Introduction: Primary premature ejaculation (PPE) is a common male neurobiological disorder. Currently, there is consensus that the impairment in central serotonin (5-HT) neurotransmission constitutes a key pathogenic factor in PPE. Selective serotonin reuptake inhibitors (SSRIs) serve as the primary pharmacological intervention; however, a comprehensive elucidation of their mechanism of action remains incomplete. Owing to significant individual variability in efficacy, SSRIs exhibit a high discontinuation rate. Hence, there is an urgent need to address the selection of SSRIs for PPE treatment.
Objective: This study aims to investigate the characteristics of tryptophan (TRP) metabolism in patients with PPE and to assess its influence on the efficacy of SSRIs.
Methods: The exploratory study included a total of 16 patients with PPE and 16 control subjects who were healthy men without any sexual dysfunction. Upon enrollment in the study, all participants underwent a thorough medical history review and physical examination. Subsequently, their serum levels of TRP, its metabolites, large neutral amino acids (LNAAs), and metabolite ratios were assessed using a liquid chromatography-mass spectrometry (LC-MS) assay. After a period of 4 weeks of dapoxetine treatment, all patients with PPE underwent reassessment using the Premature Ejaculation Diagnostic Tool (PEDT) score and intravaginal ejaculatory latency time (IELT) test.
Results: The ratio of serum TRP to other LNAAs (TRP/LNAAs) in patients with PPE was found to be significantly lower compared to the control group (P < 0.05). Conversely, the ratio of kynurenine to TRP (KYN/TRP) was observed to be significantly higher in the PPE patients compared to the control group (P < 0.05). Including the serum TRP/LNAAs ratio and KYN/TRP ratio in the prediction model yielded the highest prediction efficiency for PPE. There was a significant negative correlation between the ratio of TRP/LNAAs before the treatment and the IELT after 4 weeks of the treatment. Additionally, there was a significant positive correlation observed between the ratio of TRP/LNAAs before the treatment and the PEDT score after 4 weeks of the treatment.
Conclusions: This study demonstrates that the reduction in the TRP/LNAAs ratio and the elevation of the KYN/TRP ratio are significant characteristics associated with PPE. These findings suggest that diminished tryptophan availability in the brain and the activation of the kynurenine (KYN) pathway may play a role in the pathogenesis of PPE. The TRP/LNAAs ratio has potential as a reliable indicator of central serotonin (5-HT) levels. Considering the TRP/LNAAs ratio when selecting SSRIs for the treatment of PPE may enhance the response rate of this medication.
Keywords: kynurenine pathway; pathogenesis; primary premature ejaculation; treatment; tryptophan availability.
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