A Phase 1 interventional study of PRT13722 and Fulvestrant in Breast Cancer, Hormone-receptor-positive Breast Cancer and HER2-negative Breast Cancer, sponsored by Prelude Therapeutics Incorporated. Recruiting at 1 site in United States. Open to participants aged 18 Years and older. Per ClinicalTrials.gov, last updated 2026-11-09.
Sponsored by Prelude Therapeutics Incorporated · Phase 1, Interventional, and Treatment
This first-in-human, open-label, multicenter study will evaluate the safety, tolerability, pharmacokinetics, pharmacodynamics, and preliminary antitumor activity of PRT13722, an oral KAT6A protein degrader, in adults with locally advanced or metastatic Hormone receptor-positive, human epidermal growth factor receptor 2-negative (HR+/HER2-) breast cancer. PRT13722 will be evaluated as monotherapy and in combination with fulvestrant, abemaciclib plus fulvestrant, or imlunestrant to determine recommended dose(s) for expansion (RDE) and evaluate preliminary clinical activity.
This is a first-in-human, open-label, multicenter, dose-escalation and dose-expansion study of PRT13722, a selective oral KAT6A protein degrader, in adults with locally advanced or metastatic HR-positive, HER2-negative breast cancer. The study will evaluate the safety, tolerability, dose-limiting toxicities, pharmacokinetics, pharmacodynamics, and preliminary antitumor activity of PRT13722 administered as monotherapy and in combination with fulvestrant, abemaciclib plus fulvestrant, or imlunestrant to determine recommended dose(s) for expansion and evaluate preliminary clinical activity.
The study is conducted in three parts:
Part 1: Monotherapy dose escalation and backfill cohorts. Part 2: Combination dose-escalation cohorts with fulvestrant, abemaciclib plus fulvestrant, and imlunestrant.
Part 3: Combination dose-expansion cohorts to further evaluate safety and preliminary efficacy at the selected dose levels.
Up to approximately 240 participants are planned for enrollment across the three parts of the study.
14,825 studies on the registry are indexed under Breast Neoplasms; 3,319 are open to participants now.
This study's planned enrollment of 242 is above the median of 72 across 10,896 interventional studies indexed under Breast Neoplasms.
Browse Breast Neoplasms studies →Prelude Therapeutics Incorporated is the lead sponsor of 2 studies on the registry; 2 are open to participants now.
Counted across the registry records on this site, refreshed daily.
Go through the criteria one by one — yes, no or not sure — and take the list to your doctor or the study team. Kept in this browser only.
Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Ways of preventing pregnancy. “Highly effective” methods fail less than 1% of the time a year when used correctly — for example an IUD or an implant.
Criteria usually say which methods count and how long to keep using them after the last dose, sometimes for male participants too. Some studies accept a partner’s vasectomy if it was confirmed successful and that person is the participant’s only sexual partner.
Cancer that has spread from where it began to other parts of the body. “Metastatic” cancer is cancer that has spread this way.
HER2 is a protein, made from the ERBB2 gene, that drives the growth of some cancers. A study may look for a level of the protein, extra copies of the gene, or a particular change in it.
In breast cancer, protein tests are read as HER2-positive, HER2-low or HER2-negative; other cancers use their own rules. Check which test result the study requires.
The standard rules for measuring tumours on CT or MRI scans, to tell whether a cancer is shrinking, stable or growing during treatment.
“Measurable disease per RECIST 1.1” usually means at least one tumour 10 mm or more at its longest on a CT scan, or a lymph node 15 mm or more across its short axis.
A scale of how much an illness limits everyday activity. For people being assessed to join a study, scores run from 0 (fully active) to 4; lower is more active.
Studies most often ask for 0–1 or 0–2. Your doctor gives the score; the WHO scale uses the same steps. The full scale also has 5, which records death.
Cancer deposits in the brain from a cancer that started elsewhere in the body (CNS metastases include the spinal cord).
Studies differ: some leave out any brain metastases, some accept ones treated and stable for some weeks. Check what this study allows.
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
On an ECG, the QT interval is the time the heart’s lower chambers take to activate electrically and recover in each beat. QTc is that time adjusted for heart rate (QTcF by Fridericia’s formula, QTcB by Bazett’s). Some medicines lengthen it, and a long QTc can raise the risk of a dangerous heart rhythm.
The study may name a correction formula and a limit in milliseconds, often around 450–480.
CYP3A is a group of enzymes, mainly CYP3A4, in the liver and gut that break down many medicines. Some medicines, supplements and foods (grapefruit, St John’s wort) slow them down or speed them up, which changes how much of a study drug stays in the body.
Studies list them so they can be avoided or switched; bring a list of everything you take, including supplements.
Contraception
Ways of preventing pregnancy. “Highly effective” methods fail less than 1% of the time a year when used correctly — for example an IUD or an implant.
Criteria usually say which methods count and how long to keep using them after the last dose, sometimes for male participants too. Some studies accept a partner’s vasectomy if it was confirmed successful and that person is the participant’s only sexual partner.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Metastases
Cancer that has spread from where it began to other parts of the body. “Metastatic” cancer is cancer that has spread this way.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
HER2
HER2 is a protein, made from the ERBB2 gene, that drives the growth of some cancers. A study may look for a level of the protein, extra copies of the gene, or a particular change in it.
In breast cancer, protein tests are read as HER2-positive, HER2-low or HER2-negative; other cancers use their own rules. Check which test result the study requires.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
RECIST
The standard rules for measuring tumours on CT or MRI scans, to tell whether a cancer is shrinking, stable or growing during treatment.
“Measurable disease per RECIST 1.1” usually means at least one tumour 10 mm or more at its longest on a CT scan, or a lymph node 15 mm or more across its short axis.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
ECOG performance status
A scale of how much an illness limits everyday activity. For people being assessed to join a study, scores run from 0 (fully active) to 4; lower is more active.
Studies most often ask for 0–1 or 0–2. Your doctor gives the score; the WHO scale uses the same steps. The full scale also has 5, which records death.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Brain metastases
Cancer deposits in the brain from a cancer that started elsewhere in the body (CNS metastases include the spinal cord).
Studies differ: some leave out any brain metastases, some accept ones treated and stable for some weeks. Check what this study allows.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Clinically significant
Judged by the study doctor to matter for your health or for the study. It is a doctor’s decision, so ask the study team if you are unsure whether something counts.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
QT and QTc
On an ECG, the QT interval is the time the heart’s lower chambers take to activate electrically and recover in each beat. QTc is that time adjusted for heart rate (QTcF by Fridericia’s formula, QTcB by Bazett’s). Some medicines lengthen it, and a long QTc can raise the risk of a dangerous heart rhythm.
The study may name a correction formula and a limit in milliseconds, often around 450–480.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
CYP3A (CYP3A4) inhibitors and inducers
CYP3A is a group of enzymes, mainly CYP3A4, in the liver and gut that break down many medicines. Some medicines, supplements and foods (grapefruit, St John’s wort) slow them down or speed them up, which changes how much of a study drug stays in the body.
Studies list them so they can be avoided or switched; bring a list of everything you take, including supplements.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Participants receive PRT13722 once daily as monotherapy. This arm includes dose-escalation (Part 1A) and backfill cohorts (Part 1B) evaluating PRT13722 in participants with locally advanced or metastatic HR-positive, HER2-negative breast cancer to support the selection of RDE.
Drug: PRT13722
Participants receive PRT13722 once daily in combination with fulvestrant. This study includes a dose-escalation (Part 2A) in participants with locally advanced or metastatic HR-positive, HER2-negative breast cancer followed by dose-expansion (Part 3A) at the RDE.
Drug: PRT13722 · Drug: Fulvestrant
Participants receive PRT13722 once daily in combination with abemaciclib and fulvestrant. This study includes a dose-escalation (Part 2B) in participants with locally advanced or metastatic HR-positive, HER2-negative breast cancer followed by dose-expansion (Part 3B) at the RDE.
Drug: PRT13722 · Drug: Fulvestrant · Drug: Abemaciclib
Participants receive PRT13722 once daily in combination with imlunestrant. This study includes a dose-escalation (Part 2C) in participants with locally advanced or metastatic HR-positive, HER2-negative breast cancer followed by dose-expansion (Part 3C) at the RDE.
Drug: PRT13722 · Drug: Imlunestrant
PRT13722 is an investigational oral drug administered once daily at the assigned dose level.
Fulvestrant is an approved selective estrogen receptor degrader (SERD) administered on Cycle 1 Day 1, Cycle 1 Day 15, Cycle 2 Day 1, then once monthly as an intramuscular injection into the buttocks (gluteal muscle).
Abemaciclib is an approved oral cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor administered twice daily and used in combination with endocrine therapy for HR-positive, HER2-negative breast cancer.
Imlunestrant is an approved oral SERD administered once daily.
Dose limiting toxicity of PRT13722 (Parts 1A and 2)
Incidence of dose limiting toxicities, defined according to protocol-specified criteria
Time frame: Through cycle 1 (28 days)
Incidence of Treatment-Emergent Adverse Events [Safety and Tolerability] of PRT13722 as monotherapy and in combination (Parts 1A and 2)
Safety and tolerability assessed by the number of participants with treatment-emergent adverse events, serious adverse events, adverse events leading to dose modification, treatment interruption, or treatment discontinuation, and clinically significant laboratory abnormalities. Adverse events and selected laboratory abnormalities will be graded according to National Cancer Institute Common Terminology Criteria for Adverse Events (NCI CTCAE), Version 6.0
Time frame: Through study completion, an average of 2 years
Preliminary Antitumor Activity: Objective Response Rate (Parts 1B and 3)
Proportion of patients with best overall response of either complete response or partial response, as assessed by the investigator per RECIST v1.1
Time frame: Through study completion, an average of 2 years
Preliminary Antitumor Activity: Objective Response Rate (Parts 1A and 2)
Proportion of patients with best overall response of either complete response or partial response, as assessed by the investigator per RECIST v1.1
Time frame: Through study completion, an average of 2 years
Duration of Response
Time from first documented response to disease progression as assessed by the investigator per RECIST v1.1, or death, whichever comes first.
Time frame: Through study completion, an average of 2 years
Clinical Benefit Response
Proportion of patients with a best overall response of complete response, partial response, or durable stable disease (24 weeks or longer), as assessed by the investigator per RECIST v1.1
Time frame: Through study completion, an average of 2 years
Progression-Free Survival
Time from the date of first dose of investigational product to the date of first documented disease progression, as assessed by the investigator per RECIST v1.1, or death due to any cause
Time frame: Through study completion, an average of 2 years
Overall Survival
Time from the date of first dose of investigational product to death due to any cause
Time frame: Through study completion, an average of 2 years
Incidence of Treatment-Emergent Adverse Events [Safety and tolerability] of PRT13722 as Monotherapy and in Combination (Parts 1B and 3)
Safety and tolerability will be evaluated by incidence of adverse events leading to dose modifications, treatment interruptions, treatment discontinuations, and clinically significant laboratory abnormalities graded according to NCI CTCAE, Version 6.0
Time frame: Through study completion, an average of 2 years
Maximum Observed Plasma Concentration (Cmax)
Maximum observed plasma concentration will be calculated using non-compartmental analysis
Time frame: Cycle 1 Day 1, Cycle 1 Day 15, and Cycle 2 Day 1 (each cycle is 28 days)
Time To Maximum Concentration (Tmax)
Time to maximum concentration will be calculated using non-compartmental analysis
Time frame: Cycle 1 Day 1, Cycle 1 Day 15, and Cycle 2 Day 1 (each cycle is 28 days)
Terminal Elimination Half-life (T1/2)
Terminal elimination half-life will be calculated using non-compartmental analysis
Time frame: Cycle 1 Day 1 (each cycle is 28 days)
Area under the plasma concentration versus time curve (AUC)
Area under the plasma concentration-time curve will be calculated using non-compartmental analysis
Time frame: Cycle 1 Day 1, Cycle 1 Day 15, and Cycle 2 Day 1 (each cycle is 28 days)
Steady State Trough Concentrations
Steady state trough concentrations will be calculated using non-compartmental analysis
Time frame: Cycle 1 Day 15, and Cycle 2 Day 1 (each cycle is 28 days)
Start is estimated for Nov 2026; primary completion for Oct 2028, 23 months later.
Of 388 industry-sponsored phase 1 interventional studies under Breast Neoplasms, half reached primary completion within 39 months of starting (the median), counting those still running; a quarter within 22 and three quarters within 56. This study's planned 23 months is below that.
Counting every sponsor, half of 759 studies reached it within 44 months; a quarter within 25 and three quarters within 66.
Start moved once since we started tracking on Sep 25, 2026: Oct 2026 → Nov 2026, on Nov 9, 2026.
Earlier changes are on the record history on ClinicalTrials.gov ↗
How these figures are worked out, and which studies are left out of them.
From the registry record's own update history. This site started tracking changes on Sep 25, 2026; for anything earlier, see the record history on ClinicalTrials.gov ↗
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Prelude Therapeutics Incorporated