John A. Lawson

Structure-activity studies of β-carboline analogs

Abstract A number of β-carboline analogs have been obtained or synthesized, and their in vitro receptor affinities and in vivo antagonist activities determined. The choice of analogs was made in order to explore the importance of the N9-H, the aromatic nitrogen and the C3-ester moiety for high-receptor affinity and antagonist activity of this class of benzodiazepine antagonist. Among the analogs investigated, we describe the properties of 3-cyano-β-carboline ( lh ), the first potent β-carboline antagonist without a carbonyl at the C3-position. The results obtained indicate: (1) Specific interactions of the C3-substituent with key cationic receptor sites rather than electron-withdrawing properties are important for high-receptor affinity and antagonist activity. (2) Specific in-plane interactions of the atomatic nitrogen with a cationic receptor site, rather than stacking with neutral aromatic residues of the receptor are also important for high affinity and antagonist activity. (3) While the presence of an N9H enhances receptor affinity, interaction with an anionic receptor site does not appear essential for antagonist activity.

Pyrazolo[1,5-a]pyrimidines: Receptor binding and anxiolytic behavioral studies

Pyrazolo[1,5-a]pyrimidines (PZP) have been reported to be specific anxiolytic agents which do not potentiate ethanol or barbiturates. To further investigate these compounds, three of the most promising analogs were synthesized and a tritium-labeled analog of one of them prepared by a new synthetic procedure. These analogs did not compete with [3H]flunitrazepam or [3H]beta-carboline ethyl ester binding nor did they potentiate the [3H]flunitrazepam binding. Receptor binding studies with the [3H]PZP revealed a low affinity receptor site, distinct from that of the benzodiazepines, but with only a small fraction (20%) of specific binding. Behavioral tests using three different animal models for anxiety: muricide, approach/avoidance conflict and two-chamber exploration tests gave conflicting results, positive in the first and negative in the latter two. Furthermore, these compounds were found not to be antagonists of diazepams anticonvulsant activity. Taken together, these results, while provocative, do not support evidence that these analogs are promising specific anxiolytic agents.

In Vitro and In Vivo Profile of PPL-101 and PPL-103: Mixed Opioid Partial Agonist Analgesics with Low Abuse Potential

Opiates are still the most effective and widely used treatments for acute and chronic pain. However, the problems associated with morphine and other standard opioid analgesics severely limit their effectiveness in the clinic. PPL-101 and PPL-103 derived from morphine and morphinan ring systems contain a chiral N-substituent, which confers it with a unique combination of high-binding affinities and partial agonist activities at mu, delta, and kappa opioid receptors, leading to unique in vivo pharmacology compared to other conventional opioids. Acute antinociceptive and reward acquisition of PPL-101 and PPL-103 were assessed in mice using the tail flick assay and conditioned place preference (CPP) paradigm, respectively. The reinforcing effects of these compounds were assessed in rats using the self-administration paradigm. In mice, PPL-101 and PPL-103 produced antinociception reaching maximal effects that were equivalent to morphine at approximately 1/3 and 1/10 of morphine’s dose, respectively. PPL-101-induced antinociception was attenuated following pretreatment with the kappa antagonist JDTic, but not the mu opioid antagonist beta-FNA. In mice, PPL-101 and PPL-103 produced dose-dependent decreases in activity, similar to other kappa agonists; however, they did not produce conditioned place aversion, and in fact elicited a trend toward CPP. In rats, neither PPL-101 nor PPL-103 were self-administered when substituted for morphine and PPL-101 attenuated morphine self-administration, when administered systemically prior to the self-administration session. Collectively, these results indicate that mixed opioid receptor partial agonists can produce potent antinociceptive activity with a lack of aversion in mice and without being self-administered in rats. Compounds with this profile could be superior analgesics with greatly reduced addiction liability and fewer side-effects compared to traditional opiates.

Structure-activity studies of morphine fragments. III. Synthesis, opiate receptor binding, analgetic activity and conformational studies of spiro-[tetralin-1,4′-piperidines]

Abstract A series of 5 spiro compounds, a new class of conformationally restricted analogs of 4-alkyl-4-(m-OH-phenyl) piperidines, have been synthesized and their affinities for μ, δ and κ opioid receptor sites and in vivo analgetic activities determined. All compounds show rather low affinities for the 3 receptors, with some modulation by the N-substituent and by the position of the phenolic group. To help understand the origin of this poor affinity compared to the unrestricted 4-alkyl-4-phenyl piperidines, energy conformation calculations were performed which indicated that all the analogs favor a phenyl equatorial over a phenyl axial conformer. Significant differences in the lowest energy conformation were found between these spiro analogs and both morphine and 4-n-propyl-4-(m-OH-phenyl) piperidines, 18 and 19 which are conformationally unrestricted, closely related analogs with high μ-affinity. These differences could account for their lower affinities. To continue the search for more active members of the family, structure variations which favor a phenyl-axial conformation have been identified and proposed for further study.

Structure-activity studies of morphine fragments. II. Synthesis, opiate receptor binding, analgetic activity and conformational studies of 2-R-2(hydroxybenzyl)piperidines

Abstract In this study, a series of 2-benzyl piperidines, that can be regarded as flexible fragments of fused ring opioids, have been synthesized and their pharmacological and conformational profiles determined. These combined studies reveal that, despite the weak activity of the only analog previously reported, modifications of it can lead to compounds with significant opioid receptor affinity and analgetic activity. Conformational studies of these compounds indicate that they can bind to these receptors in either an phenyl-axial and phenyl-equatorial conformer. Features such as the position of the phenolic OH group, the nature of the other 2-substituent and of the N-substituent appear to modulate receptor recognition and activation. However, the 2-benzyl piperidines do not appear to bind or act at the opioid receptors in the same conformation or orientation as their more rigid fused ring counterparts, the benzomorphans. In general, a change from p-OH to m-OH benzyl analogs reduces efficacy and its is possible that the m-OH analogs could be promising analgesics with low physical dependence liability.