Title: Applying medicinal chemistry principles to the Olfactory Code
Description: The mammalian olfactory system is capable of decoding complex mixtures of volatile chemical odorants into identifiable percepts. While the general mode of peripheral signal transduction is largely known, the mechanism relies on a rather complicated combinatorial “olfactory code”, where each of the hundreds of expressed odorant receptors (ORs) detects multiple odorants, and a given odorant in turn activates multiple ORs (Malnic, Hirono et al. 1999). Since the first identification of mammalian ORs in 1991, the deorphanization, i.e. solving of the substrate, of ORs has proven to be a challenge. Many attempts at systematic monitoring of the olfactory code have seen marginal successes for a number of reasons. First of all, there are still no solved structures of mammalian ORs to be used for high throughput computational modeling. Second, experimental validation methods such as heterologous expression still face considerable challenges. Lastly, primary chemical features of odors that allow for OR tuning are not yet defined.

The traditional organic chemistry-based classification of odorants fails to predict biological activity, while percept-based computational analyses isolate esoteric descriptors that are difficult to chemically manipulate. Receptor level structure-activity analysis can provide a missing context to the odorant discrimination in the peripheral olfactory system. A critical finding by Manic et al (1999) indicates that each mature olfactory sensory neuron (OSN) only expresses one type of OR, allowing for high throughput screening of carefully crafted odorant panels using dissociated OSN calcium imaging. A few bioisosteric substitutions widely utilized in medicinal chemistry were used to construct odorant panels, showing greater success in defining odorant-OR interaction than previously used organic chemistry-based clustering methods. Among classical substitutions used by medicinal chemists, heteroaromatic ring exchanges are especially well tolerated when heteroatoms with a similar topological polar surface area (TPSA) are used as replacements.

Among odorants with differing TPSA, it is likely that an OR activated by analogous odorants at two extremes of the TPSA spectrum will be activated by an odorant with an intermediate TPSA. Flipping of a polar functional group, which is often used with amides in drug target replacements, is well tolerated by the ORs in esters. Furthermore, there is a predictable activation pattern relative to number of carbons in a hydrophobic chain uninterrupted by polar epitopes. Using binary mixtures, the OR activity can be further surveyed through enhancement or inhibition of OSN activation signals. Odorants activating a smaller subset of an OR population may also be binding to a larger subset of ORs, resulting in mixture inhibition. Specifically, this work indicates that extracted odorant fragments may be binding but not activating some of the OR repertoire of the original odorant. The concept of non-classical bioisosteres is applied to the OR repertoire using aliphatic and aromatic aldehydes. It appears that the specialized electronics of a fully conjugated benzene ring can in fact be dispensable, only acting as conformational restrictor of the odorant in most cases.

Not only do analogous non-conjugated systems substitute well for benzaldehyde, but so do non-cyclic odorants possessing tiglic moieties. Conformationally restricted extractions act as more faithful replacements for larger molecules in a subset of ORs. While the dissociated OSN results alone have broad implications for binding patterns of GPCRs in general, simple behavioral tests in mice using the same odorant panels indicate concrete perceptual links to medicinal chemistry-based odorant discrimination. The results from the behavioral data suggest that there may be a maximum constraint for percent OSN activation for two sequentially presented odors to be interpreted as the “same”. The results open a window to exploring other medicinal chemistry-based substitutions. Furthermore, many methodological improvements have been made over the past decade to allow for increased efficiency of deorphanization and validation of ORs.