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Just in Time: Considering Dopamine Receptor-Targeted Therapy in PD

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Released: November 02, 2026

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The introduction of selective D1/D5 receptor targeting represents a novel treatment option for patients with Parkinson's disease. This ClinicalThought reviews the roles of dopamine receptors in motor function, limitations of traditional dopamine agonists, and evidence for newer therapeutic approaches, examining how receptor selectivity, drug delivery, and tolerability may influence clinical practice.

Daniel Kremens
Daniel Kremens, MD, JD
Dopamine Receptor Therapeutics in PD

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For more than half a century, dopaminergic therapy for Parkinson's disease (PD) has largely rested on 2 pillars: levodopa, which restores dopamine nonselectively, and dopamine agonists, which traditionally have acted primarily at D2-like receptors. The approval of tavapadon in September 2026 represents a shift in this treatment paradigm. As the first approved selective D1/D5 receptor partial agonist, tavapadon directly targets the receptor family involved in the striatal direct pathway. Understanding the clinical significance of this mechanism requires revisiting the locations of D1 and D2 receptors, their signaling mechanisms, and the role of their imbalance in shaping PD symptomatology and treatment complications.

The 5 dopamine receptors fall into 2 families defined by their coupling to different G-protein subtypes: D1-like receptors (D1 and D5) and D2-like receptors (D2, D3, and D4). These receptors are distributed across multiple pathways, including the mesocortical, mesolimbic, and nigrostriatal pathways. The nigrostriatal pathway, which is particularly important in PD because of its role in motor control, primarily involves D1 and D2 receptors found in distinct neuronal populations. Dopamine modulates corticostriatal signaling to enable motor control. In the striatum, D1 receptors are concentrated on direct pathway spiny neurons. Dopamine acting at D1 receptors activates these neurons, inhibiting basal ganglia output and thereby releasing thalamocortical motor drive. D2 receptors predominate in spiny neurons of the indirect pathway; dopamine acting at these receptors tends to inhibit this braking pathway. Receptor affinity may also play a role in the differential impact of dopamine in each pathway; it is hypothesized that D2 receptors in their high-affinity state respond to low tonic dopamine levels, whereas D1 receptors are engaged mainly by phasic bursts of dopamine release. Loss of nigrostriatal neurons in PD removes dopamine from both pathways simultaneously. Direct pathway spiny projection neurons lose D1 excitation and become less active, whereas indirect pathway spiny projection neurons lose D2 inhibition and become overactive. Both changes converge on excessive output from the globus pallidus internus and substantia nigra pars reticulata, suppressing thalamic activity and contributing to bradykinesia and rigidity, hallmark motor symptoms of PD. Within this model, the direct pathway is as much a therapeutic target as the indirect one.

Traditional dopamine agonists primarily target the D2-like receptor family. For example, pramipexole, ropinirole, and rotigotine act mainly at D2/D3 receptors. Although effective, these agents generally provide less motor benefit than levodopa. Their D3 activity in the limbic striatum is thought to contribute to impulse control disorders, along with other adverse events (AEs) include somnolence, sudden sleep onset, edema, and hallucinations. These tolerability concerns have made some healthcare professionals cautious about prescribing traditional D2-like agonists. Levodopa restores dopamine signaling at both receptor families but often produces pulsatile dopaminergic stimulation, which can contribute to wearing off and levodopa-induced dyskinesia over time.

Tavapadon offers a different pharmacologic approach. It is an orally active D1/D5 partial agonist with a long half-life that supports once-daily dosing. Its selective activity at D1 and D5 receptors may help avoid some of the AEs associated with D2-like receptor stimulation. The clinical evidence supporting tavapadon comes from the phase III TEMPO development program, which included placebo-controlled trials in adults 40-80 years of age. TEMPO-1 evaluated 529 patients with early PD who received fixed doses of tavapadon at either 5 or 15 mg or placebo. At Week 26, scores on Part II of the Unified PD Rating Scale (UPDRS), which assesses patient-reported experiences of daily living, improved by 1.6 and 1.7 points with the 5-mg and 15-mg doses, respectively. These improvements were statistically significant and represent a 34% treatment difference from placebo. TEMPO-2, a flexible-dose trial, also demonstrated positive results in Part II of the UPDRS. TEMPO-3 evaluated tavapadon as adjunctive therapy in levodopa-treated patients, demonstrating an improvement in ON time without troublesome dyskinesia of 1.1 hours vs placebo. Daily OFF time was also reduced by 1.9 hours from baseline. Common AEs in the monotherapy trials included nausea, headache, dizziness, fatigue, dysgeusia, and dry mouth. In TEMPO-3, reported AEs included dyskinesia, orthostatic hypotension, and hallucinations.

The theoretical rationale for tavapadon's clinical utility follows from its effects on basal ganglia circuitry. By restoring drive to the direct pathway, it addresses the component of basal ganglia signaling not directly stimulated by traditional D2/D3 agonists. Its selectivity for D1/D5 receptors, with sparing of D2/D3 receptors, may reduce limbic and sleep-related effects that limit the use of current D2-like agonists, including impulse control disorders and somnolence. Partial agonism is another important design choice. It provides the potential for steady, submaximal D1 stimulation rather than supraphysiologic stimulation implicated in dyskinesia and tolerability concerns. Once-daily dosing and a long half-life may also promote more continuous receptor stimulation, a principle long thought to protect against motor complications. From a clinical perspective, this positions tavapadon as a potential treatment option in 3 settings. First, it may serve as initial monotherapy to delay levodopa initiation, particularly in younger patients who have a long disease course ahead of them. Second, it may be used as adjunctive therapy for patients experiencing wearing off, supported by the improvements in ON time observed in TEMPO-3. Third, it may offer an alternative for patients who cannot tolerate traditional D2/D3 agonists. In the TEMPO open-label extension study, 86.4% to 94% of patients receiving tavapadon as monotherapy did not need to initiate levodopa, and 81.3 to 87.6% of patients receiving adjunctive therapy did not need to increase their levodopa dose. When modifications were needed, patients were twice as likely to reduce their levodopa dose (~10%) than to increase it (~5%), keeping the mean daily dose under 1000 mg.

Several questions remain before these potential advantages can be fully established. First, head-to-head comparative data are lacking. The TEMPO trials compared tavapadon with placebo, rather than other dopamine agonists. As a consequence, its relative efficacy and safety remain uncertain. Long-term dyskinesia risk is another important consideration. Given the role of D1 in levodopa-induced dyskinesia, longer follow-up in the TEMPO open-label extension is essential, especially in patients receiving concomitant levodopa. Finally, whether tavapadon meaningfully reduces AEs associated with traditional D2-like agonists remains to be determined. Although lower rates of impulse control disorders and sleep-related AEs might be expected based on receptor selectivity, these potential advantages require systematic prospective assessment and real-world evidence. Taken together, these developments represent a potentially important new direction in PD treatment through selective D1/D5 receptor agonism. As clinical experience with tavapadon expands, additional evidence will help clarify how its mechanistic distinctions translate into meaningful differences in efficacy, tolerability, and treatment selection.

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