Phase I pharmacokinetic and pharmacodynamic study of SJG-136, a novel DNA sequence selective minor groove cross-linking agent, in advanced solid tumors

PURPOSE

This phase I study assessed the maximum tolerated dose (MTD), safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of SJG-136, a sequence-specific DNA cross-linking agent, in patients with advanced cancer.

EXPERIMENTAL DESIGN

In schedule A, seven patients received escalating doses of SJG-136 (6, 12, 24, and 48 μg/m(2)) daily for 5 of 21 days. Blood samples were collected for PK analysis on days 1 and 5 of cycle 1. In schedule B, SJG-136 was given daily for 3 of 21 days (N = 17; doses 20, 25, 30, and 35 μg/m(2)). Blood samples were collected on days 1 and 3 of cycles 1 and 2 for PK and PD analysis. Patients in schedule B received dexamethasone and early diuretic care.

RESULTS

Schedule A-dose-limiting toxicities included grade 3 edema, dyspnea, fatigue, and delayed liver toxicity (grade 3-4). PK analysis revealed dose-dependent increases in AUC and C(max). Substantial changes in volume of distribution at steady-state occurred after repeated dosing in some patients prior to the onset of edema. Schedule B-the same toxicities were manageable with steroid premedication and diuretic support. No significant myelosuppression occurred on either schedule. DNA interstrand cross-links correlated with systemic exposure of SJG-136 following the second dose in cycle 1 and were still detectable immediately before cycle 2.

CONCLUSIONS

The MTD of SJG-136 in this study was 30 μg/m(2) administered on a daily 3× basis with no myelosuppression effects. Coupled with supportive management, SJG-136 is now advancing to a phase II trial in ovarian cancer.

Acetyl analogs of combretastatin A-4: synthesis and biological studies

The combretastatins have received significant attention because of their simple chemical structures, excellent antitumor efficacy and novel antivascular mechanisms of action. Herein, we report the synthesis of 20 novel acetyl analogs of CA-4 (1), synthesized from 3,4,5-trimethoxyphenylacetone that comprises the A ring of CA-4 with different aromatic aldehydes as the B ring. Molecular modeling studies indicate that these new compounds possess a 'twisted' conformation similar to CA-4. The new analogs effectively inhibit the growth of human and murine cancer cells. The most potent compounds 6k, 6s and 6t, have IC(50) values in the sub-μM range. Analog 6t has an IC(50) of 182 nM in MDA-MB-435 cells and has advantages over earlier analogs due to its enhanced water solubility (456 μM). This compound initiates microtubule depolymerization with an EC(50) value of 1.8 μM in A-10 cells. In a murine L1210 syngeneic tumor model 6t had antitumor activity and no apparent toxicity.

EGFR nuclear translocation modulates DNA repair following cisplatin and ionizing radiation treatment

Epidermal growth factor receptor (EGFR) overexpression is associated with resistance to chemotherapy and radiotherapy. It modulates DNA repair after radiation-induced damage through association with the catalytic subunit of DNA protein kinase (DNA-PKcs). We investigated the role of EGFR nuclear import and its association with DNA-PKcs on DNA repair after exposure to cisplatin or ionizing radiation (IR). The model system was based on EGFR-null murine NIH3T3 fibroblasts in which EGFR expression was restored with isoforms that were wild-type (wt), derived from human cancers (L858R, EGFRvIII), or mutated in the nuclear localization signal (NLS) sequence. In cells expressing wtEGFR or EGFRvIII, there was complete unhooking of cisplatin-induced interstrand cross-links and repair of IR-induced strand breaks. In contrast, cells expressing L858R or NLS mutations showed reduced unhooking of interstrand cross-links and repair of strand breaks. Immunoprecipitation showed wtEGFR and EGFRvIII binding to DNA-PKcs, increasing 2-fold 18 hours after cisplatin therapy. Confocal microscopy and proximity ligation assay showed that this interaction in the cytoplasm and nucleus was associated with increased DNA protein kinase complex (DNA-PK) activity. Cells expressing the EGFR L858R mutation, which has constitutive kinase activity, exhibited reduced DNA repair without nuclear localization. EGFR-NLS mutants showed impaired nuclear localization and DNA-PKcs association with reduced DNA repair and DNA-PK kinase activity. In summary, EGFR nuclear localization was required for modulation of cisplatin and IR-induced repair of DNA damage. EGFR-DNA-PKcs binding was induced by cisplatin or IR but not by EGFR nuclear translocation per se. Our findings show that EGFR subcellular distribution can modulate DNA repair kinetics, with implications for design of EGFR-targeted combinational therapies.