Imaging the serotonin transporter during major depressive disorder and antidepressant treatment.(psychiatric research)(Introduction also in French)(includes statistical tables)

2005 CCNP Young Investigator Award Paper

This paper focuses on serotonin transporter 5-HTT imaging to investigate major depressive disorder (MDD) and antidepressant occupancy. Such investigations have only recently been possible as a result of major advances in ligand development. The state of the art method is [[sup.11]C] DASB PET or [[sup.11]C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile) positron emission tomography. [[sup.11]C]DASB is a breakthrough for brain imaging 5-HTT. Compared with previous radioligands, [[sup.11]C]DASB offers both high selectivity and a favourable ratio of specific binding relative to free and nonspecific binding. These characteristics contribute to valid, reliable quantitation of the 5-HTT binding potential (BP). The 5-HTT BP can be viewed as an index of 5-HTT density in a medication free state, or unblocked 5-HTT density in a medication-treated state.During major depressive episodes with no other axis I comorbidity, either no difference in regional 5-HTT BP or a trend toward elevated 5-HTT BP is typically found. During major depressive episodes (of MDD) with more severe symptoms of pessimism (dysfunctional attitudes), regional 5-HTT BP is elevated. In subjects with major depressive episodes and comorbid axis I psychiatric illnesses, decreased regional 5-HTT BP is often reported. With selective serotonin reuptake inhibitor (SSRI) treatment at doses that distinguish from placebo in the treatment of major depressive episodes, 5-HTT occupancy is approximately 80%, and there is a strong relation between plasma level and occupancy that is not predictable based on affinity alone. Implications of 5-HTT imaging findings for understanding major depressive disorder and antidepressant treatment will be discussed.

Cet article porte sur l'utilisation de l'imagerie de la 5-HTT, transporteur de la serotonine, pour etudier le trouble depressif majeur (TDM) et l'occupation des recepteurs d'antidepresseurs. Une telle etude n'a que recemment ete rendue possible par les grands progres dans le domaine des ligands. La methode de pointe dans ce domaine est la tomographie par emission de positons avec le [[sup.11]C]DASB ou [[sup.11]C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)- benzonitrile). Le [[sup.11]C]DASB represente une percee pour l'imagerie cerebrale de la 5-HTT. Comparativement aux radioligands du passe, il se caracterise tant par une haute selectivite que par un rapport favorable entre liaison specifique et libre liaison non specifique. Ce sont ces caracteristiques qui contribuent a une quantification valable et sure du potentiel de fixation (PF) de la 5-HTT. On peut voir dans ce potentiel de fixation un indice de la densite de la 5-HTT dans un etat de nonmedication ou de sa densite sans blocage dans un etat de medication. Dans les episodes depressifs majeurs sans autre comorbidite axe I, il n'y a habituellement aucune difference de PF regional ou on constate ordinairement une tendance a l'elevation de ce potentiel. Dans les episodes depressifs majeurs (de TDM) avec des symptomes aggraves de pessimisme (attitudes dysfonctionnelles), le potentiel de fixation regional de la 5-HTT. Chez les sujets en proie a de tels episodes depressifs avec troubles psychiatriques de comorbidite axe I, on signale souvent une diminution du potentiel regional. Dans le traitement d'episodes depressifs majeurs a l'inhibiteur specifique du recaptage de la serotonine (ISRS) a des doses distinctes du placebo, l'occupation des recepteurs de la 5-HTT est d'environ 80 %; on note une etroite relation entre la concentration plasmatique et l'occupation qui ne saurait etre prevue uniquement par l'affinite. Il sera question des consequences de ces donnees d'imagerie de la 5-HTT sur le plan de la comprehension du trouble depressif majeur et de son traitement aux antidepresseurs.

What properties of the serotonin transporter are important for major depressive disorder?

The serotonin transporter 5-hydroxy-tryptamine (5-HTT) is a 630 amino acid long receptor with 12 transmembrane domains. (1,2) The human 5-HTT gene is localized on chromosome 17, centred at 17q11.2. (3) Most 5-HTT are located at outer cell membranes, either perisynaptically or along axons. (4) In the human brain, the density of 5-HTT varies by region: Superior and inferior raphe nuclei > hypothalamus > thalamus (depending on the nucleus)--amygdala > putamen > caudate ~ hippocampus > insular cortex > prefrontal cortex > white matter > cerebellar cortex (except vermis). (5-7)

The serotonin transporter is coupled to sodium, chlorine and potassium transport. (3) However, the physiological role of interest of 5-HTT in major depressive disorder (MDD) and antidepressant treatment is its influence on extracellular serotonin levels. It is clear that many antidepressant drugs that bind to the serotonin transporter raise extracellular serotonin, and 5-HTT knockout mice have elevated extracellular serotonin, confirming the role of the serotonin transporter in modulating extracellular serotonin levels in vivo. (8-14)

Methods of imaging serotonin transporter in vivo

The following is a critical comparison of all 5-HTT imaging methods that have been applied in humans, with an emphasis on data relevant to humans (See Table 1). Previous comparisons have largely emphasized comparisons in baboons. (15) Although this information is valuable during radiotracer development, (16) it does not fully correspond to radioligand performance during human brain imaging because 5-HTT density can vary between animal species, (7) and the brain pharmacokinetics of 5-HTT radiotracers can differ between baboons and humans. (15,17-21)

These methods are used to derive the binding potential (BP). There are different versions of the BP but the one that is typically used is defined as follows: BP = [f.sub.2] x [B.sub.max]/[K.sub.d] . [f.sub.2] is a fraction of free and nonspecific radiotracer that interacts with the specific binding compartment. [B.sub.max] is receptor density and [K.sub.d] is the dissociation constant. BP tends to be viewed as an index of [B.sub.max] and, in the medication treated condition, it tends to be viewed as an index of receptor density not blocked by medication.

In the medication-treated state, a related measure is the 5-HTT occupancy, which can be defined as 5-HTT occupancy = (5-HTT [BP.sub.1]-5-HTT [BP.sub.2])/ 5-HTT [BP.sub.1] x 100%. 5-HTT [BP.sub.1] is the BP found in the untreated state and 5-HTT [BP.sub.2] is the BP found in the treated state.

[[sup.123]I]2-[beta]-carbomethoxy-3-[beta]-(4-iodophenyl)-tropane ([beta]-CIT) single photon emission tomography (SPECT) was once the only technique developed for measuring the 5-HTT binding potential in humans. (37,38,44) This radiotracer has almost equal affinity for the dopamine transporter, compared with the serotonin transporter. (22,23) Because dopamine transporter density is high in the substantia nigra, (45) one cannot determine whether any changes in specific binding in the midbrain in an experimental paradigm are due to 5-HTT binding in superior raphe nuclei or dopamine transporter binding in substantia nigra. That there are specific binding sites that are not 5-HTT is consistent with the low 5-HTT occupancy estimates for selective serotonin reuptake inhibitors found with this method, (30,31) compared with 5-HTT occupancy estimates with selective 5-HTT binding radiotracers. (34,35) To the best of my knowledge, there are no reliability estimates of binding potential found in the midbrain with this method. Typically, this radiotracer is used for measuring dopamine transporter BP in the striatum in humans. (44)

The PET radiotracer [[sup.11]C](+)McN5652 (trans-1,2,3,5,6,10-[beta]-hexahydro-6-[4-(methylthio)phenyl]-pyrrolo -[2,1-a]-isoquioline) shows greater selectivity for the serotonin transporter, compared with other monoamine transporters. It is estimated that this radiotracer has 1 or 2 orders of magnitude greater affinity for the serotonin transporter over the norepinephrine transporter and at least 2 orders of magnitude greater affinity for the serotonin transporter over the dopamine transporter. (24,25) [[sup.11]C](+)McN5652 has a low ratio of specific binding relative to free and nonspecific binding, which combined with modest reversibility, makes valid and reliable quantitation difficult in regions other than the thalamus, and impossible in the human cortex. (18,19,32,39) Applications of this radiotracer in illness and in treatment have mostly focused on the thalamus, using the cerebellum as a reference region with noninvasive models. (33,39,46) However, some investigators use arterial sampling to measure 5-HTT BP in other subcortical brain regions to obtain a total distribution volume (an index of total radiotracer binding) and use the cerebellar cortex region to obtain an index of free and nonspecific binding. (32)

The radiotracer [[sup.11]C] 3-amino-4-(2-dimethylaminomethylphenylsulfanyl)-benzonitrile (DASB) was a major advance because of its selectivity, reversibility, greater specific binding relative to free and nonspecific binding and reliability. (20,21,26,27,34,35,40,43,47) This radiotracer was found to be 3 orders of magnitude more selective for the 5-HTT than for the monoamine transporters and was highly selective for the 5-HTT, compared with several other screened targets. (26,27) Moreover, 92% to 95% of the specific binding to 5-HTT is displaceable by 5-HTT binding medications in animal models. (26,27) In humans, [[sup.11]C]DASB has good brain uptake (20,40); its ratio of specific binding relative to free and nonspecific binding is good and the latter has low between-subject variability. (20,21) Multiple brain regions may be assessed with noninvasive methods, (20,21,26,27,34,35,40,43,47) and the reliability of regional 5-HTT BP measures is good. (34,35,43,48) The 5-HTT BP measures are low in the cortex, but with standardized region of interest methods, good reliability of 5-HTT BP in the human cortex may be obtained. (34,35,43,48) In summary, [[sup.11]C]DASB PET imaging is the state of the art method in quantifying 5-HTT in humans.

[[sup.123]I] ADAM (2-((2-((dimethylamino)methyl)-phenyl) thio)-5-iodophenylamine) SPECT is a fourth brain imaging method that has recently been applied to investigate 5-HTT BP in humans. It has a clear advantage of selectivity over [[sup.123]I] [beta]-CIT SPECT, since most of the specific binding in most brain regions is displaceable in animal models, and it is selective for the 5-HTT over several other binding sites, including other monoamine transporters. (28,29) [[sup.123]I] ADAM has been modelled in baboons but not yet in humans. (49) The specific binding relative to free and nonspecific binding in humans is not optimal, (41) likely limiting the use of this method to assessing midbrain 5-HTT BP. However, reliability in the midbrain for 5-HTT BP measurement is good. (41)

[[sup.11]C]MADAM ([sup.11]C-N,N-Dimethyl-2-(2-amino-4 methyl-phenylthio) benzylamine) is a recently developed PET radiotracer that shows excellent selectivity over other monoamine transporters in vitro and good displacability in animal models. (50,51) Time activity curves presented show good reversibility potentially similar to [[sup.11]C]DASB but appear to have somewhat greater variability, particularly for the raphe.20,52,53 Initial reports of reliability are also promising, although the scatter in repeated-measurement (standard deviation of percent difference in repeated-measure) appears greater than what has been reported for [[sup.11]C]DASB. (34,35,43,48,52,54)

What is the optimal method of applying [[sup.11]C]DASB PET for research protocols?

For selecting regions of interest, my group recommends automated region of interest approaches with visual validation, such as those involving subroutines from linear transformations and/or nonlinear deformations applied in the spatial normalization procedure from statistical parametric mapping. (55,56) Reliability of 5-HTT BP measurement is typically excellent when such applications are applied. (35,43,54,57) For subcortical regions, manual drawing upon coregistered MRI also has excellent reliability. (34)

For a reference region, my group recommends selecting the posterior half of the cerebellar cortex, excluding vermis, excluding white matter and keeping at least one full width half the maximum from the venous sinuses and from occipital cortex. At a distance of one full width half maximum, spillover from the occipital cortex (which possesses specific binding) or venous sinuses is negligible. White matter is excluded because [[sup.11]C]DASB has different kinetics in this tissue, compared with grey matter. The vermis is excluded because it has [[sup.11]C]DASB kinetics compatible with significant specific binding. We routinely use these methods. (34,35,43,58,59)

For selecting models for region of interest methods, we endorse reference tissue approaches. (20,21,34,35,43,58,59) By applying a linear regression between 5-HTT density and total distribution volume, we estimate that the reference tissue of posterior cerebellar cortex is composed of 93% free and nonspecific binding and 7% specific binding. (6) Knowing that the true BP = distribution volume of specific binding in region of interest divided by the distribution volume of free and nonspecific binding in the cerebellar cortex, the effect of specific binding in the cerebellar cortex is quite subtle. Disease influences of even 50% magnitude on the specific binding in reference tissue translate to 3.5% changes in the distribution volume estimate of free and nonspecific binding, which ultimately results in a 3.5% bias for between-group comparisons. For occupancy studies, the nature of the occupancy equation is such that the bias from a 7% underestimate during untreated conditions is translated into a lesser bias in the overall occupancy measure (less than 2%). For example, if the striatal 5-HTT BP has a true value of 1 in the untreated condition and 0.2 in the SSRI-treated condition, the true 5-HTT occupancy is ([1-0.2]/1) = 0.8 or 80% (5-HTT occupancy = (5-HTT [BP.sub.1]-5-HTT [BP.sub.2])/ 5-HTT [BP.sub.1] x 100%). Taking into account the slight specific binding of reference tissue, the measured striatal 5-HTT BP, respectively, would be 0.93 in the untreated condition and 0.197 in the SSRI-treated condition (most of the 7% specific binding in reference tissue is blocked during treatment), leading to a measured 5-HTT occupancy of ([0.93-0.1972]/0.93) = 0.79 or 79%.

For [[sup.11]C]DASB PET, arterial methods offer no advantage for identifying subcompartments of free and nonspecific binding, because [[sup.11]C]DASB kinetics fit a single tissue compartment model in all regions. (21,60) Arterial methods do permit measurement of total distribution volume in the cerebellum, but this value is assumed to represent free and nonspecific binding, so as to quantitate binding potential measures in other regions. Thus, when arterial sampling is done, a very similar set of assumptions as compared with reference tissue models are applied.

Among the reference tissue methods, the noninvasive logan, (61) simplified reference…