Pharmacodynamic assessment of the benztropine analogues AHN-1055 and AHN-2005 using intracerebral microdialysis to evaluate brain dopamine levels and pharmacokinetic/pharmacodynamic modeling

Purpose. The benztropine (BZT) analogues bind with high affinity to the dopamine transporter (DAT) and demonstrate a behavioral and pharmacokinetic profile unlike that of cocaine. The development of a predictive pharmacokinetic/pharmacodynamic (PK/PD) model to characterize the concentration-effect relationship between the BZT analogues and brain dopamine (DA) levels is an important step in the evaluation of these compounds as potential cocaine abuse pharmacotherapies. Hence, the objective of this study was to mathematically characterize the PD of BZT analogues and cocaine, using appropriate PK/PD models. Methods. Dialysis probes were stereotaxically implanted into the nucleus accumbens of Sprague-Dawley rats ( 275 - 300 g). Extracellular fluid (ECF) DA levels were measured after intravenous administration of the BZT analogues AHN-1055 and AHN-2005, as well as cocaine using high performance liquid chromatography-electrochemical detection (HPLC-ECD). PD models were used to describe the relationship between the BZT analogues or cocaine and brain microdialysate DA, and suitability was based on standard goodness-of-fit criteria. Results. The BZT analogues produced a sustained increase in brain microdialysate DA levels in comparison to cocaine. The time of maximum concentration (T-max) for brain microdialysate DA was 2 h for AHN-1055 and 1 h for AHN-2005 compared to a Tmax of 10 min for cocaine. The duration of brain microdialysate DA elevation was similar to 12 - 24 h for the BZTs in comparison to 1 h for cocaine. An indirect model with inhibition of loss of response and a sigmoid E-max model best described the PK/PD for the BZT analogues and cocaine, respectively. The 50% of maximum inhibition (IC50) of the loss of DA was lower for AHN-2005 ( 226 +/- 27.5 ng/ml) compared to AHN-1055 ( 321 +/- 19.7 ng/ml). In addition, the EC50 for cocaine was 215 +/- 11.2 ng/ml. Conclusions. The slow onset and long duration of BZT analogue - induced DA elevation may avoid the reinforcing effects and craving of cocaine. Further, the developed models will be useful in characterizing the PK/PD of other analogues and aid in the assessment of the therapeutic efficacy of the BZT analogues as substitute medications for cocaine abuse.