You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Small Molecule Radiopharmaceuticals – A Review of Current Approaches 
Chapter Title:  Selectivity through BLA 
1. Abstract of Cited Paper (BIB001):  We here report on the synthesis and binding properties at 5-HT(7) and 5-HT(1A) receptors of ligands 3-12, that were designed according to the 'bivalent ligand' approach. Two moieties of the 5-HT(7)/5-HT(1A) ligand 4-[2-(3-methoxyphenyl)ethyl]-1-(2-methoxyphenyl)piperazine (1) were linked through their 3-methoxy substituent by polymethylene chains of variable length, with the aim to increase the affinity for 5-HT(7) receptor and the selectivity over 5-HT(1A) receptors. In the best cases, the dimers showed affinities for 5-HT(7) receptors as high as the monomer with no improvement in selectivity. Some dimers displayed 5-HT(1A) receptor affinities slightly higher than monomer 1. 
2. Abstract of Cited Paper (BIB002):  Multivalency is widely used in nature in specific recognition processes. This paper describes an approach to multivalency in the pentameric 5-HT3 receptor, a ligand-gated ion channel, which constitutes an example of intrinsically multivalent biological receptors. Owing to the picomolar Ki value, TETRA-L represents an outstanding multivalent ligand for the neurotransmitter receptor.

Literature Review:
Bivalent ligands are examples of multimeric interactions. Multimeric interactions are known to enhance the binding affinity of the ligands through multiple mechanisms, e.g., receptor clustering, chelating effect on receptors, ligand-receptor steric stabilization, and ligand accumulation near the receptor BIB003 . Overall, the effect is enhanced selectivity and enhanced binding affinity BIB002 . The multivalent concept has been extensively validated for peptides. Successful reports for multimeric peptides as diagnostics agent are included in Table 1 . Reviews regarding the development of homo-multimeric and hetero-multimeric peptidic ligands are many, and hence, for peptidic multimeric ligands readers may refer reviews BIB001  . The multimeric concept is now being extended to small molecules as well. Small molecule-based BLs are capable of multimeric interactions, thereby having higher sensitivity and selectivity.