Competition experiments with [3 H]mepyramine showed that cetirizine and its enantiomers, levocetirizine and (S)-cetirizine, bound with high affinity and stereoselectivity to human H 1 histamine receptors (K i values of 6, 3, and 100 nM, respectively). Cetirizine and levocetirizine were 600-fold more selective for H 1 receptors compared with a panel of receptors and channels. Binding results indicated that the interaction between cetirizine, its enantiomers, and histamine is compatible with a competitive behavior, in contrast with the noncompetitive profile of cetirizine and levocetirizine observed in isolated organs. Binding kinetics provided a suitable explanation for this observation, because levocetirizine dissociated from H 1 receptors with a half-time of 142 min; that of (S)-cetirizine was only 6 min, implying that the former could act as a pseudo-irreversible antagonist in functional studies. The carboxylic function of levocetirizine seemed responsible for its long dissociation time. Indeed, hydroxyl or methyl ester analogs dissociated more rapidly from H 1 receptors, with half-times of 31 min and 7 min, respectively. The importance of the carboxylic function of levocetirizine for the interaction with the H 1 receptor was further supported by the results from the mutation of Lys 191 to Ala 191 . This mutation decreased the dissociation half-time of levocetirizine from 142 to 13 min and reduced its affinity from 3 to 12 nM, whereas the affinity and dissociation kinetics of hydroxyl and methyl ester analogs were hardly affected. The mutation of Thr 194 reduced the binding stereoselectivity by selectively enhancing the affinity of the distomer.

BACKGROUND AND PURPOSEAvailable medications for chronic pain provide only partial relief and often cause unacceptable side effects. There is therefore a need for novel molecular targets to develop new therapeutics with improved efficacy and tolerability. Despite encouraging efficacy data in rodents with inhibitors of the neuronal glycine transporter-2 (GlyT2), there are also some reports of toxicity and their development was discontinued. EXPERIMENTAL APPROACHIn order to clarify the possibility of targeting GlyT2 for the treatment of pain, we have used an integrated approach comprising in vitro pharmacology, selectivity, bioavailability, in vivo efficacy and safety assessment to analyse the properties and efficacy of ALX-1393 and Org-25543, the two published GlyT2 inhibitors from which in vivo data are available. KEY RESULTSWe report that these compounds have a different set of undesirable properties that limit their usefulness as pharmacological tools. Importantly, we discover that inhibitors of GlyT2 can exert an apparent reversible or irreversible inhibition of the transporter and describe a new class of reversible GlyT2 inhibitors that preserves efficacy while avoiding acute toxicity. CONCLUSIONS AND IMPLICATIONSOur pharmacological comparison of two closely related GlyT2 inhibitors with different modes of inhibition provides important insights into their safety and efficacy profiles, uncovering that in the presence of a GlyT2 mechanism-based toxicity, reversible inhibitors might allow a tolerable balance between efficacy and toxicity. These findings shed light into the drawbacks associated with the early GlyT2 inhibitors and describe a new mechanism that might serve as the starting point for new drug development.

The diverse drug binding methods employed here are well suited to detect specific binding of LCM to CRMP-2 in the micromolar range, yet the results obtained were all negative. Results of this study suggest that LCM does not specifically bind to CRMP-2.

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