Disentangling Bradykinesia and Rigidity in Parkinson Disease: Evidence from Short- and Long-Term Subthalamic Nucleus Deep Brain Stimulation.

OBJECTIVE: Bradykinesia and rigidity are considered closely related motor signs in Parkinson disease (PD), but recent neurophysiological findings suggest distinct pathophysiological mechanisms. This study aims to examine and compare longitudinal changes in bradykinesia and rigidity in PD patients treated with bilateral subthalamic nucleus deep brain stimulation (STN-DBS). METHODS: In this retrospective cohort study, the clinical progression of appendicular and axial bradykinesia and rigidity was assessed up to 15 years after STN-DBS in the best treatment conditions (ON medication and ON stimulation). The severity of bradykinesia and rigidity was examined using ad hoc composite scores from specific subitems of the Unified Parkinson's Disease Rating Scale motor part (UPDRS-III). Short- and long-term predictors of bradykinesia and rigidity were analyzed through linear regression analysis, considering various preoperative demographic and clinical data, including disease duration and severity, phenotype, motor and cognitive scores (eg, frontal score), and medication. RESULTS: A total of 301 patients were examined before and 1 year after surgery. Among them, 101 and 56 individuals were also evaluated at 10-year and 15-year follow-ups, respectively. Bradykinesia significantly worsened after surgery, especially in appendicular segments (p < 0.001). Conversely, rigidity showed sustained benefit, with unchanged clinical scores compared to preoperative assessment (p > 0.05). Preoperative motor disability (eg, composite scores from the UPDRS-III) predicted short- and long-term outcomes for both bradykinesia and rigidity (p < 0.01). Executive dysfunction was specifically linked to bradykinesia but not to rigidity (p < 0.05). INTERPRETATION: Bradykinesia and rigidity show long-term divergent progression in PD following STN-DBS and are associated with independent clinical factors, supporting the hypothesis of partially distinct pathophysiology. ANN NEUROL 2024.

Differential profiles of serum cytokines in Parkinson's disease according to disease duration.

OBJECTIVE: Neurodegeneration and neuroinflammation are two intertwined mechanisms contributing to the pathophysiology of Parkinson's disease. Whether circulating biomarkers reflecting those two processes differ according to disease duration remains to be established. The present study was conducted to characterize the biomarkers individuals with PD with short (≤5 years) or long disease duration (>5 years). METHODS: We consecutively enrolled 104 patients with Parkinson's disease and evaluated them using validated clinical scales (MDS-UPDRS, Hoehn and Yahr staging, MMSE). Serum samples were assayed for the following biomarkers: neurofilament light chain (NfL), brain-derived neurotrophic factor (BDNF), interleukin (IL-) 1ß, 4, 5, 6, 10, 17, interferon-γ, and tumor necrosis factor α. RESULTS: Mean age of participants was 66.0 ± 9.6 years and 45 (34%) were women. The average disease duration was 8 ± 5 years (range 1 to 19 years). Patients with short disease duration (≤ 5 years) showed a pro-inflammatory profile, with significantly higher levels of pro-inflammatory IL-1ß and lower concentrations of IL-5, IL-10 and IL-17 (p < 0.05). NfL serum levels showed a positive correlation with disease duration and age (respectively rho = 0.248, p = 0.014 and rho = 0.559, p < 0.001) while an opposite pattern was detected for BDNF (respectively rho -0,187, p = 0.034 and rho = -0.245, p = 0.014). CONCLUSIONS: Our findings suggest that a pro-inflammatory status may be observed in PD patients in the early phases of the disease, independently from age.

Long-term safety and efficacy of frameless subthalamic deep brain stimulation in Parkinson's disease.

BACKGROUND: Bilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) is standard of care for Parkinson's disease (PD) patients and a correct lead placement is crucial to obtain good clinical outcomes. Evidence demonstrating the targeting accuracy of the frameless technique for DBS, along with the advantages for patients and clinicians, is solid, while data reporting long-term clinical outcomes for PD patients are still lacking. OBJECTIVES: The study aims to assess the clinical safety and efficacy of frameless bilateral STN-DBS in PD patients at 5 years from surgery. METHODS: Consecutive PD patients undergoing bilateral STN-DBS with a frameless system were included in this single-center retrospective study. Clinical features, including the Unified Parkinson's Disease Rating Scale (UPDRS) in its total motor score and axial sub-scores, and pharmacological regimen were assessed at baseline, 1 year, 3 years, and 5 years after surgery. The adverse events related to the procedure, stimulation, or the presence of the hardware were systematically collected. RESULTS: Forty-one PD patients undergone bilateral STN-DBS implantation were included in the study and fifteen patients already completed the 5-year observation. No complications occurred during surgery and the perioperative phase, and no unexpected serious adverse event occurred during the entire follow-up period. At 5 years from surgery, there was a sustained motor efficacy of STN stimulation: STN-DBS significantly improved the off-stim UPDRS III score at 5 years by 37.6% (P < 0.001), while the dopaminergic medications remained significantly reduced compared to baseline (- 21.6% versus baseline LEDD; P = 0.036). CONCLUSIONS: Our data support the use of the frameless system for STN-DBS in PD patients, as a safe and well-tolerated technique, with long-term clinical benefits and persistent motor efficacy at 5 years from the surgery.

Intraoperative Local Field Potential Beta Power and Three-Dimensional Neuroimaging Mapping Predict Long-Term Clinical Response to Deep Brain Stimulation in Parkinson Disease: A Retrospective Study.

BACKGROUND: Directional deep brain stimulation (DBS) leads allow a fine-tuning control of the stimulation field, however, this new technology could increase the DBS programming time because of the higher number of the possible combinations used in directional DBS than in standard nondirectional electrodes. Neuroimaging leads localization techniques and local field potentials (LFPs) recorded from DBS electrodes implanted in basal ganglia are among the most studied biomarkers for DBS programing. OBJECTIVE: This study aimed to evaluate whether intraoperative LFPs beta power and neuroimaging reconstructions correlate with contact selection in clinical programming of DBS in patients with Parkinson disease (PD). MATERIALS AND METHODS: In this retrospective study, routine intraoperative LFPs recorded from all contacts in the subthalamic nucleus (STN) of 14 patients with PD were analyzed to calculate the beta band power for each contact. Neuroimaging reconstruction obtained through Brainlab Elements Planning software detected contacts localized within the STN. Clinical DBS programming contact scheme data were collected after one year from the implant. Statistical analysis evaluated the diagnostic performance of LFPs beta band power and neuroimaging data for identification of the contacts selected with clinical programming. We evaluated whether the most effective contacts identified based on the clinical response after one year from implant were also those with the highest level of beta activity and localized within the STN in neuroimaging reconstruction. RESULTS: LFPs beta power showed a sensitivity of 67%, a negative predictive value (NPV) of 84%, a diagnostic odds ratio (DOR) of 2.7 in predicting the most effective contacts as evaluated through the clinical response. Neuroimaging reconstructions showed a sensitivity of 62%, a NPV of 77%, a DOR of 1.20 for contact effectivity prediction. The combined use of the two methods showed a sensitivity of 87%, a NPV of 87%, a DOR of 2.7 for predicting the clinically more effective contacts. CONCLUSIONS: The combined use of LFPs beta power and neuroimaging localization and segmentations predict which are the most effective contacts as selected on the basis of clinical programming after one year from implant of DBS. The use of predictors in contact selection could guide clinical programming and reduce time needed for it.

Predictors of Long-Term Outcome of Subthalamic Stimulation in Parkinson Disease.

OBJECTIVE: This study was undertaken to identify preoperative predictive factors of long-term motor outcome in a large cohort of consecutive Parkinson disease (PD) patients with bilateral subthalamic nucleus deep brain stimulation (STN-DBS). METHODS: All consecutive PD patients who underwent bilateral STN-DBS at the Grenoble University Hospital (France) from 1993 to 2015 were evaluated before surgery, at 1 year (short-term), and in the long term after surgery. All available demographic variables, neuroimaging data, and clinical characteristics were collected. Preoperative predictors of long-term motor outcome were investigated by performing survival and univariate/multivariate Cox regression analyses. Loss of motor benefit from stimulation in the long term was defined as a reduction of less than 25% in the Movement Disorder Society-sponsored revision of the Unified Parkinson's Disease Rating Scale (MDS-UPDRS) part III scores compared to the baseline off-medication scores. As a secondary objective, potential predictors of short-term motor outcome after STN-DBS were assessed by performing univariate and multivariate linear regression analyses. RESULTS: In the long-term analyses (mean follow-up = 8.4 ± 6.26 years, median = 10 years, range = 1-17 years), 138 patients were included. Preoperative higher frontal score and off-medication MDS-UPDRS part III scores predicted a better long-term motor response to stimulation, whereas the presence of vascular changes on neuroimaging predicted a worse motor outcome. In 357 patients with available 1-year follow-up, preoperative levodopa response, tremor dominant phenotype, baseline frontal score, and off-medication MDS-UPDRS part III scores predicted the short-term motor outcome. INTERPRETATION: Frontal lobe dysfunction, disease severity in the off-medication condition, and the presence of vascular changes on neuroimaging represent the main preoperative clinical predictors of long-term motor STN-DBS effects. ANN NEUROL 2021;89:587-597.

Frameless stereotaxy in subthalamic deep brain stimulation: 3-year clinical outcome.

BACKGROUND: In most centers, the surgery of deep brain stimulation (DBS) is performed using a stereotactic frame. Compared with frame-based technique, frameless stereotaxy reduces the duration of surgical procedure and patient's discomfort, with lead placing accuracy equivalent after the learning curve. Although several studies have investigated the targeting accuracy of this technique, only a few studies reported clinical outcomes, with data of short-term follow-up. OBJECTIVE: To assess clinical efficacy and safety of frameless bilateral subthalamic nucleus (STN) DBS in Parkinson's disease (PD) patients at 1- and 3-year follow-up. METHODS: Consecutive PD patients who underwent bilateral STN-DBS with a manual adjustable frameless system were included in the study. The data were collected retrospectively. RESULTS: Eighteen PD patients underwent bilateral STN-DBS implant and were included in the study. All patients completed 1-year observation and ten of them completed 3-year observation. At 1-year follow-up, motor efficacy of STN stimulation in off-med condition was of 30.1% (P = 0.003) and at 3-year follow-up was of 36.3%, compared with off-stim condition at 3-year follow-up (P = 0.005). Dopaminergic drugs were significantly reduced by 31.2% 1 year after the intervention (P = 0.003) and 31.7% 3 years after the intervention (P = 0.04). No serious adverse events occurred during surgery. CONCLUSIONS: Frameless stereotaxy is an effective and safe technique for DBS surgery at 1- and 3-year follow-up, with great advantages for patients' discomfort during surgery.

Should We Consider Deep Brain Stimulation Discontinuation in Late-Stage Parkinson's Disease?

BACKGROUND: Subthalamic deep brain stimulation (STN-DBS) effects may decrease with Parkinson's disease (PD) progression. There is no indication if, when, and how to consider the interruption of DBS treatment in late-stage PD. The objective of the current study was to investigate the percentage of "poor stimulation responders" among late-stage PD patients for elaborating an algorithm to decide whether and when DBS discontinuation may be considered. METHODS: Late-stage PD patients (Hoehn Yahr stage ≥4 and Schwab and England Scale <50 in medication on/stimulation on condition) treated with STN-DBS for at least 5 years underwent a crossover, double-blind, randomized evaluation of acute effects of stimulation. Physicians, caregivers, and patients were blinded to stimulation conditions. Poor stimulation responders (MDS-UPDRS part III change <10% between stimulation on/medication off and stimulation off/medication off) maintained the stimulation off/medication on condition for 1 month for open-label assessment. RESULTS: Thirty-six patients were included. The acute effect of stimulation was significant (17% MDS-UPDRS part III), with 80% of patients classified as "good responders." Seven patients were classified as "poor stimulation responders," and the stimulation was switched off, but in 4 cases the stimulation was switched back "on" because of worsening of parkinsonism and dysphagia with a variable time delay (up to 10 days). No serious adverse effects occurred. CONCLUSIONS: The vast majority of late-stage PD patients (92%) show a meaningful response to STN-DBS. Effects of stimulation may take days to disappear after its discontinuation. We present a safe and effective decisional algorithm that could guide physicians and caregivers in making challenging therapeutic decisions in late-stage PD. © 2020 International Parkinson and Movement Disorder Society.

GBA-Related Parkinson's Disease: Dissection of Genotype-Phenotype Correlates in a Large Italian Cohort.

BACKGROUND: Variants in GBA are the most common genetic risk factor for Parkinson's disease (PD). The impact of different variants on the PD clinical spectrum is still unclear. OBJECTIVES: We determined the frequency of GBA-related PD in Italy and correlated GBA variants with motor and nonmotor features and their occurrence over time. METHODS: Sanger sequencing of the whole GBA gene was performed. Variants were classified as mild, severe, complex, and risk. ß-glucocerebrosidase activity was measured. The Kaplan-Meier method and Cox proportional hazard regression models were performed. RESULTS: Among 874 patients with PD, 36 variants were detected in 14.3%, including 20.4% early onset. Patients with GBA-PD had earlier and more frequent occurrence of several nonmotor symptoms. Patients with severe and complex GBA-PD had the highest burden of symptoms and a higher risk of hallucinations and cognitive impairment. Complex GBA-PD had the lowest ß-glucocerebrosidase activity. CONCLUSIONS: GBA-PD is highly prevalent in Italy. Different types of mutations underlie distinct phenotypic profiles. © 2020 International Parkinson and Movement Disorder Society.

Central conduction abnormalities in patients receiving levodopa-carbidopa intestinal gel infusion.

In recent years, several studies have reported a relatively high frequency of polyneuropathy in patients with Parkinson's disease (PD), in particular, in patients receiving levodopa-carbidopa intestinal gel (LCIG) infusion. In spite of the several patients investigated with nerve conduction studies, no study has prospectively explored a possible central nervous system involvement of patients receiving LCIG infusion. We prospectively evaluated eight PD patients receiving LCIG infusion, who underwent neurophysiological evaluations with nerve conduction studies, visual, somatosensory and motor evoked potentials before LCIG infusion, and 1 and 6 months after. At 6 months follow-up, we found significant reduction in sural nerve SNAP amplitude, increase of central sensory conduction time N22-P40, and increases of central motor conduction time recorded from I dorsal interosseous and tibialis anterior. In PD patients with LCIG infusion, we found a subclinical neurophysiological impairment of both peripheral and central nervous system.

The treatment of dystonic tremor: a systematic review.

Tremor is one of the clinical manifestations of dystonia; however, there are no specific therapeutic trials evaluating the efficacy of treatments for dystonic tremor (DT), tremor associated with dystonia or primary writing tremor (PWT). We systematically reviewed the literature available up to July 2013 on the treatment of these tremors and retrieved the data of 487 patients published in 43 papers detailing the effects of given interventions on tremor severity. Treatment outcome was highly variable, depending on the specific type of intervention and tremor distribution. No specifically designed studies were available for the treatment of tremor associated with dystonia. As for the other tremors, drug efficacy was generally disappointing and a moderate effect was only found with anticholinergics, tetrabenazine, clonazepam, ß-blockers and primidone; levodopa was only efficacious on tremor due to dopa-responsive dystonia. The largest amount of data was available for botulinum toxin injections, which provided a marked improvement, particularly for the management of axial tremors (head or vocal cords). In refractory DTs, deep brain stimulation of several targets was attempted. Deep brain stimulation of globus pallidus internus, thalamus or subthalamic area led to a marked improvement of dystonic axial or appendicular tremors in most cases refractory to other treatments. Few other non-invasive treatments, for example, orthotic device in PWT, have been used with anecdotal success. In conclusion, considering the lack of good-quality studies, future randomised controlled trials are needed. In absence of evidence-based guidelines, we propose an algorithm for the treatment of DT based on currently available data.

Aristotle's illusion reveals interdigit functional somatosensory alterations in focal hand dystonia.

In focal hand dystonia, the cortical somatosensory representation of the fingers is abnormal, with overlapping receptive fields and reduced interdigit separation. These abnormalities are associated with deficits in sensory perception, as previously demonstrated by applying tactile stimuli to one finger at a time. What is still unknown is whether the sensory deficits can be observed when tactile perception involves more than one finger. To address this issue, we applied 'Aristotle's illusion' to 15 patients with focal hand dystonia, 15 patients with dystonia not affecting the hand (blepharospasm and cervical dystonia) and 15 healthy control subjects. In this illusion, one object touching the contact point of two crossed fingertips is perceived as two objects by a blindfolded subject. The same object placed between two parallel fingertips is correctly perceived as one. The illusory doubling sensation is because of the fact that the contact point between the crossed fingers consists of non-adjacent and functionally unrelated skin regions, which usually send sensory signals to separate spots in the somatosensory cortex. In our study, participants were touched by one sphere between the second-third digits, the second-fourth digits and the fourth-fifth digits of both hands, either in crossed or in parallel position, and had to refer whether they felt one or two stimuli. The percentage of 'two stimuli' responses was an index of the illusory doubling. Both healthy control subjects and dystonic patients presented Aristotle's illusion when the fingers were crossed. However, patients with focal hand dystonia presented a significant reduction of the illusion when the sphere was placed between the crossed fourth and fifth digits of the affected hand. This reduction correlated with the severity of motor disease at the fingers. Similar findings were not observed in non-hand dystonia and control groups. The reduction of Aristotle's illusion in non-affected fingers and its preservation in affected fingers suggests dissociation between the abnormal processing of sensory signals and the motor impairment. Based on previous evidence showing that the sensory signals coming from the fourth digit determine lower activation in the somatosensory cortex than those coming from the fifth digit, we suggest that in the crossed position, the tactile information conveyed by the fifth digit prevailed over the fourth digit, thus resulting in the perception of one stimulus. The reduction of the illusory doubling perception, therefore, may represent the functional correlate of the different level of activation between the fourth and the fifth digit in the somatosensory cortex.

Neuroplasticity in levodopa-induced dyskinesias: An overview on pathophysiology and therapeutic targets.

Levodopa-induced dyskinesias (LIDs) are a common complication in patients with Parkinson's disease (PD). A complex cascade of electrophysiological and molecular events that induce aberrant plasticity in the cortico-basal ganglia system plays a key role in the pathophysiology of LIDs. In the striatum, multiple neurotransmitters regulate the different forms of physiological synaptic plasticity to provide it in a bidirectional and Hebbian manner. In PD, impairment of both long-term potentiation (LTP) and long-term depression (LTD) progresses with disease and dopaminergic denervation of striatum. The altered balance between LTP and LTD processes leads to unidirectional changes in plasticity that cause network dysregulation and the development of involuntary movements. These alterations have been documented, in both experimental models and PD patients, not only in deep brain structures but also at motor cortex. Invasive and non-invasive neuromodulation treatments, as deep brain stimulation, transcranial magnetic stimulation, or transcranial direct current stimulation, may provide strategies to modulate the aberrant plasticity in the cortico-basal ganglia network of patients affected by LIDs, thus restoring normal neurophysiological functioning and treating dyskinesias. In this review, we discuss the evidence for neuroplasticity impairment in experimental PD models and in patients affected by LIDs, and potential neuromodulation strategies that may modulate aberrant plasticity.

The role of small intestinal bacterial overgrowth in Parkinson's disease.

Parkinson's disease is associated with gastrointestinal motility abnormalities favoring the occurrence of local infections. The aim of this study was to investigate whether small intestinal bacterial overgrowth contributes to the pathophysiology of motor fluctuations. Thirty-three patients and 30 controls underwent glucose, lactulose, and urea breath tests to detect small intestinal bacterial overgrowth and Helicobacter pylori infection. Patients also underwent ultrasonography to evaluate gastric emptying. The clinical status and plasma concentration of levodopa were assessed after an acute drug challenge with a standard dose of levodopa, and motor complications were assessed by Unified Parkinson's Disease Rating Scale-IV and by 1-week diaries of motor conditions. Patients with small intestinal bacterial overgrowth were treated with rifaximin and were clinically and instrumentally reevaluated 1 and 6 months later. The prevalence of small intestinal bacterial overgrowth was significantly higher in patients than in controls (54.5% vs. 20.0%; P = .01), whereas the prevalence of Helicobacter pylori infection was not (33.3% vs. 26.7%). Compared with patients without any infection, the prevalence of unpredictable fluctuations was significantly higher in patients with both infections (8.3% vs. 87.5%; P = .008). Gastric half-emptying time was significantly longer in patients than in healthy controls but did not differ in patients based on their infective status. Compared with patients without isolated small intestinal bacterial overgrowth, patients with isolated small intestinal bacterial overgrowth had longer off time daily and more episodes of delayed-on and no-on. The eradication of small intestinal bacterial overgrowth resulted in improvement in motor fluctuations without affecting the pharmacokinetics of levodopa. The relapse rate of small intestinal bacterial overgrowth at 6 months was 43%. © 2013 Movement Disorder Society.

Effects of deep brain stimulation of the subthalamic nucleus on patients with Parkinson's disease: a machine-learning voice analysis.

Introduction: Deep brain stimulation of the subthalamic nucleus (STN-DBS) can exert relevant effects on the voice of patients with Parkinson's disease (PD). In this study, we used artificial intelligence to objectively analyze the voices of PD patients with STN-DBS. Materials and methods: In a cross-sectional study, we enrolled 108 controls and 101 patients with PD. The cohort of PD was divided into two groups: the first group included 50 patients with STN-DBS, and the second group included 51 patients receiving the best medical treatment. The voices were clinically evaluated using the Unified Parkinson's Disease Rating Scale part-III subitem for voice (UPDRS-III-v). We recorded and then analyzed voices using specific machine-learning algorithms. The likelihood ratio (LR) was also calculated as an objective measure for clinical-instrumental correlations. Results: Clinically, voice impairment was greater in STN-DBS patients than in those who received oral treatment. Using machine learning, we objectively and accurately distinguished between the voices of STN-DBS patients and those under oral treatments. We also found significant clinical-instrumental correlations since the greater the LRs, the higher the UPDRS-III-v scores. Discussion: STN-DBS deteriorates speech in patients with PD, as objectively demonstrated by machine-learning voice analysis.

Rivastigmine as alternative treatment for refractory REM behavior disorder in Parkinson's disease.

BACKGROUND: We report on a double-blind, crossover pilot trial for the treatment of rapid eye movement behavior disorder (RBD) in 12 patients with Parkinson's disease in whom conventional therapy failed. METHODS: We employed a patch of the cholinesterase inhibitor rivastigmine at a dose of 4.6 mg/24 hours for 3 weeks compared with placebo to reduce the frequency of RBD episodes. The number of RBD episodes was monitored by diaries of bed partners. RESULTS: Rivastigmine was well tolerated in most patients, with minor side effects, mainly related to peripheral cholinergic action, and significantly reduced the mean frequency of RBD episodes during the observation time. CONCLUSIONS: The results of this pilot trial need to be confirmed by further studies on a larger number of patients.

The neurobiology of falls.

Falling is a major clinical problem; especially, in elderly population as it often leads to fractures, immobilization, poor quality of life and life-span reduction. Given the growing body of evidences on the physiopathology of balance disorders in humans, in recent years the approach of research on falls has completely changed and new instruments and new definitions have been formulated. Among them, the definition of "idiopathic faller" (i.e. no overt cause for falling in a given subject) represented a milestone in building the "science of falling". This review deals with the new determinants of the neurobiology of falling: (1) the role of motor impairment and particularly of those "mild parkinsonian signs" frequently detectable in elderly subjects, (2) the role of executive and attentive resources when coping with obstacles, (3) the role of vascular lesions in "highest level gait disorder" (a condition tightly connected with senile gait, cautious gait and frailty), (4) the role of the failure of automaticity or inter-limbs coordination/symmetry during walking and such approach would definitely help the development of screening instrument for subjects at risk (still lacking in present days). This translational approach will lead to the development of specific therapeutic interventions.

Retrospective evaluation of the dose equivalence of Botox(®) and Dysport (®) in the management of blepharospasm and hemifacial spasm: a novel paradigm for a never ending story.

Botox(®) and Dysport(®) are the preparations of botulinum neurotoxin most widely used for therapeutic purposes. Several studies have addressed the topic of the equivalency ratio (D/B ratio) to be used in clinical practice and whether a reliable value exists is still a matter of debate. To this purpose, we ideated a novel paradigm by retrospectively examining the patients affected by hemifacial spasm and blepharospasm. We compared the pairs of treatments with a switch from one brand to the other undergone by the same patient in consecutive sessions with overlapping clinical outcome. Out of 2006 treatments, we found 51 treatment pairs. D/B ratio was extremely variable (range 1.2-13.3) and in most cases (65%) it was between 1:3 and 1:5. In conclusion, even if the 1:4 ratio might be reliable for clinical purpose, a true bioequivalence between Dysport(®) and Botox(®) might not exist due to the intrinsic difference in their pharmacokinetic properties.