Lobe-Specific Gene Vector Delivery to Rat Lungs Using a Miniature Bronchoscope

An immunocompetent rat model of Mycobacterium abscessus multinodular granulomatous lung infection

Animal models that can mimic progressive granulomatous pulmonary disease (PD) due to non-tuberculous mycobacteria (NTM) have not been established in rats to date. These models could assist with the study of the pathophysiology of NTM-PD as well as the preclinical development of new therapies. In the present study, an immunocompetent rat model of progressive Mycobacterium abscessus (MABs)- PD was developed using MABs originating from a patient with cystic fibrosis. MABs was embedded in agarose beads and delivered intratracheally to the lungs of Sprague Dawley rats two times at a one-week time interval. The bacterial burden of lysed lungs, spleen and liver was assessed by calculating colony forming units (CFUs) on day 28. Lung CFUs indicated a ∼1.2-2 log10 total CFU increase compared to the initial total bacterial load instilled into the lungs. In all infected rats, multinodular granulomatous inflammatory lesions containing MABs were found in the lung. These findings support the establishment of an immunocompetent MABs PD rat model, characterised by an increase in mycobacterial burden over time and a chronic granulomatous inflammatory response to the MABs infection.

Assessment of respiratory mechanics and X-ray velocimetry functional imaging in two cystic fibrosis rat models

Two cystic fibrosis (CF) rat models, one carrying the common Phe508del mutation and the other a nonsense cystic fibrosis transmembrane conductance regulator (CFTR) mutation (knockout) were previously characterised. Although relevant CFTR mRNA reductions were present in the lung, no overt CF lung disease was observed. This study used flexiVent lung mechanic assessment and regional ventilation assessment via X-ray velocimetry (XV) functional imaging to assess the lung phenotype in both models. To determine the sensitivity of XV regional ventilation imaging, the effect of a localised physical obstruction (delivery of agar beads to part of the lungs) on lung ventilation was examined. At baseline, Phe508del and knockout CF rats had a lower inspiratory capacity, total respiratory system compliance, and static compliance than wildtype rats. Following agar bead delivery all XV ventilation parameters were altered, with substantial increases in poorly ventilated regions and ventilation heterogeneity. XV ventilation maps accurately identified locations of bead-induced airflow changes. Despite unremarkable lung histopathology, this study indicated that CF rats display altered respiratory mechanics, with CF rats needing to exert additional effort to expand and deflate their lungs due to increased stiffness. This study demonstrated the utility of XV imaging providing spatial lung ventilation information.

Inhalation delivery of nucleic acid gene therapies in preclinical drug development

INTRODUCTION

Inhaled gene therapy programs targeting diseases of the lung have seen increasing interest in recent years, though as of yet no product has successfully entered the market. Preclinical research to support such programs is critically important in maximizing the chances of developing successful candidates.

AREAS COVERED

Aspects of inhalation delivery of gene therapies are reviewed, with a focus on preclinical research in animal models. Various barriers to inhalation delivery of gene therapies are discussed, including aerosolization stresses, aerosol behavior in the respiratory tract, and disposition processes post-deposition. Important aspects of animal models are considered, including determinations of biologically relevant determinations of dose and issues related to translatability.

EXPERT OPINION

Development of clinically-efficacious inhaled gene therapies has proven difficult owing to numerous challenges. Fit-for-purpose experimental and analytical methods are necessary for determinations of biologically relevant doses in preclinical animal models. Further developments in disease-specific animal models may aid in improving the translatability of results in future work, and we expect to see accelerated interests in inhalation gene therapies for various diseases. Sponsors, researchers, and regulators are encouraged to engage in early and frequent discussion regarding candidate therapies, and additional dissemination of preclinical methodologies would be of immense value in avoiding common pitfalls.

To bead or not to bead: A review of Pseudomonas aeruginosa lung infection models for cystic fibrosis

Cystic fibrosis (CF) lung disease is characterised by recurring bacterial infections resulting in inflammation, lung damage and ultimately respiratory failure. Pseudomonas aeruginosa is considered one of the most important lung pathogens in those with cystic fibrosis. While multiple cystic fibrosis animal models have been developed, many fail to mirror the cystic fibrosis lung disease of humans, including the colonisation by opportunistic environmental pathogens. Delivering bacteria to the lungs of animals in different forms is a way to model cystic fibrosis bacterial lung infections and disease. This review presents an overview of previous models, and factors to consider when generating a new P. aeruginosa lung infection model. The future development and application of lung infection models that more accurately reflect human cystic fibrosis lung disease has the potential to assist in understanding the pathophysiology of cystic fibrosis lung disease and for developing treatments.

Comparison of physical perturbation devices for enhancing lentiviral vector-mediated gene transfer to the airway epithelium

Natural airway defences currently impede the efficacy of viral vector-mediated airway gene therapy. Conditioning airways prior to vector delivery can disrupt these barriers, improving viral vector access to target receptors and airway stem cells. This study aimed to assess and quantify the in vivo histological and gene transfer effects of physical perturbation devices to identify effective conditioning approaches. A range of flexible wire baskets with varying configurations, a Brush, biopsy forceps, and a balloon catheter were examined. We first evaluated the histological effects of physical perturbation devices in rat tracheas that were excised 10 minutes after conditioning. Based on the histological findings, a selection of devices were used to condition rat tracheas in vivo before delivering a lentiviral vector containing the LacZ reporter gene. After 7 days, excised tracheas were X-gal processed and examined en face to quantify the area of LacZ staining. Histological observations 10 minutes after conditioning found that physical perturbation dislodged cells from the basement membrane to varying degrees, with some producing significant levels of epithelial cell removal. When a subset of devices were assessed for their ability to enhance gene transfer, only the NGage® wire basket (Cook Medical) produced a significant increase in the proportion of X-gal-stained area when compared to unconditioned tracheas (8-fold, p = 0.00025). These results suggest that a range of factors contribute to perturbation-enhanced gene transfer. Overall, this study supports existing evidence that physical perturbation can assist airway gene transfer, and will help to identify the characteristics of an effective device for airway gene therapy.

Improved in-vivo airway gene transfer via magnetic-guidance, with protocol development informed by synchrotron imaging

Gene vectors to treat cystic fibrosis lung disease should be targeted to the conducting airways, as peripheral lung transduction does not offer therapeutic benefit. Viral transduction efficiency is directly related to the vector residence time. However, delivered fluids such as gene vectors naturally spread to the alveoli during inspiration, and therapeutic particles of any form are rapidly cleared via mucociliary transit. Extending gene vector residence time within the conducting airways is important, but hard to achieve. Gene vector conjugated magnetic particles that can be guided to the conducting airway surfaces could improve regional targeting. Due to the challenges of in-vivo visualisation, the behaviour of such small magnetic particles on the airway surface in the presence of an applied magnetic field is poorly understood. The aim of this study was to use synchrotron imaging to visualise the in-vivo motion of a range of magnetic particles in the trachea of anaesthetised rats to examine the dynamics and patterns of individual and bulk particle behaviour in-vivo. We also then assessed whether lentiviral-magnetic particle delivery in the presence of a magnetic field increases transduction efficiency in the rat trachea. Synchrotron X-ray imaging revealed the behaviour of magnetic particles in stationary and moving magnetic fields, both in-vitro and in-vivo. Particles could not easily be dragged along the live airway surface with the magnet, but during delivery deposition was focussed within the field of view where the magnetic field was the strongest. Transduction efficiency was also improved six-fold when the lentiviral-magnetic particles were delivered in the presence of a magnetic field. Together these results show that lentiviral-magnetic particles and magnetic fields may be a valuable approach for improving gene vector targeting and increasing transduction levels in the conducting airways in-vivo.

Effective viral-mediated lung gene therapy: is airway surface preparation necessary?

Gene-based therapeutics are actively being pursued for the treatment of lung diseases. While promising advances have been made over the last decades, the absence of clinically available lung-directed genetic therapies highlights the difficulties associated with this effort. Largely, progress has been hindered by the presence of inherent physical and physiological airway barriers that significantly reduce the efficacy of gene transfer. These barriers include surface mucus, mucociliary action, cell-to-cell tight junctions, and the basolateral cell membrane location of viral receptors for many commonly used gene vectors. Accordingly, airway surface preparation methods have been developed to disrupt these barriers, creating a more conducive environment for gene uptake into the target airway cells. The two major approaches have been chemical and physical methods. Both have proven effective for increasing viral-mediated gene transfer pre-clinically, although with variable effect depending on the specific strategy employed. While such methods have been explored extensively in experimental settings, they have not been used clinically. This review covers the airway surface preparation strategies reported in the literature, the advantages and disadvantages of each method, as well as a discussion about applying this concept in the clinic.

In vitro optimization of miniature bronchoscope lentiviral vector delivery for the small animal lung

Current gene therapy delivery protocols for small animal lungs typically utilize indirect dose delivery via the nasal airways, or bolus delivery directly into the trachea. Both methods can result in variable transduction throughout the lung, as well as between animals, and cannot be applied in a targeted manner. To minimize variability and improve lung coverage we previously developed and validated a method to visualize and dose gene vectors into pre-selected lobes of rat lungs using a mini-bronchoscope. Lentiviral (LV) vectors are known to be fragile and can be inactivated easily by temperature or the application of shear stresses. There are several ways that the bronchoscope could be configured to deliver the LV vector, and these could result in different amounts of functional LV vector being delivered to the lung. This study evaluated several methods of LV vector delivery through the bronchoscope, and how flow rates and LV vector stabilizing diluents impact LV vector delivery. NIH-3T3 cells were exposed to LV vector containing the green fluorescent protein (GFP) reporter gene using various bronchoscopic delivery techniques and the number of GFP-positive cells produced by each was quantified by flow cytometry. The results showed that directly drawing the LV vector into the bronchoscope tip resulted in 80-90% recovery of viable vector, and was also the simplest method of delivery. The fluid delivery rate and the use of stabilizing serum in the vector diluent had no effect on the viability of the LV vector delivered. These findings can be used to optimize LV vector dose delivery into individual lung lobes of small animal models.

Assessment of Lentiviral Vector Mediated CFTR Correction in Mice Using an Improved Rapid in vivo Nasal Potential Difference Measurement Protocol

Cystic Fibrosis (CF) is caused by a defect in the CF transmembrane conductance regulator (CFTR) gene responsible for epithelial ion transport. Nasal potential difference (PD) measurement is a well established diagnostic technique for assessing the efficacy of therapies in CF patients and animal models. The aim was to establish a rapid nasal PD protocol in mice and quantify the efficacy of lentiviral (LV) vector-based CFTR gene therapy. Anaesthetised wild-type (WT) and CF mice were non-surgically intubated and nasal PD measurements were made using a range of buffer flow rates. Addition of the cAMP agonist, isoproterenol, to the buffer sequence was then examined. The optimised rapid PD technique was then used to assess CFTR function produced by second and third generation LV-CFTR vectors. V5 epitope tagged-CFTR in nasal tissue was identified by immunohistochemistry. When intubated, mice tolerated higher flow rates. Isoproterenol could discriminate between WT and CF mice. Improved chloride transport was observed for the second and third generation LV-CFTR vectors, with up to 60% correction of the cAMP-driven chloride response towards WT. V5-CFTR was located in ciliated epithelial cells. The rapid PD technique enables improved functional assessment of the bioelectrical ion transport defect for both current and potential CF therapies.

The Effects of Conditioning and Lentiviral Vector Pseudotype on Short- and Long-Term Airway Reporter Gene Expression in Mice

A gene addition therapy into the conducting airway epithelium is a potential cure for cystic fibrosis lung disease. Achieving sustained lung gene expression has proven difficult due to the natural barriers of the lung. The development of lentiviral (LV) vectors pseudotyped with viral envelopes that have a natural tropism to the airway has enabled persistent gene expression to be achieved in vivo. The aims of this study were to compare the yields of hemagglutinin (HA) and vesicular stomatitis virus-glycoprotein (VSV-G) pseudotyped HIV-1 vectors produced under the same conditions by our standard LV vector production method. We then sought to measure gene expression in mouse airways and to determine whether lysophosphatidylcholine (LPC) conditioning enhances short- and long-term gene expression. C57Bl/6 mouse airways were conditioned with 10 μL of 0.1% LPC or saline control, followed 1 h later by a 30 μL dose of an HA or VSV-G pseudotyped vector carrying either the LacZ or luciferase reporter genes. LacZ expression was assessed by X-gal staining after 7 days, while lung luminescence was quantified regularly for up to 18 months by bioluminescent imaging. The HA pseudotyped vectors had functional titers 25 to 60 times lower than the VSV-G pseudotyped vectors. Conditioning the lung with LPC significantly increased the total number of LacZ-transduced cells for both pseudotypes compared to saline control. Regardless of LPC conditioning, the VSV-G pseudotype produced higher initial levels of gene expression compared to HA. LPC conditioning did not increase the number of transduced basal cells for either pseudotype compared to saline, and was not required for long-term gene expression. Both pseudotyped vectors effectively transduced the upper conducting airways of wild-type mice. The use of LPC conditioning before vector delivery was not required in mouse lungs to produce long-term gene expression, but did improve short-term gene expression.

Towards Human Translation of Lentiviral Airway Gene Delivery for Cystic Fibrosis: A One-Month CFTR and Reporter Gene Study in Marmosets

Gene therapy continues to be a promising contender for the treatment of cystic fibrosis (CF) airway disease. We have previously demonstrated that airway conditioning with lysophosphatidylcholine (LPC) followed by delivery of a HIV-1-based lentiviral (LV) vector functionally corrects the CF transmembrane conductance regulator (CFTR) defect in the nasal airways of CF mice. In our earlier pilot study we showed that our technique can transduce marmoset lungs acutely; this study extends that work to examine gene expression in this nonhuman primate (NHP) 1 month after gene vector treatment. A mixture of three separate HIV-1 vesicular stomatitis virus G (VSV-G)-pseudotyped LV vectors containing the luciferase (Luc), LacZ, and hCFTR transgenes was delivered into the trachea through a miniature bronchoscope. We examined whether a single-dose delivery of LV vector after LPC conditioning could increase levels of transgene expression in the trachea and lungs compared with control (phosphate-buffered saline [PBS]) conditioning. At 1 month, bioluminescence was detected in vivo in the trachea of three of the six animals within the PBS control group, compared with five of the six LPC-treated animals. When examined ex vivo there was weak evidence that LPC improves tracheal Luc expression levels. In the lungs, bioluminescence was detected in vivo in four of the six PBS-treated animals, compared with five of the six LPC-treated animals; however, bioluminescence was present in all lungs when imaged ex vivo. LacZ expression was predominantly observed in the alveolar regions of the lung. hCFTR was detected by qPCR in the lungs of five animals. Basal cells were successfully isolated and expanded from marmoset tracheas, but no LacZ-positive colonies were detected. There was no evidence of an inflammatory response toward the LV vector at 1 month postdelivery, with cytokines remaining at baseline levels. In conclusion, we found weak evidence that LPC conditioning improved gene transduction in the trachea, but not in the marmoset lungs. We also highlight some of the challenges associated with translational lung gene therapy studies in NHPs.

Breaching the Delivery Barrier: Chemical and Physical Airway Epithelium Disruption Strategies for Enhancing Lentiviral-Mediated Gene Therapy

The lungs have evolved complex physical, biological and immunological defences to prevent foreign material from entering the airway epithelial cells. These mechanisms can also affect both viral and non-viral gene transfer agents, and significantly diminish the effectiveness of airway gene-addition therapies. One strategy to overcome the physical barrier properties of the airway is to transiently disturb the integrity of the epithelium prior to delivery of the gene transfer vector. In this study, chemical (lysophosphatidylcholine, LPC) and physical epithelium disruption using wire abrasion were compared for their ability to improve airway-based lentiviral (LV) vector mediated transduction and reporter gene expression in rats. When luciferase expression was assessed at 1-week post LV delivery, LPC airway conditioning significantly enhanced gene expression levels in rat lungs, while a long-term assessment in a separate cohort of rats at 12 months revealed that LPC conditioning did not improve gene expression longevity. In rats receiving physical perturbation to the trachea prior to gene delivery, significantly higher LacZ gene expression levels were found when compared to LPC-conditioned or LV-only control rats when evaluated 1-week post gene transfer. This proof-of-principle study has shown that airway epithelial disruption strategies based on physical perturbation substantially enhanced LV-mediated airway gene transfer in the trachea.

Towards automated in vivo tracheal mucociliary transport measurement: Detecting and tracking particle movement in synchrotron phase-contrast x-ray images

Accurate in vivo quantification of airway mucociliary transport (MCT) in animal models is important for understanding diseases such as cystic fibrosis, as well as for developing therapies. A non-invasive method of measuring MCT behaviour, based on tracking the position of micron sized particles using synchrotron x-ray imaging, has previously been described. In previous studies, the location (and path) of each particle was tracked manually, which is a time consuming and subjective process. Here we describe particle tracking methods that were developed to reduce the need for manual particle tracking. The MCT marker particles were detected in the synchrotron x-ray images using cascade classifiers. The particle trajectories along the airway surface were generated by linking the detected locations between frames using a modified particle linking algorithm. The developed methods were compared with the manual tracking method on simulated x-ray images, as well as on in vivo images of rat airways acquired at the SPring-8 Synchrotron. The results for the simulated and in vivo images showed that the semi-automatic algorithm reduced the time required for particle tracking when compared with the manual tracking method, and was able to detect MCT marker particle locations and measure particle speeds more accurately than the manual tracking method. Future work will examine the modification of methods to improve particle detection and particle linking algorithms to allow for more accurate fully-automatic particle tracking.

Intratracheal aerosolization of viral vectors to newborn pig airways

Gene therapy for airway diseases requires efficient delivery of nucleic acids to the airways. In small animal models, gene delivery reagents are commonly delivered as a bolus dose. However, large animal models are often more relevant for the transition from preclinical studies to human trials. Aerosolizing viral vectors to the lungs of large animals can maximize anatomical distribution. Here, we describe a technique for aerosolization of viral vectors to the airways of newborn pigs. Briefly, a pig is anesthetized and intubated with an endotracheal tube, and a microsprayer is passed through the endotracheal tube. A fine mist is then sprayed into the distal trachea. Widespread and uniform distribution of transgene expression is critical for developing successful lung gene therapy treatments.

Historically, achieving uniform distribution of a gene therapy reagent in the lungs has been challenging. Here, we describe an aerosolizing technique which can be used to achieve homogenous expression of a viral vector in newborn pig lungs. Briefly, pigs are sedated and intubated with an endotracheal tube, and a microsprayer is used to aerosolize a viral vector which results in its widespread distribution in the lungs.

Methods for dynamic synchrotron X-ray respiratory imaging in live animals

Small-animal physiology studies are typically complicated, but the level of complexity is greatly increased when performing live-animal X-ray imaging studies at synchrotron and compact light sources. This group has extensive experience in these types of studies at the SPring-8 and Australian synchrotrons, as well as the Munich Compact Light Source. These experimental settings produce unique challenges. Experiments are always performed in an isolated radiation enclosure not specifically designed for live-animal imaging. This requires equipment adapted to physiological monitoring and test-substance delivery, as well as shuttering to reduce the radiation dose. Experiment designs must also take into account the fixed location, size and orientation of the X-ray beam. This article describes the techniques developed to overcome the challenges involved in respiratory X-ray imaging of live animals at synchrotrons, now enabling increasingly sophisticated imaging protocols.

Gene Therapy for Cystic Fibrosis Lung Disease: Overcoming the Barriers to Translation to the Clinic

Cystic fibrosis (CF) is a progressive, chronic and debilitating genetic disease caused by mutations in the CF Transmembrane-Conductance Regulator (CFTR) gene. Unrelenting airway disease begins in infancy and produces a steady deterioration in quality of life, ultimately leading to premature death. While life expectancy has improved, current treatments for CF are neither preventive nor curative. Since the discovery of CFTR the vision of correcting the underlying genetic defect - not just treating the symptoms - has been developed to where it is poised to become a transformative technology. Addition of a properly functioning CFTR gene into defective airway cells is the only biologically rational way to prevent or treat CF airway disease for all CFTR mutation classes. While new gene editing approaches hold exciting promise, airway gene-addition therapy remains the most encouraging therapeutic approach for CF. However, early work has not yet progressed to large-scale clinical trials. For clinical trials to begin in earnest the field must demonstrate that gene therapies are safe in CF lungs; can provide clear health benefits and alter the course of lung disease; can be repeatedly dosed to boost effect; and can be scaled effectively from small animal models into human-sized lungs. Demonstrating the durability of these effects demands relevant CF animal models and accurate and reliable techniques to measure benefit. In this review, illustrated with data from our own studies, we outline recent technological developments and discuss these key questions that we believe must be answered to progress CF airway gene-addition therapies to clinical trials.