Pharmacology of amphetamine-type designer drugs

In this thesis project, we studied the pharmacology of long-known psychostimulants and of novel amphetamine-type designer drugs in vitro. On the basis of the pharmacological characterization, we focused on the drug effects in humans, investigating the pharmacological effects of the widely used amphetamine derivative 3,4-�methylenedioxymethamphetamine (MDMA, ÒEcstasyÓ). Furthermore, we investigated potential antagonistic treatments for methamphetamine dependence.
In vitro, we characterized the pharmacology of a comprehensive set of drugs, determining their action at targets found in monoaminergic synapses. We used HEK�293 cells transfected with the respective genes for the three monoamine reuptake transporters and post-synaptic receptors. We performed pharmacological assays with radiolabeled substrates or ligands to determine drugsÕ transport modulation properties and receptor affinities. We classified novel designer drugs that were cathinone-derivatives according to their pharmacology and discriminated between three groups. Cocaine-MDMA-mixed cathinones were drugs that inhibited the dopamine (DA) and serotonin (5-HT) uptake with equal potencies to cocaine, but preferentially released transporter-mediated 5-HT like MDMA. Methamphetamine-like cathinones inhibited DA uptake preferentially over 5-HT uptake and released DA, comparable to methamphetamine. Pyrovalerone-cathinones were extremely potent DA uptake inhibitors, but non-releasers. Additionally, we found methedrone as only serotonergic cathinone, similar to MDMA and other para�methoxylated amphetamines. We described key findings upon blood-brain-barrier characteristics and differences between beta-ketonated and non�-beta-�ketonated amphetamines in receptor binding and DA/5-HT preferences. All amphetamine-type drugs inhibited norepinephrine (NE) uptake, which indicated the importance of the norepinephrine system as contributor to the effects of psychostimulants.
In humans, we assessed the role of NE and 5-HT in the mechanism of action of MDMA in two placebo-controlled, double-blind, crossover studies. To elucidate the contribution of the respective monoamines, we attenuated the MDMA-induced psychotropic and cardiovascular effects with the NE-transporter (NET) inhibitor reboxetine and also with the mixed 5-HT transporter/NET inhibitor duloxetine. We found that 5-HT primarily mediates empathogenic mood effects whereas NE is primarily involved in the MDMAÕs cardiostimulant effects. PK-PD relationships combining subjective- or cardiostimulant effects with MDMAÕs pharmacokinetics in relation to time depicted the tolerance of the drugÕs effects. Predicting drug effects with in vitro 5-�HT and NE release characteristics, we highlighted the role of NE in MDMAÕs mode of action.
We also showed that 5-HT and/or NE play a role in the MDMA-induced inappropriate secretion of antidiuretic hormone, which is involved in the development of potentially serious complications such as brain edema evolving from hyponatremia. In one of the aforesaid clinical studies, duloxetine attenuated the MDMA-induced increase of antidiuretic hormone, which was more distinctive in female than male subjects.
Finally, we addressed our research to the possibilities of pharmacological modulation of amphetamine-type drug dependence by antagonizing DA-related effects. We assessed in vitro the potencies of three DA transporter inhibitors to block methamphetamine-induced DA release, which is a known mechanism for drug dependence. The DA uptake inhibition potencies of the three assessed drugs correlated linearly with their potencies to inhibit methamphetamine-induced DA release. The strong action on the DA system and the associated abuse potential of the most potent release blocker, 3,4-�methylenedioxypyrovalerone, depicts the difficulty to find unproblematic pharmacological treatment for drug dependence.
Taken together, we characterized 5-HT, NE, and DA in amphetamine-type drug action in vitro and also illustrated their relevance for drugsÕ effects in humans.