Synthesis and evaluation of iodinated TZTP-derivatives as potential radioligands for imaging of muscarinic M2 receptors with SPET

Synthesis and in vitro evaluation of novel nortropane derivatives as potential radiotracers for muscarinic m(2) receptors

Disturbances of the cerebral cholinergic neurotransmitter system are present in neurodegenerative disorders. SPECT or PET imaging, using radiotracers that selectively target muscarinic receptor subtypes, may be of value for in vivo evaluation of such conditions. 6β-acetoxynortropane, a potent muscarinic M(2) receptor agonist, has previously demonstrated nanomolar affinity and high selectivity for this receptor. Based on this compound we synthesized four nortropane derivatives that are potentially suitable for SPECT imaging of the M(2) receptor. 6β-acetoxynortropane and the novel derivatives were tested in vitro for affinity to the muscarinic M(1-3) receptors. The original 6β-acetoxynortropane displayed high affinity (K(i) = 70-90 nM) to M(2) receptors and showed good selectivity ratios to the M(1) (65-fold ratio) and the M(3) (70-fold ratio) receptors. All new derivatives showed reduced affinity to the M(2) subtype and loss of subtype selectivity. It is therefore concluded that the newly synthesized derivatives are not suitable for human SPECT imaging of M(2) receptors.

In vivo [(123)I]CNS-1261 binding to D-serine-activated and MK801-blocked NMDA receptors: A storage phosphor imaging study in rats

Disturbances of activity of the glutamatergic neurotransmitter system in the brain are present in many neuropsychiatric disorders. The N-methyl-D-aspartate (NMDA) receptor is the most abundant receptor of the glutamatergic system. In the neurodegenerative events of Alzheimer's disease, excessive activation of NMDA receptors may contribute to neuronal death. Inhibition of NMDA receptor activation may have neuroprotective effects and (semi)quantitative imaging of the activated system may help in the selection of patients for such inhibition therapies. In this study we evaluated [(123)I]CNS-1261 binding in the rat brain. This radiotracer binds in vivo to the MK801 binding site of activated NMDA receptors. To determine the optimal time point for ex vivo assessments after bolus injection [(123)I]CNS-1261 binding in rats, we performed a time course biodistribution study using dissection techniques. [(123)I]CNS-1261 binding was also studied in the rat brain using autoradiography by means of storage phosphor imaging, with prior facilitation of NMDA receptor activation by injection of the potent coagonist D-serine and after blocking of the NMDA receptor binding site by MK801 injection in D-serine pretreated rats. Measurements of [(123)I]CNS-1261 uptake matched the distribution of similar tracers for the MK801 binding site of the NMDA receptor and revealed an optimal time point of 2 h post injection for the assessment of tracer distribution in the rat brain. The blocking experiments indicated specific binding of [(123)I]CNS-1261 to NMDA receptors but also a considerable amount of nonspecific binding. Facilitation of NMDA receptor activation by D-serine did not result in an enhancement of binding of the radiotracer in the NMDA receptor-rich rat hippocampus compared to the untreated group, as measured by autoradiography. In conclusion, our study has shown that [(123)I]CNS-1261 binding is influenced by NMDA receptor availability. However, high nonspecific binding limits quantification and small changes in receptor availability are unlikely to be detected.

Comparison of the pharmacokinetics of different analogs of 11C-labeled TZTP for imaging muscarinic M2 receptors with PET

INTRODUCTION

The only radiotracer available for the selective imaging of muscarinic M2 receptors in vivo is 3-(3-(3-[18F]fluoropropyl)thio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine) ([18F]FP-TZTP). We have prepared and labeled 3-(3-(3-fluoropropylthio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridne (FP-TZTP, 3) and two other TZTP derivatives with 11C at the methylpyridine moiety to explore the potential of using 11C-labeled FP-TZTP for positron emission tomography imaging of M2 receptors and to compare the effect of small structural changes on tracer pharmacokinetics (PK) in brain and peripheral organs.

METHODS

11C-radiolabeled FP-TZTP, 3-(3-propylthio)-TZTP (6) and 3,3,3-(3-(3-trifluoropropyl)-TZTP (10) were prepared, and log D, plasma protein binding (PPB), affinity constants, time-activity curves (TACs), area under the curve (AUC) for arterial plasma, distribution volumes (DV) and pharmacological blockade in baboons were compared.

RESULTS

Values for log D, PPB and affinity constants were similar for 3, 6 and 10. The fraction of parent radiotracer in the plasma was higher and the AUC lower for 10 than for 3 and 6. TACs for brain regions were similar for 3 and 6, which showed PK similar to the 18F tracer, while 10 showed slower uptake and little clearance over 90 min. DVs for 3 and 6 were similar to the 18F tracer but higher for 10. Uptake of the three tracers was significantly reduced by coinjection of unlabeled 3 and 6.

CONCLUSION

Small structural variations on the TZTP structure greatly altered the PK in brain and behavior in blood with little change in the log D, PPB or affinity. The study suggests that 11C-radiolabeled 3 will be a suitable alternative to [18F]FP-TZTP for translational studies in humans.