Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of Subcutaneous RE104

Abstract

Background:

This study is the first to formally evaluate in humans the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics of RE104, a prodrug of the synthetic psychedelic known as 4-hydroxy-N,N-diisopropyltryptamine or 4-OH-DiPT.

Methods:

This double-blind, randomized, placebo-controlled, phase 1 study of single subcutaneous (SC) doses of RE104 (5 to 40 mg) included 6 cohorts and a total of 48 healthy adult participants with prior experiences with hallucinogenic or psychedelic compounds.

Results:

SC doses of RE104 were generally safe up to 40 mg with no serious adverse events (AEs) or deaths. Most AEs occurred acutely under supervision and were mild to moderate. The Columbia-Suicide Severity Rating Scale score did not increase during the study, and the Assessment of Alertness and Sedation Scale was largely normal at all timepoints regardless of dose. RE104 exposure, based on Cmax, AUC_0-t, and AUC_0-inf, increased with dose from 5 to 40 mg RE104. 4-OH-DiPT appeared rapidly in plasma (median T_max ranged from 1.0 to 1.25 hours across dose groups). Mean plasma 4-OH-DiPT t_½ ranged from 2.72 hours to 4.12 hours. PKs appeared linear at the doses examined. Plasma levels of 4-OH-DiPT correlated with the Drug Effect Questionnaire and Mystical Experience Questionnaire (MEQ). Dose-related increases were observed in frequency of the MEQ 30 “complete mystical experience” responders.

Conclusions:

Single SC doses of RE104 resulted in a psychoactive experience and a favorable safety profile similar to psilocybin but with a shorter duration of psychoactive effect (3 to 4 hours). Results suggest a potential for therapeutic effect, warranting further study.

Serotonergic hallucinogens, or psychedelics, such as d-lysergic acid diethylamide (LSD), mescaline, 3,4-methylenedioxymethamphetamine (MDMA), and psilocybin, induce changes in sensory perception, emotion, thought, and sense of self, leading to remodeling of mental functions.^1–5 These effects have been described as an altered state of consciousness, often including a mystical-type experience.⁶

After decades of scientific marginalization, psychedelics are now being evaluated as potential therapeutic options for a wide variety of psychiatric conditions.⁷ Psychedelic agents have shown great therapeutic promise in depression, cancer-related distress, and addiction, as well as anxiety associated with life-threatening diseases.^7–12 For example, one dose of psilocybin (25 mg) combined with psychological support in participants with a treatment-resistant episode of major depressive disorder (MDD) resulted in an incidence of response of 37% and remission rate of 29% at 3 weeks and a sustained response in 20% of participants at week 12.¹³

Existing antidepressants (eg, serotonin reuptake inhibitors and serotonin and norepinephrine reuptake inhibitors) are hindered by several therapeutic limitations. These include a slow onset of response, suboptimal response rates, adverse effects that can persist with chronic dosing (eg, weight gain, sexual dysfunction), discontinuation symptoms, and a necessity for chronic administration to relieve symptoms and prevent relapse.^14–18 Serotonergic psychedelic compounds may offer a solution to some of these limitations by providing rapid and long-term antidepressant effects that are mediated by 5HT2A agonism.^19,20 As well as mediating the subjective psychoactive effects of classic psychedelics, agonism of the 5-HT2A receptor is also presumed to mediate the antidepressant effects.

The clinical utility of psilocybin is also not without limitations. For example, this agent induces a psychoactive state that may persist for 6 or more hours.^13,18,21 During this time, health care professionals are required to supervise patients for safety reasons, which could divert valuable medical resources when scaled up for wider use, resulting in an expensive delivery model that may limit dissemination and utilization by those in need.^13,18,21

RE104 (5-(3-(2-diisopropylaminoethyl)-1H-indol-4-oxy)-oxopentanoic acid) hydrochloride) is an investigational prodrug of the synthetic psychedelic known as 4-hydroxy-N,N-diisopropyltryptamine or 4-OH-DiPT.^22,23 Anecdotal reporting indicates dose-related psychoactive effects of oral 4-OH-DiPT that last <3 hours.^19,22 It should be noted, however, that the aqueous solubility of 4-OH-DiPT itself is very limited and thus, not readily suitable for direct subcutaneous administration. A prodrug of 4-OH-DiPT was created to improve stability, solubility and absorption relative to unmodified 4-OH-DiPT.^22,23 Preclinical studies demonstrated rapid enzymatic cleavage of RE104 to the active metabolite 4-OH-DiPT in vivo. RE104 can be injected subcutaneously rather than taken orally, and thereby circumvents the potential issues related to gastrointestinal tract transit time and individual variability of metabolism and food intake. A subcutaneous (SC) formulation was chosen to promote complete absorption, enable rapid conversion to the active metabolite, and ensure a short and reproducible psychoactive experience with the drug that facilitates clinical application as a therapeutic agent. RE104 is in clinical development for postpartum depression (PPD) and other neuropsychiatric indications such as adjustment disorder.²⁴

The clinical study presented herein is the first to formally evaluate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of RE104 for injection in healthy participants.

METHODS

The study was approved and overseen by the Central Adelaide Local Health Network Human Research Ethics Committee. This committee is constituted and operated in accordance with the principles and requirements described in the Declaration of Helsinki (Ethical Principles for Medical Research Involving Human Subjects) and with the National Health and Medical Research Council National Statement on Ethical Conduct in Research Involving Humans (2007, including current updates, 2018). The conduct of the study was in accordance with the Integrated Addendum to International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH E6 R1), Guideline for Good Clinical Practice (GCP E6 R2), annotated with comments by the Therapeutic Goods Administration (TGA; 2018). The HREC approval for the study protocol was granted on May 18, 2022. All participants gave written informed consent before enrolling in the study.

Study Population

This study involved a total of 48 healthy participants (18 to 65 years of age, inclusive) with one or more prior recreational experiences with hallucinogenic or psychedelic compounds (excluding cannabis products) occurring more than 90 days before enrollment. Participants had not experienced any previous major adverse responses to a hallucinogenic or psychedelic drug (excluding cannabis) and did not have a current or history of a significant mental disorder as assessed by a Mini-International Neuropsychiatric Interview questionnaire²⁵ and through interviews with a qualified clinical psychologist or psychiatrist.

Key exclusion criteria included clinically significant cardiovascular, neurological, pulmonary, hepatic, renal, metabolic, gastrointestinal, urologic, immunologic, endocrine, or psychiatric disease; risk or history of suicidal behavior and/or suicidal ideation; personal or family history of psychotic or bipolar disorder; use of ayahuasca, kambó, yopo, ibogaine, psilocybin, dimethyltryptamine (DMT), 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), LSD, Syrian Rue, or other psychedelic agents, or use of amphetamines, opioids, or MDMA, within 90 days of screening. Additional exclusion criteria included the use of synthetic or naturally occurring cannabinoids from 28 days before study drug administration, and agreement not to use these through to end of study; a positive cotinine (a nicotine metabolite) test, alcohol breath test, or urine screen for drugs of abuse at screening and baseline; and the use of any psychoactive medication, haloperidol, any medication with monoamine oxidase (MAO) activity, or with an agent with precipitative potential for serotonin syndrome within 28 days. Participants were also excluded if they were pregnant, breastfeeding or had a positive pregnancy test at screening or on day −1; had positive results for serum hepatitis B surface antigen, hepatitis C antibody and human immunodeficiency virus testing at screening; had an eGFR at screening ≤60 mL/min/1.73 m²; and had a positive test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2; COVID-19) within 7 days of admission to the clinical research unit (CRU).

Prohibited medications included prescription drugs taken within the 28 days or 5 half-lives (whichever was longer) before study drug administration, and/or use of over-the-counter medication in the 14 days or 5 half-lives (whichever was longer) before study drug administration. Exceptions included: hormonal contraceptives for women of childbearing potential; up to 3000 mg paracetamol per day, or up to 1200 mg ibuprofen per day for a maximum of 3 days; and acute use of topical steroids.

Study Design

This double-blind, randomized, placebo-controlled, single ascending dose phase 1 study included 6 cohorts of healthy participants (Study Protocol RE104-101 registered with Australian New Zealand Clinical Trial Registry as ACTRN12622000713796). The study comprised a screening period of up to 28 days followed by a treatment period, during which participants were admitted to the CRU 1 day before the dosing (d −1). On day 1, participants received the assigned SC starting dose of RE104 (starting at a dose of 5 mg in the first cohort) or placebo (Fig. 1) in a fed state after domiciling overnight and consuming a planned evening meal on day −1 and morning meal on day 1. Participants were discharged from the clinic on Day 2 (~24 hour post-dose) and returned to the clinic on day 10 for a follow-up visit.

FIGURE 1.

Study design.

Ascending doses were tested in 6 placebo-controlled cohorts. Each cohort was planned to comprise 6 participants receiving RE104 and 2 receiving placebo (Fig. 1). A sentinel dosing strategy was utilized for the first 2 participants (n=1 RE104; n=1 placebo) for each dose level cohort tested. In the absence of clinically significant safety signals in sentinel participants, over a minimum 24-hour post-treatment observation period, the remaining 6 participants in the cohort (5 active treatment, 1 placebo) were then dosed. A safety review committee (SRC) reviewed safety and tolerability after completion of each cohort before moving on to the next dose level.

In accordance with FDA guidelines for estimating the safe starting dose in initial clinical trials and using the no-observed-adverse-effect-level established in the RE104 nonclinical safety program, this phase 1 trial in healthy adult participants applied a 10-fold safety margin for the first cohort dosed with 5.0 mg of RE104 (FDA Guidance, 2005). The dose was then subsequently increased based on the data collected during the trial and evaluated by the SRC, which permitted dosing to continue up to 40 mg RE104. Participants in the first four cohorts received doses of 5, 10, 20, and 30 mg of RE104. Additional dose levels (cohorts 5 and 6) could be, and were, added based on emerging safety, tolerability, and PK assessments; these included 35 and 40 mg of RE104. Midway through the 40 mg cohort (cohort 6), half of the participants were reassigned to 30 mg by the SRC due to tolerability issues seen in the first 4 participants dose in the first cohort.

The current standard in clinical psychedelic research is to facilitate adequate set and setting to ensure patient safety and optimal experience. “Set” and “setting” are concepts used to describe what the study participant brings to the psychedelic situation and what they find there.⁷ “Set” refers to the conscious and unconscious parts of the individual leading up to the experience. This includes their beliefs, hopes, fears, traumas, personality, and temperament, as well as their expectations and fantasies about psychedelic experiences. In the context of clinical research, the study participant’s attitude toward the research setting, the medication, cultural attributions to psychedelics, also constitute important parts of the study participant’s set.⁷ “Setting” refers to the physical space, environment, and context in which one experiences the drug’s effects. Setting includes elements such as music, safety equipment, hospital, and inhabitants of the space (eg, session monitors, study staff, and study participants). The study participant’s relationship with the session monitor and the study team is a primary aspect of the setting.⁷ To ensure an appropriate set all participants had a preparatory session before dosing that included psychoeducation about the possible range of experiences, potential adverse reactions, and techniques and interventions used to address them by trained session monitors. Before the study drug administration, the participants were provided with a set of headphones to listen to a pre-selected sequence of music and eye covers to ensure a setting with minimal sensory impact or disruptions. Administration of the study drug was performed under direct medical supervision and participants were accompanied by session monitors trained to provide safety oversight and support during the acute experience.

Blood samples for plasma PK analysis of RE104 and 4-OH-DiPT were collected at the following timepoints: Pre-dose; post-dose at 3, 7, 15, 30, 45, 60, 75, and 90 minutes and at 2, 3, 4, 5, 6, 8, 12, 16 (day 1); and at 24 hours (day 2). Quantification of RE104 and 4-OH-DiPT in human plasma was performed using liquid chromatography with tandem mass spectrometry (LC-MS/MS), as described in a previous publication.²²

Study Endpoints

The primary endpoints focused on the safety and tolerability of RE104. This included adverse events (AEs), vital signs, laboratory values, ECG, neurological examination, Columbia-Suicide Severity Rating Scale (C-SSRS), the modified Observer’s Assessment of Alertness and Sedation Scale (MOAA/S), Rey auditory verbal learning test (RAVLT) and injection site tolerability.

C-SSRS was assessed at screening and day −1 (to exclude participants with suicidal ideation before dosing) and the “since last visit” version was to be administered at subsequent visits (to monitor participants after dosing). The MOAA/S was used to monitor a participant’s level of alertness and sedation.²⁶

The secondary endpoints addressed the pharmacokinetic (PK) properties of single escalating doses of RE104 and 4-OH-DiPT. PK endpoints comprised maximum serum concentration (C_max), time to reach C_max (T_max), time of last measurable concentration (T_last), half-life (t_½), area under the curve from time zero to infinity (AUC_0-inf), AUC from time 0 to the last measurable concentration (AUC_0-t), dose-normalized C_max (C_max/Dose), dose-normalized AUC (AUC/Dose), apparent elimination rate constant (K_el), apparent total body clearance (CL/F), and apparent volume of distribution (Vz/F).

Several exploratory endpoints evaluating the initial PD effects of single escalating SC doses of RE104 were also assessed. This included the modified Drug Effect Questionnaire (DEQ), in which participants were asked to verbally respond to indicate how strongly they felt the drug effect and how high they felt on the scale of 0 to 10 after being administered the study drug.²⁷ The Mystical Experience Questionnaire (MEQ 30) was used to assess mystical experience during a session across 4 domains of experience: mystical (including items concerning internal unity, external unity, noetic quality, and sacredness), positive mood, transcendence of time and space, and ineffability. The research shows that the presence and intensity of the mystical psychedelic experience is correlated with therapeutic efficacy in some studies, to include both symptom reduction and improved quality of life²⁸ although it has not been associated with symptom improvement in all research.²⁹ A complete mystical experience is defined as >60% of the total score (mystical, ineffability, positive mood, and transcendence of time and space domains).^30–32

Statistical Analysis

Final statistical analyses were performed after database lock on April 28, 2023. The study sample size was not based on a formal power calculation but rather on generally acceptable participant numbers for a first-in-human study. A sample size of 6 healthy participants on active treatment at each dose level was thought sufficient to adequately meet the objectives of the study, to obtain initial safety and tolerability information, to estimate PK parameters, and to explore PD effects. No formal hypothesis testing was planned for this study.

Statistical methods for the safety analyses were performed using the safety population (all-treated participants) and were primarily descriptive in nature. The plasma PK parameters of RE104 and its active entity 4-OH-DiPT were derived by noncompartmental analysis of the plasma concentration-time profiles. AUC_0-t was calculated according to the linear up/log down trapezoidal method using the measured concentration-time values above the lower limit of quantification (LLOQ). AUC_0-inf was calculated by combining AUC_0-t and extrapolated portion of the AUC (AUC_extrap). The half-lives of RE104 and 4-OH-DiPT were calculated as follows: t_1/2=ln 2/λZ.

RESULTS

Participant Disposition

A total of 48 participants met the study eligibility criteria at screening. Of these, 36 participants were administered a single SC dose of RE104 and 12 participants were administered a single SC dose of matched placebo. The disposition of all study participants by treatment group is shown in Figure 2. All participants who received 30 mg RE104 were combined into a single treatment group and participants receiving placebo in any cohort were pooled to form a placebo group for comparison.

FIGURE 2.

CONSORT diagram of participants enrolled in the study.

Virtually all participants (97.9%) who were assigned to treatment and dosed went on to complete the study as planned; the exception was 1 participant who was dosed with 10 mg RE104 but was subsequently lost to follow-up (Fig. 2). All randomized study participants (n=48; 100%) were treated and included in the Safety Analysis set, and all 36 participants who received RE104 were included in the PK analyses.

Demographics

Demographic characteristics of age, sex, race and ethnicity were broadly consistent across all cohorts and treatment groups (Table 1). The median age across all participants at screening was 35 years and ranged from 18 to 60 years. Overall, 13/48 (27.1%) participants enrolled in the study were female. All female participants in this study were women of childbearing potential except one 59-year-old female participant in Cohort 6 (35 mg RE104) who was postmenopausal as confirmed by testing of FSH levels. The majority of participants were White and identified as being not of Hispanic or Latino ethnicity (both ≥91.7% per pooled RE104 or placebo treatment groups). Mean body weight and BMI were similar across all participants and all treatment groups.

TABLE 1.

Summary of Demographics (All-Treated Analysis Set)

	RE104 5 mg (N=6)	RE104 11 mg (N=6)	RE104 20 mg (N=6)	RE104 30 mg (N=9)	RE104 35 mg (N=6)	RE104 40 mg (N=3)	Pooled RE104 (N=36)	Pooled Placebo (N=12)	All Participants (N=48)
Age (y)
n	6	6	6	9	6	3	36	12	48
Mean	37.2	29.0	46.2	34.7	38.2	26.7	36.0	40.1	37.0
SD	8.47	10.53	10.07	8.96	13.26	1.15	10.92	13.01	11.47
Sex
Female
n (%)	0	2 (33.3)	1 (16.7)	3 (33.3)	1 (16.7)	2 (66.7)	9 (25.0)	4 (33.3)	13 (27.1)
Male
n (%)	6 (100)	4 (66.7)	5 (83.3)	6 (66.7)	5 (83.3)	1 (33.3)	27 (75.0)	8 (66.7)	35 (72.9)
Ethnicity
Hispanic or Latino
n (%)	0	0	0	1 (11.1)	2 (33.3)	0	3 (8.3)	0	3 (6.3)
Not Hispanic or Latino
n (%)	6 (100)	6 (100)	6 (100)	8 (88.9)	4 (66.7)	3 (100)	33 (91.7)	12 (100)	45 (93.8)
Race
Asian
n (%)	0	0	0	0	0	1 (33.3)	1 (2.8)	0	1 (2.1)
Black or African American
n (%)	0	1 (16.7)	0	0	0	0	1 (2.8)	0	1 (2.1)
White
n (%)	6 (100)	5 (83.3)	6 (100)	9 (100)	5 (83.3)	2 (66.7)	33 (91.7)	12 (100)	45 (93.8)
Native Hawaiian or Other Pacific Islander
n (%)	0	0	0	0	0	0	0	0	0
Australian Aboriginal or Torres Strait Islander
n (%)	0	0	0	0	0	0	0	0	0
American Indian or Alaska Native
n (%)	0	0	0	0	0	0	0	0	0
Other
n (%)	0	0	0	0	1 (16.7)	0	1 (2.8)	0	1 (2.1)

Safety and Tolerability (Primary Endpoint)

Single SC administration of RE104 was generally safe and well-tolerated in healthy adult participants up to and including a dose of 30 mg, with no severe AEs, and no reports of significant or challenging experiences that were observed at the 2 higher doses tested (35 and 40 mg RE104). Across all participants, treatment-emergent adverse events (TEAEs) occurred in 22/36 (61.1%) of participants in the pooled RE104 group with 89 events, compared with 4/12 (33.3%) of participants with TEAEs in the pooled placebo group with 5 events (Table 2). Most TEAEs were mild to moderate in severity. Two participants had severe adverse events as outlined below. Regardless of severity, all TEAEs resolved. Two participants experienced adverse events of mild or moderate twitching (at 30 and 40 mg, respectively), both of which were resolved within 6 days. No participants experienced a serious AE (SAE) and no participants withdrew from the study early due to a TEAE.

TABLE 2.

Overall Summary of Treatment-Emergent Adverse Events (Safety Analysis Set).

Participants With	RE104 5 mg (N=6) n (%) E	RE104 10 mg (N=6) n (%) E	RE104 20 mg (N=6) n (%) E	RE104 30 mg (N=9) n (%) E	RE104 35 mg (N=6) n (%) E	RE104 40 mg (N=3) n (%) E	Pooled RE104 (N=36) n (%) E	Pooled Placebo (N=12) n (%) E	All Participants (N=48) n (%) E
≥1 TEAE	2 (33.3) 3	2 (33.3) 4	2 (33.3) 5	8 (88.9) 30	5 (83.3) 31	3 (100) 16	22 (61.1) 89	4 (33.3) 5	26 (54.2) 94
≥1 procedure- related TEAE	0	0	0	1 (11.1) 1	0	0	1 (2.8) 1	0	1 (2.1) 1
≥1 serious TEAE	0	0	0	0	0	0	0	0	0
≥1 severe TEAE	0	0	0	0	1 (16.7) 1	1 (33.3) 1	2 (5.6) 2	0	2 (4.2) 2
≥1 treatment -related TEAE	1 (16.7) 2	2 (33.3) 4	2 (33.3) 5	8 (88.9) 25	5 (83.3) 27	3 (100) 16	21 (58.3) 79	2 (16.7) 2	23 (47.9) 81
TEAE leading to study withdrawal	0	0	0	0	0	0	0	0	0

TEAEs were defined as adverse events that commenced at or after the study drug administration. Treatment-related AEs were defined as AEs where the relationship to study drug was reported as ‘Related’ or is missing. Participants who experienced multiple events within a category are counted only once in the specific category (n); however, each instance of the event is counted (E). Percentages (%) are calculated based on the number of participants in the analysis set (N). Percentage (%) of participants (n) in each category are calculated based on the number of participants in the analysis set (N).

AE indicates adverse event; E, number of adverse events; N, number of participants; TEAE, treatment-emergent adverse event.

The frequency and number of TEAEs increased with administration of higher dose levels of RE104 (Table 2). Across all dose levels, the majority of TEAEs reported in participants treated with RE104 were assessed as treatment-related by the Investigator [79 out of 89 events were treatment-related in 21/22 (95.5%) of participants with TEAEs]. In contrast, TEAEs in half of all placebo-treated participants were assessed as treatment-related [2 out of 5 events were treatment-related events in 2/4 (50.0%) of participants with TEAEs].

The most frequent specific TEAEs in the pooled RE104 group (Table 3) were nausea (27.8%; 10/36 participants), sinus tachycardia (25.0%; 9/36 participants), restlessness (19.4%; 7/36 participants), headache (16.6%; 6/36 participants), and agitation (11.1%; 4/36 participants). Restlessness and agitation were only reported in participants treated with the higher dose levels (30 mg and above) of RE104. Nausea occurred at the highest frequency in participants treated with 40 mg RE104 (66.7%; 2/3 participants). In participants treated with RE104, diarrhea occurred in 8.3% (3/36) of participants and vomiting and abdominal pain in 5.6% (2/36) of participants. No TEAEs of nausea, diarrhea or vomiting or abdominal pain were reported by participants in the pooled placebo group.

TABLE 3.

Summary of Treatment-Emergent Adverse Events (Safety Analysis Set).

Participants With AE	RE104 5 mg (N=6) n (%) E	RE104 10 mg (N=6) n (%) E	RE104 20 mg (N=6) n (%) E	RE104 30 mg (N=9) n (%) E	RE104 35 mg (N=6) n (%) E	RE104 40 mg (N=3) n (%) E	Pooled Placebo (N=12) n (%) E
≥1 TEAE	2 (33.3) 3	2 (33.3) 4	2 (33.3) 5	8 (88.9) 30	5 (83.3) 31	3 (100) 16	4 (33.3) 5
Nausea	1 (16.7) 1	2 (33.3) 2	0	3 (33.3) 3	2 (33.3) 2	2 (66.7) 2	0
Sinus tachycardia	0	0	0	4 (44.4) 8	3 (50.0) 7	2 (66.7) 4	0
Restlessness	0	0	0	3 (33.3) 3	3 (50.0) 3	1 (33.3) 1	0
Headache	0	1 (16.7) 1	0	1 (11.1) 2	4 (66.7) 4	0	0
Agitation	0	0	0	0	2 (33.3) 3	2 (66.7) 3	0
Muscle twitching	0	0	0	3 (33.3) 4	0	0	0
Diarrhea	0	0	1 (16.7) 1	0	2 (33.3) 2	0	0
Hyperhidrosis	0	0	1 (16.7) 1	0	1 (16.7) 1	1 (33.3) 1	0
Abdominal pain	0	0	0	2 (22.2) 2	0	0	0
Vomiting	1 (16.7) 1	0	0	0	0	1 (33.3) 1	0
Fatigue	0	0	0	2 (22.2) 2	0	0	0
Thirst	0	0	0	0	2 (33.3) 2	0	0
Tremor	0	0	0	1 (11.1) 1	0	1 (33.3) 1	0
Feeling hot	0	0	1 (16.7) 1	0	0	0	1 (8.3) 1
Myalgia	0	0	0	0	1 (16.7) 2	0	0
Dysphoria	0	0	0	0	1 (16.7) 2	0	0
Gastroenteritis	0	0	0	0	0	0	1 (8.3) 1
Chest discomfort	0	1 (16.7) 1	0	0	0	0	0
Injection site bruising	0	0	0	1 (11.1) 1	0	0	0
Folliculitis	1 (16.7) 1	0	0	0	0	0	0
Musculoskeletal injury	0	0	0	1 (11.1) 1	0	0	1 (8.3) 2
Palpitations	0	0	1 (16.7) 1	0	0	0	1 (8.3) 1
Ageusia	0	0	0	0	1 (16.7) 1	0	0
Syncope	0	0	0	1 (11.1) 1	0	0	0
Tension headache	0	0	0	1 (11.1) 1	0	0	0
Anxiety	0	0	1 (16.7) 1	0	0	0	0
Fear	0	0	0	0	0	1 (33.3) 1	0
Hallucination	0	0	0	0	1 (16.7) 1	0	0
Irritability	0	0	0	0	0	1 (33.3) 1	0
Hypotension	0	0	0	0	1 (16.7) 1	0	0

Psychiatric disorder TEAEs were more frequently observed in participants treated with the highest dose of RE104 (40 mg). All psychiatric TEAEs in participants treated with RE104 were related to the study drug. No psychiatric TEAEs were reported by participants in the pooled placebo group. Psychiatric TEAEs reported included restlessness (19.4%; 7/36 participants); and agitation (11.1%; 4/36 participants) were only reported in participants treated with the higher dose levels of RE104. All other psychiatric TEAEs each occurred in 1 participant only, including dysphoria (35 mg RE104), anxiety (20 mg RE104), mixed emotional and conduct disturbance (30 mg RE104), hallucination (35 mg RE104), fear and irritability (both reported with 40 mg RE104). Two participants had TEAEs of agitation, which were severe and occurred in the highest RE104 dose groups (35 and 40 mg) on day 1. Both events were considered by the investigator to be related to the study drug. In one participant, a 59-year-old female, moderate agitation began 40 minutes after receiving 35 mg of RE104, which became more severe over the subsequent hour despite administration of nonpharmacologic supportive grounding techniques. The event resolved ~2 hours post-dose shortly after receiving administration of the 1 mg intravenous midazolam. In the other participant, a 26-year-old female, mild agitation began 10 minutes after receiving 40 mg of RE104, and worsened in severity by 35 minutes, which persisted for an additional 25 minutes despite administration of supportive grounding techniques, finally resolving shortly after administration of 2 mg of intravenous midazolam. In both cases, the events resolved on the same day as dosing and both participants completed the study per protocol.

Injection site reactions (ISRs) occurred in 7/36 (19.4%) participants treated with a single SC injection of 20 , 30, 35, and 40 mg of RE104, and in 1 placebo participant. The frequency of ISRs did not increase with increasing RE104 dose. All ISRs were mild and transient, and none were considered by the investigator to be clinically significant.

None of the investigated safety laboratory parameters demonstrated any clinically significant change from baseline, and none met the criteria for reporting as a TEAE. There were no clinically significant ECG abnormalities noted at any time in any treatment group. No participants receiving RE104 had a Fridericia’s corrected QT interval (QTcF) >500 msec, although 1 placebo participant had a QTcF >500 msec. There were no individual changes in QTcF >60 msec. Nonsignificant ECG abnormalities occurred at a similar frequency in all cohorts receiving RE104, with no worsening compared with the frequency observed at screening/baseline and all resolved spontaneously. The frequency of nonclinically significant ECG abnormalities in the pooled placebo group was comparable to that observed with the pooled RE104 treatment group.

The C-SSRS score showed no increase during the study, indicating no apparent increased risk for suicide and suicidal behavior. Assessments of MOAA/S at all timepoints were largely normal regardless of treatment (RE104 or placebo) or dose of RE104 administered, with participants responding to their names spoken in a normal tone [mean score of 5 (SD 0) and median score of 5 (minimum/ maximum =5)].

Regardless of the treatment (RE104 or placebo) or RE104 dose administered, no clinically significant changes occurred in RAVLT recall score or in the total RAVLT score at day 10 compared with assessments conducted at day −1 and baseline. There were no clinically significant effects on vital signs that were considered treatment-related, including blood pressure effects, laboratory values, or ECG findings during the study.

Pharmacokinetics (Secondary Endpoints)

RE104 was absorbed rapidly, with a T_lag of 0 hour and median T_max values ranging from 0.25 to 0.50 hour across dose groups. After reaching peak levels, plasma RE104 concentrations seemed to decline in a mono-exponential manner in all dose groups (Fig. 3). RE104 exposure, based on geometric mean C_max, AUC_0-t, and AUC_0-inf, increased with dose, and was dose-proportional over the dose range studied (5 to 40 mg) (Table 4). Geometric mean (geometric CV%) C_max, AUC_0-t, and AUC_0-inf values ranged from 149.756 (63%) to 1781.941 (19.9%) ng/mL, 134.8 (64.9%) to 1814 (15.8%) h*ng/mL, and 135.6 (64.6%) to 1814 (15.8%) h*ng/mL, respectively.

FIGURE 3.

Mean (+SD) Plasma RE104 concentration versus time profile by dose. A, Linear scale and (B) Semi-log₁₀ scale (PK population).

TABLE 4.

Summary of RE104 Plasma PK Parameters by Dose (PK Evaluable Population)

Plasma PK Parameters	RE104 Dose
	5 mg (n=6)	11 mg (n=6)	20 mg (n=6)	30 mg (n=9)	35 mg (n=6)	40 mg (n=3)
	Summary Statistics
Cmax (ng/mL)	149.756 (63.0)	491.960 (43.8)	779.898 (15.9)	1319.347 (47.2)	1224.425 (20.1)	1781.941 (19.9)
Tmax (h)	0.500 (0.25-0.50)	0.250 (0.25-0.50)	0.375 (0.25-0.50)	0.250 (0.12-0.50)	0.375 (0.25-0.50)	0.500 (0.25-0.50)
Tlast (h)	4.000 (3.00-5.00)	5.000 (4.00-6.00)	5.000 (4.00-6.02)	5.000 (4.00-5.15)	5.542 (4.00-8.00)	6.183 (6.02-8.00)
Tlag (h)	0	0	0	0	0	0
kel (1/h)	1.391 (0.39340)	1.485 (0.29261)	1.459 (0.40487)	1.641 (0.21051)	1.248 (0.53208)	1.329 (0.15708)
t½ (h)	0.5315 (0.14595)	0.4829 (0.10037)	0.5046 (0.12989)	0.4293 (0.061541)	0.6437 (0.25484)	0.5265 (0.063774)
AUC0-t (h*ng/mL)	134.8 (64.9)	433.2 (49.4)	764.6 (16.7)	978.2 (27.6)	1187 (15.4)	1814 (15.8)
AUC0-inf (h*ng/mL)	135.6 (64.6)	433.6 (49.3)	765.3 (16.7)	978.8 (27.6)	1188 (15.5)	1814 (15.8)
%AUCext (%)	0.6066 (0.40628)	0.1050 (0.036846)	0.08935 (0.039773)	0.06384 (0.026512)	0.08987 (0.095418)	0.03467 (0.019110)
Cmax/dose (ng/mL/mg)	29.951 (63.0)	49.196 (43.8)	38.995 (15.9)	43.978 (47.2)	35.439 (20.1)	44.549 (19.9)
AUC0-t/dose (h*ng/mL/mg)	26.96 (64.9)	43.32 (49.4)	38.23 (16.7)	32.61 (27.6)	34.34 (15.4)	45.35 (15.8)
AUC0-inf/dose (h*ng/mL/mg)	27.13 (64.6)	43.36 (49.3)	38.26 (16.7)	32.63 (27.6)	34.37 (15.5)	45.36 (15.8)
CL/F (L/h)	41.76 (20.132)	25.22 (11.553)	26.42 (3.9805)	31.69 (8.9501)	29.37 (4.3424)	22.22 (3.3857)
Vz/F (L)	31.75 (19.195)	16.58 (5.4335)	19.36 (6.3849)	19.93 (7.3932)	26.60 (9.6718); 6	16.67 (0.65750); 3

AUCs, C_max, and their dose-normalized transformations are presented as geometric mean (geometric CV%); n=number of observations.

T_max, T_last, and T_lag are presented as median (min-max); n=number of observations.

Plasma RE104 t_½ (mean values ranging from ~0.43 to 0.64 h), T_last (median values ranging from ~4.000 to 6.183 h), and clearance (mean values ranging from ~22 to 42 L/h) were generally similar for all doses. Dose proportionality of plasma RE104 exposure PK parameters (ie, AUC_0-inf, AUC_0-t, and C_max) was assessed using a power model. The 95% CI for the slope of the power models for AUCs and C_max included 1 and the slope estimates were between 0.8 and 1.25, suggesting dose proportionality over the dose range studied (5 to 40 mg).

4-OH-DiPT appeared rapidly in plasma, with a T_lag of 0.000 hour and median T_max values ranging from 1.000 hour to 1.250 hours across dose groups. After reaching peak levels, plasma 4-OH-DiPT concentrations seemed to decline in a mono-exponential manner in all dose groups (Fig. 4). Geometric mean (geometric CV%) C_max, ranged from 17.175 (30.5%) to 274.200 (9.2%) ng/mL, whereas AUC_0-t and AUC_0-inf values ranged from 75.29 (60.1%) to 1376 (54.5%) h*ng/mL, and 79.89 (59.4%) to 1405 (57.1%) h*ng/mL, respectively (Table 3).

FIGURE 4.

Mean (+SD) plasma 4-OH-DiPT concentration versus time profile by dose. A, Linear scale and (B) Semi-log₁₀ scale (PK population).

Plasma 4-OH-DiPT t_½ (mean values ranging from ~2.72 to 4.12 h) and T_last (median values ranging from 12.00 to 24.00 h) were generally similar for all doses. Dose proportionality of plasma 4-OH-DiPT exposure PK parameters (Table 5) was assessed using a power model. The 95% CI for the slope of the power models for AUCs and C_max included 1 and the slope estimates were between 0.8 and 1.25, suggesting dose proportionality over the dose range studied (5 to 40 mg).

TABLE 5.

Summary of 4-OH-DiPT Plasma PK Parameters by Dose (PK Evaluable Population).

Plasma PK Parameters	RE104 Dose
	5 mg (n=6)	10 mg (n=6)	20 mg (n=6)	30 mg (n=9)	35 mg (n=6)	40 mg (n=3)
	Summary Statistics
Cmax (ng/mL)	17.175 (30.5)	57.004 (39.4)	73.045 (35.7)	127.814 (40.5)	120.192 (28.7)	274.200 (9.2)
Tmax (h)	1.125 (0.75-1.50)	1.125 (0.75-2.00)	1.125 (0.75-3.00)	1.250 (0.12-1.50)	1.000 (0.50-2.00)	1.250 (1.00-1.50)
Tlast (h)	12.000 (8.00-24.08)	24.000 (8.00-24.02)	14.075 (6.00-24.00)	15.967 (8.00-24.03)	23.842 (12.00-24.07)	24.000 (15.98-24.00)
Tlag (h)	0	0	0	0	0	0
kel (1/h)	0.2656 (0.13427)	0.2622 (0.25123)	0.3590 (0.24571)	0.2973 (0.11225)	0.3076 (0.14506)*	0.2419 (0.14377)
t½ (h)	3.416 (2.1483)	4.116 (2.2212)	3.269 (2.7176)	2.782 (1.4450)	2.718 (1.2911)*	3.677 (2.1759)
AUC0-t (h*ng/mL)	75.29 (60.1)	260.2 (64.7)	300.9 (102.6)	430.7 (49.7)	494.1 (61.9)	1376 (54.5)
AUC0-inf (h*ng/mL)	79.89 (59.4)	268.0 (66.8)	311.9 (108.4)	439.3 (50.9)	422.4 (47.6)*	1405 (57.1)
%AUCext (%)	5.702 (3.5031)	2.882 (3.5558)	3.465 (3.2720)	1.923 (2.3039)	0.7741 (0.39408)*	2.017 (2.6801)
Cmax/dose (ng/mL/mg)	3.435 (30.5)	5.700 (39.4)	3.652 (35.7)	4.260 (40.5)	3.479 (28.7)	6.855 (9.2)
AUC0-t/dose (h*ng/mL/mg)	15.06 (60.1)	26.02 (64.7)	15.05 (102.6)	14.36 (49.7)	14.30 (61.9)	34.40 (54.5)
AUC0-inf/dose (h*ng/mL/mg)	15.98 (59.4)	26.80 (66.8)	15.59 (108.4)	14.64 (50.9)	12.23 (47.6)	35.12 (57.1)

AUCs, C_max, and their dose-normalized transformations are presented as geometric mean (geometric CV%);

T_max, T_last, and T_lag are presented as median (min-max); n=number of observations.

^*N=5.

Pharmacodynamics (Exploratory Endpoints)

Modified Drug Effect Questionnaire

Initial PD response to RE104 was determined using the modified DEQ to assess subjective experience of the strength of substance effects.²⁷ Results of the modified DEQ [Item 2/are you high right now? (DEQ-high)] over time demonstrated a strong RE104 dose-dependent effect (Fig. 5).

FIGURE 5.

Mean DEQ-high scores over time by dose (all-treated analysis set).

For doses ≥30 mg RE104 peak modified DEQ-high scores ranged from 6.44 (at 1.5 h post-dose for 30 mg RE104) to 10 (at 1 h post-dose with 40 mg RE104). The mean time to peak score across these 3 highest RE104 dose levels was 1.2 hours. The mean experience duration, based on a modified DEQ-high response score of >3, ranged from 2.7 to 4.2 hours with doses ≥30 mg. For all doses, with the exception of 40 mg, modified DEQ-high scores decreased to ≤1.5 by the 4-hour post-dose timepoint; for the 40 mg RE104-treated participants, this decrease was reached by the next assessment timepoint (at 6 h). The mean experience duration for 30 mg was 3.6 hours, and all participants at that dose had a score of ≤1 at 4 hours post-dose. Only 1 participant in the pooled placebo group reported a modified DEQ-high score >3 (at 2 h post-dose on Day 1), with a duration of experience of 1 hour.

Similar trends were observed with results for the modified DEQ [Item 1/feel /do you feel a drug effect right now?(DEQ-feel)] (data not shown).

Mystical Experience Questionnaire

Initial PD response to RE104 data was determined using the MEQ 30 to measure and assess individual episodes of mystical experience. MEQ 30 responders were defined as individuals reporting a “complete” mystical experience (defined as >60% of MEQ 30 in the total score), a threshold which is considered to be predictive of clinical efficacy.^28,30 A dose-related increase in frequency of MEQ responders was observed, with 66.7%, 83.3%, and 100% of participants in the 30, 35, and 40 mg RE104 treatment groups, respectively (Fig. 6).

FIGURE 6.

Mean MEQ 30 score for individual domain and total score by RE104 dose level (safety analysis set).

DISCUSSION

This phase 1 study represents the first formal evaluation of the safety, tolerability, PK, and PD effects of the investigational therapy RE104 and its active entity 4-OH-DiPT in healthy adult volunteers.

Safety evaluation, the primary objective of this study, demonstrated that SC doses of RE104, up to and including 40 mg, were safe, with mostly mild-to-moderate, transient TEAEs that are typical of those associated with the pharmacology of psychedelic serotonergic agonists. No participants experienced a SAE and no participants withdrew from the study early due to a TEAE. The overall safety profile of RE104 seems to be consistent with those reported for other serotonergic agonists including anxiety, nausea, headaches, and physical or psychological discomfort. The TEAEs observed are likely related to its mechanism of action rather than pleiotropic effects.^13,21,33 No new safety signals were observed compared with the nonclinical toxicology findings for RE104 and the established clinical profile for classical psychedelics, like psilocybin.^13,18,21,34 The safety of various psychedelic compounds including psilocybin was recently reviewed by Holze et al.³⁵ The administration of psilocybin, LSD, and mescaline resulted in moderate blood pressure and heart rate elevations. In this study, sinus tachycardia events were asymptomatic and resolved spontaneously. In those participants who experienced sinus tachycardia, the mean duration was 74.8 minutes (range 43 to 120 min) for 30 mg RE104 (n=4), 173.3 minutes (range 28 to 303 min) for 35 mg RE104 (n=3) and 109.0 minutes (range 59 to 159 min) for 40 mg RE104 (n=2). Mean heart rate increases were 23.5 beats per minute (BPM) (8 to 37 BPM) for 30 mg RE104, 31.3 BPM (24 to 42 BPM for 35 mg RE104 and 45.0 BPM (41 to 49 BPM) for 40 mg RE104 at 1 hour and less thereafter. The effects on heart rate and blood pressure were deemed not clinically significant. Laboratory safety parameters after RE104 did not show any clinically significant change from baseline, and none met the criteria for a TEAE.

In the present study in healthy participants, RE104 was not associated with an increase in suicidality as assessed by C-SSRS score.

RE104 was absorbed rapidly after SC administration and, after reaching peak levels, plasma RE104 declined in a mono-exponential manner with a concomitant formation of 4-OH-DiPT in all dose groups. RE104 and 4-OH-DiPT exposures seemed to be dose-proportional over the dose range 5 to 40 mg, with mean plasma t½ of RE104 and 4-OH-DiPT ranging from 0.43 to 0.64 hour and 2.72 to 4.12 hours, respectively, and clearance was generally similar for all doses. The appearance of 4-OH-DiPT was observed in the first blood sample taken at 5 minutes.

Plasma levels of 4-OH-DiPT roughly correlated with the clinical assessment of drug effect by the DEQ and MEQ, with maximum drug effect occurring at the plasma peak T_max levels (1 to 1.25 hour) and duration of the subjective acute experience resolved by <4 hours for participants on ≤30 mg RE104. These responses were evident both in terms of whether participants felt high and whether participants provided responses meeting the threshold for a complete mystical experience after administration of RE104. The PK profile of the active entity of RE104, 4-OH-DiPT, was generally aligned with the PD profile, as demonstrated by the modified DEQ coinciding with the peak plasma concentration T_max and PD effects. However, the subjective drug effect declined faster than the plasma levels of 4-OH-DiPT.

Within this study, doses ≤30 mg were generally better tolerated than the higher doses of 35 and 40 mg RE104, which led to acute, severe agitation as seen in 2 participants, 1 at 35 mg and 1 at 40 mg. Similar challenging experiences have been reported to occur with other classical psychedelics including psilocybin,³⁶ particularly at higher exposures. On the basis of tolerability, a single SC injection of 30 mg RE104 was selected as the target therapeutic dose and is currently under investigation in a randomized, active dose–controlled, phase 2 trial in adult women with moderate-to-severe PPD. This dose was not associated with any serious AEs or reports of significant or challenging experiences (as noted at the 2 higher doses evaluated) and was sufficient to achieve a complete mystical experience in a majority of participants (6/9, 67%), suggesting an appropriate balance of safety, tolerability and potential for therapeutic effect, and supporting selection of this dose for future clinical proof-of-concept study.^32,37,38 At this dose, the mean acute experience duration was 3.6 hours, a convenient interval for clinical safety monitoring, representing a 50% reduction in acute psychoactive experience duration relative to historical, although not direct comparative evidence with psilocybin (6 to 8 hours).¹³

The molecular mechanisms of RE104 in achieving its pharmacodynamic effects are unclear. However, agonism of the 5-HT2A receptor presumably mediates not only the hallucinogenic effects but also the antidepressant effects of serotonergic psychedelics.^39–45 In addition, various psychedelic compounds have been shown to promote neuritogenesis, spinogenesis, and synaptogenesis as evidenced by increased dendritic arbor complexity, promote dendritic spine growth, and stimulate synapse formation in the rat cortex.⁴⁰ The structural changes in the pre-frontal cortex after psychedelics are not only associated with serotonin 5-HT_2A receptor agonism but also a stimulation of the signaling pathways involved tropomyosin receptor kinase B (TrkB) (functioning as a receptor for brain-derived neurotrophic factor) and mammalian target of rapamycin (mTOR).⁴⁰ It remains to be determined whether such effects occur in humans and mediate the antidepressant response, or whether RE104 shares these mechanisms of action.

Limitations of the current trial include the absence of an active comparator for the pharmacodynamic effects and a lack of diverse study participants (the majority of participants were White, identified as being not of Hispanic or Latino ethnicity, and male). Future studies will include the evaluation of a broader patient population to assess the safety and efficacy of targeted indications. This study in healthy adult volunteers showed that a single SC dose of RE104 at 30 mg was generally well-tolerated with robust pharmacodynamic (PD) effects that indicate potential for therapeutic effect in treatment trials, given that the intensity and quality of the subjective drug experience may be predictive of treatment response to psilocybin therapy in depression.^32,37 RE104 at 30 mg is currently being evaluated for efficacy and safety when administered under health care provider supervision in patients with PPD. In addition, a clinical lactation study is underway to assess the extent of transfer of RE104 and its major metabolites into breast milk to further characterize the safety profile of RE104 in this population. Additional studies are planned for RE104 to include other neuropsychiatric indications, such as adjustment disorder in medically ill patients.

In conclusion, single SC doses of RE104 resulted in an acute subjective experience and a favorable safety profile similar to that reported with psilocybin but with a shorter duration of acute psychoactive effect (3 to 4 h). Results from this study suggests that a single SC injection of RE104 30 mg is well-tolerated and manifests a potential for therapeutic effect, warranting further study.