Efficacy of pramipexole combined with levodopa for Parkinson's disease treatment and their effects on QOL and serum TNF-α levels.

PURPOSE: To investigate the efficacy of combining the dopamine receptor agonist pramipexole with levodopa for Parkinson's disease (PD) treatment and to measure their effects on quality of life and tumor necrosis factor (TNF)-α levels in PD patients. BASIC PROCEDURE: In total, 160 PD patients who were admitted to our hospital were equally randomized into a control treatment group (levodopa alone) and the study group (pramipexole combined with levodopa). Both groups were treated for 12 weeks.
FINDINGS: After treatment, scores from the Unified Parkinson's Disease Rating Scales (1-3), the Hamilton Depression Scale, and the Parkinson's Disease Questionnaire (PDQ-39) were significantly decreased in both groups, whereas Mini-Mental State Examination scores were significantly increased. After treatment, the study group had significantly lower scores for all scales except the Mini-Mental State Examination, for which those who received combined treatment had significantly higher scores than the control group. The incidence of adverse reactions was significantly lower in the study group than in the control group. Furthermore, after treatment, serum TNF-α levels were significantly decreased in both groups compared with pre-treatment levels.
CONCLUSION: Pramipexole combined with levodopa relieved PD symptoms and improved the quality of life of PD patients, potentially by suppressing serum TNF-α levels.

RCT Entities:   Population Interventions Outcomes

Introduction

Parkinson’s disease (PD) is a neurodegenerative disease that is second only to Alzheimer’s disease in terms of prevalence and is characterized by motor disorders among middle-aged and elderly individuals.[1] The incidence of PD is 1% to 2% among individuals in their 60s, but is 3% to 4% among those in their 80s.[2] PD patients experience a severe decline in quality of life (QOL) and lack self-care abilities, which exerts a heavy burden on their families. Importantly, the prevalence of PD is increasing worldwide with population aging.[3] The drug-based therapeutic regimens currently used for PD show varying levels of efficacy. Therefore, selecting the appropriate regimen is critical for symptomatic relief and improving patients’ QOL.

Since its introduction in the late 1960s, levodopa (a dopamine precursor and an intermediate product generated during the conversion of tyrosine to catecholamine) has become the most effective and widely used drug for PD. However, long-term treatment with levodopa is complicated by motor fluctuations. For example, after 5 years levodopa treatment, approximately 80% of young patients (age of onset between 21 and 40 years old) and 44% of elderly patients developed motor complications.[4,5] Pramipexole, a dopamine receptor agonist, was approved for the treatment of early and late PD in the United States and Europe in 1998.[6] Through a neuroprotective effect, pramipexole delays levodopa-induced motor complications in early PD, controls motor symptoms, and relieves depression in PD patients.[7] A recent study demonstrated that inflammatory cytokines are abnormally expressed in patients with neurodegenerative diseases and are involved in disease development.[8] Another study found that long-term overactivation of microglial cells in the brains of PD patients was associated with significantly increased levels of a large number of pro-inflammatory cytokines, including tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6, and interferon-γ.[9]

Although the outcomes of treatment with either pramipexole or levodopa alone for PD have been widely studied, the effect of combining pramipexole with levodopa on inflammatory cytokines and disease outcomes has not been adequately studied. Therefore, we conducted this study to compare the efficacy and safety as well as the effects on serum TNF-α levels between treatment with levodopa alone and with pramipexole combined with levodopa for PD to develop more effective and safer therapeutic options that can relieve symptoms and improve the QOL of PD patients.

Materials and methods

Ethics

This study was approved by the Medical Ethics Committee of the Third Affiliated Hospital of Soochow University. For this study, patients and their families were provided detailed information about the study, and signed informed consent forms were collected.

Inclusion and exclusion criteria

Patients aged 50 to 80 years with an educational level above primary school who fulfilled the diagnostic criteria for PD according to the UK Parkinson’s Disease Society Brain Bank[3] were included in the study. The exclusion criteria were patients with allergies or contraindications to the drugs used in this study; those with mental illnesses; those with poor treatment compliance; those with cardiac, hepatic, or renal insufficiency; and those with drug abuse. In this study, patients and their families were provided detailed information about the study, and signed informed consent forms were collected. This study was approved by the Medical Ethics Committee of the Third Affiliated Hospital of Soochow University.

Therapeutic methods

Patients in the control group were orally administered 125 mg levodopa in tablet form (batch no: H11021055; Beijing Shuguang Pharmaceutical, Beijing, China) once daily, which was gradually increased to 500 mg/day. Patients in the study group were also orally administrated 125 mg levodopa in tablet form once daily, which was gradually increased to 250 mg/day, and additionally were initiated on 0.125 mg pramipexole hydrochloride in tablet form (batch no: H20110069; Boehringer Ingelheim, Germany) thrice daily, which was gradually increased to 4.5 mg/day. Indications for treatment discontinuation were dizziness, vomiting, diarrhea, and other adverse reactions; treatment was resumed after the disappearance of adverse reactions. The treatment duration was 12 weeks for both groups.

Scoring standards

The Unified Parkinson’s Disease Rating Scale (UPDRS) 1, UPDRS2, and UPDRS3[10] were used to evaluate patients’ mental state, activities of daily living, and motor symptoms, respectively, before and after treatment, with lower scores indicating milder symptoms. The Hamilton Depression Scale (HAMD)[11] was used to evaluate the extent of depression before and after treatment, with higher scores indicating more severe depression. The Mini-Mental State Examination (MMSE)[12] was used to evaluate cognitive function, including memory, attention, and phonological competence before and after treatment. In MMSE, a score of 27 to 30 indicates normal cognitive function, whereas a score of <27 indicates cognitive impairment. The Parkinson's Disease Questionnaire (PDQ)-39[13] was used to evaluate QOL, including activities of daily living, cognition, mobility, communication, social support, and three additional dimensions before and after treatment. The PDQ-39 scale has 100 points, with higher scores indicating lower QOL. Data on the incidence of toxic side effects, including anorexia, headache, vomiting, nausea, lethargy, diarrhea, hepatic injury, and renal injury, were also collected in both treatment groups. The mean MMSE and PDQ-39 scores were used to evaluate QOL. The mean HAMD scores and the three UPDRS scores are not independent factors; the three UPDRS scores are influenced by other factors in addition to TNF-α. Therefore, results that assess correlations between TNF-α and MMSE, PDQ-39, and HAMD may not be comprehensive.

Detecting serum TNF-α levels

Serum TNF-α levels were measured by an enzyme-linked immunosorbent assay (ml077385; Shanghai Enzyme-Linked Biotechnology, Shanghai, China). Briefly, the samples and kit components were equilibrated to room temperature for 30 minutes. Then, 50 µL of recombinant human TNF-α at specific concentrations was added to derive a standard curve, and 50 µL of the samples were added to individual wells for measurement; blank wells included 50 µL of assay buffer alone. Next, 50 µL of streptavidin-conjugated horseradish peroxidase was added to each well containing the standards and samples, and the plate was covered with a microplate sealer and incubated at 37°C for 1 hour. Following five 30-second washes with 200 µL of washing liquid, 50 µL of a solution containing equal parts of chromogenic agents A and B was added, and the plate was incubated at 37°C. Finally, 50 µL of Stop solution was added to each well to stop the reaction. A Bio-Rad 680 plate reader (Bio-Rad Laboratories, Hercules, CA, USA) was used to detect the optical density of each well at 450 nm to determine serum TNF-α levels.

Statistical analysis

IBM SPSS Statistics for Windows, version 22.0 (IBM Corp., Armonk, NY, USA) was used for all statistical analyses. Count data are expressed as numbers with percentages, and the chi-square test was used to compare these data between the two groups. Measurement data are expressed as mean ± standard deviation. An independent samples t-test was used for between-group comparisons of measurement data, and a paired t-test was used for within-group comparisons of data before and after treatment. Pearson’s correlation coefficient was used to assess correlations. A P-value <0.05 was considered to indicate statistically significant differences.

Results

Patient characteristics

In total, 160 PD patients who were admitted to our hospital between March 2015 and December 2018 were randomized into two treatment groups: control (n = 80) and study (n = 80). The control group comprised 58 men and 22 women, with an average age of 61.23 ± 6.78 years and an average disease duration of 5.23 ± 1.35 years. The study group comprised 64 men and 16 women, with an average age of 63.53 ± 7.21 years and an average disease duration of 6.12 ± 1.67 years.

Comparison of general characteristics

No significant differences were found between the two groups in terms of age, sex, exercise habits, place of residence, nationality, educational level, body weight, marital status, food preference, or average disease duration (Table 1).

Table 1.

General characteristics of the treatment groups [n (%)].

Characteristic	Control group(n = 80) mean (±SD)	Study group(n = 80)mean (±SD)	χ2/F	P
Age, years			0.655	0.419
<60	34 (42.50)	29 (36.25)
≥60	46 (57.50)	51 (63.75)
Sex			1.243	0.265
Male	58 (72.50)	64 (80.00)
Female	22 (27.50)	16 (20.00)
Exercise habit			1.441	0.230
Yes	28 (35.00)	21 (26.25)
No	52 (65.00)	59 (73.75)
Place of residence			1.047	0.306
City	58 (72.50)	52 (65.00)
Countryside	22 (27.50)	28 (35.00)
Nationality			1.002	0.317
Han	69 (86.25)	73 (91.25)
National minorities	11 (13.75)	7 (8.75)
Educational level			1.270	0.260
<Senior high school	44 (55.00)	51 (63.75)
≥Senior high school	36 (45.00)	29 (36.25)
Body weight, kg			0.440	0.507
<55	30 (37.50)	26 (32.50)
≥55	50 (62.50)	54 (67.50)
Marital status			0.551	0.759
Married	65 (81.25)	63 (78.75)
Unmarried	6 (7.50)	5 (6.25)
Widowed	9 (11.25)	12 (15.00)
Food preference			1.098	0.295
Bland	60 (75.00)	54 (67.50)
Spicy	20 (25.00)	26 (32.50)
Average disease duration, years	5.23 ± 1.35	5.12 ± 1.67	0.458	0.648

SD, standard deviation.

Comparison of changes in UPDRS scores between the groups

As shown in Table 2, after treatment, all UPDRS scores were significantly decreased in both treatment groups (P < 0.05). Furthermore, all UPDRS scores were significantly lower in the study group than in the control group (P < 0.05).

Table 2.

Comparison of UPDRS scores before and after treatment (score ± SD).

Group	UPDRS1 score		UPDRS2 score		UPDRS3 score
	Before treatment	After treatment	Before treatment	After treatment	Before treatment	After treatment
Control(n = 80)	4.07 ± 1.09	2.97 ± 0.53*	21.32 ± 4.92	18.37 ± 3.55*	27.60 ± 5.10	23.15 ± 4.26*
Study(n = 80)	4.12 ± 1.05	2.16 ± 0.39*	20.41 ± 4.87	14.26 ± 3.14*	26.93 ± 5.75	19.94 ± 3.82*
t	0.296	11.010	1.176	7.756	0.780	5.018
P	0.768	<0.001	0.242	<0.001	0.437	<0.001

UPDRS, Unified Parkinson’s Disease Rating Scale; SD, standard deviation; *P<0.05 compared with scores obtained before treatment within the group.

Comparison of changes in HAMD scores between the groups

As shown in Table 3, HAMD scores were significantly decreased in both groups after treatment (P < 0.05). Additionally, the HAMD scores of the study group were significantly higher than those of the control group after treatment (P < 0.05).

Table 3.

Comparison of HAMD scores before and after treatment (score ± SD).

Group	n	Before treatment	After treatment	t	P
Control	80	19.64 ± 5.26	13.25 ± 2.46	9.842	<0.001
Study	80	19.37 ± 4.48	10.57 ± 2.77	14.943	<0.001
t	–	0.350	6.470	–	–
P	–	0.727	<0.001	–	–

HAMD, Hamilton Depression Scale; SD, standard deviation.

Comparison of changes in MMSE scores between the groups

Before treatment, there was no significant difference in MMSE scores between the groups (Table 4); however, after treatment, MMSE scores were significantly increased in both groups (P < 0.05). Importantly, MMSE scores were significantly lower in the study group than in the control group after treatment (P < 0.05).

Table 4.

Comparison of MMSE scores before and after treatment (score ± SD).

Group	n	Before treatment	After treatment	t	P
Control	80	16.12 ± 1.98	22.23 ± 1.99	19.468	<0.001
Study	80	16.23 ± 2.19	27.23 ± 2.56	29.204	<0.001
t	–	0.333	13.792	–	–
P	–	0.739	<0.001	–	–

MMSE, Mini-Mental State Examination; SD, standard deviation.

Comparison of changes in PDQ-39 scores between the groups

Before treatment, there was no significant difference in PDQ-39 scores between the groups (Table 5). In contrast, PDQ-39 scores were significantly decreased in both groups after treatment (P < 0.05). Furthermore, post-treatment PDQ-39 scores were significantly lower in the study group than in the control group (P < 0.05).

Table 5.

Comparison of PDQ-39 scores before and after treatment (score ± SD).

Group	n	Before treatment	After treatment	t	P
Control	80	46.23 ± 6.89	34.56 ± 4.58	12.616	<0.001
Study	80	45.78 ± 7.78	26.78 ± 3.45	19.968	<0.001
t	–	0.387	12.136	–	–
P	–	0.699	<0.001	–	–

PDQ, Parkinson’s Disease Questionnaire; SD, standard deviation.

Comparison of adverse reactions between the groups

No adverse reactions were observed during treatment in either group. As shown in Table 6, anorexia, headache, vomiting, nausea, lethargy, diarrhea, hepatic injury, and renal injury were observed in 12 (15.00%), eight (10.00%), four (5.00%), five (6.25%), five (6.25%), seven (8.75%), four (5.00%), and three (3.75%) patients in the control group, respectively, and in eight (10.00%), three (3.75%), two (2.50%), two (2.50%), four (5.00%), two (2.50%), one (1.25%), and two (2.50%) patients in the study group, respectively. The incidence of adverse reactions in the study group was significantly lower than in the control group (P < 0.05).

Table 6.

Comparison of adverse reactions [cases (%)].

Adverse reaction	Control group(n = 80)	Study group(n = 80)	χ2	P
Anorexia	12 (15.00)	8 (10.00)	0.914	0.339
Headache	8 (10.00)	3 (3.75)	2.441	0.118
Vomiting	4 (5.00)	2 (2.50)	0.693	0.405
Nausea	5 (6.25)	2 (2.50)	1.345	0.246
Lethargy	5 (6.25)	4 (5.00)	0.118	0.732
Diarrhea	7 (8.75)	2 (2.50)	2.943	0.086
Hepatic injury	4 (5.00)	1 (1.25)	1.858	0.173
Renal injury	3 (3.75)	2 (2.50)	0.206	0.650
Overall incidence of adverse reactions	48 (60.00)	24 (30.00)	14.545	<0.001

Comparison of serum TNF-α levels before and after treatment

Serum TNF-α levels, which were not significantly different between the groups before treatment, were significantly decreased in both groups after treatment (P < 0.05, Figure 1). Importantly, post-treatment TNF-α levels were significantly lower in the study group than in the control group (P < 0.05).

Figure 1.

Comparison of serum TNF-α levels before and after treatment in each group.

*P < 0.05 compared with values obtained before treatment within the group; #P < 0.05 compared with the control group.

TNF- α, tumor necrosis factor- α.

Correlation of serum TNF-α levels with PD severity

As shown in Figure 2, Pearson’s correlation analysis revealed that the serum TNF-α levels in the study group exhibited a significant positive correlation with post-treatment UPDRS1, UPDRS2, and UPDRS3 scores (correlation coefficient: 0.602, 0.675, and 0.685, respectively; P < 0.05).

Figure 2.

Correlation between serum TNF-α levels with UPDRS1 (a), UPDRS2 (b), and UPDRS3 (c) scores after treatment in the study group.

TNF- α, tumor necrosis factor- α; UPDRS, Unified Parkinson’s Disease Rating Scale.

Discussion

Large-scale degeneration and death of dopaminergic neurons, which is characteristic of PD, reduce endogenous striatal dopamine level, consequently leading to bradykinesia, rigidity, tremors, and postural instability in PD patients.[14] PD is currently managed by symptomatic control and drugs that act on the dopaminergic system, increasing dopamine levels, and stimulating dopamine receptors.[15] Levodopa has shown marked effectiveness as a first-line treatment for PD; however, its long-term use is associated with motor disturbances. Therefore, dopamine receptor agonists, alone or in combination with levodopa, are increasingly being used to reduce levodopa-induced motor complications.[16]

Levodopa enters the central nervous system through the blood–brain barrier and is directly converted by decarboxylation to dopamine, which is then delivered to the brain where it can reverse the degeneration and death of dopaminergic neurons and relieve the symptoms and clinical conditions of PD patients. Currently, levodopa is considered the gold standard for PD treatment.[17-19] Our data revealed that levodopa alone led to a decrease in UPDRS1, UPDRS2, UPDRS3, HAMD, and PDQ-39 scores and an increase in MMSE scores of PD patients, indicating that levodopa can relieve symptoms and improve QOL in PD patients. Prolonged treatment with increasing doses of levodopa leads to the aggravation of motor disturbances in PD patients, which actually prolongs PD. Moreover, motor disturbances gradually become more disabling and currently have are untreatable, highlighting the failure of approaches that address the medical needs of PD patients.[20] The novel dopamine receptor agonists that have recently been developed and clinically applied not only relieve the clinical symptoms of PD but also reduce the toxicity and side effects of levodopa.[21] Dopamine agonists, including pramipexole, have a longer half-life than levodopa and directly act on dopamine receptors without carrier-mediated transport into the intestinal tract or brain. Therefore, these agonists stimulate dopamine receptors for a longer period than levodopa. Additionally, their metabolism does not produce free radicals, which are considered one of the greatest hazards during levodopa treatment.[22] Pramipexole has high specificity and intrinsic activity against the D2 subfamily of dopamine receptors and shows high affinity to D2 and D3 dopamine receptor subtypes.[23,24]

Activation of D2 receptors relieves symptoms, whereas activating D3 receptors relieves depression.[25,26] In a previous study, Tayarani et al.[27] compared levodopa alone and pramipexole combined with levodopa in MPTP-treated common marmosets and revealed that the combination treatment reduced the required levodopa dosage and minimized motor disturbances while maintaining treatment efficacy. In a study on PD patients, Foster et al.[28] reported that pramipexole combined with levodopa exhibited a synergistic effect, indicating that pramipexole improved the efficacy of levodopa and led to a more effective reduction in motor complications. Another study reported that pramipexole enabled reduced levodopa doses, thus preventing complications due to excess levodopa administration.[29] In this study, PD patients who were treated with pramipexole combined with levodopa exhibited significantly lower UPDRS1, UPDRS2, UPDRS3, HAMD, and PDQ-39 scores and significantly higher MMSE scores than those who were treated with levodopa alone. Importantly, the incidence of adverse reactions was significantly lower in the study group than in the control group. Overall, these results show that pramipexole combined with levodopa was more effective than levodopa alone in relieving symptoms and improving the QOL of PD patients. The potential causes for these findings are the reduced levodopa doses made possible by pramipexole and the synergistic effect of both drugs on PD-associated biological processes.

Inhibition of inflammatory cytokines, such as TNF-α, has been shown to alleviate depression symptoms, which includes anhedonia and psychomotor inhibition, in patients with inflammatory diseases and those with depression and aggravated inflammation.[30] A previous study demonstrated that TNF-α and IL-1β levels in the striatum and hippocampus were significantly higher in rats with injury to the right medial forebrain bundle than in sham-operated rats. This study also showed that cytokine levels were normalized by treating the injured rats with ellagic acid, which also improved sports injuries to the rats by reducing neuroinflammatory levels, e.g., TNF-α and IL-1β, and protecting the brain from free radical-mediated nerve injury.[31] Finally, serum TNF-α levels, which are elevated in PD patients, are also significantly correlated with PD severity, suggesting that TNF-α is a potential biomarker for PD prognosis.[31]

Conclusions

These results indicate that serum post-treatment TNF-α levels were significantly decreased in both groups and that TNF-α levels were significantly lower in the study group than in the control group. Furthermore, post-treatment serum TNF-α levels in the study group were significantly and positively correlated with UPDRS1, UPDRS2, and UPDRS3 scores, suggesting that pramipexole combined with levodopa provides benefits in PD by reducing serum TNF-α levels.

In summary, pramipexole combined with levodopa relieved PD symptoms and patients’ QOL, potentially via suppressing serum TNF-α levels. However, there are several limitations to this study. First, the optimal dosage of pramipexole for combinatorial use with levodopa was not explored. Second, only a small number of outcome measures were assessed. Finally, the specific regulatory mechanisms of TNF-α in PD were not comprehensively discussed. Therefore, future studies are necessary to address these limitations and validate our findings.