Discovery of GLPG2737, a Potent Type 2 Corrector of CFTR for the Treatment of Cystic Fibrosis in Combination with a Potentiator and a Type 1 Co-corrector

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) protein. This epithelial anion channel regulates the active transport of chloride and bicarbonate ions across membranes. Mutations result in reduced surface expression of CFTR channels with impaired functionality. Correctors are small molecules that support the trafficking of CFTR to increase its membrane expression. Such correctors can have different mechanisms of action. Combinations may result in a further improved therapeutic benefit. We describe the identification and optimization of a new pyrazolol3,4-bl pyridine-6-carboxylic acid series with high potency and efficacy in rescuing CFTR from the cell surface. Investigations showed that carboxylic acid group replacement with acylsulfonamides and acylsulfonylureas improved ADMET and PK properties, leading to the discovery of the structurally novel co-corrector GLPG2737. The addition of GLPG2737 to the combination of the potentiator GLPG1837 and C1 corrector 4 led to an 8-fold increase in the F508del CFTR activity.

Evaluation of a novel CFTR potentiator in COPD ferrets with acquired CFTR dysfunction

RATIONALE

The majority of chronic obstructive pulmonary disease (COPD) patients have chronic bronchitis, for which specific therapies are unavailable. Acquired cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction is observed in chronic bronchitis, but has not been proven in a controlled animal model with airway disease. Furthermore, the potential of CFTR as a therapeutic target has not been tested in vivo, given limitations to rodent models of COPD. Ferrets exhibit cystic fibrosis-related lung pathology when CFTR is absent and COPD with bronchitis following cigarette smoke exposure.

OBJECTIVES

To evaluate CFTR dysfunction induced by smoking and test its pharmacological reversal by a novel CFTR potentiator, GLPG2196, in a ferret model of COPD with chronic bronchitis.

METHODS

Ferrets were exposed for 6 months to cigarette smoke to induce COPD and chronic bronchitis and then treated with enteral GLPG2196 once daily for 1 month. Electrophysiological measurements of ion transport and CFTR function, assessment of mucociliary function by one-micron optical coherence tomography imaging and particle-tracking microrheology, microcomputed tomography imaging, histopathological analysis and quantification of CFTR protein and mRNA expression were used to evaluate mechanistic and pathophysiological changes.

MEASUREMENTS AND MAIN RESULTS

Following cigarette smoke exposure, ferrets exhibited CFTR dysfunction, increased mucus viscosity, delayed mucociliary clearance, airway wall thickening and airway epithelial hypertrophy. In COPD ferrets, GLPG2196 treatment reversed CFTR dysfunction, increased mucus transport by decreasing mucus viscosity, and reduced bronchial wall thickening and airway epithelial hypertrophy.

CONCLUSIONS

The pharmacologic reversal of acquired CFTR dysfunction is beneficial against pathological features of chronic bronchitis in a COPD ferret model.

CFTR Rescue in Intestinal Organoids with GLPG/ABBV-2737, ABBV/GLPG-2222 and ABBV/GLPG-2451 Triple Therapy

Cystic fibrosis transmembrane conductance regulator (CFTR) modulators have transformed the treatment of cystic fibrosis (CF) by targeting the basis of the disease. In particular, treatment regimen consisting of multiple compounds with complementary mechanisms of action have been shown to result in optimal efficacy. Here, we assessed the efficacy of combinations of the CFTR modulators ABBV/GLPG-2222, GLPG/ABBV-2737 and ABBV/GLPG-2451, and compared it to VX-770/VX-809 in 28 organoid lines heterozygous for F508del allele and a class I mutation and seven homozygous F508del organoid lines. The combination ABBV/GLPG-2222/ABBV-2737/ABBV/GLPG-2451 showed increased efficacy over VX-770/VX-809 for most organoids, despite considerable variation in efficacy between the different organoid cultures. These differences in CFTR restoration between organoids with comparable genotypes underline the relevance of continuing to optimize the ABBV/GLPG‐Triple therapy, as well as the in vitro characterization of efficacy in clinically relevant models.

Novel Correctors and Potentiators Enhance Translational Readthrough in CFTR Nonsense Mutations

Premature-termination codons (PTCs) in CFTR (cystic fibrosis [CF] transmembrane conductance regulator) result in nonfunctional CFTR protein and are the proximate cause of ∼11% of CF-causing alleles, for which no treatments exist. The CFTR corrector lumacaftor and the potentiator ivacaftor improve CFTR function with terminal PTC mutations and enhance the effect of readthrough agents. Novel correctors GLPG2222 (corrector 1 [C1]), GLPG3221 (corrector 2 [C2]), and potentiator GLPG1837 compare favorably with lumacaftor and ivacaftor in vitro. Here, we evaluated the effect of correctors C1a and C2a (derivatives of C1 and C2) and GLPG1837 alone or in combination with the readthrough compound G418 on CFTR function using heterologous Fischer rat thyroid (FRT) cells, the genetically engineered human bronchial epithelial (HBE) 16HBE14o^- cell lines, and primary human cells with PTC mutations. In FRT lines pretreated with G418, GLPG1837 elicited dose-dependent increases in CFTR activity that exceeded those from ivacaftor in FRT-W1282X and FRT-R1162X cells. A three-mechanism strategy consisting of G418, GLPG1837, and two correctors (C1a + C2a) yielded the greatest functional improvements in FRT and 16HBE14o^- PTC variants, noting that correction and potentiation without readthrough was sufficient to stimulate CFTR activity for W1282X cells. GLPG1837 + C1a + C2a restored substantial function in G542X/F508del HBE cells and restored even more function for W1282X/F508del cells, largely because of the corrector/potentiator effect, with no additional benefit from G418. In G542X/R553X or R1162X/R1162X organoids, enhanced forskolin-induced swelling was observed with G418 + GLPG1837 + C1a + C2a, although GLPG1837 + C1a + C2a alone was sufficient to improve forskolin-induced swelling in W1282X/W1282X organoids. Combination of CFTR correctors, potentiators, and readthrough compounds augments the functional repair of CFTR nonsense mutations, indicating the potential for novel correctors and potentiators to restore function to truncated W1282X CFTR.

Discovery of GLPG2451, a Novel Once Daily Potentiator for the Treatment of Cystic Fibrosis

Cystic fibrosis (CF) is a life-threatening recessive genetic disease caused by mutations in the gene encoding for the cystic fibrosis transmembrane conductance regulator (CFTR). With the discovery of Ivacaftor and Lumacaftor, it has been shown that administration of one or more small molecules can partially restore the CFTR function. Correctors are small molecules that enhance the amount of CFTR on the cell surface, while potentiators improve the gating function of the CFTR channel. Herein, we describe the discovery and optimization of a novel potentiator series. Scaffold hopping, focusing on retaining the different intramolecular contacts, was crucial in the whole discovery process to identify a novel series devoid of genotoxic liabilities. From this series, the clinical candidate GLPG2451 was selected based on its pharmacokinetic properties, allowing QD dosing and based on its low CYP induction potential.

Discovery of 9-Cyclopropylethynyl-2-((S)-1-[1,4]dioxan-2-ylmethoxy)-6,7-dihydropyrimido[6,1-a]isoquinolin-4-one (GLPG1205), a Unique GPR84 Negative Allosteric Modulator Undergoing Evaluation in a Phase II Clinical Trial

GPR84 is a medium chain free fatty acid-binding G-protein-coupled receptor associated with inflammatory and fibrotic diseases. As the only reported antagonist of GPR84 (PBI-4050) that displays relatively low potency and selectivity, a clear need exists for an improved modulator. Structural optimization of GPR84 antagonist hit 1, identified through high-throughput screening, led to the identification of potent and selective GPR84 inhibitor GLPG1205 (36). Compared with the initial hit, 36 showed improved potency in a guanosine 5'-O-[γ-thio]triphosphate assay, exhibited metabolic stability, and lacked activity against phosphodiesterase-4. This novel pharmacological tool allowed investigation of the therapeutic potential of GPR84 inhibition. At once-daily doses of 3 and 10 mg/kg, GLPG1205 reduced disease activity index score and neutrophil infiltration in a mouse dextran sodium sulfate-induced chronic inflammatory bowel disease model, with efficacy similar to positive-control compound sulfasalazine. The drug discovery steps leading to GLPG1205 identification, currently under phase II clinical investigation, are described herein.

Biological Characterization of F508delCFTR Protein Processing by the CFTR Corrector ABBV-2222/GLPG2222

Cystic fibrosis (CF) is the most common monogenic autosomal recessive disease in Caucasians caused by pathogenic mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene (CFTR). Significant small molecule therapeutic advances over the past two decades have been made to target the defective CFTR protein and enhance its function. To address the most prevalent defect of the defective CFTR protein (i.e., F508del mutation) in CF, two biomolecular activities are required, namely, correctors to increase the amount of properly folded F508delCFTR levels at the cell surface and potentiators to allow the effective opening, i.e., function of the F508delCFTR channel. Combined, these activities enhance chloride ion transport yielding improved hydration of the lung surface and subsequent restoration of mucociliary clearance. To enhance clinical benefits to CF patients, a complementary triple combination therapy consisting of two corrector molecules, type 1 (C1) and type 2, with additive mechanisms along with a potentiator are being investigated in the clinic for maximum restoration of mutated CFTR function. We report the identification and in vitro biologic characterization of ABBV-2222/GLPG2222 (4-[(2R,4R)-4-({[1-(2,2-difluoro-1,3-benzodioxol-5-yl)cyclopropyl]carbonyl}amino)-7-(difluoromethoxy)-3,4-dihydro-2H-chromen-2-yl]benzoic acid),-a novel, potent, and orally bioavailable C1 corrector developed by AbbVie-Galapagos and currently in clinical trials-which exhibits substantial improvements over the existing C1 correctors. This includes improvements in potency and drug-drug interaction (DDI) compared with 3-(6-(1-(2,2-difluorobenzo[d][1,3]dioxol-5-yl)cyclopropanecarboxamido)-3-methylpyridin-2-yl)benzoic acid (VX-809, Lumacaftor) and improvements in potency and efficacy compared with 1-(2,2-difluoro-1,3-benzodioxol-5-yl)-N-[1-[(2R)-2,3-dihydroxypropyl]-6-fluoro-2-(1-hydroxy-2-methylpropan-2-yl)indol-5-yl]cyclopropane-1-carboxamide (VX-661, Tezacaftor). ABBV-2222/GLPG2222 exhibits potent in vitro functional activity in primary patient cells harboring F508del/F508del CFTR with an EC50 value <10 nM. SIGNIFICANCE STATEMENT: To address the most prevalent defect of the defective CFTR protein (i.e., F508del mutation) in cystic fibrosis, AbbVie-Galapagos has developed ABBV-2222/GLPG2222, a novel, potent, and orally bioavailable C1 corrector of this protein. ABBV-2222/GLPG2222, which is currently in clinical trials, exhibits potent in vitro functional activity in primary patient cells harboring F508del/F508del CFTR and substantial improvements over the existing C1 correctors.

Identifying the molecular target sites for CFTR potentiators GLPG1837 and VX-770

Identification of the binding sites for small molecules that alleviate gating defects in CFTR would assist rational drug design for the treatment of cystic fibrosis. Yeh et al. identify two potential binding sites for prototypical CFTR potentiators at the interface of CFTR’s two transmembrane domains.

The past two decades have witnessed major breakthroughs in developing compounds that target the chloride channel CFTR for the treatment of patients with cystic fibrosis. However, further improvement in affinity and efficacy for these CFTR modulators will require insights into the molecular interactions between CFTR modulators and their binding targets. In this study, we use in silico molecular docking to identify potential binding sites for GLPG1837, a CFTR potentiator that may share a common mechanism and binding site with VX-770, the FDA-approved drug for patients carrying mutations with gating defects. Among the five binding sites predicted by docking, the two top-scoring sites are located at the interface between CFTR’s two transmembrane domains: site I consists of D924, N1138, and S1141, and site II_N includes F229, F236, Y304, F312, and F931. Using mutagenesis to probe the importance of these sites for GLPG187 binding, we find that disruption of predicted hydrogen-bonding interactions by mutation of D924 decreases apparent affinity, while hydrophobic amino acids substitutions at N1138 and introduction of positively charged amino acids at S1141 improve the apparent affinity for GLPG1837. Alanine substitutions at Y304, F312, and F931 (site II_N) decrease the affinity for GLPG1837, whereas alanine substitutions at F229 and F236 (also site II_N), or at residues in the other three lower-scoring sites, have little effect. In addition, current relaxation analysis to assess the apparent dissociation rate of VX-770 yields results consistent with the dose–response experiments for GLPG8137, with the dissociation rate of VX-770 accelerated by D924N, F236A, Y304A, and F312A, but decelerated by N1138L and S1141K mutations. Collectively, these data identify two potential binding sites for GLPG1837 and VX-770 in CFTR. We discuss the pros and cons of evidence for these two loci and the implications for future drug design.

GLPG1837, a CFTR potentiator, in p.Gly551Asp (G551D)-CF patients: An open-label, single-arm, phase 2a study (SAPHIRA1)

BACKGROUND

Investigation of novel cystic fibrosis transmembrane conductance regulator (CFTR) potentiators, such as GLPG1837, for CF patients with gating mutations is challenging as trials require patients to withhold ivacaftor, the current standard of care. This study explored the feasibility of such a study and the impact of one-week ivacaftor withdrawal.

METHODS

This open-label, single-arm study aimed to enrol 32 adults ≥18 years of age with CF and at least one p.Gly551Asp (G551D) mutation. Patients received three increasing GLPG1837 dosages twice-daily for two 7-day and one 14-day period following a one-week ivacaftor washout. The primary outcome was safety; secondary outcomes were changes in sweat chloride concentration, spirometry outcomes, and pharmacokinetics.

RESULTS

Twenty-six patients enrolled; 24 completed the study. Adverse events were reported by 53.8-76.9% of patients (dosage-dependent), with respiratory adverse events most common. Mean sweat chloride concentrations decreased from 97.7 mmol/L (baseline) to 68.7 mmol/L (end of GLPG1837 treatment). In ivacaftor-pre-treated patients, mean sweat chloride concentrations rose from 42.5 mmol/L at screening to 98.5 mmol/L after ivacaftor washout. Levels were decreased following GLPG1837 treatment (to 68.8 mmol/L at treatment end). Percent predicted forced expiratory volume in 1 s declined from 73.3% at screening to 68.5% after ivacaftor washout but returned to screening level at treatment end (73.1%).

CONCLUSIONS

Patient willingness to participate in the study suggests that the need for a short period of ivacaftor withdrawal may not be a barrier to development of novel potentiators, such as GLPG1837. A one-week ivacaftor washout was generally well tolerated, but resulted in a decline in lung function, which was reversed with GLPG1837 treatment to pre-washout levels. Combined with the concentration-dependent decrease in sweat chloride concentration, results show that GLPG1837 increases CFTR activity in G551D-CF patients. FUND: This work was supported by Galapagos NV.

CLINICAL TRIAL REGISTRATION NUMBERS

NCT02707562; EudraCT 2015-003291-77.

Identification of GLPG/ABBV-2737, a Novel Class of Corrector, Which Exerts Functional Synergy With Other CFTR Modulators

The deletion of phenylalanine at position 508 (F508del) in cystic fibrosis transmembrane conductance regulator (CFTR) causes a severe defect in folding and trafficking of the chloride channel resulting in its absence at the plasma membrane of epithelial cells leading to cystic fibrosis. Progress in the understanding of the disease increased over the past decades and led to the awareness that combinations of mechanistically different CFTR modulators are required to obtain meaningful clinical benefit. Today, there remains an unmet need for identification and development of more effective CFTR modulator combinations to improve existing therapies for patients carrying the F508del mutation. Here, we describe the identification of a novel F508del corrector using functional assays. We provide experimental evidence that the clinical candidate GLPG/ABBV-2737 represents a novel class of corrector exerting activity both on its own and in combination with VX809 or GLPG/ABBV-2222.

CFTR activity is enhanced by the novel corrector GLPG2222, given with and without ivacaftor in two randomized trials

BACKGROUND

Several treatment approaches in cystic fibrosis (CF) aim to correct CF transmembrane conductance regulator (CFTR) function; the efficacy of each approach is dependent on the mutation(s) present. A need remains for more effective treatments to correct functional deficits caused by the F508del mutation.

METHODS

Two placebo-controlled, phase 2a studies evaluated GLPG2222, given orally once daily for 29 days, in subjects homozygous for F508del (FLAMINGO) or heterozygous for F508del and a gating mutation, receiving ivacaftor (ALBATROSS). The primary objective of both studies was to assess safety and tolerability. Secondary objectives included assessment of pharmacokinetics, and of the effect of GLPG2222 on sweat chloride concentrations, pulmonary function and respiratory symptoms.

RESULTS

Fifty-nine and 37 subjects were enrolled into FLAMINGO and ALBATROSS, respectively. Treatment-related treatment-emergent adverse events (TEAEs) were reported by 29.2% (14/48) of subjects in FLAMINGO and 40.0% (12/30) in ALBATROSS; most were mild to moderate in severity and comprised primarily respiratory, gastrointestinal, and infection events. There were no deaths or discontinuations due to TEAEs. Dose-dependent decreases in sweat chloride concentrations were seen in GLPG2222-treated subjects (maximum decrease in FLAMINGO: -17.6 mmol/L [GLPG2222 200 mg], p < 0.0001; ALBATROSS: -7.4 mmol/L [GLPG2222 300 mg], p < 0.05). No significant effects on pulmonary function or respiratory symptoms were reported. Plasma GLPG2222 concentrations in CF subjects were consistent with previous studies in healthy volunteers and CF subjects.

CONCLUSIONS

GLPG2222 was well tolerated. Sweat chloride reductions support on-target enhancement of CFTR activity in subjects with F508del mutation(s). Significant improvements in clinical endpoints were not demonstrated. Observed safety results support further evaluation of GLPG2222, including in combination with other CFTR modulators.

FUNDING

Galapagos NV. Clinical trial registration numbers FLAMINGO, NCT03119649; ALBATROSS, NCT03045523.

Identification and Characterization of Novel CFTR Potentiators

There is still a high unmet need for the treatment of most patients with cystic fibrosis (CF). The identification and development of new Cystic Fibrosis Transmembrane conductance Regulator (CFTR) modulators is necessary to achieve higher clinical benefit in patients. In this report we describe the characterization of novel potentiators. From a small screening campaign on F508del CFTR, hits were developed leading to the identification of pre-clinical candidates GLPG1837 and GLPG2451, each derived from a distinct chemical series. Both drug candidates enhance WT CFTR activity as well as low temperature or corrector rescued F508del CFTR, and are able to improve channel activity on a series of Class III, IV CFTR mutants. The observed activities in YFP halide assays translated well to primary cells derived from CF lungs when measured using Trans-epithelial clamp circuit (TECC). Both potentiators improve F508del CFTR channel opening in a similar manner, increasing the open time and reducing the closed time of the channel. When evaluating the potentiators in a chronic setting on corrected F508del CFTR, no reduction of channel activity in presence of potentiator was observed. The current work identifies and characterizes novel CFTR potentiators GLPG1837 and GLPG2451, which may offer new therapeutic options for CF patients.

A common mechanism for CFTR potentiators

VX-770 is a potentiator of the CFTR channel and an approved therapy for cystic fibrosis. Yeh et al. find that the apparent affinity of a new potentiator, GLPG1837, is state dependent and propose an allosteric modulation model to explain the potency and efficacy of CFTR potentiators.

Cystic fibrosis (CF) is a channelopathy caused by loss-of-function mutations of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which encodes a phosphorylation-activated and adenosine triphosphate (ATP)–gated chloride channel. In the past few years, high-throughput drug screening has successfully realized the first US Food and Drug Administration–approved therapy for CF, called ivacaftor (or VX-770). A more recent CFTR potentiator, GLPG1837 (N-(3-carbamoyl-5,5,7,7-tetramethyl-4,7-dihydro-5H-thieno[2,3-c]pyran-2-yl)-1H-pyrazole-3-carboxamide), has been shown to exhibit a higher efficacy than ivacaftor for the G551D mutation, yet the underlying mechanism of GLPG1837 remains unclear. Here we find that despite their differences in potency and efficacy, GLPG1837 and VX-770 potentiate CFTR gating in a remarkably similar manner. Specifically, they share similar effects on single-channel kinetics of wild-type CFTR. Their actions are independent of nucleotide-binding domain (NBD) dimerization and ATP hydrolysis, critical steps controlling CFTR’s gate opening and closing, respectively. By applying the two reagents together, we provide evidence that GLPG1837 and VX-770 likely compete for the same site, whereas GLPG1837 and the high-affinity ATP analogue 2′-deoxy-N⁶-(2-phenylethyl)-adenosine-5′-O-triphosphate (dPATP) work synergistically through two different sites. We also find that the apparent affinity for GLPG1837 is dependent on the open probability of the channel, suggesting a state-dependent binding of the drug to CFTR (higher binding affinity for the open state than the closed state), which is consistent with the classic mechanism for allosteric modulation. We propose a simple four-state kinetic model featuring an energetic coupling between CFTR gating and potentiator binding to explain our experimental results.

A novel, broad-spectrum inhibitor of enterovirus replication that targets host cell factor phosphatidylinositol 4-kinase IIIβ

Despite their high clinical and socioeconomic impacts, there is currently no approved antiviral therapy for the prophylaxis or treatment of enterovirus infections. Here we report on a novel inhibitor of enterovirus replication, compound 1, 2-fluoro-4-(2-methyl-8-(3-(methylsulfonyl)benzylamino)imidazo[1,2-a]pyrazin-3-yl)phenol. This compound exhibited a broad spectrum of antiviral activity, as it inhibited all tested species of enteroviruses and rhinoviruses, with 50% effective concentrations ranging between 4 and 71 nM. After a lengthy resistance selection process, coxsackievirus mutants resistant to compound 1 were isolated that carried substitutions in their 3A protein. Remarkably, the same substitutions were recently shown to provide resistance to inhibitors of phosphatidylinositol 4-kinase IIIβ (PI4KIIIβ), a lipid kinase that is essential for enterovirus replication, suggesting that compound 1 may also target this host factor. Accordingly, compound 1 directly inhibited PI4KIIIβ in an in vitro kinase activity assay. Furthermore, the compound strongly reduced the PI 4-phosphate levels of the Golgi complex in cells. Rescue of coxsackievirus replication in the presence of compound 1 by a mutant PI4KIIIβ carrying a substitution in its ATP-binding pocket revealed that the compound directly binds the kinase at this site. Finally, we determined that an analogue of compound 1, 3-(3-fluoro-4-methoxyphenyl)-2-methyl-N-(pyridin-4-ylmethyl)imidazo[1,2-a]pyrazin-8-amine, is well tolerated in mice and has a dose-dependent protective activity in a coxsackievirus serotype B4-induced pancreatitis model.

Preclinical characterization of GLPG0634, a selective inhibitor of JAK1, for the treatment of inflammatory diseases

The JAKs receive continued interest as therapeutic targets for autoimmune, inflammatory, and oncological diseases. JAKs play critical roles in the development and biology of the hematopoietic system, as evidenced by mouse and human genetics. JAK1 is critical for the signal transduction of many type I and type II inflammatory cytokine receptors. In a search for JAK small molecule inhibitors, GLPG0634 was identified as a lead compound belonging to a novel class of JAK inhibitors. It displayed a JAK1/JAK2 inhibitor profile in biochemical assays, but subsequent studies in cellular and whole blood assays revealed a selectivity of ∼30-fold for JAK1- over JAK2-dependent signaling. GLPG0634 dose-dependently inhibited Th1 and Th2 differentiation and to a lesser extent the differentiation of Th17 cells in vitro. GLPG0634 was well exposed in rodents upon oral dosing, and exposure levels correlated with repression of Mx2 expression in leukocytes. Oral dosing of GLPG0634 in a therapeutic set-up in a collagen-induced arthritis model in rodents resulted in a significant dose-dependent reduction of the disease progression. Paw swelling, bone and cartilage degradation, and levels of inflammatory cytokines were reduced by GLPG0634 treatment. Efficacy of GLPG0634 in the collagen-induced arthritis models was comparable to the results obtained with etanercept. In conclusion, the JAK1 selective inhibitor GLPG0634 is a promising novel therapeutic with potential for oral treatment of rheumatoid arthritis and possibly other immune-inflammatory diseases.

Identification of a series of compounds with potent antiviral activity for the treatment of enterovirus infections

Rhinovirus (genus enterovirus) infections are responsible for many of the severe exacerbations of asthma and chronic obstructive pulmonary disease. Other members of the genus can cause life-threatening acute neurological infections. There is currently no antiviral drug approved for the treatment of such infections. We have identified a series of potent, broad-spectrum antiviral compounds that inhibit the replication of the human rhinovirus, Coxsackie virus, poliovirus, and enterovirus-71. The mechanism of action of the compounds has been established as inhibition of a lipid kinase, PI4KIIIβ. Inhibition of hepatitis C replication in a replicon assay correlated with enterovirus inhibition.

A bacterial-two-hybrid selection system for one-step isolation of intracellularly functional Nanobodies

Camel single-domain antibody fragments or Nanobodies, are practical in a wide range of applications. Their unique biochemical and biophysical properties permit an intracellular expression and antigen targeting. The availability of an efficient intracellular selection step would immediately identify the best intracellularly performing functional antibody fragments. Therefore, we assessed a bacterial-two-hybrid system to retrieve such Nanobodies. With GFP as an antigen we demonstrate that antigen-specific Nanobodies of sub-micromolar affinity and stability above 30 kJ/mol, at a titer of 10(-4) can be retrieved in a single-step selection. This was further proven practically by the successful recovery from an 'immune' library of multiple stable, antigen-specific Nanobodies of good affinity for HIV-1 integrase or nucleoside hydrolase. The sequence diversity, intrinsic domain stability, antigen-specificity and affinity of these binders compare favorably to those that were retrieved in parallel by phage display pannings.

Dual beneficial effect of interloop disulfide bond for single domain antibody fragments

The antigen-binding fragment of functional heavy chain antibodies (HCAbs) in camelids comprises a single domain, named the variable domain of heavy chain of HCAbs (VHH). The VHH harbors remarkable amino acid substitutions in the framework region-2 to generate an antigen-binding domain that functions in the absence of a light chain partner. The substitutions provide a more hydrophilic, hence more soluble, character to the VHH but decrease the intrinsic stability of the domain. Here we investigate the functional role of an additional hallmark of dromedary VHHs, i.e. the extra disulfide bond between the first and third antigen-binding loops. After substituting the cysteines forming this interloop cystine by all 20 amino acids, we selected and characterized several VHHs that retain antigen binding capacity. Although VHH domains can function in the absence of an interloop disulfide bond, we demonstrate that its presence constitutes a net advantage. First, the disulfide bond stabilizes the domain and counteracts the destabilization by the framework region-2 hallmark amino acids. Second, the disulfide bond rigidifies the long third antigen-binding loop, leading to a stronger antigen interaction. This dual beneficial effect explains the in vivo antibody maturation process favoring VHH domains with an interloop disulfide bond.

High affinity nanobodies against the Trypanosome brucei VSG are potent trypanolytic agents that block endocytosis

The African trypanosome Trypanosoma brucei, which persists within the bloodstream of the mammalian host, has evolved potent mechanisms for immune evasion. Specifically, antigenic variation of the variant-specific surface glycoprotein (VSG) and a highly active endocytosis and recycling of the surface coat efficiently delay killing mediated by anti-VSG antibodies. Consequently, conventional VSG-specific intact immunoglobulins are non-trypanocidal in the absence of complement. In sharp contrast, monovalent antigen-binding fragments, including 15 kDa nanobodies (Nb) derived from camelid heavy-chain antibodies (HCAbs) recognizing variant-specific VSG epitopes, efficiently lyse trypanosomes both in vitro and in vivo. This Nb-mediated lysis is preceded by very rapid immobilisation of the parasites, massive enlargement of the flagellar pocket and major blockade of endocytosis. This is accompanied by severe metabolic perturbations reflected by reduced intracellular ATP-levels and loss of mitochondrial membrane potential, culminating in cell death. Modification of anti-VSG Nbs through site-directed mutagenesis and by reconstitution into HCAbs, combined with unveiling of trypanolytic activity from intact immunoglobulins by papain proteolysis, demonstrates that the trypanolytic activity of Nbs and Fabs requires low molecular weight, monovalency and high affinity. We propose that the generation of low molecular weight VSG-specific trypanolytic nanobodies that impede endocytosis offers a new opportunity for developing novel trypanosomiasis therapeutics. In addition, these data suggest that the antigen-binding domain of an anti-microbial antibody harbours biological functionality that is latent in the intact immunoglobulin and is revealed only upon release of the antigen-binding fragment.

Haemoparasites, such as African trypanosomes, have developed potent immune evasion mechanisms to avoid antibody-mediated elimination. Consequently, trypanosome surface antigen-specific immunoglobulins in the absence of complement are non-trypanocidal. In contrast, certain monovalent nanobodies (Nb), monomeric antigen-binding domains derived from camelid Heavy-Chain Antibodies (HCAb) and which have a much lower molecular weight (15 kDa) than classical antibodies (150 kDa), efficiently lyse trypanosomes both in vitro and in vivo. This is surprising as classically immunoglobulin effector functions are mediated via the Fc-domain, which is absent from the Nb. We demonstrate that the Nb-mediated trypanolysis depends on the low molecular weight, monovalency and high affinity and is associated with loss of motility, a major block to endocytosis, energy depletion and cell death. Overall, targeting the parasite surface with low molecular weight, high affinity Nbs is sufficient to exert a direct therapeutic action. Therefore, the exploitation of Nbs against African trypanosomiasis represents a novel therapeutic strategy. Furthermore, demonstration that a high affinity antigen-binding Nb or Fab fragment lacking an effector domain (i.e., Fc-domain or an attached toxin) can exert a direct biological function, suggests that intact antibodies likely harbour latent functionality which only become revealed upon removal of the Fc-domain.

Substrate-dependent modulation of enzyme activity by allosteric effector antibodies

We investigate the kinetic effects of antibody variable domain fragments derived from heavy chain antibodies (VHH domains) that behave as allosteric effectors of the nucleoside hydrolase from Trypanosoma vivax (TvNH). Strikingly, these antibodies can stimulate or inhibit TvNH steady-state activity, depending on the substrate used. This effect was investigated in greater detail using steady-state and pre-steady-state kinetic experiments. The most potent allosteric effector, VHH 1589, inhibits certain steps on the TvNH catalytic pathway (e.g. N-glycosidic bond cleavage) but increases the rates of others (e.g. substrate and product release). For the natural nucleoside 7-methyl guanosine, where product ribose release is rate determining, the net effect of VHH 1589 binding is to increase k(cat). For the poor substrate pNPR, VHH 1589 causes chemistry (O-glycosidic bond cleavage) to become rate determining and both k(cat)/K(m) and k(cat) to decrease. Thus, the substrate-dependent effects of VHH 1589 binding are caused by differences in the relative rates of chemistry with respect to subsequent steps on the catalytic pathway for these two substrates. We discuss possible mechanisms for these kinetic effects and the implications for allosteric effector drug development.

Camelid nanobodies raised against an integral membrane enzyme, nitric oxide reductase

Nitric Oxide Reductase (NOR) is an integral membrane protein performing the reduction of NO to N(2)O. NOR is composed of two subunits: the large one (NorB) is a bundle of 12 transmembrane helices (TMH). It contains a b type heme and a binuclear iron site, which is believed to be the catalytic site, comprising a heme b and a non-hemic iron. The small subunit (NorC) harbors a cytochrome c and is attached to the membrane through a unique TMH. With the aim to perform structural and functional studies of NOR, we have immunized dromedaries with NOR and produced several antibody fragments of the heavy chain (VHHs, also known as nanobodies). These fragments have been used to develop a faster NOR purification procedure, to proceed to crystallization assays and to analyze the electron transfer of electron donors. BIAcore experiments have revealed that up to three VHHs can bind concomitantly to NOR with affinities in the nanomolar range. This is the first example of the use of VHHs with an integral membrane protein. Our results indicate that VHHs are able to recognize with high affinity distinct epitopes on this class of proteins, and can be used as versatile and valuable tool for purification, functional study and crystallization of integral membrane proteins.

General strategy to humanize a camelid single-domain antibody and identification of a universal humanized nanobody scaffold

Nanobodies, single-domain antigen-binding fragments of camelid-specific heavy-chain only antibodies offer special advantages in therapy over classic antibody fragments because of their smaller size, robustness, and preference to target unique epitopes. A Nanobody differs from a human heavy chain variable domain in about ten amino acids spread all over its surface, four hallmark Nanobody-specific amino acids in the framework-2 region (positions 42, 49, 50, and 52), and a longer third antigen-binding loop (H3) folding over this area. For therapeutic applications the camelid-specific amino acid sequences in the framework have to be mutated to their human heavy chain variable domain equivalent, i.e. humanized. We performed this humanization exercise with Nanobodies of the subfamily that represents close to 80% of all dromedary-derived Nanobodies and investigated the effects on antigen affinity, solubility, expression yield, and stability. It is demonstrated that the humanization of Nanobody-specific residues outside framework-2 are neutral to the Nanobody properties. Surprisingly, the Glu-49 --> Gly and Arg-50 --> Leu humanization of hallmark amino acids generates a single domain that is more stable though probably less soluble. The other framework-2 substitutions, Phe-42 --> Val and Gly/Ala-52 --> Trp, are detrimental for antigen affinity, due to a repositioning of the H3 loop as shown by their crystal structures. These insights were used to identify a soluble, stable, well expressed universal humanized Nanobody scaffold that allows grafts of antigen-binding loops from other Nanobodies with transfer of the antigen specificity and affinity.

VHH, bivalent domains and chimeric Heavy chain-only antibodies with high neutralizing efficacy for scorpion toxin AahI'

Many efforts aim at solving the serious problems encountered with immunotherapy against scorpion envenoming. The most attractive approach consists in generating single-chain antibody fragments (scFv) as their pharmaco-kinetic properties should match closely those of the scorpion toxins. Although high affinity scFv reagents have been generated in the past, their production level, stability, and toxin neutralizing capacity remain disappointingly poor. In the current study, we identified one Nanobody (Nb), a single-domain antigen-binding fragment of a dromedary Heavy-chain antibody (HCAb) that recognizes specifically the Androctonus australis hector AahI' toxin. This Nb has excellent production, stability and solubility characteristics. With this Nb we further manufactured a tandem linked bivalent construct and assembled a HCAb with improved antigen binding due to avidity effects. All these constructs were shown in mouse models to possess a scorpion toxin neutralization capacity that exceeds by far all previous attempts with scFv-based materials, even when used at lower doses. It is therefore clear that in the near future Nanobodies will be at the core of novel serotherapeutics as they combine multiple benefits over other reagents to treat scorpion envenomed patients.

Targeting and tracing antigens in live cells with fluorescent nanobodies

We fused the epitope-recognizing fragment of heavy-chain antibodies from Camelidae sp. with fluorescent proteins to generate fluorescent, antigen-binding nanobodies (chromobodies) that can be expressed in living cells. We demonstrate that chromobodies can recognize and trace antigens in different subcellular compartments throughout S phase and mitosis. Chromobodies should enable new functional studies, as potentially any antigenic structure can be targeted and traced in living cells in this fashion.

Experimental therapy of African trypanosomiasis with a nanobody-conjugated human trypanolytic factor

High systemic drug toxicity and increasing prevalence of drug resistance hampers efficient treatment of human African trypanosomiasis (HAT). Hence, development of new highly specific trypanocidal drugs is necessary. Normal human serum (NHS) contains apolipoprotein L-I (apoL-I), which lyses African trypanosomes except resistant forms such as Trypanosoma brucei rhodesiense. T. b. rhodesiense expresses the apoL-I-neutralizing serum resistance-associated (SRA) protein, endowing this parasite with the ability to infect humans and cause HAT. A truncated apoL-I (Tr-apoL-I) has been engineered by deleting its SRA-interacting domain, which makes it lytic for T. b. rhodesiense. Here, we conjugated Tr-apoL-I with a single-domain antibody (nanobody) that efficiently targets conserved cryptic epitopes of the variant surface glycoprotein (VSG) of trypanosomes to generate a new manmade type of immunotoxin with potential for trypanosomiasis therapy. Treatment with this engineered conjugate resulted in clear curative and alleviating effects on acute and chronic infections of mice with both NHS-resistant and NHS-sensitive trypanosomes.

Molecular basis for the preferential cleft recognition by dromedary heavy-chain antibodies

Clefts on protein surfaces are avoided by antigen-combining sites of conventional antibodies, in contrast to heavy-chain antibodies (HCAbs) of camelids that seem to be attracted by enzymes' substrate pockets. The explanation for this pronounced preference of HCAbs was investigated. Eight single domain antigen-binding fragments of HCAbs (VHH) with nanomolar affinities for lysozyme were isolated from three immunized dromedaries. Six of eight VHHs compete with small lysozyme inhibitors. This ratio of active site binders is also found within the VHH pool derived from polyclonal HCAbs purified from the serum of the immunized dromedary. The crystal structures of six VHHs in complex with lysozyme and their interaction surfaces were compared to those of conventional antibodies with the same antigen. The interface sizes of VHH and conventional antibodies to lysozyme are very similar as well as the number and chemical nature of the contacts. The main difference comes from the compact prolate shape of VHH that presents a large convex paratope, predominantly formed by the H3 loop and interacting, although with different structures, into the concave lysozyme substrate-binding pocket. Therefore, a single domain antigen-combining site has a clear structural advantage over a conventional dimeric format for targeting clefts on antigenic surfaces.

Antibody repertoire development in camelids

The humoral immune response of the Camelidae is unique as these animals are the only known mammals that seem to possess functional homodimeric heavy-chain antibodies besides the classical heteromeric antibodies composed of heavy (H) and light (L) chains. By definition, the heavy-chain antibodies lack the L-chain, and it was noticed that their H-chain is devoid of the typical first constant domain (CH1) and contains a dedicated variable domain, referred to as VHH. The VHH exon is assembled from separate V-D-J gene segments. The recombined VHH region is subjected to somatic hypermutations; however, the timing and actual mechanism of the class switch from mu to the dedicated gamma-isotype remains elusive. Interestingly, antigen-specific VHHs are easily retrieved after panning of a phage-displayed rearranged V-gene pool cloned from an immunised camelid. These single-domain antigen binding entities possess a number of biophysical properties that offer particular advantages in various medical and biotechnological applications.

Neutralisation of venom-induced haemorrhage by IgG from camels and llamas immunised with viper venom and also by endogenous, non-IgG components in camelid sera

Envenoming by snakes results in severe systemic and local pathology. Intravenous administration of antivenom, prepared from IgG of venom immunised horses or sheep, is the only effective treatment of systemic envenoming. Conventional antivenoms, formulated as intact IgG, papain-cleaved (Fab) or pepsin-cleaved F(ab')2 fragments, are however ineffective against the local venom effects because of their inability to penetrate the blood/tissue barrier. We have embarked on a new research program to examine (i) whether the unusually small (15 kDa) antigen-binding fragment of camelid heavy chain IgG (V(H)H) can be exploited to neutralise the local effects of envenoming and (ii) whether a novel antivenom to treat both the systemic and local effects of envenoming can be formulated by combining anti-snake venom V(H)H and conventional F(ab')2. In this preliminary study, we demonstrate that camels and llamas respond to immunisation with Echis ocellatus venom with high antibody titres and broad antigen specificity. These encouraging immunological results were matched by the successful elimination of venom-induced haemorrhage by IgG from the venom-immunised camels and llamas. Unexpectedly, we report for the first time that camelid serum contains a non-IgG, highly potent inhibitor of venom-induced haemorrhage.

Engineering Camel Single-Domain Antibodies and Immobilization Chemistry for Human Prostate-Specific Antigen Sensing

The specificity and affinity characteristics of antibodies make them excellent probes in biosensor applications. Unfortunately, their large size, unstable behavior, and random immobilization properties create numerous problems. The single-domain antigen-binding fragment derived from heavy-chain antibodies of camelids (termed VHH) offers special advantages in terms of size, stability, and ease of generating different antibody constructs. In this study, we show the potential of those VHHs in sensing human prostate-specific antigen (hPSA) by SPR technology. Different VHH constructs were immobilized onto commercial and custom-built sensor surfaces by metal chelation, biotin-streptavidin interaction, or covalent coupling. The detection of subnanogram per milliliter hPSA concentrations could be attained on a covalently coupled three-dimensional dextran surface. Moreover, the ratio of different hPSA isoform concentrations could be assessed via a sandwich assay and resulted in the detection of clinically significant antigen concentrations within 15 min. In addition, for the first time, the intrinsic protein stability is presented as an important probe design factor, since our results reveal that higher intrinsic stability offers higher resistance to harsh regeneration conditions. In conclusion, we present VHHs as a novel class of biosensor probes rivaling conventional antibodies and their derived antibody fragments.

Identification of a Universal VHH Framework to Graft Non-canonical Antigen-binding Loops of Camel Single-domain Antibodies

Camel single-domain antibody fragments (VHHs) are promising tools in numerous biotechnological and medical applications. However, some conditions under which antibodies are used are so demanding that they can be met by only the most robust VHHs. A universal framework offering the required properties for use in various applications (e.g. as intrabody, as probe in biosensors or on micro-arrays) is highly valuable and might be further implemented when employment of VHHs in human therapy is envisaged. We identified the VHH framework of cAbBCII10 as a potential candidate, useful for the exchange of antigen specificities by complementarity determining region (CDR) grafting. Due to the large number of CDR-H loop structures present on VHHs, this grafting technique was expected to be rather unpredictable. Nonetheless, the plasticity of the cAbBCII10 framework allows successful transfer of antigen specificity from donor VHHs onto its scaffold. The cAbBCII10 was chosen essentially for its high level of stability (47 kJmol(-1)), good expression level (5 mgl(-1) in E.coli) and its ability to be functional in the absence of the conserved disulfide bond. All five chimeras generated by grafting CDR-Hs, from donor VHHs belonging to subfamily 2 that encompass 75% of all antigen-specific VHHs, on the framework of cAbBCII10 were functional and generally had an increased thermodynamic stability. The grafting of CDR-H loops from VHHs belonging to other subfamilies resulted in chimeras of reduced antigen-binding capacity.

Antigen Binding and Solubility Effects upon the Veneering of a Camel VHH in Framework-2 to Mimic a VH

Heavy chain only antibodies of camelids bind their antigens with a single domain, the VHH, which acquired adaptations relative to classical VHs to function in the absence of a VL partner. Additional CDR loop conformations, outside the canonical loop structures of VHs, broaden the repertoire of the antigen-binding site. The combined effects of part of the CDR3 that folds over the "former" VL binding site and framework-2 mutations to more hydrophilic amino acids, enhance the solubility of VHH domains and prevent VL pairing. cAbAn33, a VHH domain specific for the carbohydrate moiety of the variant surface glycoprotein of trypanosomes, has a short CDR3 loop that does not cover the former VL binding site as well as a VH-specific Trp47 instead of the VHH-specific Gly47. Resurfacing its framework-2 region (mutations Tyr37Val, Glu44Gly and Arg45Leu) to mimic that of a human VH restores the VL binding capacity. In solution, the humanised VHH behaves as a soluble, monomeric entity, albeit with reduced thermodynamic stability and affinity for its antigen. Comparison of the crystal structures of cAbAn33 and its humanised derivative reveals steric hindrance exerted by VHH-specific residues Tyr37 and Arg45 that prevent the VL domain pairing, whereas Glu44 and Arg45 are key elements to avoid insolubility of the domain.

Single Domain Antibodies Derived from Dromedary Lymph Node and Peripheral Blood Lymphocytes Sensing Conformational Variants of Prostate-specific Antigen

The importance of the lymphocyte source to generate hybridomas or to construct antibody gene libraries from which to identify potent monoclonal antibodies is understudied. However, the few comparative studies that exist seem to favor the lymph node tissue as a B-cell source. Here the peripheral blood and lymph node lymphocytes of a dromedary immunized with prostate-specific antigen (PSA) have been employed to clone two independent gene banks of the variable domains of heavy-chain antibodies (i.e. the VHHs). Several PSA-specific VHHs were retrieved after panning of these phage-displayed VHH libraries. Some of them were derived from the same B-cell lineage, possibly reflecting the restricted primary repertoire of heavy-chain antibodies. Other binders originated from different B-cell lineages and apparently converged toward a striking homologous amino acid sequence motif in their CDR3. This illustrates the strong somatic hypermutation and stringent antigen-driven selection ongoing in these animals. Although the various antigen binders exhibit a broad range of kinetic rate constants for their interaction with the PSA, leading to equilibrium constants from 70 pM to 100 nM, no significant difference existed between the binders from the two B-cell sources. The VHHs of both libraries were categorized in three groups based on nonoverlapping epitopes. Some of these VHHs could inhibit and others could enhance the proteolytic activity of the antigen. Remarkably, VHHs seem to sense or induce conformational changes on different PSA isoforms, a feature that might be exploited to study the PSA conformational flexibility and to discriminate the stages of prostate cancer.

Efficient targeting of conserved cryptic epitopes of infectious agents by single domain antibodies. African trypanosomes as paradigm

Antigen variation is a successful defense system adopted by several infectious agents to evade the host immune response. The principle of this defense strategy in the African trypanosome paradigm involves a dense packing of variant surface glycoproteins (VSG) exposing only highly variable and immuno-dominant epitopes to the immune system, whereas conserved epitopes become inaccessible for large molecules. Reducing the size of binders that target the conserved, less-immunogenic, cryptic VSG epitopes forms an obvious solution to combat these parasites. This goal was achieved by introducing dromedary Heavy-chain antibodies. We found that only these unique antibodies recognize epitopes common to multiple VSG classes. After phage display of their antigen-binding repertoire, we isolated a single domain antibody fragment with high specificity for the conserved Asn-linked carbohydrate of VSG. In sharp contrast to labeled concanavalin-A that stains only the flagellar pocket where carbohydrates are accessible because of less dense VSG packing, the single domain binder stains the entire surface of viable parasites, irrespective of the VSG type expressed. This corroborates the idea that small antibody fragments, but not larger lectins or conventional antibody fragments, are able to penetrate the dense VSG coat to target their epitope. The diagnostic potential of this fluorescently labeled binder was proven by the direct, selective, and sensitive detection of parasites in blood smears. The employment of this binder as a molecular recognition unit in immuno-toxins designed for trypanosomosis therapy becomes feasible as well. This was illustrated by the specific trypanolysis induced by an antibody::beta-lactamase fusion activating a prodrug.

Efficient tumor targeting by single-domain antibody fragments of camels

The variable domain of functional heavy chain antibodies (VHH) devoid of light chains, found in camels, constitute the smallest intact antigen-binding domain fragment. Two camel single-domain fragments, cAb-Lys2 and cAb-Lys3, recognizing an overlapping epitope of lysozyme with a dissociation constant of 2 nM and 65 nM, respectively, and a bivalent cAb-Lys3 were investigated for their ability to target transgenic tumors expressing lysozyme on their membrane. Biodistribution studies revealed that these non-immunogenic monomeric and bivalent camel single-domain antigen binders specifically target lysozyme-expressing tumors and metastatic lesions. The excess of antibody is rapidly eliminated from the blood circulation and no cAb retention was observed in normal organs. The tumor to organ cAb-ratios at 2 and 8 hr were in the (2.1-10.8):1 and (6.2-23.7):1 range, respectively. The degree and specificity of tumor retention is independent of the affinity of the recombinant camel single-domain fragments for their antigen and from their univalent monomeric (15 kDa) or bivalent format (33 kDa). This study demonstrates the successful and specific in vivo targeting of tumors by camel single-domain fragments. It may open perspectives for their future use as tumor-targeting vehicle, due to their small size, soluble behaviour and because they are non-immunogenic and interact with epitopes that are less antigenic for conventional antibodies.

Biophysical properties of camelid V(HH) domains compared to those of human V(H)3 domains

Camelidae possess an unusual form of antibodies lacking the light chains. The variable domain of these heavy chain antibodies (V(HH)) is not paired, while the V(H) domain of all other antibodies forms a heterodimer with the variable domain of the light chain (V(L)), held together by a hydrophobic interface. Here, we analyzed the biophysical properties of four camelid V(HH) fragments (H14, AMD9, RN05, and CA05) and two human consensus V(H)3 domains with different CDR3 loops to gain insight into factors determining stability and aggregation of immunoglobulin domains. We show by denaturant-induced unfolding equilibria that the free energies of unfolding of V(HH) fragments are characterized by Delta G(N-U) values between 21.1 and 35.0 kJ/mol and thus lie in the upper range of values for V(H) fragments from murine and human antibodies. Nevertheless, the V(HH) fragments studied here did not reach the high values between 39.7 and 52.7 kJ/mol of the human consensus V(H)3 domains with which they share the highest degree of sequence similarity. Temperature-induced unfolding of the V(HH) fragments that were studied proved to be reversible, and the binding affinity after cooling was fully retained. The melting temperatures were determined to be between 60.1 and 66.7 degrees C. In contrast, the studied V(H)3 domains aggregated during temperature-induced denaturation at 63-65 degrees C. In summary, the camelid V(HH) fragments are characterized by a favorable but not unusually high stability. Their hallmark is the ability to reversibly melt without aggregation, probably mediated by the surface mutations characterizing the V(HH) domains, which allow them to regain binding activity after heat renaturation.

Single-domain antibody fragments with high conformational stability

A variety of techniques, including high-pressure unfolding monitored by Fourier transform infrared spectroscopy, fluorescence, circular dichroism, and surface plasmon resonance spectroscopy, have been used to investigate the equilibrium folding properties of six single-domain antigen binders derived from camelid heavy-chain antibodies with specificities for lysozymes, beta-lactamases, and a dye (RR6). Various denaturing conditions (guanidinium chloride, urea, temperature, and pressure) provided complementary and independent methods for characterizing the stability and unfolding properties of the antibody fragments. With all binders, complete recovery of the biological activity after renaturation demonstrates that chemical-induced unfolding is fully reversible. Furthermore, denaturation experiments followed by optical spectroscopic methods and affinity measurements indicate that the antibody fragments are unfolded cooperatively in a single transition. Thus, unfolding/refolding equilibrium proceeds via a simple two-state mechanism (N <--> U), where only the native and the denatured states are significantly populated. Thermally-induced denaturation, however, is not completely reversible, and the partial loss of binding capacity might be due, at least in part, to incorrect refolding of the long loops (CDRs), which are responsible for antigen recognition. Most interestingly, all the fragments are rather resistant to heat-induced denaturation (apparent T(m) = 60-80 degrees C), and display high conformational stabilities (DeltaG(H(2)O) = 30-60 kJ mole(-1)). Such high thermodynamic stability has never been reported for any functional conventional antibody fragment, even when engineered antigen binders are considered. Hence, the reduced size, improved solubility, and higher stability of the camelid heavy-chain antibody fragments are of special interest for biotechnological and medical applications.

Phosphorylation of bovine leukemia virus Tax protein is required for in vitro transformation but not for transactivation

The Tax proteins of the oncovirinae viruses are phosphorylated transcriptional activators that exhibit oncogenic potential. The role of phosphorylation in their functional activities remains unknown. As a model for the Human T-cell leukemia virus type I (HTLV-I), Bovine Leukemia Virus (BLV) permits the characterization of viral replication and leukemogenesis in vivo. Here, we show that the BLV Tax protein is phosphorylated on serine residues 106 and 293 both in insect and in mammalian cells. These sites can also be efficiently phosphorylated by the cdc2 and MAP kinases in vitro. Mutation of these residues does not affect the capacity of the Tax protein to function as a transactivator. Indeed, the Tax proteins mutated at one or both serines increase LTR-directed viral transcription at levels similar to those obtained with wild-type Tax in cell culture. Moreover, inhibition of Tax phosphorylation by W7, a calmodulin antagonist, does not alter its transactivation activity. Thus, phosphorylation on serines 106 and 293 is not required for transactivation by Tax. However, simultaneous substitution of both serines into alanine residues destroys the capacity of Tax to cooperate with the Ha-ras oncogene to transform primary rat embryo fibroblasts and induce tumors in nude mice. When the serines were replaced with aspartic acid residues, the oncogenic potential of Tax was maintained indicating that the negative charge rather than the phosphate group itself was required for Tax oncogenicity. Finally, to assess the role of the serine residues in vivo, recombinant viruses which express the Tax mutants were constructed and injected into sheep. It appeared that the mutated proviruses replicate at levels similar to the wild-type virus in vivo. We conclude that Tax phosphorylation is dispensable for transactivation and viral replication in vivo but is required for its oncogenic potential in vitro.

The topology of the S protein in the yeast-derived hepatitis B surface antigen particles

Hepatitis B surface antigen particles are highly immunogenic and have been shown to provide a suitable support for the presentation of foreign epitopes. More information about the topology of their constitutive protein, the S (small envelope) protein, is a prerequisite to any rational attempt to replace region of this protein with foreign epitopes without modifying the assembly of the particle. The topology of the S protein within the lipid membrane was investigated here by combining extensive proteolysis of the external protein domains with proteinase K and (FTIR-ATR). The proteolytic hydrolysis of the S protein and the identification of the digestion products allowed characterization of the membrane-protected regions of the protein. FTIR spectra of the digested hepatitis B particles revealed that the peptides associated with the particles are rich in alpha-helix structure. The kinetic of 2H/H exchange provided evidence that a large fraction of the native S protein is poorly accessible to the aqueous medium.