This was a phase 1, double-blind, placebo-controlled, randomized study of repeat dosing of 5 μg, 10 μg, and 20 μg of LSD. The primary objectives were to evaluate the safety, tolerability, and pharmacokinetics of low dose LSD. Given the stage of development, this investigation aimed to explore a range of secondary outcomes related to cognition, sensory, and motor effects; a number of which are reported here.

The study was conducted in accordance with Good Clinical Practice, as required by the United Kingdom Statutory Instrument 2004 No.1031, The Medicines for Human Use (Clinical Trials) Regulations, and subsequent amendments. It was also performed in accordance with the ethical principles that have their origin in the Declaration of Helsinki. The study protocol and informed consent form were reviewed and approved by the independent ethics committee for the investigational site in accordance with International Conference on Harmonisation harmonized tripartite guideline on Good Clinical Practice and UK law. Each volunteer provided written informed consent after adequate explanation of the aims, methods, anticipated benefits, and potential hazards of the study.

Study volunteers

Healthy volunteers aged 55 to 75 years were screened within 28 days before randomization. Volunteers were assessed for eligibility based on medical history, physical examination, and inclusion and exclusion criteria. The key inclusion criterion was no LSD use in the past 5 years. Based on the modified SCID-CT, a subject with the lifetime presence of any of the following was excluded: psychotic symptoms that are not substance-induced or due to a medical condition or has a first- or second-degree relative with these disorders; any manic or hypomanic episode; lifetime presence of any major depressive episode; lifetime presence of substance abuse, or dependence on any substance in the past 5 years; current diagnosis of obsessive-compulsive disorder (OCD), dysthymic disorder, panic disorder, anorexia, and bulimia. Also excluded were volunteers who were current smokers, had blood pressure exceeding 160 mmHg (systolic) and 90 mmHg (diastolic), or were receiving concurrent medication or herbals intended for (or off label) psychiatric disorders or conditions, such as those mentioned above and non-exhaustively. In practice, none of the participants received concomitant CNS (central nervous system) medications during the trial period, except for one subject who received zopiclone for occasional insomnia in the placebo group. Volunteers were administered the Columbia-Suicide Severity Rating Scale (C-SSRS) and the SCID-CT (Clinical Trials Version; First et al. 2007) as a screening tool, modified based on guidelines described by Johnson et al. (2008) regarding human hallucinogen research.

Study design

A total of 48 subjects were included in the study. For logistical purposes, the investigation was conducted in 4 separate cohorts of 12 subjects each. Volunteers were assigned to 1 of 4 cohorts then randomly assigned to 1 of 4 dose groups: 5 μg, 10 μg, or 20 μg LSD of the active moiety or placebo in a 1:1:1:1 ratio within each cohort. Volunteers and study personnel were blinded to dose received but were aware of the dose range under investigation.

Volunteers received their assigned study dose in an in-patient setting on 6 separate occasions over 21 days with a 96-h interval. Volunteers were administered the same dose on each occasion, and each dosing day was spaced by 3 non-dosing days (i.e., doses were administered on days 1, 5, 9, 13, 17, and 21). All volunteers were kept at the clinical site for approximately 8–12 h post-dosing for testing, blood sampling, and safety monitoring. A total of 32 volunteers (i.e., 8 from each drug group) provided data for the evaluation of the pharmacokinetics (PK) on dose 1 and dose 6. A follow-up visit was conducted approximately 4 weeks after the last dose.

d-Lysergic acid diethylamide (d-LSD, HPLC purity > 99%, Onyx Scientific Limited, UK) was dissolved in ethanol at 25 mg/mL and prepared as a solution 50 μg or 2 μg d-LSD/mL in distilled water and completed to a final volume of 10 mL distilled water for oral administration. Placebo was distilled water only and was presumably indistinguishable from the LSD solution, especially given the very low dose.

Safety and tolerability assessments

Safety and tolerability evaluations consisted of adverse event (AE) monitoring, administration of the C-SSRS, blood pressure, and pulse rate at every visit from screening through to follow-up. Clinical laboratory evaluations included hematology, blood chemistry, and urinalysis at screening, baseline, doses 3 and 6, and follow-up. Electrocardiogram (ECG) parameters and physical examinations were assessed at screening and follow-up.

Treatment Emergent Adverse Events (TEAEs) that occurred in more than 2 volunteers are reported, and the incidence of AEs reported in more than 10% of volunteers was analyzed using a logistic regression model.

Pharmacodynamic assessments

Pharmacodynamic assessments reported here focus on validated cognitive tests, measurements of balance and proprioception, and subjective drug effect questionnaires. Other exploratory pharmacodynamic endpoints might be presented in future communications. No formal statistical assessment of sample size was conducted, but the number of volunteers was considered sufficient for this stage of development.

Cognition assessments

The Cambridge Neuropsychological Test Automated Battery (CANTAB) (Cambridge Cognition 2016) was performed at screening, baseline, and doses 3 and 6, between 2 and 3 h after dose administration and at the follow-up visit. Screening results were not included in analyses, as this time point was used only for familiarization of the volunteers to the battery. CANTAB tests were administered via iPad and included the following tasks:

Reaction time (RTI) measures reaction times to a visual target that appears on the screen.

Paired associates learning (PAL) measures visual memory and learning. Six boxes are presented on the screen. The volunteer must remember the pattern under each card and respond when prompted at the end of the trial.

Rapid visual information processing (RVP) measures visual attention. The volunteer is presented with single digits appearing sequentially in the center of the screen. The task is to look for a particular sequence of 3 numbers presented on the top right corner of the screen. When the sequence appears, the volunteer must tap a button.

Spatial working memory (SWM) measures spatial working memory. The volunteer is presented with boxes on the screen and is asked to find tokens that are hidden behind the boxes. The tokens can be revealed by touching the boxes and if a token is found in a box, that box will not be used again to hide other tokens in the same trial.

The CANTAB assessments resulted in 56 endpoints for analysis. Assessment scores were used to yield descriptive statistics for the 5 μg, 10 μg, 20 μg, and placebo groups at baseline, doses 3 and 6, and follow-up. Comparisons between treatment groups were performed using a one-way ANOVA with Bonferroni post hoc tests for each endpoint across time points similar to the CANTAB analysis method described in (Kuzmickienė & Kaubrys, 2015).

Balance and proprioception

Balance was assessed at screening, approximately 4 h post-dosing at every dose, and at follow-up. BTrackS™ is a balance measurement system, which computes postural sway based on a volunteer’s center of pressure (Goble et al. 2017; O’Connor et al. 2017).

Proprioception, the ability to sense stimuli arising within the body regarding position, motion, and equilibrium, was assessed at baseline and follow-up; and 2–3 h post-dosing at two of the following four doses: 2, 3, 4, and 5. The proprioception protocol is summarized in Goble (2010).

Balance and proprioception measures were analyzed using a repeated measures mixed-model (RMMM) where assessments were performed over time. The dependent variable in the model was the change from baseline value. The independent variables included dose (5 μg, 10 μg, 20 μg, or placebo) as a fixed factor, volunteers as a random term, and time (i.e., baseline, doses 1–6, follow-up) as a repeated term. The baseline value was used as a covariate term in the model to control for variable baseline performance.

Drug effect assessments

Subjective drug effect questions can be used to assess perceptual alterations and tolerability of a drug. A 22-question assessment was administered during dose days at multiple time points via visual analog scale (VAS). Time points were 15 min pre-dose and then at 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, and 8 h post-dosing. The VAS consisted of 22 questions drawn from different sources that include drug abuse liability measures: Drug Effects Questionnaire (Shram et al. 2011); Addiction Research Centre Inventory (Haertzen et al. 1963); Subjective Effects of Substances with Abuse Potential (Farré et al. 2007); and subjective scales previously used to investigate LSD (Schmid et al. 2014). Volunteers were presented with one VAS question at a time, with a slider allowing continuously varying response from 0 to 100%.

The Five Dimensional Altered States of Consciousness (5D-ASC) questionnaire is a self-report questionnaire used to measure retrospectively subjective experiences of altered states of consciousness. The 94 items were presented on a VAS scale (Dittrich 1998) and completed at approximately 7 h post-dose at every dose. The original five dimensions on which this scale measures alterations in consciousness include the following: oceanic boundlessness, dread of ego dissolution, visionary restructuralization, vigilance reduction, and auditory alterations. Changes were also assessed on the following eleven new subscales (Studerus et al. 2010): experience of unity, spiritual experience, blissful state, insightfulness, disembodiment, impaired control and cognition, anxiety, complex imagery, elementary imagery, audio-visual synesthesia, and changed meaning of percepts.

Both assessments were completed on Psytools (Delosis, London) via laptop computer.

For the VAS of subjective effects, maximum value (E max ) was used to assess differences across drug groups. E max represents the peak effects measured in the day (i.e., a single time point). The dose-response relationship for subjective effects (22 variables) and 5D-ASC (16 variables) was explored using polynomial regression models with linear, quadratic, and cubic terms. For the VAS, the E max values were averaged across dose days for each question, and for the 5D-ASC, the average value was over six doses for each of the variables.

Pharmacokinetics assessments

Blood sampling

Blood samples for plasma analysis of LSD were collected within 15 min prior to dosing and at 30 min and 1, 2, 4, 8, and 12 h post-dosing. Post-dose samples were taken ± 10% of scheduled time or ± 4-h window around the scheduled time, whichever was narrower. Immediately after blood collection, two lithium heparin tubes (1 × 4 mL and 1 × 6 mL, BD Diagnostics, Vacutainer®) were kept on wet ice until centrifuged. Within 30 min of blood collection, samples were separated by centrifugation for 10 min (1000–1200 g) at approximately 4 °C. The resultant plasma was withdrawn in approximately equal volumes of 1.5 mL into 2 appropriately labeled polypropylene tubes for PK assay and stored (within 1 h of the sampling time) at − 80 °C or lower until shipment.

Drug levels determination

Drug level determination in human plasma samples was performed at Analytical Services International Ltd. (UK).

For quantification, the LC-MS/MS system used consisted of an Applied Biosystems API4000 mass spectrometer coupled to a Agilent 1100 Series Micropump HPLC system. The HPLC system was equipped with an Agilent 1100 Series autosampler.

LSD and the internal standard (IS) LSD-D3 were separated on a 150 × 2.1 mm Alltima™ C18 5 μm stainless steel column (Grace). Mobile phase A consisted of methanol and mobile phase B was 10% methanol:formic acid solution, with a flow rate of 1.0 mL/min. The detector was an Applied Biosystems API4000 Turbo Ion Spray using Positive Ion Mode. The preparation of the calibration curve and calculation of plasma concentrations used least squares linear regression analysis with weighting factor l/× 2 obtained by the internal standard method using peak area. Computer software, Analyst 1.3.2, was used for chromatogram data analysis and quantitative calculation.

Stock solutions of LSD and IS were prepared by dissolving the accurately weighed drug in 0.1% formic acid water at concentrations of 10 μg/mL. Working solutions of LSD were freshly prepared by serially diluting the stock, to a calibration range of 200 pg/mL to 10 ng/mL, in blank control serum. A total of 100 μL plasma calibrator/control/samples was added to corresponding 5-mL polypropylene tubes. A total of 100 μL internal standard solution, 500 μL sodium hydrogen carbonate solution (saturated), and 2 mL of MTBE was added to all tubes. All tubes were mixed for a minimum of 15 min and centrifuged for a minimum of 2 min at 3500 rpm. The supernatant was transferred to clean 5-mL polypropylene tubes and evaporated to dryness in a SpeedVac. The extracts were reconstituted with 250 μL of 10% methanol/formic acid solution and allowed to stand for a minimum of 5 min before being briefly vortex-mixed. The extracts were transferred to polypropylene autosampler tubes for analysis by LC MS/MS.

Pharmacokinetic analysis

The PK parameters were derived, using non-compartmental methods, from plasma drug concentrations over time profiles for each individual volunteer. Maximum measured concentration C max and peak time T max were obtained directly from concentrations over the 12-h time profiles, as well as T first and T last (i.e., first and last quantifiable drug levels during the observation period). The lower limit of quantification (LLOQ) of LSD was 200 pg/mL. λ z (i.e., terminal elimination rate constant) was calculated by linear regression of the terminal linear portion of the loge concentration vs. time curve with a 1/Y2 weighting method, and t 1/2 (i.e., elimination half-life) was derived as ln(2)/λ z . The AUC 0-t was calculated using trapezoidal summation of the plasma measurable concentrations over the observation period. AUC 0-inf was extrapolated from AUC 0-last and the terminal elimination rate constant λ z as: AUC 0-inf = AUC 0-t + C last /λ z (m), where C last was the last measurable concentration and λ z (m) was the grand mean of all calculated λ z (here, 0.085). Recorded drug levels below the level of quantification (LLOQ), which included all values under 200 pg/mL, were treated as missing and excluded except for those recorded pre-dosing (time 0 h). For the purpose of calculating AUC 0-t when two consecutive plasma concentrations below LLOQ were encountered after T max , all subsequent values were excluded from the analysis.