Preclinical safety and tolerability of a repeatedly administered human leishmaniasis DNA vaccine

Riede, O; Seifert, K; Oswald, D; Endmann, A; Hock, C; Winkler, A; Salguero, F J; Schroff, M; Croft, S L; Juhls, C

The leishmaniases are a complex of diseases caused by protozoan parasites of the genus Leishmania with up to 1.6 million cases reported worldwide annually 1 and~350 million people at risk to develop leishmaniasis. 2 It is estimated that the most severe form, visceral leishmaniasis (VL), causes up to 40 000 deaths per year. 1 Measures to control transmission of the parasite were of limited success to date. Despite advances in treatment of VL over the last decade, drug toxicity and heat stability, difficult routes of administration and variation in drug efficacy between endemic areas remain issues to be fully solved. A preventive and therapeutic leishmaniasis vaccine prospectively represents the most cost-effective and successful measure to control the leishmaniases worldwide. 3 Immunity to the intracellular parasite Leishmania is associated with T-cell-mediated immune responses. 4 DNA vaccines are particularly able to deliver antigens into the major histocompatibility complex class I processing pathway, thereby inducing CD8 cytotoxic T-cell immune responses, 5, 6 necessary for clearance of Leishmania. In addition, CD4 T cells and B cells are activated by DNA vaccines. Hence, this vaccination technology is a promising approach for developing a leishmaniasis vaccine. 7 An appropriate preclinical safety profile of a vaccine candidate has to be established prior to initiation of clinical phase 1 studies. The US Food and Drug Administration (FDA) and the World Health Organization recommend the evaluation of distribution and persistence of DNA vaccines as well as their local reactogenicity and systemic toxicity. 8, 9 Results of corresponding studies suggest that DNA vaccines are safe, 10, 11 which has been confirmed in several clinical trials. 12 However, immunogenicity of DNA vaccines was often modest in humans, necessitating better delivery methods and improved vaccine antigen expression. 12 In addition, safety concerns prompted avoidance of antibiotic resistance genes and other selection markers for plasmid propagation. 13 State-of-the-art plasmids are consequently more efficacious, safer and smaller than classical plasmids. 14 Minimalistic Immunogenically Defined Gene Expression (MIDGE) vectors represent one of the most rigorous concepts for improvement. 15 MIDGE vectors are small linear double-stranded DNA (dsDNA) molecules, solely containing sequences required for their function in vivo and no bacterial plasmid backbone sequences as they have been shown to negatively impact transgene expression and immunogenicity. 16, 17 MIDGE-Th1 vectors are MIDGE vectors with a short peptide (PKKKRKVEDPYC) covalently attached, enhancing the immune responses to encoded antigens. 18, 19 Recently, preclinical data on biodistribution and toxicity of MIDGE and MIDGE-Th1 vectors have been published, 20, 21 indicating an excellent safety profile after a single administration.