ADMINISTRATION OF ANTICANCER DRUG S-1 INCREASES NERVE DENSITY IN THE CORNEA AND INDUCES NEUROINFLAMMATION IN TRIGEMINAL GANGLIA IN RATS
Authors: Kiyoi *Takeshi , Liu Li , He Qiang , Zheng Shijie , Nakazawa Hitomi , Uwada Jyunsuke , Masuoka Takayoshi 
Published: February 2025

Abstract

Background
Lacrimation and corneal epithelial damage have been reported in cancer patients receiving chemotherapy with S-1: a combination of tegafur, gimeracil and oteracil potassium. Furthermore, recent studies have revealed that corneal nerves contribute to preservation of ocular surface homeostasis by facilitating protection and healing of corneal epithelium, and by regulating tear secretion. Thus, the adverse effects of S-1 on ocular surface are predicted to involve abnormalities of corneal nerves, as well as the trigeminal ganglion and trigeminal nucleus in the brain stem, where cell bodies of corneal nerves and their central terminals are located.

Aims & Objectives
This study investigated the relationship between the toxic effects of S-1 on the ocular surface and neurological alteration of corneal neurons in peripheral and central nervous systems of rats.

Method
S-1 (vehicle, 2 or 5 mg/kg) was orally administered to male Wistar rats (6 weeks old) for 28 consecutive days. To evaluate ocular surface pathology, tear volume, corneal epithelial damage and number of blinks were measured every week. At the end of administration period, the corneas were collected; all nerve fibers and peptidergic nerve fibers in the cornea were stained with anti-tublin β III and anti-CGRP antibodies by a whole-mount staining technique. At the same time, the trigeminal ganglia and nuclei were also collected and embedded in paraffin. Expression of GFAP and Iba1 that are neuroinflammation marker were subsequently immunohistochemically visualized using the sections. All images were analyzed using Image J software.

Results
Administration of S-1 led to slight increases in the number of blinks and the corneal epithelial damage score, although there was no difference in tear volume between vehicle and S-1 groups. In the cornea, treatments of 2 and 5 mg/kg S-1 for 4 weeks significantly increased the densities of nerve fibers labeled by anti-tubulin β III antibody 1.28- and 1.91-fold, respectively. One week after starting S- 1 administration at 5 mg/kg, an increase in nerve density was observed. The ratio of CGRP-positive peptidergic nerves among tubulin β III-positive nerves remained consistent between the vehicle and S-1 groups. In the trigeminal ganglion, the Iba1-positive area in 5 mg/kg S-1 group significantly increased 1.58-fold compared with that in the vehicle group, whereas the GFAP-positive area tended to increase in the 5 mg/kg S-1 group. There was no statistical difference in GFAP- and Iba1-positive area in the trigeminal nucleus.

Discussion & Conclusions
Administration of S-1 induced mild corneal damage and abnormality of blink behavior in rats, although ocular symptoms in patients undergoing S-1 chemotherapy are reported to be severe. However, we found that S-1 treatment resulted in significant increases in both total and peptidergic corneal nerves that accompanied by neuroinflammation of the trigeminal ganglion. In future, we will further investigate the relationship between morphological alterations of corneal nerves and epithelial damage, and the neurophysiological changes in corneal nerves under S-1 treatment.