GRK-Biased Activation Via β2-Adrenergic Receptors in Human Skeletal Muscle (GAIN)

October 8, 2026 updated by: Morten Hostrup, PhD

G Protein-Coupled Receptor Kinase (GRK)-Biased Signaling Via β2-Adrenergic Receptors and Downstream Activation and Its Role for Protein Synthesis-Regulating Kinases, Function and Receptor Desensitization in Human Skeletal Muscle

This longitudinal mechanistic physiological study examines biased β2-adrenergic receptor (β2-AR) signaling in human skeletal muscle, with emphasis on G protein-coupled receptor kinase (GRK)-mediated pathways. Participants will receive daily oral dosing of the GRK-selective long-acting β2-agonist ATR-258 for 8 weeks. Muscle biopsies and physiological measurements will quantify GRK-, cAMP/PKA-, and β-arrestin-related up- and downstream signaling, fiber-type specificity, and potential receptor desensitization with repeated stimulation along with functional read-outs, tissue distribution, cardiovascular response and tolerability.

Study Overview

Status

Status

Indicates the current recruitment status or the expanded access status.
Active, not recruiting

Conditions

Conditions

The disease, disorder, syndrome, illness, or injury that is being studied. On ClinicalTrials.gov, conditions may also include other health-related issues, such as lifespan, quality of life, and health risks.

Intervention / Treatment

Intervention / Treatment

A process or action that is the focus of a clinical study. Interventions include drugs, medical devices, procedures, vaccines, and other products that are either investigational or already available. Interventions can also include noninvasive approaches, such as education or modifying diet and exercise.

Detailed Description

Healthy men with overweight/obesity will complete an 8-week intervention with daily oral ATR-258 (0.5-2.5 mg/day). On experimental days (Days 1, 15, 29, and 56), β2-AR signaling sensitivity will be assessed before and after stimulation (1-2, 4, and 8 hours) using blood biomarkers, hemodynamics, indirect calorimetry, and lower extremity muscle function/power testing. Muscle biopsies (vastus lateralis) will be obtained at rest and 4 hours after stimulation on Days 1, 29, and 56 to quantify downstream signaling (GRK, cAMP/PKA, β-arrestin), phosphorylation of rpS6/mTOR/Akt, β2-AR content, and muscle fiber morphology.

Study Type

Study Type

Describes the nature of a clinical study. Study types include interventional studies (also called clinical trials), observational studies (including patient registries), and expanded access.
Interventional

Enrollment (Actual)

Enrollment

The number of participants in a clinical study. The "anticipated" enrollment is the target number of participants that the researchers need for the study.
13

Phase

Phase

The stage of a clinical trial studying a drug or biological product, based on definitions developed by the U.S. Food and Drug Administration (FDA). The phase is based on the study's objective, the number of participants, and other characteristics. There are five phases: Early Phase 1 (formerly listed as Phase 0), Phase 1, Phase 2, Phase 3, and Phase 4. Not Applicable is used to describe trials without FDA-defined phases, including trials of devices or behavioral interventions.
  • Phase 2

Contacts and Locations

This section provides the contact details for those conducting the study, and information on where this study is being conducted.

Study Locations

  • Denmark
      • Copenhagen, Denmark, 2100
        • University of Copenhagen, August Krogh Section for Human & Molecular Physiology

Participation Criteria

Researchers look for people who fit a certain description, called eligibility criteria. Some examples of these criteria are a person's general health condition or prior treatments.

Eligibility Criteria

Eligibility Criteria

The key requirements that people who want to participate in a clinical study must meet or the characteristics they must have. Eligibility criteria consist of both inclusion criteria (which are required for a person to participate in the study) and exclusion criteria (which prevent a person from participating). Types of eligibility criteria include whether a study accepts healthy volunteers, has age or age group requirements, or is limited by sex.

Ages Eligible for Study

21 years to 45 years (Adult)

Accepts Healthy Volunteers

Yes

Genders Eligible for Study

Male

Description

Inclusion Criteria:

  • Healthy men
  • Age 21-45 years
  • BMI 25-35 kg/m^2
  • Body fat percentage 25-40%
  • Lean Mass Index 14-22

Exclusion Criteria:

  • Regular use of or allergy to β2-agonists
  • Serious adverse reactions to β2-agonists
  • Current smoker
  • Regular use of medication (except OTC allergy or analgesics)
  • Abnormal ECG before or after β2-AR stimulation
  • Hypertension
  • Reduced kidney function (eGFR < 90 ml/min/1.73m^2)
  • Cardiovascular, metabolic, gastrointestinal, renal, or pulmonary disease
  • Psychiatric or neurological disorders affecting compliance/safety reporting
  • Cancer history within the last 5 years
  • Substance abuse or alcohol intake >14 units/week

Study Plan

This section provides details of the study plan, including how the study is designed and what the study is measuring.

How is the study designed?

Design Details

  • Primary Purpose: Basic Science
  • Allocation: N/A
  • Interventional Model: Single Group Assignment
  • Masking: None (Open Label)

Arms and Interventions

Participant Group / Arm

Participant Group / Arm

A group or subgroup of participants in a clinical trial that receives a specific intervention/treatment, or no intervention, according to the trial's protocol.
Intervention / Treatment

Intervention / Treatment

A process or action that is the focus of a clinical study. Interventions include drugs, medical devices, procedures, vaccines, and other products that are either investigational or already available. Interventions can also include noninvasive approaches, such as education or modifying diet and exercise.
Experimental: ATR-258

Daily oral ATR-258
Drug: ATR-258

Daily oral ATR-258 for 8 weeks with repeated experimental assessment days (Days 1, 15, 29, 56).

What is the study measuring?

Primary Outcome Measures

Primary Outcome Measures

In a clinical study's protocol, the planned outcome measure that is the most important for evaluating the effect of an intervention/treatment. Most clinical studies have one primary outcome measure, but some have more than one.
Outcome Measure
Measure Description
Time Frame
Intensity of β2-AR downstream signaling pathways (e.g. GRK, cAMP/PKA, β-arrestin, rpS6) in type I and type II muscle fibers
Time Frame: Before and 4 hours after ATR-258 ingestion on day 1, 29, and 56.

Assessed in vastus lateralis biopsies at rest
Before and 4 hours after ATR-258 ingestion on day 1, 29, and 56.

Secondary Outcome Measures

Secondary Outcome Measures

In a clinical study's protocol, a planned outcome measure that is not as important as the primary outcome measure for evaluating the effect of an intervention but is still of interest. Most clinical studies have more than one secondary outcome measure.
Outcome Measure
Measure Description
Time Frame
Phosphorylation of rpS6, mTOR, and Akt in type I and type II muscle fibers
Time Frame: Day 1, 29 and, 56.

In response to ATR-258 ingestion
Day 1, 29 and, 56.
Lean/fat mass
Time Frame: Day 1, 15, 29, and 56

Measured by DXA scan
Day 1, 15, 29, and 56
Muscle function
Time Frame: Pre and post 8 weeks of daily ATR-258 ingestion. Post visit conducted 2-5 days after last day of ATR-258 ingestion.

Participants will perform lower extremity peak exercise and a one-legged knee-extensor exercise protocol to exhaustion before and after the full intervention to assess muscle power and endurance
Pre and post 8 weeks of daily ATR-258 ingestion. Post visit conducted 2-5 days after last day of ATR-258 ingestion.
Peripheral glucose clearance including OGTT-derived outcomes
Time Frame: Pre and post 8 weeks of daily ATR-258 ingestion. Post visit conducted 2-5 days after last day of ATR-258 ingestion.
Pre and post 8 weeks of daily ATR-258 ingestion. Post visit conducted 2-5 days after last day of ATR-258 ingestion.
β2-adrenergic receptor content in type I and type II muscle fibers
Time Frame: On day 1, 29 and 56

Measured by western blotting
On day 1, 29 and 56
Resting metabolic rate (incl. carbohydrate and fat oxidation)
Time Frame: Before and 1, 2, 4 and 8 hours after ATR-258 ingestion on day 1, 15, 29, and 56.

Measured by indirect calorimetry
Before and 1, 2, 4 and 8 hours after ATR-258 ingestion on day 1, 15, 29, and 56.
Femoral arterial blood flow
Time Frame: Before and 1, 2, 4 and 8 hours after ATR-258 ingestion on day 1, 15, 29, and 56.

Measured by Doppler.
Before and 1, 2, 4 and 8 hours after ATR-258 ingestion on day 1, 15, 29, and 56.
Continous ECG and heart rate
Time Frame: A 5-day baseline period, and day 1-5, day 15-20, and day 29-34 of ATR-258 administration.

Participants will wear a Holter-device for 4 x 5 days to continuously measure ECG and heart rate. The periods will be after the screening (baseline period), and after trial days on Day 1, 15, and 29.
A 5-day baseline period, and day 1-5, day 15-20, and day 29-34 of ATR-258 administration.
Muscle fiber cross-sectional area (type I and type II)
Time Frame: Day 1, 29, and 56

Measured by histochemical analysis
Day 1, 29, and 56
Maximal electrically-stimulated muscle force development and relaxation
Time Frame: Pre and post 8 weeks of daily ATR-258 ingestion. Post visit conducted 2-5 days after last day of ATR-258 ingestion.

Isometric quadriceps. Measured seated with leg in 90 degree angle and attached to strain gauge.
Pre and post 8 weeks of daily ATR-258 ingestion. Post visit conducted 2-5 days after last day of ATR-258 ingestion.

Collaborators and Investigators

This is where you will find people and organizations involved with this study.

Sponsor

Sponsor

The organization or person who initiates the study and who has authority and control over the study.
Morten Hostrup, PhD

Collaborators

Collaborators

An organization other than the sponsor that provides support for a clinical study. This support may include activities related to funding, design, implementation, data analysis, or reporting.
University of Copenhagen
Bispebjerg Hospital
Stockholm University
Atrogi AB

Study record dates

These dates track the progress of study record and summary results submissions to ClinicalTrials.gov. Study records and reported results are reviewed by the National Library of Medicine (NLM) to make sure they meet specific quality control standards before being posted on the public website.

Study Major Dates

Study Start (Actual)

Study Start

The actual date on which the first participant was enrolled in a clinical study. The "anticipated" study start date is the date that the researchers think will be the study start date.
February 19, 2026

Primary Completion (Estimated)

Primary Completion

The date on which the last participant in a clinical study was examined or received an intervention to collect final data for the primary outcome measure. Whether the clinical study ended according to the protocol or was terminated does not affect this date. For clinical studies with more than one primary outcome measure with different completion dates, this term refers to the date on which data collection is completed for all the primary outcome measures. The "anticipated" primary completion date is the date that the researchers think will be the primary completion date for the study.
December 31, 2026

Study Completion (Estimated)

Study Completion

The date on which the last participant in a clinical study was examined or received an intervention/treatment to collect final data for the primary outcome measures, secondary outcome measures, and adverse events (that is, the last participant's last visit). The "anticipated" study completion date is the date that the researchers think will be the study completion date.
December 31, 2026

Study Registration Dates

First Submitted

First Submitted

The date on which the study sponsor or investigator first submitted a study record to ClinicalTrials.gov. There is typically a delay of a few days between the first submitted date and the record's availability on ClinicalTrials.gov (the first posted date).
January 30, 2026

First Submitted That Met QC Criteria

First Submitted That Met QC Criteria

The date on which the study sponsor or investigator first submits a study record that is consistent with National Library of Medicine (NLM) quality control (QC) review criteria. The sponsor or investigator may need to revise and submit a study record one or more times before NLM's QC review criteria are met. It is the responsibility of the sponsor or investigator to ensure that the study record is consistent with the NLM QC review criteria.
February 16, 2026

First Posted (Actual)

First Posted

The date on which the study record was first available on ClinicalTrials.gov after National Library of Medicine (NLM) quality control (QC) review has concluded. There is typically a delay of a few days between the date the study sponsor or investigator submitted the study record and the first posted date.
February 19, 2026

Study Record Updates

Last Update Posted (Actual)

Last Update Posted

The most recent date on which changes to a study record were made available on ClinicalTrials.gov. There may be a delay between when the changes were submitted to ClinicalTrials.gov by the study's sponsor or investigator (the last update submitted date) and the last update posted date.
October 14, 2026

Last Update Submitted That Met QC Criteria

Last Update Submitted That Met QC Criteria

The most recent date on which the study sponsor or investigator submitted changes to a study record that are consistent with National Library of Medicine (NLM) quality control (QC) review criteria. It is the responsibility of the sponsor or investigator to ensure that the study record is consistent with the NLM QC review criteria.
October 8, 2026

Last Verified

Last Verified

The most recent date on which the study sponsor or investigator confirmed the information about a clinical study on ClinicalTrials.gov as accurate and current. If a study with a recruitment status of recruiting; not yet recruiting; or active, not recruiting has not been confirmed within the past 2 years, the study's recruitment status is shown as unknown.
October 1, 2026

More Information

Terms related to this study

Keywords

Additional Relevant MeSH Terms

Other Study ID Numbers

Other Study ID Numbers

Identifiers or ID numbers other than the NCT number that are assigned to a clinical study by the study's sponsor, funders, or others. These numbers may include unique identifiers from other trial registries and National Institutes of Health grant numbers.
  • ATR
  • H-25045258 (Other Identifier: Danish Ethics Board)

Plan for Individual participant data (IPD)

Plan to Share Individual Participant Data (IPD)?

NO

Drug and device information, study documents

Studies a U.S. FDA-regulated drug product

No

Studies a U.S. FDA-regulated device product

No

This information was retrieved directly from the website clinicaltrials.gov without any changes. If you have any requests to change, remove or update your study details, please contact register@clinicaltrials.gov. As soon as a change is implemented on clinicaltrials.gov, this will be updated automatically on our website as well.

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