Comparison of 2D and 3D cell culture models for cell growth, gene expression and drug resistance

In vitro drug screening is widely used in the development of new drugs, because they constitute a cost-effective approach to select compounds with more potential for therapy. They are also an attractive alternative to in vivo testing. However, most of these assays are done in two-dimensional culture models, where cells are grown on a polystyrene or glass flat surface. In order to develop in vitro models that would more closely resemble physiological conditions, three-dimensional models have been developed. Here, we introduce two novel fully synthetic scaffolds produced using the polymer polyhydroxybutyrate (PHB): a Solvent-Casting Particle-Leaching (SCPL) membrane; and an electrospun membrane, to be used for 3D cultures of B16 F10 murine melanoma cells and 4T1 murine breast cancer cells. A 2D cell culture system in regular tissue culture plates and a classical 3D model where cells are grown on a commercially available gel derived from Engelbreth-Holm Swarm (EHS) tumor were used for comparison with the synthetic scaffolds. Cells were also collected from in vivo tumors grown as grafts in syngeneic mice. Morphology, cell viability, response to chemotherapy and gene expression analysis were used to compare all systems. In the electrospun membrane model, cells were grown on nanometer-scale fibers and in the SCPL membrane, which provides a foam-like structure for cell growth, pore sizes varied. Cells grown on all 3D models were able to form aggregates and spheroids, allowing for increased cell-cell contact when compared with the 2D system. Cell morphology was also more similar between 3D systems and cells collected from the in vivo tumors. Cells grown in 3D models showed an increase in resistance to dacarbazine, and cisplatin. Gene expression analysis also revealed similarities among all 3D platforms. The similarities between the two synthetic systems to the classic EHS gel model highlight their potential application as cost effective substitutes in drug screening, in which fully synthetic models could represent a step towards higher reproducibility. We conclude PHB synthetic membranes offer a valuable alternative for 3D cultures.

Pre-clinical pharmacology and mechanism of action of SG3199, the pyrrolobenzodiazepine (PBD) dimer warhead component of antibody-drug conjugate (ADC) payload tesirine

Synthetic pyrrolobenzodiazepine (PBD) dimers, where two PBD monomers are linked through their aromatic A-ring phenolic C8-positions via a flexible propyldioxy tether, are highly efficient DNA minor groove cross-linking agents with potent cytotoxicity. PBD dimer SG3199 is the released warhead component of the antibody-drug conjugate (ADC) payload tesirine (SG3249), currently being evaluated in several ADC clinical trials. SG3199 was potently cytotoxic against a panel of human solid tumour and haematological cancer cell lines with a mean GI₅₀ of 151.5 pM. Cells defective in DNA repair protein ERCC1 or homologous recombination repair showed increased sensitivity to SG3199 and the drug was only moderately susceptible to multidrug resistance mechanisms. SG3199 was highly efficient at producing DNA interstrand cross-links in naked linear plasmid DNA and dose-dependent cross-linking was observed in cells. Cross-links formed rapidly in cells and persisted over 36 hours. Following intravenous (iv) administration to rats SG3199 showed a very rapid clearance with a half life as short as 8 minutes. These combined properties of cytotoxic potency, rapid formation and persistence of DNA interstrand cross-links and very short half-life contribute to the emerging success of SG3199 as a warhead in clinical stage ADCs.

Optimization of the antitumor activity of sequence-specific pyrrolobenzodiazepine derivatives based on their affinity for ABC transporters

Pyrrolobenzodiazepine (PBD) derivatives are highly potent sequence-specific DNA cross-linking agents. The present study aimed to identify key physicochemical properties influencing the interaction of a series of PBDs (four dimers and 12 monomers) with the three major human ATP-binding cassette (ABC) transporters (P-gp, ABCG2, and MRP1). Isogenic cell lines expressing P-gp and ABCG2, cell lines with acquired resistance to cytotoxic agents due to the high expression of ABC transporters, and specific inhibitors against P-gp, ABCG2, and MRP1 were used. P-gp and ABCG2 decreased the permeability of the PBD dimers across cell membranes and their interaction with DNA, reducing DNA damage and the overall cytotoxic effect. PBD monomer SG-2823 formed a conjugate with glutathione and interacted with MRP1, reducing its cytotoxic effect in A549 cells. Structure-activity relationship revealed that the interaction of PBDs with the transporters could be predicted considering the molecular weight, the lipophilicity, the number of (N + O) atoms and aromatic rings, the polar surface area, the hydrogen bonding energy, and electrophilic centers. A rational design of novel PBDs with increased potency and reduced interaction with the ABC transporters is proposed.

Abstract 5454: Novel STAT3:STAT3 small-molecule inhibitors as potential anticancer agents

Transcription factors are important targets for cancer therapy. The inhibition of protein-protein interactions (PPIs) within signalling pathways known to be key regulators of transcriptional activity is a viable approach to novel chemotherapeutic strategies. Proof-of-concept studies in cell-culture and animal models have validated the potential of small-molecule inhibitors of STAT3 signalling in cancer therapy. In particular, the protein-protein interaction between two STAT3 monomers (i.e., the dimerisation event in the signalling cascade) has been identified as a valid target to inhibit DNA-binding and the resultant transcriptional activation. Of the approximately 20 small-molecule STAT3 inhibitors reported in the literature to date, only three are described as potential STAT3:STAT3 dimerisation inhibitors, and these have IC50 values in STAT3-expressing cell lines of between 10-90 μM.

Using in silico and medicinal chemistry-based approaches based on a published X-Ray structure of STAT3 (PDB: ID-1BG1) to identify “hit” inhibitors, a focussed library (approx. 50 members) was designed around one such “hit” and synthesized employing an efficient 4-step linear approach. Library members were then entered into a screening cascade involving initial evaluation of their ability to inhibit STAT3:STAT3 interaction in a Fluorescent Polarisation (FP)-based primary PPI binding assay. “Hit” molecules from this primary assay were then studied in two cell-based assays designed to test for STAT3 selectivity. The first was a comparative MTS assay between STAT3-expressing MDA MB231 breast cancer cells and STAT3-null colon A4 cells. The second was a luciferase reporter assay designed to measure transcription inhibition in STAT3-Luc-transformed Hela cells compared to SV40-Luc-transformed Hela control cells. A potential “lead” molecule, RH06, emerged from this screening cascade with potentially selective STAT3 inhibitory activity in the low micromolar (i.e., ∼1 µM) region. RH06 is currently being studied for its effect on STAT3 and pSTAT3 signal activation, and its potentially differential effect on upstream and downstream mediators (i.e., JAK2, Bcl-xl, Cyclin D1 and pSTAT1).

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 5454.

Solution structure of a 2:1 C2-(2-naphthyl) pyrrolo[2,1-c][1,4]benzodiazepine DNA adduct: molecular basis for unexpectedly high DNA helix stabilization

The naturally occurring pyrrolo[2,1- c][1,4]benzodiazepine (PBD) monomers such as sibiromycin, anthramycin, and tomaymycin form stable covalent adducts with duplex DNA at purine-guanine-purine sites. A correlative relationship between DNA-binding affinity, as measured by enhanced thermal denaturation temperature of calf thymus DNA ( T m), and cytotoxicity is well documented for these naturally occurring compounds and a range of synthetic analogues with sibiromycin having the highest Delta T m value (16.3 degrees C), reflecting favorable hydrogen-bonding interactions between the molecule and DNA bases. We report here that, surprisingly, the structurally simple synthetic C2-(2-naphthyl)-substituted pyrrolo[2,1- c][1,4]benzodiazepine monomer ( 5) has a Delta T m value (15.8 degrees C) similar to that of sibiromycin and significantly higher than the values for either anthramycin (13.0 degrees C) or tomaymycin (2.6 degrees C). 5 also has similar cytotoxic potency to sibiromycin which is widely regarded as the most potent naturally occurring PBD monomer. To investigate this, we have used NMR in conjunction with molecular dynamics to study the 2:1 adduct formed between 5 and the DNA duplex d(AATCTTTAAAGATT) 2. In contrast to the hydrogen-bonding interactions which predominate in the case of sibiromycin and anthramycin adducts, we have shown that the high binding affinity of 5 is due predominantly to hydrophobic (van der Waals) interactions. The high-resolution 2D NOESY, TOCSY, and COSY data obtained have also allowed unequivocal determination of the orientation of the PBD molecule (A-ring toward 3'-end of covalently bound strand), the stereochemistry at the C11 position of the PBD (C11 S), and the conformation of the C2-naphthyl ring which extends along the floor of the minor groove thus optimizing hydrophobic interactions with DNA. These results provide opportunities for future drug design in terms of extending planar hydrophobic groups at the C2 position of PBDs to maximize binding affinity.

Synthesis of a novel C2-aryl pyrrolo[2,1-c][1,4]benzodiazepine-5,11-dione library: Effect of C2-aryl substitution on cytotoxicity and non-covalent DNA binding

A 23-member C2-aryl pyrrolo[2,1-c][1,4]benzodiazepine-5,11-dione (PBD dilactam) library has been synthesized using Suzuki coupling, and the effect of base upon racemisation at the C11a-position during the cross-coupling reaction studied. Three library members (21, 30 and 33) were sufficiently cytotoxic in the NCI's preliminary screen to warrant further evaluation, and one (30, R=p-Br) was found to be cytotoxic at the sub-micromolar level in the A498 renal cancer cell line. DNA thermal denaturation studies suggested that this activity may be associated with non-covalent DNA interaction, and also demonstrated that introduction of C2-C3 unsaturation and addition of C2-aryl functionalities to the PBD dilactam skeleton significantly enhanced helix stabilisation compared to the unsubstituted PBD dilactam (6).

Parallel Synthesis of a Novel C2-Aryl Pyrrolo[2,1-c][1,4]benzodiazepine (PBD) Library

A 66-member C2-aryl pyrrolo[2,1-c][1,4]benzodiazepine (PBD) library has been successfully synthesized in parallel via Suzuki coupling using PS-PPh3Pd (catalyst) and PS-DEAM (scavenger) under microwave radiation. Library members were obtained in sufficient yield (up to 91%) and purity (85-98% crude) for biological evaluation.