During the last decades, oligonucleotides (ONs) have emerged as an alternative therapeutic modality for a variety of diseases. They can target disease mechanisms that are considered undruggable by small molecular drugs and, in comparison to small molecular drug development, they can be developed rapidly from target identification to clinical use. However, their therapeutic potential is hindered by rapid renal clearance, widespread biodistribution, and poor cellular uptake. Spherical nucleic acids (SNAs) are an alternative covalent formulation for the delivery of therapeutic ONs. They are large enough to avoid renal clearance and are readily taken up to various cell types. Despite many beneficial properties their targeted systemic delivery remains a challenge. Tissue specific ligands can be utilized to facilitate targeted delivery but commonly used methodologies for SNA synthesis provide poor control over the valency and site of ligand attachment.

In this thesis, a controlled synthesis to obtain conjugates of well-defined [60]fullerene-based molecular SNAs (MSNAs) is described. The developed two-step assembly allows heterofunctionalization of MSNAs which can be utilized for site-specific ligand and labelling group integration. Various analytical methods were deployed to confirm the structural integrity and homogeneity of the synthesized MSNAs. Applicability of the MSNAs for site-specific radiolabelling was demonstrated and the effect of backbone chemistry, ligand decoration, and degree of labelling on the biodistribution was studied by positron emission tomography/computed tomography (PET/CT). The developed synthesis strategy was also applied for preparation of site-specific conjugates of glycan engineered antibodies (Ab) and MSNAs. Antigen binding properties, Ab-mediated endocytosis, and anti-proliferative effects of the conjugates on breast carcinoma cells were studied. Also, isopeptide bond formation between a peptide-ON conjugate and a recombinant protein was studied as an alternative methodology for generating Ab-ON conjugates (AOCs). The synthetic methodologies developed in this thesis can be used in the development of novel site-specific Ab-MSNA conjugates for diagnostic and therapeutic applications.