The effect of thioctic acid on allodynia in a rat vincristine-induced neuropathy model

Abstract

Objective

To investigate the antiallodynic effects of thioctic acid in vincristine-induced neuropathy in rats.

Methods

Neuropathy was induced in Sprague–Dawley rats via vincristine intraperitoneal injection. After 15 days, rats were investigated for the presence of mechanical and cold allodynia, and those with allodynia received intraperitoneal injection with normal saline or 1, 5, or 10 mg/kg thioctic acid. Mechanical and cold allodynia were assessed before treatment and at 15, 30, 60, 90, 150 and 180 min after treatment.

Results

Mechanical and cold allodynia were reduced by thioctic acid injection. The duration of effect increased with thioctic acid dose.

Conclusion

Thioctic acid may be an effective treatment for vincristine-induced neuropathy.

Introduction

The anticancer agent vincristine induces painful distal sensorimotor neuropathy in∼ 60% of patients, resulting in discontinuation of treatment and reduced health-related quality-of-life.^1,2 Since the pathogenesis of vincristine induced neuropathy is not clearly understood, various treatments are prescribed empirically, including tricyclic antidepressants, anticonvulsants, opioids, nonsteroidal anti-inflammatory agents and α-adrenergic agonists.^3–5 The efficacy of these drugs in the treatment of neuropathy is limited, however, and some have poor safety profiles.^6–8

Thioctic acid (α-lipoic acid) is a biological antioxidant ⁹ that has been shown to improve nerve blood flow and distal nerve conduction velocity in a rat model of diabetic neuropathy. ¹⁰ Thioctic acid is an effective treatment for diabetic sensorimotor polyneuropathy ¹¹ and diabetic peripheral neuropathy, ¹² and also has been shown to improve functional activity in patients with chronic low back pain. ¹³ Vincristine-induced polyneuropathy and diabetic polyneuropathy develop in similar ways, as both involve inflammation and oxidative stress.^14–16 The aim of the present study was to investigate the antiallodynic effects of thioctic acid in a vincristine-induced rat neuropathy model.

Materials and methods

Animals

Sprague–Dawley rats (n = 40; body weight 200–250 g) were housed and fed according to guidelines on the care and use of laboratory animals, as adopted by the Institutional Animal Care and Use Committee, Department of Laboratory Animals, Bucheon St Mary’s Hospital, Catholic University of Korea, Bucheon City, Republic of Korea, who approved the study. Rats were housed (2–3 animals per plastic cage) and maintained on a 12-h light/dark cycle, with free access to food and water.

Neuropathy model

Vincristine was used to induce neuropathy as described. ¹⁰ Briefly, 0.1 ml/kg per day vincristine sulphate (Hopira, Lake Forest, IL, USA) was injected intraperitoneally for 5 days, paused for 2 days, then resumed for an additional 5 days. Allodynia was assessed on day 15 from the beginning of the injections via Semmes–Weinstein aesthesiometer (Stoelting, Wood Dale, IL, USA). Rats that showed a foot withdrawal response when a 4 -g filament was applied to the hindpaw were considered allodynic.

Treatment

Allodynic rats were allocated between four groups: normal saline (n = 6; 1 ml/kg normal saline intraperitoneal injection); TA1 (n = 8; 1 mg/kg thioctic acid intraperitoneal injection [Thiotacid® injection; Dai han, Kyeongido, Korea]); TA 5 (n = 8; 5 mg/kg thioctic acid intraperitoneal injection); and TA 10 (n = 8; 10 mg/kg thioctic acid intraperitoneal injection). The volume of all injections was 1 ml/kg. Thioctic acid dose was determined by preliminary experiments. Mechanical and cold allodynia were assessed before treatment and at 15, 30, 60, 90, 120, 150 and 180 min after treatment.

Mechanical allodynia

Mechanical allodynia was assessed using a series of von Frey filaments (1.0, 1.4, 2.0, 4.0, 6.0, 8.0, 10.0, 15.0 and 26.0 g), as described. ¹⁷ The left hind paw of each animal was stimulated and the withdrawal threshold was defined as the minimal pressure level (in g) that elicited a withdrawal response (lifting, shaking or licking). If there was no response to any filament, the withdrawal threshold was recorded as 26.0 g.

Cold allodynia

Cold allodynia was assessed using acetone, as described. ¹⁷ Acetone was applied to the plantar surfaces of the left hind paw and withdrawal responses were noted. Testing was repeated 5 times with 3–5 min between each test. The intensity of cold allodynia was expressed as withdrawal frequency: (n paw withdrawal responses/n trials) × 100.

Statistical analyses

Data were presented as mean ± SD. Between-group and between-timepoint differences were evaluated using repeated-measures analysis of variance followed by post hoc Dunn’s test. Statistical analyses were performed using SAS version 9.3 (SAS Institute, Cary, NC, USA), and P-values < 0.05 were considered statistically significant.

Results

The effects of thioctic acid on mechanical allodynia are shown in Table 1. There was no significant change in withdrawal threshold in the normal saline group at any timepoint after injection. Withdrawal frequency was significantly higher at 15 min after1  mg/kg thioctic acid injection compared with normal saline. Compared with normal saline, 5  mg/kg thioctic acid significantly increased withdrawal threshold for 60 min, and 10 mg/kg thioctic acid significantly increased withdrawal threshold for 90 min (P < 0.05 for each comparison; Table 1).

Table 1.

Effect of intraperitoneal injection with normal saline or thioctic acid on mechanical allodynia withdrawal threshold in vincristine-induced neuropathy in a rat model.

Timepoint	Normal saline n = 6	Thioctic acid
		1 mg/kg n = 8	5 mg/kg n = 8	10 mg/kg n = 8
Before injection	1.2 ± 0.1	1.2 ± 0.3	1.1 ± 0.2	1.0 ± 0.0
After injection, min
15	3.1 ± 1.3	10.6 ± 9.2*	20.3 ± 7.4*	24.3 ± 3.8*
30	1.5 ± 0.4	2.0 ± 1.4	14.1 ± 8.9*	15.9 ± 8.9*
60	1.6 ± 0.7	1.2 ± 0.3	10.6 ± 9*	12.3 ± 8.9*
90	1.4 ± 0.7	1.1 ± 0.2	1.7 ± 1.8	11.2 ± 9.5*
120	1.4 ± 0.5	1.0 ± 0.2	1.1 ± 0.1	7.1 ± 9.1
150	1.2 ± 0.2	1.0 ± 0.0	1.1 ± 0.2	7.3 ± 11.6
180	1.0 ± 0.0	1.1 ± 0.1	1.0 ± 0.0	7.3 ± 11.6

Data presented as mean ± SD.

*

P < 0.05 vs normal saline; repeated measures analysis of variance.

The effects of thioctic acid on cold allodynia are shown in Table 2. There was no significant change in withdrawal threshold in the normal saline or 1 mg/kg thioctic acid groups at any timepoint after injection. Compared with normal saline, 5 mg/kg thioctic acid significantly decreased withdrawal frequency for 30 min, and 10 mg/kg thioctic acid significantly decreased withdrawal frequency for 90 min (P < 0.05 for each comparison; Table 2).

Table 2.

Effect of intraperitoneal injection with normal saline or thioctic acid on cold allodynia withdrawal frequency in vincristine induced neuropathy in a rat model.

Timepoint	Normal saline n = 6	Thioctic acid
		1 mg/kg n = 8	5 mg/kg n = 8	10 mg/kg n = 8
Before injection	90.0 ± 11.0	87.5 ± 10.4	92.5 ± 10.4	95.0 ± 9.3
After injection, min
15	80.0 ± 12.6	62.5 ± 12.8	35.0 ± 20.7*	17.5 ± 27.1*
30	80.0 ± 17.9	70.0 ± 26.2	46.3 ± 31.6*	30.0 ± 15.1*
60	80.0 ± 21.9	80.0 ± 26.2	52.5 ± 30.1	40.0 ± 23.9*
90	83.3 ± 15.1	82.5 ± 16.7	70.0 ± 21.4	50.0 ± 21.4*
120	83.3 ± 15.1	85.0 ± 20.7	82.5 ± 12.8	62.5 ± 31.1
150	86.7 ± 10.3	87.5 ± 14.9	85.0 ± 14.1	65.0 ± 23.3
180	90.0 ± 11.0	90.0 ± 10.7	92.5 ± 10.4	87.5 ± 21.2

Data presented as mean ± SD.

*

P < 0.05 vs normal saline; repeated measures analysis of variance.

Discussion

The present study showed that thioctic acid has antiallodynic properties in a vincristine-induced neuropathy model, in accordance with findings in oxaliplatin-induced neuropathy.^18,19 Thioctic acid has no effect on cisplatin-induced mechanical hyperalgesia, ²⁰ suggesting that treatment should be tailored for each chemotherapeutic drug. We used relatively small doses of thioctic acid (1, 5 and 10 mg/kg) compared with other researchers (50 and 100 mg/kg), ¹⁸ due to differences in pain intensity incurred by oxaliplatin and vincristine.

The antiallodynic action of thioctic acid has not been fully elucidated. Oxidative stress has an important role in several neuropathic pain models, including vincristine induced neuropathy, and spinal or sciatic nerve ligation.^21,22 Concentrations of thiobarbituric acid reactive substances (TBARS) and superoxide anions were increased by vincristine treatment.^23,24 It has been reported that lipoic acid treatment reversed indices of oxidative stress, including TBARS. ²⁵ It is possible that the antioxidant properties of thioctic acid are responsible for its antiallodynic effects.

Inflammatory processes play an important role in the development of vincristine-induced neuropathy. ²⁶ In rats, vincristine treatment resulted in significantly increased tumour necrosis factor (TNF)-α concentrations in sciatic nerve tissue. ²⁷ Concentrations of nitric oxide (NO) metabolites, neuronal nitric oxide synthase (nNOS) and cyclic guanosine monophosphate (cGMP) were decreased in spinal cord tissue from vincristine-induced hyperalgesic mice, ²⁸ suggesting that changes in NO and cGMP may induce thermal hyperaglesia in vincristine-induced neuropathic pain.

Thioctic acid has known anti-inflammatory effects:^29,30 it significantly reduces interleukin-1β and TNF-α levels in experimental acute pancreatitis (in rats) and inhibits the increase in inducible NO synthase expression and activity in morphine-dependent rat brain.^31,32 These anti-inflammatory actions may be responsible for the antiallodynic effects of thioctic acid in neuropathy.

In conclusion, intraperitoneal administration of thioctic acid reduced mechanical and cold allodynia in a vincristine-induced rat neuropathy model. It is possible that the antioxidant and anti-inflammatory properties of thioctic acid are responsible for its antiallodynic effects. Further studies are required to elucidate the mechanism of action of thioctic acid fully, and clinical studies must be conducted before this agent is used to treat patients with neuropathy.