The D1/D5 Dopamine Partial Agonist PF-06412562 in Advanced-Stage Parkinson's Disease: A Feasibility Study

Prevalence of Late-Stage Parkinson's Disease in the US Healthcare System: Insights from TriNetX

BACKGROUND

Patients in late-stage Parkinson's disease (PDLS) are caregiver-dependent, have low quality of life, and higher healthcare costs.

OBJECTIVE

To estimate the prevalence of PDLS patients in the current US healthcare system.

METHODS

We downloaded the 2010-2022 data from the TriNetX Diamond claims network that consists of 92 US healthcare sites. PD was identified using standard diagnosis codes, and PDLS was identified by the usage of wheelchair dependence, personal care assistance, and/or presence of diagnoses of dementia. Age of PDLS identification and survival information were obtained and stratified by demographic and the disability subgroups.

RESULTS

We identified 1,031,377 PD patients in the TriNetX database. Of these, 18.8% fitted our definition of PDLS (n = 194,297), and 10.2% met two or more late-stage criteria. Among all PDLS, the mean age of PDLS identification was 78.1 (±7.7) years, and 49% were already reported as deceased. PDLS patients were predominantly male (58.5%) with similar distribution across PDLS subgroups. The majority did not have race (71%) or ethnicity (69%) information, but for the available information >90% (n = 53,162) were White, 8.2% (n = 5121) Hispanic/Latino, 7.8% (n = 4557) Black, and <0.01% (n = 408) Asian. Of the PDLS cohort, 71.6% identified with dementia, 12.9% had personal care assistance, and 4.8% were wheelchair-bound.

CONCLUSIONS

Late-stage patients are a significant part of the PD landscape in the current US healthcare system, and largely missed by traditional motor-based disability staging. It is imperative to include this population as a clinical, social, and research priority. © 2024 The Author(s). Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Rationale and Development of Tavapadon, a D1/D5-Selective Partial Dopamine Agonist for the Treatment of Parkinson's Disease

Currently, available therapeutics for the treatment of Parkinson's disease (PD) fail to provide sustained and predictable relief from motor symptoms without significant risk of adverse events (AEs). While dopaminergic agents, particularly levodopa, may initially provide strong motor control, this efficacy can vary with disease progression. Patients may suffer from motor fluctuations, including sudden and unpredictable drop-offs in efficacy. Dopamine agonists (DAs) are often prescribed during early-stage PD with the expectation they will delay the development of levodopa-associated complications, but currently available DAs are less effective than levodopa for the treatment of motor symptoms. Furthermore, both levodopa and DAs are associated with a significant risk of AEs, many of which can be linked to strong, repeated stimulation of D2/D3 dopamine receptors. Targeting D1/D5 dopamine receptors has been hypothesized to produce strong motor benefits with a reduced risk of D2/D3-related AEs, but the development of D1-selective agonists has been previously hindered by intolerable cardiovascular AEs and poor pharmacokinetic properties. There is therefore an unmet need in PD treatment for therapeutics that provide sustained and predictable efficacy, with strong relief from motor symptoms and reduced risk of AEs. Partial agonism at D1/D5 has shown promise for providing relief from motor symptoms, potentially without the AEs associated with D2/D3-selective DAs and full D1/D5-selective DAs. Tavapadon is a novel oral partial agonist that is highly selective at D1/D5 receptors and could meet these criteria. This review summarizes currently available evidence of tavapadon's therapeutic potential for the treatment of early through advanced PD.

Fluoroalkoxylated C-3 and C-9 (S)-12-bromostepholidine analogues with D1R antagonist activity

To illuminate the tolerance of fluoroalkoxylated groups at the C-3 and C-9 positions of tetrahydroprotoberberines (THPBs) on D1R activity, C-3 and C-9 fluoroalkoxylated analogues of (S)-12-bromostepholidine were prepared and evaluated. All compounds examined were D1R antagonists as measured by a cAMP assay. Our structure-activity studies herein indicate that the C-3 position tolerates a 1,1-difluoroethoxy substituent for D1R antagonist activity. Compound 13a was the most potent cAMP-based D1R antagonist identified and was also found to antagonize β-arrestin translocation in a TANGO assay. Affinity assessments at other dopamine receptors revealed that 13a is selective for D1R and unlike other naturally-occurring THPBs such as (S)-stepholidine, lacks D2R affinity. In preliminary biopharmaceutical assays, excellent BBB permeation was observed for 13a. Further pharmacological studies are warranted on (S)-stepholidine congeners to harvest their potential as a source of novel, druggable D1R-targeted agents.

Characterization of behavioral changes in T-maze alternation from dopamine D1 agonists with different receptor coupling mechanisms

RATIONALE

Dopamine D1 receptor agonists have been shown to improve working memory, but often have a non-monotonic (inverted-U) dose-response curve. One hypothesis is that this may reflect dose-dependent differential engagement of D1 signaling pathways, a mechanism termed functional selectivity or signaling bias.

OBJECTIVES AND METHODS

To test this hypothesis, we compared two D1 ligands with different signaling biases in a rodent T-maze alternation task. Both tested ligands (2-methyldihydrexidine and CY208243) have high intrinsic activity at cAMP signaling, but the former also has markedly higher intrinsic activity at D1-mediated recruitment of β-arrestin. The spatial working memory was assessed via the alternation behavior in the T-maze where the alternate choice rate quantified the quality of the memory and the duration prior to making a choice represented the decision latency.

RESULTS

Both D1 drugs changed the alternate rate and the choice latency in a dose-dependent manner, albeit with important differences. 2-Methyldihydrexidine was somewhat less potent but caused a more homogeneous improvement than CY208243 in spatial working memory. The maximum changes in the alternate rate and the choice latency tended to occur at different doses for both drugs.

CONCLUSIONS

These data suggest that D1 signaling bias in these two pathways (cAMP vs β-arrestin) has complex effects on cognitive processes as assessed by T-maze alternation. Understanding these mechanisms should allow the identification or discovery of D1 agonists that can provide superior cognitive enhancement.

Role of dopamine agonists in Parkinson's disease therapy

Dopamine agonists are an important component of Parkinson's therapy. When weighing up the various therapy options, therapy with levodopa has recently been increasingly preferred due to its stronger efficacy and the ostensibly lower rate of side effects. The advantage of the lower incidence of motor complications during therapy with dopamine agonists was neglected. The occurrence of side effects can be explained by the different receptor affinity to the individual dopaminergic and non-dopaminergic receptors of the individual dopamine agonists. However, the different affinity to individual receptors also explains the different effect on individual Parkinson symptoms and can, therefore, contribute to a targeted use of the different dopamine agonists. Since comparative studies on the differential effect of dopamine agonists have only been conducted for individual substances, empirical knowledge of the differential effect is of great importance. Therefore, the guidelines for the treatment of Parkinson's disease do not consider the differential effect of the dopamine agonists. The historical consideration of dopamine agonists within Parkinson's therapy deserves special attention to be able to classify the current discussion about the significance of dopamine agonists.

Advances in the Therapeutic Use of Non-Ergot Dopamine Agonists in the Treatment of Motor and Non-Motor Symptoms of Parkinson's Disease

Dopamine (DA) agonists, as an excellent dopamine replacement therapy for patients with early and advanced Parkinson's disease (PD), play a vital role in controlling motor and several nonmotor symptoms. Besides, the application of DA agonists may delay levodopa therapy and the associated risk of motor complications. Indeed, each DA agonist has unique pharmacokinetic and pharmacodynamic characteristics and therefore has different therapeutic efficacy and safety profile. The comorbidities, significant non-motor manifestations, concomitant medications, and clinical features of PD individuals should guide the selection of a specific DA agonist to provide a more patient-tailored treatment option. Thorough knowledge of DA agonists helps clinicians better balance clinical efficacy and side effects. Therefore, this review refers to recent English-written articles on DA agonist therapy for PD patients and summarizes the latest findings on non-ergot DA agonists as well as the advantages and disadvantages of each compound to help clinicians in the selection of a specific DA agonist. In addition, novel D1/D5 partial agonists and new formulations of DA agonists are also discussed.

Dose-Dependent Regulation on Prefrontal Neuronal Working Memory by Dopamine D1 Agonists: Evidence of Receptor Functional Selectivity-Related Mechanisms

Low doses of dopamine D1 agonists improve working memory-related behavior, but high doses eliminate the improvement, thus yielding an ‘inverted-U’ dose-response curve. This dose-dependency also occurs at the single neuron level in the prefrontal cortex where the cellular basis of working memory is represented. Because signaling mechanisms are unclear, we examined this process at the neuron population level. Two D1 agonists (2-methyldihydrexidine and CY208,243) having different signaling bias were tested in rats performing a spatial working memory-related T-maze task. 2-Methyldihydrexidine is slightly bias toward D1-mediated β-arrestin-related signaling as it is a full agonist at adenylate cyclase and a super-agonist at β-arrestin recruitment, whereas CY208,243 is slightly bias toward D1-mediated cAMP signaling as it has relatively high intrinsic activity at adenylate cyclase, but is a partial agonist at β-arrestin recruitment. Both compounds had the expected inverted U dose-dependency in modulating prefrontal neuronal activities, albeit with important differences. Although CY208,243 was superior in improving the strength of neuronal outcome sensitivity to the working memory-related choice behavior in the T-maze, 2-methyldihydrexidine better reduced neuron-to-neuron variation. Interestingly, at the neuron population level, both drugs affected the percentage, uniformity, and ensemble strength of neuronal sensitivity in a complicated dose-dependent fashion, but the overall effect suggested higher efficiency and potency of 2-methyldihydrexidine compared to CY208,243. The differences between 2-methyldihydrexidine and CY208,243 may be related to their specific D1 signaling. These results suggest that D1-related dose-dependent regulation of working memory can be modified differentially by functionally selective ligands, theoretically increasing the balance between desired and undesired effects.

A dopamine D1 agonist vs. methylphenidate in modulating prefrontal cortical working memory

Methylphenidate is used widely to treat symptoms of attention-deficit/hyperactivity disorder (ADHD), but like other stimulants has significant side effects. This study utilized a rodent model (spontaneously hypertensive rat) of spatial working memory (sWM) to compare the effects of methylphenidate with the novel dopamine D₁-like receptor agonist 2-methyldihydrexidine. Acute oral administration of methylphenidate (1.5 mg/kg) caused sWM improvement in half of the tested rats, but impairment in the others. Both improvement or impairment were eliminated by administration of the D₁ antagonist SCH39266 directly into the prefrontal cortex (PFC). Conversely, 2-methyldihydrexidine showed greater sWM improvement compared to methylphenidate without significant impairment in any subject. Its effects correlated negatively with vehicle-treated baseline performance (i.e., rats with lower baseline performance improved more than rats with higher baseline performance). These behavioral effects were associated with neural activities in the PFC. Single neuron firing rate was changed, leading to the alteration in neuronal preference to correct or error behavioral responses. Overall, 2-methyldihydrexidine was superior to methylphenidate in decreasing the neuronal preference, prospectively, in the animals whose behavior was improved. In contrast, methylphenidate, but not 2-methyldihydrexidine, significantly decreased neuronal preference, retrospectively, in those animals who had impaired performance. These results suggest that a D₁ agonist may be more effective than methylphenidate in regulating sWM-related behavior through neural modulation of the PFC, and thus may be superior to methylphenidate or other stimulants as ADHD pharmacotherapy. Significance Statement Methylphenidate is effective in ADHD by its indirect agonist stimulation of dopamine and/or adrenergic receptors, but the precise effects on specific targets are unclear. We compared methylphenidate to a dopamine D₁ receptor-selective agonist by investigating effects on working memory occurring via neural modulation in the prefrontal cortex. The data suggest that pharmacological treatment selectively targeting the dopamine D₁ may offer a superior approach to ADHD pharmacotherapy.

The Signaling and Pharmacology of the Dopamine D1 Receptor

The dopamine D1 receptor (D1R) is a Gα_s/olf-coupled GPCR that is expressed in the midbrain and forebrain, regulating motor behavior, reward, motivational states, and cognitive processes. Although the D1R was initially identified as a promising drug target almost 40 years ago, the development of clinically useful ligands has until recently been hampered by a lack of suitable candidate molecules. The emergence of new non-catechol D1R agonists, biased agonists, and allosteric modulators has renewed clinical interest in drugs targeting this receptor, specifically for the treatment of motor impairment in Parkinson's Disease, and cognitive impairment in neuropsychiatric disorders. To develop better therapeutics, advances in ligand chemistry must be matched by an expanded understanding of D1R signaling across cell populations in the brain, and in disease states. Depending on the brain region, the D1R couples primarily to either Gα_s or Gα_olf through which it activates a cAMP/PKA-dependent signaling cascade that can regulate neuronal excitability, stimulate gene expression, and facilitate synaptic plasticity. However, like many GPCRs, the D1R can signal through multiple downstream pathways, and specific signaling signatures may differ between cell types or be altered in disease. To guide development of improved D1R ligands, it is important to understand how signaling unfolds in specific target cells, and how this signaling affects circuit function and behavior. In this review, we provide a summary of D1R-directed signaling in various neuronal populations and describe how specific pathways have been linked to physiological and behavioral outcomes. In addition, we address the current state of D1R drug development, including the pharmacology of newly developed non-catecholamine ligands, and discuss the potential utility of D1R-agonists in Parkinson's Disease and cognitive impairment.

Functional Selectivity of Dopamine D1 Receptor Signaling: Retrospect and Prospect

Research progress on dopamine D₁ receptors indicates that signaling no longer is limited to G protein-dependent cyclic adenosine monophosphate phosphorylation but also includes G protein-independent β-arrestin-related mitogen-activated protein kinase activation, regulation of ion channels, phospholipase C activation, and possibly more. This review summarizes recent studies revealing the complexity of D₁ signaling and its clinical implications, and suggests functional selectivity as a promising strategy for drug discovery to magnify the merit of D₁ signaling. Functional selectivity/biased receptor signaling has become a major research front because of its potential to improve therapeutics through precise targeting. Retrospective pharmacological review indicated that many D₁ ligands have some degree of mild functional selectivity, and novel compounds with extreme bias at D₁ signaling were reported recently. Behavioral and neurophysiological studies inspired new methods to investigate functional selectivity and gave insight into the biased signaling of several drugs. Results from recent clinical trials also supported D₁ functional selectivity signaling as a promising strategy for discovery and development of better therapeutics.

Current Therapies in Clinical Trials of Parkinson's Disease: A 2021 Update

Parkinson's disease (PD) is a progressive neurodegenerative disorder that currently has no cure, but treatments are available to improve PD symptoms and maintain quality of life. In 2020, about 10 million people worldwide were living with PD. In 1970, the United States Food and Drug Administration approved the drug levodopa as a dopamine replacement to manage PD motor symptoms; levodopa-carbidopa combination became commercialized in 1975. After over 50 years of use, levodopa is still the gold standard for PD treatment. Unfortunately, levodopa therapy-induced dyskinesia and OFF symptoms remain unresolved. Therefore, we urgently need to analyze each current clinical trial's status and therapeutic strategy to discover new therapeutic approaches for PD treatment. We surveyed 293 registered clinical trials on ClinicalTrials.gov from 2008 to 16 June 2021. After excluded levodopa/carbidopa derivative add-on therapies, we identified 47 trials as PD treatment drugs or therapies. Among them, 19 trials are in phase I (41%), 25 trials are in phase II (53%), and 3 trials are in phase III (6%). The three phase-III trials use embryonic dopamine cell implant, 5-HT_1A receptor agonist (sarizotan), and adenosine A_2A receptor antagonist (caffeine). The therapeutic strategy of each trial shows 29, 5, 1, 5, 5, and 2 trials use small molecules, monoclonal antibodies, plasma therapy, cell therapy, gene therapy, and herbal extract, respectively. Additionally, we discuss the most potent drug or therapy among these trials. By systematically updating the current trial status and analyzing the therapeutic strategies, we hope this review can provide new ideas and insights for PD therapy development.

D1, not D2, dopamine receptor activation dramatically improves MPTP-induced parkinsonism unresponsive to levodopa

Levodopa is the standard-of-care for Parkinson's disease, but continued loss of dopamine neurons with disease progression decreases its bioconversion to dopamine, leading to increased side effects and decreased efficacy. In theory, dopamine agonists could equal levodopa, but no approved oral "dopamine agonist" matches the efficacy of levodopa. There are consistent data in both primate models and in Parkinson's disease showing that selective high intrinsic activity D1 agonists can equal levodopa in efficacy. There are, however, no data on whether such compounds would be effective in severe disease when levodopa efficacy is low or absent. We compared two approved antiparkinson drugs (levodopa and the D2/3 agonist bromocriptine) with the experimental selective D1 full agonist dihydrexidine in two severely parkinsonian MPTP-treated non-human primates. Bromocriptine caused no discernible improvement in parkinsonian signs, whereas levodopa caused a small transient improvement in one of the two subjects. Conversely, the full D1 agonist dihydrexidine caused a dramatic improvement in both subjects, decreasing parkinsonian signs by ca. 75%. No attenuation of dihydrexidine effects was observed when the two subjects were pretreated with the D2 antagonist remoxipride. These data provide evidence that selective D1 agonists may provide profound antiparkinson symptomatic relief even when the degree of nigrostriatal degeneration is so severe that current drugs are ineffective. Until effective disease-modifying therapies are discovered, high intrinsic activity D1 agonists may offer a major therapeutic advance in improving the quality of life, and potentially the longevity, of late stage Parkinson's patients.