A Phase 1 interventional study of G1T48 and Palbociclib in Carcinoma, Ductal, Breast, Breast Cancer Female and Breast Neoplasm, sponsored by G1 Therapeutics, Inc.. Completed at 15 sites in 7 countries. Open to female participants aged 18 Years and older. Per ClinicalTrials.gov, last updated 2026-09-21.
Sponsored by G1 Therapeutics, Inc. · Phase 1, Interventional, and Treatment
This is a study to investigate the potential clinical benefit of G1T48 as an oral selective estrogen receptor degrader (SERD) alone and in combination with palbociclib, a cyclin dependent kinase 4/6 (CDK 4/6) inhibitor, in patients with estrogen receptor-positive, HER2-negative metastatic breast cancer.
The study is an open-label design, consisting of 3 parts: dose-finding portion including food effect (Part 1), G1T48 monotherapy expansion portion (Part 2), and G1T48 in combination with palbociclib expansion portion (Part 3). All parts include 3 study phases: Screening Phase, Treatment Phase, and Survival Follow-up Phase. The Treatment Phase begins on the day of first dose with study treatment and completes at the Post-Treatment Visit. Approximately, 184 patients may be enrolled in the study.
For Parts 1 and 2, patients must satisfy 1 of the following criteria for prior therapy:
For Part 3, patients must satisfy 1 of the following criteria for prior therapy:
Exclusion Criteria:
Patients in Part 1 will receive a single oral dose of G1T48 on Cycle 1 Day -3 and will begin once-daily dosing on Cycle 1 Day 1. The initial dose cohort shall receive an identified starting dose and subsequent cohorts shall receive higher doses based on the safety and PK (pharmacokinetic(s)) data obtained from the previous dose levels.
Drug: G1T48
In Part 1, additional G1T48 cohort(s) of 8 patients may be enrolled to assess the effect of different fat content meals (eg, high fat, moderate fat, or low-fat) on the rate and extent of the absorption of G1T48. Patients will receive a single oral dose of G1T48 on Cycle 1 Day -10 and on Cycle 1 Day -3. Patients will begin G1T48 once-daily dosing on Cycle 1 Day 1.
Drug: G1T48
Patients in Part 2 will receive G1T48 once-daily at the dose determined in Part 1.
Drug: G1T48
Patients in Part 3 will receive G1T48 once-daily at the dose determined in Part 2 in combination with palbociclib once-daily on Days 1 to 21 of each 28-day cycle.
Drug: G1T48 · Drug: Palbociclib
oral SERD
Also known as: Rintodestrant
CDK 4/6 Inhibitor
Also known as: Ibrance
Dose Limiting Toxicity
Time frame: Cycle 1 Day -3 to Cycle 1 Day 28
Recommended Phase 2 dose
G1T48 alone and in combination with palbociclib; progression-free survival (PFS)
Time frame: 12 months
Number of Treatment Related Adverse Event, including Abnormal Laboratory Events
All AEs, including clinical laboratory, vitals signs, physical examinations and ECGs will be analyzed in all patients receiving study drug(s) from the signing of the informed consent until 30 days after the last dose of study medication(s).
Time frame: 21 months
Tumor response based on RECIST, Version 1.1
G1T48 alone and in combination with palbociclib;
Time frame: 21 months
Effect of food on bioavailability of G1T48
Time frame: Part 1, Cycle 1 Day -10 to Cycle 1 Day 1.
Pharmacokinetics of G1T48 and metabolites: Maximum Plasma Concentration (Cmax)
Time frame: Part 1, Cycle 1 Day -3 to Cycle 2 Day 1. Part 2, Cycle 2 Day 1 to Cycle 3 Day 1. Part 3, Cycle 2 Day 1 to Cycle 3 Day 1.
Pharmacokinetics of G1T48 and metabolites: Area under Curve - plasma concentration (AUC)
Time frame: Part 1, Cycle 1 Day -3 to Cycle 2 Day 1. Part 2, Cycle 2 Day 1 to Cycle 3 Day 1. Part 3, Cycle 2 Day 1 to Cycle 3 Day 1.
Pharmacokinetics of G1T48 and metabolites: Plasma: terminal half life (T1/2)
Time frame: Part 1, Cycle 1 Day -3 to Cycle 2 Day 1. Part 2, Cycle 2 Day 1 to Cycle 3 Day 1. Part 3, Cycle 2 Day 1 to Cycle 3 Day 1.
Pharmacokinetics of G1T48 and metabolites: Plasma - Volume of distribution
Time frame: Part 1, Cycle 1 Day -3 to Cycle 2 Day 1. Part 2, Cycle 2 Day 1 to Cycle 3 Day 1. Part 3, Cycle 2 Day 1 to Cycle 3 Day 1.
Pharmacokinetics of palbociclib: Plasma - Trough concentration
Time frame: Part 3, Cycle 2 Day 1 to Cycle 3 Day 1.
This study is completed, as verified in Sep 2026. You cannot join it, but the record below documents what was studied.
Get an email when the registry record changes — status, dates, results — or when someone posts here.
Sign in to followQuestions and observations about this study, from anyone following it. Not medical advice, and not a channel to the study team — their contact details are on the registry record.
Sign in to join the discussion. Reading takes no account; posting does. You choose a display name, and a pseudonym is the default.
Nothing here yet. If you are running this trial, taking part in it, or weighing whether to, this is the place to say so.
G1 Therapeutics, Inc.