2.2 Phase I/IIa clinical study
2.2.1 Subjects
To be eligible to participate in the Phase I clinical study male and female subjects were required to be aged 25 years and above, with no previous exposure to serotonergic psychedelic substances, and registered with a general practitioner within the UK. Subjects were also required to have a body mass index (BMI) 18.0–30.9 kg/m2, no clinically relevant physical findings, electrocardiogram (ECG) or clinical laboratory parameters at the screening visit.
Subjects with a current or past diagnosis of a mental health disorder as defined by the American Psychiatric Association Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-5), a history of suicide attempts, or any first- or second-degree relative with a psychotic or bipolar disorder were excluded from the study.
2.2.2 Study design and treatment
The Phase I study was a randomised, double-blind, placebo-controlled, parallel-group dose-escalation trial (ClinicalTrials.gov: NCT04673383; EudraCT: 2020-000251-13). The study was conducted to meet criteria of European Medicines Agency (EMA) guidelines and Good Clinical Practice. The study was approved by the UK Medicines and Healthcare Products Regulatory Agency (MHRA) and London Brent ethics committee.
Due to the psychedelic effects of DMT and the absence of comprehensive safety and tolerability data from previous clinical studies, EMA guidelines for risk identification and risk mitigation were followed,22 together with scientific advice from the MHRA. Additionally, the study was designed in accordance with published guidelines regarding the clinical assessment of hallucinogenic substances in humans.23
At the screening visit, subjects provided written informed consent and attended a structured interview with the study psychiatrist using the Mini-International Neuropsychiatric Interview. All subjects participated in individual or group preparation sessions, including advice on what to expect and how to respond to the psychedelic experience at screening and again the day prior to study drug administration.
Subjects were admitted to the clinical pharmacology unit (Hammersmith Medicines Research [HMR], London, UK) the day before study drug administration. No intake of alcohol or caffeine for 24 hours before and during the treatment period was permitted. On the day of treatment, study drug was administered in a room set up according to best practice principles for psychedelic studies,23 including soft lighting, soft furnishings, music and photographs/art depicting scenes of nature; subjects were also asked to wear an eye mask during study drug administration. A therapist and a psychiatrist were present in the room, with additional clinical staff supervising study drug administration and blood sampling.
Subjects were required to remain in the clinical unit overnight for further psychological and safety evaluations and discharged the next morning dependent on satisfactory assessment outcomes. Follow-up was conducted by phone or video call for up to 3 months after study treatment.
Four single ascending dose levels of SPL026 were studied, dependent on tolerability and the PK profile of each dose. Further details of the clinical study and psychotherapy methodology will be published elsewhere.
SPL026 drug substance and drug product were manufactured in the UK in accordance with Good Manufacturing Practice (GMP) with 2.5 mg/mL DMT free base in 10 ml aqueous sterile solution; placebo consisted of the same ingredients and volume with the exception of the active substance. Active and placebo treatments were identical in appearance and administered in the same volume.
2.2.3 Dosing procedure
Study drug was administered as a continuous 10-minute IV infusion, split into 2 phases. A single cannula was inserted in the forearm vein with two separate syringes and two separate syringe pumps, connected by a 3-way tap in order to provide different infusions rates for the 2 phases of infusion. The first syringe pump infused study drug over 5 minutes (Phase 1), followed by another 5-minute infusion from the second syringe pump (Phase 2).
There were 4 dosing regimens of SPL026 (Table 1) and each dose cohort was randomised so that 6 subjects received SPL026 and 2 placebo (doses are stated as free base DMT unless otherwise indicated). Following the completion of each dose level, the infusion regimen for the next dose level was assessed according to PK modelling (described below).
2.2.5 PK modelling
The dosing regimen used in this study was selected based on PK modelling with the aim of generating a slower onset of the psychedelic experience, whilst achieving the same peak plasma concentrations attained in previous clinical studies. A one-compartment model was determined to best describe the PK of IV administered DMT fumarate data.22,23 The resulting model parameters were used to simulate multiple infusion regimens of total DMT doses ranging from 9–21.5 mg. Each infusion regimen comprised a 6 mg dose given over the first 5 min to support the intent of a gradual onset of psychedelic experience, followed by a second infusion designed to elicit further psychedelic effects (see the Supplementary Information for full details). Simulated PK profiles and associated peak plasma concentration (Cmax) predictions are presented in Figure S1 and Table S2.
The plasma concentration data from Cohorts 1, 2 and 4 were also modelled using a one-compartment infusion model. The model was considered an appropriate fit for the individual plasma-concentration data from Cohort 1 (mean parameter errors <15%), Cohort 2 (mean parameter errors <20%) and Cohort 4 (mean parameter errors <20%) (Table S2, Table S3). The resulting mean dataset parameters were used to generate predictions at different infusion times and rates prior to dose escalation (Figure S2). Cohort 3 data was deemed inappropriate for such modelling due to multiple missing samples (described in the Results section).
Compartmental and simulated PK analyses were conducted using PCModfit version 6.9 for Windows running with Microsoft Excel (2019).24
2.2.6 PK
Blood samples were drawn using a cannula inserted in the forearm vein (the opposite side to where study treatment was administered) pre-dose and at a nominal 2, 5, 6, 7, 10 (end of IV infusion), 11, 13, 15, 30, 60, 120 and 240 min after the start of the infusion.
Plasma concentrations at actual timepoints for each subject were analysed. The following PK parameters were calculated using all available data for each evaluable subject: Cmax; time to peak plasma concentration (Tmax), area under the plasma concentration–time curve from time zero to time of last measurable concentration (AUClast); area under the plasma concentration–time curve from time zero to infinity (AUCinf); terminal half-life (t1/2); clearance (CL); apparent volume of distribution during terminal phase after IV administration (Vz); volume of distribution at steady state after IV administration (Vss); and mean residence time after IV administration (MRTinf).
Determination of SPL026 and IAA in human plasma was conducted using a validated ultra high-performance LC-MS/MS system (Pharmaron UK Ltd., Hoddesdon, UK). The internal standard used for DMT was DMT-d8 and control blank was human plasma (K2EDTA; BiolVT Ltd.). The calibration range to be validated for DMT was 0.0619 ng/mL (lower limit of quantification [LLOQ]) to 310 ng/mL, and 2.00–1000 ng/mL for IAA.
2.2.7 Statistical analyses
The PK concentration population comprised subjects who received at least 1 dose of study treatment and for whom a blood sample had been analysed. The PK parameter population comprised subjects in the PK concentration population for whom PK parameters could be derived.
Actual sampling times were used to derive PK parameters and missing data were not imputed. Plasma concentrations of DMT below LLOQ of the LC-MS/MS were either treated as zero (if they occurred before Tmax) or considered as missing.
Plasma concentrations and PK parameters were summarised by treatment, using descriptive statistics. For log-transformed parameters, the primary measure of central tendency was the geometric mean and other parameters, it was the arithmetic mean or median.
Dose proportionality was assessed using analysis of variance (ANOVA), linear regression and the power model for Cmax and AUClast parameters. Dose-normalised and log-transformed PK parameters were compared between dose cohorts using one-way ANOVA. The linearity between log transformed Cmax, and AUClast vs SPL026 was determined25using a linear regression model, setting the administered dose as the independent variable and each PK parameter as the dependent variable. The linear relationship between Cmax and AUClast dose were assessed using the power model (log PK parameter = α + β * log(dose) + ε), where α=intercept, β=slope. The power model was fitted by restricted maximum likelihood (REML) mixed effect model, with intercept and log(dose) as fixed effects. The dose proportionality of each PK parameter was confirmed if the 90% confidence interval (CI) of β (log PK parameter vs log dose) included the value 1.0.
The relationship between BMI, weight and age vs dose-normalised Cmax and concentration at 5 minutes (C5min) was determined by linear regression analysis to confirm that flat dosing (rather than weight-based dosing) of SPL026 was appropriate.
PK analysis was conducted by the Statistics and Data Management Department at HMR, using WinNonlin version 8.1 or higher. Descriptive statistics were derived using SAS version 9.4 or higher, including mean, standard deviation (SD), median, minimum and maximum values. Additionally, for PK variables percent coefficient of variation (%CV) and 95% CI of the arithmetic mean were derived. The adjusted coefficient of determination (R2) measured the goodness-of-fit for the linear regression model (where values range from 0-1, 0 indicating that the predictor variable accounts for no variation in the dependent variable, and 1 predictor variable accounts for all variation in dependent variable values). Post-hoc analyses were performed using IBM SPSS version 28.0 and tested at the p<0.05 significance level.