The Impact of Post-Tear Collection Storage on Tear Film Substance P Concentration

Tear film substance P in patients treated with neurotoxic chemotherapy

Neurotoxic chemotherapy has been shown to be associated with reduced corneal nerves and ocular surface discomfort. Substance P is a neuropeptide expressed by sensory nerves including those in the densely innervated cornea. It is involved in both pain signaling and the regulation of epithelial and neural health. While its levels in tear fluids have been used as a neuropathic biomarker in diabetes, investigations of tear concentrations of substance P in chemotherapy-induced peripheral neuropathy have not been explored. The current cross-sectional study assessed substance P expression in tears of patients following neurotoxic chemotherapy treatment. Patients treated with paclitaxel (n = 35) or oxaliplatin (n = 30) 3-24 months prior to assessment were recruited along with healthy controls (n = 25). Flush tear collection, in-vivo corneal confocal microscopy and neurotoxicity assessments were also conducted. Enzyme-linked immunosorbent assays were used to measure substance P concentrations in collected tears, while total protein content (TPC) was measured with the bicinchoninic acid method (BCA). General linear models were used for statistical analysis. Substance P concentration was reduced in paclitaxel-treated patients [Median (Interquartile range, IQR): 1.11 (0.20-2.24) ng/ml)] compared to the oxaliplatin group [4.28 (1.01-10.73) ng/ml, p = 0.02]. Substance P expressed as a proportion of TPC was also lower in the paclitaxel group [0.00006 (0.00001-0.00010) %] compared to the oxaliplatin group [0.00018 (0.00008-0.00040) %, p = 0.005]. Substance P concentration and its percentage in TPC were also reduced in the paclitaxel group when compared to healthy controls [4.61 (1.35-18.51) ng/ml, p = 0.02; 0.00020 (0.00006-0.00060) %, p = 0.04, respectively]. Higher cumulative dose of paclitaxel was correlated with a reduction in substance P concentrations (r = -0.40, p = 0.037), however no associations were found with corneal nerve parameters or neuropathy severity (p > 0.05). While these findings show evidence for the dysregulation of tear film substance P following paclitaxel treatment, longitudinal studies should be conducted to investigate how substance P levels in tears change during treatment.