Multicompartment, numerical model of cellular events in the pharmacokinetics of gene therapies

Whole-Body Disposition and Physiologically Based Pharmacokinetic Modeling of Adeno-Associated Viruses and the Transgene Product

To facilitate model-informed drug development (MIDD) of adeno-associated virus (AAV) therapy, here we have developed a physiologically based pharmacokinetic (PBPK) model for AAVs following preclinical investigation in mice. After 2E11 Vg/mouse dose of AAV8 and AAV9 encoding a monoclonal antibody (mAb) gene, whole-body disposition of both the vector and the transgene mAb was evaluated over 3 weeks. At steady-state, the following tissue-to-blood (T/B) concentration ratios were found for AAV8/9: ∼50 for liver; ∼10 for heart and muscle; ∼2 for brain, lung, kidney, adipose, and spleen; ≤1 for bone, skin, and pancreas. T/B values for mAb were compared with the antibody biodistribution coefficients, and five different clusters of organs were identified based on their transgene expression profile. All the biodistribution data were used to develop a novel AAV PBPK model that incorporates: (i) whole-body distribution of the vector; (ii) binding, internalization, and intracellular processing of the vector; (iii) transgene expression and secretion; and (iv) whole-body disposition of the secreted transgene product. The model was able to capture systemic and tissue PK of the vector and the transgene-produced mAb reasonably well. Pathway analysis of the PBPK model suggested that liver, muscle, and heart are the main contributors for the secreted transgene mAb. Unprecedented PK data and the novel PBPK model developed here provide the foundation for quantitative systems pharmacology (QSP) investigations of AAV-mediated gene therapies. The PBPK model can also serve as a quantitative tool for preclinical study design and preclinical-to-clinical translation of AAV-based gene therapies.

Application of physiologically-based pharmacokinetic models for therapeutic proteins and other novel modalities

The past two decades have seen diversification of drug development pipelines and approvals from traditional small molecule therapies to alternative modalities including monoclonal antibodies, engineered proteins, antibody drug conjugates (ADCs), oligonucleotides and gene therapies. At the same time, physiologically-based pharmacokinetic (PBPK) models for small molecules have seen increased industry and regulatory acceptance.This review focusses on the current status of the application of PBPK models to these newer modalities and give a perspective on the successes, challenges and future directions of this field.There is greatest experience in the development of PBPK models for therapeutic proteins, and PBPK models for ADCs benefit from prior experience for both therapeutic proteins and small molecules. For other modalities, the application of PBPK models is in its infancy.Challenges are discussed and a common theme is lack of availability of physiological and experimental data to characterise systems and drug parameters to enable a priori prediction of pharmacokinetics. Furthermore, sufficient clinical data are required to build confidence in developed models.The PBPK modelling approach provides a quantitative framework for integrating knowledge and data from multiple sources and can be built on as more data becomes available.

Current progress and limitations of AAV mediated delivery of protein therapeutic genes and the importance of developing quantitative pharmacokinetic/pharmacodynamic (PK/PD) models

While protein therapeutics are one of the most successful class of drug molecules, they are expensive and not suited for treating chronic disorders that require long-term dosing. Adeno-associated virus (AAV) mediated in vivo gene therapy represents a viable alternative, which can deliver the genes of protein therapeutics to produce long-term expression of proteins in target tissues. Ongoing clinical trials and recent regulatory approvals demonstrate great interest in these therapeutics, however, there is a lack of understanding regarding their cellular disposition, whole-body disposition, dose-exposure relationship, exposure-response relationship, and how product quality and immunogenicity affects these important properties. In addition, there is a lack of quantitative studies to support the development of pharmacokinetic-pharmacodynamic models, which can support the discovery, development, and clinical translation of this delivery system. In this review, we have provided a state-of-the-art overview of current progress and limitations related to AAV mediated delivery of protein therapeutic genes, along with our perspective on the steps that need to be taken to improve clinical translation of this therapeutic modality.

Integrating mitosis, toxicity, and transgene expression in a telecommunications packet-switched network model of lipoplex-mediated gene delivery

Gene delivery systems transport exogenous genetic information to cells or biological systems with the potential to directly alter endogenous gene expression and behavior with applications in functional genomics, tissue engineering, medical devices, and gene therapy. Nonviral systems offer advantages over viral systems because of their low immunogenicity, inexpensive synthesis, and easy modification but suffer from lower transfection levels. The representation of gene transfer using models offers perspective and interpretation of complex cellular mechanisms,including nonviral gene delivery where exact mechanisms are unknown. Here, we introduce a novel telecommunications model of the nonviral gene delivery process in which the delivery of the gene to a cell is synonymous with delivery of a packet of information to a destination computer within a packet-switched computer network. Such a model uses nodes and layers to simplify the complexity of modeling the transfection process and to overcome several challenges of existing models. These challenges include a limited scope and limited time frame, which often does not incorporate biological effects known to affect transfection. The telecommunication model was constructed in MATLAB to model lipoplex delivery of the gene encoding the green fluorescent protein to HeLa cells. Mitosis and toxicity events were included in the model resulting in simulation outputs of nuclear internalization and transfection efficiency that correlated with experimental data. A priori predictions based on model sensitivity analysis suggest that increasing endosomal escape and decreasing lysosomal degradation, protein degradation, and GFP-induced toxicity can improve transfection efficiency by three-fold. Application of the telecommunications model to nonviral gene delivery offers insight into the development of new gene delivery systems with therapeutically relevant transfection levels.

A novel nonviral gene delivery system: multifunctional envelope-type nano device

In this review we introduce a new concept for developing a nonviral gene delivery system which we call "Programmed Packaging." Based on this concept, we succeeded in developing a multifunctional envelope-type nano device (MEND), which exerts high transfection activities equivalent to those of an adenovirus in a dividing cell. The use of MEND has been extended to in vivo applications. PEG/peptide/DOPE ternary conjugate (PPD)-MEND, a new in vivo gene delivery system for the targeting of tumor cells that dissociates surface-modified PEG in tumor tissue by matrix metalloproteinase (MMP) and exerts significant transfection activities, was developed. In parallel with the development of MEND, a quantitative gene delivery system, Confocal Image-assisted 3-dimensionally integrated quantification (CIDIQ), also was developed. This method identified the rate-limiting step of the nonviral gene delivery system by comparing it with adenoviral-mediated gene delivery. The results of this analysis provide a new direction for the development of rational nonviral gene delivery systems.

Influence of lipid components on gene delivery by polycation liposomes: Transfection efficiency, intracellular kinetics and in vivo tumor inhibition

Transfection efficiency of non-viral gene vectors is influenced by many factors, including chemical makeup, cellular uptake pathway and intracellular delivery. To investigate the effect of lipid saturation on transfection efficiency of polycation liposomes (PCLs), a soybean phospholipids (SPL), egg phospholipids (EPL) and hydrogenated soybean phosphatidylcholine (HSPC) series was used to prepare PCLs. Testing these PCLs in a luciferase assay indicated that with increasing saturation (SPL<EPL<HSPC), the ability to confer gene expression decreased. The effect of protamine combined with these PCLs was also studied in different cell lines. Improved transfection because of protamine incorporation was dependent on lipid saturation and on the cell line tested. The kinetics of cellular uptake and intracellular distribution was studied using flow cytometry and laser scanning confocal microscope, which showed that naked oligonucleotide (ODN) and PCLs/ODN complexes became equilibrium after 4h incubation. PCLs containing SPL (PCLs-S) and 1,2-dieleoyl-sn-glycero-3-phosphoethanolamine (PCLs-D) increased uptake rates by 2.20- and 5.45-fold, respectively. Furthermore, pCMV-IL-12 transfection mediated by PCLs-D showed excellent tumor inhibition efficiency compared with control and naked pCMV-IL-12 treatments in vivo.

Gene transfer to the mammalian reproductive tract

This review summarizes the results of research on gene transfer to the mammalian genital tract. Gene transfer experiments have been developed during the last 2 decades and have been applied using in vitro, ex vivo and in vivo procedures. (i) In vitro methods have been applied to the uterine epithelial cells with the principal purpose of analysing some pathological change occurring in the uterus. In the male tract, epididymal cell lines have been used to evaluate the expression of particular genes and the function of specific proteins. (ii) Ex vivo methods have been applied to both the uterus and the vas deferens in humans, and good transgene expression has been recorded. (iii) In vivo gene transfer in the female tract has been employed in the uterus and oviduct using gene injections or electroporation methods. The glandular epithelium of both organs can be transfected efficiently, and transfection efficiency depends on the hormonal stage of the animal. The best expression occurred during pseudopregnancy and meta-estrus periods, when high progesterone and low estradiol concentrations occur. In the male tract, in vivo methods have been applied to mouse vas deferens and epididymis. In both organs, patches of epithelial regions appeared to express the transgenes. Furthermore, the secretions of both organs were also modified using gene constructions that led to the expression of some secretory proteins. In summary, gene modifications in the epithelium of the mammalian reproductive tract have been successful employing different technologies. Further improvements in transfection efficiency would help provide new insights into the physiology of these reproductive organs. Furthermore, the use of these methods could also be used to modify the fertility of mammals.

Gene therapy: a pharmacokinetic/pharmacodynamic modelling overview

Since gene therapy started over 20 years ago, more than one-thousand clinical trials have been carried out. Nonviral vectors present interesting properties for their clinical application, but their efficiency in vivo is relatively low, and further improvements in these vectors are needed. Elucidating how nonviral vectors behave at the intracellular level is enlightening for vector improvement and optimization. Model-based approach is a powerful tool to understand and describe the different processes that gene transfer systems should overcome inside the body. Model-based approach allows for proposing and predicting the effect of parameter changes on the overall gene therapy response, as well as the known application of the pharmacokinetic/pharmacodynamic modelling in conventional therapies. The objective of this paper is to critically review the works in which the time-course of naked or formulated DNA have been quantitatively studied or modelled.

In-vivo gene transfer induces transgene expression in cells and secretions of the mouse cauda epididymis

Mouse cauda epididymis were in-vivo transfected using the lipid FuGENE 6 as gene vector. Two gene constructions were employed: the p-GeneGRIP which codifies for the Green Fluorescent Protein (GFP) and the pSEAP-control that expresses an alkaline phosphatase as a secretion. Transfection was detected by fluorescence and appeared in the nucleus and cytoplasm of epithelial cells. Transfection was observed in 39.70% of cells after 2 days and in 31.77% after 7 days, and then diminished progressively. Moreover, the presence of the transgene in the DNA isolated from treated epididymides was observed by polymerase chain reaction. GFP gene expression appeared in large areas of the cauda epididymis and it was observed exclusively in the cytoplasm of epithelial cells. GFP gene expression occurred during 2 weeks after gene injection and occupied 32.24, 29.98 and 22.37% of the area of the tubules when analyzed 2, 7 and 15 days after gene injection. The cauda was also analyzed in toto and showed similar results. The use of the pSEAP-control gene showed that cauda epididymis secretions can also be modified by the transfection procedure. A significant increase of alkaline phosphatase activity appeared in the epididymal fluids 7 days after gene injection. These results indicate that transfection procedures could be an important tool in the future to study epididymal physiology or to change the fertilizing ability of spermatozoa.

Quantitative comparison of intracellular unpacking kinetics of polyplexes by a model constructed from quantum dot-FRET

A major challenge for non-viral gene delivery is gaining a mechanistic understanding of the rate-limiting steps. A critical barrier in polyplex-mediated gene delivery is the timely unpacking of polyplexes within the target cell to liberate DNA for efficient gene transfer. In this study, the component plasmid DNA and polymeric gene carrier were individually labeled with quantum dots (QDs) and Cy5 dyes, respectively, as a donor and acceptor pair for fluorescence resonance energy transfer (FRET). The high signal-to-noise ratio in QD-mediated FRET enabled sensitive detection of discrete changes in polyplex stability. The intracellular uptake and dissociation of polyplexes through QD-FRET was captured over time by confocal microscopy. From quantitative image-based analysis, distributions of released plasmid within the endo/lysosomal, cytosolic, and nuclear compartments formed the basis for constructing a three-compartment first-order kinetics model. Polyplex unpacking kinetics for chitosan, polyethylenimine, and polyphosphoramidate were compared and found to correlate well with transfection efficiencies. Thus, QD-FRET-enabled detection of polyplex stability combined with image-based quantification is a valuable method for studying mechanisms involved in polyplex unpacking and trafficking within live cells. We anticipate that this method will also aid the design of more efficient gene carriers.

Intracellular kinetics of non-viral gene delivery using polyethylenimine carriers

PURPOSE

Polymeric nucleic acid carriers are designed to overcome one or more barriers to delivery. High molecular weight polyethylenimine (PEI) shows high transfection efficiency but exhibits high cytotoxicity (Fischer et al. Biomaterials, 24:1121-1131 (2003); Peterson et al. Bioconjug. Chem., 13:845-854 (2002)). Nontoxic water-soluble lipopolymer (WSLP) was previously developed using branched poly(ethylenimine) (PEI, mw 1,800) and cholesteryl chloroformate (Han, Mahato, and Kim. Bioconjug. Chem., 12:337-345 (2001)) and is an effective non-viral gene carrier with transfection levels equal or above high molecular weight PEI with a lower cytotoxicity profile. To understand how differences in these polymeric carriers influence transfection, we studied the pharmacokinetics of polymer gene carriers at the cellular level.

MATERIALS AND METHODS

Cells were exposed in vitro to different polymeric carriers and the transport of the carriers into different cellular compartments was determined using cellular fractionation and real-time quantitative PCR. A multi-compartment mathematical model was applied to time series measurements of the trafficking of plasmids across each cellular barrier.

RESULTS

Our result indicates that the chemical modification of WSLP increased the rate parameter for endosomal escape significantly compared to conventional PEI carriers thereby increasing the overall transfection efficiency.

CONCLUSIONS

These results are consistent with the goal of endosomal destabilization of the carrier design. This method provides a quantitative means for assessing different polymer construct designs for gene delivery.

Molecular and cellular barriers limiting the effectiveness of antisense oligonucleotides

Antisense oligonucleotides present a powerful means to inhibit expression of specific genes, but their effectiveness is limited by factors including cellular delivery, biochemical attack, and poor binding to target. We have developed a systems model of the processes required for an antisense oligonucleotide to enter, gain access to its target mRNA, and exert activity in a cell. The model accurately mimics observed trends in antisense effectiveness with the stability of the oligonucleotide backbone and with the affinity/kinetics of binding to the mRNA over the time course of inhibition. By varying the model parameters within the physically realizable range, we note that the major molecular and cellular barriers to antisense effectiveness are intracellular trafficking, oligonucleotide-mRNA binding rate, and nuclease degradation of oligonucleotides, with a weaker dependence on total cellular uptake than might be expected. Furthermore, the model may serve as a predictive tool to design and test strategies for the cellular use of antisense oligonucleotides. The use of integrated mathematical modeling can play a significant role in the development of antisense and related technologies.

Model-based inference of gene expression dynamics from sequence information

A dynamic model of prokaryotic gene expression is developed that makes considerable use of gene sequence information. The main contribution arises from the fact that the combined gene expression model allows us to access the impact of altering a nucleotide sequence on the dynamics of gene expression rates mechanistically. The high level of detail of the mathematical model is considered as an important step towards bringing together the tremendous amount of biological in-depth knowledge that has been accumulated at the molecular level, using a systems level analysis (in the sense of a bottom-up, inductive approach). This enables to the model to provide highly detailed insights into the various steps of the protein expression process and it allows us to access possible targets for model-based design. Taken as a whole, the mathematical gene expression model presented in this study provides a comprehensive framework for a thorough analysis of sequence-related effects on the stages of mRNA synthesis, mRNA degradation and ribosomal translation, as well as their nonlinear interconnectedness. Therefore, it may be useful in the rational design of recombinant bacterial protein synthesis systems, the modulation of enzyme activities in pathway design, in vitro protein biosynthesis, and RNA-based vaccination.

Clinical trials of a new class of therapeutic agents: antisense oligonucleotides

Antisense oligodeoxynucleotides (ODNs) are short stretches of DNA complementary to a target mRNA. The ODNs selectively hybridise to their complementary RNA by Watson-Crick base pairing rules. In theory, the use of antisense ODNs provides a method to specifically inhibit the intracellular expression of any disorder whose genetic aetiology is well known. For this reason, researchers thought that if antisense drugs proved to be so specific there would be no side effects. However, toxicity-related problems arose in initial animal studies of antisense drugs in the early 1990s and since then companies have been using these compounds cautiously. In order to be useful therapeutically, an ODN must (a) exhibit reasonable stability in the physiological environment, (b) be taken up and retained in adequate quantities by the target cells, (c) specifically bind target mRNA with high affinity, (d) have an acceptable therapeutic ratio, free of unwanted toxic and non-specific side effects and (e) be easily synthesised in sufficient quantities to allow clinical use. Most of these criteria have already been met by ODNs recently used in this way. This review describes certain therapeutic applications of antisense techniques currently under investigation in oncology, haematopathology and inflammatory diseases.

Engineering synthetic vectors for improved DNA delivery: insights from intracellular pathways

Significant progress has been made in the area of nonviral gene delivery to date. Yet, synthetic vectors remain less efficient by orders of magnitude than their viral counterparts. Research continues toward unraveling and overcoming various barriers to the efficient delivery of DNA, whether in plasmid form encoding a gene or as an oligonucleotide for the selective inhibition of target gene expression. Novel components for overcoming these hurdles are continually being incorporated into the design of synthetic vectors, leading to increasingly more virus-like particles. Despite these advances, general principles defining the design of synthetic vectors are yet to be developed fully. A more quantitative analysis of the cellular uptake and intracellular processing of these vectors is required for the rational manipulation of vector design. Mathematical frameworks with a more conceptual basis will help obtain an integrated perspective on these complex systems. In this review, we critically examine the progress made toward the improved design of synthetic vectors by the strategic exploitation of intracellular mechanisms and explore newer possibilities to overcome obstacles in the practical realization of this field.

In vitro transfection of the human vas deferens using DNA-liposome and DNA-neutral lipid complexes

OBJECTIVE

To transfect the human vas deferens in vitro.

DESIGN

Prospective study, description of a procedure.

SETTING

Research center and university hospital.

PATIENT(S)

Seven fertile men undergoing vasectomies or vasovasostomies.

INTERVENTION(S)

Human vas deferens pieces were transfected in vitro using the p-GeneGrip gene construction, which codifies for the green fluorescent protein (GFP). Lipofectamine or GenePorter were employed as gene vectors.

MAIN OUTCOME MEASURE(S)

After vas deferens epithelium transfection, we described the vas deferens foreign gene expression. Maximum transfection occurred in 14.7% of the vas deferens epithelial cells. After using GenePorter, we observed green fluorescent protein gene expression in 40% of samples, which occupied 9.86% of the epithelial area. After Lipofectamine treatment, transgene expression occurred in 33% of the samples and occupied 9.05% of the epithelial area.

CONCLUSION(S)

The human vas deferens epithelium has the potential to be modified by gene transfection.

Rational design and engineering of delivery systems for therapeutics: biomedical exercises in colloid and surface science

Engineering delivery systems of therapeutic agents has grown into an independent field, transcending the scope of traditional disciplines and capturing the interest of both academic and industrial research. At the same time, the acceleration in the discovery of new therapeutic moieties (chemical, biological, genetic and radiological) has led to an increasing demand for delivery systems capable of protecting, transporting, and selectively depositing those therapeutic agents to desired sites. The vast majority of delivery systems physically reside in the colloidal domain, while their surface properties and interfacial interactions with the biological milieu critically determine the pharmacological profiles of the delivered therapeutic agents. Interestingly though, the colloidal and surface properties of delivery systems are commonly overlooked in view of the predominant attention placed on the therapeutic effectiveness achieved. Moreover, the development and evaluation of novel delivery systems towards clinical use is often progressed by serendipity rather than a systematic design process, often leading to failure. The present article will attempt to illustrate the colloid and interfacial perspective of a delivery event, as well as exemplify the vast opportunities offered by treating, analysing and manipulating delivery systems as colloidal systems. Exploring and defining the colloid and surface nature of the interactions taking place between the biological moieties in the body and an administered delivery vehicle will allow for the rational engineering of effective delivery systems. A design scheme is also proposed on the way in which the engineering of advanced delivery systems should be practiced towards their transformation from laboratory inventions to clinically viable therapeutics. Lastly, three case studies are presented, demonstrating how rational manipulation of the colloidal and surface properties of delivery systems can lead to newly engineered systems relevant to chemotherapy, gene therapy and radiotherapy.

A model for the analysis of nonviral gene therapy

Further understanding of the mechanisms involved in cellular and intracellular delivery of transgene is needed to produce clinical applications of gene therapy. The compartmental and computational model designed in this work is integrated with data from previous experiments to quantitatively estimate rate constants of plasmid translocation across cellular barriers in transgene delivery in vitro. The experimental conditions between two cellular studies were held constant, varying only the cell type, to investigate how the rates differed between cell lines. Two rate constants were estimated per barrier for active transport and passive diffusion. Translocation rates of intact plasmid across the cytoplasmic and nuclear barriers varied between cell lines. CV1 cells were defined by slower rates (0.23 h(-1) cytoplasmic, 0.08 h(-1) nuclear) than those of the HeLa cells (1.87 h(-1) cytoplasmic, 0.45 h(-1) nuclear). The nuclear envelope was identified as a rate-limiting barrier by comparing the rate of intact plasmid translocation at each barrier. Slower intact plasmid translocation in CV1 cells was correlated with a reduced absolute capacity for transgene efficiency in comparison with HeLa cells. HeLa cells were three times more efficient than CV1 cells at producing green fluorescent protein per intact plasmid delivered to the nucleus. Mathematical modeling coordinated with experimental studies can provide detailed, quantitative understanding of nonviral gene therapy.

In vivo transfection of the mouse vas deferens

To explore the possibility that specific characteristics of the epithelium of the male tract can be modified, transfections of the mouse vas deferens have been performed using in vivo injections of cationic DNA/liposome complexes. Gene transfer was done employing the reporter genes pEGFP-C1 encoding Green Fluorescent Protein (GFP) and pCMV-nls-beta encoding the nuclear beta-Galactosidase (beta-Gal). Foreign gene expression reached a maximum of 6.8% in the epithelial cells of the vas after treatment with the nuclear beta-Gal gene construction and of 13.3% after employing the GFP gene construction. Expression of the GFP gene appeared from one week up to three months following injection, and it appeared as patches of modified cells along the epithelium. Results from immunocytochemistry and Western Blotting support the conclusion that transfection of epithelial cells was achieved. We have also transfected the vas using gene constructions that express secretory proteins--specifically, the reporter system pSEAP-control that expresses a secretory form of human placental alkaline phosphatase, and the pGFP-Ctk-37 that expresses a secretion form of GFP. In both cases, the fluids expressed from the transfected vas showed a significant increase of alkaline phosphatase activity after pSEAP transfection and the presence of GFP protein when pGFP-Ctk-37 gene construction was employed. Our results indicate that the vas can be transfected in vivo using liposomes as vectors of foreign genes and that the vas fluid contents can be modified.

Quantitative analysis of synthetic gene delivery vector design properties

As intracellular gene delivery pathways are highly complex combinations of multiple potentially rate-limiting cellular and molecular processes, approaches to the design of synthetic delivery vectors focusing on any single barrier individually will likely be suboptimal. We offer here an "integrative systems" approach to vector characterization and design, combining quantitative experiment and computational modeling studies of vector uptake and trafficking kinetics. This model is validated using data for delivery of a green fluorescent protein (GFP)-encoding plasmid by means of Lipofectamine, permitting specification of model parameter values. The model is then used to make a priori predictions on the effect of polymer length in polyplex vectors, with additional parameter values determined from previous independent experimental studies of plasmid release. Comparison with data on GFP expression via these polyplex vectors shows that the model successfully predicts an experimentally observed biphasic dependence of expression efficiency on polymer length and quantifies the contributions of competing effects yielding the optimal intermediate polymer length. Finally, we use the model to predict potential effects of incorporating nuclear localization sequences in these kinds of synthetic vectors, and find that the degree of benefit from these will depend on the values of other key system properties including the vector unpackaging rate constant. Thus, we demonstrate the usefulness of a bioengineering, integrative-systems modeling approach to improved vector design and analysis.

Safety and pharmacokinetics of naked plasmid DNA in the skin: studies on dissemination and ectopic expression

Gene therapy using naked DNA injected into muscle and skin is increasingly being used for vaccination and treatment purposes. Favorably, naked plasmid DNA does not exhibit the various limitations inherent to viral vectors, such as the elicitation of adverse immune responses and the risk of insertional mutagenesis. In order to assess the distribution and safety of naked plasmid DNA in a relevant animal model, we analyzed if intracutaneously injected plasmid DNA was transported to other organs and if ectopic expression occurred. When a "superdose" of a marker plasmid was injected intradermally, most organs were found transiently to contain the plasmid DNA for several days, whereas integration into the host genome was not detected. With the exception of ovary, however, mRNA expression only occurred in the skin, regional lymph nodes, and muscular tissues. From a safety standpoint, skin gene therapy with naked plasmid DNA can be considered safe due to the rapid biodegradation of plasmid DNA and the exclusive and transient expression of foreign genes in tissues known to take up DNA.

Receptor-mediated targeting of gene delivery vectors: insights from molecular mechanisms for improved vehicle design

One way to deliver transgenes to cells in a selective manner is to target the delivery vehicles, or vectors, to specific cell-surface receptors as a first step toward ultimate transport of the gene to the nucleus for expression. While selective delivery, although often to undesired cell types, occurs naturally for some viral vectors and can be achieved for nonviral vehicles, current understanding and control of the delivery mechanism is inadequate for many therapeutic applications. The complicated nature of receptor-mediated transgene uptake and transport requires improved analysis to more effectively evaluate delivery vehicles. As receptor-mediated pathways for gene delivery typically involve vector binding, internalization, subcellular trafficking, vesicular escape, nuclear translocation, and unpackaging for transcription, each of these processes offer mechanisms that can be exploited to enhance targeted gene delivery via properly designed vehicles. For the purpose of this review, current targeted gene delivery vehicles are divided into three approaches: viral, synthetic, and hybrid vectors. Each approach possesses advantages as well as disadvantages at the present time for in vitro and in vivo application, and provides particular challenges to overcome in order to gain significantly improved targeted delivery properties. Quantitative experiments and mathematical modeling of the gene delivery pathway will serve to provide insight into molecular mechanisms and rate-limiting steps for effective gene expression. Information on molecular mechanisms obtained by such methodologies can then be applied to specific vectors, whether viral, synthetic, or hybrid, allowing for the creation of targeted, effective, and safe gene therapeutics.

Delivery of recombinant gene products to the central nervous system with nonautologous cells in alginate microcapsules

Somatic gene therapy using nonautologous recombinant cells immunologically protected with alginate microcapsules has been successfully used to treat rodent genetic diseases. We now report the delivery of recombinant gene products to the brain in rodents by implanting microencapsulated cells for the purpose of eventually treating neurodegenerative diseases with this technology. Alginate-poly-L-lysine-alginate microcapsules enclosing mouse C2C12 myoblasts expressing the marker gene human growth hormone (hGH) at 95+/-20 ng/million cells/hr were implanted into the right lateral ventricles of mice under stereotaxic guidance. Control mice were implanted similarly with nontransfected but encapsulated cells. Delivery of hGH to the different regions of the brain at various times postimplantation was examined. At 7, 28, 56, and 112 days postimplantation, hGH was detected at high levels around the implantation site and also at lower levels in the surrounding regions, while control mice showed no signal. Immunohistochemical staining of the implanted brains showed that on days 7, 56, and 112 postimplantation, hGH was localized in the tissues around the implantation site. Mice implanted with encapsulated but nontransfected cells showed no signal. Hence, the feasibility of using encapsulated nonautologous cells to deliver recombinant gene products to the brain for extended periods may allow the application of this technology to the treatment of neurodegenerative genetic disorders.

Biodistribution of radiolabeled lipid-DNA complexes and DNA in mice

The tissue biodistribution and expression of [33P]DNA-1-[2-[9-(Z)-octadecenoyloxy]ethyl]]-2-[8](Z)-heptadece nyl]-3 -[hydroxyethyl]imidazolinium chloride (DOTIM):cholesterol complexes and 33P-radiolabeled DNA expressing chloramphenicol acetyl transferase (CAT; 4.7 kB) were studied after intravenous (iv) injection in ICR mice. Mice were injected with 200 microL of complex containing DNA at 3 mg/kg or DNA alone. One group received 8 microCi of radioactivity and were sacrificed at 5 and 20 min, and 1, 2, 4 and 24 h post-dose (n = 4/time point). A second group received the equivalent of 3.9 microCi of radioactivity and were sacrificed at 20 min, and 2 and 24 h for subsequent whole body autoradiographic analysis (WBA; n = 2/time point). The tissue distribution of intact DNA was assessed by Southern blot at 24 h post-dose, whereas the integrity of complexes and DNA incubated in heparinized whole blood was studied separately. In further studies, the time course of expression in lung tissue over a 48-h period was examined, and the relative lung-expression of purified open circular (OC) versus supercoiled (SC) DNA at 24 h was evaluated. Approximately 42% of the radioactivity was found in the lungs 5 min after injection and about half this percentage was found in the liver. By 2 h, only 5% remained in the lungs, but 48% was present in the liver. No other tissue accumulated >5% of the dose throughout the duration of the study. WBA radiograms confirmed the tissue distribution results and highlighted significant accumulation of radioactivity in bone over time. Southern Blot analysis demonstrated intact DNA in many tissues 24 h after dosing. In contrast, the majority of DNA incubated in blood was degraded within 2 h, although the complexes afforded some protection relative to DNA alone. The OC DNA expressed equivalently to SC DNA in lung tissue (OC = 1035 +/- 183 pg; SC = 856 +/- 257 pg/mg soluble protein, n = 6, mean +/- SEM) at 24 h, and detectable levels of CAT were present within 2 h of dosing (21.3 +/- 7.2 pg, n >/= 8, mean +/- SD). The results confirm that DNA-DOTIM:cholesterol complexes are initially deposited in the lungs after iv administration.

Polylysine-based transfection systems utilizing receptor-mediated delivery

Receptor-mediated gene transfer is a promising gene delivery technique. It employs a DNA-binding polycation, such as polylysine, to compact plasmid DNA to a size that can be taken up by cells (<100-200 nm). To allow internalization by receptor-mediated endocytosis, cell binding ligands, such as asialoglycoproteins or galactose for hepatocytes, anti-CD3 and anti-CD5 for T-cells, and transferrin, have been covalently attached to polylysine. Intracellular barriers for successful gene transfer include release of DNA complexes from endosomes or lysosomes, nuclear import of DNA complexes, and disassembly of the DNA-polylysine particles. Release of particles from internal vesicles has been achieved by the addition of lysosomotropic agents or glycerol to the transfection medium, or by the incorporation of endosomolytic compounds, such as viruses or membrane active peptides. This technique has already been used to transfect certain organs in vivo, including liver and lung.

Pharmacokinetic considerations in somatic gene therapy

It is anticipated that gene therapies will be useful for achieving long durations of action with little temporal fluctuation in the level of the therapeutic gene product, localized effects in specific tissues or cell types, and levels of biological products that can be regulated over time by drugs or physiological events. The effective clinical application of gene therapies will require detailed consideration of the pharmacokinetics of the gene and its gene product. This requires understanding not only the apparent kinetics of the bioactive gene product, but the intrinsic kinetics of each step involved in gene delivery, gene expression and the bioavailability of the gene product. Numerical models are described for three different approaches to gene therapy: (i) those that involve permanent integration of a transgene into the target cell, (ii) those that involve transient residence of the transgene within the cell, and (iii) those that allow control over gene expression by regulatory factors or administered drugs. Experimental studies describing the dynamics and kinetics of DNA in vivo and early pharmacokinetic studies of viral and non-viral systems are reviewed.

Cationic lipid-based gene delivery systems: pharmaceutical perspectives

Gene delivery systems are designed to control the location of administered therapeutic genes within a patient's body. Successful in vivo gene transfer may require (i) the condensation of plasmid and its protection from nuclease degradation, (ii) cellular interaction and internalization of condensed plasmid, (iii) escape of plasmid from endosomes (if endocytosis is involved), and (iv) plasmid entry into cell nuclei. Expression plasmids encoding a therapeutic protein can be, for instance, complexed with cationic liposomes or micelles in order to achieve effective in vivo gene transfer. A thorough knowledge of pharmaceutics and drug delivery, bio-engineering, as well as cell and molecular biology is required to design optimal systems for gene therapy. This mini-review provides a critical discussion on cationic lipid-based gene delivery systems and their possible uses as pharmaceuticals.

Genetic therapy. Past, present, and future

The early stages of genetic therapy present challenges for clinicians and basic scientists. Clinicians must become familiar with new terminology and concepts, and must keep a perspective on the new field in the face of inflated claims and high-profile failures. Basic scientists must continually return to disease models and to patients to determine what are the proper safety issues and relevant efficacy questions for specific diseases and vector systems. And in an era of instant information, all concerned parties must be careful about how progress is communicated to colleagues, patients, and the lay public.

Development of targeted delivery systems for nucleic acid drugs

Our increased understanding of disease pathogenesis is the basis for developing novel nucleic acid drugs. The main challenge encountered in this development is how to maintain therapeutically meaningful concentrations of the drugs in the vicinity of their targets for the desired periods. The intrinsic difficulty arises from the fact that nucleic acid drugs are not readily transported across membranes. Hence, their delivery and transport characteristics at the whole body, organ and cellular levels need to be thoroughly examined. Liposomes and receptor-mediated polycation systems are promising carriers for their delivery in vivo. There are many barriers to be overcome for successful antisense and gene therapies. Along with other factors, disposition, stability against nucleases, binding to cell surface receptor and internalization, and intracellular trafficking affect the in vivo delivery and efficacy of nucleic acid drugs. This review article discusses the delivery and transport of these compounds.

Quantitative analysis of transient gene expression in mammalian cells using the green fluorescent protein

The green fluorescent protein (Gfp) has been used as a reporter, along with flow cytometric analysis, to follow the dynamics of gene expression in transiently transfected mammalian cells. Gene transfer conditions for lipofection were optimized. The highest fraction of transfectants were obtained when lipid-DNA complexes were formed with 6 microliters lipid and 1 microgram DNA for chinese hamster ovary (CHO) cells and with 9 microliters lipid and 2 micrograms DNA for NIH/3T3 cells. Chinese hamster ovary cells were monitored for Gfp expression and growth for 6 days following transfection. An initial decrease in viability for 36 h was observed after which cell growth followed exponential kinetics with increasing viability. Intracellular accumulation of recombinant protein peaked at 24 h post-transfection and then decreased with first order kinetics at a rate comparable to the specific growth rate. It appears that dilution by growth accounts for the decrease of Gfp in the biomass. Immunofluorescent staining of Gfp and subsequent flow cytometric analysis of transfected cells revealed a linear correlation between the green fluorescence and immunofluorescence. This indicates that green fluorescence is a quantitative measure of intracellular Gfp in single cells in spite of the dynamics of post-translational modifications involved in the conversion of expressed protein into its fluorescent form. A structured model has been formulated to describe the observed kinetics of gene expression and fluorophore formation. The model accurately predicts experimental trends and suggests that the fraction of non-fluorescent Gfp is significant only during the initial period of gene expression.

Nonviral gene therapy: the promise of genes as pharmaceutical products

Although most research on gene therapy has focused on the use of recombinant viruses to deliver genes to cells in vivo, progress also has been made toward developing nonviral, pharmaceutical formulations of genes for in vivo human therapy. Various methods for nonviral gene therapy have been proposed. Some approaches are aimed at developing "artificial viruses" that attempt to mimic the process of viral infection using synthetic materials. Others apply the theory and methods of advanced, particulate drug delivery to deliver DNA to select somatic targets. These approaches employ DNA complexes containing lipid, protein, peptide, or polymeric carriers as well as ligands capable of targeting the DNA complex to cell-surface receptors on the target cell and ligands for directing the intracellular trafficking of DNA to the nucleus. Nonviral systems have been used to deliver genes to the lung, liver, endothelium, epithelium, and tumor cells and have been shown to be generally safe. More than a dozen clinical trials are currently underway using nonviral systems for disease indications including cystic fibrosis and cancer. Future advances in nonviral systems will be based on an emerging appreciation of the biological constraints on the fate and function of DNA within the body and within the cell.

Cancer gene therapy using plasmid DNA: pharmacokinetic study of DNA following injection in mice

The fate of plasmid DNA complexed with cationic lipids delivered intravenously in mice was evaluated at selected timepoints up to 6 months postinjection. Blood half-life and tissue distribution of plasmid DNA and potential expression in tissues were examined. Southern blot analyses of blood indicated that intact plasmid DNA was rapidly degraded, with a half-life of less than 5 min for intact plasmid, and was no longer detectable at 1 hr postinjection. Southern analyses of tissue demonstrated that intact DNA was differentially retained in the lung, spleen, liver, heart, kidney, marrow, and muscle up to 24 hr postinjection. After 7 days, no intact plasmid DNA was detectable by Southern blot analysis; however, the plasmid was detectable by the polymerase chain reaction (PCR) in all tissues examined at 7 and 28 days postinjection. At 6 months postinjection, femtogram levels of plasmid were detected only in muscle. Immunohistochemical analyses did not detect encoded protein in the tissues harboring residual plasmid at 1 or 7 days postinjection.

Non-viral gene therapy

Non-viral gene therapies are currently under development that employ drug-delivery methods for targeting genes to selected cells in the body, where they express therapeutic gene products. Various methods have been described for non-viral gene therapy, ranging from the direct intramuscular injection of purified DNA to the systemic administration of formulations comprising DNA and lipids, proteins, peptides, or polymers. Products for non-viral gene therapies are designed both for direct administration to patients by conventional routes and for expression of a therapeutic product over a finite period of time in a manner similar to conventional medicines. Initial preclinical and clinical studies indicate that non-viral gene delivery methods exhibit safety profiles similar to conventional pharmaceutical or biological products. Clinical trials have been proposed, or are currently under way, to assess the applicability of non-viral gene therapy for a variety of disorders, including cystic fibrosis, cancer, and peripheral vascular disease. Non-viral techniques may soon allow gene therapy to be applied in clinical practice alongside conventional medicines for the treatment of common diseases.

In vivo gene transfer into rabbit thyroid follicular cells by direct DNA injection

Direct injection of DNA expression vectors into muscle leads to expression of encoded recombinant gene products in mature muscle cells. This phenomenon is not shared by most other organs. We have surveyed various organs in the rabbit to identify other cell types that would express DNA vectors after direct injection. We observed that thyroid follicular cells were capable of acquiring plasmid DNA and expressing recombinant gene products after direct interstitial injection of plasmid vectors into the thyroid gland. The level of expression of a chloramphenicol acetyltransferase (CAT) reporter gene in thyroid tissue was similar to that seen in muscle tissue three days after injection in controlled experiments. Using a beta-galactosidase reporter gene, expression was localized to thyroid follicular cells. CAT activity decreased with first-order kinetics and a half-life t1/2 of 40 hr. DNA was identified in thyroid tissue by polymerase chain reaction (PCR) analysis and displayed first-order elimination kinetics with a half-life t1/2 of 10 hr. The persistence of the gene and gene product in the thyroid was significantly different from that observed after injection of DNA vectors into muscle or delivery of DNA vectors to the liver using asialoglycoprotein/polylysine/DNA complexes, suggesting that there are significant differences in the process of DNA uptake or compartmentalization in these experimental systems. These results introduce the possibility of developing the thyroid as a novel target for treating certain thyroid or systemic diseases using DNA vectors.