Therapeutic viral vectors are common tools for gene delivery in research and clinical cell and gene therapy applications. These vectors have been widely used due to their efficiency in invading cells and introducing their genetic material. LUXTURNA (from Spark Therapeutics, Inc.), ZYNTEGLO (from bluebird bio, Inc.), and ZOLGENSMA (from Novartis Gene Therapies, Inc.), are some examples of multiple cell and gene therapy products approved by FDA in the past few years. In spite of the successful outcomes of these drugs in preclinical and clinical studies in the past few years, there are still some challenges that limit these approaches from attaining their full potential. These include pre-existing immunity to viral vectors due to previous natural exposure to different serotypes of these viruses, and low efficiency of the vectors in transducing the right cells or tissues. Currently, to overcome these challenges, potential participants with high titers of neutralizing Abs are excluded from cell and gene therapy clinical trials and vectors are administered in high doses in eligible treatment recipients which results in more drug associated toxicity. To minimize cell and gene therapy associated immunotoxicities and enhance their therapeutic benefit, we are proposing the use of uniquely designed excipients which act as engager compounds to mediate specific and efficient attachment of therapeutic vectors to the target cells, therefore reducing required dose of the virus and the subsequent toxicity and immunogenicity of the vectors.