Safety and pharmacokinetics of the non-hormonal male contraceptive YCT-529

Abstract

Background

Since nearly half of all pregnancies in the US and worldwide are unintended, there is a critical need for additional contraceptive options for men and women. After a hiatus in non-hormonal male contraceptive development of about half a century, the new chemical entity YCT-529 – a retinoic acid receptor-α antagonist - is being developed as a non-hormonal oral male contraceptive to decrease sperm count by impairing retinoic acid signaling in the testes.

Methods

Here, we report the results of the first in human Phase 1a clinical trial with YCT-529 to assess its safety, tolerability, pharmacokinetics and pharmacodynamics, and its potential effects on heart rate, inflammatory biomarkers, sexual desire and mood. Sixteen male volunteers were enrolled to receive single oral doses of 10, 30, 90 or 180 mg of YCT-529 in the fasted state. Volunteers also received 30 mg in the fed state to study the effect of food on the pharmacokinetics of YCT-529.

Results

Single doses of up to 180 mg of YCT-529 had no effects on heart rate, hormone (follicle-stimulating hormone, luteinizing hormone, and testosterone), sex hormone-binding globulin or inflammatory biomarker levels, sexual desire or mood. Further, there was no clear food effect on the pharmacokinetics of YCT-529.

Conclusions

Overall, YCT-529 was well tolerated in this single ascending dose study (ClinicalTrials.gov registration: NCT06094283), which is a substantial requirement in contraceptive development.

Plain language summary

Since nearly half of all pregnancies in the world are unintended, there is a critical need for additional contraceptive options for men and women. The small molecule YCT-529 stops sperm production, and we are developing YCT-529 as a non-hormonal oral contraceptive for men. To study the safety of YCT-529 in humans, we conducted a Phase 1a clinical trial where 16 healthy men received either placebo or escalating single doses of YCT-529 to assess its safety and tolerability. YCT-529 was well tolerated, and no adverse effects were noted. The positive results from this first clinical trial laid the groundwork for a second trial, where men receive YCT-529 for 28 days and 90 days, to study safety and changes in sperm parameters.

Introduction

Nearly half of all pregnancies in the US and worldwide are unintended, demonstrating a need for additional contraceptive options¹. Even though men state that contraception should be a shared responsibility, their options are limited to condoms, which have a 13% failure rate² and vasectomy, which is a surgical procedure with uncertain reversibility³. This demonstrates the lack of highly effective and reversible options. A series of bis(dichloroacetyl) diamines (BDADs) including WIN 18,446, and gossypol were studied in men at the end of the 1950s and early 1980s, respectively, as potential oral male contraceptives because of their ability to reduce sperm count^4,5. However, further development was abandoned because of side effects – BDADs so far are incompatible with alcohol causing the disulfiram reaction⁶ and gossypol may lead to hypokalemia⁷. These findings underscore the need for a careful analysis of a compound’s safety. After a hiatus in non-hormonal male contraceptive development of about half a century, YCT-529 - a retinoic acid receptor-alpha (RAR-α) antagonist that decreases sperm count by impairing retinoic acid (RA) signaling in the testes - is currently being developed as an oral, non-hormonal male contraceptive.

Retinoid signaling through the intracellular metabolite all trans-retinoic acid (ATRA) – a metabolite of vitamin A (retinol) - is crucial for male germ cell development and differentiation⁸. All trans-retinoic acid binds to the nuclear retinoic acid receptors (RARs)-α, β, and γ RAR-α has been validated as a contraceptive target with male RARA1 knockout mice being sterile, but otherwise normal⁹. Importantly, the resulting male sterility is reversible^{10, 11–12}. The results from our initial preclinical studies with sexually mature male mice and nonhuman primates (NHPs) showed that oral administration of YCT-529 once daily for 28 days and 14 days, respectively, resulted in impaired spermatogenesis that fully recovered¹³. Additionally, no adverse events were noted in mice that were dosed for 42 days or NHPs that were dosed for 108 days in total. Our IND-enabling studies with male rats and dogs confirmed efficacy, reversibility and safety with a 22-40X safety margin. Based on its compelling nonclinical efficacy and safety profile, we initiated a double-blind, placebo controlled, first in human single ascending dose Phase 1a clinical study with YCT-529. Here, we present the results of its safety, tolerability, pharmacokinetics, and pharmacodynamics in healthy male subjects demonstrating that YCT-529 was – as expected from a single dose administration - well tolerated and became bioavailable at concentrations that were efficacious in the animal species we tested. Even though sperm parameters were not assessed, the study results are relevant as they establish safety and tolerability of YCT-529 as a non-hormonal oral contraceptive drug.

Methods

Study participants

Study participants were healthy, vasectomized, men aged 32–59 years, with BMIs of 21.9–31.1 kg/m² who met all inclusion criteria. Participants were asked to abstain from alcohol, grapefruit juice, and other medications during the study period (Supplementary Data 1). Of the 40 screened subjects, 16 (40.0%) subjects were enrolled, and 24 (60.0%) subjects failed to meet the entry criteria. Table 1 summarizes demographics and baseline characteristics of the enrolled subjects.

Table 1. Demographic and Clinical Characteristics of Subjects at Baseline

Characteristic	All fasting placebo (n = 4)	All fasting active (n = 12)	All fed active (n = 8)
	Values presented as mean ± SD (range) or number (%)
Age and weight
Age at study entry, years	43 ± 6 (36–48)	46 ± 8 (32–59)	47 ± 7 (38–59)
Height, cm	179 ± 7 (169–184)	177 ± 7 (167–189)	178 ± 7 (167–189)
Body weight at study entry, kg	84.5 ± 9.5 (76.8–98.4)	86.2 ± 11.4 (69.3–111)	84.5 ± 13.9 (69.3–111)
BMI, kg/m2	26.4 ± 2.1 (24.4–29.1)	27.7 ± 3.4 (21.9–31.1)	26.8 ± 3.9 (21.9–31.1)
Ethnicity
Not Hispanic or Latino	3 (75)	12 (100)	8 (100)
Hispanic or Latino	1 (25)	0	0
Race
White	4 (100)	12 (100)	8 (100)
Smoking history and alcohol consumption
Subject does not smoke	3 (75)	10 (83)	7 (88)
Subject previously smoked	1 (25)	2 (17)	1 (13)
No alcohol consumption	0	2 (17)	2 (25)
No excessive alcohol consumption	4 (100)	10 (83)	6 (75)
Hormones and SHBG
T, nmol/L	17.6 ± 3.9 (10.8–23.1)	18.1 ± 5.3 (10.8–31.5)	15.2 ± 5.3 (8.2–23.4)
FSH, IU/L	4.4 ± 2.0 (2.3–7.7)	4.3 ± 2.0 (1.8–8.2)	4.0 ± 1.8 (2.1–7.4)
LH, IU/L	5.7 ± 2.4 (1.7–8.4)	4.3 ± 1.2 (1.8–6.8)	5.0 ± 2.1 (2.9–5.6)
SHBG, nmol/L	32.4 ± 10.8 (23–50)	36.9 ± 11.9 (20–56)	34.9 ± 13.2 (21–55)
Inflammatory biomarkers
TNF-alpha, pg/mL	9.2 ± 1.7 (6.3–11.8)	9.7 ± 1.8 (6.0–12.7)	9.5 ± 1.2 (7.8–11.1)
IL-6, pg/mL	2.5 ± 1.6 (1.5–5.1)	2.5 ± 1.9 (1.5–9.7)	2.0 ± 1.3 (1.5–5.3)
E-selectin, ng/mL	14.3 ± 5.3 (8.1–22.1)	16.2 ± 5.6 (8.2–16.1)	16.0 ± 5.6 (9.0–25.3)
vWF, IU/mL	1.2 ± 0.4 (0.7–1.6)	0.9 ± 0.3 (0.4–1.4)	0.9 ± 0.2 (0.7–1.5)
hsCRP, mg/L	1.1 ± 1.2 (0.3–3.9)	1.1 ± 1.0 (0.3–3.8)	0.6 ± 0.3 (0.3–1.0)

BMI body mass index, FSH follicle stimulating hormone, hsCRP high-sensitivity C reactive protein, IL-6 interleukin 6, IU international unit, LH luteinizing hormone, n the number of subjects in the analysis set, SD standard deviation, SHBG sex hormone binding globulin, T testosterone, TNF-alpha tumor necrosis factor-alpha, vWF von Willebrand Factor.

Study design

A phase 1a first-in-human trial evaluated single oral ascending-doses (SAD) of YCT-529 (10–180 mg) in 16 healthy males across 2 cohorts. The first subject was enrolled on November 20, 2023, the last subject was enrolled on March 5, 2024, and the study was completed on June 18, 2024. The primary objective was to assess safety and tolerability. The secondary objectives were to evaluate pharmacokinetics (PK), pharmacodynamics (PD) and the food effect on the PK of YCT-529. The exploratory objectives were to evaluate the effect of YCT-529 on placebo-corrected change from baseline QTc interval, and inflammatory biomarkers. The cohort size was estimated based on usual standards in first in human studies to ensure data in a minimum of 6 evaluable subjects per cohort¹⁴. An evaluable subject was someone who had completed the planned safety assessments up to Day 15 and PK assessments up to 144 hours after dosing.

The study received approval from the Riverside Research Ethics Committee and the Medicines and Healthcare products Regulatory Agency (MHRA), was conducted at Quotient Sciences’ Phase 1 unit in Nottingham, UK and is registered at www.clinicaltrials.gov as NCT06094283. All relevant ethical regulations were followed, and all study participants provided signed and written informed consent before any study procedures.

The study consisted of 3 periods (Fig. 1). Periods 1 and 2 were double-blind, randomized, placebo controlled, and the cohorts were split into sentinel and main groups and dosed in the fasted state. The sentinel groups consisting of 2 subjects (1 active and 1 placebo) were dosed 72 h prior to the main group, which included 6 subjects (1 placebo and 5 active). Cohort 1 received a single dose of 10 mg of YCT-529 in Period 1 (only 5 subjects received YCT-529), followed by 30 mg in Period 2. Cohort 2 first received 90 mg and then 180 mg of YCT-529. The washout period between Periods 1 and 2 was at least 14 days. In Period 3, a total of 8 participants (no sentinels) from both cohorts received a 30 mg dose after consuming a high-fat, high calorie breakfast 30 min prior to administration. Throughout all periods of the study, subjects fasted for 4 h after dosing.

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Fig. 1

Phase 1a study design.

Schematic overview of Part 1 (10 and 30 mg, fasted), Part 2 (90 and 180 mg, fasted) and Period 3 (30 mg, fed).

Periods 1 and 2 for both cohorts followed the same study design: Subjects underwent screening from Day −28 to Day −2, were admitted in the morning of Day −1 and dosed on Day 1, remained onsite until 48 hours post-dose (Day 3) and had follow-up visits on Days 5, 7, 9 and 15 (± 1 day). A final phone call was conducted on Day 22 (± 2 days) of the last period to check on their wellbeing.

Study drug, drug product and administration

YCT-529 sodium salt was manufactured by Quotient Sciences (Alnwick, UK) under Good Manufacturing Practices (GMP) conditions. YCT-529 sodium salt was filled into Swedish orange Size 0 hard gelatin capsules at strengths that yielded 10, 30 and 90 mg of YCT-529 free acid. Matching placebo capsules contained microcrystalline cellulose. Study drug (YCT-529 or placebo) was administered with 240 mL of water. Water was allowed ad libitum during the study, except for between 1 h before and after dosing.

Study assessments

The primary outcome measures included the incidence and nature of any adverse events (AEs), dose-limiting AEs and serious AEs; vital signs; 12-lead and Holter ECG assessments; clinical safety laboratory assessments (hematology, coagulation, serum chemistry, and urinalysis). The secondary outcome measures were plasma PK and serum PD (testosterone [T], follicle stimulating hormone [FSH], luteinizing hormone [LH] and sex hormone binding globulin [SHBG] levels). The exploratory outcome measures were serum levels of interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), E-selectin and high-sensitive C-reactive protein (hsCRP), and plasma levels of von Willebrand factor (vWF). A psychosexual diary was utilized to monitor for potential changes in sexual desire and mood¹⁵.

For each cohort, the Dose Escalation Review Committee (DERC) evaluated safety and PK data up to Day 15, along with available PD data, to determine the subsequent dose level (Fig. 1).

UPLC/MS-MS and PK parameter assessment

Plasma concentrations of YCT-529 were determined by ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC/MS-MS) by Quotient Sciences - Alnwick. Human plasma samples were spiked with reserpine (internal standard) and mixed with 5 mM ammonium acetate to precipitate proteins. Solid phase extraction was performed with a 1.7 μm Acquity UPLC BEH C18 Column (Waters, Milford, MA), and samples were analyzed using a Thermo TSQ Quantum or TSQ Vantage mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).

The flow rate was 0.5 mL/min with a gradient profile mobile phase A of 10 mM ammonium acetate/acetonitrile (98:2) and mobile phase B of 100% acetonitrile with turbo ion spray source in negative mode. The total run time was 5.0 min. The retention time was 3.04 min for YCT-529, and 3.02 min for reserpine. Parent/daughter mass to charge ratio (m/z) transitions were 434.4 → 390.4 and 607.7 → 181.1 for YCT-529 and reserpine, respectively. The range of the calibration curve was 2–500 ng/mL calibration curve, and 2.0 ng/mL was the lower limit of quantification (LLOQ). Method validation further demonstrated no hemolysis effect up to and including 1%, and frozen stability for 114 days at −20 °C and after 4 freeze-thaw cycles, and stability at room temperature for 21 h.

Plasma PK parameters were estimated where possible by non-compartmental analysis methods using Phoenix® WinNonlin® software (v8.3, Certara USA, Inc., USA).

Laboratory tests

All safety laboratory tests including hormone, SHBG and inflammatory biomarker analyses were performed at the onsite certified laboratory. Blood samples were collected to assess levels of hormones (T, FSH, LH), SHBG and inflammatory biomarkers (hsCRP, IL-6, E-selectin, vWF, TNF-alpha). Whole blood was collected into EDTA-K2 anticoagulant tubes for vWF analysis. For all other parameters, whole blood was collected into serum separator tubes. The serum was separated by centrifugation at 3500 x g for 10 min at 4 °C and stored frozen at −80 °C until analysis. Testosterone, FSH, LH, SHBG and IL-6 were analyzed with electrochemiluminescence (ECLIA), TNF-alpha and hsCRP with CLIA, and vWF with immunoturbidimetry. Each sample was measured in duplicates, and the average was calculated.

Statistics and reproducibility

All measurements were taken from distinct samples from 4 subjects in the placebo group, 5 subjects in the 10 mg YCT-529 fasting group, 6 subjects in the 30, 90 and 180 mg YCT-529 fasting groups, and 8 subjects in the 30 mg YCT-529 fed group. No sample was measured repeatedly. Graphs were created with Origin 2024 version 10.2.3.403.

AEs, laboratory variables, vital signs, 12-lead ECGs, and PD

No formal statistical analysis was performed for the respective assessments and descriptive statistics were utilized.

Concentration-QTc interval statistical analysis of Holter ECG data

Formal statistical analyses were performed to investigate the relationship between plasma concentrations of YCT-529 and ΔQTcF using linear mixed effects modelling techniques using PROC MIXED in SAS. Only time points were included in the model where both a QTcF value and a plasma concentration were available.

By time point statistical analysis of Holter ECG Data

Formal statistical analysis was performed on ΔQTcF, ΔHR, ΔPR interval, and ΔQRS duration using PROC MIXED in SAS.

Assessment of dose proportionality

The PK parameters C_max, AUC_0-t and AUC_inf underwent natural logarithmic transformation, and dose proportionality was assessed using SAS Software procedure PROC MIXED with the following power model: log (AUC or Cmax) = μ + β * log(dose), with μ = intercept, and β = slope. Resulting values were then brought into relation with the critical region defined as^16,17: 1 + ln (ΘL) / ln (r) <β <1 + ln (ΘH) / ln (r), with ΘL = 0.50, ΘH = 2.00, r = ratio highest dose level/lowest dose level, and β = slope from the power model. If the 90% CI for β lay entirely within the critical region, dose proportionality was concluded. 2^β was used to estimate the increase of PK parameters when doubling the dose.

Assessment of food effect

Formal statistical analysis using mixed effect modelling techniques was performed on C_max, AUC_0-t and AUC_inf upon administration of 30 mg YCT-529 in the fasting and fed states. The null hypothesis was that there is no difference between fed and fasting conditions. Only subjects who completed both the fed and fasting periods and who had reliable PK parameter estimates were included in the statistical analysis. Intra subject variability (CVw) values were calculated as follows: CVw = 100 × (exp(Mean Square Error) −1)^1/2 using PROC MIXED in SAS.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Results

Adverse events, vital signs and clinical safety laboratory assessments

Overall, no participant withdrew from the study, no study stopping criteria were reached, and no adverse events (AEs) of special interest, severe or serious treatment-emergent AEs (TEAEs) or TEAEs leading to study drug withdrawal were reported.

Fifty percent of subjects who received placebo (2 subjects), 50% of subjects who received YCT-529 in the fasted state (6 subjects) and 25% of subjects who received YCT-529 in the fed state (2 subjects) reported TEAEs, of which headache and respiratory tract infection were most common (Table 2). All TEAEs were temporary and fully resolved. Further, all TEAEs were classified as not related to YCT-529 except for one; asymptomatic cardiac arrhythmia of low clinical concern detected by Holter monitoring was classified as possibly related to YCT-529 in one subject of the 90 mg YCT-529 group. The cardiac findings - frequent ventricular extrasystoles with several couplets and one slow run (triplet), and several atrial couplets and one triplet - were isolated, asymptomatic, not accompanied by laboratory or clinical findings, mild, temporary, self-limiting, and resolved without intervention. This participant was evaluated by a cardiologist who examined the subject and reviewed his echocardiogram and Holter results and felt there was no evidence of cardiac disease or heart damage from his exposure to the study drug.

Table 2. Treatment-Emergent Adverse Events

MedDRA System Organ Class, Preferred Term and Severity	All placebo fasting n = 4		All active fasting n = 12		All active fed n = 8
	Mild N (%) [E]	Unrelated N (%)	Mild N (%) [E]	Unrelated N (%)	Mild N (%) [E]	Unrelated N (%)
Infections and Infestations
Viral upper respiratory tract infection	0	0	1 (8.3) [1]	1 (8.3)	1 (12.5) [2]	1 (12.5)
COVID-19	0	0	0	0	1 (12.5) [1]	1 (12.5)
Nasopharyngitis	0	0	1 (8.3) [1]	1 (8.3)	0	0
Oral herpes	0	0	1 (8.3) [1]	1 (8.3)	0	0
Pharyngitis	0	0	1 (8.3) [1]	1 (8.3)	0	0
Rhinitis	0	0	1 (8.3) [1]	1 (8.3)	0	0
Upper respiratory tract infection	1 (25.0) [1]	1 (25.0)	0	0	0	0
Musculoskeletal and connective tissue disorders
Arthralgia	0	0	0	0	1 (12.5) [1]	1 (12.5)
Musculoskeletal stiffness	0	0	0	0	1 (12.5) [1]	1 (12.5)
Myalgia	0	0	1 (8.3) [1]	1 (8.3)	0	0
Nervous system disorders
Headache	1 (25.0) [7]	1 (25.0)	0	0	0	0
Lethargy	0	0	1 (8.3) [1]	1 (8.3)	0	0
Cardiac disorders
Arrythmia	0	0	1 (8.3) [1]	1 (8.3)	0	0
Gastrointestinal disorders
Diarrhea	1 (25.0) [1]	1 (25.0)	0	0	0	0
General disorders and administration site conditions
Fatigue	0	0	0	0	1 (12.5) [1]	1 (12.5)
Immune system disorders
Seasonal allergy	0	0	1 (8.3) [2]	1 (8.3)	0	0
Psychiatric disorders
Libido decreased	1 (25.0) [1]	1 (25.0)	0	0	0	0
Skin and subcutaneous tissue disorders
Acne	0	0	1 (8.3) [1]	1 (8.3)	0	0
TOTAL	[10]	[10]	[11]	[11]	[6]	[6]

An AE was defined as treatment-emergent if the first onset or worsening of the event occurred after the first administration of study drug and before the final Follow-up Phone Call.

E denotes how many times the event was reported, N the number of subjects reporting at least 1 event, and n the number of subjects in analysis set.

Administration of YCT-529 at doses of 10–180 mg (fasted) and 30 mg (fed) showed no clinically relevant changes in vital signs (Table 3), hematology (Table 3 and Supplementary Data 2), coagulation (Table 3 and Supplementary Table 1), serum chemistry (Table 3 and Supplementary Data 3), or urinalysis (Table 3) up to 14 days post-dose compared to baseline. One subject who received placebo had increased levels of creatine kinase (CK), lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) on Day 8 post dose. Compared to baseline, CK was increased 92-fold (8277 IU/L vs. 90 IU/L) and declined substantially over the following 6 days to 306 IU/L (3.4-fold above baseline). Levels of LDH were increased 2-fold (277 IU/L vs. 132 IU/L), and ALT was increased 4.5-fold (86 IU/L vs. 19 IU/L). Markers of kidney injury (e.g., creatinine) were not elevated at any time point, and the subject was asymptomatic (e.g., no reporting of myalgia) One subject who received 10 mg of YCT-529 in the fasted state had a 3.5-fold increase in CK 14 days post dose compared to baseline (906 IU/L vs. 256 IU/L), which declined over the following 4 days below baseline levels (219 IU/L). These results indicate that all doses of YCT-529 (10, 30, 90 and 180 mg [fasted] and 30 mg [fed]) were well tolerated in this single ascending dose Phase 1a study.

Table 3. Summary of Changes from Baseline of Safety Assessments up to 14 Days Post Dose

	10 mg	30 mg	90 mg	180 mg	30 mg
	fasted				fed
ECG concentration-QTc analysis	no clinically relevant changes				N/A
ECG timepoint analysis (QTcF interval)	no clinically relevant changes				N/A
Vital signs	no clinically relevant changes
Hematology	no clinically relevant changes
Coagulation	no clinically relevant changes
Serum chemistry	no clinically relevant changes
Urinalysis	no clinically relevant changes

N/A not applicable, QTc corrected QT interval, QTcF QT interval corrected for heart rate using Fridericia’s correction.

Holter ECG assessments

Cardiac safety was assessed with ECGs and Holter monitoring. To evaluate whether YCT-529 had proarrhythmic potential after single dose administration of 10–180 mg in the fasted state, Holter ECGs were extracted for exposure-response and QTc prolongation analyses. In the concentration-QTc analysis, the upper bounds of the 90% CI remained below 10 msec (the threshold of regulatory concern¹⁸) at all dose levels at their respective geometric mean C_max. At 180 mg, the highest dose studied, the estimated ΔΔQTcF was −6.12 msec (90% CI: −13.12, 0.89 msec) (Fig. 2a, b). These results demonstrate that a single dose of up to 180 mg of YCT-529 had no impact on QTcF. Timepoint analysis (QTcF interval) confirmed these findings: the greatest time-matched and placebo-corrected changes from baseline QTcF were observed at 1–2 h post-dose across all doses, and the upper bound of the 90 % CI for time-matched and placebo-corrected changes from baseline QTcF did not exceed 10 msec for any dose levels at any timepoint (Fig. 2c, d and Table 3; time-matched but not placebo-corrected durations of QTcF intervals are shown in Supplementary Fig. 1). At 180 mg, the highest dose studied, the estimated ΔΔQTcF 2 h post dose was 1.7 msec (90% CI: -1.9, 5.3 msec). Additionally, no effects on HR interval, PR interval or QRS duration were observed (Supplementary Table 2).

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Fig. 2

Holter ECG exposure-response and timepoint analyses.

Concentration-response analysis: (a) Individual changes in QTcF from baseline at the respective plasma concentrations of YCT-529 after single dose administration of YCT-529. Shown are individual changes at 10 mg (dark grey triangles; n = 49), 30 mg (black squares; n = 60), 90 mg (pink diamonds; n = 60) and 180 mg (blue circles; n = 60), or matching placebo (light grey stars; n = 80) in the fasting state. Also shown are the model-predicted regression line (solid line) with upper and lower bounds of the two-sided 90% confidence interval (dashed lines). b Estimated ΔΔQT and two-sided 90% CIs based on geometric mean Cmax and β as the slope of the power model. Timepoint analysis (QTcF interval). c Placebo-corrected changes of QTcF from baseline as least square means ± 2-sided 90% CI after single dose administration of YCT-529 at 10 mg (light grey), 30 mg (dark grey), 90 mg (black) and 180 mg (blue) in the fasting state. The dotted line depicts the upper bound of the two-sided 90% confidence interval that was required to be <10 msec to conclude the absence of a clinically relevant effect of YCT-529 on QTcF. Shown are means ± SEM of n = 8 subjects (placebo), n = 5 subjects (10 mg YCT-529) and n = 6 subjects (30, 90 and 180 mg YCT-529) per time point. d Largest estimated ΔΔQTcF intervals with 90% CI at the fasted dose levels studied.

PK in the fasted and fed states, and dose proportionality

PK in the fasted state

Following single oral dose administration of 10–180 mg of YCT-529, maximum plasma concentrations (C_max) occurred with a median T_max of 8.00 h (10, 90, and 180 mg) and 4.03 h (30 mg) post-dose (Fig. 3a, c and Supplementary Table 3). Terminal slopes were reliably determined for all subjects and geometric mean terminal elimination half-lives (T_1/2) ranged between 51.41 and 75.72 hours across all 4 dose levels.

[See PDF for image]

Fig. 3

Exposure-time relationship after single oral dose administration of YCT-529 in the fasting and fed state.

a 336-hour PK profile of YCT-529 after oral administration of YCT-529 at 10 mg (light grey), 30 mg (black), 90 mg (blue) and 180 mg (orange) in the fasting state. Shown are individual data points and geometric mean concentrations ± geometric SD of n = 5 subjects (10 mg YCT-529) and n = 6 subjects (30, 90 and 180 mg YCT-529) per time point. b 336-hour PK profile of YCT-529 after oral administration of YCT-529 at 30 mg in the fasting (grey) and fed (blue) state. Shown are individual data points and geometric mean concentrations ± geometric SD of n = 6 subjects (fasting) and n = 8 subjects (fed). c Geometric mean concentrations ± geometric SD over 336 hours in all treatment groups.

PK in the fed state and food effect

Four subjects of Cohort 1 and 4 subjects of Cohort 2 received 30 mg of YCT-529 after a high fat breakfast to study the food effect on the PK of YCT-529. Maximum plasma concentrations occurred with a median T_max of 10.00 hours post-dose (Fig. 3b, c and Supplementary Table 3). Terminal slopes were reliably determined for all subjects and the geometric mean T_1/2 was 65.08 hours. In the 4 subjects who received 30 mg of YCT-529 in the fed and fasted states, relative bioavailability values (Fr_el) were 160.36% (C_max), 136.52% (AUC_0-t), and 133.89% (AUC_inf) indicating that peak and overall exposure following a high fat breakfast were increased compared to the fasted state (Supplementary Table 4).

Dose proportionality in the fasted state

To evaluate if there is a linear increase in exposure with increases in dose, we determined dose proportionality. Dose proportionality could not be concluded over the entire dose range (10-180 mg) with power model analysis (Supplementary Table 5). This was most notable following dose escalation from 10 to 30 mg. Therefore, dose proportionality was also assessed over a reduced dose range (30–180 mg). Power model analysis indicated that exposure (AUC_0-t and AUC_inf) increased 2-fold upon doubling the dose; C_max increased 2.3-fold. These results indicate that AUC_0-t and AUC_inf increased dose proportionally, whereas the increase in C_max was slightly higher than dose proportional.

PD effects on hormones and SHBG

YCT-529 is being developed as a non-hormonal male contraceptive and therefore should have no effects of hormonal homeostasis. To evaluate whether YCT-529 had any effects on hormone levels, we assessed free serum concentrations of testosterone (T), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) as well as sex hormone binding globulin (SHBG) up to 14 days post dose. Mean free serum T, FSH, LH and SHBG concentrations remained within reference ranges at all time points in all subjects per treatment group (Supplementary Table 6). Changes from baseline 1 and 14 days post-dose were minimal and not statistically significant in all treatment groups.

PD effects on inflammatory biomarkers

Published data suggests that RA plays a role in vascular inflammation¹⁹. For safety reasons, we therefore included the assessment of high-sensitivity C-reactive protein (hsCRP), an inflammatory biomarker that is correlated with cardiovascular risk²⁰, as well as surrogate biomarkers of early endothelial cell activation (IL-6, vWF and E-selectin), and TNF-alpha as a main inducer of inflammation²¹. Mean E-selectin, hsCRP and vWF concentrations remained within reference ranges at all time points in all subjects per treatment group (Supplementary Data 4). Changes from baseline 1 and 14 days post-dose were minimal and not statistically significant in all treatment groups. Mean ± SD IL-6 concentrations were briefly above the upper range of normal (7.0 pg/mL) 24 hours post-dose at 10 mg YCT-529, fasted (9.04 ± 4.43) and at 30 mg YCT-529, fed (8.55 ± 7.20 pg/mL). In the 10 mg group, 3 out of 5 subjects receiving YCT-529 had IL-6 levels above the upper range of normal (11.0, 15.2 and 9.0 pg/mL), and in the 30 mg fed group, 4 out of 6 subjects receiving YCT-529 had IL-6 concentrations of 7.4, 16.0, 14.5 and 19.5 pg/mL. No subject had a clinically relevant out-of-range IL-6 concentration at any time point. With the exception of very few time points TNF-alpha concentrations were consistently (including baseline) above the upper range of normal (8.1 pg/mL) in all subjects but were not clinically relevant.

Psychosexual diary

To assess sexual desire, sexual function and mood, subjects were required to complete a self-reported psychosexual diary¹⁵ once daily from baseline through Day 15 (Supplementary Data 5). The scores for Sexual Desire (Question 1) generally increased from baseline over time, with similar patterns in placebo and YCT-529 groups. Enjoyment of sexual activity (Question 2), mood scores (Question 3) and sexual experience, erection fullness, and duration (Questions 4, 5, 6) showed minimal or no changes from baseline in both placebo and active groups.

Discussion

Compared to drugs that cure a disease, the safety bar for contraceptives is much higher. The primary end exploratory endpoints of this Phase 1a clinical trial were therefore focused on safety assessments, such as occurrence of AEs, and changes in hematology, coagulation, serum chemistry parameters, inflammatory biomarkers, or mood. For safety reasons, the originally planned highest dose of 200 mg of YCT-529 was reduced to 180 mg, to not exceed exposure levels (C_max and AUC) that were deemed safe in our preclinical IND-enabling studies. There were no notable differences in the proportion of subjects reporting TEAEs between YCT-529 and placebo groups. Viral upper respiratory tract infection was the most reported TEAE by two subjects in the active groups. The majority of reported TEAEs were generally consistent with a Phase 1 healthy volunteer trial. The only TEAE that was considered possibly related to YCT-529 was a cardiac arrhythmia of short duration (frequent ventricular extrasystoles with several couplets and one slow run [triplet], several atrial couplets and one triplet) detected by Holter ECG assessment in one subject of Cohort 2 who first received 90 mg and then 180 mg of YCT-529. The first events were reported approximately 15 minutes prior to dosing with 180 mg. At that time, the previous dose of 90 mg had washed out already according to PK data. Additional events occurred approximately 220 minutes after dosing with 180 mg. Due to the temporality of dosing time and the occurrence of additional events after dosing with 180 mg, causality to the study drug could not be completely ruled out. The subject was fully investigated by a cardiologist and the outcome was unremarkable with no additional follow-up needed. All TEAEs resolved prior to the end of the study without intervention.

Retinoic acid receptor-α seems to be the major RAR to mediate neutrophil granulocyte maturation. It inhibits differentiation when it is not bound to RA (unliganded state) and promotes differentiation when bound to RA (liganded state)²². Absolute neutrophil counts were within range of normality (2.0–7.5 × 10⁹/L) in all subjects at all time points except for in one placebo subject who had lower counts in general and in one subject with 1.63 × 10⁹/L 14 days after receiving 30 mg of YCT-529 in the fed state. These results demonstrate that YCT-529 at single doses of up to 180 mg, despite a long half-life had no impact on maturation or differentiation of neutrophils.

Sporadic slight elevations in CK concentrations were noted at various time points (including screening) in 3 subjects who received placebo and in 6 subjects who received YCT-529. Notably, one subject who received placebo had a CK concentration of 8277 IU/L (reference range 38–204 IU/L) at 8 days post-dose in Period 2. All CK elevations were transient and associated with strenuous exercise.

To address potential effects of YCT-529 on cardiac safety – first and foremost prolongation of the cardiac corrected QT (QTc) interval – we performed exposure response modeling of intensive ECG data collected by Holter monitoring. This is in line with the most recent ICH guideline that is putting more emphasis on the outcome of the concentration-response analysis as the small number of subjects in a first in human study renders the per time point analysis less robust²³. A time-matched and drug-induced effect on the heart rate (HR) QTc beyond an upper bound limit of 10 msec is a threshold of regulatory concern¹⁸. The results of the concentration-response and timepoint analyses showed that YCT-529 did not prolong the time-matched and placebo-controlled QTcF beyond 10 msec at any dose level studied. Notably, at 180 mg (the highest dose tested), the upper bounds of the 90% CI were 0.89 msec (concentration response analysis) and 5.3 msec (time point analysis at 2 h post dose). These results indicate that administration of YCT-529 at single doses of 10-180 mg neither prolonged the time-matched and placebo-controlled QTcF to a clinically relevant extent (> 10 msec) nor affected HR, PR interval, and QRS duration. Even though cardiac arrythmia of short duration was detected by Holter monitoring in one subject of Cohort 2, this assessment holds true because the cardiologists who examined this subject further concluded that after reviewing all data and performing an echocardiogram that no further intervention was needed.

Assessment of PK parameters showed that following oral single dose administration of YCT-529, the median T_max in the fasted state was 8 hours (10, 90, and 180 mg), and 4 hours (30 mg). It is unclear why there was greater variability in the first cohort of men, as this could not be correlated with relevant demographic parameters, such as BMI. We therefore believe that intra- and intersubject variability is the result of metabolic differences. To study the food effect on the PK of YCT-529, 4 subjects of Cort 1 and 4 subjects of Cohort 2 received an oral dose of 30 mg after consuming a high fat breakfast. Compared to the fed state, the absorption of 30 mg YCT-529 in the fed state was delayed with a median T_max of 10 hours compared to 4 hours. In the 4 subjects who received 30 mg with and without food, C_max, AUC_0-t, and AUC_inf were increased by 60.4%, 36.5%, and 33.9%, respectively, compared with the fasted state. While these results combined with the delayed T_max make it seem as if food had an impact on the PK of YCT-529, the CIs for the food effect comparisons were very wide resulting in a low degree of precision for the food effect estimate. Various reasons can account for the high variability noted in the fed state, such as individual physiological differences, i.e., gastric pH²⁴, and small cohort size. Furthermore, high variability in exposure is not uncommon when using a simple drug in capsule formulation, as there are no excipients to modify the release rate or attempt to overcome physiological differences.

Dose proportionality is an important factor in drug development as it ultimately gives important information about drug clearance. We analyzed dose proportionality and overall, plasma exposure in the fasted state increased with dose. Over the full dose range (10–180 mg), the increase was less than dose proportional (C_max increased 1.84-fold, AUC_0-t 1.65-fold and AUC_inf 1.64-fold). Over the reduced dose range (30–180 mg), C_max increased 2.26-fold (greater than dose proportional), while AUC_0-t and AUC_inf increased dose proportionally (2-fold and 1.96-fold, respectively) indicating that drug clearance was constant after single dose administration of 30, 90 or 180 mg of YCT-529. Inconsistent dose proportionality is likely due to inter-subject variability of exposure for reasons discussed above, which was high across the 10, 30, and 90 mg dose levels (geometric CV% values 41.7–77.8%), and moderate at 180 mg (geometric CV% value 31.5%).

Exposure levels (AUC) at efficacious dose levels were 7713 h*ng/mL (AUC_0-inf) in the mouse after single dose administration and 29,085 h*ng/mL (AUC_0-48) in the NHP after 28-day administration of YCT-529¹³. Exposure levels (AUC_0-24) at efficacious dose levels on Day 28 were 2660 h*ng/mL in the rat and 21,200 h*ng/mL in the dog (Supplementary Data 6). Exposure levels (AUC_0-24) in men after single dose administration in this Phase 1a study were 2390 h*ng/mL (10 mg), 3520 h*ng/mL (30 mg), 8430 h*ng/mL (90 mg), and 27,300 h*ng/mL (180 mg). These results demonstrate that exposure levels in men after single dose administration reach levels associated with efficacy in animal species. Even though the long half-life of 51-76 hours could warrant dosing every 2–3 days, we are dosing men once daily in our ongoing Phase 1b/2a clinical trial (NCT06542237) to assess the PK profile of YCT-529 upon repeat-dose administration and to guide the dosing regimen in subsequent clinical trials.

Our preclinical studies with mice and NHPs demonstrated that hormone levels remained unchanged after dosing for 28 and 108 days, respectively¹³. To confirm this in men, levels of serum free T, FSH and LH, as well as SHBG were assessed following oral administration of 10–180 mg YCT-529 (fasted) and 30 mg YCT-529 (fed). As expected, hormone and SHBG concentrations remained consistent supporting the fact that YCT-529 does not change hormone levels.

Since RA might play a role in vascular inflammation¹⁹, we assessed concentrations of hsCRP (inflammatory biomarker correlated with cardiovascular risk²⁰), IL-6, vWF and E-selectin (surrogate biomarkers of early endothelial cell activation) and TNF-alpha (main inducer of inflammation²¹). Levels of hsCRP transiently increased above the upper range of normal (5 mg/L) in 2 subjects on Day 14 post dose. In one subject who received placebo, the hsCRP concentration was 12 mg/L (compared to 1 mg/L at baseline). In one subject who received 30 mg YCT-529 (fed), hsCRP levels were 16.4 mg/L (compared to 1 mg/L at baseline) and back at 0.8 mg/L levels 11 days later. In general, hsCRP values < 1 mg/L indicate low relative cardiovascular risk²⁰. Except for those 2 excursions, and baseline concentrations >1 mg/L in 8 subjects (2 receiving placebo, 6 receiving YCT-529), overall hsCRP levels were <1 mg/L indicating that YCT-529 did not change hsCRP levels to clinically relevant levels. This was expected after single dose administration of YCT-529. Twenty-four hours post-dose, mean increases in IL-6 from baseline were observed across all treatments, with the greatest effect observed in the 30 mg YCT-529 fed group. This is in line with the finding that a high fat diet can increase the expression of pro-inflammatory cytokines, such as IL-6^25,26. Increases were transient and mean IL 6 concentrations returned to near-baseline levels in all treatment groups by Day 14 post-dose. YCT-529 and IL-6 concentrations did not correlate, and similar IL-6 changes were observed in the fasted placebo and YCT-529 groups.

Clinical studies with hormone-based approaches have been shown to be highly effective in the majority of study participants; however, they bear several side effects, such as acne, weight gain and mood changes²⁷. To prove our hypothesis that YCT-529 as a non-hormonal therapeutic should not affect sexual functions or mood, we asked all subjects to complete a self-reported psychosexual diary¹⁵. Following administration of YCT-529 or placebo, sexual desire, sexual function and mood remained comparable to baseline scores. The results from this first in human study were in line with the expectation that sexual function and mood would remain unchanged after single dose administration of YCT-529 and because it was not expected to alter testosterone levels.

Conclusion

Administration of single oral doses of 10–180 mg YCT-529, fasted, and 30 mg, fed, is well tolerated by healthy male subjects. YCT-529 does not demonstrate an adverse cardiac safety profile, or change in hematology, coagulation or serum chemistry parameters, hormone or proinflammatory cytokine levels, sexual drive and mood, and shows good bioavailability (with no clear food effect). These results warrant further clinical development of YCT-529. A 28-day and 90-day repeat dose Phase 1b/2a trial is ongoing (NCT06542237).

Acknowledgements

We thank all the subjects for participating in this study and trying an untested drug knowing the uncertainty of benefit for themselves but the potential benefit for others. We are grateful to all the Quotient staff members who were involved in drug synthesis and this study for their endless support that was essential for the success of this study. We are thankful to all our investors who made this research possible.

Author contributions

The sponsor, YourChoice Therapeutics (N.M., S.W.M., K.H.M., E.P.R., R.C.S., A.B.) conceived, designed and oversaw the study. At the clinical contract research organization (CRO), Quotient Sciences, S.S. was the Principal Investigator, E.D. and G.W. were responsible for the PK analyses, D.H. was responsible for the statistical analyses, and H.W. and S.B. were responsible for overseeing the study and liaising between Sponsor and CRO. A.B. acquired funding. N.M. wrote the first draft. All authors participated in data interpretation, vouched for data accuracy and critical review of the paper.

Peer review

Peer review information

Communications Medicine thanks Stephanie Page, John Amory and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Data availability

The data generated in this study are included in this article and in Supplementary Fig. 1, Supplementary Tables 1–6 and Supplementary Data 2–5. The numerical data (source data) plotted in the graphs in Figs. 2a, c, 3a, b, as well as exposure data in rats and dogs at efficacious dose levels are summarized in Supplementary Data 6. The clinical study protocol, reporting and analysis plan, and/or individual de-identified data, along with an informed consent form, may be made available upon a direct request to the corresponding authors. After consideration and the approval of such a request, data will be shared through a secure online platform.

Competing interests

The authors declare the following competing interests: N.M. is co-founder and CSO of YourChoice Therapeutics. A.B. is co-founder and CEO of YourChoice Therapeutics. K.M. is an employee of YourChoice Therapeutics. S.W.M. and E.P.R. are paid consultants of YourChoice Therapeutics. N.M., S.W.M., K.H.M., E.P.R., R.C.S., A.B. conceived, designed and oversaw the study. N.M. wrote the first draft.

Supplementary information

The online version contains supplementary material available at https://doi.org/10.1038/s43856-025-01004-4.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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