Absolute quantification by positron emission tomography of the endogenous ligand

The results of several recent papers have shown a significant influence of the endogenous neurotransmitters on the exogenous ligand kinetics measured by positron emission tomography. For example, several groups found that the percentage of D2 receptor sites occupied by the endogenous dopamine ranged from 25% to 40% at basal level. An obvious consequence of this significant occupancy is that the ligand-receptor model parameters, usually estimated by a model that does not rake into account the endogenous ligand (EL) kinet ics, can be significantly biased. In the current work, the authors studied the biases obtained by using the multiinjection approach. The results showe d that in the classical ligand-receptor model, the receptor concentration i s correctly estimated and that only the apparent affinity is biased by not taking the EL into account. At present, all absolute quantifications of the EL have been obtained through pharmacologic manipulation of the endogenous transmitter concentration, which is often too invasive a method to be used in patients. A theoretical reasoning showed that a noninvasive approach is necessarily based on both the apparent affinity measurement and on a multi region approach. The correlation between the receptor concentration and the apparent affinity, previously observed with some ligands, verifies these t wo conditions: thus, the authors suggest that this correlation could be the result of the EL effect. To test this assumption experimentally, the effec t of reserpine-induced dopamine depletion on the interactions between the D 2 receptor sites and the FLB 457 is studied. With untreated baboons, the ap parent FLB 457 affinity was smaller in the receptor-rich regions (striatum) than in the receptor-poor regions. This discrepancy disappeared after dopa mine depletion, strongly suggesting that this affinity difference was relat ed to the EL effect. Therefore, the purpose of the current study was to tes t the ability to quantify the EL based on the observed correlation between the receptor concentration and the apparent affinity. This approach offers a method fur estimating the percentage of receptor sites occupied by the EL and, if its affinity is known, the free EL concentration. From the data ob tained using FLB 457 with baboons, the authors found that approximately 53% of the D2 receptor sites are occupied by dopamine in the striatum and that the free dopamine concentration is approximately 120 nmol/L at basal level . This approach is transferable to patients, because the experimental data are obtained without pharmacologically induced modification of the EL.