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:  Receptor Imaging 
1. Abstract of Cited Paper (BIB001):  A rapid one-step 18 F labeling reaction with fluoridealuminum complex, which is based on chelation chemistry, has received a surge of interest for 18 F radiolabeling of peptides. In this study, a non-peptidic bivalent integrin αvβ3 antagonist (bivalent-IA) was conjugated with 1,4,7-triazacy- clononane-1,4-diiacetic acid (NODA). A novel 18 F labeled radiotracer, 18 F-bivalent-IA, was developed via one step 18 F� AlF/NODA chelation reaction in aqueous phase with high radiochemical yield (65�75%, decay corrected) and good specific activity (750�850 mCi/μmol). The tumor integrin targeting efficiency and in vivo pharmacokinetic profile of 18 F-bivalent-IA were evaluated in U-87 MG (integrin positive) and MDA-MB- 231 (integrin negative) models by small-animal PET/CT scan followed by a biodistribution study. The PET/CT and ROI results showed high tumor uptake of 18 F-bivalent-IA in U-87 MG tumor-bearing mice from 5 to 120 min p.i. with good contrast, and the U-87 MG tumor uptake values (6.35 ± 0.67%ID/g, at 1 h p.i.) were 6 times higher than those of MDA-MB-231 tumor (1.05 ± 0.12%ID/g, at 1 h p.i.) (P < 0.0001) which correlated with the integrin αvβ3 expression in tumor tissues confirmed by immunohistochemistry. Co-injection of the 18 F-bivalent-IA with 6 nmol (6 μg) of nonradioactive bivalent-IA effectively blocked tumor uptake demonstrating the integrin αvβ3-specificity. In conclusion, the first 18 F labeled non-peptidic bivalent integrin αvβ3 targeting radiotracer, 18 F-bivalent-IA, was developed and proved to be a highly potent and specific PET radiopharmaceutical for noninvasive imaging of integrin αvβ3, which plays a critical role in tumor angiogenesis and metastasis.

Literature Review:
In another study of Hazari et al. BIB001 , bis-MPP ( Figure 2E ) derivative has been synthesized to image serotonin receptors. The duplication of the pharmacophores leads to a supra-additive increase in binding and potency as compared to monovalent analog. Thus, the bis-compound had sub-nanomolar affinity for the receptor, 1000 times more selectivity for 5HT1A as compared to D2, 5-HTT, or 5HT2A. The compound was validated as PET imaging agent. For the imaging of αvβ3, a non-peptidic BL was reported by Wang et al. BIB002 (Figure 2F ). This molecule consisted of two units of antagonist 4-[2-(3,4,5,6-tetrahydropyrimidine-2-lamino)-ethyloxy]benzoyl-2-(S)-aminoethylsulfonyl-amino-h-alanine (IA) and radiolabeled using 18 F-AlF/NODA chelation reaction. NIR imaging probe for αvβ3 (25): Figure 2L was reported for cancer imaging. The non-peptidic small molecule bivalent antagonist demonstrated improved binding avidity relative to the monovalent ligand.