Dr. Kaddurah-Daouk is an inventor on patents in the metabolomics field. She has received funding or consultancy fees for BMS, Pfizer Inc., AstraZeneca and Lundbeck. Dr. Ranga Krishnan has served as consultant for CeNeRx, Lundbeck, foundation, Pfizer Inc. He has direct and indirect ownership interest in CeNeRx, Corcept, Orexigen, Atentiv, sonexa and Protalix. Additionally he is an inventor on patents in the metabolomics field. Wayne Matson is an inventor on patents in the metabolomics field. Dr. A. Rush has received consulting fees from Advanced Neuromodulation Systems, Best Practice Project Management, Otsuka, University of Michigan and Brain Resource; has received consultant/speaker fees from Forest Pharmaceuticals and Singapore College of Family Physicians; has received consultant fees and is a stockholder of Pfizer; has received author royalties from Guilford Publications, Healthcare Technology Systems and the University of Texas Southwestern Medical Center, meeting travel grant from CINP and has received research support from the National Institute of Mental Health. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors.
Conceived and designed the experiments: RKD JAR RRK. Analyzed the data: HZ EC. Contributed reagents/materials/analysis tools: WM SS SM OF. Wrote the paper: RKD HZ MBB SHB. Contributed study samples: EP MD.
Therapeutic response to selective serotonin (5-HT) reuptake inhibitors in Major Depressive Disorder (MDD) varies considerably among patients, and the onset of antidepressant therapeutic action is delayed until after 2 to 4 weeks of treatment. The objective of this study was to analyze changes within methoxyindole and kynurenine (KYN) branches of tryptophan pathway to determine whether differential regulation within these branches may contribute to mechanism of variation in response to treatment. Metabolomics approach was used to characterize early biochemical changes in tryptophan pathway and correlated biochemical changes with treatment outcome. Outpatients with MDD were randomly assigned to sertraline (n = 35) or placebo (n = 40) in a double-blind 4-week trial; response to treatment was measured using the 17-item Hamilton Rating Scale for Depression (HAMD17). Targeted electrochemistry based metabolomic platform (LCECA) was used to profile serum samples from MDD patients. The response rate was slightly higher for sertraline than for placebo (21/35 [60%] vs. 20/40 [50%], respectively, χ2(1) = 0.75, p = 0.39). Patients showing a good response to sertraline had higher pretreatment levels of 5-methoxytryptamine (5-MTPM), greater reduction in 5-MTPM levels after treatment, an increase in 5-Methoxytryptophol (5-MTPOL) and Melatonin (MEL) levels, and decreases in the (KYN)/MEL and 3-Hydroxykynurenine (3-OHKY)/MEL ratios post-treatment compared to pretreatment. These changes were not seen in the patients showing poor response to sertraline. In the placebo group, more favorable treatment outcome was associated with increases in 5-MTPOL and MEL levels and significant decreases in the KYN/MEL and 3-OHKY/MEL; changes in 5-MTPM levels were not associated with the 4-week response. These results suggest that recovery from a depressed state due to treatment with drug or with placebo could be associated with preferential utilization of serotonin for production of melatonin and 5-MTPOL.
Selective serotonin (5-HT) reuptake inhibitors (SSRIs) are the most commonly prescribed antidepressant medications for the treatment of Major Depressive Disorder (MDD). The primary target for SSRIs is the 5-HT transporter, inhibition of the transporter initiates multiple changes within different branches of the tryptophan pathway; yet antidepressant effects are very complex, and several pathways have been implicated in their mechanism of action, in addition to affecting serotoninergic neurotransmission, SSRIs also influence other systems, such as dopamine
Response to current therapies in treating MDD varies considerably, and onset of antidepressant therapeutic action typically does not occur until after 2 to 4 weeks of treatment which delays clinicians from knowing whether an antidepressant is going to work for a particular patient
Metabolomics tools enable identification and quantification of hundreds to thousands of compounds that can report on changes in biochemical pathways
The 75 patients in this report were a subset of the 165 patients who entered a randomized, double-blind, flexible dosing, placebo-controlled study performed at 12 clinical sites. Sertraline dosing was started at 50 mg/day at baseline (week 0), with dose increased up to 100 mg/day at week 1 and up to 150 mg/day at week 2, as seen needed by the treating clinician. Subjects selected for this study were those with serum samples and HAMD17 scores available at baseline and 1 week and at 4 weeks (+/−1 week) after treatment. Finally, the subset of participants in this study largely overlaps with those used in our previous study
Study participants were outpatients, 18–65 years of age, from various clinical sites across the United States. Patients had a primary diagnosis of nonpsychotic MDD by the criteria specified in the Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, with symptoms of depression present for at least one month prior to screening, and a total baseline score >22 on the 17-item HAMD17 at screening. A complete description of the inclusion and exclusion used in this study can be found in
Samples were analyzed using a liquid chromatography electrochemical array (LCECA) platform that has been extensively validated and used in our prior studies in neurodegenerative and psychiatric disorders
Pathway | Metabolite | Abbreviation |
Tryptophan | 3-Hydroxykynurenine | 3-OHKY |
5-Hydroxyindoleacetic Acid | 5-HIAA | |
5-Hydroxytryptophan | 5-HTP | |
NA-5-HT | ||
Anthranillic Acid | ANA | |
Indole-3-lactic Acid | I-3-LA | |
Melatonin | MEL | |
Serotonin | 5-HT | |
Tryptophan | TRP | |
5-Methoxytryptophol | 5-MTPOL | |
Tryptophol | TRPOL | |
Kynurenine | KYN | |
Tyrosine | 3- |
3-OMD |
4-Hydroxyphenylacetic Acid | 4-HPAC | |
3,4-Dihydroxyphenylacetic Acid | DOPAC | |
3,4-Dihydroxymandelic Acid | DIOHMAL | |
Homogentistic Acid | HGA | |
Homovanillic Acid | HVA | |
L-DOPA | LD | |
Methoxyhydroxyphenlyglycol | MHPG | |
Tyrosine | TYR | |
Vanillylmandelic Acid | VMA | |
Phenylalanine | 4-Hydroxybenzoic Acid | 4-HBAC |
4-Hydroxyphenyllactic Acid | 4-HPLA | |
Purine | 7-Methylxanthine | 7-MXAN |
Guanosine | GR | |
Guanosine Mono Phosphate | GRMP | |
Hypoxanthine | HX | |
Uric Acid | UA | |
Xanthine | XAN | |
Xanthosine | XANTH | |
Cysteine, Glutathione | Glutathione (reduced) | GSH |
Cysteine | CYS | |
Antioxidants | Delta-Tocopherol | DTOCO |
Alpha-Tocopherol | ATOCO | |
One Carbon Metabolism | Methionine | METH |
Other | Vanillic Acid | VANA |
Data preprocessing: We first removed the metabolites that have more than 40% missing values. Data from one subject who had more than 50% metabolite data missing was excluded from analysis. Most metabolites have right-skewed distribution; log transformation was thus applied to the metabolite concentrations at all three time points to induce normality of the data. Principal components analysis (PCA; SIMCA-P+12.0) was used to identify outliers. The analysis was performed for each time point. All metabolites were first standardized to have mean 0 and standard deviation 1. Subjects with ≥3 standard deviations away from the center of data cloud within two principal components (PC's) scores scatter plot were considered outliers and were removed from further analysis.
Paired t-tests were used to examine which metabolites changed significantly from baseline to week 1 and from baseline to week 4. These analyses were conducted separately for the sertraline and placebo groups. Between groups t-tests were used to compare baseline to one week and baseline to four week change scores between the sertraline and placebo groups.
Pearson correlation analysis was used to examine associations of changes in metabolite levels with concurrent change in depressive symptoms. Analyses were conducted separately for one and four week change. Correlations of the sertraline group and placebo group were compared using the two-sample
Signatures of response to sertraline and placebo were identified by using paired t-tests to compare week 4 metabolite levels to baseline metabolite levels in the responders and non-responders to therapy. Participants were considered responders if they showed a ≥50% percent reduction in HAMD17 scores after four weeks of treatment. These analyses were also performed to evaluate changes in ratios of individual metabolites within (e.g., 5-HT/5-HTP) and between branches of tryptophan pathway (e.g., KYN/MEL).
Pearson correlation analysis was used to examine associations of the pre-treatment levels of tryptophan pathway metabolites and ratios to the percent change in HAMD17 scores after four weeks of treatment.
For all the above systematic univariate tests, q-values
The sertraline and placebo groups did not differ significantly in age (44±11.3 years vs. 40±12.9 years, p = 0.15), gender (24/35 [69%] vs. 28/40 [70%] female, p = 0.89) or race (26/35 [74%] vs. 31/40 [78%] white, p = 0.75). The response rate was slightly higher for sertraline than for placebo, but this difference was not statistically significant (21/35 [60%] vs. 20/40 [50%] responders, χ2(1) = 0.75, p = 0.39). Age, gender, or race were not associated with percent change in HAMD17 scores in either sertraline or placebo group (
Several metabolites changed significantly from baseline to week one in the group receiving sertraline (
Abbreviations: HIOMT = hydroxyindole-
Compound | Pathway | Sertraline | Placebo | Comparison | |||||
Change | P-value | Q-value | Change | P-value | Q-value | P-value | Q-value | ||
TRPOL | Tryptophan | 0.033 | 0.74 | 0.64 | 0.062 | 0.8 | |||
5-HT | Tryptophan | ||||||||
5-HIAA | Tryptophan | −0.036 | 0.40 | 0.45 | 0.031 | 0.67 | |||
5-HTP | Tryptophan | −0.11 | 0.21 | 0.34 | −0.019 | 0.84 | 0.63 | 0.49 | 0.91 |
5-MTPOL | Tryptophan | 0.041 | 0.50 | 0.52 | 0.016 | 0.81 | 0.62 | 0.78 | 0.94 |
5-MTPM | Tryptophan | −1.8 | 0.075 | 0.22 | 0.31 | 0.91 | |||
KYN | Tryptophan | −0.045 | 0.23 | 0.35 | 0.043 | 0.21 | 0.29 | 0.084 | 0.80 |
NA-5-HT | Tryptophan | 0.0079 | 0.90 | 0.71 | 0.06 | 0.35 | 0.41 | 0.56 | 0.91 |
TRP | Tryptophan | 0.0036 | 0.92 | 0.71 | 0.063 | 0.067 | 0.14 | 0.23 | 0.86 |
3-OHKY | Tryptophan | −0.074 | 0.20 | 0.34 | 0.096 | 0.087 | 0.16 | 0.035 | 0.67 |
MEL | Tryptophan | 0.34 | 0.10 | 0.23 | 0.3 | 0.22 | 0.30 | 0.89 | 0.97 |
LD | Tyrosine | 0.081 | 0.37 | 0.43 | −0.22 | 0.043 | 0.11 | 0.032 | 0.67 |
HVA | Tyrosine | 0.053 | 0.46 | 0.48 | 0.019 | 0.8 | 0.62 | 0.74 | 0.94 |
VMA | Tyrosine | 0.11 | 0.029 | 0.13 | 0.041 | 0.41 | 0.45 | 0.31 | 0.91 |
DOPAC | Tyrosine | −0.13 | 0.22 | 0.34 | 0.051 | 0.67 | 0.59 | 0.26 | 0.86 |
MHPG | Tyrosine | −0.17 | 0.33 | 0.42 | 0.051 | 0.82 | 0.62 | 0.43 | 0.91 |
TYR | Tyrosine | 0.083 | 0.10 | 0.23 | 0.68 | 0.92 | |||
3-OMD | Tyrosine | −0.12 | 0.16 | 0.30 | 0.14 | 0.17 | 0.26 | 0.05 | 0.73 |
4-HPAC | Tyrosine | 0.29 | 0.0044 | 0.057 | 0.15 | 0.072 | 0.15 | 0.26 | 0.86 |
DIOHMAL | Tyrosine | 0.18 | 0.091 | 0.23 | 0.59 | 0.91 | |||
HGA | Tyrosine | 0.93 | 0.97 | ||||||
XAN | Purine | −0.19 | 0.029 | 0.13 | 0.56 | 0.91 | |||
HX | Purine | −0.07 | 0.38 | 0.44 | 0.17 | 0.86 | |||
GR | Purine | −0.23 | 0.31 | 0.40 | −0.13 | 0.56 | 0.56 | 0.76 | 0.94 |
7-MXAN | Purine | −0.099 | 0.74 | 0.64 | −0.1 | 0.72 | 0.59 | 0.99 | 0.98 |
GRMP | Purine | −0.011 | 0.90 | 0.71 | −0.033 | 0.69 | 0.59 | 0.85 | 0.97 |
XANTH | Purine | 0.082 | 0.16 | 0.30 | −0.022 | 0.62 | 0.59 | 0.15 | 0.86 |
UA | Purine | 0.011 | 0.71 | 0.59 | 0.032 | 0.67 | |||
METH | One Carbon Metabolism | 0.097 | 0.084 | 0.23 | 0.14 | 0.056 | 0.13 | 0.64 | 0.91 |
4-HPLA | Phenylalanine | 0.83 | 0.97 | ||||||
4-HBAC | Phenylalanine | −0.085 | 0.77 | 0.65 | 0.19 | 0.46 | 0.49 | 0.48 | 0.91 |
ATOCO | Antioxidant | 0.21 | 0.29 | 0.40 | 0.27 | 0.86 | |||
DTOCO | Antioxidant | 0.3 | 0.30 | 0.40 | 0.60 | 0.91 | |||
CYS | Cysteine, Glutathione | −0.034 | 0.21 | 0.34 | 0.039 | 0.25 | 0.32 | 0.089 | 0.8 |
GSH | Cysteine, Glutathione | −0.013 | 0.44 | 0.47 | −0.0067 | 0.72 | 0.59 | 0.81 | 0.96 |
Changes in absolute concentrations of metabolites (log-transformed) after one week are shown; positive values – up-regulated metabolites, negative values– down-regulated metabolites. Significant changes are shown in bold. See
Compound | Pathway | Sertraline | Placebo | Comparison | |||||
Change | P-value | Q-value | Change | P-value | Q-value | P-value | Q-value | ||
TRPOL | Tryptophan | 0.061 | 0.5 | 0.49 | −0.015 | 0.88 | 0.46 | 0.57 | 0.92 |
5-HT | Tryptophan | −0.018 | 0.78 | 0.45 | |||||
5-HIAA | Tryptophan | −0.06 | 0.31 | 0.36 | 0.015 | 0.79 | 0.45 | 0.36 | 0.91 |
5-HTP | Tryptophan | 0.12 | 0.1 | 0.16 | 0.0022 | 0.98 | 0.5 | 0.28 | 0.91 |
5-MTPOL | Tryptophan | 0.13 | 0.089 | 0.14 | 0.092 | 0.19 | 0.21 | 0.68 | 0.99 |
5-MTPM | Tryptophan | −0.058 | 0.43 | 0.36 | 0.09 | 0.9 | |||
KYN | Tryptophan | 0.045 | 0.16 | 0.22 | 0.04 | 0.26 | 0.26 | 0.92 | 1 |
NA-5-HT | Tryptophan | 0.041 | 0.45 | 0.45 | 0.019 | 0.74 | 0.45 | 0.78 | 1 |
TRP | Tryptophan | 0.052 | 0.091 | 0.16 | 0.68 | 0.99 | |||
3-OHKY | Tryptophan | 0.044 | 0.41 | 0.42 | 0.4 | 0.91 | |||
MEL | Tryptophan | 0.17 | 0.5 | 0.38 | 0.37 | 0.91 | |||
LD | Tyrosine | 0.12 | 0.19 | 0.24 | 0.042 | 0.67 | 0.43 | 0.55 | 0.92 |
HVA | Tyrosine | 0.1 | 0.18 | 0.23 | 0.021 | 0.81 | 0.45 | 0.47 | 0.91 |
VMA | Tyrosine | −0.015 | 0.79 | 0.62 | −0.0078 | 0.88 | 0.46 | 0.92 | 1 |
DOPAC | Tyrosine | −0.014 | 0.9 | 0.67 | 0.12 | 0.21 | 0.22 | 0.37 | 0.91 |
MHPG | Tyrosine | −0.21 | 0.34 | 0.37 | 0.11 | 0.56 | 0.39 | 0.27 | 0.91 |
TYR | Tyrosine | 0.67 | 0.99 | ||||||
3-OMD | Tyrosine | 0.079 | 0.4 | 0.42 | 0.13 | 0.27 | 0.26 | 0.75 | 1 |
4-HPAC | Tyrosine | 0.13 | 0.13 | 0.17 | 0.27 | 0.91 | |||
DIOHMAL | Tyrosine | 0.16 | 0.054 | 0.11 | 0.11 | 0.91 | |||
HGA | Tyrosine | 0.55 | 0.92 | ||||||
XAN | Purine | −0.11 | 0.086 | 0.15 | 0.56 | 0.92 | |||
HX | Purine | −0.0057 | 0.95 | 0.69 | 0.011 | 0.87 | 0.46 | 0.88 | 1 |
GR | Purine | 0.0002 | 1 | 0.7 | −0.18 | 0.51 | 0.38 | 0.63 | 0.99 |
7-MXAN | Purine | 0.083 | 0.78 | 0.62 | 0.051 | 0.82 | 0.45 | 0.93 | 1 |
GRMP | Purine | 0.052 | 0.62 | 0.54 | −0.16 | 0.096 | 0.16 | 0.13 | 0.91 |
XANTH | Purine | 0.047 | 0.35 | 0.38 | 0.0063 | 0.88 | 0.46 | 0.54 | 0.92 |
UA | Purine | −0.015 | 0.66 | 0.43 | 0.34 | 0.91 | |||
METH | One Carbon Meta bolism | 0.48 | 0.91 | ||||||
4-HPLA | Phenylalanine | 0.85 | 1 | ||||||
4-HBAC | Phenylalanine | 0.12 | 0.69 | 0.59 | 0.13 | 0.7 | 0.45 | 0.99 | 1 |
ATOCO | Antioxidant | 0.24 | 0.91 | ||||||
DTOCO | Antioxidant | 0.27 | 0.91 | ||||||
CYS | Cysteine, Glutathione | −0.0014 | 0.97 | 0.69 | −0.044 | 0.27 | 0.26 | 0.42 | 0.91 |
GSH | Cysteine, Glutathione | 0.014 | 0.75 | 0.62 | −0.013 | 0.74 | 0.45 | 0.65 | 0.99 |
Changes in absolute concentrations of metabolites (log-transformed) after four week are shown; positive values – up-regulated metabolites, negative values– down-regulated metabolites. Significant changes are shown in bold. See
Among the patients receiving placebo, a number of metabolites showed significant changes from baseline to the first week of treatment (
A comparison of the treatment signatures of sertraline and placebo revealed some interesting patterns (
Metabolic changes in responders and non-responders are presented in
We also examined associations between metabolite changes and treatment outcome. Following one week of sertraline treatment, changes in 5-HT and KYN were associated with symptoms reduction from baseline (r = −0.38, p = 0.017 for 5-HT; r = 0.32, p = 0.047 for KYN). For placebo group, changes in DOPAC levels correlated with the treatment response (r = −0.51 and p = 0.00059). A direct comparison of one-week correlations between the two groups, however, did not reveal any significant differences.
After four weeks of treatment, changes in metabolites from tryptophan pathway correlated with the percent change in HAMD17 scores in the placebo group, including 5-MTPOL, MEL and 5-HIAA (r = 0.34, p = 0.028; r = 0.32, p = 0.036 and r = −0.31, p = 0.040, respectively). Following four weeks of treatment changes in 4-HPAC had different (p = 0.042) correlations with outcome between sertraline (r = 0.27) and placebo (r = −0.20) groups.
Analysis of metabolic changes that correlates with treatment outcome suggests that different branches within tryptophan metabolism might be regulated differently in responders and non-responders to drug and to placebo. Therefore we performed more targeted and comprehensive analysis of the tryptophan pathway, with a focus on the methoxyindole and KYN branches in responders and non-responders to sertraline and placebo (
Panels A and B show correlations of metabolites at baseline with the four week treatment response in the sertraline and placebo groups, respectively. The correlations are color coded as indicated by the color bar. Panels C and D show four week metabolic changes unique to responders in the sertraline and placebo groups respectively. Abbreviations: HIOMT = hydroxyindole-
Correlations of pre-treatment metabolite levels and ratios of metabolites to four week percent change in HAMD17 scores in the sertraline treatment condition are presented in
Metabolites | Correlation | p-value | q-value |
5-MTPM | 0.41 | 0.013 | 0.15 |
5-HT | 0.29 | 0.093 | 0.35 |
5-MTPOL | −0.24 | 0.16 | 0.35 |
NA-5-HT | −0.24 | 0.17 | 0.35 |
MEL | −0.22 | 0.20 | 0.35 |
5-HIAA | −0.19 | 0.28 | 0.36 |
5-HTP | 0.077 | 0.66 | 0.40 |
TRP | 0.051 | 0.77 | 0.42 |
Ratios | Correlation | p-value |
5-HTP/TRP | 0.072 | 0.68 |
5-HT/TRP | 0.19 | 0.28 |
NA-5-HT/TRP | −0.28 | 0.10 |
5-HIAA/TRP | −0.11 | 0.54 |
5-MTPOL/TRP | −0.2 | 0.25 |
MEL/TRP | −0.26 | 0.14 |
5-MTPM/TRP | 0.32 | 0.058 |
5-HT/5-HTP | 0.15 | 0.40 |
NA-5-HT/5-HTP | −0.21 | 0.23 |
5-HIAA/5-HTP | −0.17 | 0.34 |
5-MTPOL/5-HTP | −0.21 | 0.24 |
MEL/5-HTP | −0.31 | 0.068 |
5-MTPM/5-HTP | 0.22 | 0.19 |
NA-5-HT/5-HT | −0.33 | 0.051 |
5-HIAA/5-HT | −0.33 | 0.054 |
5-MTPOL/5-HT | −0.25 | 0.14 |
5MTPM/5-HT | 0.11 | 0.53 |
MEL/NA-5-HT | −0.2 | 0.26 |
5-MTPOL/5MTPM | −0.28 | 0.11 |
Binary analysis was used to examine changes in metabolites in responders and non-responders in the four week sertraline group (
Analysis of ratios of metabolites between the kynurenine and methoxyindole branches of tryptophan pathway revealed profound differences between responders and non-responders to sertraline (
Ratio | Responder | Nonresponder | ||
Estimate | p-value | Estimate | p-value | |
KYN/TRP | −0.006 | 0.75 | −0.017 | 0.40 |
KYN/5-HTP | 0.0057 | 0.88 | −0.076 | 0.11 |
KYN/5-HT | 0.95 | |||
KYN/NA-5-HT | −0.019 | 0.48 | 0.027 | 0.55 |
KYN/MEL | −0.28 | −0.085 | 0.59 | |
KYN/5-MTPOL | −0.062 | 0.090 | −0.012 | 0.86 |
3-OHKY/TRP | 0.013 | 0.67 | −0.04 | 0.20 |
3-OHKY/5-HTP | 0.025 | 0.62 | −0.099 | |
3-OHKY/5-HT | 0.97 | |||
3-OHKY/NA-5-HT | −0.00032 | 0.99 | 0.0038 | 0.94 |
3-OHKY/MEL | −0.27 | −0.11 | 0.49 | |
3-OHKY/5-MTPOL | −0.043 | 0.21 | −0.035 | 0.67 |
Similarly to sertraline, we examined associations of tryptophan pathway metabolites and ratios of metabolites to HAMD17 changes in the patients treated with placebo. The results of these analyses are summarized in
Metabolites | Correlation | p-value | q-value |
5-MTPOL | −0.40 | 0.0075 | 0.49 |
MEL | −0.16 | 0.29 | 0.99 |
5-HT | −0.15 | 0.33 | 0.99 |
5-HTP | 0.093 | 0.55 | 0.99 |
5-MTPM | 0.069 | 0.66 | 0.99 |
TRP | −0.044 | 0.78 | 0.99 |
5-HIAA | 0.028 | 0.86 | 0.99 |
NA-5-HT | −0.021 | 0.90 | 0.99 |
Ratios | Correlation | p-value |
5-HTP/TRP | 0.14 | 0.38 |
5-HT/TRP | −0.15 | 0.34 |
NA-5-HT/TRP | −0.044 | 0.78 |
5-HIAA/TRP | 0.091 | 0.56 |
MEL/TRP | −0.1 | 0.52 |
5MTPM/TRP | 0.039 | 0.81 |
5-HT/5-HTP | −0.12 | 0.45 |
NA-5-HT/5-HTP | −0.15 | 0.32 |
5-HIAA/5-HTP | −0.072 | 0.65 |
5-MTPOL/5-HTP | −0.28 | 0.071 |
MEL/5-HTP | −0.22 | 0.16 |
5MTPM/5-HTP | −0.04 | 0.80 |
NA-5-HT/5-HT | 0.081 | 0.61 |
5-HIAA/5-HT | 0.14 | 0.37 |
5-MTPOL/5-HT | −0.15 | 0.32 |
MEL/5-HT | −0.059 | 0.71 |
5MTPM/5-HT | 0.29 | 0.062 |
MEL/NA-5-HT | −0.16 | 0.31 |
MEL/5MTPM | −0.18 | 0.25 |
NA-5-HT/5MTPM | −0.00076 | 1.00 |
5-HIAA/5MTPM | 0.057 | 0.72 |
5-MTPOL/5MTPM | −0.29 | 0.059 |
Analyses of change in responders and non-responders to placebo revealed that there were significant four week increases in 5-MTPOL and MEL in responders to placebo (
Responder | Nonresponder | |||
Ratio | Estimate | p-value | Estimate | p-value |
KYN/TRP | −0.0085 | 0.67 | −0.0027 | 0.89 |
KYN/5-HTP | 0.043 | 0.39 | −0.0051 | 0.93 |
KYN/5-HT | −0.0063 | 0.89 | 0.05 | 0.075 |
KYN/NA-5-HT | −0.0023 | 0.95 | 0.018 | 0.48 |
KYN/MEL | −0.37 | 0.19 | 0.18 | |
KYN/5-MTPOL | −0.099 | 0.079 | 0.038 | 0.28 |
3-OHKY/TRP | 0.011 | 0.74 | 0.034 | 0.27 |
3-OHKY/5-HTP | 0.063 | 0.22 | 0.032 | 0.60 |
3-OHKY/5-HT | 0.014 | 0.79 | 0.086 | |
3-OHKY/NA-5-HT | 0.018 | 0.66 | 0.055 | 0.056 |
3-OHKY/MEL | −0.35 | 0.22 | 0.11 | |
3-OHKY/5-MTPOL | −0.079 | 0.16 | 0.074 | 0.075 |
Correlating pre-treatment metabolite levels to the four-week treatment outcome did not yield any metabolites within the tyrosine pathway that significantly correlate with response to treatment with drug or placebo. An examination of changes in metabolites in responders and non-responders in the sertraline group yielded 4-HPAC and DIOHMAL to be significantly increased in responders (
The results of our recent metabolomics study indicated that pretreatment serum metabolic signatures of MDD patients allowed prediction of response to sertraline
In the sertraline group, we found a marked decrease in level of 5-HT from baseline to both one and four weeks of the study, which is significantly larger than that in the placebo group. This observation is consistent with previous reports on the effects of SSRIs on blood 5-HT levels and most likely reflects inhibition of 5-HT transporter and decreased levels of 5-HT in the platelets following chronic SSRI treatment
In addition to decreased 5-HT and 5-HIAA levels, we observed changes in other metabolites within the tryptophan pathway, both at baseline and after treatment, and these effects were different for responders and non-responders. There was a significant and positive correlation between baseline 5-MTPM levels and treatment response at four weeks, indicating that higher levels of 5-MTPM before treatment were associated with a more favorable response. Levels of 5-MTPM were significantly reduced over the four week treatment period, but only among good responders to sertraline. Additionally, significant differences in the ratios between several metabolites within the tryptophan pathway were observed. The ratios of 5-HIAA, NA-5-HT and MEL to both 5-HT and 5-MTPM were significantly lower in responders to sertraline, compared to non-responders. In the responders to sertraline levels of 5-MTPOL and MEL were significantly increased over the four week treatment period, while in the non-responders levels of 5-MTPOL, MEL and 5-MT were not affected. Neither KYN nor the other metabolites within the kynurenine branch of tryptophan pathway were significantly altered in the group receiving sertraline. Analysis of the ratios showed an increase in KYN/5-HT and 3-OHKY/5-HT ratios following four week treatment both in responders and non-responders to sertraline. The ratios in KYN/MEL and 3-OHKY/MEL were significantly decreased only in the responders. Taken together, these observations suggest that more favorable outcome of the sertraline treatment is associated with the effect on methoxyindole branch of tryptophan pathway, compared to the effect on the kynurenine branch. Increased ratios in KYN/5-HT and 3-OHKY/5-HT might seem to contradict this suggestion. However, levels of KYN and its metabolites, and of methoxyindoles in plasma reflect their respective levels in cerebrospinal fluid
Several studies implicated methoxyindole and kynurenine branches of tryptophan metabolism in the pathogenesis of depression and in mechanisms of action SSRIs
Although melatonin has been extensively studied as a product of the pineal gland other methoxyindoles are less well studied; both central and peripheral effects have been described for these compounds. 5-MTPOL is regulated during the circadian cycle similarly to melatonin
Limitations of this study include a relatively small sample size and relatively short duration of treatment. Longer treatment periods should help to determine which changes occur earlier, better define the relation between response and biochemical changes, and better discern the ability of early changes to predict latter outcomes. In future studies we will focus on effects of SSRIs and placebo on other pathways, this should extend findings reported and explain variability that exists between responders and non-responders in more details. Additionally correlations between peripheral and central metabolic changes induced by drug and placebo should be investigated. This can lead to the identification of peripheral biomarkers that are disease related and that can be more easily measured. Additionally, a potential limitation of this study is lack of the group of MDD patients without any treatment. While without such group of subjects the possibility still exists that response to placebo could not be distinguished from spontaneous recovery, justification of such clinical trial design could be problematic due to the ethical concerns; future studies are necessary to address this question.
In conclusion, our results suggest a potential role for methoxyindoles in mechanisms of recovery from depressed state, a finding that needs to be followed upon where samples should be collected during the day and night hours to define how the circadian cycle, production of methoxyindoles, changes in sleep patterns and hormonal axis are modified in responders and non-responders to SSRIs and to placebo.
Subject selection criteria.
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Association of demographic variables with percent change in HAMD17 score.
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Metabolic changes after one week treatment with sertraline and placebo in responders (A) and in non-responders (B).
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Metabolic changes after four week treatment with sertraline and placebo in responders (A) and in non-responders (B).
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We acknowledge Jon Kilner, MS, MA for formatting for publication.