Identifying individual differences of fluoxetine response in juvenile rhesus monkeys by metabolite profiling

Abstract

Fluoxetine is the only psychopharmacological agent approved for depression by the US Food and Drug Administration for children and is commonly used therapeutically in a variety of neurodevelopmental disorders. Therapeutic response shows high individual variability, and severe side effects have been observed. In the current study we set out to identify biomarkers of response to fluoxetine as well as biomarkers that correlate with impulsivity, a measure of reward delay behavior and potential side effect of the drug, in juvenile male rhesus monkeys. The study group was also genotyped for polymorphisms of monoamine oxidase A (MAOA), a gene that has been associated with psychiatric disorders. We used peripheral metabolite profiling of blood and cerebrospinal fluid (CSF) from animals treated daily with fluoxetine or vehicle for one year. Fluoxetine response metabolite profiles and metabolite/reward delay behavior associations were evaluated using multivariate analysis. Our analyses identified a set of plasma and CSF metabolites that distinguish fluoxetine- from vehicle-treated animals and metabolites that correlate with impulsivity. Some metabolites displayed an interaction between fluoxetine and MAOA genotype. The identified metabolite biomarkers belong to pathways that have important functions in central nervous system physiology. Biomarkers of response to fluoxetine in the normally functioning brain of juvenile nonhuman primates may aid in finding predictors of response to treatment in young psychiatric populations and in progress toward the realization of a precision medicine approach in the area of neurodevelopmental disorders.

Figure 1

Study design and analysis strategy. (a) Study design. Thirty-two juvenile male rhesus monkeys with high- and low-activity MAOA genotypes were used in our study. At 1 year of age, animals were randomized to receive either fluoxetine or an equivalent volume of vehicle per day (n=8 per group). After 1 year of dosing, monkeys were assessed with impulsivity test to evaluate the fluoxetine effect on the behavioral level and blood and cerebrospinal fluid (CSF) specimens were collected for metabolomics analysis. (b) Analysis strategy. To identify metabolite biomarkers for the response to fluoxetine, multivariate linear regression model was applied for plasma and CSF metabolite profiling data. Analysis of variance was used to examine the difference of impulsivity outcomes between fluoxetine-treated and control animals. To identify metabolites associated with impulsivity, we performed multivariate linear regression of the impulsivity outcomes (average screen intervals) on the intensity of each metabolite in plasma and CSF. MAOA, monoamine oxidase A.

Figure 2

Plasma and CSF metabolites in response to fluoxetine treatment. (a) Partial least square (PLS) analysis demonstrates separation of metabolite profiling between fluoxetine- (green) and vehicle-treated (red) monkeys in both plasma and CSF specimens. (b) Two metabolite candidates that respond to fluoxetine treatment in both plasma and CSF. 5-aminovaleric acid lactam (left) displayed increased metabolite level in fluoxetine-treated group. Hypoxanthine's levels (right) are dependent on treatment-by-genotype interaction. The upper and lower borders of the box represent the 25th and 75th percentile, respectively; the solid line represents the median; upper and lower whiskers are minimum and maximum without outliers, respectively; points represent outliers. CSF, cerebrospinal fluid; MAOA, monoamine oxidase A.

Figure 3

Fluoxetine increases juvenile rhesus monkey impulsivity. The number of screen intervals elapsing before the monkey responded were averaged across 40 trials of the test session. Fluoxetine, but not MAOA genotype, influenced performance (P=0.024). The upper and lower borders of the box represent the 25th and 75th percentile, respectively; the solid line represents the median; upper and lower whiskers are minimum and maximum without outliers, respectively. Points represent the average screen intervals across 40 trials of individual monkeys in each group. MAOA, monoamine oxidase A.

Figure 4

Molecular pathways. (a) ‘Alanine, Aspartate, Glutamate' metabolism and (b) ‘Nicotinate, Nicotinamide' metabolism pathways are implicated in fluoxetine treatment response, associated with impulsivity and dependent on MAOA genotype. Metabolites identified in response to fluoxetine treatment or associated with impulsivity are indicated. Connected pathways and metabolites relevant for psychiatric disorders are highlighted. CSF, cerebrospinal fluid; MAOA, monoamine oxidase A.

Figure 4

Molecular pathways. (a) ‘Alanine, Aspartate, Glutamate' metabolism and (b) ‘Nicotinate, Nicotinamide' metabolism pathways are implicated in fluoxetine treatment response, associated with impulsivity and dependent on MAOA genotype. Metabolites identified in response to fluoxetine treatment or associated with impulsivity are indicated. Connected pathways and metabolites relevant for psychiatric disorders are highlighted. CSF, cerebrospinal fluid; MAOA, monoamine oxidase A.