Half-life of receptor occupancy--a meaningless concept

AZD5213: a novel histamine H3 receptor antagonist permitting high daytime and low nocturnal H3 receptor occupancy, a PET study in human subjects

The histamine H3 receptor represents an appealing central nervous system drug target due to its important role in the neurobiology of cognition and wake-sleep regulation. The therapeutic benefit of H3 antagonists/inverse agonists may be hampered by disruption of sleep that has been observed in humans with prolonged high H3 receptor occupancy (H3RO), extending into night-time. AZD5213 is a highly selective H3 antagonist (in vitro inverse agonist) developed to achieve a pharmacokinetic profile permitting circadian fluctuations of H3RO. Its efficacy has been demonstrated in rodent behavioural models of cognition. In human subjects, AZD5213 was safe and well tolerated following repeated doses (1-14 mg/d) and demonstrated a short (∼5 h) half-life. In this PET study H3RO was measured using the radioligand [11C]GSK189254 ([11C]AZ12807110) in seven young male volunteers following single doses of AZD5213 (0.05-30 mg). H3RO was calculated using the Lassen plot method. The plasma concentrations and the affinity constant (K i,pl 1.14 nmol/l, corresponding to the plasma concentration required to occupy 50% of available receptors) were used to estimate the H3RO time-course. AZD5213 showed dose and concentration dependent H3RO ranging from 16 to 90%. These binding characteristics and the pharmacokinetic profile of AZD5213 indicate that high daytime and low night-time H3RO could be achieved following once daily oral dosing of AZD5213. Fluctuations of H3RO following circadian rhythm of the histamine system may be expected to reduce the risk of sleep disruption while maintaining daytime efficacy. AZD5213 may thus be an optimal compound to evaluate the clinical benefit of selective H3 antagonism in cognitive disorders.

Mechanisms of action of antipsychotic drugs of different classes, refractoriness to therapeutic effects of classical neuroleptics, and individual variation in sensitivity to their actions: Part I

Many issues remain unresolved about antipsychotic drugs. Their therapeutic potency scales with affinity for dopamine D2 receptors, but there are indications that they act indirectly, with dopamine D1 receptors (and others) as possible ultimate targets. Classical neuroleptic drugs disinhibit striatal cholinergic interneurones and increase acetyl choline release. Their effects may then depend on stimulation of muscarinic receptors on principle striatal neurones (M4 receptors, with reduction of cAMP formation, for therapeutic effects; M1 receptors for motor side effects). Many psychotic patients do not benefit from neuroleptic drugs, or develop resistance to them during prolonged treatment, but respond well to clozapine. For patients who do respond, there is a wide (>ten-fold) range in optimal doses. Refractoriness or low sensitivity to antipsychotic effects (and other pathologies) could then arise from low density of cholinergic interneurones. Clozapine probably owes its special actions to direct stimulation of M4 receptors, a mechanism available when indirect action is lost.

The plasma-occupancy relationship of the novel GABAA receptor benzodiazepine site ligand, alpha5IA, is similar in rats and primates

BACKGROUND AND PURPOSE

alpha5IA (3-(5-methylisoxazol-3-yl)-6-[(1-methyl-1,2,3-triazol-4-yl)methyloxy]-1,2,4-triazolo[3,4-a]phthalazine) is a triazolophthalazine with subnanomolar affinity for alpha1-, alpha2-, alpha3- and alpha5-containing GABA(A) receptors. Here we have evaluated the relationship between plasma alpha5IA concentrations and benzodiazepine binding site occupancy in rodents and primates (rhesus monkey).

EXPERIMENTAL APPROACH

In awake rats, occupancy was measured at various times after oral dosing with alpha5IA (0.03-30 mgxkg(-1)) using an in vivo {[(3)H]flumazenil (8-fluoro 5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester)} binding assay. In anaesthetized rhesus monkeys, occupancy was measured using {[(123)I]iomazenil (ethyl 5,6-dihydro-7-iodo-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester)} gamma-scintigraphy and a bolus/infusion paradigm. In both rat and rhesus monkey, the plasma drug concentration corresponding to 50% occupancy (EC(50)) was calculated.

KEY RESULTS

In rats, alpha5IA occupancy was dose- and time-dependent with maximum occupancy occurring within the first 2 h. However, rat plasma EC(50) was time-independent, ranging from 42 to 67 ngxmL(-1) over a 24 h time course with the average being 52 ngxmL(-1) (i.e. occupancy decreased as plasma drug concentrations fell). In rhesus monkeys, the EC(50) for alpha5IA displacing steady-state [(123)I]iomazenil binding was 57 ngxmL(-1).

CONCLUSIONS AND IMPLICATIONS

Rat plasma EC(50) values did not vary as a function of time indicating that alpha5IA dissociates readily for the GABA(A) receptor in vivo. These data also suggest that despite the different assays used (terminal assays of [(3)H]flumazenil in vivo binding in rats and [(123)I]iomazenil gamma-scintigraphy in anaesthetized rhesus monkeys), these techniques produced similar plasma alpha5IA EC(50) values (52 and 57 ngxmL(-1) respectively) and that the plasma-occupancy relationship for alpha5IA translates across these two species.

The role of emission tomography in pharmacokinetic and pharmacodynamic studies in clinical psychopharmacology

Position Emission Tomography (PET) and Single Photon Emission Tomography (SPECT) can be used for both pharmacokinetic and pharmacodynamic measures in vivo in man. As such they have a wide range of applications including description of neurochemical changes in disease, occupancy, brain effects of medicines and discovery and validation of biomarkers. The power of these tools is in their chemical specificity and sensitivity, and in the ability to describe processes in vivo, thus documenting the effects of genetic and environmental interactions. The future of these technologies is dependent on an investment in bringing out and validating new radiotracers.