Abstract 1763: Gastric cancer antibody fragment drug-conjugates (FDCs): Succeeding in solid tumors where ADCs fail

Antibody Drug Conjugates (ADCs) are failing due to 3 critical limitations: Low potency, ineffective solid-tumor penetration and poor tolerability. The industry is full of approaches where full-length Immunoglobulins have been engineered to carry defined numbers of payloads with higher-loadings of less potent payloads appearing to be well-tolerated. However, antibody fragments (e.g. single-chain Fcs-scFvs), which have many advantages including rapid tumor penetration, faster clearance, inexpensive manufacture, have been technologically challenging to apply in oncology. Our novel approach enables scFvs to have a high Drug:Antibody loading ratio (DAR) whilst retaining effective binding and other favorable biophysical properties, leading to a new product class tailored for solid tumors. Antikor has two FDC products in development for solid tumors, notably gastric: anti-HER2 FDC (ANT-043) and a second target (ANT-045) which will be disclosed during this presentation. ANT-043 has pM potencies in a range of HER2-expressing cell-lines, including trastuzumab-resistant models, excellent tumor ablation effects in breast, ovarian and gastric cancer xenograft models and superior tolerability compared to an ADC in rat toxicology studies at a dose of 1mg/kg/weekly. Quantitative payload tumor uptake and fluorescent immuno-histological studies demonstrate superior solid tumor penetration across the entire tumor and diffusion from blood vessels. In collaboration with our partners, Essex Biotechnology, Antikor is taking ANT-043 into IND-enabling studies for clinical development. ANT-045, which emerged from Antikor’s proprietary FDC ‘discovery engine’, is progressing towards IND-filing and updated data will illustrate how ANT-045 could have a broader patient benefit in gastro-intestinal cancers. ANT-045 has excellent in vitro cell-kill potency (pM IC50s) and excellent stability and superior in vivo tumor cure efficacy, compared to a leading clinical stage benchmark ADC. This presentation will focus on Antikor’s FDC discovery platform (stable high-DAR scFv-display libraries, tailored linker-payloads and design features) that has the potential to generate first-in-class products for difficult to treat solid tumors for patient benefit and promising to succeed where ADCs have failed to deliver.

Citation Format: Mahendra P. Deonarain, Gokhan Yahioglu, Ioanna Stamati, Bryan Edwards, Soraya Diez-Posada, Isabel Perez-Castro, Anja Pomowski, Laura Bouche, Ashleigh Stewart, Monika Maciuszek, Sam Ness, Malcolm Ngiam, Quinn Xue. Gastric cancer antibody fragment drug-conjugates (FDCs): Succeeding in solid tumors where ADCs fail [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1763.

Abstract 909: Antibody fragment drug-conjugates (FDCs)-application of ANT-043 and ANT-045 in solid tumors

Despite the recent resurgence, Antibody Drug Conjugates (ADCs) are failing to address challenging cancer indications due to 3 critical limitations: Low potency, ineffective solid-tumour penetration and poor tolerability. The industry is well-served by approaches where full-length Immunoglobulins are designed to carry defined numbers of payloads. However, antibody fragments (e.g. single-chain Fvs-scFvs), which have many advantages including rapid tumour penetration, faster clearance, inexpensive manufacture, have been technologically challenging to apply in oncology. Our novel approach enables scFvs to have a high Drug:Antibody loading ratio (DAR) whilst retaining effective binding and other favourable biophysical properties, leading to a new product class tailored for solid tumours. Antikor has two ‘first-in-class' FDC products in development for solid tumours: anti-HER2 FDC (ANT-043) a follow-up product against a second target (ANT-045) which will be disclosed during this presentation. ANT-043 has demonstrated excellent tumour ablation effects in breast, ovarian and gastric cancer xenograft models and superior tolerability compared to an ADC. Quantitative payload tumour uptake by mass spectrometry and fluorescent immuno-histological studies demonstrate superior solid tumour penetration across the entire tumour and diffusion from blood vessels and rapid kinetic uptake. In collaboration with our partners, Essex Biotechnology, ANT-043 is moving into IND-enabling studies for clinical development. Our new flagship product, ANT-045, which emerged from our proprietary FDC ‘discovery engine', is progressing well and new data will illustrate how ANT-045 could have a broader patient benefit in gastro-intestinal cancers. Like ANT-043, ANT-045 has potent in vitro cell-kill properties, excellent stability and drug-like features and illustrates how linker-payload design is critical for the tailoring properties of this emerging format of ADC. This presentation will focus on ANT-045's remarkable development and show that this FDC demonstrates superior efficacy and tolerability compared to an ADC when equated on an equi-mass, equi-molar and equi-payload basis. ANT-045 could be used to switch ‘immunologically-cold' tumour ‘hot' to benefit from checkpoint inhibitor therapy and data will be presented to support this concept.

Citation Format: Mahendra P. Deonarain, Gokhan Yahioglu, Ioanna Stamati, Bryan Edwards, Soraya Diez-Posada, Anja Pomowski, Ashleigh Stewart, Isabel Perez-Castro, Laura Bouche, Triin Jurgenson, Monika Maciuszek, Sam Ness, Malcolm Ngiam, Quinn Xue. Antibody fragment drug-conjugates (FDCs)-application of ANT-043 and ANT-045 in solid tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 909.

Current strategies for the discovery and bioconjugation of smaller, targetable drug conjugates tailored for solid tumor therapy

Introduction: Antibody-Drug Conjugates (ADCs) have undergone a recent resurgence with 5 product approvals over the last 2 years but for those close to the field, it's been repeated cycles of setbacks and new innovations. A new wave of innovation is in the type of format used to deliver the cytotoxic payloads, with smaller bio-molecules being designed to have more optimal penetration and elimination properties tailored for solid tumors.Areas covered: In this review, the authors cover many of the recently described smaller-format drug conjugates (including formats such as diabodies, Fabs, scFvs, domain antibodies) with an emphasis on the types of conjugation technologies used to attach the chemical linker-payload.Expert opinion: Smaller formats are highly influenced by the structure of the linker-payload, arguably more-so than larger ADCs, so careful consideration is needed where solublising and pharmacokinetic modulation is required. High-quality conjugates are being developed with in vivo tumor efficacy and tolerability properties competitive with ADCs and with a few formats already in clinical development, we expect the pipeline to expand and to reach the market.

Tackling solid tumour therapy with small-format drug conjugates

The pharmacokinetic-pharmacodynamic relationship is extremely complex and tumour drug penetration is one key parameter influencing therapeutic efficacy. In the context of antibody-drug conjugates (ADCs), which has undergone many innovation cycles and witnessed many failures, this feature is being addressed by a number of alternative technologies. Immunoglobulin-based ADCs continue to dominate the industrial landscape, but smaller formats offer the promise of more-effective cytotoxic payload delivery to solid tumours, with a higher therapeutic window afforded by the more rapid clearance. To make these smaller formats viable as delivery vehicles, a number of strategies are being employed, which will be reviewed here. These include identifying the most-appropriate size to generate the larger therapeutic window, increasing the amount of functional, cytotoxic payload delivered through conjugation or half-life extending technologies or other ways of extending the dosing without inducing toxicity.

Miniaturised 'antibody'-drug conjugates for solid tumours?

With Antibody-Drug Conjugate strategies firmly focussed on the precise conjugation to the large protein Immunoglobulin-G format, it is easy to miss the more recent technological innovations in small-format drug conjugates. Here, the targeting ligand can be at 50-95% reduced in size, or even smaller if peptidic in nature. Antibody domains or alternative binding scaffolds, chemically-modified with ultra-potent cytotoxic payloads offer an alternative approach for oncology therapeutics, promising a wider therapeutic window by virtue of superior solid tumour penetration properties and more rapid system clearance. Many of the traditional ADC concepts still apply, but as these miniaturised ADCs enter the clinic over the next 2-3 years, we will learn whether these new features translate to patient benefits.

Using antibody directed phototherapy to target oesophageal adenocarcinoma with heterogeneous HER2 expression

Early oesophageal adenocarcinoma (OA) and pre-neoplastic dysplasia may be treated with endoscopic resection and ablative techniques such as photodynamic therapy (PDT). Though effective, discrete areas of disease may be missed leading to recurrence. PDT further suffers from the side effects of off-target photosensitivity. A tumour specific and light targeted therapeutic agent with optimised pharmacokinetics could be used to destroy residual cancerous cells left behind after resection. A small molecule antibody-photosensitizer conjugate was developed targeting human epidermal growth factor receptor 2 (HER2). This was tested in an in vivo mouse model of human OA using a xenograft flank model with clinically relevant low level HER2 expression and heterogeneity. In vitro we demonstrate selective binding of the conjugate to tumour versus normal tissue. Light dependent cytotoxicity of the phototherapy agent in vitro was observed. In an in vivo OA mouse xenograft model the phototherapy agent had desirable pharmacokinetic properties for tumour uptake and blood clearance time. PDT treatment caused tumour growth arrest in all the tumours despite the tumours having a clinically defined low/negative HER2 expression level. This new phototherapy agent shows therapeutic potential for treatment of both HER2 positive and borderline/negative OA.

Small-Format Drug Conjugates: A Viable Alternative to ADCs for Solid Tumours?

Antibody-Drug Conjugates (ADCs) have been through multiple cycles of technological innovation since the concept was first practically demonstrated ~40 years ago. Current technology is focusing on large, whole immunoglobulin formats (of which there are approaching 100 in clinical development), many with site-specifically conjugated payloads numbering 2 or 4. Despite the success of trastuzumab-emtansine in breast cancer, ADCs have generally failed to have an impact in solid tumours, leading many to explore alternative, smaller formats which have better penetrating properties as well as more rapid pharmacokinetics (PK). This review describes research and development progress over the last ~10 years obtained from the primary literature or conferences covering over a dozen different smaller format-drug conjugates from 80 kDa to around 1 kDa in total size. In general, these agents are potent in vitro, particularly more recent ones incorporating ultra-potent payloads such as auristatins or maytansinoids, but this potency profile changes when testing in vivo due to the more rapid clearance. Strategies to manipulate the PK properties, whilst retaining the more effective tumour penetrating properties could at last make small-format drug conjugates viable alternative therapeutics to the more established ADCs.

Upregulation of mucin glycoprotein MUC1 in the progression to esophageal adenocarcinoma and therapeutic potential with a targeted photoactive antibody-drug conjugate

BACKGROUND

Mucin glycoprotein 1 (MUC1) is a glycosylated transmembrane protein on epithelial cells. We investigate MUC1 as a therapeutic target in Barrett's epithelium (BE) and esophageal adenocarcinoma (EA) and provide proof of concept for a light based therapy targeting MUC1.

RESULTS

MUC1 was present in 21% and 30% of significantly enriched pathways comparing BE and EA to squamous epithelium respectively. MUC1 gene expression was x2.3 and x2.2 higher in BE (p=<0.001) and EA (p=0.03). MUC1 immunohistochemical expression increased during progression to EA and followed tumor invasion. HuHMFG1 based photosensitive antibody drug conjugates (ADC) showed cell internalization, MUC1 selective and light-dependent cytotoxicity (p=0.0006) and superior toxicity over photosensitizer alone (p=0.0022).

METHODS

Gene set enrichment analysis (GSEA) evaluated pathways during BE and EA development and quantified MUC1 gene expression. Immunohistochemistry and flow cytometry evaluated the anti-MUC1 antibody HuHMFG1 in esophageal cells of varying pathological grade. Confocal microscopy examined HuHMFG1 internalization and HuHMFG1 ADCs were created to deliver a MUC1 targeted phototoxic payload.

CONCLUSIONS

MUC1 is a promising target in EA. Molecular and light based targeting of MUC1 with a photosensitive ADC is effective in vitro and after development may enable treatment of locoregional tumors endoscopically.

A HER2 selective theranostic agent for surgical resection guidance and photodynamic therapy

In many cancers early intervention involves surgical resection of small localised tumour masses. Inadequate resection leads to recurrence whereas overzealous treatment can lead to organ damage. This work describes production of a HER2 targeting antibody Fab fragment dual conjugated to achieve both real time near-infrared fluorescent imaging and photodynamic therapy. The use of fluorescence emission from a NIR-dye could be used to guide resection of tumour bulk, for example during endoscopic diagnosis for oesophago-gastric adenocarcinoma, this would then be followed by activation of the photodynamic therapeutic agent to destroy untreated localised areas of cancer infiltration and tumour infiltrated lymph nodes. This theranostic agent was prepared from the Fab fragment of trastuzumab initially by functional disulfide re-bridging and site-specific click reaction of a NIR-dye. This was followed by further reaction with a novel pre-activated form of the photosensitiser chlorin e6 with the exposed fragments' lysine residues. Specific binding of the theranostic agent was observed in vitro with a HER2 positive cell line and cellular near-infrared fluorescence was observed with flow cytometry. Specific photo-activity of the conjugates when exposed to laser light was observed with HER2 positive but not HER2 negative cell lines in vitro, this selectivity was not seen with the unconjugated drug. This theranostic agent demonstrates that two different photo-active functions can be coupled to the same antibody fragment with little interference to their independent activities.

Emerging formats for next-generation antibody drug conjugates

INTRODUCTION

Antibody drug conjugates now make up a significant fraction of biopharma's oncology pipeline due to great advances in the understanding of the three key components and how they should be optimised together. With this clinical success comes innovation to produce new enabling technologies that can deliver more effective antibody-drug conjugates (ADCs) with a larger therapeutic index.

AREAS COVERED

There are many reviews that discuss the various strategies for ADCs design but the last 5 years or so have witnessed the emergence of a number of different antibody formats compete with the standard whole immunoglobulin. Using published research, patent applications and conference disclosures, the authors review the many antibody and antibody-like formats, discussing innovations in protein engineering and how these new formats impact on the conjugation strategy and ultimately the performance. The alternative chemistries that are now available offer new linkages, stability profiles, drug:antibody ratio, pharmacokinetics and efficacy. The different sizes being considered promise to address issues, such as tumour penetration, circulatory half-life and side-effects.

EXPERT OPINION

ADCs are at the beginning of the next stage in their evolution and as these newer formats are developed and examined in the clinic, we will discover if the predicted features have a clinical benefit. From the commercial activity, it is envisaged that smaller or fragment-based ADCs will expand oncological applications.

Generation of a selectively cytotoxic fusion protein against p53 mutated cancers

BACKGROUND

A significant number of cancers are caused by defects in p21 causing functional defects in p21 or p53 tumour-suppressor proteins. This has led to many therapeutic approaches including restoration by gene therapy with wild-type p53 or p21 using viral or liposomal vectors, which have toxicity or side-effect limitations. We set out to develop a safer, novel fusion protein which has the ability to reconstitute cancer cell lines with active p21 by protein transduction.

METHODS

The fusion protein was produced from the cell-translocating peptide Antennapedia (Antp) and wild-type, full-length p21 (Antp-p21). This was expressed and refolded from E. coli and tested on a variety of cell lines and tumours (in a BALB/c nude xenograft model) with differing p21 or p53 status.

RESULTS

Antp-p21 penetrated and killed cancer cells that do not express wild type p53 or p21. This included cells that were matched to cogenic parental cell lines. Antp-p21 killed cancer cells selectively that were malignant as a result of mutations or nuclear exclusion of the p53 and p21 genes and over-expression of MDM2. Non-specific toxicity was excluded by showing that Antp-p21 penetrated but did not kill p53- or p21- wild-type cells. Antp-p21 was not immunogenic in normal New Zealand White rabbits. Recombinant Antp peptide alone was not cytotoxic, showing that killing was due to the transduction of the p21 component of Antp-p21. Antp-p21 was shown to penetrate cancer cells engrafted in vivo and resulted in tumour eradication when administered with conventionally-used chemotherapeutic agents, which alone were unable to produce such an effect.

CONCLUSIONS

Antp-p21 may represent a new and promising targeted therapy for patients with p53-associated cancers supporting the concept that rational design of therapies directed against specific cancer mutations will play a part in the future of medical oncology.

Site-specific polysialylation of an antitumor single-chain Fv fragment

Protein pharmacokinetic modulation is becoming an important tool in the development of biotherapeutics. Proteins can be chemically or recombinantly modified to alter their half-lives and bioavailability to suit particular applications as well as improve side effect profiles. The most successful and clinically used approach to date is chemical conjugation with poly(ethylene glycol) polymers (PEGylation). Here, therapeutic protein half-life can be increased significantly while retaining biological function, reducing immunogenicity and cross-reaction. Naturally occurring alternatives to such synthetic polymers could have major advantages such as lower side effects due to biodegradability and metabolism. Polysialic acid (PSA) has been investigated as a pharmacokinetic modulatory biopolymer with many successful examples in preclinical and clinical development. Single-chain Fvs (scFvs) are a choice antibody format for human therapeutic antibody discovery. Because of their small size, they are rapidly eliminated from the circulation and often are rebuilt into larger proteins for drug development and a longer half-life. Here we show that chemical polysialylation can increase the half-life of an antiplacental alkaline (PLAP) and anticarcinoembryonic antigen (CEA) scFv (F1 and MFE-23, respectively) 3.4-4.9-fold, resulting in a 10.6-15.2-fold increase in blood exposure. Amine-directed coupling of the MFE-23 scFv reduced its immunoreactivity 20-fold which was resolved by site-specific polysialylation through an engineered C-terminal thiol residue. The site-specifically polysialylated MFE-23 scFv demonstrated up to 30-fold improved tumor uptake while displaying favorable tumor:normal tissue specificity. This suggests that engineering antibody fragments for site-specific polysialylation could be a useful approach to increase the half-life for a variety of therapeutic applications.

Novel photosensitisers derived from pyropheophorbide-a: uptake by cells and photodynamic efficiency in vitro

Photodynamic Therapy (PDT) is a minimally invasive procedure used for treating a range of neoplastic diseases, which utilises combined action of light and a PDT drug called a photosensitiser. The efficiency of this treatment depends crucially on the properties of the photosensitiser used, namely on its efficient uptake by cells or by the surrounding vasculature, intracellular localisation, minimal dark toxicity and substantial phototoxicity. In this report we compare the spectroscopic properties, cell uptake and in vitro phototoxicity of two novel hydrophilic photosensitisers derived from pyropheophorbide-a (PPa). Both new photosensitisers have the potential to form bioconjugates with antibody fragments for targeted PDT. We find that the photophysical properties of both new photosensitisers are favourable compared to the parent PPa, including enhanced absorption in the red spectral region and substantial singlet oxygen quantum yields. Both molecules show efficient cellular uptake, but display a different intracellular localisation. Both new photosensitisers exhibit no significant dark-toxicity at concentrations of up to 100 microM. The phototoxicity of the two photosensitisers is strikingly different, with one derivative being 13 times more efficient than the parent PPa and another derivative being 18 times less efficient in SKOV3 ovarian cancer cells. We investigate the reasons behind such drastic differences in phototoxicity using confocal fluorescence microscopy and conclude that intracellular localisation is a crucial factor in the photodynamic efficiency of pheophorbide derivatives. These studies highlight the underlying factors behind creating more potent photosensitisers through synthetic manipulation.

Characterisation of a panel of anti-tetanus toxin single-chain Fvs reveals cooperative binding

An approach for enhancing antibody affinity is to engineer Chelating Recombinant Antibodies (CRAbs) which consist of two tandemly linked single-chain Fvs (scFvs) that bind to distinct non-overlapping epitopes on the antigen molecule leading to a synergistic decrease in K_D. In order to develop this technology, the aim of this present study was to identify scFvs which can simultaneously bind to the tetanus toxin heavy chain C-terminal sub-domain (H_c), characterise their bio-physical properties and determine their functional efficacy.

Over 50 antibodies specific for Hc were isolated from a human scFv phagemid library and found to bind specifically to the C-terminal sub-domain of H_c (H_cC clones), the N-terminal sub-domain (HcN clones) or junctional epitopes on the whole Hc fragment only (HcJ clones). Fifteen clones were assayed in a pairwise competition binding study. The revealed, with few exceptions, that H_cC clones were able to simultaneously bind to the toxin with H_cN or H_cJ clones. All other combinations competed for binding. Interestingly, we also observed cooperative binding with many non-competing scFv pairings which may impact upon the binding mechanism of CRAbs. We found that 14/15 clones neutralised toxin activity in a ganglioside binding assay and this effect was strongly related to affinity. This included clones that did not bind to the H_cC sub-domain which is responsible for direct interaction with gangliosides on nerve cells. For 7 scFvs that underwent further characterisation we found broad variations in propensity for multimerisation, affinity and potency.

The diverse array of clones characterised in this paper can be used to construct CRAbs and will prove useful in further characterisation of toxin biology and in measuring the effects of polyclonal antibody therapy.

Antibodies targeting cancer stem cells: a new paradigm in immunotherapy?

Antibody targeting of cancer is showing clinical and commercial success after much intense research and development over the last 30 years. They still have the potential to delivery long-term cures but a shift in thinking towards a cancer stem cell (CSC) model for tumor development is certain to impact on how antibodies are selected and developed, the targets they bind to and the drugs used in combination with them. CSCs have been identified from many human tumors and share many of the characteristics of normal stem cells. The ability to renew, metabolically or physically protect themselves from xenobiotics and DNA damage and the range of locomotory-related receptors expressed could explain the observations of drug resistance and radiation insensitivity leading to metastasis and patient relapse.Targeting CSCs could be a strategy to improve the outcome of cancer therapy but this is not as simple as it seems. Targets such as CD133 and EpCAM/ESA could mark out CSCs from normal cells enabling specific intervention but indirect strategies such as interfering with the establishment of a supportive niche through anti-angiogenic or anti-stroma therapy could be more effective.This review will outline the recent discoveries for CSCs across the major tumor types highlighting the possible molecules for intervention. Examples of antibody-directed CSC therapies and the outlook for the future development of this emerging area will be given.

Natural killer cell signal integration balances synapse symmetry and migration

Imaging immune surveillance by natural killer (NK) cells has revealed that integration of activating and inhibitory signals determines whether or not NK cells stop to kill the target cell or retain a migratory configuration.

Natural killer (NK) cells discern the health of other cells by recognising the balance of activating and inhibitory ligands expressed by each target cell. However, how the integration of activating and inhibitory signals relates to formation of the NK cell immune synapse remains a central question in our understanding of NK cell recognition. Here we report that ligation of LFA-1 on NK cells induced asymmetrical cell spreading and migration. In contrast, ligation of the activating receptor NKG2D induced symmetrical spreading of ruffled lamellipodia encompassing a dynamic ring of f-actin, concurrent with polarization towards a target cell and a “stop” signal. Ligation of both LFA-1 and NKG2D together resulted in symmetrical spreading but co-ligation of inhibitory receptors reverted NK cells to an asymmetrical migratory configuration leading to inhibitory synapses being smaller and more rapidly disassembled. Using micropatterned activating and inhibitory ligands, signals were found to be continuously and locally integrated during spreading. Together, these data demonstrate that NK cells spread to form large, stable, symmetrical synapses if activating signals dominate, whereas asymmetrical migratory “kinapses” are favoured if inhibitory signals dominate. This clarifies how the integration of activating and inhibitory receptor signals is translated to an appropriate NK cell response.

Immune cells survey their local environment and an immunological response can be activated when an appropriate target cell or antigen-presenting cell is recognised by key cell surface molecules. Just how the multitude of protein–protein interactions work to regulate this decision is an ongoing question. Imaging technology has provided key insights, demonstrating that immune cell activation is often accompanied by the segregation of proteins at immune synapses. Natural killer (NK) cells are lymphocytes that can recognise and kill virally infected or tumour-transformed cells via the formation of a synapse that facilitates secretion of cytotoxic granules directed at the target cells. Key to understanding target cell recognition by NK cells is to establish how the balance of activating and inhibitory signals at the synapse leads to an appropriate response, e.g., to kill or spare a target cell. We found that when activating ligands are dominant on a target cell, NK cells stop migrating and spread lamellipodia across the target cell to form a large symmetrical synapse. If inhibitory signals dominate, the symmetry of the NK cell spreading response is broken and the stop signal is reversed, which reduces the time spent in contact with the target cell. Thus, NK cell activating and inhibitory signals regulate NK cell synapse symmetry and migration to determine whether an NK cell will kill or move on.

Single-chain Fv phage display propensity exhibits strong positive correlation with overall expression levels

Background

Single chain Fvs (scFvs) are widely applied in research, diagnostics and therapeutic settings. Display and selection from combinatorial libraries is the main route to their discovery and many factors influence the success of this process. They exhibit low thermodynamic stability, resulting in low levels of premature cytosolic folding or aggregation which facilitates sec YEG-mediated translocation and phage in E. coli. However, there is little data analysing how this is related to and influenced by scFv protein expression.

Results

We characterised the relationship between overall scFv expression and display propensity for a panel of 15 anti-tetanus toxin scFvs and found a strong positive correlation (Rho = 0.88, p < 0.005) between the two parameters. Display propensity, overall expression and soluble localisation to the periplasm and extracellular fractions were clone specific characteristics which varied despite high levels of sequence homology. There was no correlation between display of scFv or its expression in non-fused (free) form with soluble scFv localisation to the periplasm or culture supernatant. This suggests that divergence in the fate of scFv-pIII and non-fused scFv after translocation to the periplasm accounts for the observed disparity. Differential degrees of periplasmic aggregation of non-fused scFv between clones may affect the partitioning of scFv in the periplasm and culture supernatant abrogating any correlation. We suggest that these factors do not apply to the scFv-pIII fusion since it remains anchored to the bacterial inner membrane as part of the innate phage packaging and budding process.

Conclusion

We conclude that in the absence of premature cytosolic aggregation or folding, the propensity of a scFv to be displayed on phage is directly related to its overall expression level and is thus indirectly influenced by factors such as codon bias, mRNA abundance or putative DNA motifs affecting expression. This suggests that scFvs capable of high overall expression and display levels may not produce high yields of non phage-fused soluble protein in either the periplasmic or extracellular fractions of E. coli. This should be considered when screening clones selected from combinatorial libraries for further study.

The nucleotide and amino acid sequences of the anti-tetanus toxin scFvs have been deposited in the EMBL data base: accession numbers-C1: AM749134, C2: AM749135, C3: AM749136, C4: AM749137, C5: AM749138, N1: AM749139, N2: AM749140, N3: AM749141, N4: AM749142, N5: AM749143 J1; AM749144, J2: AM749145, J3: AM749146, J4: AM749147, J5: AM749148.

Recombinant antibodies for cancer therapy

BACKGROUND

Recombinant antibodies have evolved into successful therapeutics with 10 approved for cancer and more in the pipeline. Four of the top ten cancer therapy drugs are recombinant antibodies.

OBJECTIVES

To survey the current state-of-the-art highlighting the reasons for this success and looking ahead to the next generation of antibody therapy.

METHODS

An analysis was carried out to identify preclinical and clinical examples and the underlying concepts and mechanisms that have shown how to design better therapies.

RESULTS/CONCLUSIONS

Greater understanding of the molecular basis of cancer has led to improved antibodies and a greater selection of targets. Fine tuning of successful antibodies through modification of glycosylation, affinity, size and other parameters are paying dividends. Fc-engineering is likely to be predominant in the near future but conjugates, fragments and fusion proteins will continue to be developed and find their place in the arsenal of antibody therapeutics.

Modulation of antibody pharmacokinetics by chemical polysialylation

Chemical coupling of a variety of polymers to therapeutic proteins has been studied as a way of improving their pharmacokinetics and pharmacodynamics in vivo. Conjugates have been shown to possess greater stability, lower immunogenicity, and a longer blood circulation time due to the chemicophysical properties of these hydrophilic long chain molecules. Naturally occurring colominic acid (polysialic acid, PSA) has been investigated as an alternative to synthetic polymers such as poly(ethylene glycol) (PEG) due to its lower toxicity and natural metabolism. Antibodies and their fragments are a good example of the types of proteins which benefit from pharmacokinetic engineering. Here, we chemically attached differing amounts and differing lengths of short (11 kDa) and longer (22 kDa) chain colominic acid molecules to the antitumor monoclonal antibody H17E2 Fab fragment. Different coupling ratios and lengths were seen to alter the electrophoretic mobility of the Fab fragment but have a minor effect on the antibody immunoreactivity toward the placental alkaline phosphatase (PLAP) antigen. Polysialylation generally increased Fab fragment blood half-life resulting in higher tumor uptake in a KB human tumor xenograft mouse model. One H17E2 Fab-PSA conjugate had over a 5-fold increase in blood exposure and over a 3-fold higher tumor uptake with only a marginal decrease in tumor/blood selectivity ratio compared to the unconjugated Fab. This conjugate also had a blood bioavailability approaching that of a whole immunoglobulin.

Targeted photodynamic therapy with multiply-loaded recombinant antibody fragments

Current photodynamic therapy (PDT) of cancer is limited by inefficiencies involved in specifically targeting photosensitizers to tumors. Although antibodies are being explored as targeting vehicles, they present significant challenges, particularly in terms of pharmacokinetics and drug-coupling. We describe here a novel and effective system to covalently attach multiple photosensitizer molecules (both preclinical, pyropheophorbide-a and clinically approved, verteporfin photosensitizers) to single-chain Fvs. Further, we demonstrate that not only do the resulting photoimmunoconjugates retain photophysical functionality, they are more potent than either free photosensitizer, effectively killing tumor cells in vitro and in vivo. For example, treatment of human breast cancer xenografts with a photoimmunoconjugate comprising an anti-HER-2 scFv linked to 8-10 molecules of pyropheophorbide-a leads to significant tumor regression. These results give an insight into the important features that make scFvs good carriers for PDT drugs and provide proof of concept of our unique approach to targeted photodynamic therapy (tPDT). This promises to significantly improve on current photodynamic therapies for the treatment of cancer.

Phage display of chelating recombinant antibody libraries

High affinity and discriminating specificity are important parameters for any successful antibody based targeting strategy. We herein describe a system for the construction and subsequent selection of affinity-optimised chelating recombinant antibodies (CRAbs) from a randomised filamentous phage-display inter-scFv linker library. Using a simple, robust and highly degenerate tandem scFv cloning strategy a phage-display library of CRAbs with varied inter-scFv linkers was constructed and characterised. The library consisted of two single-chain Fvs (scFvs) of well characterised anti-lysozyme antibodies D1.3 and HyHEL-10(TF), specific for distinct non-overlapping epitopes, separated by flexible polypeptide linkers of varying lengths and sequences. The use of a stringent affinity-based selection strategy quickly led to the enrichment of CRAbs with a restricted set of linker lengths (16-21 amino acids) which agrees very closely with previously described crystal structure data, affinity measurements and mathematical modelling. This CRAb linker phage-display selection strategy is a widely applicable approach for the selection of very high affinity CRAbs for pairs of scFvs against potentially any target antigen, complementing the more arbitrary affinity maturation approaches based on random mutagenesis.

Characterisation and internalisation of recombinant humanised HMFG-1 antibodies against MUC1

The humanised HMFG-1 immunoglobulin has been extensively developed as a clinical immunotherapeutic agent for MUC1 expressing tumours. We have constructed a single-chain Fv (scFv) and Fab fragment from this antibody and shown that both these species retain their specificity for MUC1. The scFv was less stable and less soluble than the Fab. Detailed analyses of the binding kinetics of the whole IgG and Fab fragment show that the affinity for MUC1 synthetic peptides is low (approximately 100 nM for the IgG and 10 μM for the Fab), with particularly low but similar dissociation rate constants (0.031–0.095 s⁻¹). Binding to native antigen on the cell surface is over two orders of magnitude better. Confocal immunofluorescence microscopy shows that both the IgG and Fab are internalised rapidly (the IgG is internalised within 15 min) and colocalise to early endosomes. This work provides an appreciation of the binding, internalising and trafficking kinetics, important for the development of future therapeutics based on this antibody.

Impact of antibody framework residue VH-71 on the stability of a humanised anti-MUC1 scFv and derived immunoenzyme

Anti-MUC1 single-chain Fv (scFv) fragments generated from the humanised antibody huHMFG1 had adequate antigen-binding properties but very poor stability irrespective of the applied linker or domain orientation. Mutagenesis of heavy-chain framework residue V_H-71, previously described as a key residue for maintaining the CDR-H2 main-chain conformation and thus important for antigen binding, markedly stabilised the scFv while having only a minor effect on the binding affinity of the molecule. Because of its improved stability, the engineered fragment exhibited immunoreactivity with tumour cells even after 7 days of incubation in human serum at 37°C. It also showed, in contrast to the wild-type scFv, a concentration-dependent binding to the target antigen when displayed on phage. When fusing the scFv to the recombinant ribonuclease rapLRI, only the fusion protein generated with the stable mutant scFv was able to kill MUC1⁺ tumour cells with an IC₅₀ of 80 nM. We expect this novel immunoenzyme to become a promising tool for the treatment of MUC1⁺ malignancies.

Antibody-guided enzyme therapy of cancer producing cyanide results in necrosis of targeted cells

A number of enzyme/prodrug activation approaches for the treatment of cancer have been reported to date with varying success. We describe progress in the development of a system based on a beta-glucosidase enzyme in combination with a naturally occurring "prodrug," the sugar linamarin, which releases the cytotoxin cyanide. A recombinant fusion protein, composed of an scFv (MFE-23) reactive against carcinoembryonic antigen (CEA) and a plant-derived beta-glucosidase (linamarase), was produced and its cytotoxic potential was investigated. The fusion protein was expressed in a supersecretory mutant strain of Saccharomyces cerevisiae and purified by affinity chromatography. Extensive functional in vitro characterisation of the fusion protein showed that it retained antigen binding activity but that its catalytic activity was impaired, a problem not related to its fusion with the scFv. Nevertheless, we demonstrated complete tumour cell killing at doses of prodrug that are completely nontoxic to nontargeted cells. Preliminary in vivo characterisation showed that extensive glycosylation of the fusion protein caused its rapid clearance through the hepatic route. Aggregational properties also led to poor pharmacokinetics. Furthermore, we present some data analysing the mode of cell death resulting from exposure to this system. Enzymic catalysis of the substrate generates cyanide, a metabolic poison that asphyxiates cells and leads them to a necrotic-like cell death. This system has been called antibody-guided enzyme nitrile therapy (AGENT).

A recombinant cytotoxic chimera based on mammalian deoxyribonuclease-I

A number of mammalian proteins with suitable biological activities have been considered for use in targeted tumour therapy. Deoxyribonuclease-I (DNase-I), an endonuclease that degrades double-stranded DNA, represents an attractive candidate for tumour targeting since it is normally non-toxic yet could be highly cytotoxic when redirected to the cell nucleus. Our aim was to investigate the cytotoxic potential of mammalian DNase-I and its possible use in tumour-targeting strategies for cancer therapy. A chimeric molecule comprising a scFv reactive against the human placental alkaline phosphatase (hPLAP) and bovine pancreatic DNase-I was designed and investigated. The development of a tightly controlled system for the bacterial expression of DNase-I and its chimera is described. The production and purification of active DNase-I from the soluble cell fraction and significant yields from the insoluble fraction by isolation and refolding are described. The construction, expression, purification and in vitro characterisation of an anti-PLAP scFv-DNase-I chimera is also described. This molecule was shown to possess both antigen-binding and DNA-degrading activity in in vitro assays, thus combining the specific cell-targeting properties of the scFv and the potent, highly catalytic activity of the endonuclease. Furthermore, this chimeric molecule was highly cytotoxic in vitro in cells expressing the PLAP antigen. Targeting mammalian DNase-I provides a novel therapeutic strategy for selective cell killing, with the promise of less systemic toxicity and immunogenicity than currently used immunotoxins.

Design, characterization and anti-tumour cytotoxicity of a panel of recombinant, mammalian ribonuclease-based immunotoxins

Bovine seminal ribonuclease (BSRNase) is an unusual member of the ribonuclease superfamily, because of its remarkable anti-tumour and immunosuppressive properties. We describe here the construction, expression, purification and characterization of a panel of six immunotoxins based upon this enzyme and show that we can increase its anti-tumour activity by over 2 x 10(4)-fold. This is achieved by improving tumour cell targeting using a single-chain Fv (scFv) directed against the oncofetal antigen placental alkaline phosphatase. As well as the simple scFv-BSRNase fusion protein, we have constructed five other derivatives with additional peptides designed to improve folding and intracellular trafficking and delivery. We find that the molecule most cytotoxic to antigen (PLAP)-positive cells in vitro is one that contains a C-terminal 'KDEL' endoplasmic reticulum retention signal and a peptide sequence derived from diphtheria toxin. All these molecules are produced in Escherichia coli (E. coli) as insoluble inclusion bodies and require extensive in vitro processing to recover antigen binding and ribonuclease activity. Despite incomplete ribonuclease activity and quaternary assembly, these molecules are promising reagents for specific chemotherapy of cancer and are potentially less harmful and immunogenic than current immunotoxins.

Redesigned anti-human placental alkaline phosphatase single-chain Fv: soluble expression, characterization and in vivo tumour targeting

Although much progress has been made in the production of recombinant antibodies and their fusions, there are still problems with solubility and folding. Useful antibodies produced from cloned hybridomas do not always result in scFvs behaving favourably. We report here further work on an scFv (H17E2) against the oncofetal antigen human placental alkaline phosphatase. The overall expression was greatly improved and the H17E2 scFv was redesigned by manipulation of the interdomain linker, resulting in much higher expression levels of the soluble scFv in its active conformation at 0.2-0.5 mg/l of bacterial culture. We show that the new soluble version of this scFv has similar characteristics to the refolded version in terms of antigen and tumour cell binding, stability and in vivo pharmacokinetics. The final tumour uptake behaviour of these scFvs is superior to that of the parental whole antibody with respect to tumour:organ ratios, but still requires further development before considering it as a suitable molecule for clinical use in ovarian or testicular cancer.

Construction, expression and characterisation of a single chain anti-tumour antibody (scFv)-IL-2 fusion protein

We describe the production and preliminary characterisation of a fusion protein between interleukin-2 and a single-chain Fv version of the H17E2 anti-placental alkaline phosphatase (PLAP) antibody. This molecule could be used to target interleukin-2 to PLAP-expressing tumours.

Genetic delivery of enzymes for cancer therapy

For many years, antibodies have been examined as means to deliver cytotoxic proteins to kill target cells (immunotoxins). More recently, there have been studies on enzymes that convert prodrugs to active drugs to kill target cells. The advances in gene therapy strategies now allow one to deliver the gene for the protein or enzyme as an alternative. This technique, although in its infancy, promises to overcome some of the problems associated with antibody-mediated delivery. Thymidine kinase and cytosine deaminase are some of the enzymes currently being exploited in this way, but more are on the horizon. However, more research is still needed to enable full exploitation of the transcriptional differences between tumour and normal cells so that more existing cancers can be treated in this way.

DNA vaccination for cancer treatment

The recent finding that inoculation with plasmids encoding a variety of proteins leads to T cell and antibody responses in vivo against these proteins provides a novel means of active specific immunisation by plasmid vaccination. The demonstration that both major histocompatibility complex (MHC) Class I- and Class II-mediated interactions can be elicited may make this approach suitable for development of tumour vaccines. Plasmids may prove to be an efficient way to build 'subunit' and multi-subunit vaccines based on the genetic changes that occur in carcinogenesis. Expression of DNA encoding fragments of tumour-specific proteins as neo-antigens or surrogate antigens in a novel context may be a means of breaking immunological tolerance and lead to the generation of tumour-specific immune responses.

Reductive and oxidative half-reactions of glutathione reductase from Escherichia coli

Glutathione reductase catalyzes the reduction of glutathione disulfide by NADPH and has a redox active disulfide and an FAD cofactor in each monomer. In the reductive half-reaction, FAD is reduced by NADPH and electrons pass from the reduced flavin to the redox active disulfide. The oxidative half-reaction is dithiol-disulfide interchange between the enzyme dithiol and glutathione disulfide. We have investigated the reductive and oxidative half-reactions using wild-type glutathione reductase from Escherichia coli and in an altered form of the enzyme in which the active site acid-base catalyst, His439, has been changed to an alanine residue (H439A). H439A has 0.3% activity in the NADPH/GSSG assay. The replacement affects both the oxidative half-reaction, as expected, and the reductive half-reaction--specifically, the passage of electrons from reduced flavin to the disulfide. Reduction of H439A by NADPH allows direct observation of flavin reduction. The NADPH-FAD charge transfer complex is formed in the dead time. Reduction of FAD, at a limiting rate of 250 s-1, is observed as a decrease at 460 nm and an increase at 670 nm (FADH(-)-NADP+ charge transfer). Subsequent passage of electrons from FADH- to the disulfide (increase at 460 nm and a decrease at 670 nm) is very slow (6-7 s-1) and concentration independent in H439A. The monophasic oxidative half-reaction is very slow, as expected for reduced H439A.(ABSTRACT TRUNCATED AT 250 WORDS)

Targeting enzymes for cancer therapy: old enzymes in new roles

Enzymes which traditionally have played no role in cell-directed cytotoxicity are finding their way into schemes for prodrug activation and immunotoxins owing to such useful enzymatic activity. Alkaline phosphatase, carboxypeptidases, beta-glucosidases and beta-lactamases among many others are being utilised to regenerate potent anti-cancer drugs or toxic small molecules from precursors in a bid to enhance their activity in tumours. These prodrug activation systems require the pretargeting of the enzyme to the surface of a tumour cell, usually by an antibody or its immunoreactive fragment. A recent novel approach proposes the intracellular delivery of appropriate enzymes, such as phosphodiesterases, to particular cellular compartments. There, enzyme activity can cause substantive damage resulting in cell death. Cell targeting of mammalian phosphodiesterase promises to improve upon conventional immunotoxins because of their increased cytotoxicity when targeted to the appropriate compartment and their expected lack of, or lower, immunogenicity in clinical use.

Genetically engineered antibodies for diagnostic pathology

Antibody genes can be cloned, genetically manipulated, and expressed in both homologous and heterologous expression systems to produce viable antigen-binding proteins complete with natural effector functions. Manipulation of antibody genes permits the expression of fusion proteins or truncated proteins that retain antigen-binding activity. The new antibody technologies are becoming increasingly sophisticated, permitting the alteration of antigen-binding responses, the transfer of antigen specificity between antibodies, and the expression of minimal-size antigen-binding protein domains. These new molecules have been made mostly for studies on function or to provide molecules suited for in vivo diagnosis and therapy; very few have been specifically designed for, or used for, diagnostic histopathology. We describe here the adaptation of small antibody derivatives for use in immunohistochemistry. Molecules suitable for this purpose need only to possess specific antigen-binding ability and some means of detection of antigen-bound material. Detection could be by recognition of a genetically fused flag or tag epitope, by the fusion of an enzyme whose activity can be assayed, or by fusion with a protein that can interact with pre-existing histopathological reagents.

Targeting phosphodiesterases as a strategy for killing tumor cells

Ribonucleases (RNases) are being employed as alternative cytotoxic proteins to the conventionally used ones such as ricin and Pseudomonas exotoxin. Mammalian RNases are attractive enzymes because of their comparable cytotoxicity when suitably directed and the likelihood of lower immunogenicity compared to plant and bacterial toxins. Bovine seminal RNase (BSRNase) is a member of the RNase superfamily, but differs in many interesting ways. Unlike the rest of the family it is dimeric, and possesses antitumor and immunosuppressive properties. These features make it a choice candidate for a single-chain antibody (scFv) based immunotoxin. This work describes preliminary data on the construction, expression in Escherichia coli and characterization of a tumor-specific scFv (directed against human placental alkaline phosphatase)-BSRNase chimeric molecule. It is shown that the created molecule has RNA degrading activity and antigen-binding activity when refolded from bacterial inclusion bodies.

Deoxyribonuclease I (DNAse I). A novel approach for targeted cancer therapy

A number of phosphodiesterases, some of which possess additional biological activities (e.g., antitumor, immunosuppressive, and so on), have been considered for use in targeted tumor therapy. We propose Deoxyribonuclease I (DNase I), a compact, monomeric enzyme, as a very attractive candidate for targeting to tumor cells. Only a small amount of enzyme targeted to a cell needs to enter the nucleus in order to degrade the chromosomal DNA, making a cell incapable of further replication. We describe preliminary data on the construction of a potent single-chain antibody (scFv) immunotoxin based on bovine pancreatic DNAse I. The use of a mammalian enzyme should be much less toxic and less immunogenic than current immunotoxins and may expand the current limits of immunotoxin therapy.

A universal antibody-derived targeting agent

We report bacterial expression of a single-chain antibody (ScFv) reactive against the haptens 4-hydroxy-3 nitrophenylacetic acid (NP) and 4-hydroxy-3-iodo-5-nitrophenylacetic acid (NIP) that is suitable for targeting to mammalian cells in vitro in a novel two-step targeting strategy. Hapten-derivatized primary antibodies of known specificity, bound to target cells, can capture the ScFv. Specificity resides in the interaction of the primary targeting antibody with the target and the interaction of the ScFv for NP/NIP, since the ScFv does not bind cells and nonderivatized antibodies bound at cells cannot capture the ScFv. The ScFv described here can therefore be considered as a universal agent for delivery of drugs, toxins, or radionuclides to any cell type for which a previously characterized antibody exists.

Cooperativity induced by a single mutation at the subunit interface of a dimeric enzyme: glutathione reductase

When glycine418 of Escherichia coli glutathione reductase, which is in a closely packed region of the dimer interface, is replaced with a bulky tryptophan residue, the enzyme becomes highly cooperative (Hill coefficient 1.76) for glutathione binding. The cooperativity is lost when the mutant subunit is hybridized with a wild-type subunit to create a heterodimer. The mutation appears to disrupt atomic packing at the dimer interface, which induces a change of kinetic mechanism. A single mutation in a region of the protein remote from the active site can thus act as a molecular switch to confer cooperativity on an enzyme.

Engineering surface charge. 1. A method for detecting subunit exchange in Escherichia coli glutathione reductase

The gene gor encoding Escherichia coli glutathione reductase was mutated to create a positively charged N-terminal extension consisting of five arginine residues followed by a factor Xa cleavage site to the enzyme polypeptide chain. The modified protein assembled in vivo to yield a dimeric enzyme with kinetic parameters indistinguishable from those of wild-type glutathione reductase. The N-terminal extension could not be released by treatment with factor Xa but could be removed by exposure to trypsin, again without effect on the enzyme activity. The modified enzyme was readily separated from the wild-type enzyme by means of ion-exchange chromatography or nondenaturing polyacrylamide gel electrophoresis. Incubation of the modified and wild-type enzymes, separately or as a mixture, with NADH led to their partial inactivation, and activity was restored by exposure to 1 mM reduced glutathione. No hybrid dimer was formed in the mixture of modified and wild-type enzymes, as judged by polyacrylamide gel electrophoresis, strongly suggesting that the inactivation induced by NADH was not due to dissociation of the parental dimers. The addition of otherwise benign positively or negatively charged extensions to the N- or C-terminal regions of the constituent polypeptide chains of oligomeric enzymes offers a simple route to detecting hybrid formation and the causative subunit dissociation and exchange.

Engineering surface charge. 2. A method for purifying heterodimers of Escherichia coli glutathione reductase

Two gor genes encoding different mutants of Escherichia coli glutathione reductase have been expressed in the same E. coli cell, leading to the creation of a hybrid form of the enzyme dimer. One of the gor genes carried, in addition to various directed mutations, a 5' extension that encodes a benign penta-arginine "arm" added to the N-terminus of the glutathione reductase polypeptide chain [Deonarain, M.P., Scrutton, N.S., & Perham, R.N. (1992) Biochemistry (preceding paper in this issue)]. This made possible, by means of ion-exchange chromatography or nondenaturing polyacrylamide gel electrophoresis, the facile separation of the hybrid enzyme from the two parental forms. Moreover, the two subunits in the hybrid enzyme could be made to carry different mutations. In this way, glutathione reductases with only one active site per dimer were generated: the effects of replacing tyrosine-177 with glycine in the NADPH-binding site, which greatly diminishes the Km for glutathione and switches the kinetic mechanism from ping-pong to ordered sequential, and of replacing His-439 with glutamine in the glutathione-binding site, which greatly diminishes the Km for NADPH, were both found to be restricted to the one active site carrying the mutations. This system of generating separable enzyme hybrids is generally applicable and should make it possible now to undertake a more systematic study of catalytic mechanism and assembly for the many enzymes with quaternary structure.

Active site complementation in engineered heterodimers of Escherichia coli glutathione reductase created in vivo

By directed mutagenesis of the cloned Escherichia coli gor gene encoding the dimeric flavoprotein glutathione reductase, Cys-47 (a cysteine residue forming an essential charge-transfer complex with enzyme-bound FAD) was converted to serine (C47S) and His-439 (required to facilitate protonation of the reduced glutathione) was converted to glutamine (H439Q). Both mutant genes were placed in the same plasmid, pHD, where each of them came under the control of a strong tac promoter. This was designed to achieve equal over-expression of both genes in the same E. coli cell. The parental homo-dimers show no (C47S) or very little (H439Q) activity as glutathione reductases. The formation in vivo of heterodimers, carrying one crippled and one fully functional active site, was detected by absorbance spectroscopy and fluorescence emission spectrometry of enzyme-bound FAD and by active site complementation. The fractional distribution of homo- and hetero-dimers was in accord with that expected for a random association of enzyme subunits. In a homo-dimer, the H439Q mutation leads to a big fall in the value of Km for NADPH which binds some 1.8 nm from the point of mutation (Berry, A., Scrutton, N.S. & Perham, R. N. Biochemistry 28, 1264-1269 (1989)). However, the one active site in the H439Q/C47S hetero-dimer exhibited kinetic parameters similar to those of the wild-type enzyme. Thus, the effect of the H439Q mutation must be retained within the active site that accommodates it and is not transmitted through the protein to the second active site across the subunit interface.(ABSTRACT TRUNCATED AT 250 WORDS)

Directed mutagenesis of the redox-active disulphide bridge in glutathione reductase from Escherichia coli

Directed mutagenesis of the gor gene from Escherichia coli encoding the flavoprotein glutathione reductase was used to convert the two cysteine residues that comprise its redox-active disulphide bridge to alanine (C42A) and serine (C47S) residues. A double mutant (C42AH439A) was also created in which His-439, the proton donor/acceptor in the glutathione-binding site, was additionally converted into an alanine residue. The C42A and C47S mutants were both unable to catalyse the reduction of glutathione by NADPH. The C42A mutant retained the transhydrogenase activity of the wild-type enzyme, whereas the C47S mutant was also inhibited in this reaction. These results support the view that in the catalytic mechanism of E. coli glutathione reductase, the thiolate form of Cys-42 acts as a nucleophile to initiate disulphide exchange with enzyme-bound glutathione and that the thiolate form of Cys-47 generates an essential charge-transfer complex with enzyme-bound FAD. Titration of the C42A and C42AH439A mutants indicated that the imidazole side-chain of His-439 lowered the pKa of the charge-transfer thiol (Cys-47) from 7.7 to 5.7, enhancing its ability to act as an anion at neutral pH. Several important differences between these mutants of E. coli glutathione reductase and similar mutants (or chemically modified forms) of other members of the flavoprotein disulphide oxidoreductase family were noted, but these could be explained in terms of the different redox chemistries of the enzymes concerned.

Alternative proton donors/acceptors in the catalytic mechanism of the glutathione reductase of Escherichia coli: the role of histidine-439 and tyrosine-99

The cloned Escherichia coli gor gene encoding the flavoprotein glutathione reductase was placed under the control of the tac promoter in the plasmid pKK223-3, allowing expression of glutathione reductase at levels approximately 40,000 times those of untransformed cells. This greatly facilitated purification of the enzyme. By directed mutagenesis of the gor gene, His-439 was changed to glutamine (H439Q) and alanine (H439A). The tyrosine residue at position 99 was changed to phenylalanine (Y99F), and in another experiment, the H439Q and Y99F mutations were united to form the double mutant Y99FH439Q. His-439 is thought to act in the catalytic mechanism as a proton donor/acceptor in the glutathione-binding pocket. The H439Q and H439A mutants retain approximately 1% and approximately 0.3%, respectively, of the catalytic activity of the wild-type enzyme. This reinforces our previous finding [Berry et al. (1989) Biochemistry 28, 1264-1269] that direct protonation and deprotonation of the histidine residue are not essential for the reaction to occur. The retention of catalytic activity by the H439A mutant demonstrates further that a side chain capable of hydrogen bonding to a water molecule, which might then act as proton donor, also is not essential at this position. Tyr-99 is a further possible proton donor in the glutathione-binding pocket, but the Y99F mutant was essentially fully active, and the Y99FH439Q double mutant also retained approximately 1% of the wild-type specific activity.(ABSTRACT TRUNCATED AT 250 WORDS)