Disposition and Absolute Bioavailability of Oral Imlunestrant in Healthy Women: A Phase 1, Open‐Label Study

Abstract

Imlunestrant (LY3484356) is a next‐generation orally bioavailable selective estrogen receptor degrader being investigated for the treatment of estrogen receptor–positive advanced breast and endometrial cancers. This Phase 1, open‐label, 2‐part study evaluated the disposition and absolute bioavailability of [¹⁴C]‐imlunestrant in 16 US‐based healthy women (aged 36‐65 years) of non–childbearing potential. Part 1 participants (N = 8) received an oral dose of 400‐mg [¹⁴C]‐imlunestrant solution (100 µCi). Part 2 participants (N = 8) received an oral dose of 2 × 200‐mg imlunestrant tablets followed by approximately 45 µg [¹⁴C]‐imlunestrant (approximately 1 µCi) given as a 15‐minutes infusion 4 hour later. Blood, fecal, and urine samples were collected. Total radioactivity was primarily eliminated in feces (97.3%) with trace amounts recovered in urine (0.278%), suggesting minimal renal clearance. Imlunestrant accounted for most of the radioactive dose in feces (61.8%), followed by metabolite M2 (20.9%), metabolites M5 + M10 (coeluted), M7, M8, M9, and M11 (5.1% or less for each). Absolute bioavailability of imlunestrant after oral administration relative to intravenous administration was 10.9% based on dose‐normalized area under the concentration–time curve from time zero to infinite time. Imlunestrant was well tolerated as an oral solution or as a tablet/intravenous dose. Eight participants reported mild/moderate treatment‐related adverse events that resolved by the end of the study.

Estrogen receptor‐positive (ER+) breast cancer comprises approximately 80% of all breast cancers. ¹ , ² , ³ Selective estrogen receptor degraders (SERDs) are drugs that target ERα for proteasome‐dependent degradation. ⁴ , ⁵ Fulvestrant is a first‐generation SERD approved for the treatment of ER+/human epidermal growth factor receptor 2‐negative metastatic breast cancer in postmenopausal women after disease progression on prior endocrine therapy. ⁶ , ⁷ , ⁸ However, fulvestrant has modest efficacy in the second‐line treatment of metastatic ER+ mutant breast cancer, including tumors treated with CDK4/6 inhibitors. ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵ , ¹⁶ Fulvestrant also demonstrated limited efficacy in patients with ESR1 mutant breast cancer. ⁷ , ⁸ , ⁹ , ¹⁰ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ Further, fulvestrant is poorly soluble and requires intramuscular injections (500 mg administered as 2 250‐mg/5‐mL injections on Days 1, 15, and 29, then once monthly thereafter). ²¹ , ²² , ²³ Therefore, a more potent oral SERD is needed for the treatment of ER+ breast cancer. ²⁴ , ²⁵ , ²⁶

Several novel oral SERDs, which aim to overcome the limitations of fulvestrant, are currently in clinical development (eg, elacestrant, imlunestrant, and giredestrant). ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ Imlunestrant (LY3484356) is a next‐generation orally bioavailable, brain‐penetrant SERD, and pure ER antagonist. ³⁰ Imlunestrant has potent activity against both wild‐type and mutant ER and inhibits ER‐dependent transcription and cell growth. ³⁰ In the Phase 1a/b EMBER trial in patients with ER+ breast cancer or endometroid endometrial cancer, imlunestrant demonstrated a safety profile of low‐grade treatment‐emergent adverse events (AEs) and a favorable pharmacokinetic (PK) profile (NCT04188548). ³¹ Imlunestrant (200‐1200 mg once daily) was investigated as a single agent or in combination with abemaciclib, everolimus, alpelisib, trastuzumab, or aromatase inhibitors for the treatment of ER+ metastatic breast cancer and endometroid endometrial cancer. ³² After oral administration of imlunestrant (200‐1200 mg once daily), a dose‐proportional increase was observed in the maximum plasma concentration (C_max) and area under the concentration–time curve (AUC) from time zero to infinity (AUC0‐∞). ³² The median time to maximum observed concentration (t_max) was approximately 4 hours, and the geometric mean t_1/2 ranged from 25 to 30 hours. ³² At doses of 400 mg or greater, imlunestrant demonstrated exposures that were above the biologically effective concentration range. ³² In the ongoing Phase 3 EMBER‐3 trial, imlunestrant (400 mg once daily) is being compared with fulvestrant or exemestane, and in a combination of imlunestrant‐abemaciclib for the treatment of ER+/human epidermal growth factor receptor 2‐negative locally advanced or metastatic breast cancer previously treated with an aromatase inhibitor with or without a CDK4/6 inhibitor (NCT04975308). ³³

Unlike intravenously administered drugs, orally delivered drugs may have a bioavailability less than 100% due to incomplete absorption, gut wall metabolism, or elimination during the first pass through the liver. ³⁴ The absolute bioavailability (fraction of the drug that reaches systemic circulation after extravascular administration) and disposition (drug metabolism and elimination pathways) after the oral delivery of imlunestrant have not been determined. Information obtained from disposition and absolute bioavailability studies can be used to optimize the treatment dose, assess which of the drug metabolites need further testing in toxicology studies (eg, metabolites accounting for 10% or more of the total circulating drug exposure), and plan appropriate clinical pharmacology studies (eg, drug–drug interaction studies and studies in patients with renal or hepatic impairment). ³⁵ , ³⁶ Here, we report the results of a Phase 1 study investigating the disposition and oral bioavailability of imlunestrant in healthy postmenopausal women of non–childbearing potential.

Subjects and Methods

Study Design and Participants

This Phase 1, open‐label, 2‐part, single‐dose study of imlunestrant (LY3484356) was conducted in the United States (Labcorp Clinical Research Unit, Madison, WI) between August 2021 and May 2022. Key objectives of Part 1 of this study were to evaluate the disposition, mass balance, PK, and metabolism of imlunestrant. The main objective of Part 2 of this study was to determine the oral bioavailabilty of imlunestrant.

Participants were women of non–childbearing potential aged 36‐65 years (body mass index, 18‐35 kg/m²) who were healthy as determined by medical history, physical examination, and laboratory assessments at the time of screening (Supplemental Appendix). Participants in Part 1 did not participate in Part 2. This study was conducted in accordance with the International Ethical Guidelines for Biomedical Research Involving Human Subjects (2002), the International Conference on Harmonization Good Clinical Practice Guidelines, and the Declaration of Helsinki. The protocol was reviewed and approved by an institutional review board (Western Institutional Review Board–Copernicus Group [previously designated as Western Institutional Review Board], Puyallup, WA). All participants provided written informed consent.

Study Interventions

All participants were screened within 28 days before enrollment, were admitted to the clinical research unit (CRU) on Day −1, and were administered treatment on Day 1 after an overnight fast of 10 hours or more. All participants received a safety follow‐up call approximately 7 days after discharge from the CRU (Figure 1). The total duration of the study was up to 59 days in Part 1 and up to 46 days in Part 2.

Figure 1.

Study design. In Part 1, blood, fecal, and urine samples were collected until ≥90% of the radioactive dose was recovered and ≤1% of the radioactive dose/day was recovered in urine and feces for 2 consecutive days or until discharge from the CRU (Day 22). CRU, clinical research unit; IV, intravenous

Participants in Part 1 of the study received a single oral dose of 400‐mg [¹⁴C]‐imlunestrant solution (100 µCi) (Table S1). Participants were discharged from the CRU between Days 12 and 22, provided 90% or more of the radioactive dose was recovered and 1% or less of the radioactive dose per day for 2 consecutive days was recovered in the excreta (urine and feces combined). Sample collection continued until discharge criteria were met or until discharge from the CRU (Day 22). For participants experiencing vomiting within 4 hour after dosing with [¹⁴C]‐imlunestrant, vomitus was collected.

Participants in Part 2 of the study received a single dose of 400 mg nonradiolabeled imlunestrant administered as two 200‐mg tablets followed by a single dose of 45 µg (approximately 1 µCi) [¹⁴C]‐imlunestrant administered as a 15‐minutes intravenous infusion 4 hour later. Participants remained in the CRU until Day 9. Blood samples were collected during the infusion and for at least 2 hour after infusion from the arm contralateral to the infusion site.

Sample Collection

Part 1: Blood Sampling

Blood samples for determining the plasma concentrations of [¹⁴C]‐imlunestrant and total radioactivity were collected less than 30 minutes before dosing and at 0.5, 1, 1.5, 2, 3, 4, 6, 8, 10, 12, 24, 48, 72, 96, 120, 144, 168, 192, 216, 240, and 264 hour (Day 12) after dosing. If participants did not meet the discharge criteria by Day 12, additional samples were collected at 24‐hour intervals until the end of treatment or discharge from the CRU. Blood samples for the profiling of imlunestrant metabolites in circulation were collected before dosing and at 1, 2, 4, 8, 12, 24, 48, 72, 96, 120, 144, 168, 192, 216, 240, and 264 hour (Day 12) after dosing.

Part 1: Urine, Feces, and Expired Air Sampling

Urine samples for determining the concentration of total radioactivity and profiling of imlunestrant metabolites were collected from 12 hour before dosing (−12‐0 hour before dosing); from 0 to 6, 6 to 12, and 12 to 24 hour after dosing; and at 24‐hours intervals up to and including 264 hours (Day 12) after dosing.

Fecal samples for determining the concentration of total radioactivity and profiling of imlunestrant metabolites were collected from 0 to 24 hour after dosing and for 24‐hour intervals up to and including 264 hours after dosing (Day 12).

Sample collection (urine and feces) and participants’ residence in the CRU continued until discharge criteria were met or until the day of discharge from the CRU (Day 22).

Expired air samples were collected before dosing and at 1, 2, 4, 8, and 24 hour after dosing.

Part 2: Blood Sampling

Blood samples for determining the plasma concentrations of imlunestrant were collected at less than 30 minutesbefore oral dosing and at 0.5, 1, and 2 hour after dosing. Blood samples for determining the plasma concentrations of [¹⁴C]‐imlunestrant and total radioactivity were collected at 4 hour after oral dosing (ie, just before start of intravenous dosing). Blood samples for determining the plasma concentrations of imlunestrant and [¹⁴C]‐imlunestrant were collected at 5, 15, 20, 30, and 45 minutes after the start of intravenous infusion and at 1, 2, 3, 4, 6, 8, 10, 12, 24, 48, 72, 96, 120, 144, 168, and 192 hour (Day 9) after intravenous dosing.

Synthesis of Metabolites M1 and M2

Glucuronide M1 (LSN3517019) was synthesized by coupling of the parent compound LY34834356 with protected bromoglucuronic acid followed by the deprotection step (Figure S1). Sulfate M2 (LSN3527840) was prepared by the treatment of LY3484356 with the sulfur trioxide trimethylamine complex.

Quantification of Imlunestrant and Total Radioactivity in Plasma

Plasma concentrations of nonradiolabeled imlunestrant in Parts 1 and 2 were measured using validated high‐performance liquid chromatography (HPLC) with tandem mass spectrometry at Labcorp Bioanalytical Services, LLC (Indianapolis, IN). The HPLC/mass spectrometry system comprised of a Shimadzu HPLC system and an API 4000 mass spectrometer (Sciex) with an electrospray ionization source in positive mode. A Waters XBridge BEH C18 column (2.1 × 50 mm, id 5 µm; Waters Corp.) was used for separation. The mobile phase A was a water phase containing 20 mM of ammonium formate and 0.1% of formic acid. The mobile phase B was acetonitrile and 0.1% formic acid. The gradient elution was as follows: 0‐0.01 minutes (B 30%); 0.01‐0.2 minutes (B 30%); 0.2‐1.2 minutes (B 75%); 1.2‐1.3 minutes (B 95%); 1.5‐1.9 minutes (B 95%); 1.9‐2.0 minutes (B 20%); 2.0‐2.5 minutes (B 20%); 2.5‐3.2 minutes (B 90%); 3.2‐3.4 minutes (B 90%); and 3.4‐3.6 minutes (B 30%). The flow rate was 0.75 mL/min. The column temperature was 40°C, and the autosampler temperature was maintained at 5°C. Quantification was accomplished by monitoring the transition of m/z of 525.2 → 485.15 for imlunestrant and m/z of 529.2 → 489.15 for LSN3514100 (internal standard). The lower limit of quantification was 0.500 ng/mL, and the upper limit of quantification was 500.000 ng/mL.

Plasma concentrations of total radioactivity (Parts 1 and 2) and [¹⁴C]‐imlunestrant (Part 2) were analyzed at Pharmaron ABS, Inc. (Germantown, MD). Concentrations of total radioactivity and [¹⁴C]‐imlunestrant were measured using liquid scintillation counting (LSC) or accelerator mass spectrometry (AMS) and liquid chromatography (LC) and AMS (HPLC/AMS), respectively. For the measurement of total radioactivity, plasma samples were mixed with Ultima Gold liquid scintillant (MilliporeSigma) and analyzed in a TriCarb 3180 TR/SL or TriCard 5110 TR liquid scintillation counter (Perkin Elmer). Total radioactivity in the plasma was measured by AMS using a SSAMS‐250 mass spectrometer (National Electrostatics Corp.) for samples that could not be quantified for radioactivity using LSC. For the measurement of [¹⁴C]‐imlunestrant, LC fractions were collected using an Agilent 1290 Infinity LC system (Agilent Technologies) and analyzed using an SSAMS‐250 mass spectrometer (National Electrostatics Corp.). Plasma samples were spiked with the internal standard (¹²C‐imlunestrant) and extracted with acetonitrile. The supernatants were reduced to dryness with nitrogen and reconstituted in 10 mM ammonium formate pH 9.0:methanol (50:50 v/v). A Waters Acquity BEH C18 column (2.1 × 100 mm, id 1.7 µm; Waters Corp.) was used for separation. The mobile phase A was a water phase containing 10 mM of ammonium formate pH 9.0 and the mobile phase B was methanol. The gradient elution was as follows: 0‐2 minutes (B 70%); 2‐10.3 minutes (B 80%); 10.3‐12 minutes (B 90%); 12‐14 minutes (B 90%); 14‐14.1 minutes (B 70%); and 14.1‐19 minutes (B 70%). The flow rate was 0.25 mL/min. The column temperature was 40°C, and the autosampler temperature was maintained at 5°C. HPLC eluent fractions were collected at the retention time of imlunestrant, subjected to graphitization, and analyzed by AMS.

Quantification of Radioactivity in Urine, Feces, and Expired Air

Urine, feces, and expired air samples obtained from participants in Part 1 were analyzed at Labcorp Early Development Laboratories, Inc. The amount of radioactivity was measured using Model 2900TR or 2910TR liquid scintillation counters (Packard Instrument Co.). Urine samples containing 0.2% Triton X‐100 (v/v) were mixed with Ultima Gold XR scintillation cocktail (MilliporeSigma) and analyzed by LSC. Feces samples were weighed and homogenized in a weighed amount of acetonitrile:water (1:1 v/v). Duplicate weighed subsamples (0.2 g) were combusted in a Model 307 Sample Oxidizer (Packard Instrument Co.). The produced ¹⁴CO₂ was trapped in Carbo‐Sorb, mixed with PermaFluor, and analyzed by LSC. Ultima Gold XR scintillation cocktail (MilliporeSigma) (10 mL or greater) was added to vials containing the breath test solution and analyzed by LSC.

Metabolite Profiling

Plasma samples obtained from participants in Part 1 of the study were analyzed at Pharmaron ABS, Inc. Plasma samples were extracted twice with acetonitrile (3 times the volume of plasma), evaporated to dryness, and reconstituted with 10 mM ammonium formate pH 9.0:methanol (80:20 v/v) for metabolic profiling. A Waters Acquity BEH C18 column (2.1 × 100 mm, id 1.7 µm; Waters Corp.) was used for separation. The mobile phase A was a water phase containing 10 mM ammonium formate (pH 9.0), and the mobile phase B was methanol. The gradient elution was as follows: 0‐2 minutes (B 20%); 2‐60 minutes (B 90%); 60‐65 minutes (B 90%); 65‐65.1 minutes (B 20%); and 65.1‐80 minutes (B 20%). The flow rate was 0.25 mL/min. The column temperature was 40°C and autosampler temperature was maintained at 5°C. Plasma radiochromatograms were generated by collecting HPLC fractions, followed by AMS analysis of each fraction or fractions pooled across regions using an SSAMS‐250 (National Electrostatics Corp.).

Fecal samples obtained from participants in Part 1 were analyzed at Q2 Solutions, Indianapolis, IN). Fecal samples were extracted twice with acetonitrile (4 volumes). The supernatants from the 2 extractions were combined, and the total volume was adjusted to 11 mL using 80:20 acetonitrile:H₂O. The combined supernatant (5 mL) was evaporated to dryness under nitrogen and reconstituted in methanol. The reconstituted samples were centrifuged, and the supernatants were used for radioprofiling. Fecal samples were profiled by LC and high‐resolution mass spectrometry (LC/HRMS). The LC/HMRS system comprised of a Shimadzu HPLC system and a Thermo QExactive HF system (Thermo Fisher Scientific) with an electrospray ionization source in positive mode. The eluent from the HPLC was split between a mass spectrometer and a fraction collector. Collected fractions were radioassayed offline, and the data were used to construct radiochromatograms for quantification. Urine samples were not radioprofiled as the urinary elimination of imlunestrant‐related total radioactivity was negligible.

Metabolite Identification

Plasma and feces samples from participants in Part 1 were analyzed at Q2 Solutions. The LC/HRMS system used to identify imlunestrant metabolites consisted of a Shimadzu HPLC system and a Thermo QExactive HF system (Thermo Fisher Scientific) equipped with a heated electrospray ionization in positive mode. Data analyses were conducted using Xcalibur Software (Thermo Fisher Scientific). Metabolites observed in the AMS radioprofiles were identified provisionally by retention time matching of available standards. Metabolites in plasma were identified if they were 5% or more of total radioactivity in the AUC pool. Metabolites in feces were identified if they were 1% or greater of the dose (totaled across time points). A metabolite number (eg, M1, M2, M3; based on metabolites of imlunestrant identified from in vitro and in vivo studies in animals) was assigned to each metabolite.

Pharmacokinetic Assessments

PK parameter estimates were determined using noncompartmental procedures using Phoenix WinNonlin Version 8.1.1 software (Certara USA, Inc.). The PK parameters calculated included AUC to the time of the last quantifiable concentration (AUC0‐t_last), AUC_0‐∞, C_max, t_max, t_1/2 associated with the terminal rate constant (λz) in noncompartmental analysis, apparent total body clearance of drug calculated after extravascular administration, total body clearance of drug calculated after intravenous administration, and volume of distribution during the terminal phase after intravenous administration.

Oral bioavailability (F) in Part 2 was calculated as follows:

$$\mathrm{F}\%=\frac{\left[\mathrm{AUC}\left(0-\infty \right),\mathrm{Imlunestrant}\right]\times [\mathrm{Dose},[14\mathrm{C}]-\mathrm{imlunestrant}]}{\left[\mathrm{AUC}\left(0-\infty \right),[14\mathrm{C}]-\mathrm{imlunestrant}\right]\times [\mathrm{Dose},\mathrm{Imlunestrant}]}\times 100\%$$

The ratios of plasma imlunestrant to plasma total radioactivity were calculated for each time point in Part 1. The ratios of exposure of plasma imlunestrant to plasma total radioactivity were calculated based on AUC_0‐∞ and C_max in Parts 1 and 2. The percentage and cumulative percentage of radiolabeled dose excreted in expired air were also calculated for Part 1.

Safety Assessments

Safety and tolerability assessments included monitoring for AEs and serious AEs (SAEs) up to Day 59 in Part 1 and Day 46 in Part 2. All treatment‐emergent AEs (TEAEs), SAEs, and discontinuations due to AEs after oral or intravenous administration of imlunestrant were recorded. The frequency of TEAEs was summarized by treatment, Medical Dictionary for Regulatory Activities Version 24.0 system organ class, and preferred term. AEs were categorized as mild, moderate, or severe based on the investigator's assessment. Vital signs and laboratory evaluations (chemistry, hematology, and urinalysis) were monitored throughout the study.

Statistical Methods

No formal sample size calculation was made, and the number of participants enrolled was based on previous studies. ³⁶

The safety population included all participants who received any study treatment. Mass balance analyses included all participants who received a full dose of study treatment and had total radioactivity concentration (urinary and fecal) data available. PK population included all participants who received a full dose of study treatment and had at least 1 valid postdose analytical result for PK parameter estimation (ie, no missing samples at critical time points, or protocol deviations, or relevant AEs such as vomiting that would affect PK analysis).

Results

Participants

Sixteen participants (N = 8 each in Parts 1 and 2) were enrolled in the study. The mean age of the participants was 53 (range, 36‐65) years and mean (standard deviation [SD]) body mass index was 27.5 (3.9) kg/m² (Table S2).

Two participants who received a single oral dose of [¹⁴C]‐imlunestrant solution in Part 1 experienced vomiting. One participant experienced vomiting approximately 3 hours after dosing (Day 1). One participant experienced vomiting approximately 0.75 hours after dosing (Day 1). This participant withdrew on Day 15. One participant in Part 1 withdrew on Day 24 for personal reasons. Participant withdrawals in Part 1 were not study related. All participants in Part 2 completed the study.

Pharmacokinetic Evaluations

Part 1: Mass Balance and Excretion

The recovery of radioactivity after a single oral dose of 400 mg [¹⁴C]‐imlunestrant solution (100 µCi) was greater than 90% in the 5 participants with evaluable data (Figure 2). Three participants were excluded from the analysis: One participant was excluded due to the elimination of a large portion of the dose as vomitus (ie, a significant dose of radioactivity was eliminated in the vomit) and 2 participants due to early termination.

Figure 2.

Mean (±SD) cumulative radioactive dose recovered after a single 400‐mg oral dose of [¹⁴C]‐imlunestrant (100 µCi) solution (Part 1). Based on 5 participants with evaluable data. Three participants were excluded due to early termination or elimination of a large portion of the dose as vomitus. SD, standard deviation.

The overall mean recovery of total radioactivity over 480 hours was 97.5%. Over 90% of the radioactivity was recovered in the feces in the first 192 hours after the administration of imlunestrant. Total radioactivity was primarily excreted via the feces (97.3%; range, 90.0%‐101.0%) and urine (0.278%; range, 0.109%‐0.497%). Radioactivity in urine was detected up to 12 hours after dosing in all 5 participants, 24 hours in 4 participants, 48 hours in 2 participants, and 120 hours in 1 participant. Radioactivity in the feces was detected up to 240 hours after dosing in 5 participants, 264 hours in 4 participants, and 480 hours in 1 participant. Radioactivity was detected in the breath of 1 participant 8 and 24 hours after dosing.

Part 1: PK Assessments

PK assessments were conducted using data from 6 participants. Samples from the 2 participants who withdrew were collected up to 3 and 72 hours after dosing and were excluded from the analyses.

After the oral administration of 400 mg [¹⁴C]‐imlunestrant solution (100 µCi) in the fasted state, total radioactivity, and imlunestrant appeared rapidly in plasma with a median t_max of 2 (range, 1.0‐3.0) hours (Table 1 and Figure 3). Imlunestrant was eliminated with a mean (SD) t_1/2 of 33.7 (14.7) hours, whereas total radioactivity decreased with a mean (SD) t_1/2 of 114.5 (39.3) hours.

Table 1.

Summary of Imlunestrant, [¹⁴C]‐Imlunestrant, and Total Radioactivity PK Parameters

	Arithmetic mean (SD)
	Part 1 (N = 8) 400 mg [14C]‐imlunestrant (100 µCi) Oral		Part 2 (N = 8) 400 mg imlunestrant a oral + 45 µg [14C]‐imlunestrant (≈1 µCi) IV
	Plasma imlunestrant (n = 6 b )	Plasma total radioactivity (n = 6 b )	Plasma imlunestrant (n = 8)	Plasma [14C]‐imlunestrant (n = 8)	Plasma total radioactivity (n = 8)
AUC0‐tlast (ng•h/mL or ng•hEq/mL)	3270 (1100)	17,100 (5890)	1900 (821)	1.92 (0.441)	6.40 (0.956)
AUC0‐∞ (ng•h/mL or ng•hEq/mL)	3310 (1110)	18,900 (6840)	1960 (843)	2.01 (0.498)	7.54 (1.27)
Cmax (ng/mL or ng•hEq/mL)	117 (50.7)	107 (20)	38.7 (28.6)	0.585 (0.118)	0.869 (0.152)
tmax (h) c	2.00 (1.0‐3.0)	2.00 (1.5‐3.0)	4.37 (2.0‐16.0)	0.25 (0.1‐0.3)	0.25 (0.3‐0.3)
t1/2 (hours)	33.7 (14.7)	114.5 (39.3)	32.1 (7.3)	32.5 (16.6)	74.5 (14.8)
CL/F (L/h)	135 (55.3)	NA	240 (116)	NA	NA
CL (L/h)	NA	NA	NA	23.8 (6.09)	NA
Vz (L)	NA	NA	NA	1020 (282)	NA
F	NA	NA	NA	10.9 (3.58)	NA

AUC_0‐∞, area under the concentration versus time curve from time zero to infinity; AUC_0‐tlast, area under the concentration versus time curve from time zero to time t, where t is the last time point with a measurable concentration; CL, total body clearance of drug calculated after intravenous administration; CL/F, apparent total body clearance of drug calculated after extravascular administration; C_max, maximum observed drug concentration; F, absolute bioavailability; IV, intravenous; NA, not applicable; PK, pharmacokinetic; SD, standard deviation; t_1/2, half‐life associated with the terminal rate constant (λz) in noncompartmental analysis; t_max, time of maximum observed drug concentration; V_z, volume of distribution during the terminal phase after intravenous administration

^a Administered as a single dose of two 200‐mg tablets (nonradiolabeled).

^b Eight participants were enrolled in Part 1. One participant withdrew from the study on Day 15 and 1 participant on Day 24; these participants were excluded from the analyses.

^c Median (range).

Figure 3.

Arithmetic mean (SD) plasma concentration profile of (A) total radioactivity and [¹⁴C]‐imlunestrant (100 µCi) (Part 1), and (B) total radioactivity, imlunestrant, and [¹⁴C]‐imlunestrant (∼1 µCi) (Part 2). IV, intravenous; SD, standard deviation.

Plasma concentrations of imlunestrant were quantifiable up to 168 hours after dosing in 6 participants, 192 hours in 4 participants, 216 hours in 3 participants, and 408 hours in 1 participant. Interindividual variability (coefficient of variation ([CV]%) was 38% for AUC_0‐tlast, 38% for AUC_0‐∞, and 48% for C_max. Total radioactivity in the plasma was quantifiable up to 264 hours after dosing in 6 participants, 384 hours in 2 participants, and 480 hours in 1 participant. Interindividual variability (CV%) was 37% for AUC_0‐tlast, 39% for AUC_0‐∞, and 18% for C_max. The geometric mean ratio of plasma imlunestrant versus total radioactivity was 0.19 for AUC_0‐tlast, 0.18 for AUC_0‐∞, and 0.20 for C_max, indicating that most of the total radioactivity was due to the metabolites of imlunestrant (Table S3).

Part 1: Metabolism

Five participants were included in metabolite profiling and identification after the oral administration of 400 mg [¹⁴C]‐imlunestrant solution (100 µCi) in the fasted state (samples from the same 5 participants that were used for the mass balance and excretion analyses were used for metabolite profiling and identification). Three participants were excluded due to early termination or elimination of a large portion of the dose as vomitus.

Radioprofiles of plasma samples revealed the presence of unchanged imlunestrant and 3 metabolites (M1 [LSN3517019], M2 [LSN3527840], and M12) (Figure 4). Imlunestrant and metabolite M1 accounted for most of the radioactivity in circulation (20% each) (Table 2). Metabolite M2 accounted for 4.5% and M12 for 4.6% of the radioactivity in plasma. Imlunestrant and its 3 metabolites were detectable in plasma up to 264 hours. Metabolites M1 and M2 were analyzed for activity, and neither was found to be pharmacologically active.

Figure 4.

Pathways for the metabolism of imlunestrant (LY3484356) in humans (Part 1).

Table 2.

Summary of Imlunestrant Metabolites in the Plasma and Feces After an Oral Dose of 400 mg [¹⁴C]‐Imlunestrant Solution (100 µCi)

Compound	Chemical formula	Metabolic pathway	[M+H]+ (m/z)	Plasma % AUC0‐264 h	Feces % dose
Imlunestrant	C29H25F4N2O3	Parent	525.1789	19.8	61.8
M1	C35H33F4N2O9 +	Glucuronide conjugate of imlunestrant	701.2120	19.9
M2	C29H25F4N2O6S+	Sulfate conjugate of imlunestrant	605.1362	4.47	20.9
M5 + M10a	C29H27F4N2O4 + (M5) C23H15F3NO3 + (M10)	Addition of O+2H and reduction to an alcohol (M5) O‐dealkylation to bisphenol (M10)	543.1905 (M5) 410.0999 (M10)	‐	1.1
M7	C25H19F3NO4 +	N‐dealkylation of the azetidine ring and reduction to an alcohol	454.1257	‐	4.7
M8	C25H17F3NO5 +	Oxidation of M7 to a carboxylic acid	468.1054	‐	5.1
M9	C25H19F3NO7S+	Conjugation of M7 with sulfate	534.0833	‐	1.1
M11	C25H17F3NO8S+	Conjugation of M8 with sulfate	548.0625	‐	0.7
M12	C29H25F4N2O7S+	Mono‐oxidation on the ethoxy‐azetidine moiety, sulfate conjugation of the phenol	621.131	4.64	‐
Total				48.8	95.4

AUC_0‐264, area under the concentration–time curve from time zero to 264 hours.

Urine was not radioprofiled as the urinary elimination of imlunestrant was negligible (0.278% of the total radioactivity excreted).

Radioprofiles of fecal extracts from the 5 participants showed the presence of unchanged imlunestrant and 7 metabolites (M2, M5, and M10 [coeluting], M7, M8, M9, and M11) (Figure 4). Imlunestrant accounted for 61.8% and the metabolite M2 for 20.9% of the radioactive dose in the feces (Table 2). Metabolites M5/M10 (coeluting), M7, M8, M9, and M11 individually accounted for a mean of 5.1% or less of the radioactive dose.

Part 2: Absolute Bioavailability

After the oral administration of 400 mg of imlunestrant as two 200‐mg tablets (nonradiolabeled) in the fasted state, imlunestrant appeared steadily in plasma with a median t_max of 4.37 (range, 2.0‐16.0) hours and was eliminated with a mean (SD) t_1/2 of 32.1 (7.3) hours (Table 1 and Figure 3). Interindividual variability (CV%) was 45% for AUC_0‐tlast, 45% for AUC_0‐∞, and 59% for C_max. C_max of total radioactivity and [¹⁴C]‐imlunestrant was achieved within 15 minutes (range, 0.25‐0.32 and 0.08‐0.32 hours, respectively) after the start of intravenous infusion with 45 µg of [¹⁴C]‐imlunestrant (1 µCi). The mean (SD) t_1/2 was 32.5 (16.6) hours for [¹⁴C]‐imlunestrant and 74.5 (14.8) hours for total radioactivity. Total radioactivity in the plasma was quantifiable in all participants (N = 8) up to 192 hours after the start of intravenous infusion. Interindividual variability (CV%) was 24% for AUC_0‐tlast, 26% for AUC_0‐∞, and 21% for C_max. Based on the dose‐normalized AUC_0‐∞ ratio for oral versus intravenous dosing, the mean (SD) of oral bioavailability was 10.9% (3.58; range, 6.80‐18.2) after administration of imlunestrant as an oral dose of two 200‐mg tablets in the fasted state.

Safety

All participants (N = 8; 100%) in Part 1 experienced TEAEs (Table S4). The most common TEAEs included diarrhea (5 events), nausea (2 events), and vomiting (2 events). Three participants in Part 2 reported 5 TEAEs (nausea, headache, dysgeusia, supraventricular extrasystoles, and infusion site pain). All TEAEs were considered mild or moderate in severity and resolved by the end of the study. There were no SAEs, deaths, or discontinuations due to AEs.

Discussion

Imlunestrant is a next‐generation, oral SERD that is being investigated for the treatment of ER+ advanced breast and endometrial cancers. ³⁰ , ³¹ , ³⁷ , ³⁸ This Phase 1 study showed that the primary route of elimination of imlunestrant and its metabolites was via the feces (97.3%) with low urinary excretion (0.278%). Expired radioactivity after administration of [¹⁴C]‐imlunestrant was detected in the breath of only 1 participant. After a single oral dose of 400 mg [¹⁴C]‐imlunestrant solution, the overall recovery of the radioactive dose in excreta was greater than 90%. Unchanged imlunestrant accounted for 61.8% of the radioactive dose in the feces, indicating incomplete absorption of imlunestrant from the gastrointestinal tract or possible hydrolysis of its glucuronide or sulfate metabolites in the feces. Radioactivity was recovered in the feces after 24 hours following oral administration of [¹⁴C]‐imlunestrant solution, with substantial recovery occurring between 24 and 96 hours. This delay may be due to the biliary excretion of a substantial fraction of the dose as metabolites, which were then hydrolyzed back to the parent compound before elimination. This hypothesis is supported by the observation that a substantial fraction of orally administered imlunestrant appeared as a glucuronide metabolite in the bile from cannulated rats and was then metabolized by the gut flora back to imlunestrant (data not shown). However, the fraction of total imlunestrant present in the feces due to metabolite hydrolysis could not be determined from this human study.

The oral bioavailability of imlunestrant administered as a tablet in the fasted state was 10.9% (Part 2), and the oral bioavailability of imlunestrant dosed as a solution in the fasted state was 17.0% (Part 1, data not shown). Imlunestrant had a moderate first‐pass extraction, as approximately 35%‐50% of the drug that was absorbed went through first‐pass metabolism in the gut. Approximately 30% of the imlunestrant that passed through the portal vein was extracted by first‐pass metabolism in the liver. Therefore, the extent of first‐pass metabolism of imlunestrant in the gut and liver can be considered moderate based on the estimated values of 53%‐65%. In vitro studies have shown that imlunestrant is a substrate of enzymes such as cytochrome P450 (CYP) 3A4 and uridine 5′ diphospho‐glucuronosyltransferases (UGTs; including UGT1A1, UGT1A8, UGT1A3, UGT1A9, and UGT1A10) that are abundantly expressed in the gastrointestinal tract. This is similar to the metabolism of estrogen‐like molecules such as ethinylestradiol and raloxifene, which undergo substantial first‐pass glucuronidation and sulfation in the gastrointestinal tract. ³⁹ , ⁴⁰ , ⁴¹

Nine metabolites of imlunestrant (M1 [LSN3517019], M2 [LSN3527840], M5/M10 [coeluting], M7, M8, M9, M11, and M12) were identified in this study. Imlunestrant and metabolite M1 were the predominant drug‐related components in the plasma, each accounting for approximately 20% of the circulating radioactivity. Metabolites M2 and M12 constituted approximately 4.5% (for each) of the circulating radioactivity in the plasma. Metabolites M1 and M2 were analyzed in vitro for activity, and neither was found to be pharmacologically active. Imlunestrant accounted for 61.8% and metabolite M2 for 20.9% of the radioactive dose in the feces. Radioactivity recovered in the feces did not contain the glucuronide metabolite M1. This may be explained by metabolite M1 being excreted into the gastrointestinal tract and then hydrolyzed back to imlunestrant. Therefore, the parent drug recovered in the feces may have included the unabsorbed imlunestrant as well as imlunestrant resulting from the hydrolysis of M1. Imlunestrant has a moderate to high permeability and low solubility similar to Biopharmaceutics Classification System Class 2 compounds and is an in vitro substrate of the efflux transporter P‐glycoprotein. ⁴² Inhibition of P‐glycoprotein by quinidine resulted in a decrease in the systemic exposure of imlunestrant that was not considered clinically meaningful. ⁴² Consequently, the effect of glucuronidation or P‐glycoprotein–mediated transport on the bioavailability and systemic clearance of imlunestrant could not be determined. M5+M10, M7, M8, M9, and M11 (5.1% or less for each) were other imlunestrant metabolites detected in the feces.

A single oral dose of 400 mg of imlunestrant in the fasted state was well‐tolerated by healthy women. TEAEs were mild or moderate and resolved by the end of the study, and no SAEs were observed.

This study is limited by the small number of participants (N = 8 for each in Parts 1 and 2, with analyses in Part 1 based on 5 participants with evaluable data). Three participants were excluded due to early termination or elimination of a large portion of the dose as vomitus. However, the sample size was deemed appropriate for the current investigation based on published studies. The study was carried out in healthy postmenopausal women of non–childbearing potential. The disposition and oral bioavailability of imlunestrant may be different in patients with breast cancer. However, subsequent population PK modeling analyses showed a substantial overlap between the PK profiles of healthy participants and patients with metastatic breast cancer or endometrial cancer in Phase 1‐3 studies (data not shown). This finding indicates that the impact of the study population on the PK of imlunestrant was minimal. Oral bioavailability was estimated after participants were administered imlunestrant in the fasted state and may be different in the fed state. Further, interactions of imlunestrant with foods or other drugs need to be investigated.

Conclusions

The oral bioavailability of imlunestrant in the fasted state was 10.9%. Imlunestrant and its metabolites were primarily eliminated via the feces. Studies evaluating disposition in patients with renal impairment may not be necessary, as the radioactive dose recovered in the urine was minimal (0.278%). Additionally, only 12.7% of the metabolites resulting from oxidation were excreted in the feces. Imlunestrant was primarily cleared through O‐glucuronidation, O‐sulfation, and CYP3A4‐mediated oxidation. Therefore, as the activity of the CYP and UGT enzymes may be reduced in hepatic insufficiency, studies are ongoing to evaluate the safety and PK of imlunestrant in severe hepatic impairment. ⁴²

Conflicts of Interest

Authors are employees and stockholders of Eli Lilly and Company.

Funding

This study was funded by Eli Lilly and Company.

Supporting information

Supporting Information

Data Availability Statement

Eli Lilly and Company provides access to all individual participant data collected during the trial, after anonymization, except for PK or genetic data. Data are available to request 6 months after the indication studied has been approved in the United States and European Union and after primary publication acceptance, whichever is later. Access is provided after a proposal has been approved by an independent review committee identified for this purpose and after receiving a signed data‐sharing agreement. After a proposal is approved, data and documents, including the study protocol, will need to be provided in a secure data‐sharing environment. For details on submitting a request, see the instructions provided at www.vivli.org.