Chemical and Combined Genetic and Chemical Modifications of Adenovirus Vector Capsids to Overcome Barriers for In Vivo Vector Delivery

Florian Kreppel


Ad Vectors and Gene Therapy

More than 2000 clinical gene therapy trials have been approved to date (see http:// and several of them have successfully demonstrated that the delivery of genes can be a causative treatment with significant clinical benefit for patients suffering from genetic disorders. Among the most successful trials were those addressing the genetic immunodeficiencies severe combined immunodeficiency-adenosine deaminase deficiency (SCID-ADA) (Aiuti et al. 2002, 2009; Gaspar et al. 2006, 2011), X-linked severe combined immunodeficiency 1 (SCID-X1), X-linked chronic granulomatous disease (X-CGD) (Ott et al. 2006, 2007; Kang et al. 2011), Wiskott-Aldrich syndrome (Aiuti et al. PMID 23845947), the brain demyelinating disease X-linked adrenoleukodystrophy (ALD) (Cartier et al. 2009, 2012) and metachromatic leukodystrophy (Biffi et al. PMID 23845948), the genetic retinal disorder Leber’s congenital amaurosis type 2 (LCA2) (Bainbridge et al. 2008, 2015; Hauswirth et al. 2008; Maguire et al. 2008; Testa et al. 2013), and lipoprotein lipase deficiency (Gaudet et al. 2010, 2012, 2013).

In the light of these very remarkable recent successes it is important to note that a wide variety of different diseases may be treated by gene therapy protocols. Such diseases include malignant tumors and disseminated metastases, genetic diseases of lung, skeletal muscle, and liver, diseases that could be treated by using secretory organs such as liver or muscle as production sites for therapeutic proteins, and infectious diseases that may be prevented or cured or whose progression may be halted by genetic vaccination. However, the treatment of all of these diseases either requires or least benefits from systemic vector delivery to exploit the enormous potential of gene therapy. A comprehensive vector and disease-specific understanding of vector-host interactions is of paramount importance prior to safe and efficacious systemic vector delivery.

Ad-based vectors are the most frequently used vector type in clinical trials (see They have been used in more than 490 clinical trials and have been delivered to thousands of patients. With one tragic exception (Raper et al. 2003), Ad vectors have been shown to be well-tolerated and safe. In addition, the world’s first nationally licensed gene therapy products Gendicine and Oncorine are based on Ad (Peng 2005; Wilson 2005; Raty et al. 2008; Liang 2012).

However, data from clinical and preclinical trials have also pointed out that the clinical applicability of Ad vectors is currently limited by numerous vector-host interactions. These interactions lead to rapid vector neutralization, acute toxicity, and vector sequestration and mistargeting. From a virological perspective the wild type human Ad type 5 (Ad5) may be considered one of the best-characterized viruses. Yet it is an enormous challenge to describe and understand the complexity of interactions between the Ad vector and a patient’s organism after in vivo vector delivery on a molecular level. Such comprehensive understanding is mandatory before Ad vectors can become clinical routine for a large number of patients and a wide variety of diseases.

The multitude of interactions between Ad vectors and patients after in vivo vector delivery imposes a large number of barriers that negatively influence efficient and specific gene delivery and determine the toxicity of the vectors to a large degree. While clinical studies and data from animal models have already revealed a large number of such barriers, only few of them are understood in great detail. Furthermore, in particular with the ongoing development of Ad vectors based on different types and species, it appears very likely that numerous barriers have not even been identified yet.

The aim of this chapter is to outline the complexity of Ad vector-host interactions as far as possible today and to figure out the utility of chemical and combined genetic and chemical Ad vector modifications as tools to analyze and overcome efficacy-limiting barriers for Ad vector-mediated gene transfer. Since most data in the past were generated with Ad5-based vectors, a strong focus will be on this Ad type, which can serve as an excellent paradigm to also understand interactions of other Ad types. Most if not all of the barriers described below impact on the efficacy of in vivo delivered Ad vectors independent of the route of delivery.

In the early years of Ad vector development numerous approaches tried to manipulate the vector tropism and vector-host interactions by genetic means. However, the introduction of small ligand motifs (for targeting) or point mutations (for detargeting) was typically not sufficient to overcome the multitude of biological in vivo barriers and to enable successful clinical vector application. Therefore, this chapter focuses on chemical and combined genetic and chemical capsid modifications, which hold great promise for efficacious in vivo delivery of Ad vectors. The reader should note that in parallel to chemical capsid modifications, type switching strategies, chimeric vectors, and approaches of directed evolution (Bauzon and Hermiston 2012) have currently been developed with great success. While these are not a direct subject of this chapter it will become obvious that such strategies can beneficially be combined with chemical approaches to create improved Ad-based vectors now (Nguyen et al. 2016) and in the future.

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