Small Molecule Radiopharmaceuticals – A Review of Current Approaches <s> Applications of BLA <s> Neurotransmitter receptors (neuroreceptors) are classified into two types, G protein-coupled receptors (GPCRs) and ligand-gated ion channels. The former occupies a small part of the large GPCR superfamily, whereas the latter consists of three superfamilies. In these superfamilies, humans and rodents share almost the same set of neuroreceptor genes. This neuroreceptor gene set is good material to examine the degree of selective constraint exerted on each member gene of a given superfamily. If there are any neuroreceptor genes under the degree of selective constraint that is very different from that of the other member genes, they may have influenced the functional features characteristic of human neural activities. With the aim of identifying such neuroreceptor genes, we collected sequence data of orthologous neuroreceptor genes for humans, mice, and rats by database searches. This data set included ortholog pairs for 141 kinds of neuroreceptor genes, which covered almost the whole set of neuroreceptor genes known to be expressed in the human brain. The degree of selective constraint was estimated by computing the ratio (d(N)/d(S)) of the number of nonsynonymous substitutions to that of synonymous substitutions. We found that the d(N)/d(S) ratio ranged widely and its distribution fitted a gamma distribution. In particular, we found that four neuroreceptor genes are under the significantly relaxed selective constraint. They are an ionotropic glutamate receptor subunit NMDA-2C, two GABA(A) receptor subunits, i.e., GABA(A)-epsilon and GABA(A)-theta, and a dopamine receptor D4. Interestingly, these neuroreceptors have been reported to be associated with cognitive abilities such as memory and learning, and responsiveness to novel stimuli. These cognitive abilities can influence the behavioral features of an individual. Thus, it suggests that the relaxed-constraint neuroreceptor genes have evolved, assuring that the nervous system responds to a variety of stimuli with proper flexibility. <s> BIB001 </s> Small Molecule Radiopharmaceuticals – A Review of Current Approaches <s> Applications of BLA <s> Dopamine D2 receptor homodimers might be of particular importance in the pathophysiology of schizophrenia and, thus, serve as promising target proteins for the discovery of atypical antipsychotics. A highly attractive approach to investigate and control GPCR dimerization may be provided by the exploration and characterization of bivalent ligands, which can act as molecular probes simultaneously binding two adjacent binding sites of a dimer. The synthesis of bivalent dopamine D2 receptor ligands of type 1 is presented, incorporating the privileged structure of 1,4-disubstituted aromatic piperidines/piperazines (1,4-DAPs) and triazolyl-linked spacer elements. Radioligand binding studies provided diagnostic insights when Hill slopes close to two for bivalent ligands with particular spacer lengths and a comparative analysis with respective monovalent control ligands and unsymmetrically substituted analogues indicated a bivalent binding mode with a simultaneous occupancy of two neighboring binding sites. <s> BIB002 </s> Small Molecule Radiopharmaceuticals – A Review of Current Approaches <s> Applications of BLA <s> G-protein-coupled receptors (GPCRs) represent the largest family of membrane proteins involved in cellular signal transduction and are activated by various different ligand types including photons, peptides, proteins, but also small molecules like biogenic amines. Therefore, GPCRs are involved in diverse physiological processes and provide valuable drug targets for numerous diseases. Emerging body of evidence suggests that GPCRs exist as monomers or cross-react forming dimers and higher-ordered oligomers. In this Perspective we will review current biochemical and biophysical techniques to visualize GPCR dimerization, functional consequences of homo- and heterodimers, and approaches of medicinal chemists to target these receptor complexes with homo- and heterobivalent ligands. <s> BIB003
A BL functions best when multiple binding pockets are present in the target. Depending on the pharmacophores, a BL can target one or multiple biomarkers. Tumor targeting can benefit from the high binding avidity and selectivity of BL. Furthermore, hetero-BL can result in more specificity as it targets different receptors simultaneously. Receptor-based imaging, especially for neuroreceptors, can also benefit from the bivalent approach. Many receptors/ neuroreceptors belong to G-protein coupled receptor (GPCR) family BIB001 . After the reports about the existence of GPCRs as oligomers and higher-orders started pouring (11), BLs were successfully developed and validated against them. The approach has been of high relevance in the design and development of second generation antipsychotics BIB002 BIB003 . A BL can target both homo-and hetero-dimeric receptor systems depending on the pharmacophores. Another target for BLs is β-amyloid plaques because of the presence of multiple binding sites (14) .